SYNTHESIS AND PROPERTIES OF BRANCHED
SEMI-CRYSTALLINE THERMOSET RESINS
Hans Claesson
Royal Institute of Technology
Fibre and Polymer Technology
Stockholm 2003
Akademisk avhandling
Som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framlägges till offentlig
granskning för avläggande av teknisk doktorsexamen fredagen den 5 september 2003, kl.
13.00 i kollegiesalen, administrationsbyggnaden, Valhallavägen 79, Kungliga Tekniska
Högskolan, Stockholm. Avhandling försvaras på engelska.
To my Wife
Laura
Abstract
This thesis describes the synthesis and characterization of branched semi-crystalline
polymers. Included in this work is the SEC characterization of a series of dendrimers.
The branched semi-crystalline polymers were synthesized in order to investigate the
concept of their use as powder coatings resins. This concept being that the use of branched
semi-crystalline polymers in a UV-cured powder coating system may offer a lower
temperature alternative thus allowing the use of heat sensitive substrates and the added benefit
of a reduced viscosity compared to linear polymers.
A series of branched poly(ε-caprolactone)’s (PCL) (degree of polymerization: 5-200)
initiated from hydroxyl functional initiators were synthesized. The final architectures were
controlled by the choice of initiator structure; specifically the dendritic initiators yielded starbranched PCL’s while the linear initiator yielded comb-branched PCL’s. The dendritic
initiators utilized were: (1) a 3rd-generation Boltorn H-30, commercially available
hyperbranched polyester with approximately 32 hydroxyl groups, (2) a 3rd-generation
dendrimer with 24 hydroxyl groups, and (3) a 3rd-generation dendron with 8 hydroxyl groups.
Linear PCL was synthesized for comparison. All dendritic initiators are based on 2,2bis(methylol) propionic acid. The comb-branched polymers were initiated from a modified
peroxide functional polyacrylate. The resins were end-capped with methylmethacrylate in
order to produce a cross-linkable system. The polymers and films were characterized using 1H
NMR, 13C NMR, SEC, DMTA, DSC, FT-IR, FT-Raman, rheometry and a rheometer coupled
to a UV-lamp to measure cure behavior.
The star-branched PCL’s exhibited considerably lower viscosities than their linear
counterparts with the same molecular weight for the molecular region investigated (2-550 kg
mol-1). It was also found that the zero shear viscosity increased roughly exponentially with M.
The PCL star-branched resins are semi-crystalline and their melting points (Tm) range from
34-50°C; films can be formed and cured below 80°C. The viscoelastic behaviour during the
cure showed that the time to reach the gel point, a few seconds, increased linearly with
molecular weight. The crossover of G’ and G’’ was used as the gel point. Measurement of
mechanical properties of films showed that the low molecular weight polymers were
amorphous while those with high molecular weight were crystalline after cure.
The polymerization of 5,5-dimethyl-1,3-dioxane-2-one (NPC) from oligo- and
multifunctional initiators was evaluated utilizing coordination and cationic polymerization.
Two tin based catalysts, stannous(II) 2-ethylhexanoate and stannous(II) trifluoromethane
sulfonate, were compared with fumaric acid. Fumaric acid under bulk conditions resulted in
lower polydispersity and less chance of gelling. The synthesis of star-branched polymers was
confirmed by SEC data. The star polymers exhibited a Tg at 20-30°C and a Tm at about
100°C.
All semi-crystalline resins exhibited a fast decrease in viscosity at Tm. Blends of combbranched semi-crystalline resins and amorphous resins exhibited a transition behavior inbetween that of pure semi-crystalline resins and that of amorphous resins.
The SEC characterization of a series of dendrimers with different cores and terminal
groups showed that the core had an impact on the viscosimetric radius of the core while the
terminal groups appeared to have no effect.
Keywords: star-branched, semi-crystalline, comb-branched, ring-opening polymerization,
poly(ε-caprolactone), dendritic, thermoset, low temperature curing, powder coating, UVcuring, poly(5,5-dimethyl-1,3-dioxane-2-one), size exclusion chromatography, rheology,
dendritic aliphatic polyester
I
Sammanfattning
Avhandlingen beskriver syntesen och karakteriseringen av en serie förgrenade
delkristallina polymerer. Inkluderat i detta arbete är också SEC karakteriseringen av en serie
av dendrimerer.
De förgrenade delkristallina polymererna syntetiserades och karakteriserades för att
undersöka ett koncept för deras användning som pulverbindemedel. Konceptet är att en
delkristallin struktur tillsammans med UV-härdning kan resultera i en lägre härdtemperatur
och på så vis möjliggöra användningen av pulverfärger på värmekänsliga substrat. En grenad
struktur kan också ge en lägre viskositet.
En serie av förgrenade poly(ε-kaprolakton)er (PKL) (polymerisationsgrad: 5-200) initierad
från hydroxyfunktionella initiatorer syntetiserades. Den slutliga arkitekturen styrdes av valet
av initiator. De dendritiska initiatorerna resulterade i stjärnförgrenad PKL och den linjära
initiatorn resulterade i kamförgrenad PKL. De dendritiska initiatorerna som användes var: (1)
en 3je-generations Boltorn H-30, kommersiellt tillgänglig hyperförgrenad polyester med
cirka 32 hydroxylgrupper, (2) en 3je generations dendrimer med 24 hydroxylgrupper, (3) en
3rd-generations dendron med 8 hydroxylgrupper. Linjär PKL syntetiserades för jämförelse.
Alla dendritiska initiatorer är baserade på 2,2-bis(metylol) propionat syra. Den kamförgrenade
polymeren initierades från en modifierad epoxid funktionell polyakrylat. Polymererna
funktionaliserades med metylmetakrylat för att ge en tvärbindningsbar polymer. Polymererna
och filmerna karakteriserades med 1H NMR, 13C NMR, SEC, DMTA, DSC, FT-IR, FTRaman, reometer, och en reometer utrustad med en UV-lampa för att mäta härdbeteende.
De stjärnförgrenade polymererna uppvisade en betydligt lägre viskositet än de linjära
polymererna med samma molekylvikt (M) (i området 2-550 kg mol-1). Det observerades
också att gräns skjuvviskositeten (η0) ökade exponentiellt med M, vilket var förväntat.
PKL stjärnpolymererna är delkristallina och smältpunkten (Tm) är 34-50°C; filmer kan
framställas och härdas under 80°C. Det viskoelastiska beteendet under härdning av de
stjärnförgrenade polymererna visade att tiden till att nå gelpunkten, några sekunder, ökade
linjärt med molekylvikten hos polymeren. G’=G’’ användes for att bestämda gelpunkten.
Mätningen av de mekaniska egenskaperna hos härdade filmerna av de stjärnförgrenade
polymererna visade att de filmer tillverkad från polymerer med låg molekylvikt var amorfa
medan de med hög molekylvikt var delkristallina.
Polymerisationen av 5,5-dimetyl-1,3-dioxan-2-one (NPK) från multifunktionella initiatorer
utvärderades med koordinations- och katjonpolymerisation. Två tennbaserade katalysatorer
och fumarsyra jämfördes. Fumarsyra in bulkreaktion resulterade i lägst polydispersitet och
mindre risk för gelning jämfört med de tennbaserade katalysatorerna. Syntesen av
stjärnförgrenade polymerer bekräftades av SEC-analys. Stjärnpolymererna hade ett Tg på 2030°C och ett Tm på ca. 100°C.
Alla delkristallina polymerer uppvisade en snabb minskning av viskositeten vid Tm.
Blandningar av delkristallina kampolymerer och amorfa polymerer uppvisade ett
smält/mjuknings beteende mellan det av en ren delkristallin och amorf polymer.
SEC karakteriseringen av en serie av dendrimerer med olika kärnmolekyler och terminala
grupper visade att viskositetradien påverkades av kärnmolekylen men inte av de terminala
grupperna.
II
List of Papers
This thesis is a summary of the following papers:
I
“Synthesis and Characterisation of Star Branched Polyesters with Dendritic
Cores and the Effect of Structural Variations on Zero Shear Rate Viscosity” H.
Claesson, E. Malmström, M. Johansson & A. Hult Polymer (2002), 43(12),
3511-3518.
II
“Semi-Crystalline Thermoset Resins: Tailoring Rheological Properties in Melt
using Comb Structures with Crystalline Grafts” H. Claesson, C. Scheurer, E.
Malmström, M. Johansson, A. Hult, W. Paulus & R. Schwalm Submitted to:
Progress in Organic Coatings.
III
“Star-Branched Poly(neopentyl carbonate)s” P. Löwenhielm, H. Claesson, E.
Malmström & A. Hult Manuscript.
IV
“Rheological Behaviour during UV-curing of a Star-Branched Polyester” H.
Claesson, M. Doyle, E. Malmström, M. Johansson, J-A. E. Månsson & A. Hult.
Progress in Organic Coatings (2002), 44(1), 63-67.
V
“Synthesis and Characterization of Bis-MPA Dendrimers with Different Core
and Terminal Groups” M. Malkoch, H. Claesson, P. Löwenhielm, E. Malmström
& A. Hult Manuscript.
III
Table of Contents
Abstract……………………………………………………………..…….. I
Sammanfattning…………………………………………...……………… II
List of Papers…………………………………………………………....…III
1. Purpose of the Study……………...………….………………………… 1
2. Background…………………………………..………………………… 2
2.1 Powder Coatings...……………………………………………..….……… 2
2.1.1 UV-Cured Powder Coatings……….………………………………. 4
2.2 Branched Polymers...………………………………...…………………… 6
2.2.1 Dendritic Polymers………………………………………………… 6
2.2.1.1 Dendrimers…….....…………………………………………... 6
2.2.1.2. Hyperbranched Polymers..……..……………………………. 8
2.2.2 Star-Branched Polymers…………….……………………………... 8
2.2.3 Comb-Branched Polymers…………………………………………. 9
2.3 Ring-Opening Polymerization………………………….………………… 9
2.3.1 Coordination Insertion Ring-Opening Polymerization……………..10
2.3.2 Cationic Ring-Opening Polymerization………………………….... 10
2.4 Rheology in the Molten State…………………………………………….. 10
2.4.1 Rheological Behavior of Linear Polymers………………………… 10
2.4.2 Rheological Behavior of Branched Polymers………………...…… 11
3. Syntheses and Chemical Characterization…………………………..…. 13
3.1 Monomers………………………………………………………………… 13
3.2 Star-Branched Poly(ε-caprolactone)’s……………………………………. 13
3.2.1 Synthesis…………………………………………………………… 14
3.2.2 NMR Characterization…………………………………………..…. 15
3.2.3 SEC Characterization……………………………………..……….. 16
3.3 Star-Branched Poly(neopentyl carbonate)’s………..……………....…….. 18
3.3.1 Synthesis………………………………………………………..….. 19
3.3.1.1 Monomer Synthesis………………………………………….. 19
3.3.1.2 Polymer Synthesis……………………………………………. 19
3.3.2 NMR Characterization………………………………………...…… 20
3.3.3 SEC Characterization…………………………………………..….. 22
3.3.4 Catalyst Evaluation………………………………………………… 23
3.3.5 Thermal Characterization…………………………………….……. 25
3.4 Comb-Branched Poly(ε-caprolactone)’s…………………………...…….. 26
3.4.1 Synthesis………………………………………………………….... 26
3.4.2 IR Characterization…………………………..…………………….. 27
3.4.3 SEC Characterization…………………………...…………………. 28
3.5 Bis-MPA Dendrimers…………………………………………………….. 29
3.5.1 SEC Characterization………………………...……………………. 30
4. Rheological Characterization………………………………………..…. 35
4.1 Zero Shear Viscosity of Star-Branched Poly(ε-caprolactone)’s…………. 35
4.2 UV-curing Rheological Behavior of Star-Branched Poly(ε-caprolactone). 36
4.3 Dynamic Viscosity from Solid to Molten State………………………..…. 42
4.3.1 Comb Poly(ε-caprolactone) and Blends…………….……………. 43
5. Film Characterization…………………………………………………... 45
5.1 Mechanical Properties of Star-Branched Poly(ε-caprolactone) Films…… 45
5.2 Comb Poly(ε-caprolactone) Films……………........................................... 46
5.2.1 Powder and Film Preparation……………………………………… 46
5.5.2 Film Properties……………………………………………….……. 47
6. Conclusions……………………………………………………….……. 49
7. Suggestions of Further Work………………………………...………… 51
Acknowledgements…………………………………….…………………. 52
References…………………………………………………….……….….. 54
Appendix A – Structures of the Different Star-Branched Polymers
Appendix B – SECuc and Viscosity Data of all Star-Branched PCL’s
Appendix C – Synthetic Scheme of Boltorn-PNPC
1. Purpose of the Study
Thermoset resins are very important in industry were high demands are set on the final
properties; applications such as adhesives, molding compounds and coatings. One area were
thermoset resins have a significant share of the market in comparison to thermoplastic resins
is in the area of powder coatings. Since the introduction of powder coatings, the industry has
been striving to improve coating technology to widen its application to new markets such as
wood coatings. UV-cured powder coating systems were recently introduced as an alternative
to conventional thermal curing in an effort to capitalize on this new market. The goal of this
body of work was to investigate a new powder coating concept, which would result in a
reduction of the curing temperature. The approach was to investigate how changes in
macromolecular architecture, molecular composition, molecular weight and introduction of
crystallinity affect the properties relevant to powder coating applications, which include
rheological, cure, and final film properties. In addition, the effects of molecular weight and
architecture on resin crystallinity was also a subject of interest as was the relationship between
resin structure and resin properties before, during and after curing. To obtain better overall
knowledge of the relationship between architecture, functionality and properties in a crossfield approach, this included dendritic, star and comb polymers.
The specific purpose of this work was to:
Investigate the effect of star-branching on zero shear viscosity.
Investigate the UV-curing performance of star-branched polymers.
Synthesize comb-branched polymers and evaluate the film properties of the pure resin
and blends with conventional resins.
Develop a polymer better suited for a low temperature curing thermoset resin, i.e. with
a Tm and Tg proving storage stability and good film properties.
Evaluate a series of dendrimers with different cores and terminal groups utilizing a
triple detection SEC.
1
2. Background
This chapter briefly reviews the areas of interest covered by this thesis. First the basic
concepts, applications, and problems associated with powder coatings are introduced. This is
followed by a presentation of the different polymerization techniques utilized to synthesize
branched polymers. Finally, there is a review of the various macromolecular architectures and
their rheological behavior.
2.1 Powder Coatings
Powder coatings in their powder form can be either thermoplastic or thermosetting. The
first powder coatings, developed in the 1950’s, were thermoplastic and were applied to
preheated metal substrates.1 Thermoplastic powder coatings, by definition, are melted to form
a film at elevated temperature and solidify upon cooling. Film formation is a result of the
melting and coalescence of powder particles. In order for thermoplastic powder coatings to
achieve good mechanical properties, high molecular weight of the resin is required.
Coalescence and leveling are mainly surface tension driven and the level of viscosity the main
property affecting the rate of film formation.2,3 The manufacture of thermoplastic powder
coatings is relatively simple and raw materials are normally commodity polymers with overall
acceptable properties such as polyvinyl chloride, polyolefines, nylons and polyesters.4 The
main disadvantages with these coatings are high fusion temperature, low pigmentation levels,
and poor adhesion to metal substrates. In spite of these general shortcomings, some of them
display outstanding properties such as solvent resistance (polyolefines), outstanding
weathering resistance (polyvinylidene chloride) and exceptional abrasion resistance (nylon).
Low price and ease of handling are other advantages.2,5
A few years after the development of thermoplastic powder coatings, Shell Chemicals
developed the first thermosetting powder coatings. During film formation of traditional
thermosetting powder coatings, a thermally activated reaction takes place, resulting in the
formation of a cross-linked polymer. Their introduction solved many of the problems
associated with thermoplastic powder coatings. In the late 1960’s and early 1970’s when new
laws and regulations in industrialized countries gave powder coating technology a “bump”
forward.
The main components of thermosetting polymers are a primary resin and a cross-linker.4
Film formation is traditionally accompanied by chemical cross-linking (curing) that
commences when the system is heated. Curing affects both the viscosity and the flow. Caution
must be taken so that curing does not restrict film formation and leveling, i.e. curing must
start well above the glass transition temperature (Tg) of the resin. During curing a threedimensional network is formed by the low molecular weight resin and the cross-linker.
Network formation is dependent on the average degree of functionalization of the resin/crosslinker system. It is therefore necessary to control the degree of functionalization (fn), Tg and
the number average molecular weight (Mn) in order to obtain the desired network.6 If fn is
equal to or slightly over two, the final structure, after full conversion, will be a high molecular
weight linear or branched polymer which may have good mechanical properties but poor
solvent resistance. On the other hand, if fn is too high, the final structure will have excessive
cross-link density and may be brittle with poor mechanical properties. Systems can be tailored
to obtain desired properties by changing the chemical structure and cross-link density.
Powder coating formulation can be difficult since a number of their properties have
conflicting needs: (1) minimization of pre-mature cross-linking during production; (2)
2
stability against coalescence during storage; (3) coalescence, degassing, and leveling, i.e. film
formation, at the lowest temperature possible; and (4) cross-linking at the lowest temperature
possible.4 Coalescence of the powder during storage can be avoided if the resin has a high Tg.
On the other hand, a low Tg allows coalescence and leveling at a lower temperature since the
rule of thumb is that the lowest feasible curing temperature is 70-80°C above the Tg of the
powder.
Viscosity controls the flow and leveling of a powder coating. A low viscosity promotes
leveling while a high viscosity impedes leveling. The main driving force for leveling is the
surface tension, which is similar for most resins. Of the remaining parameters affecting
leveling, such as mean film thickness, and particle size, shape and distribution, viscosity is the
only parameter controlled by molecular architecture, weight, and chemical composition.
During leveling the shear rate is very low.3,4,7 Thus, the zero shear viscosity critically
influences leveling.
Over the years the terminology of the different systems has grown and become confused.
For clarity, table 2.1 shows some of the major classes of powder coatings along with some
details about each.
Table 2.1 Overview of the most common powder coating systems.5
Common
name
Primary resin
Cross-linker
Curing temp. (°C)
Epoxy
Bis-phenol A epoxy
Polyamines, anhydrides or phenolics
1808
Hybrid
COOH-functional
polyester
Bis-phenol A epoxy
160-2005
COOH-functional
polyester
Triglycidylisocyanurate or
hydroxyalkylamin
OH-functional
polyester
Blocked-isocyanate or amino acid
Epoxy-functional
acrylic
Dibasic acid
OH-functional acrylic
Blocked-isocyanate or amino resin
Acrylate-functional
resin
Free radical
1205
Epoxy-functional
resin
Cationic
1209
Polyester
Acrylic
UV*-cure
180-20011
130-1805
* Ultra Violet
The main advantages associated with powder coatings compared to solvent- or water-borne
coatings include: near zero volatile organic emission, high application speed, reduced energy
consumption, easy clean-up, recycling of over spray (>95% utilization), durable finishes,
possibility to apply thick films, and electrostatic application of 3D substrates. The powder
coating system also requires less skill and training to operate, substantially reduced
flammability and low toxicity.2,10,11,12These advantages have led to continuous growth of the
3
powder coating market during the last few decades. However, powder coatings do have some
limitations, namely: increased risk of dust explosions, inability to coat large or heat sensitive
substrates, some appearance limitations, major components must be solid which results in
material limitations, low production and application flexibility due to the clean-up needed
between color changes.4 In addition, the application techniques of thermoplastic and
thermoset powder coatings are commonly fluidized bed and electrostatic spray, respectively,
thus limiting their use to industrial settings.10
2.1.1 UV-Cured Powder Coatings
Traditionally, the powder coating cross-linking reaction has been controlled by
temperature. Normal curing temperatures are 160-200°C, and are thus not suitable for
application on heat sensitive substrates such as wood and plastic. Curing temperature is
determined by the combination of storage stability and the film formation process. Powder
coatings must be storable at 30°C without the resin particles fusing, thus the glass transition
temperature, Tg, of the polymer must be 60°C or higher. Film formation for amorphous resins
requires a temperature at least 50°C above Tg, giving a minimum curing temperature of about
110°C.13 One way to lower the curing temperature is by the introduction of semi-crystalline
material with a suitable melting temperature.14 The advantage of using a crystalline resin is
the rapid melting, versus the slow softening of an amorphous resin (figure 2.1).
Heat
Onset
of Cure
Leveling/
Curing
Leveling/
Curing
Thermally Cured
System
Leveling
UV-curing
UV Cured
System
Figure 2.1 Film formation process for a thermally cured system (left) versus an UV cured
system (right). The UV-curing allows for greater control due to the nature of initiation of
cross-linking.
Aside from having the appropriate resin, low curing temperature also requires an initiating
system that can be activated at low temperatures while stable at room temperature. This is
difficult to achieve with conventional thermal initiators, which normally obey an Arrhenius
relationship with respect to reaction rate as a function of temperature. One alternative is for
4
the onset of cure to be controlled by the use of ultra violet (UV) initiation, which may result in
smoother coatings when desired (figure 2.2).
In recent years UV-curing of powder coatings has obtained increased attention in industrial
research11,15 as this technique allows fast curing at lower temperatures than conventional
powder coatings.16 The research was triggered by the possibility of coating heat sensitive
substrates. Other advantages include shorter cycle times, improved storage stability, no
premature reaction during manufacturing, and better leveling since viscosity does not increase
until UV-irradiated.10 Film formation can be performed at low temperatures (90-140°C) and
cured in a matter of seconds with UV.4,10 Although UV-curing offers many advantages, it
does have limitations such as working film thickness, interferences due to UV absorbance by
pigments, and difficulty in curing complex shapes.10 Though pigmented coatings can be
cured, the clear coatings are the most interesting. Powder coatings with bis phenol A epoxy as
the binder are cationically UV cured. Acrylated epoxy resins, with or without acrylated
polyesters or unsaturated maleic resins as binders, are cured via a free radical mechanism.
Log(Viscosity)
Crystalline resin
UV-cured
Amorphous resinthermally cured
Onset of UV-cure
Temperature/Time
Figure 2.2 A schematic diagram of viscosity as a function of temperature/time for a thermally
cured amorphous resin and an UV-cured crystalline resin. The onset of cure can be
controlled when using an UV-curing system while the thermally cured system starts to cure
almost immediately.
5
2.2 Branched Polymers
There are a many different types of branched polymers. Dendritic (extremely branched),
pom-pom, H-, comb- and star-branched polymers are just some examples. Due to the complex
architecture of branched polymers, their properties differ from their linear counterparts. The
rheological properties of branched polymers have been studied extensively, especially the star
polymers where they have served as models to increase the general understanding of branched
macromolecules.
2.2.1 Dendritic Polymers
Dendritic polymers, comprised of dendrimers and hyperbranched polymers (figure 2.3), are
synthesized from ABx monomers. An ABx monomer consists of two different functional
groups, A and B, where there are two or more B’s for every A (figure 2.4). The final structure
is dependent upon the growth process. A controlled growth yields a dendrimer while an
uncontrolled growth yields a hyperbranched polymer.
Dendrimer
Hyperbranched polymer
Figure 2.3 Schematic representations of a dendrimer and a hyperbranched polymer.
2.2.1.1 Dendrimers
The term “dendrimer” was first coined by Tomalia et al. to describe a large family of
regularly branched poly(amidoamines).17 Dendrimers contain bonds that converge to a single
point with each repeating unit containing a branch junction and with the final molecule
featuring a very large number of identical chain ends (figure 2.3). The interest in dendrimers
is driven by their possibly unique rheological, mechanical and compatibility properties.17,,18,19
Dendrimers are synthesized under controlled conditions and display a perfect branching
pattern. The divergent and convergent growth approaches are the two different stepwise
procedures used to synthesize dendrimers. In the divergent growth approach, successfully
employed by Tomalia et al.17,18 and Newkome et al.20, the ABx monomers are added layer by
layer (or generation by generation) to a multifunctional core molecule. This process leads to
larger and larger dendritic molecules with an ever-increasing number of chain ends and
functional groups. The convergent growth approach, developed by Hawker and Fréchet21,
begins at the chain end and proceeds inwards through successive additions of dendritic
6
molecules to a single building block. In a final reaction, the completed dendrons or arms are
attached to a multi-functional core. While it has been shown that the two approaches can give
exactly the same structure, the growth process are essentially opposite. One disadvantage of
the divergent approach is that it generates a large number of functional end groups which may
not all react to form the next completely substituted layer (generation). This makes separation
of fully functionalized and almost fully functionalized species impossible. On the other hand,
the convergent growth approach offers better control over each step of the synthesis. With
each reaction, fewer functional groups are involved and purification is simplified due to the
large differences in molar mass between product and by-products. Multiple synthetic steps are
required to produce high molecular weight polymers with either approach making the
synthesis of dendrimers tedious and expensive.
However, recently Fréchet et al. developed a method for the divergent synthesis of
aliphatic polyester (bis-MPA) dendrimers utilizing an anhydride building block.22 This
approach proved to be highly efficient and circumvented the previous time-consuming
purification problems.
2.2.1.2 Hyperbranched Polymers
The main features distinguishing hyperbranched macromolecules from dendrimers are the
ability to synthesize these structures in one step and that they contain linear units. The linear
units produce characteristics such as broad molecular weight distribution and irregular
branching (figure 2.4).
Hyperbranched macromolecules, first reported by Kim and Webster23, have been studied in
detail since 1989. However, Flory discussed the fundamental concepts underlying their
synthesis more than 40 years ago.24 Flory predicted that ABx monomers with one reactive
group of type A and x reactive groups of type B would polymerize readily and give a soluble,
easy to process (low viscosity), three-dimensional structure free of cross-links.
B
Dendritic unit
B
B
B
B
+
Core molecule
A
B
B
A
B
B
B
B
B
B
B
B
B
B
B
Terminal unit
B
Linear unit
B
B
B
B
B
B
B
B
B
B
B
B
Core molecule
B
Figure 2.4 Schematic of the synthesis, structure and different repeating units of a
hyperbranched polymer.
Hyperbranched macromolecules contain three types of repeating unit dependent on degree
of substitution; dendritic, linear and terminal (figure 2.4). The dendritic unit is composed of
fully substituted AB2 monomers, the linear unit has one reacted and one unreacted B group,
and the terminal unit has two unreacted B groups. The degree of branching (DB), used to
characterize hyperbranched polymers, defined by Fréchet et al25, follows:
7
DB =
∑ Dendritic units + ∑ Terminal units
∑ All repeating units
Dendrimers have a DB of 1 since they contain only dendritic and terminal units. The
comparison of two hyperbranched polymers with the same chemical composition but different
DB has shown that solubility increases with the degree of branching while the melt viscosity
is inversely related.26
Hyperbranched polymers are usually prepared in a single-step polymerization and are thus
not as tedious to synthesize as dendrimers. Even though theoretically one-step growth of a
hyperbranched macromolecule could lead to a “perfect” dendrimer, it has never been
encountered due to the kinetics of the polymerization techniques used. A living
polymerization may lead to more dendrimer-like structures. To obtain a high molecular
weight hyperbranched polymer, several conditions must be fulfilled e.g. the reactive groups,
A and B, should only react with each other and side reactions should be kept to a minimum
preventing deactivation and cross-linking.27 Hult et al.28 used p-toluene-sulfonic acid to
catalyze the bulk synthesis of hyperbranched aliphatic polyester in the molten state. 2,2Bis(methylol)propionic acid (bis-MPA) was used as the monomer and 2-ethyl-2(hydroxymethyl)-1,3-propanediol as the core molecule. Voit et al. have performed additional
work with the same monomer.29
2.2.2 Star-Branched Polymers
Star polymers are one of the simplest forms of branched polymers. They consist of a core
molecule onto which linear polymers are coupled or grafted from. Synthesis can be divided
into three general approaches described in figure 2.5. The first route is the core-first method
where polymer chains are grown directly from a multifunctional core (route A). The other two
routes utilize the arm-first method, where preformed linear polymers are linked to a
multifunctional coupling agent or a diene (routes B and C).30
One of the most common approaches is anionic polymerization of monodisperse arms, then
attachment to a chlorosilane functional core molecule, which can vary in degree of
functionalization (route B), permitting good control of molecular weight and polydispersity.31
Other synthetic approaches include atom transfer radical polymerization,32,33 nitroxidemediated polymerization,34 ring-opening polymerization (ROP) utilizing coordination
insertion,35 ring-opening metathesis polymerization36 and radical addition-fragmentation
chain-transfer polymerization.37
Some of the uses for star polymers include various coating applications such as antifouling
coatings, conductive coatings and low volatile organic content coatings.38,39,40 Star polymers
are also used as colloidal stabilizers, additives to improve impact resistance and reduce
permeability, and in medical/surgical devices.41,42,43
8
Monomer
Multifunctional
initiator
+
Living prepolymer
Multifunctional
core moiety
R
+
Living prepolymer
Diene
Polymerization
Coupling
reaction
Linking
reaction
A
B
C
Figure 2.5 Schematic illustrations of the three approaches for the synthesis of a starbranched polymer.
2.2.3 Comb-Branched Polymers
Comb-branched polymers consist of a backbone with polymer side chains.44 The backbone
and side chains can either be of the same or different chemical composition (graft
copolymers). Some of the first graft copolymers were acrylonitrile-butadiene-styrene
copolymer (ABS), high impact polystyrene (HIPS) and non-ionic emulsifiers. In the case of
ABS and HIPS, the copolymerization yielded a better result than merely physical blending
due to the low compatibility between the components.45,46
The synthetic approaches for comb polymers are the same as for star polymers.
2.3 Ring-Opening Polymerization
Ring-opening polymerization (ROP) can proceed through a number of different
mechanisms depending on the type of monomer and catalyst involved. Of the wide range of
polymers produced via ROP, some have gained industrial significance; for example
poly(caprolactam) and poly(ethylene oxide).47 ROP of cyclic esters, such as ε-caprolactone
(CL) and L,L-dilactide, is gaining industrial interest due to their degradability.48 The
mechanisms of interest of interest in this work are coordination insertion and cationic.
9
2.3.1 Coordination Insertion Ring-Opening Polymerization
Coordination insertion ring-opening polymerization is an effective route to obtain welldefined polyesters. Stannous(II) 2-ethylhexanoate (Sn(Oct)2) is a common catalyst for the
ROP of lactones and lactides.49 There are two main proposed mechanisms for the ROP of
cyclic esters using Sn(Oct)2 and a hydroxyl functional co-initiator; complex formation
between the monomer and hydroxyl group prior to ROP and formation of a tin-alkoxide prior
to initiation.50,51
The main advantage of ROP of cyclic esters (and amides) versus a condensation reaction of
the equivalent hydroxy carboxylic acid is absence of water formation.
2.3.2 Cationic Ring-Opening Polymerization
A broad range of heterocyclic compounds can undergo cationic ring-opening
polymerization (CROP).48 CROP propagates either through an activated monomer or an
activated chain end mechanism. In both cases the propagation involves the formation of a
positively charged species.52 A major drawback of CROP is the occurrence of unwanted side
reactions, thus limiting the molecular weight of the final product.53,54
2.4 Rheology in the Molten State
Rheology is defined as the science of the deformation and flow of matter. In the molten
state linear polymers often exhibit pronounced viscoelastic properties, such as shear thinning,
extension thickening, viscoelastic normal stresses, and time-dependant rheology. All of these
properties are due to the physical nature of most polymers, which is long and easily distorted.
Viscosity is a measure of a fluid’s resistance to flow and describes the internal friction of a
moving fluid. Viscosity is an extremely important property of polymer melts and solutions
since it controls the possibility of processing. If the viscosity is too high, processing will be
difficult or impossible. In the case of powder coatings (and other types of coatings), viscosity
controls leveling.
2.4.1 Rheological Behavior of Linear Polymers
The main factor affecting viscosity in the molten state is the molecular weight. Figure 2.6
shows the well-known plot of how zero shear viscosity (η0) relates to molecular weight (M).55
η0 depends on M as η0 ∝ M below the critical molecular weight (Mc) and η0 ∝ M3.4± 0.1 above
Mc.
10
Log(η0)
η0∝M3.4
η0∝M1
MC
Log(M)
Figure 2.6 The relationship between zero shear viscosity (η0) and molecular weight (M) for a
linear polymer with low polydispersity.
The steep increase in viscosity above Mc is due to entanglements. Entanglements restrict
molecular motion by preventing chains from passing one another and moving perpendicular to
their own molecular contour. In order to relax stress, individual polymer chains have to move
along their own contour in a snake-like fashion. This snake-like motion is called reptation.56
For linear polymers the onset of entanglement starts at their Mc, which for highly flexible
polymers ranges between 300-600 atoms in the main chain.57
2.4.2 Rheological Behavior of Branched Polymers
Dendritic polymers are essentially free of entanglements and exhibit a spherical shape at
higher generations due to sterical and branching considerations. This results in unique
rheological properties, which is one of their most prominent features.58,59 In contrast to chaintype macromolecules, which exhibit a non-Newtonian shear thinning behavior at a critical
shear, dendritic macromolecules exhibit Newtonian flow behavior in solution and bulk
regardless of the number of generations.60
The rheological properties of star-branched polymers have also been extensively
investigated. Due to their simplicity, they have served as models for the development of new
or refinement of existing rheology theories.61,62 The rheological behavior of star polymers
differs from linear and dendritic polymers, since their relaxation modes are different.
Entangled star polymers cannot relax through reptation since one end is attached to a core
moiety. The arms relax through primitive path fluctuations63,64 and constraint release.65
During primitive path fluctuations, some times called “breathing modes”, a polymer arm is
drawn back and then re-extended into a new tube. Constraint release occurs when surrounding
arms fluctuate. Tube widening by constraint release is analogous to the addition of a low
molecular weight solvent and is therefore called “dynamic dilution”.66
As previously mentioned, η0 depends on M as η0 ∝ M below Mc and η0 ∝ M3.4 above Mc.
However, for star polymers the viscosity increases exponentially with molecular weight67 and
hence no Mc is found to coincide with the onset of entanglement. Also, the η0 of a star
polymer is not dependent on the total M, but on the arm M.68 This makes it possible to
11
increase the molecular weight without changing the viscosity by increasing the number of
arms.
Comb polymers are normally less defined than star polymers but studies have shown that
the η0 of comb polystyrenes is lower than that of a linear polymer with the same molecular
weight.69,70 In addition, Sherrington et al. prepared a large number of branched poly(ethylene
terephthalate)s (PET) by addition of a branching agent and a chain stopper. The branched
PET’s exhibited both lower solution and melt viscosities in spite of a significantly higher
molecular weight than the linear model polymer.71
12
3. Syntheses and Chemical Characterization
This chapter covers the synthesis and chemical characterization of the different branched
polymer structures, which appear in this work. The chemical characterizations include nuclear
magnetic resonance (NMR), size exclusion chromatography (SEC), Fourier-transform
infrared spectroscopy (FT-IR) and FT-Raman. Triple detection (SEC3) and universal
calibration (SECUC) were utilized in the SEC characterization. Also covered in this chapter is
the SEC characterization of a series of dendrimers.
3.1. Monomers
The monomers used as the starting material in the synthesis of the different architectures
are shown in figure 3.1. 2,2-Bis(methylol)propionic acid (bis-MPA), a crystalline dihydroxy
carboxylic acid, was used as the monomer in the synthesis of the dendron and dendrimer. It is
also the monomer for the hyperbranched polymer Boltorn H-30 (commercially produced by
Perstorp).72 ε-Caprolactone (CL), a liquid cyclic ester, was used to synthesize the arms of both
the star-branched and comb poly(ε-caprolactone)’s (PCL). 5,5-Dimethyl-1,3-dioxane-2-one,
also known as neopentyl carbonate (NPC), was the crystalline cyclic carbonate used in the
synthesis of the star-branched poly(neopentyl carbonate) (PNPC). The hydroxyl functional
initiators used in the synthesis of the PNPC stars and the comb polymer backbone are not
included in figure 3.1, however the structures of the initiators are described in section 3.3.1.2
and the composition of the backbone is described in section 3.4.
O
OH
O
O
O
HO
O
O
OH
Bis-MPA
ε-Caprolactone
Neopentylene carbonate
Figure 3.1 Monomers used in the synthesis of the various macromolecular architectures.
3.2 Star-Branched Poly(ε-caprolactone)’s
Variations were made in the architectures of a group of star-branched PCL’s in order to
investigate the effect of branching on zero shear viscosity. ε-Caprolactone was chosen as the
monomer due to its ease of polymerization and to the semi-crystallinity of the formed
polymer.
13
3.2.1 Synthesis
The synthesis of star polymers can be divided into three parts: synthesis of the core
molecules, grafting, and end capping. The core molecules include the third generation bisMPA dendron and dendrimer which were synthesized according to a procedure by Hult et
al.73 The final core moiety was a pseudo third-generation, bis-MPA based, hyperbranched
molecule, similar to a polymer developed by Hult et al.,28,74now commercially available under
the trade name Boltorn and kindly supplied by Perstorp AB.
Grafts were accomplished by ring-opening polymerization (ROP) of CL onto hydroxyfunctional cores. The reactions were performed in bulk at 110°C with Sn(Oct)2 as the catalyst
with the degree of polymerization (DP) controlled by the monomer to initiator feed ratio.
Finally, the hydroxy-functional end groups of poly(ε-caprolactone) (PCL) were end-capped
with methacrylate groups (scheme 3.1).
HO
OH
OH
HO
O
OH
+
HO
OH
HO
OH
Sn(Oct)2
110°C
O
OH
HO
HO
O
n
OH
O
O
O
O
HO
O
O
O
O
O
nO
O
O
O
O
O n
OH
O O
nO
O
O
n
O
O
O
O
O
n
O
O
O
O
HO
n
O
O
O
n
nO
O
O
HO
O
O
OH
O
n
O
OH
OH
Scheme 3.1 General outline of the synthesis of star polymers where the large center circle
represents a dendritic core molecule. The final end capping is not included in the scheme.
The various star-branched polymers differed with respect to the length of the PCL grafts and
the type of core molecule. Throughout, the initiator and monomer were carefully dried, as
14
traces of water would initiate homo-polymerization. The structures of the different star
polymers are presented in Appendix A.
3.2.2 NMR Characterization
The 1H NMR spectra of the polymers were used to calculate the DP of the PCL grafts and
the molecular weight (table 3.1). The DP was calculated by comparing the integrals of the
protons on the methylene adjacent to the hydroxyl end-group (A) relative to those on the
methylene next to the carbonyl carbon (C) (figure 3.2). Although it is also possible to perform
the calculation by comparison with the methylene next to the oxygen in the repeat unit (B),
this method of calculation is inferior due to the overlapping shift of the core molecules. The
accuracy of the DP value obtained for polymers with high DP is reduced due to the reduced
relative size of the peak from the protons next to the hydroxyl group.
B
C
32.5
1.0
Integral
4.1
4.0
3.9
3.8
3.7
3.6
3.5
(ppm)
O
B
C
R
n
O
O
OH
C
A
A
Integral
7.5
32.5
7.0
6.5
6.0
5.5
5.0
4.5
1.0
4.0
3.5
34.6
3.0
2.5
2.0
1.5
1.0
0.5
(ppm)
Figure 3.2 1H NMR spectrum of a dendron-PCL with an average DP of about 35.
It has been shown that the 13C NMR shift of the quaternary carbon in the repeat unit of the
initiating species is sensitive to the degree of substitution.28 This quaternary carbon resonates
at 50.6 ppm if both hydroxyl groups are unreacted, at 48.8 ppm if one hydroxyl group
remains, and at 46.8 ppm if both hydroxyl groups are reacted. This resonance change is useful
in assessing the success of the grafting reaction. 13C NMR spectra of the polymers showed
that those with shorter arms still contained unreacted hydroxyl groups (figure 3.3).
15
51
50
49
48
47
46
45
44
(ppm)
13
Figure 3.3 C NMR spectrum of the quaternary carbons from the bis-MPA repeating unit in
the hyperbranched core of Boltorn-PCL, DP8. The peak at 46.8 ppm shows the presence of
fully functionalized bis-MPA hydroxyl groups. The peak at 48.8 ppm shows the presence of
partially functionalized bis-MPA end groups. The absence of a peak at 50.6 ppm shows that
there are no end groups with both hydroxyl groups unreacted. The integral calculation of
functionalization shows that approximately four hydroxyl groups remain unreacted. (The
peaks are shifted downfield 0.8 ppm due to an uncalibrated spectrum.)
Full functionalization was observed at varying DP for the various structures analyzed (figure
3.4). The dendron and dendrimer were fully reacted at a DP of about 13-15 while the
hyperbranched polymer was fully substituted at a DP of about 20. The spectra further showed
that in all cases only one of the hydroxyl groups, 48.8 ppm, was unreacted. This lack of
functionalization was probably due to the statistical nature of the reaction since the star
polymers with longer arms showed no trace of incomplete functionalization of end-groups.
51
50
49
48
47
46
45
44
(ppm)
13
Figure 3.4 C NMR spectrum of Boltorn-PCL, DP of 32. Only one peak at 46.8 ppm
indicates that all hydroxyl groups have reacted. (The peak is shifted downfield 0.8 ppm due to
an uncalibrated spectrum.)
3.2.3 SEC Characterization
Size exclusion chromatography (SEC) was performed to determine molecular weight and
distribution. The SEC apparatus equipped with a triple detector array, including differential
refractive index, differential viscometer and right (and low) angle laser light scattering
detector (RALLS). This set-up permitted molecular weight to be determined using
conventional, universal (SECUC), and triple detection calibration (SEC3), simultaneously.75
16
SECUC is based on the fact that VH ∝ [η]M ,where is the hydrodynamic volume and [η] is the
intrinsic viscosity, for a wide range of polymers and complex architectures including e.g.
dendritic-, H-, star-, comb- and co-polymers.76,77,78 This makes SECUC a useful tool for the
determination of molecular weight since there is no influence from the architecture or
chemical composition of the analyzed compounds.
The molecular weights, especially the weight average molecular weight (Mw), obtained
from SEC3, are in general, very close to those from SECUC, which is to be expected since both
methods are independent of architecture and chemical composition. However, light scattering
(SEC3 method) is dependent on the refractive index increment, which is low for PCL in
tetrahydrofuran (the mobile phase), resulting in poor signal to noise ratio, especially for the
low molecular weight fraction. The agreement of the SECUC data with the number average
molecular weight (Mn) from 1H NMR data is, in general, good at low DP. This agreement is
reduced at high DP since accurate end-group analysis becomes more difficult with increasing
DP (table 3.1). In addition, the SEC determination of Mn is very sensitive to errors and
difficult o determine exactly. Polydispersity generally increased with increasing molecular
weight. This is due to intra- and intermolecular transesterification that occurs at high
conversion and molecular weight.79
Table 3.1 SEC and 1H NMR data of the star-branched PCL characterized with SEC3 and
SECUC. Mark-Houwink (MH) equation: [η] = kM α . The MH α value is a polymer
conformation parameter. The α value decreases with the compactness of the structure.
(degree of polymerization DP, polydispersity index PDI)
DP
aim
DP
1
H
NMR
1
Mna
H NMR
Boltorn-PCL
50
51
189 900
70
79
292 200
Dendrimer-PCL
12
14
41 100
15
14
41 100
20
24
68 500
35
42
117 700
60
51
142 300
Dendron-PCL
18
15
14 600
40
46
42 900
80
81
74 900
Linear PCL
15
17
2 200
50
45
5 400
80
82
9 600
A
17
1 900
B
39
4 400
C
117
13 300
a
g mol-1
SEC
SECUC
PDI
SEC
SECUC
MH α
value
Branching
average
239 000
303 800
1.58
1.54
236 300
308 700
9.13
7.86
0.263
0.204
39
46
41 300
42 300
67 300
99 900
109 200
1.44
1.39
1.31
1.60
1.47
42 000
43 700
72 100
104 700
122 300
1.04
1.03
1.01
1.14
1.30
0.771
0.775
0.742
0.758
0.787
18
19
24
22
23
15 800
47 800
68 000
1.27
1.39
1.30
16 000
43 000
66 400
1.01
1.27
1.71
0.822
0.836
0.817
6.8
8.2
8.5
3 000
6 400
13 300
2 800
6 600
16 900
1.23
1.24
1.53
1.20
1.53
1.38
2 300
5 200
10 300
2 200
5 100
14 200
1.01
1.03
1.46
1.02
1.46
1.41
0.888
0.860
0.885
0.865
0.923
0.908
-
Mwa
3
PDI
3
Mwa
17
The polydispersity index (PDI) values obtained from SEC3 are similar for all samples whereas
there is a broad distribution/range of PDI’s obtained from SECUC. This can be attributed to the
insensitivity of the RALLS detector (SEC3) to low molecular weight polymers with a low
refractive index increment, resulting in too narrow distribution of the disperse polymers
(Boltorn-PCL). In addition, when considering the multifunctional initiator moieties, Boltorn a
polydisperse hyperbranched polymer, the monodisperse dendron and dendrimer it is obvious
that the SEC3 PDI’s are incorrect. The PDI of Boltorn H-30 in N,N-dimethylformamide and
THF utilizing SECUC is, according to a study performed by Månson et al. at least 2.80
The Mark-Houwink (MH) data permits the calculation of the number of arms on a star
polymer. Polymers with long chain branches such as star- and comb-branched polymers have
reduced hydrodynamic radius and intrinsic viscosity [η] compared to linear polymers of the
same molar mass. By comparing the intrinsic viscosity of a branched polymer and a linear
reference polymer it is possible to estimate the degree of branching. The number of arms was
calculated using the Zimm-Stockmayer equations.81 Through the comparison of the intrinsic
viscosity of the branched polymer ([η]b) and a linear equivalent ([η]l), the branching index g
is calculated:
 [η ] 
g =  b 
 [η ]l 
where ε is a form factor dependent on the type of branched polymer.82,83 The form factor is
generally considered to be ∼0.75. This definition of g is less exact than the ratio of the radius
of gyration (RG), however it is more practical to use due to the problems associated with
measuring RG over the whole molecular weight distribution. The number of arms is then
calculated using81
g = (3 f − 2 ) / f 2
1/ ε
where f is the number of arms. The calculated number of arms is in good agreement with the
theoretical number of arms for the star polymers with a dendron and dendrimer core moieties.
Their number of arms is 8 and 24 respectively. At lower DP the measured number of arms is
lower than the theoretical value. This was expected since the 13C NMR showed that
functionalization at low DP’s was incomplete. The Boltorn-PCL with theoretically 32 arms,
however, displayed a higher number of arms. This can be contributed to the nature of Boltorn,
a hyperbranched polymer with a lower degree of structural uniformity.
Appendix B lists SEC as determined by SECUC and viscosity data of all star-branched
PCL.
3.3 Star-Branched Poly(neopentyl carbonate)’s
One of the goals of this work was to identify a solid material suitable for low temperature
curing. Since the thermal properties of branched poly(ε-caprolactone)’s are unsuitable for
applications as solid coatings (Tg = -55°C Tm = 35-50°C). An interesting candidate for this is
the polymer of 5,5-dimethyl-1,3-dioxane-2-one or neopentyl carbonate (NPC). The polymer
has a reported Tg of 20-30°C and Tm of 100-130°C.84,85,86,87 An additional feature of
polycarbonates is increased resistance to hydrolysis compared to polyesters.88 While there are
no references of branched poly(NPC) (PNPC) in the open literature, there is at least one claim
of a star-branched PNPC that can be found in the patent literature. The patent claims synthesis
of stars from NPC and pentaerythritol catalyzed by stannous(II) 2-ethylhexanoate (Sn(Oct)2)
although no detailed characterization is disclosed.89
18
3.3.1 Synthesis
3.3.1.1 Monomer Synthesis
Preparation of NPC from diethyl carbonate and neopentyl glycol as previously described
by Sarel et al.90 References to production of NPC are also found in patent literature.91,92 The
reaction proceeded in two steps. In the first step, oligomer or prepolymer was formed which
was subsequently subjected to pyrolysis and ring closure resulting in a cyclic monomer. The
synthetic route used in this work was similar to the patented procedures in that it omits an
extraction step. Sn(Oct)2 was used as catalyst to ensure formation of prepolymer and increase
the rate of ring closing depolymerization. All volatile by-products and reagents including
residual neopentyl glycol were removed under reduced pressure and NPC was obtained by
pyrolysis at 210°C followed by distillation (scheme 3.2). This elaborated method does not
require extraction and the monomer could be conveniently prepared in 200 g scale in one pot
with a 70% yield.
O
O
OH
HO
+
O
Sn(Oct)2,130°C
O
-EtOH
Prepolymer
Pyrolysis, 210 °C
O
O
Scheme 3.2 Synthetic route for NPC.
3.3.1.2 Polymer Synthesis
An experimental series was designed in order to find favorable conditions for the synthesis
of star-branched PNPC’s with cores consisting of various polyols. Polymers with three and
four arms were synthesized from trimethylolpropane (TMP) (3OH), di-TMP and etoxylated
pentaerythritol (PP50) (4OH). TMP and di-TMP represent polyols with primary hydroxyl
groups in neopentylic positions. PP50, on the other hand, has approximately 5 ethylene oxiderepeating units per molecule and the hydroxyl groups in this polyol are less sterically
crowded. Star-branched polymers were also synthesized from a Boltorn H30 (3rd pseudo
generation) (Appendix C). In addition, linear PNPC by initiation from n-BuOH was
synthesized and used as a reference compound.
HO
HO
OH
HO
HO
O
O
OH
OH
TMP
OH
O
OH
O
OH
Di-TMP
O
O
HO
PP50
Figure 3.5 Three and four functional core molecules.
Previously reported work suggests tin(II) compounds and mild organic acids as promising
catalysts for realization of the target structures. Kricheldorf et al. reported the synthesis of
19
linear PNPC with a molecular weight of up to 250 kg mol-1 by a coordination insertion
mechanism utilizing Sn(Oct)2.85 Hedrick et al. have reported stannous(II) trifluoromethane
sulfonate (Sn(OTf)2) as an efficient catalyst for polymerization of lactones under milder
conditions than Sn(Oct)2.93 NPC has also been polymerized cationically. There are two
proposed mechanisms, the activated monomer mechanism and the activated chain end
mechanism. Early work on cationic polymerization of NPC reports side reactions causing
decarboxylation that result in formation of ether groups in the growing chains. Reported
initiator systems consist of strong acids, such as triflic acid or boron trifluoride,86,94,95 and
more recently living systems, such as triethyl borate/HCl.Et2O.87 However, the use of strong
acids can increase the risk of side reactions during polymerization, and the nature of the alkyl
borate initiating species prevents initiation from a multifunctional scaffold. The catalysts
chosen for this study were Sn(Oct)2, Sn(OTf)2 and fumaric acid (pKa= 3.02). The choice of
fumaric acid was inspired by the work of Endo et al. who recently synthesized ε-caprolactone
star-branched polymers utilizing this catalyst.96
3.3.2 NMR Characterization
1
H NMR was employed to monitor conversion and estimate the degree of polymerization
(DPNMR) (figure 3.6). Conversion was calculated by comparing the peak integral at 1.12 ppm
(b) with the peak at 0.99 ppm (b’). The peak 1.12 ppm (b) corresponds to the protons of the
methyl groups in the monomer and the peak at 0.99 ppm (b’) represents the methyl groups of
the polymer backbone. The DPNMR was calculated by comparing the integrals of the repeating
unit peak at 0.99 ppm (b’) and the peak at 0.93 ppm (d), corresponding to the methyl groups
in the terminal repeat unit. The DPNMR is thus equal to I0.99/I0.93+1. This calculation can also
be performed on the integral at 3.34 ppm (c) that corresponds to the CH2 in the α position to
the terminal hydroxyl group. In this case DPNMR is equal to I0.99/(3×I3.34)+1. If di-TMP is
present as the initiating polyol, the protons located next to the ether bond in di-TMP yield a
resonance peak in the vicinity of 3.3 ppm, which makes it impossible to calculate DP using
the peak integral at 3.34 ppm (c). However, the peaks arising from the ether bond in di-TMP
indicate where peaks would likely emerge if ether bonds were formed during the
polymerizations. Since decarboxylation and the formation of undesirable ether bonds is
known to occur in cationic polymerization of cyclic carbonates.86,95 The fact that the integral
of the 3.3 ppm resonance peak did not increase during the polymerization from di-TMP and it
was not found in any of the other prepared polymers, it is evident that no ether bonds were
formed during polymerization.
20
O
R
O
O
a´
O
O
c
O
n
b´
O H
d
b´
a´
O
a
O
O
b
a
6.5
6.0
5.5
5.0
4.5
d
HBP
Boltorn
Boltorn
HBP
7.0
b
c
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
(ppm)
Figure 3.6 1H NMR spectra of a Boltorn-PNPC containing unreacted monomer.
The degree of substitution of the hydroxyl groups of Boltorn was investigated using 13C
NMR. As previously mentioned, Hult et al. have shown that the 13C NMR resonance
corresponding to the quaternary carbon of the bis-MPA repeat unit is dependent on the degree
of substitution.28 The resonance shifts for the fully substituted, mono-substituted and unsubstituted Boltorn are 46, 48 and 50 ppm respectively. A distinct peak was detected at 46
ppm accompanied by a smaller peak at 48 ppm, suggesting that the majority of the hydroxyl
groups of Boltorn were substituted (figure 3.7).
50
48
46
(ppm)
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
(ppm)
Figure 3.7 13C NMR spectra of a Boltorn-PNPC. The enlarged area shows the quaternary
carbons.
21
3.3.3 SEC Characterization
A universal calibration (SECUC) method was used to study the obtained star polymers
while triple detection including light scattering was excluded due to a poor signal to noise
ratio. All polymers were synthesized using fumaric acid as the catalyst.
The relationship between intrinsic viscosity the degree of branching is discussed in section
3.2.3. This relationship can also be used to rank polymers according to branching by
comparing the Mark-Houwink (MH) plots of the respective polymers.97,98 The MH plots for
linear, 4-arm and multi-arm star-branched polymers are depicted in figure 3.8. A trend is
clearly seen with decreasing slope in the order linear, four arm and multiarm polymer. This
suggests successful grafting of PNPC onto the multifunctional initiator molecules.
-0.25
PP50-PNPC
4 arm
star
-0.59
-0.92
Hyperbranched
Boltorn-PNPC
-1.26
Linear -PNPC
-1.60
2.91
3.26
3.62
3.98
4.34
4.69
5.05
Relative response
Log[M]
Figure 3.8 Mark-Houwink plot of linear and star-branched PNPC.
9.0
13.0
17.0
20.0
Retention volume (mL)
Figure 3.9 SEC traces of PNPC polymerized from Boltorn (left), PP50 (center) and n-BuOH
(right).
The SEC traces of the same polymers are shown in figure 3.9. The Boltorn-PNPC is
bimodal, most likely linear or polymers with a few arms; this can be seen in the figure 3.8 in
22
that the low molecular weight region has a different slope. The four arm star and the linear
polymers are unsymmetrical, an indication of side reactions.
3.3.4 Catalyst Evaluation
Linear PNPC initiated from n-BuOH were synthesized in order to evaluate the catalytic
performance of Sn(OTf)2 in toluene under mild conditions. The degree of polymerization
(DP) controlled by the monomer to initiator feed ratio The DP aimed for (DPaim) were 20 and
100. The polymerizations were performed in toluene at 50°C in order to prevent n-BuOH
from boiling. The polymerizations were run to 90% conversion; the yields after precipitation
in cold methanol were 70 and 60% for the DPaim 20 and 100 respectively (table 3.2). The low
yield is the result of side reactions that result in a fraction of soluble oligomers, and this
decrease in yield was most evident at the higher monomer to initiator ratio. The SEC trace
was bimodal at the high DPaim with moderate PDI in both cases.
The efficiency of the catalysts to form star polymers was evaluated by reacting NPC with diTMP in presence of the respective catalysts: Sn(Oct)2, Sn(OTf)2 and fumaric acid (table 3.2).
The polymerizations were performed in bulk at 130°C in order to maintain low viscosity at
high conversions. The polymerizations utilizing Sn(Oct)2 and Sn(OTf)2 exhibited high PDI’s,
indicating poor control. The PDI was significantly lower for the polymer synthesized using
fumaric acid. As fumaric acid resulted in the lowest PDI it was selected for further study. A
linear relationship between conversion and molecular weight was observed for conversion up
to 80-90% (figure 3.10). Polydispersities were fairly low (1.2) at conversions below 50% and
increased gradually up to 2-2.5 above 90% conversion. The observed increase in Mw and
broadening of PDI show an increase in side reactions at high conversion. However, observed
side reactions in this reaction were significantly lower than observed for the tin catalysts
(table 3.2). All polymerizations showed Mn values lower than the theoretical values. The
deviation of Mn from the theoretical value was greater at higher DP’s, and may to some extent
be attributed to spontaneous thermal polymerization or initiation from impurities. An
experiment was therefore performed to determine the extent of thermal polymerization or
decomposition of monomer; NPC was heated at 130°C for 24 hours resulting in 6%
conversion according to 1H NMR.
6000
2
1.9
Mn, NMR
Mn, SEC
PDI
4000
1.8
1.7
1.6
3000
1.5
PDI
Mn (g mol-1)
5000
1.4
2000
1.3
1.2
1000
1.1
0
0
20
40
60
80
1
100
Conversion (%)
1
Figure 3.10 Mn obtained from H NMR, SEC and PDI as a function of conversion for the
polymerization of NPC with a DPaim of 40 by fumaric acid at 130°C.
23
Table 3.2 Data of the ROP of NPC with hydroxyl functional initiators in the presence of different catalysts.
a
Initiator
Catalyst
DPaim
n-BuOH
n-BuOH
Di-TMP
Di-TMP
Di-TMP
TMP
TMP
PP50
PP50
Boltorn
Boltorn
Boltorn**
Boltorn
Sn(OTf)2
Sn(OTf)2
Sn(Oct)2
Sn(OTf)2
Fumaric acid
Fumaric acid
Fumaric acid
Fumaric acid
Fumaric acid
Fumaric acid
Fumaric acid
Fumaric acid
Fumaric acid
20
100
10
10
10
5
20
10
20
5
10
10
30
Mna
theo.
2 700
13 100
5 400
5 400
5 400
2 100
7 800
5 600
10 700
23 000
42 000
42 000
120 000
Temp.
(°C)
50
50
130
130
130
130
130
130
130
130
130
130
130
Time
(h)
48
48
16
4
9
5
9
9
20
4
6
24
24
Conv.
(%)
93
91
85
90
70
83
47
75
90
73
80
80
92
Yield
(%)
70
60
60
70
55
68
35
60
75
*
65
-
75
g mol-1. * No precipitation. ** Data in this row is from a fractional precipitation of the sample above it.
24
Mna
NMR
1 800
2 100
4 700
4 800
3 400
1 900
3 400
3 900
7 600
22 000
30 000
30 000
47 000
Mna
SEC
2 400
3 800
4 700
4 900
3 100
1 800
3 400
4 700
6 600
8 500
25 000
4 000
Mna
SEC
2 700
4 700
18 400
13 800
4 300
2 900
4 500
6 000
9 000
52 000
99 000
20 000
PDI
α
1.2
1.2
3.9
2.8
1.4
1.6
1.3
1.3
1.3
6
4
5
0.69
0.70
0.43
0.30
0.39
0.30
0.12
0.46
0.46
0.12
0.28
0.12
The polymers derived from PP50 showed lower polydispersities at high conversion (90%)
compared to polymers based on initiators with neopentylic hydroxyl groups (table 3.2). The
SECUC values obtained for the three and four arm star polymers were close to those obtained
by 1H NMR. Polymerizations initiated from Boltorn in the presence of fumaric acid had
aliquots taken for 1H NMR analysis. The analysis of synthesis with a DPaim of 10 showed that
the molecular weight increased up to a DP of 7-8. Additional polymerizations were performed
with DPaim of 5, 10 and 30. Conversions were kept below 90% to avoid gelling.
3.3.5 Thermal Characterization
The precipitated polymers were subjected to differential scanning calorimetry (DSC)
analysis that comprised two melting/crystallization cycles with heating and cooling rates of
10°C/min. In the first cycle the sample was heated from 25 to 140°C and then cooled to 30°C. The sample was then heated to 140°C to complete the procedure. On the second
heating, melting endotherms were only observed for three and four arm star polymers with an
arm length of 8-10 repeating units or more (figure 3.11). A one-hour annealing segment at
50°C was therefore added prior to the second heating but no or little effect was observed. A Tg
was also observed between 20-30°C. Some endotherms were bimodal or very broad on the
second heating. Boltorn-PNPC DPaim=30 was the only multiarm star that exhibited an
endotherm in the second heating. The two distinct melting endotherms that may be attributed
to the presence of linear polymer (figure 3.11).
Endo
Linear PNPC
Di-TMP-PNPC
PP50-PNPC
Boltorn-PNPC
-50
0
50
100
150
Temperature (°C)
Figure 3.11 DSC traces for linear and star polymers, 2nd heating after annealing 50°C, 1 h.
25
3.4 Comb-Branched Poly(ε-caprolactone)’s
The comb-branched polymers were synthesized as complement to the star-branched
polymers. The following study of the comb polymers included investigation of the
performance as a powder coating and the overall effect of varying the architecture.
3.4.1 Synthesis
The comb polymers were synthesized from a backbone, ER 065. ER 065 is a polyacrylate
resin obtained from BASF AG, or more specifically, a copolymer consisting of 20 wt.%
glycidyl methacrylate, 5 wt.% of butyl acrylate, 55 wt.% methyl methacrylate and 20 wt.%
styrene. The oxirane group of glycidyl methacrylate provided the reactive site for the first
reaction with bis-MPA, creating the hydroxyl functional resin (HR) necessary for the ROP of
CL. The reaction between oxirane and bis-MPA was not only convenient but also efficient
since the procedure was merely a melt mixing (155°C) of the components and the catalyst,
tetrabutyl ammoniumbromid (TBAB). In addition, this reaction increased the functionality
since each oxirane group was reacted with bis-MPA, which has two hydroxyl groups (Scheme
3.3).
n
O
O
n
O
+
O
HO
O 1wt% TBAB, 155 °C
HO
OH
OH
O
O
O
OH
OH
O
O , Sn(Oct) , 110 °C
2
O
O
HO
mO
O
O
HO
O
O
O
O
O
O
m
m =10 or 20
O
O
O
n
HO
, TEA, DMAP
O
O
O
O
O
mO
O
O
O
O
O
m
O
O
O
O
O
HO
n
O
Scheme 3.3 Synthetic scheme of a comb polymer. (tetrabutylammoniumbromid-TBAB,
triethylamine-TEA, N,N-dimethylaminopyridine-DMAP)
26
The PCL side chains were grafted onto bis-MPA and end-capped using the procedure
described for star-branched PCL in section 3.3. The different comb polymers and their
precursors are presented in table 3.3.
Table 3.3 A list of the different resins synthesized and tested. The resins were polymerized and
end-capped batch wise (B = batch).
Description
Resin
ER 065
HR
Comb-PCL10, B1-B4
Comb-PCL20, B1-B4
Comb-PCL10M, B1-B2
Comb-PCL20M, B1-B2
Starting material, epoxide functional amorphous resin
ER 065 functionalized with bis-MPA, hydroxy functional
ε-Caprolactone polymerized from HR, DPaim=10
ε-Caprolactone polymerized from HR, DPaim=20
PCL10 end capped with methacrylic anhydride
PCL20 end capped with methacrylic anhydride
3.4.2 IR Characterization
The reaction between oxirane and bis-MPA was monitored with FT-IR and FT-Raman99,100
spectroscopy (FT- Fourier Transform). Samples were taken every five minutes until the
reaction was complete. Figure 3.12 shows the FT-IR spectra of the individual components, ER
065 and bis-MPA, and allowing their comparison with the reaction mixture after 30 minutes.
The carboxyl acid peak of bis-MPA shifts from 1683 to 1721 cm-1. This shift corresponds to
the formation of an ester, i.e. the same shift as the acrylate carbonyls. The broad hydroxyl
signal at 3200–3600 cm-1 of bis-MPA is decreased due to the low concentration of bis-MPA
in the final reaction mixture (85 wt. % ER 065 + 15 wt. % bis-MPA).
1722
701
1136
759
ER 065
989
1452
907 843
1384
2949
1023
1683
630
1232
1044
996
935 868
3358
A
1306
Bis-MPA
2946
1141
908
790
1455 1386
1721
1138
701
759
Reaction mixture
after 30 minutes
2948
4000,0
3600
3200
2800
2400
2000
1800
cm-1
988
1452
842
1385
1600
1400
1200
1000
800
600,0
Figure 3.12 IR spectra of ER 065, bis-MPA and of the reaction mixture, 85 wt.% ER 065 and
15 wt.% bis-MPA, after 30 minutes. After 30 minutes the peak at 1683 cm-1, corresponding to
the C=O stretch of bis-MPA, is no longer present and the reaction is completed.
27
The reaction was also monitored with FT Raman. The upper spectrum in figure 3.13 shows
the spectrum of ER 065 with the epoxide peak at 1255 cm-1. The lower spectrum shows the
modified ER 065 (HR) after a reaction time of 30 minutes with no epoxide signal.
2948
1000
ER 065
1447
3056
809
1600
1185
1031
619
1255
1726
1582
2949
Epoxide
Int
1000
Reaction mixture after
30 minutes
3058
1451
1601
1184
1730
4000,0
3600
3200
2800
2400
2000
1800
1600
Raman Shift / cm-1
1031
810
1155
1400
1200
619
1000
800
600
521
400,0
Figure 3.13 FT Raman spectra of the starting resin ER 065 (upper) and the reaction mixture,
85 wt.% ER 065 and 15 wt.% bis-MPA, after a reaction time of 30 minutes (lower).
Comparison of the two spectra shows the disappearance of the epoxide signal at 1255 cm-1.
3.4.3 SEC Characterization
Normally, 1H NMR is a useful tool for calculating the DP for poly(ε-caprolactone). In this
case however, the signals from the copolymer backbone overlaps with the signal from the
methylene group next to the hydroxyl end group making accurate structure analysis difficult.
On the other hand, SEC analysis supported the result of successful grafting of bis-MPA. The
molecular weight increased during this first reaction from a Mw = 8200 g/mol (ER 065) to a
Mw = 10 600 g/mol (HR). This increase correlates fairly well with the calculated theoretical
Mw = 9800 g/mol for HR. SEC analysis also showed an increase in molecular weight, from a
value of 10 600 g/mol to 33 900-38 500 g/mol for the PCL10 batches and to 55 200-62 800
g/mol for the PCL20 batches. The calculated theoretical molar mass values for PCL10 and
PCL20 are 36 200 g/mol and 62 600 g/mol respectively (table 3.4).
28
Table 3.4 SEC data for the comb polymers tested, listed by synthesis batch (B = batch). In the
last synthetic step, the different batches were mixed in order to reduce the number of
methacrylation reactions (PCL10M, B1-B2 and PCL20M, B1-B2). APA is an acrylate
functional polyacrylate derived from ER 065.
Sample
Mn
ER 065
APA
HR
PCL 10 B1
PCL 10 B2
PCL 10 B3
PCL 10 B4
PCL 20 B1
PCL 20 B2
PCL 20 B3
PCL 20 B4
PCL10M B1
PCL10M B2
PCL20M B1
PCL20M B2
3 000
3 200
3 100
4 900
4 300
5 300
6 200
10 000
12 700
10 600
10 500
7 100
7 770
9 660
13 500
3
SEC
8 200
10 500
10 600
38 500
33 900
37 100
36 900
62 600
55 200
62 000
62 800
43 180
46 280
65 030
71 500
Mw
Theoretical
9000
9 800
36 200
36 200
36 200
36 200
62 600
62 600
62 600
62 600
37 800
37 800
64 200
64 200
PDI
2.8
3.3
3.5
7.8
7.9
7.0
6.0
6.3
4.3
5.9
6.0
6.1
6.0
6.7
5.3
3.5 Bis-MPA Dendrimers
In the early 1990’s, Hult et al. presented the first hydroxyl functional hyperbranched
aliphatic polyester, as previously mentioned. This polyester was synthesized from the core
moiety 2-ethyl-2-(hydroxymethyl)-1,3-propanediol (TMP) and the AB2 monomer 2,2bis(hydroxymethyl)propionic acid (bis-MPA).101 Similar hyperbranched compounds are now
commercially available under the trade name Boltorn®. This work led to an attempt to
synthesize the corresponding hydroxyl functional dendrimer with TMP as the core and bisMPA as the inherent structure. This attempt failed for a couple of reasons. First there were
problems associated with steric hindrance in the final coupling between the dendrons
(wedges) and the small core (TMP). Second there were difficulties monitoring the coupling
reaction due to the absence of an UV active moiety in the core or the pre-synthesized
dendrons.
For this work, three sets of aliphatic polyester dendrimers based on 2,2bis(hydroxymethyl)- propionic acid (bis-MPA) were evaluated using SEC. Two of the sets
had benzylidene (Bz) terminal groups and either a trimethylol propane (TMP) or triphenolic
(Ar) core moiety. The last set had acetonide terminal groups and a triphenolic core moiety
(figure 3.14). The synthesis of a dendrimer with a TMP core was made possible by the use of
an anhydride building block.
29
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O
O
O
O
O
O
O
O
O O
O
O
O
O
O
O O
O
O
O
O
O
O O
O O
O
O O
O
O
O
O
O
O
O O
O
O
O O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OO O
O
O O
OO
O
O
O
O
O
O O
O
O
O
O
O
O
O
O
O
O
OO
O
O
O
O
O O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O
O
O
O O
OO
O O
Bz-[G#4]-TMP
O
O
O
O
O
O
O
OO
O
O
OO
O
O
O
O
O
O
O
O
O
O O
O
O
O
O O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Acetonide-[G#4]-Ar
O
O
O
O O
O
O O
O
O
O
O
O
O
OO O
O
O
O
O
O
OO O
O O
O
O
O
O
O O
O O
O
O
O
O
O O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O
O
O
O
O
O
O
O
O
O
O O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O
OO
O O O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O
O
O
O
OO
O
O
O O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O O
OO
O
O
O
O
O
Bz-[G#4]-Ar
Figure 3.14 Shown above are the 4th generation dendrimers as representatives for the three
sets of dendrimers. (Bz= benzylidene terminal groups, TMP= trimethylol propane core moiety
and Ar= triphenolic core moiety.)
3.5.1 SEC Characterization
The SEC characterization of the dendrimers was performed using SEC3 and SECUC,
previously described in section 3.2.3.
The molar masses obtained with SECUC in the first analysis run (R1) of the dendrimers were
close to theoretical values (table 3.5). A second run (R2) was conducted with an additional
column in order to verify the reproducibility. The results from SECUC in R2 confirmed the
previous R1 results.
It was observed that Bz-[G#4]-TMP eluted slower than the other dendrimers of the same
generation, acetonide-[G#4]-Ar and Bz-[G#4]-Ar, which eluted at the same retention volume
(figure 3.15). To elucidate this further the viscosimetric radius (RV) was calculated from
matrix assisted laser desorption ionization time of flight (MALDI-TOF) molecular weights
and intrinsic viscosity ([η]) data from the SEC evaluation using the Einstein equivalent sphere
model,

 3
[η ]M 
Rv = 

 10πN A
1
3
that is based on the Stokes–Einstein relationship for the viscosity of suspended spheres. RV
and hydrodynamic radius (RH) are usually identical.102,103 This agreement was observed
between RV and RH values obtained by right angle laser light scattering (RALLS), with some
minor discrepancies. These discrepancies were exacerbated at lower generations (RV/RH=
1.00±0.02, [G#3-5], 0.97±0.08, [G#1-2]).
30
MALDI TOF
Dendrimer
a
g
SEC
a
Mcalc
M
(g mol-1)
(g mol-1)
-1
PDId
Mn (g mol )
SECUC b
SEC3 c
SECUC
b
R1e
R2f
R1e
R2f
R1e
R2f
RGg
(Å)
RV h
(Å)
Bz-[G#1]-Ar
919
917
740
730
900
1150
1.00
1.10
6.70
8.10
Bz-[G#2]-Ar
1880
1877
1670
1750
1840
2110
1.00
1.08
8.80
10.5
Bz-[G#3]-Ar
3802
3708
3830
3790
3960
3870
1.00
1.06
11.3
13.4
Bz-[G#4]-Ar
7646
7636
7640
8710
7540
7760
1.01
1.04
14.1
16.3
Bz-[G#1]-TMP
747
747
730
-
752
-
1.01
-
5.70
7.00
Bz-[G#2]-TMP
1708
1706
1630
1580
1750
1950
1.00
1.08
7.90
9.42
Bz-[G#3]-TMP
3630
3623
3680
3680
3860
4070
1.00
1.08
10.6
12.3
Bz-[G#4]-TMP
7474
7452
7350
7689
7540
7770
1.01
1.07
14.0
15.9
Bz-[G#5]-TMP
15162
15099
14690
16000
14550
14920
1.02
1.04
17.1
20.1
Acetonide-[G#1]-Ar
774
-
-
834
-
1100
-
1.08
7.40
7.40
Acetonide-[G#2]-Ar
1592
1588
1780
1850
1600
2080
1.00
1.07
7.70
9.78
Acetonide-[G#3]-Ar
3226
3221
3400
3730
3230
3920
1.00
1.03
10.4
12.6
Acetonide-[G#4]-Ar
6493
6486
7340
7300
5710
7080
1.01
1.01
13.3
16.2
+ b
c
d
e
f
Measured M-Ag ; Universal calibration method; Tripel detection calibration method; Polydisperisty index; single separation column; two separation columns;
radius of gyration; h Viscosimetric radius.
Table 3.5 MALDI-TOF and SEC data for dendrimers consisting of different: generations, end-groups and core molecules.
31
110
Bz-[G#4]-TMP
___
Bz-[G#4]-Ar
Acetonide-[G#4]-Ar
U
87
Relative response
_______
__
____ ___
×
64
41
18
-5
8.0
8.5
8.9
9.3
9.8
10.2
Elution volume (mL)
Figure 3.15 The retention volume for all 4th generation dendrimers: of Bz-[G#4]TMP, Bz-[G#4]-Ar and Ac-[G#4]-Ar.
The plot of RV against M (figure 3.16) shows the size of the three sets of dendrimers
and an additional set of acetate-[G]-Ar dendrimers. The acetate-[G]-Ar were
originally synthesized by Hult et al. and their self diffusion constant was studied
utilizing pulsed field-gradient spin-echo (PGSE)-1H NMR in CDCl3.73 The published
RV values of acetate-[G]-Ar were close to the RV data obtained for the dendrimers in
this study. However, direct comparison of the results is not possible as the current
work utilized THF as the solvent and it may have affected the RV. The terminal
groups, acetonide and Bz appear to have no significant impact on the RV of the
studied generations. However, Bz-[G]-TMP exhibits a slightly smaller size and this is
related to the smaller core molecule.
The ratio RG/RV = 3 / 5 ≈ 0.77 for a sphere of uniform density. There have been a
number of publications utilizing this ratio for dendrimer analysis, especially for
poly(propylenimine) (PPI) dendrimers, using low angle light scattering.104,105,106 For
PPI dendrimers, the RG/RV ratio was to found to exhibit a semi quantitative
relationship.106 It was also determined that the solvent may have a large impact on the
measurement of RG and RV.
32
20
V
R (Å)
Bz-[G]-Ar
Bz-[G]-TMP
Acetonide-[G]-Ar
Acetate-[G]-Ar
10
9
8
7
1000
4
-1
Molecular weight (g mol )
10
Figure 3.16 RV as a function of molecular weight. The acetate-[G]-Ar data
determined by PGSE-1H NMR in CDCl3 are from a previous study by Hult et al.73
SEC3 evaluation also yielded the radius of gyration (RG) of the dendrimers
calculated using the Flory-Fox107 and Ptitsyn-Eisner equation:108
1
1
 1  2  [η ]M  3
RG =   

6  F 
(
)
where F = 2.86 ⋅ 10 21 1 − 2.63e + 2.68e 2 , e = (2α − 1) / 3 , and α is the exponent of the
Mark-Houwink equation: [η] = kM α . The data obtained suggests spheres of uniform
density, values fairly close to 0.77, for all of the dendrimers except acetonide-[G#1]Ar. However, in view of the use of α from the Mark-Houwink equation to obtain the
RG and the fact that determining α accurately requires a polydisperse sample, the RG
data is to be regarded as uncertain. In addition, the Flory-Fox and Ptitsyn-Eisner
equation is derived for linear and flexible polymers.
The molecular weights obtained with light scattering were, in general, close to the
theoretical and MALDI-TOF values. Some of the deviation of the SEC values from
the theoretical values can be explained by a combination of factors, such as the small
amount of compounds available for analysis and the low concentrations used (4-6 mg
ml-1). Acetonide-[G]-Ar exhibited a poor signal to noise ratio due to a low refractive
index increment, also contributing to experimental error.
33
For polymers in general, there is a linear logarithmic relationship between [η] and
molecular weight, but for dendrimers the viscosity starts to decline at a certain
generation due to conformational change to a globular structure.58 This reduction in
[η] was observed for Bz-[G]-Ar and Bz-[G]-TMP (figure 3.17) of the 4th and 5th
generation, respectively. However, acetonide-[G]-Ar did not display this characteristic
reduction of [η], at least not below the 5th generation, whereas the [η] of Bz-[G]-Ar
was at its maximum at the 3rd generation. Theoretical calculations have predicted
increased crowding at the surface as the number of generations increase.109 For
acetonide-[G]-Ar, this crowding might occur at a higher generation due to the less
bulky terminal acetonide groups as compared to the bulky benzylidene groups found
in Bz-[G]-Ar.
[η] (dL/g)
0.04
0.03
Bz-[G]-Ar
Bz-[G]-TMP
Acetonide-[G]-Ar
1000
10
4
-1
Molecular weight (g mol )
Figure 3.17 Intrinsic viscosity ([η]) as a function of molecular weight.
34
4. Rheological Characterization
Rheological behavior is important since it controls the deformation and flow
properties of polymers. In order to evaluate the flow properties of branched semicrystalline resins, a number of rheological experiments were preformed. Zero shear
viscosity measurements were performed to evaluate the flow properties at very low
shear rates such as those present at coating leveling. UV-curing behavior was also
studied in order to investigate curing performance. Lastly dynamic viscosity
measurements were made to study the relationship of complex viscosity and
temperature, an important feature of semi-crystalline, solid coating materials.
4.1 Zero Shear Viscosity of Star-Branched Poly(ε-caprolactone)’s
The viscoelastic properties of polymers are strongly influenced by chain
entanglement.110 Chain entanglement occurs naturally in a melt when critical
molecular weight, Mc, is reached. Architecture also has a pronounced effect on the
viscosity of a polymer (figure 4.1). A linear PCL was synthesized and analyzed for
comparison. The linear PCL was found to follow the power law, using the molecular
weight obtained from SECUC, given by the expressions:
η0 = 4.65×10-8 M1.99 M<Mc, Mc=7500 g mol-1
η0 = 2.80×10-13 M3.34 M>Mc
The deviation from η0 ∝ M for M<Mc can be attributed the lack of correction made
for the change in the monomeric friction coefficient.55,57 Correction of the curve is not
within the scope of this study. The Mc value is equivalent to about 450 atoms in the
main chain, which is in the range expected for a highly flexible chain. 55,57 This type
of measurements are normally performed on polymers synthesized by anionic
polymerization, which results in polymers with narrow PDI.
35
1000
10
0
η (Pa s)
100
Linear PCL, Mw<Mc
Linear PCL, Mw>Mc
Dendron-PCL
Dendrimer-PCL
Boltorn-PCL
1
0.1
1000
10
4
5
10
6
10
-1
M (g mol )
w
Figure 4.1 Zero shear viscosity as a function of Mw obtained from SECUC.
As mentioned previously, the zero shear viscosity (η0) increases exponentially with
molecular weight for star-branched polymers.67 The viscosity of a star polymer with 4
or more arms is dependent on the arm length or degree of polymerization (DP) while
independent of the number of arms and the total molecular weight.68,111 This can be
seen for star-branched polymers when plotting viscosity as a function of arm
molecular weight (figure 4.2). Arm molecular weight (Ma) was calculated from Mw by
subtracting Mcore and then dividing by the theoretical number of arms. In general, data
scatter can be explained by the degree of functionalization at low M and by
polydispersity. In addition, there is some uncertainty regarding the number of
hydroxyl groups on Boltorn since its structure is less exact than the dendrimer.
36
1000
Dendron-PCL
Dendrimer-PCL
Boltorn-PCL
0
η (Pa s)
100
10
1
100
4
1000
10
5
10
-1
M (g mol )
a
Figure 4.2 Viscosity as a function as Ma obtained from SEC.
Measurements were conducted at 90°C so that the viscosity values of all of the
samples were in the measurable range of the equipment.
4.2 UV-Curing Rheological Behavior of Star-Branched Poly(ε-caprolactone)’s
During resin cross-linking, the system reaches the gel point at which time the
reactive system loses the ability to flow. This is one of the most important kinetic
characteristics of cure. According to Flory, the gel point is characterized by the
appearance of a macromolecule with an infinitely large molecular weight.111 Some
fast curing systems reach 50% conversion in 0.3 s.112 Rapid conversion such as this
makes it difficult to use classical methods and equipment to monitor the reaction.
Månson et al. have developed a new method of measuring the viscoelastic behavior of
fast curing systems.113 Using a commercial rheometer coupled to an UV-light source,
the viscoelastic behavior can be measured in ultra fast reacting systems. This novel
set-up is particularly suited for determining gel point, vitrification, if it occurs, and
modulus evolution of fast UV-curing systems.
Rheological behavior during UV-curing was measured on four different samples of
Boltorn-PCL. The molecular weight of the Boltorn-PCL star polymers ranged from 77
000 to 193 000 g mol-1 and the distribution ranged from 1.3 to 1.8 (table 4.1).
As previously mentioned, the gel point is the most important factor influencing the
processing of thermosets since flow becomes restricted at this point. At the gel point,
the material transitions from a liquid to a solid state and thereafter deformation may
cause damage to the formed network.
37
Table 4.1 Data collected on Boltorn-PCL material used in the study of the UV-curing
rheological behavior. The SEC data was obtained using conventional calibration.
Sample
Boltorn-PCL
DP,
aim
DP,
NMR
Mw, NMR
(g mol-1)
Mw, SEC
(g mol-1)
Mn/Mw
DP20
DP30
DP40
20
30
40
50
20
30
38
52
76 600
113 000
142 200
193 300
70 700
96 100
105 400
153 000
1.25
1.25
1.39
1.78
DP50
According to Winter and Chambon114,115, the gel point can be determined from
dynamic mechanical measurements of the frequency dependence of the loss tangent
(tan δ). The gel point is defined as the point at which tan δ is independent of the
frequency, i.e. at the intersection of tan δ at different frequencies. Figure 4.3 is a plot
of tan δ for the different frequencies of Boltorn-PCL DP30. Although a trend in gel
times was recorded for the resins, no clear intersections could be observed. It was
therefore impossible to draw any conclusions from this data concerning the gel point.
The ambiguity was probably due to a slight mistiming when superimposing the tan δ
curves from the four different frequencies, an occurrence that becomes more evident
when dealing with gel times on the order of 0.4 seconds. Normally, this problem is
solved with simultaneous multi-wave measurements. However, the sampling rate on
normal equipment is approximately every 8 seconds, far too slow for the system
investigated here.
18
40 Hz
16
14
60 Hz
Tan δ
12
10
80 Hz
8
99 Hz
6
4
2
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Time (s)
Figure 4.3 Tan δ of Boltorn-PCL DP30 at 40, 60, 80 and 99 rad s-1. UV irradiation
commenced at t=2 s.
38
To overcome this sampling limitation, a data reduction program developed by
Månson et al. allowing a sampling rate of every 0.01 second was used.116 This
procedure consisted of applying arbitrary wave shaped, strain controlled oscillations
at four different frequencies on four distinct samples, extracting the tan δ curves and
superposing them to seek the crossover. However, since this method ultimately
yielded ambiguous results, it was preferred to use the crossover point of the storage
modulus, G’, and the loss modulus, G’’, to determine the gel point.117,118 Although
this method of identifying the gel point is common industrial practice, it is not
supported by theoretical arguments except for in the case of one class of polymers119
Those which exhibit power law relaxation G(t)~t-1/2 when reaching the gel point.
Examples are stoichiometrically balanced network polymers and network polymers
with excess cross-linker, but only at temperatures high above Tg. When the crossing
of G’ and G’’ was applied to the investigated system, the gel point was reached within
seconds and showed no clear frequency dependence (figure 4.4). The time to reach gel
point increased linearly with DP. This relationship follows kinetic theory, since the
concentration of methacrylate end groups decreases with increasing Mw.
4.5
4
Time at tan δ =1 (s)
3.5
3
2.5
2
1.5
1
0.5
0
0
10
20
30
40
50
60
Degree of Polymerization
Figure 4.4 Time to G’ and G’’ crossover for 40 (+), 60 (Ο), 80 (−) and 99 (U) rad s-1.
UV irradiation commenced at t=2 s.
In order to obtain clear experimental results for the initial part of the reaction, large
strain was used due to the low viscosity of the resins. Figure 4.5 shows a run that was
used to obtain the G’ G’’ crossover point for Boltorn-PCL, DP50. However, the
higher strain caused either the samples to fracture or the apparatus to overload, as the
modulus increased, thus full cure was not achieved. Hence, the strain was reduced in
order to measure the evolution of G’ and G’’ to full cure (figure 4.6). Complete
leveling off of the modulus required 20 to 30 s. A drop in G’’ was observed shortly
after crossing G’ to which there was no apparent explanation.
39
5
G'
G''
10000
4
1000
3
100
2
tan δ
10
tan δ
G', G'' (Pa)
100000
1
1
0
0
1
2
3
4
5
6
7
Time (s)
Figure 4.5 G’ and G’’ for Boltorn-PCL, DP50, measured at 40 rad s-1. UV
irradiation commenced at t = 2 s.
10000000
G', DP30
G', DP20
G', DP40
1000000
G', G'' (Pa)
100000
G', DP50
G'', DP30
G'', DP20
G'', DP40
G'', DP50
10000
1000
100
10
1
0
10
20
30
40
50
60
70
Time (s)
Figure 4.6 Evolution of G’ and G’’ to full cure.
Since no change in Tg occurred during cross-linking, the cured material was
rubbery at the curing temperature, 75°C. The modulus at the rubber plateau decreases
with increasing DP (figure 4.7) since the distance between cross-links increases. Upon
40
cooling, the cross-linked material crystallized. Upon cooling, the cross-linked material
crystallizes to a varying extent depending on the DP, with the degree of crystallization
increases with increasing DP (table 2). Calculations of degree of crystallization were
carried out using 166.5 J g-1 as the heat of fusion for a 100% crystalline material.120
There is also an increase of the melting temperature with increasing molecular weight.
1.8
1.6
1.4
G' (MPa)
1.2
1
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
60
Degree of Polymerization
Figure 4.7 G’ as a function of DP after full cure at 75°C.
Table 4.2 DSC data of the cross-linked films.
Sample
Boltorn-PCL20
Boltorn-PCL30
Boltorn-PCL40
Boltorn-PCL50
Degree of
crystallisation (%)
29
41
42
50
Onset of
melting (°C)
34.3
41.3
47.9
50.2
Peak of
melting (°C)
45.3
51.7
53.6
58.6
41
4.3 Dynamic Viscosity from Solid to Molten State
An important feature of the semi-crystalline resins is their fast transition from the
solid to the molten state. This characteristic will permit the use of lower curing
temperatures since the modulus of the resin is maintained until the melting
temperature (Tm) is reached.
Figure 4.8 depicts the complex viscosity (η*) as a function of temperature of a
number of different synthesized resins. The measurement of η* during the melting
transition of semi-crystalline resins shows a rapid decrease in viscosity over a narrow
temperature range, as compared to the slow softening of the amorphous acrylate
functional polyacrylate resin (APA). This rapid reduction in viscosity of semicrystalline resins gives them an advantage when compared to conventional amorphous
resins with thermal initiators, which soften slowly from Tg until the point where the
cross-linking reaction starts to increase the viscosity. For example, the semicrystalline PNPC displays a melting temperature almost ideal for a low temperature
curing powder coating resin, just below 100°C.
APA
Comb PCL DP10
Comb PCL DP20
Boltorn-PCL DP10
Boltorn-PCL DP20
Di-TMP-PNPC DP8
8
10
6
η* (Pa s)
10
4
10
100
1
0
50
Temperature (°C)
100
150
Figure 4.8 Dynamic viscosity as a function of temperature for 4 types of different
resins.
42
4.3.1 Comb Poly(ε-caprolactone)’s and blends
To further widen the scope of this work, blends were prepared of the amorphous
resin APA and the comb-PCL’s.
9
10
APA
APA:Comb-PCLM DP20, 7:3
APA:Comb-PCLM DP10, 3:2
APA:Comb-PCLM DP20, 1:1
Comb-PCLM DP20
Comb-PCLM DP10
8
10
7
10
6
η* (Pa s)
10
5
10
4
10
1000
100
10
0
50
100
150
Temperature (°C)
Figure 4.9 Complex viscosity as function of temperature for some of the different
resins and resin blends.
The measurement of η* of the blends revealed a rheological behavior which falls
between those of its individual components (figure 4.9). The complex viscosity for
PCL10M started to decrease before that of PCL20M, which is consistent with the Tm
results from the calorimetric data (table 4.3). The melting points of the comb-PCL’s
are lower than that of linear PCL, which exhibits a Tm of around 65°C. The 1:1 blends
exhibit some crystalline behavior while the 3:2 and 7:3 blends behave as if fully
amorphous. The samples containing crystalline resin were all cooled the same way,
from 140 to –10°C, before the measurements began. This cooling did not allow
enough time for these PCL in the blends to crystallize to its full extent and it is
therefore likely that the resins would display behavior close to that seen with 1:1
blends. The η* at higher temperatures is also lower for the blends, facilitating leveling
and resulting in smoother films.
43
Table 4.3 DSC-data of the synthesized semi-crystalline resins.
a
44
Sample
Tg
Tm, peak a
DCb % a
ER 065
PCL10 B2
PCL10 B2
PCL20 B2
PCL20 B2
50c
-54
-55
-54
-55
36
38
47
47
29
34
35
35
Second heating, bDegree of crystallinity, c BASF AG
5. Film Characterization
5.1 Mechanical Properties of Star-Branched Poly(ε-caprolactone)
Film properties were evaluated after cross-linking. The polyester and the
photoinitiator Irgacure 184™ (1% by wt.) were added to just enough butyl acetate to
dissolve the polymer. The mixture was then applied with a 700 µm applicator onto a
glass plate. After evaporation of solvent at RT, the film was melted at 70 °C and then
immediately UV-cured with 5 subsequent passes under the UV-lamp, giving a total
dose of 500 mJ cm-2.
The dynamic mechanical properties of six cross-linked Boltorn-PCL films were
examined at temperatures between -70 and 100°C (table 5.1). The average film
thickness varied between 60-130 µm. All samples had an onset of Tg close to –50°C
which was as expected for CL based polymers. The dynamic mechanical thermal
analysis (DMTA) data shows that the cross-linking of the films reduced the possibility
for the PCL-chains to crystallize when the films were cooled. The resins with short
PCL-chains, DP 8 or less, did not crystallize at all in the cross-linked state, while
resins having longer PCL-chains, crystallized to some extent. In previous work,
calorimetric data of similar films have shown that the degree of crystallization in the
cross-linked films of long-chained resins is approximately 50% as compared to the
uncured resin.121
Table 5.1 DMTA of cross-linked Boltorn PCL films, where E is the Young’s modulus.
Boltorn-PCL
E (MPa)
E (MPa)
E (MPa)
(DPNMR)
(t=-55 °C)
(t=23°C)
(Rubber-plateau)
5
8
20
30
38
52
1770
1600
1350
380
440
1410
15
16
94
45
79
250
15
16
2.3
0.9
~1
~1
The effect of crystallization in the cured film is clearly seen in figure 5.1, where
the semi-crystalline film maintains a higher modulus above Tg as compared to the
amorphous film. Above the melting point, the modulus for crystalline resins drops to a
level lower than that for amorphous resins since the crystalline resins have a lower
cross-link density. The results show that the structure of a resin has a pronounced
effect on the final film properties and, thus, by altering the structure a resin can be
tailored for a specific application.
45
Young's modulus (MPa)
10000
1000
Semi-crystalline
100
Amorphous
10
1
-80
-60
-40
-20
0
20
40
60
80
100
120
Temperature (°C)
Figure 5.1 Young’s modulus as a function of temperature. The semi-crystalline and
the amorphous films have DP’s of 52 and 5, respectively.
5.2 Properties of Comb Poly(ε-caprolactone) Films
The study of comb polymers was made to investigate the overall effect on film
properties of varying the graft length. In addition, blends of amorphous and semicrystalline resins were evaluated with respect to length of the grafts and blend ratio.
To give some insight into these relationships, an epoxy functional acrylic resin (ER
065) was modified into a semi-crystalline thermosetting resin (section 3.4).
5.2.1 Powder and Film Preparation
All resins were melt-mixed with photoinitiator (Irgacure 2959, 2 wt.%).
Pulverization was performed using dry ice since the resins were slightly tacky at room
temperature. The resulting powder was sifted through a 150 µm sieve and applied to a
steel substrate with an electrostatic spray gun. The combination of the low Tm and low
Tg of poly(ε-caprolactone) made grinding, sieving and spraying tricky and storing at
room temperature impossible. The powders were fused for 15 minutes at 140°C. This
temperature was chosen so that all the films and resins, amorphous, semi-crystalline
and mixtures thereof, would undergo the same fusing conditions. Another set of films
was fused in an IR oven at 140°C or 160°C for two minutes. However, the 2 minute
exposure was not long enough to produce an even surface, a requirement for the
mechanical testing. The curing was initially performed using two different pieces of
equipment, a UV-Minicure with a single high-pressure mercury lamp and a high
power irradiator, HL-60-3X1, using three lamps, two high-pressure mercury lamps
and a gallium doped mercury lamp.
46
Table 5.2 Cure conditions and mechanical properties of the cured films. All films
were fused in a convection oven at 140°C for 15 minutes. No orange peel texture was
observed for any of the films and the gloss was poor for all films. The cross-cut range
from Gt 0-Gt 5 where Gt 0 is the best. All films were prepared from resins from the
second batch
Sample
Curing
Comb-PCL10M
Comb-PCL10M
Comb-PCL20M
Comb-PCL20M
UV Minicure
HL-60-3X1
UV Minicure
HL-60-3X1
Erichsen test
[mm]
8.4
8.6
9.4
8.8
Diameter at
break [mm]
4
6
2
3
Cross-cut
test
Gt 5
Gt 5
Gt 5
Gt 4
The films cured with higher irradiation intensity, HL-60-3X1, were slightly more
brittle, as exhibited by the Erichsen and bending test results (table 5.2). Higher
irradiation intensity results in a greater number of radicals, which in turn leads to a
shorter kinetic chain length and decreased cross-link density. Films cured using the
UV-Minicure were tacky, a sign of oxygen inhibition. The high intensity system
produced films without a tacky surface and was therefore used to cure all films for
which film properties were to be evaluated. It is also evident from the Erichsen and
bending tests, tables 5.2 and 5.3, that PCL20 exhibits a greater flexibility, as expected,
than the PCL10 resin due to the lower cross-link density of PCL20.
5.2.2 Film Properties
Different ratios of Comb-PCL10M:APA and of Comb-PCL20M:APA were
evaluated to identify which blends showed the best characteristics (table 5.3). More
blends were made with PCL10 as it was less tacky, which made processing easier.
The Comb-PCL10M:APA 4:6 blend exhibited the best overall properties. The results
show that it is possible to use the highly branched structures to modify a system and
improve its properties. The fact that APA has acrylate functionality and the semicrystalline resin has methacrylate end groups, will give different polymerization rates.
How this affects the system is not known.
47
Table 5.3 Film properties of the evaluated films.
Sample,
Blend ratio
Erichsen
test[mm]
Diameter of
break [mm]
Cross-cut
test
Gloss
Surface
APA
0.3
-
Gt 5
Good
Orange peel
Comb-PCLM
10:APA, 3:7
4.3
-
Gt 3
Fair
Smooth
CombPCLM
20:APA, 3:7
4.4
-
Gt 3
Fair
Smooth
CombPCLM
10:APA, 4:6
5.3
8
Gt 1
Fair
Smooth
CombPCLM
10:APA, 1:1
7.3
6
Gt 4
Fair
Smooth
CombPCLM
20:APA, 1:1
5.8
6
Gt 4
Fair
Smooth
CombPCLM 10
8.6
6
Gt 5
Poor
Smooth
CombPCLM 20M
8.8
3
Gt 4
Poor
Smooth
One factor to note with these resins is the fact that even a high molecular weight
semi-crystalline resin will help the leveling of the film. The APA resin by itself is not
able to produce a film without orange peel while the PCL20M resin and blends
thereof with a molecular weight of about 60 000 g/mol produce smooth films.
Addition of a high molecular weight resin can also reduce the shrinkage upon cure,
which in turn will reduce internal stress buildup.
An important point to keep in mind is that there is a conflict between cross-linking
density and crystallinity. The grafts have to be long enough to ensure crystallinity and
yet short enough not to crystallize after cross-linking. The cross-link density of this
type of resin is low, which makes the final films flexible. On the other hand, this is
not necessarily a problem since the substrates for low temperature curing, paper,
plastic materials and wood, are inherently flexible.
The problems associated with use of a pure semi-crystalline resin, the process
problems and poor mechanical properties, might be avoided by using grafts of a
crystalline polymer with the right combination of Tg and Tm, preferably in the range
of 30-40°C and 80-100°C, respectively.
48
6. Conclusions
The concept of aUV-curable branched semi-crystalline thermoset resin was
evaluated. The poly(ε-caprolactone) system confirmed the anticipated advantages,
include low zero shear viscosity, rapid reduction of viscosity at the melting
temperature, and fast curing.
It was shown that the molecular weight can be increased about one order of
magnitude using a star-branched polyester with aproximately30 arms over that of a
linear polyester while maintaining the zero shear viscosity. The high molecular weight
in the range of interest, up to 10-20 Pa s of zero shear viscosity, can reduce or
eliminate unwanted penetration of a porous substrate. In addition, the semi-crystalline
resins exhibit a fast decrease in viscosity around the melting temperature as compared
with the slow softening of an amorphous resin. After leveling, UV initiated cure is
completed in a matter of seconds. Even though the time to gel increased linearly with
increasing molecular weight when using the G’ and G’’ crossover as the gel point,
time to gel was less than 2 s. Dynamic mechanical data show that the rubber plateau
modulus decreases with increasing length of the poly(ε-caprolactone) grafts. An
important point to keep in mind is that there is a conflict between cross-linking
density and crystallinity. The grafts have to be long enough to ensure crystallinity and
yet short enough not to crystallize after cross-linking. The low cross-link density and
glass transition temperature makes the star-branched poly(ε-caprolactone) films
flexible and soft. In the cured films the degree of crystallization depends on the length
of the PCL-grafts. Post cure crystallization can be avoided by using short grafts,
around DP=10, for star PCL’s. The dynamic viscosity measurements of the melting
transition clearly showed the advantage of a semi-crystalline system, specifically the
rapid decrease in viscosity at Tm.
Thermosetting comb-polymers with semi-crystalline grafts were also synthesized
in a straightforward manner requiring only three steps from a starting epoxide
functional amorphous resin. The synthesis allowed good control of the structure and
molecular weight. The resulting semi-crystalline resins displayed relatively high
crystallinity and a considerably depressed melting point compared to linear poly(εcaprolactone). The low melt viscosity resulted in smooth films. The flexibility of the
films containing a blend of semi-crystalline and amorphous resin was also improved
compared to the films of pure amorphous resin. This was due to a decrease in crosslink density and the low Tg of the poly(ε-caprolactone) grafts. The main drawback
was the poor storage stability of the semi-crystalline powders and the blends. In
addition, the mechanical properties of the model poly(ε-caprolactone) systems are not
ideal. However, the concept provides advantages, such as low viscosity, high
molecular weight, improved storage stability, low toxicity and absence of a low
molecular weight binder, compared to conventional powder coating systems.
The branched poly(neopentylene carbonate) on the other hand displays a Tm
around 100°C and a Tg around 20-30°C which is more suitable for a low temperature
curing powder coating. The dynamic viscosity measurements also showed that the
resins maintained a high ‘viscosity’ until close to melting. This should allow for good
storage stability and reasonable mechanical properties. However, the control of the
polymerization was low; as a consequence the yield was also low.
SEC molecular weight data collected on the series of dendrimers using SECUC and
SEC3 were close to the theoretical and MALDI-TOF values. Calculations of the
viscosimetric radii showed a difference in size for Bz-[G]-TMP compared to the
49
acetonide/Bz-[G]-Ar dendrimers. This difference is associated to the difference in size
of the core molecule. It was also showed that the terminal unit had no effect on the
viscosimetric radius. The ratio Rg/RV suggests that that the dendrimers behave as
uniform-density spheres. The intrinsic viscosity of acetonide-[G]-Ar revealed a
delayed viscosity maximum compared to Bz-[G]-Ar, which might be explained by its
less congested surface.
50
7. Suggestions of further work
For the possible use of highly branched semi-crystalline resins in powder coating
application very little work has been carried out, to our knowledge. Since there are a
number of advantages a more thorough investigation of this concept is warranted
using other monomers to produce systems with better overall properties.
The branched poly(neopentylene carbonate) displays promising powder coating
properties which warrants a further investigation into their cross-linked film
properties.
In addition, the highly crystalline poly(ω-pentadecalactone) with a Tm of around 90°C
might be a suitable candidate for coating application, however the Tg is very low.
There are also a number of different dendritic initiators available. A hyperbranched
hydroxyl functional polyether might provide an increased outdoor stability. This
particular core is readily available within the group. In the near future a
hyperbranched polycarbonate might be available for evaluation.
Another possible study of these polymers is as rheological additives in established
coating systems.
51
Acknowledgements
I would like to express my gratitude to my supervisor Professor Anders Hult for
accepting me as his graduate student, providing guidance, sharing his extensive
knowledge and making the group a great place to work and develop.
I would also like to thank the senior members of the Hult group, my co-supervisor,
Associate Professor Eva Malmström and Associate Professor Mats Johansson; Eva,
for always being enthusiastic, and for her insightful help, without her this work would
have been much harder; Mats for his support, interesting discussions and input to my
work.
Financial support provided by Perstorp AB and AB Wilhelm Becker is gratefully
acknowledged.
Professor Jan-Anders Månson and Marc Doyle, both co-authors, are gratefully
acknowledged for making my visit to Lausanne productive and enjoyable.
I am indebted to Dr. Christian Kugge at The Institute for Surface Chemistry,
Thomas Larsson and especially Björn Atthoff in the Hillborn group at Uppsala
University, Department of Material Chemistry, for providing access to and guidance
in using their rheometer.
I would like to thank Professors Ann-Christine Albertsson, Bengt Stenberg and
Sigbritt Karlsson for organizing and leading the Department of Polymer Technology.
Especially, Ulf Gedde and Mikael Hedenqvist are acknowledged for there the
interesting discussions
The members of Professor Hult’s group, both past and present; Curzio, the Italian
Chorizio, Johan, I still haven’t seen you play with the band, Henrik (I), the king of
dendrimers and cheap shots, Anna, Daniel, Cecilia, Geraldine, Linda S., Thierry, Phil,
Claire, Helene, Linda, my favorite diploma worker-good luck with your PhD, Emma
and Josefina are thanked, if not their help, for the many fun times spent together.
Some people in the group require some special recognition:
Andreas Krupicka for always being a good sport and friend, who also engaged me
in interesting discussions regarding everything from polymer mechanics to the secret
of a really good cup of coffee.
Peter Löwenhielm for always trying my patience and for his ability to create
complete disorder in the whole lab, single-handedly. During the years there have also
been interesting collaborations that made this thesis possible. You’re the man!
The “time-martyr” of the group, Michael “I work better at night (zzzz…)”
Malkoch, sucking the life-force out of the group in the last days of his thesis writing,
is thanked for making the world and the group a better place. See ya in California!
Super host Robert “I know everything” Vestberg for arranging great parties,
frequently; making sure everyone gets their ration of “Labimner”.
Andreas “I know the rest” Nyström for his extensive knowledge in most matters and
his first-rate proofing.
All my friends in the department, former and present, are also thanked for making
it a great place to work. Special thanks go to Eugenia, Micke “The party never ends”
Krook, Ana, the excellent hostess of many gatherings, Kamyar with whom I shared an
52
office and Guillaume with whom I never want to share an office, and Richard “The
Nano Particle” Olsson for the interesting conversations about everything from science
to heavy metal.
I would also like to thank the administrative personnel of the department,
especially Inger Lord and Margareta Andersson for their valuable assistance with all
administrative matters.
I would like to give special thanks to Dr. Mikael Trollsås and Dr. James L. Hedrick
for accepting me as a diploma worker and later as an intern at IBM/CPIMA and for
introducing me to the world of hands-on polymer chemistry. Micke also introduced
me to the pleasure of wine (and the questionable one of grappa); I hope to see you on
the tennis court again. The time spent in California was fun and instructive and not
only did it result in me continuing my education for a PhD.
During my time in the California I meet the love of my life, Laura my wife, who
has been supporting me in all aspects of life including but also in my research with
discussions and the excellent proofing of all my writing, that sometimes makes as
little sense as me….
Last but not the least; I would like to express my sincere thanks to my parents,
Annika and Ingemar for all the support and encouragement over the years as a student
to reach this goal. I also want to thank my siblings Lisa and Anders, and all my
relatives all over the world for their support and encouragement of me becoming a
plastic doctor.
53
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57
APPENDIX A
H
n
H
O
O
H
n
O
H
O
O
O O
O
H
H
O
H
H
O
O n
O
O
O
n
n
O
O
O
n
H
n
O
O
HO
O
O
O
O
O
O
O
O
O
O
O
O
OH
O
O
O
O
O
O
O
O
O
O
O
O
O
O
n
O
O
O
H
H
O
O
O
O
H
O
O
n
O
H
O
O
O
n
O
n
O
O
O
O
n
O
O
O
H
O
n
O
O
H
O
O
n
n
H
Boltorn-PCL
H
n
O
O
O
n
O
O
O
O
O O
O
n
O
H
O
O
O
O
O
O
O
O
n
O
H
O
O
O
H
n
O
O
O
O
O
n
H
O
O
O
O
n
O
O
O
O
O
H
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
n O
O
n O
O
O
O
H
O
O
n
O
H
O
O
n
O
O
O
O
O
O
O
H
O
O
O
n
n
O
O
nO
O
H
O
H
H
H
H
n
H
O
H
O
H
n
O
H
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n O
O
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H
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n
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n
O
O
H
n
O
n O
O
O
O
O
O n
H
O
O
O
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n
O
n
O
O
O
O
O
O
O
O
O
H
O
O
O
n
O
H
O
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n
O
O
O
O
O
O
O
n
O
O
O
O
O
O
O
O
O
n
H
n H
H
Dendrimer-PCL
n H
H
H
n
O
n H
n H
O
n
O
O
O
O
H
H
O
O
O
H
O
O
O
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n
O
O O
O
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O
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OO
O
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n
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n
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n
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O
O
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O
O
O
O
O
O
H
H
O
n
O
O
O
O
O
O
n
O
O
n
n H
H
Dendron-PCL
H
n
O
O
O
O
O
H
n
O
O
n
O
O
O
O
H
O
O
O
O
n
O
O
n H
Linear PCL
H
APPENDIX B
Boltorn-PCL
DP,
DP, 1H- Mw, 1H-NMR
-1
Mw, SEC
Mw/Mn
-1
aim
NMR
(g mol )
(g mol )
5
7
10
12
15
20
25
30
35
40
45
50
50
55
55
70
5
8
10
13
16
22
22
32
36
40
46
51
54
55
56
79
21 900
32 800
40 100
49 300
62 000
84 000
84 000
120 500
135 100
149 700
171 600
189 900
200 800
204 500
208 100
292 100
29 500
43 100
50 800
66 700
72 200
99 400
107 100
126 800
144 200
168 000
199 800
193 900
187 700
220 500
216 100
279 900
Mw, SEC
η0
Mz,
-1
(g mol )
(Pa s)
2.99
3.38
3.36
3.16
3.32
3.11
3.68
6.39
5.25
6.92
7.58
8.45
9.88
7.62
8.35
7.68
54 400
82 200
90 300
118 000
124 700
189 200
199 100
237 000
271 000
328 900
420 600
380 600
365 900
410 400
419 300
545 900
1.18
1.32
2.48
4.24
4.69
10.3
13.8
21.0
45.0
79.8
100
135
129
235
137
460
Mw/Mn
Mz,
η0
Dendrimer-PCL
DP,
DP, 1H- Mw, 1H-NMR
-1
-1
aim
NMR
(g mol )
(g mol )
10
12
15
18
20
25
30
35
40
45
60
15
14
14
21
24
30
37
42
49
51
51
43 800
41 100
41 100
60 300
68 500
84 900
104 000
117 700
136 900
142 300
142 300
32 300
39 400
38 200
58 400
67 600
86 200
94 300
96 500
116 500
119 500
114 100
-1
1.35
1.43
1.43
1.50
1.18
1.31
1.33
1.50
1.60
1.56
2.00
(g mol )
(Pa s)
36 500
45 300
43 900
67 720
76 600
102 700
114 600
113 500
138 800
141 900
143 400
1.34
3.18
3.30
6.10
6.85
12.2
38.3
44.0
47.0
63.0
65.0
Dendron-PCL
DP,
DP, 1H-
Mn, 1H-NMR
-1
Mw, SEC
Mw/Mn
-1
aim
NMR
(g mol )
(g mol )
10
13
18
15
17
20
20
20
25
35
37
40
80
11
14
15
16
19
20
21
22
23
35
39
41
81
11 000
13 700
14 600
15 500
18 300
19 200
20 100
21 000
21 900
32 900
36 500
38 400
74 900
11 900
14 600
15 300
16 400
18 300
20 400
22 000
22 700
22 200
32 600
36 000
38 700
65 400
Mw, 1H NMR
Mw, SEC
η0
Mz
-1
(g mol )
(Pa s)
1.05
1.15
1.13
1.11
1.14
1.14
1.16
1.17
1.25
1.28
1.68
1.41
2.02
12 900
16 000
16 600
17 500
19 800
22 100
23 900
25 100
25 300
36 900
45 600
46 600
88 400
1.05
1.73
1.58
2.10
2.17
2.66
3.85
5.40
6.00
12.0
40.0
46.0
505
Mw/Mn
Mz,
η0
Linear-PCL
DP,
DP, 1H
-1
-1
aim
NMR
(g mol )
(g mol )
14
15
16
20
20
22
24
30
50
40
50
54
60
66
60
70
80
90
100
110
190
180
200
17
17
19
21
24
25
25
35
45
46
52
55
62
66
54
72
82
96
106
120
183
184
202
2 160
2 160
2 390
2 620
2 960
3 070
3 070
4 210
5 350
5 470
6 150
6 490
7 290
7 750
6 380
8 400
9 570
11 120
12 300
13 900
21 100
21 200
23 300
2 000
2 060
2 210
2 420
2 810
3 080
3 180
4 380
5 150
5 170
6 290
6 230
7 050
7 620
7 480
9 500
10 800
12 900
13 300
15 800
24 200
24 800
28 000
1.19
1.15
1.17
1.10
1.18
1.30
1.18
1.25
1.11
1.13
1.18
1.22
1.25
1.13
1.26
1.27
1.30
1.37
1.29
1.43
1.46
1.54
1.65
-1
(g mol )
(Pa s)
2 340
2 330
2 540
2 710
3 220
3 810
3 660
5 210
5 830
5 750
7 210
7 300
8 370
8 430
8 920
11 400
13 300
16 300
16 300
20 500
31 900
33 900
39 800
0.182
0.198
0.210
0.247
0.327
0.364
0.420
0.777
1.10
1.11
1.69
1.81
2.16
2.70
2.76
4.90
7.90
13.0
13.7
26.0
120
134
215
APPENDIX C
OH
HO
O
HO
OH
O
O
OH OH
HO
O
O
O
O
O
O
O
O
O
O
OH
O
O
OH
OH
O
OH
OH
O
O
O
HO
HO
OH
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OH
O
O
O
OH
OH
O
O
OH
HO
O
O
O
HO
OH
O
O
HO
OH
O
O
O
OH
O
O
Hyperbranched polymer
Boltorn H30
OH
O
O
HO
OH
OH
O
O
O
NPC
Catalyst
OH
O
O
OH
n
HO
O
O
O
O
O
O
n
n
O
O
O
O
n
O
O
O
O
O
O
O
O
n
O
O
O
n
O
O
HO
O
n
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
n
O
n
OH
O
OH
O
O
O
O
OH
O
O
n
O
O
O
OH
O
O
O
O
n
OH
O
O
n
O
O
O
O
O
O
O
n
O
O
O
OH
O
HO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
n
O
O
O
O
O
O
O
O
O
O
OH
O
O
O
O
O
O
O
OH
n
O
O
O
O
O
O
O
O
O
O
O
O
O
OO
O
n
HO
O
n
O
HO
OH O
O
O
O
O
O
O
O
O
OH
O
O
O
O
O
O
O
O
O
O
O
O
O HO
O
O
O
OH
O
O
O
HO
n
O
O
O
O
O
O
O
HO
O
OH
O
O
OH
OH
Synthetic Scheme for Boltorn-PNPC
OH