MACROMOLECULAR STRUCTURE EVOLUTION OF GIANT MOLECULES VIA
“CLICK” CHEMISTRY: ASYMMETRIC GIANT GEMINI SURFACTANTS
BASED ON POLYHEDRAL OLIGOMERIC SILSESQUIOXANE
A Thesis
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Hao Su
May, 2014
MACROMOLECULAR STRUCTURE EVOLUTION OF GIANT MOLECULES VIA
“CLICK” CHEMISTRY: ASYMMETRIC GIANT GEMINI SURFACTANTS
BASED ON POLYHEDRAL OLIGOMERIC SILSESQUIOXANE
Hao Su
Thesis
Approved:
Accepted:
Advisor
Dr. Stephen Z. D. Cheng
Dean of the College
Dr. Stephen Z. D. Cheng
Faculty Reader
Dr. Toshikazu Miyoshi
Dean of the Graduate School
Dr. George R. Newkome
Department Chair
Dr. Coleen Pugh
Date
ii
ABSTRACT
Physical properties of giant molecules are intimately dependent upon their primary
chemical structures and molecular topologies and thus, precise controls of these
structures and topologies are an important pursuit in developing new functional
materials. In this paper, we report the rational molecular design and cascading “click”
synthesis of three types of polyhedral oligomeric silsesquioxane (POSS)-based
asymmetric giant gemini surfactants (AGGSs) including (1) consisting of two polymer
tails differing in chain lengths and two identical heads, (2) having two distinct functional
heads and two similar tails, and (3) possessing two tails of different lengths and two
distinct head groups. The syntheses were achieved in a convenient, efficient, and modular
way, allowing the precise control and rigorous tuning of both of polymer tail composition
and POSS surface chemistry in these AGGSs. The synthetic processes can be rapidly
accomplished within about 5 hours and only require non-chromatographic purification.
Our study expands the scope of synthetically available giant surfactants along with the
route of macromolecular structure evolution of giant molecules inspired by further
decreasing the symmetry of POSS-based giant molecules using “click” chemistry
iii
ACKNOWLEDGEMENTS
First of all, I would like to express fully respect to my advisor, Dr. Stephen, Z. D.
Cheng, for giving me a opportunity to study and work with him, and for his kind help and
tremendous support. His confidence, passion and dedication to science really inspired me
a lot and urge me to be a good researcher. He taught a lot not only in academic research
but also being a better person in real life.
I would like to give me thanks to my reader, Dr. Toshikazu Miyoshi, who also take
care of me a lot. I am very honor to have a such dedicated thesis reader. Although, I am
not his student, he often asked me about my research progress and gave me a lot of
encouragements.
I would like to highlight my thank to Dr. Yiwen Li for advice, encouragement, and
friendship. He was the one who really brought me into the research and taught my a lot
through the approximate two years. I was very lucky to have such good mentor and friend
in Dr. Cheng’s group. I also would like to extend my thanks to Mr. Zhao Wang, who also
taught me a lot for his research discussions, support and friendship. Moreover, Special
thanks give to Dr. Xuehui Dong, Dr. Kan Yue, Mr. Xueyan Feng, Mr. Shuo Zhang, Mr.
Hao Liu, Mr. Mingjun Huang and Mr. Pengtao Lu for guidance and friendship. I would
also like to further extend thanks to rest of the many Cheng’s group members. I was also
very grateful to Ms. Kai Guo and Prof. Chrys Wesdemiotis for the MALDI-TOF mass
spectral analysis.
iv
I would like to extend a special thank to Dr. Qiang Fu at SCU for giving me the
opportunity to start my research and recommend me to Dr. Cheng’s group. I also wish to
thank Dr. Nanying Ning, who taught me a lot in the undergraduate research.
Last but not least, I would like to thank my parents and all family members. I am
most appreciative of their deep love and strong support. Without this, I will never go so
far. Also, another special thank are given to my friends both in Akron and China. They
are those who I spend my youth with.
v
TABLE OF CONTENTS
Page
LIST OF FIGURES…………...............................................................................viii
LIST OF TABLES………….................................................................................x
LIST OF SCHEMES…………..............................................................................xi
CHAPTER
I. INTRODUCTION…………...............................................................................1
II. EXPERIMENTAL SECTION…………………………….………..................7
2.1 Chemicals and Solvents....................................................................................7
2.2 Characterization................................................................................................8
2.3 Synthetic Procedure..........................................................................................10
2.3.1 HO-N3..............................................................................................10
2.3.2 BocNH-O-N3...................................................................................11
2.3.3 NH2-O-N3........................................................................................12
2.3.4 PS48-(tBAPOSS)-N3........................................................................12
2.3.5 PS48-(tBAPOSS)-(tBAPOSS)-PS32 (AGGS1)................................13
2.3.6 PS48-(tBAPOSS)-(APOSS)-PS48 (AGGS2)....................................13
2.3.7 PS48-(tBAPOSS)-(APOSS)-PS32 (AGGS3)....................................14
III. RESULTS AND DISCUSSIONS…………………….....................................15
3.1 Rational Design of AGGSs...............................................................................15
3.2 Facile Synthesis of Macromolecular “Clickable” Precursor........................... 18
3.3 Facile Synthesis of AGGSs..............................................................................26
vi
IV. CONCLUSIONS……………………………………........................................33
REFERENCES………………………………………………...............................34
vii
LIST OF FIGURES
Figure
Page
1. 1H NMR spectra of (a) PS48-(tBAPOSS)-N3, (b) PS48-(tBAPOSS)(tBAPOSS)-PS32, (c) PS48-(tBAPOSS)-(APOSS)-PS48, and (d)PS48
-(tBAPOSS)-(APOSS)-PS32……………................................................................21
2. 13C NMR spectra of (a) PS48-(tBAPOSS)-N3, (b) PS48-(tBAPOSS)
-(tBAPOSS)-PS32, (c) PS48-(tBAPOSS)-(APOSS)-PS48, and (d)
PS48-(tBAPOSS)-(APOSS)-PS32…………….........................................................22
3. UV spectra of (a) CHO-(VPOSS)-DIBO (black curve), (b) PS48(tBAPOSS)-N3 (red curve), (c) PS48-(tBAPOSS)-(tBAPOSS)PS32 (blue curve),(d) PS48-(tBAPOSS)-(APOSS)-PS48 (purple curve),
and (e) PS48-(tBAPOSS)-(APOSS)-PS32 (green curve)…………….......................23
4. FTIR spectra of (a) PS48-N3 (black curve), (b) PS48-(tBAPOSS)-N3
(red curve), and (c) PS48-(tBAPOSS)-(tBAPOSS)-PS32 (blue curve).…………….23
5. MALDI-TOF mass spectra of PS48-(tBAPOSS)-N3. The full spectrum
was obtained in positive linear mode……………...................................................24
6. SEC overlays for polymers: PS48-N3 (black curve), NH2-O-PS32
(red curve), PS48-(tBAPOSS)-N3 (blue curve), and PS48-(tBAPOSS)
-(tBAPOSS)-PS32 (green curve)……………...........................................................24
7. FTIR spectra of (a) PS48-(tBAPOSS)-N3 (black curve), (b) PS48(tBAPOSS)-(APOSS)-PS48 (red curve), and (c) PS48-(tBAPOSS)(APOSS)-PS32 (blue curve)……………..................................................................29
8. SEC overlays for polymers: PS48-(tBAPOSS)-N3 (black curve), NH2
-O-PS48 (red curve), and PS48-(tBAPOSS)-(APOSS)-PS48 (blue curve)...….…..….30
9. SEC overlays for polymers: PS48-(tBAPOSS)-N3 (black curve), NH2
-O-PS32 (red curve), and PS48-(tBAPOSS)-(APOSS)-PS32 (blue curve)…......….....32
viii
LIST OF TABLES
Table
Page
1. Summary of molecular characterization and
physical parameters of products…………………………………………..................25
ix
LIST OF SCHEMES
Scheme
Page
1. Schematic illustrations of various kinds of giant surfactants…………................6
2. General Synthetic Approaches for POSS-Based AGGSs…………......................6
3. Synthetic Approach for NH2-O-N3…………........................................................18
x
CHAPTER I
INTRODUCTION
Broadly diversified macromolecular chemical structures, topologies and shapes
within the field of nanotechnology have paid the critical importance and grown
considerably in recent years, yet the overwhelming majority of self-assembled soft matter
nanostructures
intended for energy and medicine applications highly rely on the
uniformity of macromolecules.1,
2
For many biomacromolecules (i.e. protein or nucleic
acid) constructed by various monomeric units (amino acids or nucleotides), on the other
hand, their secondary/tertiary structures and biological functions are critically dependent
on the precisely-controlled distribution of monomeric unit sequence. In contrast to natural
polymers such as DNA and proteins, rapid and precise synthesis of the primary chemical
structures (i.e. functionalities and block sequences) and control of topologies of most
synthetic polymers remain a grand challenge in polymer chemistry.1, 3, 4
In the recent years, the emergence of giant molecules provides new opportunities to
achieve the exquisite level of “precision” via modular, robust, and efficient chemistries.5
Giant molecules describe the precisely-defined macromolecules made from molecular
nanoparticles subunits or their conjugates with other molecular nano-building blocks.5 In
particular, giant surfactants describe a unique class of giant molecules which are
consisted of polymers-tethered nanoparticles with precise chemical compositions and
1
macromolecular topologies.5-8 It captures the essential structural features of the
corresponding class of small-molecule surfactants but having size amplification into
several nanometers. Along with the macromolecular structure evolution of giant
molecules inspired by breaking of the symmetry, more types of giant surfactants with
various macromolecular architectures thus can be designed and synthesized in analogy to
their small-molecule counterparts, such as giant lipids with symmetric/asymmetric tails,9,
10
giant gemini surfactants,11, 12 giant bolaform surfactants,12 and multiheaded/multitailed
giant surfactants.12,
13
Additionally, giant surfactants are also useful as attractive
nanobuilding blocks for advanced functional materials by virtue of their self-assembled
engineering capabilities to construct supramolecular structures with sub-10 nm feature
sizes.6, 7 This class of unique macromolecules bridges the gap between small-molecular
surfactants and block copolymers, which are two most important amphiphilic molecules,
by possessing a duality of both two in terms of their self-assembly behaviors in the bulk
and solution.
So far, most of the efforts to self-assemble giant surfactants have focused on the
introduction of functional polyhedral oligomeric silsesquioxane (POSS) nanoparticles as
the head, which relies on readily established surface chemistries.14-19 In addition, POSS
allows the precise control of shape, symmetry, tethered number and localization of
functional groups on the surface, which are highly desirable for the design and synthesis
of novel giant molecules.5, 20 By subsequently decreasing the symmetry of POSS cage, it
could be critical to develop a library of readily available POSS-based giant surfactants
with an exact number of surface functional groups and polymer tails at precise locations.
An ongoing challenge in developing new types of POSS-based giant surfactants is the
2
elaboration of new methodologies for their surface functionalization. On the other hand,
tremendous success of connecting various types of tails onto the chemical functionalized
POSS heads has been achieved by both of “grafting-from”9,
strategies.10,
12, 13, 22
21
and “grafting-onto”
The former one sometimes has difficulties in generating perfectly-
controlled grafted polymer tail compositions.9 Therefore, when considering the further
development of diverse POSS-based giant surfactants with more complex molecular
structures, “grafting-onto” strategy seems to be a more reliable and general approach
towards various giant surfactants with precisely-controlled molecular mass, chemical
compositions, and topological architectures.22
The introduction of “click” chemistry into “grafting-onto” strategy has brought a
wide range of opportunities for POSS-based giant surfactants synthesis with these highly
efficient, robust, and modular reactions as an ideal methodology for the rapid and precise
preparation of giant molecules.5 Within this “click” toolbox of reactions are Cu(I)catalyzed [3+2] azide-alkyne cycloaddition (CuAAC),23 strain-promoted azide-alkyne
cycloaddition (SPAAC),24, 25 thiol-ene “click” coupling (TECC) reaction26-29 and oxime
ligation.30, 31 The sequential combinations of these “click” reactions for the synthesis of
giant surfactants with complex molecular and topological structures have already been
proven to be successful in the macromolecular structure evolution of giant molecules. For
example, our group has demonstrated a sequential triple “click” chemistry methodology
based on the consecutive SPAAC, CuAAC, and TECC to prepare POSS-based
multiheaded/multitailed giant surfactants.13 Most recently, the synthetic potential of
“click” chemistry has been exploited and integrated for the efficient one-pot synthesis of
single-tailed and asymmetric multitailed giant surfactants.10 These promising results
3
suggest that the integration of different orthogonal “click” reactions may present
numerous opportunities for greater structural and topological versatilities by further
deceasing the macromolecular symmetry and controls in the construction of preciselydefined giant surfactants.
Small-molecular gemini surfactants might be one of the most important nonconventional surfactants during the past decade.32-35 It is so-called “gemini” since this
amphiphilic compound possesses two conventional surfactant sub-units connected
together via a spacer arm. This class of materials possesses high surface activities and
behaves unusual aggregation properties in solution.32 Specifically, considerable effort has
been exercised to design and synthesize various asymmetric gemini surfactants (AGSs)
(also called heterogemini or heterodimeric surfactants).36 AGSs are usually made up of
two different amphiphilic molecules, still connected at the level of, or close to, the head
groups via a spacer. Notably the amphiphilic moieties might differ by the length and/or
nature of the alkyl chains and by the nature of the head group. It was found that both of
the dissymmetry of AGSs and hydrophobic chain length could have a great influence on
the micellization process of small molecular surfactants.37 In addition, it is also observed
that the self-assembly behavior of giant surfactants is particularly sensitive to their
macromolecular architectures and topological variations.5, 7
Correspondingly, design and synthesis of giant gemini surfactants have also been
initiated. Our investigations have shown that the spacer length could greatly affect the
solution self-assembly behaviors of symmetric giant gemini surfactants containing two
carboxylic acid-functionalized POSS (APOSS) as well as two identical polystyrene (PS)
4
tails.11 It has been found that giant gemini surfactant with longer spacer exhibited more
stretched PS tail conformation in its micellar cores. Therefore, by further breaking the
symmetry of giant gemini surfactants, our current interests becomes focusing on precisely
building up various asymmetric giant gemini surfactants (AGGSs) via established “click”
approaches and systematically study on their self-assembly behaviors and structureproperty relationships. Notably, if we consider the functional POSS cage as one
polymeric block with precisely determined surface interaction and fixed volume, POSSbased AGGSs can thus also be recognized as a class of unique H-shape star block
polymers.38, 39 Nevertheless, the synthesis of well-defined AGGSs presents an immense
challenge if our pervious sequential “click” method are employed, especially when both
of the heads and tails are of different compositions. Multiple step (> 5 steps) construction
and extensive purifications are necessarily required, which might be a extremely timeconsuming and low-yield synthetic process.10
In this thesis, we strive to develop a general, convenient, and facile methodology
towards three kinds of POSS-based AGGSs (Scheme 1) in about 5 hours, including (1)
AGGS1 which consists of two polystyrene (PS) chains of different chain length tethered
with two identical functional POSS heads; (2) AGGS2 possessing two similar PS chains
tethered with two distinct functional POSS heads; and (3) AGGS3 which contains two
different PS tails tethered with two different functional POSS heads. The syntheses could
be achieved in a modular and efficient fashion by repeatedly using one-pot “click”
approach (Scheme 2), allowing ~ 100% conversion and non-chromatography purification.
Most importantly, both of the polymer chain compositions and POSS surface
5
functionalities could be precisely controlled and independently tuned during the
cascading synthetic process.
Scheme 1. Schematic illustrations of various kinds of giant surfactants: (a) giant
surfactant; (b) giant lipid; (c) symmetric giant gemini surfactant; (d) asymmetric giant
gemini surfactant with different polymer tails (AGGS1); (e) asymmetric giant gemini
surfactant with different head groups (AGGS2); and (f) asymmetric giant gemini
surfactant with different tails and heads (AGGS3).
Scheme 2. General Synthetic Approaches for POSS-Based AGGSs: (a): (i) PSn-N3, THF,
25
C, 2 hours, without any purifications; (ii) NH2-O-N3, TsOH, R1-SH (t-butyl
mercaptoacetate), DMPA, THF, 25
C, 30 min, 82%. (b): (i) CHO-(VPOSS)-DIBO,
THF, 25 C, 2 hours, without any purifications; (ii) NH2-O-PSm, TsOH, R2-SH (t-butyl
mercaptoacetate or 2-mercaptoacetic acid), DMPA, THF, 25
C, 30 min, 88% for
AGGS1, 87% for AGGS1, and 84% for AGGS3.
6
CHAPTER II
EXPERIMENTAL SECTION
2.1 Chemicals and Solvents
Styrene monomers (Aldrich, 99%) was purified by distillation from calcium hydride
under reduced pressure prior to use. Tetrahydrofuran (THF, Certified ACS, EM Science),
methanol (Fisher Scientific, reagent grade), ethyl acetate (Fisher Scientific), toluene
(Certified ACS), dichloromethane (Certified ACS), chloroform (Certified ACS), N, Ndimethylformamide (DMF, Sigma-Aldrich, anhydrous 99.8 %) and hexanes (Certified
ACS) were used after distillation. Cuprous bromide (CuBr, Aldrich, 98 %) was freshly
purified by stirring in acetic acid overnight, washed with acetone, and dried in vacuum.
2-Mercaptoacetic acid (Aldrich, > 98 %) was distilled under reduced pressure before use.
Octavinyl
POSS
(VPOSS,
Hybrid
Plastics,
>
97
%),
N,N,N’,N’’,N’’-
pentamethyldiethylene-triamine (PMDETA, Aldrich, 99 %), 4-formylbenzoic acid
(Aldrich, 97 %), 2-bromoisobutyryl bromide (Aldrich, 98 %), ethylene glycol (Aldrich,
anhydrous,
99.8%),
2-hydroxyethyl
2-bromoisobutyrate
(Aldrich,
95
%),
4-
(bromomethyl)benzoic acid (Sigma, 97 %), (boc-aminooxy) acetic acid (Aldrich, 98 %),
2,2-dimethoxy-2-phenylacetophenone (DMPA, Acros Organics, 99 %), 1-thioglycerol
(Sigma, > 99 %), N, N'-diisopropylcarbodiimide (DIPC, Acros Organics, 99 %), 4(dimethylamino) pyridine (DMAP, Aldrich, 99 %), hydrogen chloride solution (Aldrich,
4M in dioxane), sodium azide (Aldrich, > 99 %), succinic anhydride (Aldrich, > 99 %),
7
p-toluenesulfonic acid (TsOH, Aldrich, 98.5 %), and triethylamine (Et3N, Aldrich, > 99
%) were used as received. Silica gel (VWR, 230-400 mesh) was activated by heating to
140
C for 12 hrs. 4-(Dimethylamino) pyridinium toluene-p-sulfonate (DPTS),11 t-butyl
mercaptoacetate,40
4-(azidomethyl)benzoic
acid
(COOH-N3),41
azido-end-capped
polystyrene (PSn-N3),11 aminoxy-functionalized polystyrene (NH2-O-PSm),10 and CHO(VPOSS)-DIBO10 were synthesized as reported, respectively.
2.2 Characterization
Size exclusion chromatographic analyses (SEC) for the synthesized polymers were
performed using a Waters 150-C Plus instrument equipped with three HR-Styragel
columns [100 Å, mixed bed (50/500/103/104 Å), mixed bed (103, 104, 106 Å)], and a triple
detector system. The three detectors included a differential refractometer (Waters 410), a
differential viscometer (Viscotek 100), and a laser light scattering detector (Wyatt
Technology, DAWN EOS, λ = 670 nm). THF was used as eluent with a flow rate of 1.0
mL/min at 30
C.
All 1H and 13C NMR spectra were acquired in CDCl3 (Aldrich, 99.8 % D) utilizing a
Varian Mercury 300 NMR and 500 NMR spectrometer. The 1H NMR spectra were
referenced to the residual proton signals in the CDCl3 at δ 7.27 ppm; while the 13C NMR
spectra were referenced to 13CDCl3 at δ 77.00 ppm.
Infrared spectra were obtained on an Excalibur Series FT-IR spectrometer
(DIGILAB, Randolph, MA) by casting films on KBr plates from solutions with
8
subsequent drying at 40 - 50
C. The spectroscopic data were processed using Win-IR
software.
UV-Vis absorption spectra were collected using a Shimadzu 1750 UV-Vis
spectrometer. The test sample was dissolved in chloroform at a concentration of 50
μmol/mL and transferred to a quartz cuvette for measurement.
Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass
spectra were acquired on a Bruker Ultraflex-III TOF/TOF mass spectrometer (Bruker
Daltonics, Inc., Billerica, MA) equipped with a Nd:YAG laser (355 nm). The spectra
were measured in positive linear mode. The instrument was calibrated prior to each
measurement with external PMMA or PS standards at the molecular weight under
consideration. The compound trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB, Aldrich, > 98 %) served as matrix and was prepared in CHCl3 at a
concentration of 20 mg/mL. The cationizing agent sodium trifluoroacetate or silver
trifluoroacetate was prepared in MeOH/CHCl3 (v/v = 1/3) at a concentration of 5 mg/mL
or 10 mg/mL. The matrix and cationizing salt solutions were mixed in a ratio of 10/1
(v/v). All samples were dissolved in CHCl3 at a concentration of 10 mg/mL. The sample
preparation followed the procedure of depositing 0.5 μL of matrix and salt mixture on the
wells of a 384-well ground-steel plate, allowing the spots to dry, depositing 0.5 μL of
each sample on a spot of dry matrix/salt, and adding another 0.5 μL of matrix and salt
mixture on top of the dry sample (sandwich method). After solvent evaporation, the plate
was inserted into the MALDI mass spectrometer. The attenuation of the Nd:YAG laser
was adjusted to minimize undesired polymer fragmentation and to maximize the
sensitivity.
9
Electrospray ionization mass spectrometry (ESI MS) experiments were performed
on a Waters Synapt HDMS instrument quadrupole/time-of-flight (Q/TOF) mass
spectrometer (Waters, Milford, MA). The following ESI parameters were selected: ESI
capillary voltage, 3.0 kV; sample cone voltage, 70 V; extraction cone voltage, 3.2 V;
desolvation gas flow, 500 L/h (N2); trap collision energy (CE), 6 eV; transfer CE, 4 eV;
trap gas flow, 1.5 mL/min (Ar); sample flow rate, 5 μL/min; source temperature, 80 °C;
desolvation temperature, 150 °C. The sprayed solutions were prepared by dissolving ~0.1
mg of sample in 1 mL of CHCl3/MeOH (v/v, 50/50) containing 1% (v/v) NaTFA solution
(1 mg/mL in MeOH). The data analysis was conducted with the MassLynx 4.1 program
of Waters.
Thin-layer chromatographic analyses of the functionalized polymers were carried
out by spotting samples on flexible silica gel plates (Selecto Scientific, Silica Gel 60, F254 with fluorescent indicator) and developing using toluene or its mixture with other
polar solvents.
2.3 Synthetic Procedures
2.3.1 HO-N3
To a 100 mL round-bottomed flask equipped with a magnetic stirring bar was added
4-(azidomethyl)benzoic acid (COOH-N3) (177 mg, 1.0 mmol), ethylene glycol (620 mg,
10.0 mmol) and DPTS (295 mg, 1.0 mmol), followed by the addition of 30 mL freshly
dried CH2Cl2 to fully dissolve the solids. The mixture was capped by a rubber septum,
cooled to 0 oC and stirred for 10 min before DIPC (189 mg, 1.5 mmol) was added
10
dropwise via a syringe. The mixture was allowed to warm up to room temperature and
stirred for another 12 hr. The white precipitates was then filtered off and the filtrate was
washed with water and brine, dried over Na2SO4. After solvent removal, the residue was
purified by flash chromatography on silica gel using CH2Cl2 as the eluent to afford the
product (150 mg, 68 %). 1H NMR (300 MHz, CDCl3, ppm, δ): 8.05 (d, 2H, -OCCC2H2-),
7.36 (d, 2H, -CH2CC2H2-), 4.43 (t, 2H, -OCH2CH2OH), 4.38 (s, 2H, -CH2N3), 3.93 (t, 2H,
-OCH2CH2OH), 2.74 (s, 1H, -OCH2CH2OH).
13
C NMR (75 MHz, CDCl3, ppm, δ):
166.48, 140.68, 130.18, 129.74, 127.92, 66.68, 61.06, 54.18.
2.3.2 BocNH-O-N3
To a 100 mL round-bottomed flask equipped with a magnetic stirring bar was added
HO-N3 (221 mg, 1.0 mmol), (boc-aminooxy) acetic acid (201 mg, 1.05 mmol) and DPTS
(295 mg, 1.0 mmol), followed by the addition of 30 mL freshly dried CH2Cl2 to fully
dissolve the solids. The mixture was capped by a rubber septum, cooled to 0 oC and
stirred for 10 min before DIPC (189 mg, 1.5 mmol) was added dropwise via a syringe.
The mixture was allowed to warm up to room temperature and stirred for another 12 hr.
The white precipitates was then filtered off and the filtrate was washed with water and
brine, dried over Na2SO4. After solvent removal, the residue was purified by flash
chromatography on silica gel using CH2Cl2/EtOAc (v/v = 8/1) as the eluent to afford the
product (342 mg, 83 %). 1H NMR (300 MHz, CDCl3, ppm, δ): 8.07 (d, 2H, -OCCC2H2-),
7.82 (s, 1H, -CH2ONH-), 7.39 (d, 2H, -CH2CC2H2-), 4.56 (m, 4H, -OCH2CH2O-), 4.47 (s,
2H, -CH2ONH-), 4.42 (s, 2H, -CH2N3), 1.48(s, 9H, -OOCC(CH3)3). 13C NMR (75 MHz,
11
CDCl3, ppm, δ): 169.40, 165.83, 156.25, 140.82, 130.21, 129.46, 127.96, 82.01, 72.47,
62.80, 62.50, 54.17, 28.11.
2.3.3 NH2-O-N3
BocNH-O-N3 (300 mg, 0.73 mmol) was dissolved in 10 mL of hydrogen chloride
solution. The solution was stirred for 4 h, followed by the addition of excess Et3N to
adjust the pH. The white precipitates was then filtered off and the filtrate was then
evaporated under vacuum. The residue was purified by flash chromatography on silica
gel using CH2Cl2/EtOAc (v/v = 1/3) as the eluent to afford the product as a white powder
(180 mg, 84 %). 1H NMR (300 MHz, DMSO-d6, ppm, δ): 8.00 (d, 2H, -OCCC2H2-), 7.53
(d, 2H, -CH2CC2H2-), 4.79 (s, 2H, -CH2ONH2), 4.58 (s, 2H, -CH2N3), 4.50 (m, 4H, OCH2CH2O-), 3.58 (s, 2H, -ONH2).
13
C NMR (75 MHz, DMSO-d6, ppm, δ): 168.12,
165.77, 141.77, 130.16, 129.49, 128.94, 70.69, 63.37, 53.49. MS (ESI MS): Calcd. for
C12H15N4O5 295.10, Found: 294.76 (M∙H)+.
2.3.4 PS48-(tBAPOSS)-N3
CHO-VPOSS-DIBO (100 mg, 0.0852 mmol) and PS48-N3 (Mn,
NMR
= 5.1 kg/mol,
435 mg, 0.0852 mmol) were dissolved in 8 mL of THF. The mixture was stirred at room
temperature for 2 hours, followed by the addition of NH2-O-N3 (25 mg, 0.0852 mmol),
TsOH (5 mg, 0.0290 mmol), t-butyl mercaptoacetate (126 mg, 0.852 mmol), and DMPA
(2 mg, 0.005 mmol). The solution was then irradiated under UV 365 nm for 30 minutes,
12
after which it was precipitated into cold methanol solution three times. The sample PS48(tBAPOSS)-N3 was collected and dried under vacuum overnight to afford a white powder
(531 mg; Yield: 82 %). SEC: Mn = 7.9 kg/mol, PDI = 1.02.
2.3.5 PS48-(tBAPOSS)-(tBAPOSS)-PS32 (AGGS1)
PS48-(tBAPOSS)-N3 (Mn,
NMR
= 7.6 kg/mol, 150 mg, 0.0197 mmol) and CHO-
VPOSS-DIBO (23 mg, 0.0197 mmol) were dissolved in 5 mL of THF. The mixture was
stirred at room temperature for 2 hours, followed by the addition of NH2-O-PS32 (Mn,
NMR=
3.5 kg/mol, 69 mg, 0.0197 mmol), TsOH (5 mg, 0.0290 mmol), t-butyl
mercaptoacetate (29 mg, 0.197 mmol), and DMPA (2 mg, 0.005 mmol). The one pot
solution was then irradiated under UV 365 nm for 30 minutes, after which it was
precipitated into cold methanol solution three times. The sample PS48-(tBAPOSS)(tBAPOSS)-PS32 was collected and dried under vacuum overnight to afford a white
powder (238 mg; Yield: 88 %). SEC: Mn = 14.1 kg/mol, PDI = 1.01.
2.3.6 PS48-(tBAPOSS)-(APOSS)-PS48 (AGGS2)
PS48-(tBAPOSS)-N3 (Mn,
NMR
= 7.6 kg/mol, 150 mg, 0.0197 mmol) and CHO-
VPOSS-DIBO (23 mg, 0.0197 mmol) were dissolved in 5 mL of THF. The mixture was
stirred at room temperature for 2 hours, followed by the addition of NH2-O-PS48 (Mn, NMR
= 5.1 kg/mol, 100 mg, 0.0197 mmol), TsOH (5 mg, 0.0290 mmol), 2-mercaptoacetic acid
(18 mg, 0.197 mmol), and DMPA (2 mg, 0.005 mmol). The one pot solution was then
13
irradiated under UV 365 nm for 30 minutes, after which it was precipitated into cold
methanol solution three times. The sample PS48-(tBAPOSS)-(APOSS)-PS48 was collected
and dried under vacuum overnight to afford a white powder (250 mg; Yield: 87 %). SEC:
Mn = 15.0 kg/mol, PDI = 1.10.
2.3.7 PS48-(tBAPOSS)-(APOSS)-PS32 (AGGS3)
PS48-(tBAPOSS)-N3 (Mn,
NMR
= 7.6 kg/mol, 150 mg, 0.0197 mmol) and CHO-
VPOSS-DIBO (23 mg, 0.0197 mmol) were dissolved in 5 mL of THF. The mixture was
stirred at room temperature for 2 hours, followed by the addition of NH2-O-PS32 (Mn, NMR
= 3.5 kg/mol, 69 mg, 0.0197 mmol), TsOH (5 mg, 0.0290 mmol), 2-mercaptoacetic acid
(18 mg, 0.197 mmol), and DMPA (2 mg, 0.005 mmol). The one pot solution was then
irradiated under UV 365 nm for 30 minutes, after which it was precipitated into cold
methanol solution three times. The sample PS48-(tBAPOSS)-(APOSS)-PS32 was collected
and dried under vacuum overnight to afford a white powder (218 mg; Yield: 84 %). SEC:
Mn = 14.4 kg/mol, PDI = 1.06.
14
CHAPTER III
RESULTS AND DISCUSSION
3.1 Rational Design of AGGSs
Structurally, there are total five molecular segments to build up a AGGS: the first
polymer tail, the first functional POSS head, a spacer group, the second functional POSS
head, and the second polymer tail. To prepare such giant surfactants with complex
macromolecular architectures, quite a few possible techniques for joining those five
molecular pieces to each other can be potentially used. However, many of these methods
are quite sophisticated, usually requiring the delicate handling of highly reactive reagents
under tightly controlled conditions. In contrast, “click” chemistries may provide the
suitable and excellent chemical approaches to achieve the goal since they are easy to
perform, give rise to the target products in high yields with limited or no byproducts,
work well under various experimental conditions, and are often unaffected by the nature
of the functionalities being connected to each other.4, 42 Significant advances in “click”
chemistry-based synthetic methodology promise to improve the availability of a widerange of POSS-based giant surfactants for further structure-property investigation.
Furthermore, the combined utilization of multiple orthogonal “click” reactions presents
many opportunities to design and synthesize well-defined multifunctional giant
surfactants without sacrificing synthetic simplicity or efficiency.5 In particular, the
15
application of cascading one-pot “click” approach enable the rapid and precise
preparation
of
multi-component
POSS-based
giant
molecules
with
structural
complexity.10
The success of one-pot “click” methodology highly relies on suitable and versatile
“clickable” nanobuilding blocks based on functional POSS cages. In our pervious work,
CHO-(VPOSS)-DIBO, an important precursor possessing three kinds of “click”
functionalities (a vinyl group for TECC,
4-dibenzocyclooctyne (DIBO) motif for
SPAAC, and an aldehyde group for oxime ligation), has proven itself very useful as a
powerful precursor for the modular and facile one-pot synthesis of POSS-based
multifunctional giant surfactants under mild condition.10 In this study, we would like to
continue to employ this readily accessible and highly versatile POSS-based building
block for future exploration of various AGGSs. Considering that CHO-(VPOSS)-DIBO
could incorporate three different kinds of molecular segments during one-pot reaction
(functional POSS head was also regarded as a new molecular segment), so we are able to
rapidly and efficiently construct such AGGSs containing five distinct molecular species
by repeating one-pot “click” methodology two times.10 Notably, macromolecular
intermediate attaching with “molecular click adaptor/switch” after the first step one-pot
“click” reaction can be recognized as a new macromolecular “clickable” precursor for
joining further functionlization during the synthetic procedure.12
In small-molecular AGS system, the dissymmetry factor(s) of head or(and) tail play
a significant role in the self-assembly of gemini surfactants in the aqueous solution.36 In
this work, to emphasize the asymmetric issue of AGGs and further decreasing the
symmetry level of giant gemini surfactants, we specifically design POSSs with different
16
surface functionalities (t-butyl mercaptoacetate functionalized POSS (tBAPOSS) vs. 2mercaptoacetic acid functionalized POSS (APOSS)) and hydrophobic PS tails with
different chain length (5.1k vs. 3.5k). In particular, Dhead (dissymmetry of head)
(0  Dhead  100%) of AGGSs could be quantitatively estimated using eqn (1) as follows:
M R1POSS  M R2 POSS
Dhead 
 100%
M R1POSS  M R2 POSS
(1)
where M R1POSS and M R2 POSS indicate the molecular mass of R1POSS and R2POSS,
respectively. Similarly, Dtail (dissymmetry of tail) (0  Dtail  100%) of AGGSs can be
calculated from M PS1 (molecular mass of PS tail 1) and M PS2 (molecular mass of PS tail 2)
as in eqn (2):
Dtail 
M PS1  M PS2
M PS1  M PS2
 100%
(2)
In addition, similar to many other giant surfactants, the weight fraction of heads
(Whead) or tails (Wtail) offers another important physics parameter to determine the selfassembly behaviors of AGGSs, which could be directly derived from the eqn (3) and (4)
as followed:
Whead 
( M R1POSS  M R2 POSS )
Whead 
M AGGS
( M PS1  M PS2 )
M AGGS
 100%
(3)
 100%
(4)
Notably, tBAPOSS was selectively introduced as a modal functional head into
AGGS
system
due
to
several
reasons:
17
(1)
t-butyl
acetate-based
molecular/macromolecular clusters (e.g. poly (tert-butyl acrylate) (PtBA)) are immiscible
with PS, generally resulting in microphase separation between different domains;43-46 (2)
multi-component macromolecules consisting of PS block, poly(acrylic acid) (PAA) block,
and PtBA block or its analogies (e.g. poly(methyl acrylate) (PMA)) usually could selfassemble into a broad range of intricate nanostructures in dilute solution, which makes
our AGGSs more interesting;47, 48 (3) it is also very easy to tuning Dhead of AGGSs by
converting tBAPOSS head into APOSS head via controlled acidolysis.49
3.2 Facial Synthesis of Macromolecular “Clickable” Precursor
In Scheme 2, PS48-(tBAPOSS)-N3 was conveniently achieved by sequential multistep one-pot “click” reaction as a macromolecular intermediate. By taking advantages of
the “molecular click adaptor” molecule (NH2-O-N3), which could be served as a common
“clickable” precursor for three different kinds of AGGSs using the second one-pot
“click” reaction. In general, “click” adaptors are quite useful for further expanding the
scope of “click” methodology in the design and synthesis of giant surfactants.12 For NH2O-N3, it consists of two kinds of functional groups that are of completely orthogonal
“click” reactivities (CuAAC/SPAAC and oxime ligation), which could be employed
facilely to convert one“click” functionality to the other. The molecular structure (scheme
3) and purity of NH2-O-N3 is supported by both of NMR techniques (see the part of
synthetic procedures) and ESI-MS spectrum (found 294.76 Da versus calcd. 295.10 Da
for (M∙H)+).
18
Scheme 3. Synthetic Approach for NH2-O-N3: (i) ethylene glycol, DPTS, DIPC, dry
CH2Cl2, 0 C, 68%; (ii) (boc-aminooxy) acetic acid, DPTS, DIPC, dry CH2Cl2, 0 C,
83%; (iii) hydrogen chloride solution, 25 C, 84%.
The synthesis of PS48-(tBAPOSS)-N3 was directly performed by stoichiometric
mixing of CHO-(VPOSS)-DIBO (1.0 equiv) as the versatile “clickable” building block,
azido-functionalized PS chain as the tail 1 (PS48-N3, Mn, NMR = 5.1 kg/mol, PDI = 1.08, 1.0
equiv) in a common solvent (such as CHCl3 or THF). The one pot solution was then
stirred at room temperature for about 2 hours. Without any further treatment or
purification, a few regents including NH2-O-N3 as the spacer of final AGGS (also
“clickable” adaptor) (1.0 equiv), t-butyl mercaptoacetate as a functional thiol (10.0
equiv), p-toluenesulfonic acid (TsOH) as the catalyst for oxime ligation (5 mg), and 2,2dimethoxy-2-phenylacetophenone (DMPA) as the photoinitiator for TECC (2 mg) were
then added into the solution followed by another 30 min of irradiation under UV 365 nm.
The purification does not need any chromatographic processes, and all the byproducts
can be easily removed by repeated precipitation into cold methanol.
Results from various instrumental characterizations including 1H NMR (Figure 1a),
13
C NMR (Figure 2a), UV-vis (Figure 3), FT-IR (Figure 4), MALDI-TOF mass
spectrometry (Figure 5) and SEC (Figure 6) fully support the success of this one-pot
“click” reaction. First of all, compared with NMR results of CHO-(VPOSS)-DIBO, the
19
disappearance of the vinyl protons in the resonance range of δ (6.16-5.84) ppm in the 1H
NMR spectrum (Figure 1a) and sp2 carbon signals at δ 137.08 and 128.52 ppm (Figure 2a)
in the 13C NMR spectrum of PS48-(tBAPOSS)-N3 fully confirm the complete thiol-ene
functionalization of VPOSS cage. This result strongly agrees with the new characteristic
resonances emerging in the range of δ 3.19 and 2.79 ppm in the 1H NMR spectrum,
which is attributed to the protons of the newly-formed thiol ether bonds. Secondly, the
success of SPAAC reaction is revealed by the occurrence of two new distinct
characteristic peaks at δ 6.26 and 6.11 ppm corresponding to the proton (a) on the DIBO
in Figure 1a, shifted from the single peak at δ 5.58 ppm in the 1H NMR spectrum of
CHO-(VPOSS)-DIBO reported before.10,
13
In addition, The complete disappearance of
the strong absorbance for strained alkyne in DIBO at ~ 306 nm in the UV–vis absorbance
spectrum (Figure 3a and 3b) also indicates the near quantitative conversion efficiency of
“click” cycloaddition.10 Thirdly, the characteristic peak at δ 10.09 ppm corresponding to
free aldehyde proton in CHO-(VPOSS)-DIBO is completely missing in the new 1H NMR
spectrum of resulting macromolecular precursor, suggesting ~ 100% conversion of oxime
ligation reaction. The successful incorporation of the azido-functionalized spacer after the
one-pot reaction could be both demonstrated by benzyl azide protons (c) at δ 4.40 ppm in
the 1H NMR spectrum of (Figure 1a) as well as a strong characteristic vibrational band at
around 2100 cm-1 for azide group in the FT-IR spectrum (Figure 4). Moreover, the SEC
overlay shows a single symmetric distribution for PS48-(tBAPOSS)-N3 (Mn = 7.9 kg/mol,
PDI = 1.02) shifted to a lower retention volume relative to that of PS48-N3 (Mn = 5.2
kg/mol, PDI = 1.08) due to a increase in total molecular weight (Figure 6 and Table 1).
Finally, in the MALDI-TOF mass spectrum (Figure 5), a single narrow molecular weight
20
distribution can be clearly observed under the positive linear mode despite the relatively
high molecular weight of PS48-(tBAPOSS)-N3. Although mono-isotopic resolution is not
possible in this molecular weight range, the average molecular weights of the peaks
match well with the calculated values (e.g., for 46-mer with the formula of
C475H525N5NaO39S8Si8, observed m/z 7433.89 Da vs. calcd. 7432.45 Da). Therefore, all
the results above unambiguously confirm the molecular structures and uniformity of
resulting intermediate product after the first one-pot reaction, which may serve as a
versatile precursor for the further construction of various POSS-based AGGSs via
another one-pot “click” reaction.
21
Figure 1. 1H NMR spectra of (a) PS48-(tBAPOSS)-N3, (b) PS48-(tBAPOSS)-(tBAPOSS)PS32, (c) PS48-(tBAPOSS)-(APOSS)-PS48, and (d) PS48-(tBAPOSS)-(APOSS)-PS32.
22
Figure 2. 13C NMR spectra of (a) PS48-(tBAPOSS)-N3, (b) PS48-(tBAPOSS)-(tBAPOSS)PS32, (c) PS48-(tBAPOSS)-(APOSS)-PS48, and (d) PS48-(tBAPOSS)-(APOSS)-PS32.
23
Figure 3. UV spectra of (a) CHO-(VPOSS)-DIBO (black curve), (b) PS48-(tBAPOSS)-N3
(red curve), (c) PS48-(tBAPOSS)-(tBAPOSS)-PS32 (blue curve), (d) PS48-(tBAPOSS)(APOSS)-PS48 (purple curve), and (e) PS48-(tBAPOSS)-(APOSS)-PS32 (green curve).
Figure 4. FTIR spectra of (a) PS48-N3 (black curve), (b) PS48-(tBAPOSS)-N3 (red curve),
and (c) PS48-(tBAPOSS)-(tBAPOSS)-PS32 (blue curve).
24
Figure 5. MALDI-TOF mass spectra of PS48-(tBAPOSS)-N3. The full spectrum was
obtained in positive linear mode.
Figure 6. SEC overlays for polymers: PS48-N3 (black curve), NH2-O-PS32 (red curve),
PS48-(tBAPOSS)-N3 (blue curve), and PS48-(tBAPOSS)-(tBAPOSS)-PS32 (green curve).
25
Table 1. Summary of molecular characterization and physical parameters of products.*
Sample
PS48-N3
Mn, NMR
Mn, SEC
(g/mol) a
(g/mol) b
5.1 k
5.2 k
PDI
Whead c
Wtail d
Dhead e
Dtail
1.0
-
-
-
-
-
-
-
-
-
-
-
-
21.5%
67.1%
-
-
f
8
NH2-O-PS32
3.5 k
3.9 k
1.0
9
NH2-O-PS48
5.1 k
5.2 k
1.0
5
PS48-(tBAPOSS)-N3
7.6 k
7.9 k
1.0
2
PS48-(tBAPOSS)-
13.7 k
14.1 k
1.0
23.9%
62.8%
0
18.6%
(tBAPOSS)-PS32
14.6 k
15.0 k
1
19.8%
69.9%
13.4%
0
21.9%
65.2%
13.4%
18.6%
PS48-(tBAPOSS)-
1.1
(APOSS)-PS48
0
PS48-(tBAPOSS)-
13.2 k
14.4 k
1.0
(APOSS)-PS32
6
* (a) Molecular weight calculated based on 1H NMR; (b) molecular weight measured
from SEC; (c) weight fraction of heads calculated based on eqn (3); (d) weight fraction of
tails calculated based on eqn (4); (e) dissymmetry of head calculated based on eqn (1); (f)
dissymmetry of tail calculated based on eqn (2).
26
3.3 Facile Synthesis of AGGSs
Holding the azide group with high reactivity, the macromolecular “clickable”
precursor, PS48-(tBAPOSS)-N3, could join the next one-pot reaction for the exploration of
different AGGSs. In this work, three typical AGGSs with different Dhead and Dtail, can be
simply achieved during a sequential multi-step one-pot reactions using distinct polymeric
tail (NH2-O-PSm) and functional thiol ligand (R2-SH), respectively.
Small molecular gemini surfactant possessing two identical head groups and two
hydrophobic tails of different chain length is regarded as one of the most important
AGSs.37, 50 Oda and co-workers observed that both of overall chain length and alkyl chain
symmetry can strongly influence the micellar morphologies and the physical properties of
various phases formed by this kind of AGSs in solution.37 Therefore, it is of great interest
to synthesize and study the counterpart in POSS-based giant surfactant, which consists of
two polymer tails differing in chain length and two similar functional POSS heads
(AGGS1) (Scheme 1d). In this new system, two key structural elements, Wtail/Whead and
Dtail (since Dhead = 0 in AGGS1), are both important parameters to determine the
spontaneous curvatures of macromolecules, resulting in different thermodynamic stable
micelles in solution. Indeed, the latter factor is particularly significant sometimes,
allowing one to assess a library of topological isomers by varying different tail lengths
for the quantitative structure-property investigations.
Although AGGS1 might be prepared by our previously reported sequential “click”
method, time-consuming synthesis via multiple steps and extensive chromatographic
purifications are generally required. Herein, we decide to employ one-pot “click”
approach again to future construct PS48-(tBAPOSS)-(tBAPOSS)-PS32, the model product
27
for AGGS1. The synthesis starts with the mixing equimolar amounts of macromolecular
precursor, PS48-(tBAPOSS)-N3 (Mn,
NMR
= 7.6 kg/mol, PDI = 1.02) obtained from last
one-pot reaction, and “click” building block, CHO-VPOSS-DIBO, in a common solvent.
The mixture was stirred at room temperature for about 2 hours, immediately followed by
the addition of amino-oxy chain-end functionalized PS tail 2 for AGGS1 (NH2-O-PS32,
Mn, NMR= 3.5 kg/mol, PDI = 1.09), a functional thiol for the second POSS head (t-butyl
mercaptoacetate, 10.0 equiv) and a few milligrams of TsOH and DMPA as catalysts for
oxime ligation and TECC, respectively. The whole solution was then irradiated under UV
365 nm for about half an hour, and the final product was collected as a white powder in a
good yield (~ 88%) after repeated precipitation into cold methanol.
Similar to PS48-(tBAPOSS)-N3, both of 1H (Figure 1b) and
13
C NMR (Figure 2b)
results confirmed the complete consumption of the vinyl groups by TECC after the
reaction by the disappearance of the corresponding proton and carbon peaks, respectively.
In addition, the absence of aldehyde proton at δ 10.09 ppm in the 1H NMR spectrum of
PS48-(tBAPOSS)-(tBAPOSS)-PS32 (Figure 1b) also indicate the successful oxime ligation
during the one-pot reaction. Moreover, the disappearance of the strong characteristic
vibrational band for the azide group at ~ 2100 cm-1 in the FT-IR spectrum (Figure 4c) as
well as the absorbance for the DIBO motif at 306 nm in the UV-vis spectrum (Figure 3c)
suggested that the successful stoichiometric SPAAC reaction. This could also be
supported by the complete shifting of resonance signals for the protons (c) adjacent to
azide group from δ 4.40 ppm to δ 5.75 ppm in the 1H NMR spectrum (Figure 1b). The
increased overall molecular weight due to the incorporation of another two
macromolecular segments during the one-pot reaction was also reflected by the clear shift
28
in retention volume of PS48-(tBAPOSS)-(tBAPOSS)-PS32 (Mn = 14.1 kg/mol, PDI = 1.01)
compared with PS48-(tBAPOSS)-N3 (Mn = 7.9 kg/mol, PDI = 1.02) in SEC overlay
(Figure 6). Although MALDI-TOF mass spectroscopy failed to provide a good result for
this sample, probably due to its low ionization ability and the relatively high molecular
weight, the precisely-defined macromolecular structure of the resulting model product for
AGGS1, PS48-(tBAPOSS)-(tBAPOSS)-PS32 is also fully validated by all other
characterization data (Wtail = 62.8%, Dtail = 18.6%).
Small molecular AGSs differing in nature of head groups represent another common
structural variance in non-conventional surfactants.51 Recent advance revealed that this
kind of gemini compound exhibited a critical micelle concentration (cmc) value below
that of corresponded symmetrical gemini surfactant.51 This interesting phenomenon thus
inspires us to explore similar AGGS2 possessing two different functional POSS heads
and identical PS hydrophobic tails. It could be speculated that two key structural factors
including Wtail/Whead and Dhead may have a great influence on the phase behaviors of
AGGS2 in solution.
The synthetic approach for model product of AGGS2, PS48-(tBAPOSS)-(APOSS)PS48, again takes advantage of the one-pot “click” strategy by using a combination of
several orthogonal “click” reactions. The whole procedure is quite similar to the one
towards PS48-(tBAPOSS)-(tBAPOSS)-PS32, but using different amino-oxy chain-end
functionalized PS tail 2 (NH2-O-PS48, Mn, NMR= 5.1 kg/mol, PDI = 1.05) and functional
thiol for the second POSS head (2-mercaptoacetic acid). The final product was fully
characterized by various techniques including 1H NMR (Figure 1c), 13C NMR (Figure 2c),
UV-vis spectrum (Figure 3), FT-IR (Figure 7) and SEC (Figure 8) to confirm the
29
macromolecular structure and uniformity. Notably, the installation of the APOSS
nanocage as the head 2 for AGGS2 was supported by the observation of characteristic
peak for protons (i) of (-CH2COOH) at δ 3.32 ppm in the 1H NMR spectrum (Figure 1c)
and a strong absorbance band at around 3300 cm-1 in the FT-IR spectrum FT-IR (Figure
7b). In addition, similar to the general characterizations of PS48-(tBAPOSS)-(tBAPOSS)PS32, the success of TECC, oxime ligation and SPAAC reactions during the one-pot
synthetic process could be directly proved by no vinyl protons and carbons existing in
NMR spectra (Figure 1c and Figure 2c), no aldehyde proton appeared at δ 10.09 ppm in
the 1H NMR spectrum (Figure 1c), and the disappearance of the vibrational band at ~
2100 cm-1 in the FT-IR spectrum (Figure 7) and the absorbance peak at 306 nm in the
UV-vis spectrum (Figure 3), respectively. Moreover, from the SEC overlay shown in
Figure 8, it is evident that the SEC trace of the final product, PS48-(tBAPOSS)-(APOSS)PS4 (Mn = 15.0 kg/mol, PDI = 1.10), shifted to a smaller retention volume after the onepot “click” reaction. Therefore, it can be concluded that the model AGGS2 has been
successfully synthesized by repeating one-pot “click” methodology (Wtail = 69.9%, Dhead
= 13.4%).
30
Figure 7. FTIR spectra of (a) PS48-(tBAPOSS)-N3 (black curve), (b) PS48-(tBAPOSS)(APOSS)-PS48 (red curve), and (c) PS48-(tBAPOSS)-(APOSS)-PS32 (blue curve).
Figure 8. SEC overlays for polymers: PS48-(tBAPOSS)-N3 (black curve), NH2-O-PS48
(red curve), and PS48-(tBAPOSS)-(APOSS)-PS48 (blue curve).
The Further development of AGGS3 containing two distinct functional POSS heads
and two polymer tails with unequal chain length is nontrivial. This multi-component
31
system seems much more complex than the former ones since there are total three
structural factors (Wtail/Whead, Dhead, and Dtail) all have impacts on the self-assembly
behaviors of AGGS3. In general, it will be very difficult to use traditional organic and
polymer chemistry method to prepare such complex asymmetric geminis (including both
of AGGS3 and corresponded small molecular surfactants). However, our recent
cascading synthetic strategy are able to achieve the model product for AGGS3, PS48(tBAPOSS)-(APOSS)-PS32, precisely and efficiently.
The synthesis of PS48-(tBAPOSS)-(APOSS)-PS32 could be simply conducted in 2.5
hours via the same way towards PS48-(tBAPOSS)-(tBAPOSS)-PS32 but using different
thiol ligand for the second POSS head (2-mercaptoacetic acid). Similar to the two
established model AGGSs, the successful organization of distinct molecular segments
into PS48-(tBAPOSS)-(APOSS)-PS32 via different three types of “click” reactions was
unambiguously supported by 1H NMR (Figure 1d), 13C NMR (Figure 2d), FT-IR (Figure
7c), and UV-vis spectrum (Figure 3e). Furthermore, the SEC chromatogram of PS48(tBAPOSS)-(APOSS)-PS32 (Figure 9) exhibits a monomodal, symmetric peak with a
narrow molecular weight distribution (Mn = 14.4 kg/mol, PDI = 1.06). There is a clear
shift in retention volume compared to that of PS48-(tBAPOSS)-N3 (Mn = 7.9 kg/mol, PDI
= 1.02), consistent with the molecular weight increase as a result of the incorporation of
new macromolecular blocks. All those results fully established the chemical structure and
uniformity of final product, and several important structural factors were also determined
(Wtail = 65.2%, Dhead = 13.4%, Dtail = 18.6%).
Above all, our new cascading synthetic strategy by repeating one-pot “click”
methodology enable the precise, modular and facile synthesis of multi-component
32
macromolecules with complex molecular structures such as various kinds of AGGSs.
This strategy is quite straightforward and general, almost maintaining all the superior
features from one-pot “click” methodology, including minimum set-up, simple work-up,
short reaction time (about 5 hours, about 2.5 hours for each step one-pot synthesis), mild
operation conditions and non-chromatographic purifications. It is thus promising to
further expand the scope of this strategy for the generation of numerous complex welldefined giant surfactants possessing multiple functional POSS heads tethered with
diverse polymer tails.
Figure 9. SEC overlays for polymers: PS48-(tBAPOSS)-N3 (black curve), NH2-O-PS32
(red curve), and PS48-(tBAPOSS)-(APOSS)-PS32 (blue curve).
33
CHAPTER VI
CONCLUSIONS
In summary, following the macromolecular structure evolution of giant surfactants
by further decreasing their molecular symmetries, we have successfully reported the
molecular design and facile syntheses of three kinds of novel POSS-based AGGSs by
repeating one-pot “click” methodology. This strategy is a general, robust, and efficient
approach to precisely control and easily tune each molecular segment of AGGSs. The
whole procedure is usually completed within 5 hours and does not require any
chromatography methods for purification. Several important molecular parameters in
AGGSs, such as overall molecular mass, surface functionalities of the nanoparticle,
weight fraction of heads/tails, dissymmetry of head/tail can be systematically varied
during the repeated one-pot reaction. This strategy could be further employed to develop
more kinds of diverse POSS-based giant surfactants with complex macromolecular
architectures for systematic structure-property relationship investigations. Moreover, the
cascading one-pot synthetic strategy based on a series of orthogonal “click” reactions
might also provide a few new opportunities to construct novel H-shape ABCDE star
block copolymers.
34
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