Master of Chemical Engineering Thesis
Poly(ionic liquid) (co)polymers
via controlled radical polymerization
KyungWon Stacey Lee
260325992
Department of Chemical Engineering
McGill University
Montréal, Québec, Canada
A thesis submitted to McGill University in partial fulfillment
of the requirements of the degree of Master of Engineering
December 2014
Abstract
Polymer gel electrolytes, also known as ion gels, are attractive for next- generation solidstate electrolytes due to their avoidance of leakage and flammability issues related to
organic solvent-based electrolytes. Ion gel electrolytes are formed by swelling polymers
in ionic liquids or synthesizing ionic liquid block copolymers. Such materials can be
tuned into smart materials for energy transport and storage by incorporating thermoreversible groups. In order to impart such functionalities and to perform consistently and
predictably, it is imperative for these polymers to be derived from well-defined
structures. Controlled radical polymerization (CRP) techniques were employed as the
synthesis method for such tailored microstructured polymers that form the core of the
studies presented in this thesis.
In the initial study of the novel ionic liquid block copolymers, this thesis presents the
synthesis and characterization of two different poly(ionic liquid) block copolymers.
Through the study of 2-acrylamido-2-methyl-1-propanesulfonate tris[2-(2-methoxyethoxy)ethylene] ammonium salt (AMPS-oxyethylene ammonium salt) polymer, it was
aimed to synthesize well-controlled and well-defined structure AMPS oxyethylene
ammonium salt polymer via two main CRP techniques, nitroxide-mediated
polymerization (NMP) and reversible-addition-fragmentation chain transfer radical
polymerization (RAFT), and to prove its ability as a macroinitiator to form a diblock
copolymer. Unfortunately the gel permeation chromatography (GPC) molecular weight
analysis in common organic solvents was not able to generate meaningful data about the
i
molecular weight distribution. The reason and details are explained in Chapter 3.
In the second study, a poly(ionic liquid) block copolymer was synthesized. First, the
block copolymer poly(2-hydroxyethyl methacrylate - ran - styrene) - b - poly(styrene)
(poly(HEMA-ran-S)-b-PS) was synthesized by NMP, then converted quantitatively into
its corresponding poly(2-bromoethyl methacrylate - ran- styrene) - b - poly(styrene)
(poly(BrEMA-ran-S)-b-PS), the precursor towards ionic liquid containing-polymer. The
brominated block copolymer was subsequently functionalized with a nucleophile, 1methylimidazole,
becoming
poly(1-[(2-methacryloyloxy)ethyl]-3-methylimidazolium
bromide - ran - styrene) - b - poly(styrene) (poly([MEMIm][Br]-ran-S)-b-PS) block
copolymer. The micro-phase separation and morphology of the poly(ionic liquid) diblock
copolymer was investigated by transmission electron microscopy (TEM). The TEM
image indicated weak microphase separated morphology with no long-range periodic
order, shown in Chapter 4.
ii
Résumé
Des gels de polymères électrolytes, aussi connu comme des gels ioniques, sont attrayants
pour les électrolytes à l'état solide de la prochaine génération en raison qu’ils peuvent
éviter des fuites et leur inflammabilité réduite, liées aux électrolytes à base de solvants
organiques. Des gels ioniques sont formés par un gonflement de polymères dans des
liquides ioniques ou synthétisé de copolymères à blocs liquides et ioniques. Ces
matériaux peuvent être accordés dans la catégorie des matériaux intelligents pour le
transport et stockage de l'énergie en incorporant des groupes thermoréversible. Afin de
conférer ces fonctionnalités et de la synthèse de façon constante et prévisible, il est
impératif pour ces polymères d’avoir des structures bien définies. La Polymérisation
radicalaire (CRP) a été utilisée comme méthode de synthèse de ces polymères
microstructurés sur mesure qui forment le noyau des études présentées dans cette thèse.
L'étude initiale de ces nouveaux copolymères ioniques et liquides, cette thèse présente la
synthèse et la caractérisation des copolymères de deux différents poly (liquides ioniques)
de bloc. Grâce à l'étude de 2-acrylamido-2-méthyl-1-propanesulfonate de tris[2-(2méthoxy-éthoxy)éthylène] ammonium sel (sel d'ammonium AMPS-oxyéthylène)
polymère, on vise à synthétiser de manière bien contrôlée et bien structuré et défini
AMPS polymère de sel d'ammonium oxyéthylénés via deux principales techniques de
PCR, la polymérisation des nitroxydes (NMP) et via la polymérisation radicalaire de
transfert de chaîne réversible (RAFT), et de prouver sa capacité en tant que macroinitiateur pour former un copolymère. Malheureusement, la chromatographie par
permeation de gel (GPC) analyse de la masse moléculaire dans des solvants organiques
iii
communs n’était pas en mesure de produire des données significatives sur la distribution
de poids moléculaire. La raison et les détails sont expliqués dans le chapitre 3.
Dans la deuxième partie, un copolymère séquencé poly (liquide ionique) a été synthétisé.
Tout d'abord, le poly copolymère séquencé (méthacrylate de 2- hydroxyéthyle - ran styrène) - b - poly (styrène) (poly (HEMA-ran-S) -b-PS) a été synthétisé par la NMP, puis
convertie quantitativement en son poly correspondant (2-bromoéthyl méthacrylate styrène aléa-) - b - poly (styrène) (poly(BrEMA-ran-S)-b-PS), le précurseur vers le
liquide ionique contenant du polymère. Le copolymère substitué avec la bromure a
ensuite été fonctionnalisé avec un nucléophile, le 1-méthylimidazole, devenant poly (1 [(2- méthacryloyloxy) éthyl] bromure -3-méthylimidazolium - ran - styrène) - b – poly
(styrène)
(poly([MEMIm][Br]-ran-S))
copolymère
PS
-b-bloc.
La
séparation
microphasique et la morphologie du copolymère dibloc poly (liquide ionique) a été étudié
par microscopie électronique à transmission (MET). L'image MET indique une
morphologie de microphases faibles sans ordre à longue distance périodique, comme
présenté dans le chapitre 4.
iv
Acknowledgement
I would like to convey my deepest thankfulness to my research supervisor Prof. Milan
Marić and Prof. Phillip Servio for their continuous and excellent guidance, support and
encouragement I received throughout my time at McGill. Prof. Milan Marić’s extensive
knowledge in the field of polymers was an invaluable resource for my learning. His
dedication was greatly appreciated.
I want to thank also to all my research group members; especially Chi who trained me on
polymer synthesis and characterization, and guided me through the degree, and Xeniya
and Hanno who had enlightening discussions and suggestions.
I would also like to thank the McGill department of Chemical Engineering for providing
excellent research facilities and office space. Special thanks go to Ms. Emily Musgrave,
Ms. Louise Miller-Aspin, and Mr. Frank Caporuscio.
I would like to express my gratitude to the Department of Chemistry and the Facility for
Electron Microscopy Research (FEMR) at McGill for the use of the analytical equipment;
Dr. Frederick Morin for NMR training and help, Mr. Petr Fiurasek in the Centre for Self
Assembled Chemical Structures for his assistance with the FTIR, TGA and DSC, and Dr.
Kelly Sears, Ms. Jeannie Mui and Dr. David Liu for their help with transmission electron
microscopy analysis.
v
I would like to thank the Natural Sciences and Engineering Research Council of Canada
(NSERC), Fonds Québécois de la Recherche sur la Nature et les Technologies (FQRNT),
and the Department of Chemical Engineering at McGill University for funding the
research.
Finally I would like to thank my family and friends for always being there for me.
vi
Table of Contents
Abstract............................................................................................................................... i Résumé .............................................................................................................................. iii Acknowledgement ............................................................................................................. v 1. Introduction ................................................................................................................... 3 2. Background and Literature Review ............................................................................ 5 2.1 Ionic Liquids .........................................................................................................................5 2.1.1 Physical and Chemical Properties of Ionic Liquid .........................................................6 2.1.1.1 Melting Points ........................................................................................................................ 6 2.1.1.2 Molecular Structures and Interactions ................................................................................... 7 2.1.1.3 Non-Volatility and Thermal Stability .................................................................................... 7 2.2 Polymerization ......................................................................................................................8 2.2.1 Controlled Radical Polymerization .................................................................................9 2.2.1.1 Nitroxide Mediated Radical Polymerization ....................................................................... 11 2.2.1.2 Atom Transfer Radical Polymerization ............................................................................... 13 2.2.1.3 Reversible-Addition-Fragmentation Chain-Transfer Radical Polymerization .................... 15 2.3 Self-Assembly of Block Copolymers .................................................................................15 3. Controlled synthesis of 2-acrylamido-2-methyl-1-propanesulfonate tris[2-(2methoxyethoxy)ethyl] ammonium salt polymer via CRP....................................... 18 Abstract .....................................................................................................................................18 3.1 Introduction ........................................................................................................................18 3.2 Experimental section ..........................................................................................................21 3.2.1 Materials .......................................................................................................................21 3.2.2 Synthesis of AMPS-ammonium salt monomer .............................................................21 3.2.3 Synthesis of AMPS-ammonium salt polymer via NMP ...............................................22 3.2.4 Synthesis of random copolymer of AMPS-ammonium salt and styrene via NMP ......24 3.2.5 Synthesis of AMPS-ammonium salt polymer via RAFT .............................................24 3.2.6 Synthesis of block copolymer of AMPS-ammonium salt and styrene .........................25 3.2.7 Characterization using Nuclear Magnetic Resonance (NMR) and Gel Permeation
Chromatography (GPC) .........................................................................................................26 1
3.3 Results and Discussion .......................................................................................................29 3.3.1 Homopolymerization of AMPS-ammonium salt monomer via NMP ..........................29 3.3.2 Random copolymerization of AMPS-ammonium salt and styrene via NMP ...............31 3.3.3 Homopolymerization of AMPS-ammonium salt polymer via RAFT ...........................32 3.3.4 Block copolymerization of AMPS-ammonium salt and styrene using RAFT .............33 3.3.4 Molecular weight and dispersity analysis using GPC ...................................................35 3.4 Future work ........................................................................................................................36 3.5 Conclusion ...........................................................................................................................36 4. Synthesis of poly(2-hydroxyethyl methacrylate) via NMP and ionic liquid
functionalization ......................................................................................................... 38 Abstract .....................................................................................................................................38 4.1 Introduction ........................................................................................................................39 4.2 Experimental Section .........................................................................................................42 4.2.1 Materials .......................................................................................................................42 4.2.2 Synthesis of random copolymer of HEMA and styrene via NMP ................................43 4.2.4 Synthesis of diblock copolymer poly(2-bromoethyl methacrylate-ran-styrene)-bpoly(styrene) ..........................................................................................................................45 4.2.5 Synthesis of imidazolium-based poly(ionic liquid) block copolymer, poly(1-[(2methacryloyloxy)ethyl]-3-methylimidazolium bromide-ran-styrene)-b-poly(styrene) ........47 4.2.6 Characterization using Nuclear Magnetic Resonance (NMR) ......................................48 4.2.7 Gel Permeation Chromatography (GPC) ......................................................................50 4.3 Results and Discussion .......................................................................................................51 4.3.1 Random copolymerization of HEMA and styrene ........................................................51 4.3.2 Chain Extension of HEMA/styrene random copolymer ...............................................53 4.3.3 Bromination of diblock copolymer poly(HEMA-ran-styrene)-b-poly(styrene) ...........55 4.3.4 Quaternization with 1-methylimidazole .......................................................................57 5. Conclusion ................................................................................................................... 61 6. Considerations for Future Work ............................................................................... 62 References ........................................................................................................................ 63 2
1. Introduction
The need to store energy is becoming increasingly urgent, since it is essential if
renewable energy sources are to become a dominant supply. The interest in energy
storage has increased largely in response to the energy market conditions since the 1970s
that featured high oil and natural gas prices, regulatory restrictions on thermal power
plants such as the Kyoto Protocol, and dependence on low efficiency steam plants for
peak electricity demand
[1]
. Deployment of energy storage, however, over the past two
decades has been limited by low natural gas prices, availability of high efficiency hydro
power plants, and limited cost reductions in storage technologies. In addition, the
economically challenging cost to design, operate, and manage, as well as utility risk
aversion including market uncertainty have also limited storage development
[2]
.
Nonetheless, energy storage technologies have drawn attention again recently due to
heavily increased expectation on renewable energy. According to the National
Renewable Energy Laboratory, energy storage is one of the potentially important
enabling technologies supporting commercialization of renewable energy, especially
solar power and wind [3].
Among the vast amount of possible materials and technologies in a large-scale energy
storage such as lead-acid batteries and liquid electrolyte vanadium-redox or Regenesys
flow batteries
[4]
, polymer gel electrolytes, also known as ion gel electrolytes, are
currently of great interest as prospective alternatives to conventional liquid electrolytes,
especially due to their ability of achieving comparable high ionic conductivity and the
avoidance of leakage and flammability issues related to organic solvent-based
3
electrolytes
[5,6]
. Ion gel electrolytes are a new class of gel-type polymeric networks,
formed by 1) swelling polymers with significant loadings of ionic liquids
[7]
, 2)
polymerizing ionic liquid monomers, or 3) using a quaternization reaction of non-ionic
liquid monomers with nucleophiles post-polymerization. Such polymer can be connected
to another block (segment) of polymer with distinct characteristics (i.e. amphilicity
between the respective segments) forming a block copolymer. Block copolymers can
form periodic ordered structures via self-assembly. Self-assembled block copolymers are
attractive for next-generation solid-state electrolytes due to high dimensional stability,
excellent processability, flexibility, transparency, and improved safety by forming
nanostructured ion conducting channels, lamellae, or spheres [5-8].
Starting with this Introduction, this thesis consists of a literature review on the
fundamental topics, and two projects where both are introducing new methods to
synthesize novel ionic liquid containing polymers and their integration into block
copolymers. In Chapter 2, a brief background and literature review on ionic liquids,
controlled radical polymerization techniques, and self-assembly of block copolymers will
be provided. Chapter 3 describes the synthesis of ionic liquid polymer via controlled
radical polymerization of ionic liquid monomer, 2 - acrylamido - 2 - methyl - 1 propanesulfonate tris[2 - (2 - methoxy - ethoxy)ethylene] ammonium salt (AMPS oxyethylene ammonium salt) and Chapter 4 details the study of poly(ionic liquid) block
copolymer synthesized by nitroxide mediated polymerization of 2-hydroxyethyl
methacrylate (HEMA) and quaternization.
4
2. Background and Literature Review
2.1 Ionic Liquids
Ionic liquids are molten salts that are in the liquid state at ambient temperature. They are
composed of poorly coordinated and bulky organic cations and either inorganic or
organic anions, which results in these salts being liquid below 100 °C, or even at ambient
temperature
[9]
. Most common cations for ionic liquid extraction and synthesis are based
on the imidazolium or pyridinium ring with one or more alkyl groups attached to the
nitrogen or carbon atoms in the ring structure. Common inorganic anions include halide
ions, tetrafluoroborate (BF4-), tetrachloroaluminate (AlCl4-), hexafluorophosphate (PF6-),
and bis(perfluoromethyl-sulfonyl)imide (CF3SO2)2N-
[10]
. The molecular structure of
some common cations and anions are shown in Figure 1.
N
N
N
CnH2n+1
H3C
CnH2n+1
Alkylimidazolium
F
F
F
Cl
B
F
Tetrafluoroborate
F
Cl
O
F
F3C
P
Al
Cl
F
Alkylpyridinium
Cl
Tetrachloroaluminate
F
F
S
O
N
S
CF3
O
O
F
Hexaflouorophospate Bis(perfluoromethyl-sulfonyl)imide
Figure 1 Molecular structures of cations and anions in common ionic liquids
The
earliest
truly
room
temperature
ionic
liquids
ethylammonium
nitrate
(C2H5)NH3+NO3- were discovered in 1914 by Paul Walden [11]. With the development of
5
convenient ionic liquid synthesis methods in the 1970-80s, the number of ionic liquids
with novel structures has dramatically grown. The number of newly synthesized ionic
liquids already exceeds more than 500 and is still increasing
[10,12-16]
. Ionic liquids have
recently been widely explored as functional materials because of their unique
combination of physical and chemical properties
[17]
, such as high ionic conductivity,
chemical and thermal stability, and a wide electrochemical window, which make ionic
liquids attractive electrolytes for electrochemical devices
[5,18]
. In addition, the large
variety of cations and anions affords great flexibility of property-tuning towards different
applications [19].
2.1.1 Physical and Chemical Properties of Ionic Liquid
2.1.1.1 Melting Points
As mentioned in the previous section, the melting point of ionic liquids should be below
100 °C. The main reasons for the reduced melting point are the charge distributions on
the ions, hydrogen bonding ability, the symmetry of the ions, and the van der Waals
interactions. For instance, the melting point of one of the most common inorganic salts,
sodium chloride (NaCl), is 801 °C under ambient pressure. For 1-propyl-3methylimidazolium chloride (C7H13N2Cl), its melting point is only 60 °C
[10]
. From the
comparison above, it is clear that the most dramatic reduction in melting point is caused
by replacing the small inorganic cations by larger, more asymmetric organic cations. Also
the fact that at least one ion has a delocalized charge and is organic prevents the
formation of a stable crystal lattice which is more ordered and consequently harder to
break down to the liquid state [9].
6
2.1.1.2 Molecular Structures and Interactions
The molecular structure, polarity, and molecular interactions of ionic liquids provide
them the ability to dissolve a wide range of organic and inorganic compounds. Including
not only molecular interactions existing in conventional organic solvents such as
hydrogen bonding, dipole-dipole, and van der Waals interactions but also ionic
electrostatic interactions between charged molecules, ionic liquids are very miscible with
many polar substances. In addition, some selected ionic liquids can simultaneously
dissolve organic and inorganic substances that are usually poorly soluble in conventional
solvents (e.g. cellulose material, wool, and carbon nanotubes). These versatile features of
ionic liquids as solvents offer numerous opportunities for the development of new
extraction processes and chemical syntheses [10, 20-22].
2.1.1.3 Non-Volatility and Thermal Stability
Another important property of ionic liquids is that they have no distinguishable vapor
pressure
[17]
. The non-volatility of ionic liquids provides an easy process to separate low
molecular weight volatile compounds from the catalyst or substrates dissolved in ionic
liquids
[23]
. In production of polymers, the non-volatile ionic liquid can be recycled after
precipitation of polymer and simple purification steps. The unreacted volatile monomers
can also be reused by evaporation from the ionic liquid mixture
[24]
. Negligible vapor
pressure results in very high thermal stability as well. The first thermal event on heating
of the ionic liquids is thermal decomposition, often beginning around 400 °C with
minimal vapor pressure [17]. Possessing both chemical and thermal stability, ionic liquids
have been sought as environmentally safer and economically viable replacements for
organic solvents [17, 25].
7
2.2 Polymerization
The basic definition of a polymer is a substance composed of molecules that have long
sequences of one or more species of atoms or groups of atoms linked to each other by
covalent bonds. The process to form polymers by linking monomer molecules through
chemical reactions is known as polymerization [26]. For example, polystyrene is produced
by polymerization of styrene, and typically a few thousand monomers are comprised in a
chain. The macromolecular nature of polymers sets them apart from other materials and
gives them unique properties. One of the important advantages of polymers compared to
metals is their flexibility, comparatively light in weight, and widely tunable physical
properties since polymers are easy to functionalize and process. Further, it is easily
possible to combine the specific properties of the polymers into a single material by
simple copolymerization [26,27].
Polymerizations can be usually classified into several categories based on the reaction
mechanism. The most common types are step-wise, ionic, and free radical
polymerization. One of the remarkable advantages of ionic polymerization, that was
invented by a British and American polymer chemist Michael Szwarc in 1956, is the
ability to precisely control the molecular weight distribution and microstructure, as well
as the ability to produce ‘living’ polymers
[84,85]
. Polymers are considered living if all
chains are nearly the same chain length at a given instance and are able to grow without
any side-reactions or termination. The initiation in ionic polymerization is assumed to be
instantaneous and the propagation is free of termination and chain transfer as long as
monomer is available and no impurity is present. However, this technology is highly
sensitive to the presence of impurities such as moisture and air, and consequently it
8
requires very strict control of reagent purity and reaction conditions. For this reason,
ionic polymerization is relatively expensive and used industrially in a few niche
applications [26-28].
In contrast, free radical polymerization is a common technique to synthesize high
molecular weight polymers in industry. It is an ideal polymerization method for industrial
synthesis because it can easily be implemented on a large scale with numerous types of
monomer in various media and methodologies: aqueous media, suspension, emulsion,
etc. Free radical polymerization is similar to ionic polymerization in the sense that there
are three essential stages such as initiation, propagation, and termination, but the
initiation is much slower compared to ionic polymerization. Also, termination and chain
transfers occur constantly throughout the reaction leading to polymer products with
relatively broader molecular weight distribution and non-uniform structure [26-28].
2.2.1 Controlled Radical Polymerization
The advantages of ionic polymerization and free radical polymerization were finally
combined into controlled radical polymerization (CRP). In this process, the initiation is as
fast as the one in ionic polymerization. Also, it is controlled to reduce the contribution of
irreversible termination reactions, to yield well-defined polymers with desired molecular
weight, and to narrow molecular weight distribution
[29]
. Being adapted from radical
polymerizations, CRP provides simplicity and reaction conditions easily implemented
industrially. It can even be done in dispersed aqueous media, which is not possible for
ionic polymerization. However, the significant difference between conventional free
radical polymerization and controlled ‘living’ radical polymerization is the establishment
9
of a rapid dynamic equilibrium between a very small amount of chain-growing free
radicals and a large excess of the deactivated species
[28]
. The basic principle underlying
this technique is to suppress termination so that it becomes insignificant by creating the
activation/growth/deactivation
cycle,
and
reversibly
trapping
and
temporarily
deactivating the chain radicals. The various types of controlled radical polymerization
follow one of three general strategies, which are illustrated in Figure 2.
(a) Strategy 1: Dissociation - combination (DC)
C
H2
H
C
C
H2
End
CH +
End
X
X
CH2 = CHX
Propagation
(b) Strategy 2: Atom transfer (AT)
C
H2
H
C
End +
C
H2
A
CH +
A-End
X
X
CH2 = CHX
Propagation
(c) Strategy 3: Degenerative chain transfer (CT)
[1]
C
H2
H
C
X
End + [2]
C
H2
CH
[2]
X
C
H2
H
C
End + [1]
C
H2
X
CH2 = CHX
Propagation
CH
X
CH2 = CHX
Propagation
Figure 2 Basic principles of the three strategies for controlled radical
polymerization [26, 28]
Three major CRP techniques are known as nitroxide mediated polymerization (NMP) [3039]
, atom transfer radical polymerization (ATRP)
[40-43]
, and reversible addition10
fragmentation transfer (RAFT)
[44-47]
, which will be described briefly in the following
sections.
2.2.1.1 Nitroxide Mediated Radical Polymerization
One of the three major CRP techniques is know as nitroxide mediated radical
polymerization (NMP). This polymerization process employs the dissociationcombination strategy shown in Figure 2(a). The principles of NMP involve reversible
trapping and release of the propagating chain radical by the nitroxide. In the trapped form,
the end group structure is an alkoxyamine in which the C-O bond is weak and dissociates
to regenerate the chain radical and nitroxide [26]. Figure 3 shows the basic principle of the
nitroxide mediate radical polymerization.
C
H2
H
C
R1
O
N
R2
X
Alkoxyamine end-group
kdiss
kcomb
C CH
H2
X
R1
+
O
R2
kp
CH2 = CHX
Propagation
N
Nitroxide
kt
Bimolecular Termination
Figure 3 Basic principle of nitroxide mediated radical polymerization [26]
As indicated in Figure 3, the dissociation-combination equilibrium strongly favours the
alkoxyamine chain end (the rate coefficient for dissociation, kdiss is much smaller than the
rate coefficient for combination, kcomb) such that instantaneously the propagating chain
radical concentration is sufficiently low and so the rate of bimolecular termination, kt, is
insignificant for a significant portion of the polymerization [26, 28].
11
Early in the development of NMP, styrene-based polymers were successfully synthesized
with a high level of control using the first generation nitroxide TEMPO (2,2,6,6,
tetramethyl piperidinyl-1-oxy) [26]. Unfortunately, TEMPO was not the ideal nitroxide. It
was not appropriate for homopolymerization of other types of monomers such as
acrylates, methacrylates and acrylamides
[26-28]
. From the late 1990s, the development of
so-called second-generation acyclic nitroxides such as TIPNO and SG1, which are
capable of achieving living-like conditions in radical polymerizations of other monomers
[26]
, has made the nitroxide mediate polymerization more viable as a CRP method. The
chemical structures of three nitroxides, TEMPO, TIPNO (2,2,5 trimethyl-4-phenyl-3azahexane-3-nitroxide) and SG1 (N-(2-methylpropyl)-N-(1,1-diethylphosphono-2,2dimethylpropyl)-N-oxyl, and associated alkoxyamines are depicted in Figure 4.
(a) Nitroxides
O
N
O
TEMPO
N
N
O
O
P
O
O
SG1
TIPNO
(b) Alkoxyamines
O
N
N
O
O
N
P
O
O
O
O
OH
Styryl-TEMPO
Styryl-TIPNO
BlocBuilder®
Figure 4 Chemical structures of some (a) nitroxides and (b) alkoyamines [26]
BlocBuilder® is a trade name for a unimolecular initiator that is effectively the
alkoxyamine of methacrylic acid and SG1.
12
2.2.1.2 Atom Transfer Radical Polymerization
Atom transfer radical polymerization (ATRP) is very similar to NMP in the sense that the
chain growth process is through a series of activation-propagation-deactivation cycles,
the equilibrium of the reaction is controlled, and finally, the equilibrium strongly favours
the dormant state and consequently the rate of bimolecular termination is massively
reduced [26]. The main difference of ATRP compared to NMP is that a transition metal
catalyst in the form of a halide compound and complexed by ligands and an organic alkyl
halide are used as an activator and an initiator, respectively. The basic principle of ATRP
is shown in Figure 5.
(a) Initiation
+
R-X
R
MtXzLm
+
MtXz+1Lm
(b) Activation with Monomer
R1
R1
R
+
MtXz+1Lm
+
H2C
C
R
R2
C C
H2
R2
+
MtXz+1Lm
+
MtXz+1Lm
(c) Activation, Propagation and Deactivation
R1
R1
R
C C X
H2
R2
+
MtXzLm
kact
R
kdeact
C C
H2
R2
kp
CH2 = CR1R2
Propagation
kt
Bimolecular Termination
Figure 5 Basic principles of atom transfer radical polymerization [26, 28]
Initiation proceeds via single-electron transfer from the metal halide catalyst (MtXzLm) to
the halogen atom in the R-X bond, where Mt is a transition metal such as Cu, Ni, Pd, Rh,
Ru and Mo, X is a halogen atom usually Cl and Br
[28]
, and L is a ligand. This process
yields a radical R! and an oxidized metal complex by capturing the released halogen
13
atom. Then a monomer is introduced to the radical R! in order to produce a chain radicalended chain. Eventually, propagation of the chain radical is deactivated by the reverse
process in which the oxidized transition metal complex (MtXz+1Lm) abstracts the halogen
atom back to the propagating radical forming a new C-X bona at the chain end
[26]
. As
mentioned earlier, the equilibrium strongly favours the C-X bonds such that the
instantaneous concentration of chain radical is sufficiently low to make the termination
process insignificant.
The main advantage of the ATRP is that the reactions are very flexible concerning the
presence of functional groups on both monomer and initiator. A wide range of monomers
such as styrenics, acrylates, methacrylates, acrylonitrile, acrylamides and 2-vinylpyridine
has been well-control polymerized by ATRP. In contrast to 2-vinylpyridine,
polymerization of 4-vinylpyridine had presented a very challenging problem for ATRP
since both monomer 4-vinylpyridine and poly(4-vinylpyridine) are strong coordinating
ligands that can compete for the binding of the metal catalysts in these system. A
possibility of the formation of pyridine-coordinated metal complexes could arise in the
polymerization solution where the large excess amount of monomer present over the
employed ligand [68]. ATRP reactions can be carried out either in bulk or in solution, and
also in heterogeneous systems. However, ATRP methods are known to be not useful for
monomers containing acid groups such as acrylic acid and methacrylic acid since acidic
protons cannot be tolerated, so acidic monomers can be polymerized only in their ionic
form [26].
14
2.2.1.3 Reversible-Addition-Fragmentation Chain-Transfer Radical Polymerization
The essential difference between the reversible-addition-fragmentation chain-transfer
radical (RAFT) polymerization and conventional free radical polymerization is the
addition of highly active chain transfer agents, usually dithio compounds. This dithio
transfer agent fragments during the chain-transfer process to release a new radical and
generate a new dithio compound via the reversible addition-fragmentation mechanism
illustrated below in Figure 6 [26-28].
S
S
R
R +
C
Z
S
C
A
S
A
Z
S
+ A
C
R
S
Z
Figure 6 Addition-fragmentation mechanism in RAFT polymerization [26,28]
The overall reaction process consists of the following steps: initiation, additionfragmentation, re-initiation, and equilibration in which the fundamental step is the
reversible trapping of the majority of the propagating chain radicals into dormant
thiocarbonyl compounds, thereby reducing the possibility of bimolecular termination
reactions [28].
2.3 Self-Assembly of Block Copolymers
Self-assembly is a fundamental process in nature whereby small chemical units are
introduced into a new environment, reach an equilibrium state, and produce much more
organized larger-structures and patterns
[48]
. A block copolymer is a macromolecule that
15
consists of two or more distinct polymers connected covalently at the molecular level.
Micro-phase separation occurs between the constituent blocks of sufficient length when
they have chemical incompatibility [49]. Under certain conditions of solvent, temperature,
concentration and composition, a block copolymer will self-assemble. The scale of this
type of phase separation is much smaller usually at tens of nanometers than the macrophase separation because of the restriction from the covalent linkage
[50]
. Leibler found
the critical condition for phase separation in well-defined AB diblock copolymers as
𝜒!" 𝑁 = 10.5 [51], where 𝜒!" is the Flory enthalpic interaction parameter between the two
polymeric blocks, and N represents the copolymer total degree of polymerization. When
this criterion is satisfied, the AB diblock copolymer will self-assemble into different
ordered morphologies depending on the relative length of the blocks. Figure 7 shows the
possible morphologies and phase diagram for the microphase separation of AB diblock
copolymer.
Figure 7 Possible morphologies and phase diagram for microphase-separated AB
diblock copolymers in terms of 𝝌𝑵 versus 𝝓𝑩 (volume fraction of B segment) [52]
16
The self-assembly of block copolymers is a fascinating feature because it provides the
building blocks for numerous new materials that need to have sophisticated
microstructures to perform a particular function. For instance, self-assembled block
copolymers are of great interest as solid- state polymer electrolytes to benefit ion
transport by forming nanostructured ion-conducting channels
[5,8]
. A block copolymer
consisting of two different monomers can have architectures such as diblock (AB),
triblock (ABA), pentablock (ABABA), multiblock (ABn) and star diblocks (ABnX), each
of which gives rise to different morphologies [53]. To be able to manipulate these building
blocks effectively, precise control of the polymerization process is required.
17
3. Controlled synthesis of 2-acrylamido-2-methyl-1propanesulfonate tris[2-(2-methoxyethoxy)ethyl]
ammonium salt polymer via CRP
Abstract
In this section, a poly(ionic liquid) was synthesized by polymerizing an ionic liquid
monomer. 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), the polymerizable
component, is converted to the monomeric ionic liquid ammonium salt by addition of
tris[2-(2-methoxyethoxy)ethyl]-amine. AMPS oxyethylene ammonium salt polymer is
then synthesized by controlled radical polymerization, NMP and RAFT technique, in
order to obtain the ‘living’ polymer with controlled molecular weight and narrow
molecular weight distribution. Furthermore, a random copolymer of AMPS oxyethylene
ammonium salt polymer/styrene (fstyrene = 90 mol%) and a block copolymer comprising
an AMPS ionic liquid polymer block and a polystyrene block were also synthesized.
3.1 Introduction
Polymerizing ionic liquid monomers is the most efficient method to form the polymer gel
electrolytes. Without extra substitution and/or quaternization reactions, polymerized ionic
liquid polymers provide a unique potential to display structural stability (self-assembly)
and ion transport in single-ion conductors with ionic liquid based polymer chemistry [5,55].
Through the well-designed controlled synthesis, ionic liquid moieties are covalently
attached to the polymer backbone or side chains, while the counterions are mobile [71].
18
2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) was found to show high proton
conductivity due to the sulfonic acid groups in its chemical structure, thus it had been
previously used to study the proton-conducting electrolyte membrane, or to convert it
into ionic liquid monomer
[55,69]
. Ricks-Laskoski and Snow reported the synthesis of an
ionic liquid polymer using AMPS and its electrochemical properties (electric field
actuation) [55]. They reacted 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) with
tris[2-(2-methoxyethoxy)ethyl]-amine to obtain the quaternary ammonium salt monomer.
The liquid ammonium sulfonate monomer is then polymerized to produce an ionic liquid
polymer via conventional free radical polymerization at 70°C for 18 hr. Here, in this
study, two controlled radical polymerization techniques were applied instead of
conventional free radical polymerization, in order to obtain well-defined architecture
‘living’ polymer with predictable molecular weights and narrow molecular weight
distribution (Figure 8), with the eventual goal to control the path of ion conduction – via
a self-assembled block copolymer. Among the three main methodologies of controlled
radical polymerization (CRP), reversible addition fragmentation transfer (RAFT)
polymerization has been referred to as the most significant commercial technique by Lai
et al. because it only involves organic substances and works very well with most acrylic
derivatives, including acrylic acid
[70]
. RAFT, however, requires sulfur-based agents that
are difficult to synthesize. Nitroxide mediated radical polymerization (NMP) attracts
much interest here, because it is relatively simple compared to RAFT or ATRP, in that
post-processing modification is less intensive. Here, NMP and RAFT polymerization
techniques were applied to synthesize AMPS oxyethylene ammonium salt ionic liquid
polymer and their kinetic behavior was compared.
19
O
(a)
O
O
NH
O
NH
S
O
N
O
H
S
O
O
O
O
O
AMPS oxyethylene
ammonium salt monomer
Tris[2-(2-methoxyethoxy)ehtyl]-amine
(b)
O
O
O
OH
AMPS
O
equi molar
25 °C
N
+
O
O
O
O
O
HO
O N
O
S
O P
OH
O
RAFT C12H25S
NMP
O
S
DMP
RAFT agent
BlocBuilder®
NMP mediator
O
O
HO
S
OH
SG1
S
n
O
NH
n
S
O
C12H25
O
O
O
O
NH
O
O
O
O
N
O
O
H
S
N
O
O
O
O
O
H
S
O
O
O
Figure 8 Reaction scheme for (a) AMPS oxyethylene ammonium salt monomer
synthesis [55] and (b) NMP and RAFT polymerization
20
3.2 Experimental section
3.2.1 Materials
2-acrylamido-2-methyl-1-propanesulfonic
acid
(AMPS,
99%),
tris[2-(2-
methoxyethoxy)ethyl]-amine (95%), 2,2′-Azobis(2-methylpropionitrile) (AIBN, 98%), 2(dodecylsulfanylthiocarbonylsulfanyl)-2-methylpropionic acid (DMP, 98% HPLC),
Styrene (≥99%, contains stabilizer), aluminum oxide (Al2O3, Brockmann, type 1,
basic/neutral), and calcium hydride (CaH2, 95%, reagent grade) were purchased from
Sigma-Aldrich and used as received. Diethyl ether (anhydrous, ≥99%, certified ACS
reagent grade), tetrahydrofuran (THF, ≥99.9%, HPLC grade), acetone (≥99.5%, certified
ACS reagent grade) and methanol (≥99.8%, certified ACS reagent grade) were obtained
from Fisher Scientific and used as received. Tris[2-(2-methoxyethoxy)ethyl]-amine and
styrene were purified and dehydrated by passing through a column of basic aluminum
oxide mixed with 5 wt% calcium hydride, and were stored in a sealed flask under a head
of nitrogen in a refrigerator until needed. 2-({tert-butyl[1-(diethoxyphosphoryl)-2,2dimethylpropyl]amino}oxy)-2-methylpropionic acid, also known as BlocBuilder® (99%)
was
obtained
from
Arkema
and
{tert-butyl[1-(diethoxyphosphoryl)-2,2-
dimethylprophyl]amino} nitroxide (SG1, >85%) was kindly donated by Noah Macy from
Arkema. Both BlocBuilder® and SG1 were used as received and stored at 5°C.
3.2.2 Synthesis of AMPS-ammonium salt monomer
AMPS, a white crystalline compound, was mixed with an equimolar quantity of purified
tris[2-(2-methoxyethoxy)ethyl]-amine, transparent dark brown liquid, in a nitrogen
21
purged 25-mL three-neck round-bottom glass flask. According to Ricks-Laskoski and
Snow, tris[2-(2-methoxyethoxy)ethyl]-amine was chosen here because 1) AMPS would
be dissolved in without the need of a solvent, and 2) its oxyethylene substituents would
protect the protonated ionic center(−NH+−) from coordinating with the sulfonated anion
and forming solid salts instead of molten salts
[55]
. In their procedure, the mixture was
stirred at ambient temperature for 8 hr or until the AMPS particles were completely
dissolved. In spite of the reason 1), the complete dissolution of AMPS (1.865 g, 8.999
mmol) in the tertiary amine (2.933 g, 9.068 mmol) required more than 24 hr at a room
temperature. Heating up to 35°C still required excessive dissolution times. In order to
reduce the reaction time, the minimum amount of methanol (3 mL to produce 4.775 g of
AMPS-ammonium monomer) was added into the reaction. Methanol was selected, as
both AMPS and the tertiary amine solution were soluble in it. After 20 hr of total
dissolution and drying in vacuum oven to get rid of residual methanol, 3.362 g AMPSammonium salt monomer (6.335 mmol, 70% yield) was produced as transparent light
amber oil. The characterization from the proton nuclear magnetic resonance spectrometer
1
H NMR spectrum is shown in a section 3.2.7.
3.2.3 Synthesis of AMPS-ammonium salt polymer via NMP
Without further purification, AMPS-ammonium salt monomers were polymerized using
NMP. The polymerizations were performed in a 25-mL three-neck round-bottom glass
flask fitted with a reflux condenser, a magnetic stir bar, and a thermal well. The amount
of initiator, BlocBuilder®, was calculated according to the amount of monomer and a
22
target molecular weight, Mn,target, (molecular weight at 100 % conversion) of 25 kg mol-1
as shown below
𝑚!! =
𝑚!"#"!$% 𝑀!,!"#$%!
𝑀!! − 1
where 𝑚!! is the mass of BlocBuilder®, 𝑚!"#"!$% is the mass of monomer used in the
reaction, and 𝑀!! is the molecular weight of BlocBuilder® (381.4 g mol-1). For a specific
example, 3.057 g of AMPS-ammonium salt monomer (5.761 mmol), 0.0474 g of
BlocBuilder® (0.124 mmol), and 0.0032 g of SG1 (0.011 mmol, 10 mol% of
BlocBuilder®) were charged into a reactor. The oxygen was removed by bubbling ultrapure nitrogen for 30 minutes at room temperature, followed by heating up and carrying
out the reaction at 90°C for 2 hr, while maintaining the nitrogen purge. Samples were
taken by syringe in a predetermined time interval for NMR analysis. It was noticed that
the viscosity of sample was increasing with the reaction time. The resulting polymer,
much more viscous and transparent/dark amber mixture compared to the monomer, was
dissolved in acetone, precipitated in cold diethyl ether, and became white flocculants.
The precipitant was quickly collected in an ice-cold Buchner funnel via suction filtration.
When the white solid particles warmed to room temperature, the flocculants/crystals
turned into transparent, amber oil. The precipitation process was repeated twice and then
the crude product was dried under vacuum to remove any remaining solvents. Again, the
characterization of polymer via 1H NMR spectrum is presented in a section 3.2.7.
23
3.2.4 Synthesis of random copolymer of AMPS-ammonium salt and
styrene via NMP
A statistical copolymer of AMPS-ammonium salt monomer and styrene was synthesized
via NMP. Two main reasons of copolymerization are to obtain ionic liquid properties
with much less loading amount of ionic liquid monomer (10 mol% of ionic liquid in feed)
and to detect a GPC RI signal as the homopolymer of AMPS-ammonium salt was not
detectable. Well-defined (co)polystyrene can be synthesized through NMP process, and
its GPC analysis is well-established
[57]
. The feed composition of styrene was 90 mol%
and the target molecular weight at complete conversion was set to 25 kg mol-1 (degree of
polymerization, DP of 170). The reaction was performed using the same setup and
procedures as the homopolymerization described earlier. In the copolymerization,
however, no free SG1 was added and the reaction was carried out at 120°C for 2.5 hr.
The final polymer was dissolved in the minimum amount of THF, precipitated in cold
methanol, decanted and dried under vacuum at room temperature. The yield was 86% and
the final composition was 93 mol% polystyrene based on NMR analysis. More detailed
kinetic result is presented in a section 3.3.2.
3.2.5 Synthesis of AMPS-ammonium salt polymer via RAFT
In order to synthesize the ‘living’ homopoly(ionic liquid) block, RAFT polymerization
technique was adapted instead of NMP. 2,2′-Azobis(2-methylpropionitrile) (AIBN) and
2-(dodecylsulfanylthiocarbonylsulfanyl)-2-methylpropionic acid (DMP) were used as
initiator and chain transfer agent, respectively. The trithiocarbonated chain transfer agent
was prepared according to a previously reported procedure by Lai et al.
[70]
, it has
24
previously been employed in sysntehsis of alkyl acrylates, alkyl acrylamides, acrylic acid,
and styrene. The amount of monomer was calculated by following the formulation
reported by Cao et al. The target molecular weight was kept same as the previous
reaction as 25 kg mol-1 [58].
𝑚!"#"!$% =
𝑀!,!"#$%! − 𝑀!"#
× 𝐷𝑀𝑃
𝐶𝑜𝑛𝑣.
where 𝑀!"# is the molecular weight of DMP chain transfer agent (364.6 g mol-1),
𝐷𝑀𝑃 is the concentration of DMP, and 𝐶𝑜𝑛𝑣. is the theoretical conversion of the
monomer which was assumed to be 1 (complete conversion). The typical concentration
ratio of chain transfer agent and initiator, 𝐷𝑀𝑃 / 𝐴𝐼𝐵𝑁 , was reported as 5:1 according
to Cao et al.
[58]
, so the same ratio was applied here. In an example, 3.007 g of AMPS-
ammonium salt monomer (5.666 mmol), 0.004 g of AIBN (0.024 mmol), and 0.045 g of
DMP (0.122 mmol) were mixed in the a 25-mL three-neck round-bottom glass flask with
the reflux condenser on. The reactor was heated up to 70°C for 2.5 hr while stirring and
nitrogen purging was maintained. The precipitation process was the same as previously
described.
3.2.6 Synthesis of block copolymer of AMPS-ammonium salt and
styrene
AMPS-ammonium ionic liquid polymer synthesized with RAFT technique was chain
extended with styrene to produce the block copolymer poly(AMPS-ammonium)-bpoly(styrene). The RAFT formulation introduced earlier was used to calculate the amount
of second monomer, styrene, except AMPS-ammonium salt polymer was used as macro25
chain transfer agent instead of DMP. Molecular weight of AMPS-ammonium salt
polymer was calculated to be 10 kg mol-1 based on the 42 % conversion measured by
NMR analysis. The ratio of 𝐶ℎ𝑎𝑖𝑛 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑎𝑔𝑒𝑛𝑡 / 𝐴𝐼𝐵𝑁 was kept as 5. Targeting
the molecular weight of final block copolymer to be 25 kgmol-1 (Mn,target, molecular
weight at 100% conversion of second monomer), 1.212 g of AMPS-ammonium salt
polymer was mixed with 0.004 g of AIBN (0.026 mmol) and 1.810 g of styrene (17.4
mmol) in the reactor, heated up to 80°C for 16 hr. NMR analysis of the final sample
indicated that 93 % of styrene monomer was converted so the molecular weight of the
block copolymer was calculated to be 24 kg mol-1. The final product was purified by
precipitation in cold diethyl ether, twice re-dissolved in THF and precipitated into diethyl
ether. For comparison, a block copolymer poly(styrene)-b-poly(AMPS-ammonium salt)
was also synthesized to differentiate the characteristics. RAFT chain extension of AMPSammonium salt into polystyrene was performed using the same setup and polymerization
procedure as described earlier. Characterization of copolymer is shown in a section 3.3.4.
3.2.7 Characterization using Nuclear Magnetic Resonance (NMR) and
Gel Permeation Chromatography (GPC)
Chemical structure of monomer and polymer, monomer conversion, and copolymer
composition were characterized by the proton nuclear magnetic resonance spectrometer.
1
H NMR spectra were recorded on a Varian Gemini 2000 spectrometer operating at
400MHz. Chemical shifts are reported in ppm relative to residual solvent resonances
(1H). Deuterated chloroform CDCl3 was used without further purification. Conversion of
AMPS-ammonium salt monomer was calculated using the three vinyl protons (δ = 5.3–
26
5.4, 5.9 – 6.0, and 6.0 – 6.1 ppm) of the monomer and a proton adjacent to the
quaternized tertiary amine on cation of both monomer (a) and polymer (b) (δ = 9.8 – 10.4
ppm) shown in Figure 9. Note that in Figure 9b both monomer and polymer peaks
appeared since the sample was taken before the precipitation/purification process. A lot
of monomers were still remained in the sample.
(a)
27
(b)
Figure 9 1H NMR spectra of AMPS-ammonium salt (a) monomer and (b) polymer
Number-average molecular weight Mn and dispersity, Đ of all polymer products were
characterized by gel permeation chromatography (GPC) (manufactured by Waters
Breeze) using HPLC grade THF as a mobile phase at 40 °C and a flow rate of 0.3 mL
min-1 in this study. The GPC was equipped with three Waters Styragel HF columns
(molecular weight measurement ranges: HR1: 102 – 5×103 g mol-1, HR2: 5×102 – 2×104 g
mol-1, HR3: 5×103 – 6×105 g mol-1) and a guard column. The GPC was also equipped
with both 2487 ultraviolet (UV) and 2410 reflective index (RI) detectors. The results
reported in this study were obtained from the RI detector. All molecular weight
measurements were calibrated with poly(methyl methacrylate) (PMMA) narrow
molecular weight distribution standards.
28
3.3 Results and Discussion
3.3.1 Homopolymerization of AMPS-ammonium salt monomer via NMP
Homopolymerization of AMPS-ammonium salt monomer was performed using NMP to
examine the level of control and to confirm if AMPS-ammonium salt polymer is indeed a
good candidate as a macroinitiator for synthesizing block copolymer. The kinetic results
shown in Figure 10 indicate that the scaled conversion (ln[(1-x)-1]) followed the first
order kinetics with respect to monomer concentration as expected from a controlled
polymerization. The derivation of the scaled conversion ln[(1-x)-1] comes from the
differential equation and the integral rate law of the first order reaction kinetics.
𝑅𝑎𝑡𝑒 = −
𝑑𝐴
= 𝑘 𝐴 𝐄𝐪𝐮𝐚𝐭𝐢𝐨𝐧 𝟏
𝑑𝑡
where 𝑅𝑎𝑡𝑒 is the reaction rate in units of mol/time, 𝐴 is the molar concentration of
reactant A in units of mol, 𝑡 is the reaction time, and 𝑘 is the reaction rate coefficient
constant in units of 1/time. Rearranging the Equation 1,
𝑑𝐴
= −𝑘𝑑𝑡 𝐄𝐪𝐮𝐚𝐭𝐢𝐨𝐧 𝟐
[𝐴]
Then integral both sides of the equation,
[!]
𝑑𝐴
=−
[!]! [𝐴]
!
𝑘𝑑𝑡 𝐄𝐪𝐮𝐚𝐭𝐢𝐨𝐧 𝟑
!!
Upon integration when 𝑡! = 0,
ln
[𝐴]
= −𝑘𝑡 𝐄𝐪𝐮𝐚𝐭𝐢𝐨𝐧 𝟒
[𝐴]!
An overall conversion 𝑥 in a batch reaction,
29
𝑥=
[𝐴]! − 𝐴
𝐴
=1−
𝐄𝐪𝐮𝐚𝐭𝐢𝐨𝐧 𝟓
[𝐴]!
𝐴!
Rearrange Equation 5 and substitute into Equation 4,
ln(1 − 𝑥) = −𝑘𝑡 𝐄𝐪𝐮𝐚𝐭𝐢𝐨𝐧 𝟔
Finally, the linear equation between scaled conversion and time is derived.
ln[(1 − 𝑥)!! ] = 𝑘𝑡 𝐄𝐪𝐮𝐚𝐭𝐢𝐨𝐧 𝟕
Figure 10 Kinetic plot for bulk homopolymerization of AMPS-ammonium salt
initiated by BlocBuilder® at 90 °C
The R2 value of the trendline is 0.97 which represents the very close linear relationship
between ln[(1-x)-1] and reaction time where x represents the conversion of AMPSammonium salt monomer to polymer. The molecular analysis with the plot of Mn versus
conversion x, however, needs to be done for more detailed kinetic and mechanism study.
30
3.3.2 Random copolymerization of AMPS-ammonium salt and styrene
via NMP
The homopolymerization of AMPS-ammonium salt using NMP was attempted at 90°C in
bulk. The reaction mixture became very viscous shortly after the temperature reached
90°C because the monomer itself was viscous. Since the final homo-polymer of AMPSammonium salt could not have been analyzed by GPC, the ionic liquid monomer was
copolymerized with styrene. AMPS-ammonium salt/styrene random copolymerization
was performed at 120°C in bulk. The initial amount of styrene in the feed was 90 mol%
and the kinetic result is shown in Figure 11.
Figure 11 Kinetic plot for random copolymerization of AMPS-ammonium salt and
styrene at 120°C with initial feed composition with respect to styrene of 90 mol%
The random copolymerization of AMPS-ammonium salt with styrene did not obey the
first order kinetics so as controlled polymerization since it failed to achieve a linear
relationship between ln[(1-x)-1] and reaction time where x represents the average
conversion of AMPS-ammonium salt monomer and styrene. The R2 value of the linear
31
trendline was only 0.83. The first sample at time 0 was taken when the reaction solution
stabilized at the reaction temperature. Although, in this experiment, the heating rate was
relatively slow at 2 °C min-1, and the time 0 sample (when temperature reached 120°C)
was taken almost 1 hour after the heating started. That is why the conversion at time 0
was already 24 % and the intercept does not go through the origin. It was also noticed
that styrene monomer was already 78 % converted after 0.5 hr and was completely
converted after 1 hr. However, a kinetic plot from a conventional free radical
polymerization would have rapidly increased in ln[(1-x)-1] over short period of time in
the beginning of reaction and the rate of reaction decreased once the maximum monomer
conversion is reached.
3.3.3 Homopolymerization of AMPS-ammonium salt polymer via RAFT
Homopolymerization of AMPS-ammonium salt monomer was also performed using
RAFT. The reaction kinetics is shown in Figure 12. The kinetic plot indicates that
AMPS-ammonium salt monomer can be polymerized by RAFT technique and the
polymerization rate was first order with respect to monomer concentration showing a
stronger linear relation (R2 = 0.98) between ln[(1-x)-1] and reaction time plot as shown in
Figure 12 below.
32
Figure 12 Kinetic plot for RAFT polymerization of AMPS-ammonium salt at 70°C
However, the final conversion of AMPS-ammonium salt monomer using NMP technique
at 120 °C for 2 hr was 53 % while that using RAFT technique at 70 °C for 2.5 hr was 42
%. NMP process result a higher conversion of monomer in a shorter time, but at higher
temperature, comparing to RAFT process. Molecular weight data from the GPC analysis
is required to differentiate the mechanism, chain length, and weight distribution between
two polymerization techniques.
3.3.4 Block copolymerization of AMPS-ammonium salt and styrene
using RAFT
The block copolymers poly(AMPS-ammonium)-b-poly(styrene) and poly(styrene)-bpoly(AMPS-ammonium) were synthesized using RAFT polymerization. The ‘living’
homopolymer of AMPS-ammonium salt synthesized by RAFT was chain extended with
33
styrene. The molar composition of final block copolymer was calculated to be 44 % of
AMPS-ionic liquid block, using NMR peaks of purified polymer sample.
Figure 13 1H NMR spectrum of poly(styrene)-b-poly(AMPS-ammonium)
It was noticed in NMR analysis shown in Figure 13, however, that AMPS-ammonium
salt monomers were still appearing in the block copolymer sample after the precipitation
process. The purification process needed to be repeated at least 3 times in order to
remove unreacted monomer completely. Fractionation was also performed for the chain
extended block copolymer in an attempt to remove dead chains with lower molecular
weight and remaining monomers. The block copolymer was dissolved in a minimal
amount of THF and then diethyl ether was added drop-wise while the solution was stirred
until the solution turned cloudy and higher molecular weight (or longer chained) polymer
34
precipitate was observed. The polymer was then collected and dried. After 3 times of
fractionation, the ionic liquid monomers were completely removed shown in NMR
spectrum.
3.3.4 Molecular weight and dispersity analysis using GPC
For the number-average molecular weight Mn and dispersity Đ analysis, the purified
polymer samples from all the polymerization were prepared for GPC, along with the
monomer sample and samples taken during the polymerization reaction. However, GPC
RI detector was not able to detect signals for any of AMPS-ammonium salt samples. The
refractive indexes, ηD, of the final polymer sample from homopolymerization using NMP
was measured and compared to one of HPLC grade THF. If isorefractive, then analysis
using the RI detector in the particular solvent would not be feasible. The refractive index
of THF at 25 °C and wavelength of 589 nm (sodium D line), however, was found as
1.4050
[56]
, and that of AMPS-ammonium salt polymer was measured by refractometry
(Abbe) as 1.4746, which were pronouncedly different. No signal in GPC RI detector
might have been due to sticking or affinity of the AMPS groups onto the columns (the
AMPS monomer is still fairly polar). In Ricks-Laskoski and Snow’s study, they used the
conventional free radical polymerization technique, which is known to produce polymers
with relatively broader molecular weight distribution (dispersity) and non-uniform
structure. They reported the molecular weight of the polymer products calculated by
using the conversion data from NMR analysis. This method, however, is not accurate as
GPC, and also does not provide the dispersity. To prove the polymerization was truly
controlled in this study, the dispersity analysis needs to be done.
35
3.4 Future work
It is critical to get molecular weight and dispersity analysis done in order to continue to
develop ‘living’ controlled structure AMPS-based ionic liquid copolymer. GPC analysis
can be tried with HPLC grade DMF or chloroform solvent. Molecular weight analysis
can also be attempted by using multi-angle static light scattering method. Fundamental
studies on kinetics and mechanism of polymerization reaction with AMPS-ammonium
salt monomer via NMP should be considered. When the truly ‘living’ controlled AMPSammonium salt polymer is synthesized, a series of AMPS-based poly(ionic liquid) block
copolymer with styrene at various poly(ionic liquid) compositions with the goal of
understanding the relationship between the poly(ionic liquid) composition and the
microphase separation morphology, and also between the nanostructured morphology and
its influence on the electron transport properties.
3.5 Conclusion
In this study, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) ionic liquid
polymers were synthesized by NMP and RAFT polymerization techniques. Random
copolymer of AMPS and 90 mol% styrene was also synthesized via NMP to obtain the
well-controlled polymer. Random copolymerization of AMPS and styrene via NMP was
also likely not controlled. RAFT homopolymerization of AMPS resulted in first-order
kinetic plots in the relationship between the scaled conversion and reaction time. The
block copolymer of AMPS and styrene, poly(AMPS)-b-poly(styrene), was successfully
36
synthesized by subsequent chain extension using RAFT technique producing the molar
composition of 44 % AMPS and 56 % styrene. Molecular weight analyses were
attempted with GPC, however the RI detector could not detect the polymer in any
sample.
37
4. Synthesis of poly(2-hydroxyethyl methacrylate) via
NMP and ionic liquid functionalization
Abstract
In this Chapter, imidazolium-functionalized poly(ionic liquid) diblock copolymer, a
precursor towards polymer gel electrolytes, was synthesized. The block copolymer
consisting of hydrophilic HEMA block and hydrophobic styrene block was synthesized
via NMP, and the side chain hydroxyl groups were replaced with imidazolium functional
groups in order to produce the ionic liquid containing diblock copolymer. First, 2hydroxyethyl methacrylate (HEMA) was random copolymerized with 10 mol% styrene
via NMP using N-succinimidyl ester functional BlocBuilder® unimolecular initiator
(NHS-BB). A block copolymer poly(HEMA-ran-styrene)-b-poly(styrene) (poly(HEMAran-S)-b-PS) was synthesized by subsequent chain extension with pure styrene. The
block copolymer with Đ of 1.64, Mn, poly(HEMA-ran-styrene) = 17.3 kg mol-1, Mn, poly(styrene) = 10
kg mol-1 (relative to PMMA standard) was brominated with trimethylsilyl bromide, and
then followed in a quaternization reaction with 1-methylimidazole, in order to convert the
hydroxyl groups in the HEMA units into ionic liquid moiety and have bromides as
counterions. The micro phase separation and morphology of the poly(ionic liquid)
diblock copolymer was investigated by transmission electron microscopy (TEM).
38
4.1 Introduction
Ionic liquids are molten salts that are comprised entirely of ions. Due to their unique
physicochemical properties such as negligible vapor pressure, and chemical and thermal
stability, ionic liquids exist as liquid at ambient temperature
[9]
. Ionic liquids have
attracted considerable research interest over the past decade not only for the properties
listed above, but also electrochemical stability, non-flammability, optical transparency,
and high ionic conductivity. Recent fundamental studies of ionic liquids have focused on
developing them as a new class of materials for a number of applications, especially
electrolytes for electrochemical devices
[17]
. Polymerized ionic liquid block copolymers,
also known as ion gel electrolytes, are the smart applications of the properties of ionic
liquid. In this novel material, the lack of mechanical integrity in ionic liquids has been
supplied by combining it with well-designed controlled-structure polymer at the nanoscale. Polymer gel electrolytes are generally prepared by physically cross-linking selfassembly
[72]
. The self-assembly of block copolymers can result in a range of
nanostructures, such as lamellae, bicontinuous gyroid, hexagonal cylinders, or bodycentered cubic spheres, where morphology and domain size are tunable based on
polymer/ionic liquid composition in block copolymers
[5,73]
. The self-assembled
nanostructured morphologies of block copolymers and their influence on electrochemical
properties are the most important aspect to be researched for the utilization of poly(ionic
liquid) block copolymer as solid-state polymer gel electrolytes.
In order to synthesize well-controlled complex polymer architectures, controlled radical
polymerization methods including NMP, ATRP, and RAFT have been applied
62,75]
[54,57,61,
. Appukuttan et al. recently published a report of the polymerization of 239
hydroxyethyl acrylate (HEA) via ATRP in order to convert poly(HEA) into ionic liquid
polymer with bromination and subsequent quaternization reaction
[59]
. Here, adapting
their process to the methacrylic analog, 2-hydroxyethyl methacrylate (HEMA)-based
block copolymer was synthesized by NMP. HEMA is widely used since late 1970s as a
major component in the manufacture of soft contact lenses, drug delivery and hydrogel
for a variety of applications [74,75]. However, very little work has been done regarding the
applications for controlled-structure HEMA-based block copolymers. There have been
several studies with anionic polymerization method. Nagasaki et al. in 1995 successfully
reported
the
synthesis
of
‘living’
(ProHEMA) via anionic polymerization
poly(2-(trimethylsiloxy)ethyl
[76]
methacrylate)
. This technique, however, requires several
extra/lengthy steps for protection of the alcohol functionality. More recently, ATRP and
RAFT polymerization techniques were applied to synthesize well-defined poly(HEMA).
Robinson et al. in 2001 achieved the efficient controlled polymerization of HEMA using
ATRP at ambient temperature with good living character
[54]
. Islam et al. in 2012
published their findings in synthesis of random copolymer of HEMA and methyl
methacrylate (MMA) by RAFT
[77]
. Fundamental studies on synthesis of poly(HEMA)
via NMP and its kinetics and mechanism of polymer reaction remain relatively
unexplored. Generally, NMP regarded as limited applicability compared to ATRP and
RAFT, since only styrenics and alkyl acrylates monomers have actually been
polymerized under ‘living’ controlled conditions in the presence of nitroxide [78]. Despite
this, NMP technique was chosen in this study, because it does not require any metal
catalyst or sulfur-based agent as ATRP or RAFT requires, respectively. NMP also offers
routes to highly controlled microstructures, with defined chain lengths and chain end
40
fidelity, which is desirable for customizing material properties. Cunningham and his
colleagues have successfully synthesized 2-hydroxyethyl acrylate (HEA), which is very
similar to HEMA, via NMP using SG1-based alkoxyamine initiator and have chainextended to produce amphiphilic block copolymers
[62]
. Homopolymerization of
methacrylates such as HEMA, however, is very difficult to control by NMP. According
to Charleux and co-workers, methacrylic esters have very high activation-deactivation
equilibrium constant, K, which does not lead to controlled polymerization in the presence
of nitroxide in polymerization process, resulting in polymers with broad molecular
weight distribution and low conversion [34]. Using a small concentration of monomer that
is controllable by NMP, such as 10 mol% of styrene as co-monomer, which has much
lower equilibrium constant compared to most methacrylates, was suggested synthesizing
a ‘living’ random copolymer via NMP. In this theory, the chain ends will be mostly
styrenic that leads to better control of the methacrylate-rich polymers.
In
this
work,
a
imidazolium-based
(poly([MEMIm][Br]-ran-S)-b-PS)
poly(ionic
composed
of
liquid)
random
block
copolymer
copolymer
of
1-[(2-
methacryloyloxy)ethyl]-3-methylimidazolium bromide and styrene using nitroxide
mediated polymerization was synthesized. It was prepared by quaternization with a
nucleophile 1-methylimidazole after the polymerization of the HEMA-based block
copolymer poly(HEMA-ran-S)-b-PS. 1-methylimidazole is one of the common precursor
to some ionic liquids. Imidazolium-based poly(ionic liquid)s and their properties and
phase separation in block copolymer form have been studied thoroughly by Bailey and
his colleagues
[78,79]
. The kinetics and molecular weight analysis of HEMA
polymerization, molecular weight shift in chain extension, change in chemical structure
41
and glass transition temperature after bromination and quaternization, and microphase
separation and morphology were identified with a combination of analytical techniques
including NMR, GPC, FTIR, DSC, and TEM.
4.2 Experimental Section
4.2.1 Materials
N-hydroxysuccinimide (NHS, 98%), N,N’-dicyclohexylcarbodiimide (DCC, 99.9%),
dimethyl sulfoxide-d6 (DMSO-d6, 99.9 atom% D), trimethylsilyl bromide (TMSBr, 97%),
aluminum oxide (Al2O3, Brockmann, type 1, basic/neutral), and calcium hydride (CaH2,
95%, reagent grade) were purchased from Sigma-Aldrich. Pentane (99%, HPLC grade),
diethyl ether (anhydrous, ≥99%, certified ACS reagent grade), dichloromethane (99.9%,
anhydrous) and methanol (≥99.8%, certified ACS reagent grade) were obtained from
Fisher Scientific. N,N-dimethylformamide (DMF, ≥99.8%, ACS reagent grade) was
obtained from Acros Organics. Pyridine (≥99%, ACS grade) and acetic anhydride (≥97%,
ACS grade) were purchased from ACP Chemicals. Ruthenium tetroxide (RuO4, 0.5%
stabilized aqueous solution) was obtained from Electron Microscopy Sciences. All above
listed compounds were used as received. 2-hydroxyethyl methacrylate (HEMA, ≥99%),
Styrene (≥99%, contains stabilizer, Sigma-Aldrich) and tetrahydrofuran (THF, ≥99.9%,
certified) were purified and dehydrated by passing through a column of aluminum oxide
(neutral for HEMA, basic for styrene and THF) mixed with 5 wt% calcium hydride, and
were stored in a sealed flask under a head of nitrogen in a refrigerator until needed. 242
({tert-butyl[1-(diethoxyphosphoryl)-2,2-dimethylpropyl]amino}oxy)-2-methylpropionic
acid, also known as BlocBuilder® (99%) was obtained from Arkema and {tert-butyl[1(diethoxyphosphoryl)-2,2-dimethylprophyl]amino} nitroxide (SG1, >85%) was kindly
donated by Noah Macy from Arkema. Both BlocBuilder® and SG1 were used as received
and stored at 5°C.
4.2.2 Synthesis of random copolymer of HEMA and styrene via NMP
The preparation of the random copolymer poly(HEMA-ran-styrene) is shown in Figure
14a below.
O
(a)
n
+
SG1
O
m
NHS - BB
O
O
O
n+m
N
OH
OH
Styrene
O
O
O
HEMA
poly(HEMA-ran-styrene)
(b)
O
O
O
P
O
N
HO
+
DCC
O
O
+
O
P
DCU
O
N
O
O
N
O
OH
N
O
O
N-hydroxysuccinimide
BlocBuilder®
(c)
NHS - BlocBuilder
O
O
O
O
O
O
n+m
SG1
O
k
N
O
O
O
N
O
+
O
k
SG1
O
OH
OH
poly(HEMA-ran-styrene)
O
n+m
Styrene
poly(HEMA-ran-styrene)-b-poly(styrene)
43
Figure 14 Reaction scheme for (a) synthesis of random copolymer of 2hydroxymethyl methacrylate and styrene via NMP using N-hydroxysuccinimideyl
functionalized BlocBuilder(NHS-BB) as initiator, (b) synthesis of NHS-BlocBuilder,
and (c) synthesis of diblock copolymer containing HEMA/styrene random
copolymer block and poly(styrene) block by NMP
The N-hydroxysuccinimidyl-functionalized BlocBuilder initiator (NHS-BlocBuilder) was
synthesized via coupling of BlocBuilder® and N-hydroxysuccinimide following
procedures first described by Vinas et al. and adapted by Zhang (Figure 14b)
[60, 61]
. The
copolymerization was conducted in a 25-mL three-neck round-bottom glass flask fitted
with a reflux condenser. Targeting the molecular weight of final product to be 15 kgmol-1
(Mn,target, molecular weight at 100% conversion), 0.176 g of NHS-BlocBuilder (0.367
mmol) and 0.011 g of free SG1 (0.037 mmol, 10 mol% of NHS-BlocBuilder) were
dissolved in the monomer mixture of 4.944 g of HEMA (37.988 mmol, fHEMA = 90 mol%)
and 0.397 g of styrene (3.816 mmol), then mixed with 5.462 g of DMF as solvent (5.762
mL, 74.730 mmol, 50 wt% of total mixture). The solution was transferred into the
reaction flask, which was completely sealed with rubber septa. The air and oxygen was
removed by bubbling ultra-pure nitrogen for 30 minutes at room temperature, followed
by heating up and carrying out the reaction at 90°C for 5 h, while maintaining the
nitrogen purge. Samples were taken by syringe in a predetermined time interval for NMR
analysis and for GPC measurement after modification. The resulting copolymer was
twice precipitated in diethyl ether, re-dissolved in DMF, decanted and dried under
vacuum at room temperature to obtain the final purified copolymer (2.398 g, 48 % yield).
44
4.2.3 Synthesis of diblock copolymer poly(HEMA-ran-styrene)-bpoly(styrene)
Diblock copolymer was prepared by the macroinitiator method. The final purified
copolymer poly(HEMA-ran-styrene), as macroinitiator, which was initiated by NHSBlocBuilder and capped with SG1, was directly heated with styrene monomer at 120°C
for 5 hr to give diblock copolymer poly(HEMA-ran-styrene)-b-poly(styrene). The
synthesis of diblock copolymer was performed using the same setup and similar
polymerization procedures as the synthesis of the random copolymer described earlier.
Thus, for an experiment with target molecular weight of block copolymer being 92 kg
mol-1, 0.931 g of poly(HEMA-ran-styrene) macroinitiator (0.054 mmol, Mn=17 kg mol-1,
Đ = 1.5) and 4.091 g of fresh styrene (39.234 mmol) were mixed with 5.887 g of DMF
(6.210 mL, 80.545 mmol) in a 25-mL three-neck round-bottom glass flask, deoxygenated
by nitrogen bubbling for 30 minutes at room temperature before heating to 120°C. The
reaction was stopped after 5 hr, and the chain-extended block copolymer was precipitated
in 50/50 v/v mixture of methanol and diethyl ether, decanted, and dried in a vacuum oven
at room temperature overnight. The purified block copolymer was analyzed by 1H NMR
and GPC after acetylation (see below for the method) in order to find its mole
composition and the Mn and Đ, respectively.
4.2.4 Synthesis of diblock copolymer poly(2-bromoethyl methacrylateran-styrene)-b-poly(styrene)
The following procedures for the bromination of the HEMA units in poly(HEMA-ranstyrene)-b-poly(styrene) was adapted from Appukuttan et al. with slight modification [59].
45
(a)
O
O
O
O
n+m
N
O
O
k
SG1
O
TMSBr
O
O
CH2Cl2, 25°C
n+m
N
O
O
k
SG1
O
Br
OH
poly(BrEMA-ran-styrene)-b-poly(styrene)
(b)
O
O
O
O
O
n+m
N
O
O
k
SG1
O
CH3C3H3N2
O
n+m
N
O
O
k
SG1
O
THF, 85°C
N Br
Br
poly(BrEMA-ran-styrene)-b-poly(styrene)
H3C
N
PIL-b-Poly(styrene)
Figure 15 Reaction scheme for (a) synthesis of diblock copolymer poly(BrEMA-ranstyrene)-b-poly(styrene) by bromination with trimethylsilyl bromide, and (b)
synthesis of poly ionic liquid by quaternization with 1-methylimidazole
poly([MEMIm][Br]-ran-S)-b-PS
The bromination procedure is shown in Figure 15a. The side chain hydroxyl groups on
the HEMA units within poly(HEMA-ran-styrene)-b-poly(styrene) were replaced by
bromide groups using trimethylsilyl bromide (TMSBr). The reaction was performed with
the purified block copolymer and its anti-solvent dichloromethane in the ice bath. For a
specific example, 0.483 g of poly(HEMA-ran-styrene)-b-poly(styrene) (0.018 mmol) was
suspended in 5 mL of dichloromethane (6.625 g, 78.005 mmol) at 0°C. Excess amount of
trimethylsilyl bromide (0.958 g, 0.825 mL, 6.255 mmol; 3 equivalents with respect to the
number of hydroxyl groups in poly(HEMA-ran-styrene)) was added drop-wise and the
solution was slowly warmed to room temperature. Transferring TMSBr out of the
46
Sure/Seal™ bottle to the reactor was performed with caution because it was highly
reactive with moisture in the atmosphere. Addition to the reactor was also done slowly
drop-wise and carefully since the mixing process was spontaneous and exothermic.
Temperature was increased by 3-4 °C as a drop added in, so next drop was waited until
the temperature came down to 0 °C. It was noticed that the mixture at room temperature
was clear transparent solution with the polymer in the bottom. After 24 hr of stirring, the
brominated
diblock
poly(styrene)
copolymer
poly(2-bromoethyl
(poly(BrEMA-ran-S)-b-PS)
became
methacrylate-ran-styrene)-bcompletely
soluble
in
dichloromethane, and the mixture turned yellow with no solids suspended in the course of
the reaction. The final product was precipitated into cold methanol, washed twice with
methanol, and dried under vacuum at room temperature overnight, resulting in yellow
solid particles.
4.2.5 Synthesis of imidazolium-based poly(ionic liquid) block copolymer,
poly(1-[(2-methacryloyloxy)ethyl]-3-methylimidazolium
bromide-ran-
styrene)-b-poly(styrene)
The quaternization reactions were based on the procedures described by Stancik et al. and
Appukuttan et al.
[8, 59]
. The process was adapted with slight modification in this study
shown in Figure 15b. 0.160 g of the brominated diblock copolymer, poly(BrEMA-ran-S)b-PS, prepared as described earlier was dissolved completely in the minimum amount of
THF (approximately 2.125 mL) appearing as orange-yellow colour. Excess amount of 1methylimidazole (approximately 5 times more than the number of bromide groups in the
polymer chain) was mixed with the polymer solution. The mixture turned clear after
47
adding the nucleophile component. The solution was heated to 80°C for 24 h with a
reflux condenser under nitrogen atmosphere followed by cooling to ambient temperature.
The poly(1-[(2-methacryloyloxy)ethyl]-3-methylimidazolium bromide-ran-styrene)-bpoly(styrene) (poly([MEMIm][Br]-ran-S)-b-PS) was then precipitated in diethyl ether,
decanted, dissolved in methanol, re-precipitated in diethyl ether, and the process was
repeated two times to remove excess 1-methylimidazole. The 83% yield resultant solid
was then dried under vacuum at room temperature overnight. Characterization by DSC
and TEM of quaternized block copolymer can be found in a section 4.3.4.
4.2.6 Characterization using Nuclear Magnetic Resonance (NMR)
1
H NMR spectra were recorded on a Varian Gemini 2000 spectrometer operating at
400MHz in order to characterize chemical structure, monomer conversion, and
copolymer composition. Chemical shifts are reported in ppm relative to residual solvent
resonances (1H). Deuterated DMSO-d6 was used without further purification. Conversion
of HEMA was calculated using the two protons adjacent to the ester oxygen of the
monomer (δ = 4.0 – 4.1 ppm) and those corresponding to the polymer (δ = 3.8 – 4.0
ppm).
48
(a)
.
(b)
Figure 16 1H NMR spectra of (a) HEMA and (b) poly(HEMA-ran-styrene)
49
4.2.7 Gel Permeation Chromatography (GPC)
Number-averaged molecular weight Mn and dispersity, Đ of all polymer products were
characterized by GPC (Waters Breeze) using a mixture of DMF and 0.05 M LiBr as a
mobile phase at 50 °C and a flow rate of 0.1 mL min-1. The GPC was equipped with two
ResiPore 250×4.6 mm columns and one ResiPore Guard 50×4.6 mm column from
Polymer Laboratories Inc. (Varian) (molecular weight operating range: 2×102 – 4×105 g
mol-1, Nominal Particle Size: 3µm, Typical Column Efficiency >80,000 p/m)
[80]
. The
GPC was also equipped with both 2487 ultraviolet (UV) and 2410 reflective index (RI)
detectors. The results reported in this study were obtained from the RI detector. All
molecular weight measurements were calibrated with poly(methyl methacrylate)
(PMMA) narrow molecular weight distribution standards. The polymer samples,
however, were modified in order to make poly(HEMA-ran-styrene) soluble in the GPC
solvent by acetylation of the hydroxyl groups using acetic anhydride. This is due to the
polarity of polymer sample and due to its affinity to the GPC column. The following
method was adapted from Bian et al.
[62]
. For a typical GPC sample, 0.02 g of
poly(HEMA-ran-styrene) was dissolved in 0.5 mL of pyridine and stirred with 0.1 mL of
acetic anhydride at room temperature overnight. The acetylated copolymer sample,
poly(AcEMA-ran-styrene) was obtained after being precipitated in methanol and after
decanting the solvent, dried in a vacuum oven at room temperature overnight
50
4.3 Results and Discussion
4.3.1 Random copolymerization of HEMA and styrene
Figure 17 depicts the kinetic plots (a) and number-average molecular weight (Mn) versus
conversion plots (b) for the random copolymerization of HEMA with styrene (fHEMA,0 =
90 mol%) in DMF at 90°C using NHS-BlocBuilder as initiator and 10 mol% additional
free SG1 nitroxide.
(a)
R2 = 0.9938 51
(b)
Figure 17 Random copolymerization of HEMA and styrene in DMF at 90°C (a)
kinetic plot of ln[(1-x)-1] versus reaction time (x = monomer conversion) and (b)
number-average molecular weight (Mn) and dispersity (Đ) versus conversion
Under these conditions, the polymerization reaction followed first-order kinetics as
expected from a controlled polymerization (Figure 17a). The molecular weight of random
copolymer increased linearly with conversion and the dispersity Đ remained low, below
1.50 throughout the reaction shown in Figure 14b. These results indicate that the
HEMA/styrene random copolymerization using NHS-BlocBuilder was well controlled.
As shown in Figure 14b, Mn of HEMA/styrene copolymers increased fairly linearly with
monomer conversion. On the contrary, it does not follow the theoretical prediction (solid
line). This disagreement is likely due to low initiating efficiency. Previous studies on
52
copolymerizations with methacrylates via NMP have found the issue associated with low
initiating efficiency. For instance, Charleux et al. reported that the initiating efficiency
was only 75% at about 60% conversion for copolymerization of MMA and styrene using
BlocBuilder® [34]. The low initiating efficiency is caused by incomplete initiation of NHSBlocBuilder due to irreversible termination of primary and/or oligo-radicals. With a
higher conversion, Mn drifting upward was observed, which could be a result from the
decreasing number of living chains through unexpected self-termination of two polymer
chains. Consequently, the molecular weight distribution (or dispersity Đ) at a higher
conversion was found to be greater than that found at a lower conversion. Note that all
reported Mns in this report were raw GPC data values (acetylated polymer samples),
which were relative to PMMA standards. The difference, however, between the reported
values and actual (theoretical) Mn of HEMA/styrene copolymers was also considered.
The actual Mn of copolymer was calculated using universal calibration with MarkHouwink coefficients for PMMA, poly(styrene), and poly(HEMA) in DMF (KPMMA =
1.60×10-4 dL g-1, αPMMA = 0.66, KPSt= 0.71×10-4 dL g-1, αPSt= 0.716 [63]; KPHEMA= 1.06×104
dL g-1, αPMMA = 0.70
[64]
;) and the differences were negligible as the maximum error
percentage was 15%. The correction was accounted for in the graph in Figure 17b.
4.3.2 Chain Extension of HEMA/styrene random copolymer
The final purified copolymer poly(HEMA-ran-styrene) (90 mol% HEMA) as a
macroinitiator was chain extended with styrene to examine its ‘livingness’ in DMF
solvent at 120°C. The addition of a styrene second block was chosen to obtain diblock
copolymer with a hydrophilic block (HEMA/styrene) and a hydrophobic block (styrene).
53
Mn of the diblock copolymer poly(HEMA-ran-S)-b-PS was 27 kg mol-1 (relative to
PMMA standard) after approximately 5 h or reaction time (83 mol% HEMA). It is 10 kg
mol-1 higher than the Mn of macroinitiator, indicating about 10% conversion of styrene
monomer with respect to the target molecular weight (92 kg mol-1), which agrees with the
NMR analysis.
P(HEMA3ran3st)"
Detector'Signal'(a.u.)'
1"
P(HEMA3ran3St)3b3P(St)"
0.8"
0.6"
0.4"
0.2"
0"
15"
17"
19"
21"
Reten3on'Time'(min)'
23"
25"
Figure 18 GPC traces of macroinitiator poly(HEMA-ran-styrene) (solid line) and
diblock copolymer poly(HEMA-ran-styrene)-b-poly(styrene) (dashed line)
The chain extension result was also demonstrated in the GPC chromatograms shown in
Figure 18. As one can see, the chromatogram of chain extended block copolymer was
fairly monomodal and shifted to the lower retention time compared to that of the
macroinitiator. This indicates that most chains were re-initiated and polymerized with the
second monomer. The shoulder on the right end in the diblock copolymer’s GPC trace
implied existence of some un-reacted macroinitiators. Since the Mns of macroinitiator and
final chain extended diblock copolymer were similar (17 kg mol-1 and 27 kg mol-1,
respectively), it was not easy to get rid of un-reacted macroinitator through precipitation
54
and/or fractionation. The dispersity Đ of the diblock copolymer was 1.64, which was
broader than that of macroinitiator (Đ = 1.49 for poly(HEMA-ran-S), Figure 14b). The
increase in molecular weight distribution may be due to the slow initiation, the
incomplete removal of the unreactive macroinitiator, existence of dead chains in the
macroinitiator, and/or occurrence of termination reactions during the chain extensions
(dead oligo-radicals).
4.3.3 Bromination of diblock copolymer poly(HEMA-ran-styrene)-bpoly(styrene)
The change in hydroxyl group concentration in the poly(BrEMA-ran-S)-b-PS from
poly(HEMA-ran-S)-b-Ps was monitored using Fourier Transform Infrared Spectroscopy
(FTIR) as shown in Figure 19.
55
Transmission#(a.u.)#
$$$$$$P(HEMA"ran"styrene)$
$$$$$$P(HEMA"ran"styrene)"b"P(styrene)$
$$$$$$P(BrEMA"ran"styrene)"b"P(styrene)$$
4000#
O"H$
3600#
3200#
C"H$
C=O$
Ester$
2800#
2400#
2000#
1600#
Wavenumber#(cm:1)#
C"O$
Ester$
1200#
Poly$
(Styrene)$
800#
400#
Figure 19 FT IR spectra of poly(HEMA-ran-styrene) (top), block copolymer
poly(HEMA-ran-styrene)-b-poly(styrene) (middle), and brominated block
copolymer poly(BrEMA-ran-styrene)-b-poly(styrene) (bottom)
Comparing the signals from macroinitiator (top) and block copolymer (middle), a
stronger poly(styrene) peak was noted at 650 – 700 cm-1 wavenumber in block copolymer,
while O-H, C-H, and C-O peaks from ester structure in HEMA remained constant. It was
noticed that C=0 peak at 1700 cm-1 in diblock copolymer was much more intense than
that in macroinitiator. This could be due to the remaining DMF solvent, NHSBlocBuilder, or any impurities in the diblock copolymer sample. Most importantly, the
hydroxyl group O-H peak around 3200 – 3600 cm-1 was pronouncedly decreased in the
brominated block copolymer as expected. The intensity of the peak from functional group
of C-Br at 500 – 600 cm-1 was difficult to identify since it was overlapped with other
peaks.
56
4.3.4 Quaternization with 1-methylimidazole
Imidazolium functionalized ionic liquid block copolymer poly([MEMIm][Br]-ran-S)-bPS was synthesized by a quaternization reaction with 1-methylimidazole in THF solvent.
The brominated block copolymer was dissolved in THF and was mixed with excess 1methylimidazole. The colour of solution went from yellow to transparent after adding 1methylimidazole. It was observed that the reaction was noticeably sensitive at the speed
of stir bar rotating and temperature. The mixing process produced bubbles which was not
expected, and the rate of bubbling was proportional to the stirring rate and/or the reaction
temperature. This bubbling could have been caused by undesired side reaction producing
some gas compounds. However, it was not noticed if the gas was released or not, since
there was N2 gas purging during the reaction. The final product was viscous and sticky,
and dried into very thin, dark brown.
The glass transition temperature (Tg) of the imidazolium-based poly(ionic liquid) block
copolymer was determined and compared to the macroinitiator, poly(HEMA-ran-S)-bPS, and poly(BrEMA-ran-S)-b-PS using differential scanning calorimeter (DSC; TA
Instruments, Q2000 V24.4) over a temperature range of -20 to 150 °C at a
heating/cooling rate of 10 °C/min under a N2 environment using a heat/cool/heat method.
Tg was determined using the midpoint method from the second thermogram heating
cycle.
57
Figure 20 Differential scanning calorimeter thermogram and Tg comparison of
poly(HEMA-ran-S) (1), poly(HEMA-ran-S)-b-PS (2), poly(BrEMA-ran-S)-b-PS (3),
and poly([MEMIm][Br]-ran-S)-b-PS (4).
Figure 20 shows glass transition temperatures (Tgs) of the macroinitiator poly(HEMAran-S), diblock copolymer poly(HEMA-ran-S)-b-PS, brominated diblock copolymer
poly(BrEMA-ran-S)-b-PS, and imidazolium-based poly(ionic liquid) diblock copolymer
poly([MEMIm][Br]-ran-S)-b-PS. As one can see, Tgs of the functional blocks such as
poly(HEMA-ran-S), poly(BrEMA-ran-S), and poly([MEMIm][Br]-ran-S) decreases
from 69 °C to 37 °C as the side chain compound changes, knowing that the higher Tgs
(around 100 °C) of the diblock copolymer samples are polystyrene
[81]
. There were two
distinct Tgs determined in diblock copolymer samples. This suggests that there is
microphase separation in these diblock copolymers.
58
Another evidence of microphase separation morphology was obtained by transmission
electron microscopy (TEM). Figure 21 shows TEM image of the imidazolium-based
poly(ionic liquid) diblock copolymer, poly([MEMIm][Br]-ran-S)-b-PS with 83 mol% of
the imidazolium-based polymer block.
Figure 21 TEM image of poly([MEMIm][Br]-ran-S)-b-PS
The solid-state poly(ionic liquid) block copolymer sample was cut into extremely thin
slice, placed on the copper grid, and coated with carbon using the cryo-ultramicrotomy
method by Ms. Jeannie Mui at the Facility for Electron Microscopy Research (FEMR) at
McGill. The cryo-sectioned sample was then reacted with ruthenium tetroxide in order to
stain the polystyrene block for compositional contrast between two polymer block
phases. The FEI Tecnai™ 12 BioTwin 120 kV transmission electron microscopy with
AMT XR80C CCD camera system was used to obtain the image shown in Figure 21. The
TEM image clearly indicates a microphase separated morphology with no long-range
59
periodic order. The hexagonally packed cylindrical structure with poly(ionic liquid)
domain (brighter area in TEM image) was expected with the given composition of 83
mol% poly(ionic liquid) and 17 mol% polystyrene. This weak microphase separation
could have been attributed to the partial miscibility between the imidazolium-based
random copolymer with styrene block and the polystyrene. The final block copolymer
composition by 1H-NMR spectroscopy in CDCl3 and DMSO-d6 or molecular weight
analysis by GPC in DMF yield the uncertainty of the analysis because of poor solubility
of the imidazolium-based methacrylate polymer block in those solvents.
60
5. Conclusion
This study reported 2-hydroxyethyl methacrylate-rich copolymerizations via nitroxide
mediated polymerization method with the feed composition of 90 mol% HEMA using
styrene as a controlling co-monomer and NHS-BlocBuilder as an initiator. The
copolymerization was controlled to relatively high conversion ~65% based on the linear
increase in Mn versus conversion and monomodal molecular weight distribution with
relatively low Đ at ~1.45. This SG1-capped random copolymer poly(HEMA-ran-styrene)
was an effective and truly ‘living’ macroinitiator for subsequent chain extension with
styrene to synthesize block copolymer poly(HEMA-ran-styrene)-b-poly(styrene) as
shown by GPC and NMR.
Finally, the conversion of the hydroxyl groups from HEMA in the block copolymer into
bromide and subsequent quaternization with 1-methylimidazole were successful as
observed with FTIR and DSC. However, the morphology of the imidazolium-based
poly(ionic liquid) block copolymer (83 mol% PIL) was weak as no long-range periodic
order was observed with TEM. This poly(ionic liquid) block copolymer represents an
initial iteration to make polymer ionic gel electrolytes and this study appears attractive
for further fundamental exploration. Future work will be directed towards determination
of the mechanical, morphological and rheological properties, as well as the ionic
conductivity.
61
6. Considerations for Future Work
First, small-angle X-ray scattering of the imidazolium-based poly(ionic liquid) block
copolymer needs to be investigated to confirm the morphology observed with TEM. It is
beneficial to synthesize a series of polymerized ionic liquid block copolymers at various
poly(ionic liquid) compositions with the goal of understanding the dynamics of the selfassembly process. Also, comparison of morphologies with different molar composition
should be studied to obtain the possible nanostructures with ion-conducting channels in a
potential polymer ionic gel electrolyte. It is expected that the bicontinuous gyroid
morphology structure would be the most useful and interesting for the ion-conducting
channel
[82,83]
. The gyroid morphology has monodisperse pore diameters in which all
channels and struts are fully interconnected, which is ideal for achieving continuous
connectivity of all electronically active phases to the device electrode. According to
Edward et al. columns, cylindrical, spheres, disks, or lamellae morphologies usually
exhibit only an in-plane continuity in thin film [78,82]. Quaternization reaction with various
nucleophile such as pyridine and triethylamine and comparison between various
poly(ionic liquid) block copolymers could help the future study of polymer gel
electrolytes as well. The use of the succinimidyl ester group of NHS-BlocBuilder can be
further explored. The N-succinimidyl ester can be easily coupled with amine-containing
compounds by single-step coupling
[65]
, which would be helpful to design stimuli-
responsive poly(ionic liquid) block copolymers.
62
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