Download SYNTHESIS OF HIGH MOLECULAR WEIGHT POLY (METHYL

SYNTHESIS OF HIGH MOLECULAR WEIGHT POLY (METHYL
METHACRYLATE) BY ARGET ATRP
A Thesis
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Jialin Qiu
August, 2015
SYNTEHSIS OF HIGH MOLECULAR WEIGHT POLY (METHYL
METHACRYLATE) BY ARGET ATRP
Jialin Qiu
Thesis
Approved:
Accepted:
_____________________
Advisor
Dr. Kevin A. Cavicchi
____________________
Department Chair
Dr. Sadhan C. Jana
_____________________
Dean of the College
Dr. Eric J. Amis
_____________________
Advisor
Dr. Bryan D. Vogt
_____________________
Committee Member
Dr. Sadhan C. Jana
_____________________
Interim Dean of the Graduate
Dr. Rex D. Ramsier
____________________
Committee Member
Dr. Alamgir Karim
_____________________
Date
ii
ABSTRACT
Synthesis of poly (methyl methacrylate) by activators regenerated by electron
transfer (ARGET) atom transfer radical polymerization (ATRP) was extensively
studied. It was observed that when the target molecular weight is higher, side
reactions and terminations becomes more prominent. In this study, a facile
method was used to synthesize high molecular weight PMMA by ARGET ATRP.
The effect of solvent, catalyst and ligand were discussed systematically. Low
molecular weight polyethylene glycol (PEG) (600 g/mol) as solvent significantly
increased the reaction rate and provides molecular weight as high as 96 kDa
within 2h reaction, Đ as low as 1.2. PEG can stabilize the catalyst by complexing
to avoid the side reactions such as the reactions between catalyst and chain end.
The terminal hydroxyl group could potentially increase the reaction rate. It was
also observed that the results have PEG molecular weight dependence. As PEG
molecular weight increased, viscosity increased which resulted in poor polymer
chain diffusion. However PEG-metal stability constant increased when PEG
molecular weight increased which provided better control. These factors
competitively affected the polymerization kinetics. Optimum solvent was observed
using PEG (600 Da). The comparison between two copper based catalysts CuBr2
iii
and CuCl2 showed that shorter bond length of Cu-Br leaded to a faster reaction
rate. For the comparison of ligands, N,N,N′,N′,N′′-pentamethyldiethylenetriamine
(PMDETA) as a tridentate ligand had weaker reducing ability but stronger
coordination ability compared to tetramethylethylenediamine (TMEDA) as
bidentate ligand.
Synthesized PMMA was used as macroinitiator to synthesize poly (methyl
methacrylate)-block-polystyrene (PMMA-PS) block copolymer by bulk ARGET
ATRP. With the increasing amount of styrene, PMMA diffusion became better and
prematurely termination was reduced. PMMA-PS with molecular weight of 770
kDa and Đ of 1.32 was successfully synthesized.
iv
ACKNOWLEGEMENTS
I would like to appreciate all those people who have helped me during my master
study in University of Akron.
Firstly I would like to give my great gratitude to my advisors, Prof. Bryan Vogt and
Prof. Cavicchi, for their support in the research. They show their patience when
teaching me how to do research. They provided me many ideas and show me the
research method. I cannot work efficiently without their help.
I would like to thank Prof. Karim for being my committee member and his most
valuable comments on the thesis.
There are a lot of people in our group who has helped me a lot and give me
sincere suggestions when I got problems in research. I would like to thank Zhe
Qiang, Junyan Wang, Guodong Deng, Jehoon Lee, Changhuai Ye for their help
and discussion with me.
And I would like to thank all my classmates and friends in University of Akron and
University of Akron and Donghua University. They offer me the opportunity for
doing research here. I got to learn much through 3+2 program. Lastly, I would like
v
to thank my family members: grandparents and parents. They supported me to
study abroad and encourage me all the time.
vi
TABLE OF CONTENTS
Page
LIST OF FIGURES ................................................................................................ x
LIST OF SCHEMES ............................................................................................. xii
LIST OF TABLES ................................................................................................ xiii
CHAPTER
I. INTRODUCTION ................................................................................................ 1
1.1Introduction ................................................................................................... 1
1.1.2 Polymerization Method to Synthesize Poly (methyl methacrylate) ....... 2
1.1.3 High Molecular Weight Poly (methyl methacrylate) Synthesis ............ 11
II. SYNTHESIS OF HIGH MOLECULAR WEIGHT PMMA.................................. 16
2.1 Motivation ................................................................................................... 16
2.2 Introduction ................................................................................................ 17
2.2.2 Use of PEG as Solvent ....................................................................... 19
2.2.3 Use of PEG as Solvent to Synthesize High Molecular Weight PMMA 22
2.2.4 Copper Based Catalyst ....................................................................... 22
2.2.5 Nitrogen Based Ligand ........................................................................ 25
2.3 Experimental Section ................................................................................. 27
2.3.1 Materials .............................................................................................. 27
vii
2.3.2 ARGET ATRP of MMA in PEG (600) with PMDETA as Reducing Agent
and Ligand ................................................................................................... 28
2.3.3 Characterization of PMMA .................................................................. 28
2.4 Results and Discussion ............................................................................. 29
2.4.1 Scheme of Reactants .......................................................................... 29
2.4.2 Effect of Solvents ................................................................................ 32
2.4.3 Effect of Catalyst ................................................................................. 41
2.4.4 Effect of Ligand ................................................................................... 45
2.5 Conclusions ............................................................................................... 49
III. SYNTHESIS OF PMMA-PS BY ARGET ATRP.............................................. 51
3.1 Introduction ................................................................................................ 51
3.1.1 Fractional Precipitation ........................................................................ 52
3.2 Experimental Section ................................................................................. 57
3.2.1 Materials .............................................................................................. 57
3.2.2 Fractional Precipitation ........................................................................ 57
3.2.3 Chain Extension of PMMA Macroinitiator with Styrene ....................... 57
3.2.4 Characterization of PMMA-PS ............................................................ 58
3.3 Results and Discussion ............................................................................. 58
3.3.1 Fraction Precipitation of PMMA ........................................................... 58
3.3.2 Effect of Styrene Amount .................................................................... 60
3.3.2 Effect of PMMA Macroinitiator Molecular Weight ................................ 64
viii
3.4 Conclusions ............................................................................................... 66
CONCLUSIONS .................................................................................................. 68
REFRENCES ...................................................................................................... 70
ix
LIST OF FIGURES
Figure
Page
1.1 Scheme of high-pressure AGET ATRP method ..................................... 14
2.1 X-ray structure of CuCl2 and CuBr2 ........................................................ 24
2.2 UV-visible spectra of Cu (II) Br2 with (A) TMEDA, (B) PMDETA ............ 26
2.3 Solubility test of CuBr2 in (A) acetonitrile, (B) PEG-600, and (C) anisole
.............................................................................................................. 34
2.4 Molecular weight evolution affected by PEG molecular weight .............. 37
2.5 Kinetic plots affected by PEG molecular weight ..................................... 38
2.6 Molecular weight dispersity (Đ) affected by PEG molecular weight ....... 39
2.7 Molecular weight evolution affected by catalyst ..................................... 42
2.8 Kinetic plots affected by catalyst ............................................................ 43
2.9 Molecular weight dispersity (Đ) affected by catalyst .............................. 44
2.10 Molecular weight evolution affected by ligand ...................................... 46
2.11 Kinetic plots affected by ligand ............................................................. 47
2.12 Molecular weight dispersity (Đ) affected by ligand ............................... 48
3.1 Process of fractional precipitation .......................................................... 54
3.2 GPC traces of PMMA (a) before fractionation, (b) after fractionation .... 55
3.1 Molecular weight (Mn) and Đ (PDI) before and after fractionation.......... 56
x
3.3 GPC traces of PMMA before and after fractionation .............................. 59
3.4 GPC traces of PMMA-PS ....................................................................... 61
3.5 NMR spectra of PMMA-PS .................................................................... 63
xi
LIST OF SCHEMES
Scheme
Page
1.1 Illustration of the RAFT Mechanism ................................................................ 4
1.2 Mechanism of ATRP ....................................................................................... 7
1.3 Mechanism of ARGET ATRP ........................................................................ 10
2.1 Reaction equation of ARGET ATRP of MMA ................................................ 29
2.2 Chemical structures of (A) acetonitrile, (B) PEG, and (C) anisole ................. 30
2.3 Chemical structure of (A) PMDETA, (B) TMEDA .......................................... 31
xii
LIST OF TABLES
Table
Page
2.1. Effect of solvents on the ARGET ATRP of MMA using PMDETA ................ 33
2.2 Effect of solvents on the ARGET ATRP of MMA using TMEDA ................... 33
3.2 Molecular weight (Mn) and Đ before and after fractionation .......................... 60
3.3 Effect of styrene amount ............................................................................... 62
3.4 Effect of macroinitiator molecular weight ....................................................... 65
xiii
CHAPTER I
INTRODUCTION
1.1 Introduction
Poly (methyl methacrylate) (PMMA) is a polymer that has been extensively
investigated in a number of applications, such as electrospinning1, preparation of
carbon nanotube/PMMA composites2 and fabrication of high-refractive-index thin
films.3 Among its applications, it is observed that when changing the molecular
weight (MW) of PMMA, its property can be changed.
1.1.1 High Molecular Weight Poly (methyl methacrylate).
The preparation of an electrolyte using high MW PMMA nanocomposite was
reported. The electrolyte containing high MW PMMA nanocomposite shows a
stable lithium interfacial resistance over three weeks of storage time. The superior
stability is favored for electrochemical applications.4 PMMA can be fabricated into
microfluidics. Pore formations were observed in PMMA with MW higher than 96.7
kDa because of the high softening temperature of the polymer. The size of pores
decreased with an increased MW but the number of pores is more extensive for
higher MW PMMA.5 Moreover, high MW PMMA can be used to produce block
copolymers such as poly (methyl methacrylate)-block-polystyrene (PMMA-PS).
High MW PMMA-PS thin films have higher toughness surface than the low MW
PMMA-PS thin films because of the entanglement of long polymer chains.6 High
1
MW block copolymer also provides a route to solve the problem of dendrite
growth in Li-polymer batteries.7 High MW PMMA-PS thin films bring a large
domain space which is applicable to nanolithographic pattern transfer to target
substrate and provides large feature size on hundreds of nanometers scale.8
1.1.2 Polymerization Method to Synthesize Poly (methyl methacrylate)
Several polymerization methods including ionic polymerization and free radical
polymerization have been used to synthesize PMMA. Anionic polymerization
contains an anionic active center while free radical polymerization has a free
radical source. In free radical polymerization, reversible addition-fragmentation
chain transfer (RAFT), atom transfer radical polymerization (ATRP) and ARGET
ATRP are discussed in detail.
Anionic Polymerization
Anionic polymerization is a living polymerization technique which includes a
anionic active center.9 Where living means that chain termination does not occur
until the addition of a terminating agent.10 Ideally the growing chain is always
reactive and adding additional monomers. Based on the properties above, it has
two important features:
1. In the absence of termination and chain transfer, the degree of polymerization
of resulting polymer at 100% conversion is controlled by the molar ratio of
monomer to initiator.
2
2. When the initiation rate is much faster than the propagation rate, all chains are
initiated at the same time and polymerize in the same monomer environment.
This condition plus the previous condition gives chains with low molecular
weight dispersity.11
In a living polymerization the monomer is fully consumed until the new monomer
is added allowing the synthesis of block copolymers. Anionic polymerization has
also been used to produce PMMA-PS with low Đ. The nucleophilicity between
monomer helps to avoid the chain termination or transfer.12 the challenge of
anionic polymerization is the very strict requirements of the reaction condition as
the anion is sensitive to both oxygen and water and requires purification of all the
polymerization reagents and inert atmosphere conditions.13, 14
Reversible Addition-fragmentation Chain Transfer (RAFT)
Reversible Addition-Fragmentation chain Transfer （RAFT） polymerization is one
of several living radical polymerization techniques. The same as other free radical
polymerizations, the free radical in RAFT usually comes from a thermochemical
initiator or the interaction of gamma of UV radiation with some reagents.15
3
公式 1 Scheme 1.1 Illustration of the RAFT Mechanism
Scheme 1.1 Illustration of the RAFT Mechanism.16 Reproduced with permission
from ref 16.
4
RAFT polymerization goes through the process from initiation to termination.
Initiation: The polymerization is initiated by a free radical source e.g. AIBN that
can decompose into free radicals. The monomer reacts with the free radical
producing a propagating radical chain.17
Propagation: In the propagation process, the monomer is added to the growing
chain through the propagating radical.
RAFT pre-equilibrium: The propagating radical reacts with the RAFT agent to
produce a RAFT adduct radical through a fragmentation reaction. Either the
starting radical or a polymeric RAFT agent is produced by the RAFT adduct
radical losing either the polymeric species or the R group, respectively, through a
reversible process.18
Re-initiation: The radical produced from RAFT adduct propagates with the
monomer and form another reactive polymer chain.
Main-RAFT equilibrium: In an ideal situation, the radicals have the same
opportunity to propagate with the species, which have not been terminated
through a rapid exchange process. Thus, the chains have the same opportunity to
grow and will end up with narrow Đ.
Termination: The same as other free radical polymerization, during the RAFT
polymerization process the growing chain will terminate. One kind of the
termination in RAFT is known as bi-radical termination.19, 20 The termination is
caused by adduct reacting with other radicals. However ideally the RAFT adduct
is well-hindered and will not undergo termination.
5
Atom Transfer Radical Polymerization (ATRP)
Atom transfer radical polymerization (ATRP) contains several components: the
initiator (R-X) that contains an alkyl halogen bond; the catalyst that typically is a
transition metal species (Mtn) which has two available oxidation states and is used
in its relative lower oxidation state; and the ligand that combines with the catalyst
through a covalent or ionic bond to form a metal complex (Mtn/L). The metal
species is responsible for the homo-cleavage of the alkyl halogen bond in the
initiator. During this process, a free radical is produced and propagates with
monomer to generate a propagating polymer chain. After that, metal complex will
be oxidized to its high oxidation state (Mtn+1/L) and act as a deactivator in the
reaction.
6
R-X+CuI X/L
Ka
Kda
R+CuII X2/L
+M kt
KP
公式 2 Scheme 1. 2 Mechanism of ATRP
Scheme 1.2 Mechanism of ATRP.
7
R-R+CuII X2/L Metal complex in its low oxidation state is activator and is responsible for the
generation of the reaction. Metal complex being oxides to its higher oxidation
state is deactivator, which prevents the reaction going on. By controlling the ratio
of activator to deactivator, propagation rate and termination rate can be
controlled.21 The intermittent and repeated activation/deactivation cycle makes
the majority of the polymer chains grow at a constant rate.22
The molecular weight dispersity (Đ) of polymers synthesized by ATRP is
calculated by:
Đ= Mw/Mn = 1+1/DPn+ ([RX]0 kp/kdeact[Mtn+1X/L])(2/Conv-1)
(DPn is degree of polymerization; Conv is conversion [RX]0 is concentration of
initiator before reaction)
Typically, the Đ decreases with increasing conversion and deactivator
concentration. Different monomers as well as the choice of different ligands,
catalyst, initiator, solvent and reacting condition will affect rate of polymerization.
The lower oxidation state metal catalyst is sensitive to the oxygen in the air and
hard to handle or preserve. Therefore, additional steps must be taken to prevent
oxidation of the catalyst during the setup and running of an ATRP
polymerization.23
Activators regenerated by electron transfer (ARGET) ATRP
8
Activators regenerated by electron transfer (ARGET) ATRP has been investigated
by Matyjaszewski’s group.24 Different from ATRP, transition metal catalyst is used
in its deactivated, higher oxidation state. To activate the catalyst, a reducing agent
is introduced to continuously reduce the catalyst to its activated state and
regenerate the persistent radicals. The amount of catalyst needed for the reaction
is also significantly decreased by adding the reducing agent because of the
continuous reduction process.25 This makes metal complex catalyst easier to
remove. The using of high oxidation state metal also makes the reaction system
less oxygen sensitive and easy to prepare. Reducing agents commonly used in
ARGET ATRP includes phenol, sugar, ascorbic acid, tin (II) 2-ethylhexanoate (Sn
(EH)2), and Cu0.26 A number of papers have discussed the synthesis of PMMA by
ARGET ATRP25, 27, 28, 29. Systematic study has been done to discuss the effect of
reagents on the reaction kinetic.22, 25, 30, 31, 32
9
R-X+CuI X/L
Ka
Kda
R+CuII X2/L
+M kt
KP
R-R+CuII X2/L
Reducing agent
公式 3 Scheme 1.3 Mechanism of ARGET ATRP
Scheme 1.3 Mechanism of ARGET ATRP.
10
1.1.3 High Molecular Weight Poly (methyl methacrylate) Synthesis
Anionic Polymerization
Anionic polymerization has been used to synthesize high MW PMMA and its
related block copolymers. The living process with no termination makes the high
MW achievable. PMMA with a MW over 800 kDa and Đ lower than 1.2 has been
synthesized by anionic polymerization.33 The use of initiators composed of
organolithium compound and a large excess of lithium trimethylsilanolate
(Me3SiOLi) provides a superior control on the MW and Đ. However this method
requires extensive purification of both the polymerization equipment and each
reagent to achieve moisture and oxygen free conditions. For example, anionic
polymerization is many times carried out in glass ampoules filled with dried
nitrogen.33
Free Radical Polymerization
(a) Conventional Free Radical Polymerization
Conventional free radical polymerization is considered to be the easiest and
fastest way to synthesize a polymer. One of its advantages is that it is less
sensitive to media impurities compared to anionic polymerization. Conventional
free radical polymerization can be applied to a broad range of monomers. These
features significantly reduce the cost and make it easy to carry out. Conventional
free radical polymerization using ionic liquids as solvent has been reported to
produce PMMA with MW 8x1053 Da and higher. Rates of polymerization are
enhanced using ionic liquid compared to the ones using organic solvents. The
11
rapid polymerization rate and high MW is due to the diffusion-controlled
termination brought by ionic liquids.34 However PMMA synthesize by this method
has a broad Đ around 2, which can be a potential problem for its application such
as the block copolymer synthesis.
(b) Atom Transfer Radical Polymerization (ATRP)
In ATRP, side reactions can take place between any two of the reagents such as
monomer and catalyst, growing chains and solvent, etc. Side reactions occur
more often under elevated temperature by heterolytic cleavage of C-X bond in
initiator or oxidation of the radical to a carbocation.35, 36 PMMA with molecular
weights ranging from 1 kDa to 150 kDa have been successfully synthesized by
ATRP.37Past attempts to achieve high MW PMMA by ATRP showed that when
the MW was over 100 kDa, Đ was greater than 1.5. Termination and other side
reactions become more and more prominent when a higher MW is targeted.
(c) Activators Regenerated by Electron Transfer (ARGET) Atom Transfer Radical
Polymerization (ATRP)
Some high MW polymers, which cannot be produced by ATRP, can be
successfully synthesized by ARGET ATRP.38 It is mainly because the side
reactions are reduced because of the drastically smaller amount of Cu (II) species
required. The suppression of the side reactions between chain end and catalyst
improves the control over the chain-end functionality. To synthesize high MW
PMMA, high initiator efficiency and low extents of chain transfer and termination
reactions are required. Several methods have been reported to synthesize high
MW PMMA by ATRP. The high-pressure method is one of these methods. Under
12
high
pressure,
the
propagation
rate
is
enhanced
and
termination
is
suppressed.39,40 Figure 1.1 schematically shows the synthesis of high molecular
weight PMMA by high pressure AGET ATRP method.
13
图 1 Figure 1.1 Scheme of high-pressure AGET ATRP method
Figure 1.1 Scheme of high-pressure AGET ATRP method.39 Reproduced with the
permission from ref 39.
14
The MW of up to 850 kDa with narrow Đ around 1.11 was reported by
polymerization under high pressure. However this method requires the
high-pressure conditions not routinely available.
Without using high pressure, purchasing or synthesizing an efficient initiator also
make the synthesis high MW PMMA achievable.41 Initiators such as
2-bromo-2-methylpropionate (BMPE) have been used to synthesize PMMA with a
MW of 300 kDa and Đ as low as 1.2. However the preparation of the initiator
requires complicated synthesis step or high costs to purchase. Another possible
route to synthesize high MW PMMA employs multiple steps of isolation and
purification of the polymer at intermediate stages and uses a macroinitiator for
chain extension. PMMA with MW of 100 kDa and narrow Đ of 1.15 has been
achieved by chain extension method. However the macroinitiator containing dead
chains may remains in the polymer, which can be problematic, such as for block
copolymer synthesis.42
15
CHAPTER II
SYNTHESIS OF HIGH MOLECULAR WEIGHT PMMA
2.1 Motivation
High MW poly (methyl methacrylate) PMMA gain more and more attention and
put in to applications such as preparation of electrolyte4 and microfluidics.5
Synthesis of high MW PMMA can be carried out both in anionic polymerization13
and free radical polymerizations43. However the strict experimental conditions for
anionic polymerization make it more difficult to carry out in a lab. The preferred
route to synthesize high MW PMMA is by free radical polymerization. It has been
observed that when target MW if over 100 kDa, side reactions and terminations
become more pronouncing that causes broad Đ.37 ARGET (activator regenerated
by electron transfer) ATRP has been investigated by Matajaszewski’s group and
provides another possible route to achieve high MW.39 It is reported that high MW
PMMA can be synthesized using ARGET ATRP by using the high pressure up to
6 kbar44, efficient initiator41, or multiples steps including a chain extension
process.42 Polyethylene glycol (PEG) is a new solvent that has been used in
ARGET ATRP.45 Use of PEG as solvent efficiently increase reaction rate
andreduce terminations and side reaction. As these effects are similar to those
needed for the synthesis of well-defined, high MW polymers method to synthesize
high MW PMMA by ARGET ATRP using PEG as a solvent was investigated in
16
this thesis. A systematic study on the effect of reagents was performed to
understand the best conditions for the synthesis of high MW PMMA. This includes
the choice of solvent, catalyst and ligand.
2.2 Introduction
In order to synthesize high MW polymer, high propagation rate and suppressed
termination are crucial. The following methods have been used to synthesize high
MW PMMA by ATRP.
2.2.1 High Molecular Weight PMMA Synthesis Methods
High pressure ATRP has been demonstrated to synthesize high MW PMMA with
low Đ. ATRP of MMA under pressures up to 500 MPa has been carried out with
small amount of metal complex. The resulting PMMA has a MW of 3600 kDa and
Đ as low as 1.24.40 Also reversible additional-fragmentation chain transfer (RAFT)
polymerization under high pressure has been reported.46 47 PMMA with a MW of
up to 1250 kDa with Đ lower than 1.2 has been successfully synthesized with
reaction times less than a few hours by high pressure RAFT. The main purpose of
high pressure is to increase the propagation rate. The polymerization becomes
faster even though the concentration of free radicals is low.
However the high-pressure method is not always helpful. For example, PMMA
synthesized from high pressure RAFT has a high Đ of 1.61 when MW is 150 kDa.
The reason is considered to be a decreased chain-transfer constant to the RAFT
agent or that the gel point is reached.48 The reaction apparatus used in
17
high-pressure polymerization is rarely available in lab and it may cause safety
issue. These drawbacks make the higher-pressure method difficult to implement.
Recently high MW PMMA was synthesized by ARGET ATRP at ambient
pressure.38 The MW of PMMA can reach over 1000 kDa with a narrow Đ of 1.2.
This method includes cumyl dithiobenzoate (CDB) as the initiator/chain transfer
agent and copper powder as the reducing agent. It was demonstrated that the
catalysts in ATRP and radicals could activate alkyl dithioesters acting as an alkyl
pseudohalides ATRP initiaitor.49, 50 A drawback to this method is that
cumyldithiobenzoate, while commercially available, is still extremely expensive.
Another possible route to synthesize high MW PMMA is to enhance the initiator
efficiency. An efficient initiator results in a fast initiation rate and a reduced
termination rate, which makes the synthesis of high MW PMMA in one-pot
possible. PMMA with MW higher than 350 kDa with Đ as low as 1.2 is synthesized
using phenyl 2-bromo-2-methyl propionate (BMPE) as an initiator by ATRP.41 As
it is observed the kinetics follow a linear relationship with monomer concentration
indicating termination and side reactions are negligible.41 However, additional
steps need to be taken to synthesize initiator.
High MW PMMA can be synthesized with a new initiation system through a
reverse ATRP process. The new initiation system consisting of
1,1,2,2-tetraphenyl-1, 2-ethanediol (TPED)/FeCl3/PPh3 enables the synthesis of
PMMA with MW of 172 kDa and Đ as low as 1.13. The decomposition of the
initiator TPED causes the formation of monomer radicals. Propagation processes
18
involves the reaction between monomer and the monomer radicals. The
subsequent propagation process obeys the ATRP mechanism. PMMA
synthesized using this method is with α-hydrogen and ω-chlorine atom end
groups. Polymerization initiated with TPED alone provides much broader Đ
because of the terminations between propagating species. So the bi-end group is
thought to be responsible for the good control over high MW.42 Similarly,
synthesis of new initiator takes additional steps.
Synthesis of high MW PMMA also can be carried out in multiple steps. PMMA
with MW of 100 kDa and narrow Đ of 1.15 has been achieved by a chain
extension method. However the macroinitiator containing dead chains may be
remaining in the polymer after the chain extension process. This drawback makes
a problem for the further application of synthesize polymer.42
Each of the previously described methods is able to synthesis high MW PMMA,
however they either require specialized reaction equipment, non-commercially
available chemicals, or multiple step reactions. Therefore, an alternative method
using simple reaction equipment, one-pot conditions, and inexpensive,
commercially available materials is desirable.
2.2.2 Use of PEG as Solvent
ARGET ATRP can be carried out in both solutions and bulk. It also can be
conducted in homogenous or heterogeneous reaction conditions.25 The choice of
solvent therefore affects the polymerization kinetics. According to literature,
anisole, acetonitrile, and DMF etc. have been widely used as solvent in ARGET
19
ATRP.51 These solvents are hazardous and require careful handling for recycling
and disposal. Meanwhile some catalyst such as CuBr2 has poor solubility in these
solvents. These problems are solved by low MW polyethylene glycol (PEG).
The first use of PEG as solvent in organic reaction was carried out in the Heck
reaction.52 By comparing PEG with other conventional solvents such as DMA,
DMSO, CH3CN, PEG is unique in producing a single regioisomer with 80/20 E/Z
diastereoselection. They found that PEG can not only act as solvent but also a
ligand that helps with the formation of C-C bond.52
The use of PEG as solvent for ATRP polymerization was first carried out in the
polymerization of MMA mediated by Cu(II). The polymerization rate in PEG-400
was faster compared with the polymerization in conventional solvent (e.g.
toluene). The explanation is considered to be the metal complexation by PEG and
ligand and the polarity of the solvent. With the increase of solvent polarity, the
polymerization rate increases. The catalyst favors to form a more ”loose” structure
in solvent with higher polarity.53 Also the terminal hydroxyl group in PEG-400
could potentially enhance the polymerization rate.54
Based on the application of PEG as solvent in polymerization, PEG-600 was
firstly used as solvent in AGET ATRP of MMA with TMEDA acting as ligand and
reducing agent. 10 kDa PMMA with Đ lower than 1.2 was successfully
synthesized and is confirmed the living characterization of polymerization.55
20
Interestingly PEG cannot only act as a solvent, but also as a ligand. ATRP of
MMA using iron catalyst was carried out in PEG without additional ligand. Iron
chloride formed homogenous metal complex with PEG. The effect of PEG
structure and MW was investigated. However the Đ is above 1.30. The
asymmetric GPC trace of the macroinitiator leads to an unsatisfying chain
extension.56
Similarly, AGET ATRP of MMA was carried out in PEG using FeCl3 as catalyst
without additional ligands. Compared with other polar solvents such as DMF,
MeCN and DMSO, PMMA obtained in PEG exhibited a higher reaction rate and
lower Đ, which indicates higher initiator efficiency in PEG. Side reactions were
also suppressed using PEG as solvent. As the amount of PEG increased, the
reaction rate increased, plateaued, and then decreased. The explanation is
consider to be a comprehensive result of stronger complexing ability between
catalyst and PEG as ligand, better solubility of catalyst in PEG and lower radical
concentration. They also investigated that the reaction kinetics has a PEG MW
dependency. As PEG MW increases, the conversion of MMA increases then
decreases in the same reaction time.45 With the increase of PEG MW,
microviscosity also increases. PMMA chains are not fully solvated in high
microviscosity PEG.57 So some polymers stop growing at certain chain length. It
has been determined that polymerization rate decreases with the increase of
solvent microviscosity.58, 59, 60, 61 The stability constants of PEG-metal complex
increased with the PEG MW. As MW of PEG increases, PEG forms more stable
21
metal complex and helps produce more free radical to speed up the reaction.62, 63,
64, 65
2.2.3 Use of PEG as Solvent to Synthesize High Molecular Weight PMMA
PEG has been used to produce PMMA with good control by ARGET ATRP.
However to the best of our knowledge, no work has been done about using PEG
as solvent producing high MW PMMA by ARGET ATRP. For the synthesis of high
MW PMMA, high initiator efficiency, suppressed termination and decreased side
reaction are the crucial points. According to previous work, PEG is able to
produce high propagation rate and stabilize the catalyst by complexing. Therefore
the synthesis of high MW PMMA with PEG as the solvent was investigated. Prior
to the discussion of these results, the next two sections provide background
information on the catalyst and ligand systems used in these studies.
2.2.4 Copper Based Catalyst
In ATRP, the salt of a transition metal, such as copper, iron and nickel, are widely
used as a catalyst. A copper catalyst is superior because of its low cost. Copper
salts with different halide counter-ions exhibit a different structure when combined
with a ligand which affects the propagation rate of the polymerization.
It was established earlier that mixed halide initiator/catalyst system ethyl
α-bromoisobutyrate (EBiB)/CuCl provides better control compared to pure halide
initiator system EBiB/CuBr in ATRP of MMA in nonaqueous media. In the initiator,
the C-Br bond is weaker than the C-Cl bond, which leads to a faster initiation rate.
22
Figure 2.8 shows the X-ray structure of two different catalysts. The Cu-Br bond is
longer and more stable than Cu-Cl bond, which leads to a faster exchange rate
with initiator and form a more stable complex.66, 67 The propagation rate of the
polymerization is slower when the polymer chain end contains the stronger C-Cl
bond. In addition the C-Br bond would causes unfavorable side reactions.68 In
contrast with those results, it has been observed that ATRP of MMA using the
EBiB/CuBr initiation system gives better control over the EBiB/CuBr system in
aqueous media. Considering the stability of complexes in aqueous media, the
loss of halide ion from the complexes will be greater in the CuCl system. The loss
of ions leads to a decreased concentration of deactivator and a decreased
deactivation rate. The Cu2+ generated by the loss of ion causes oxidation
termination.69 In addition, it is argued that the α-methyl group in MMA imposes a
steric hindrance in Br atoms transfer.70, 71, 72
23
图 2 Figure 2.1 X-ray structure of CuCl2 and CuBr2
Figure 2.1 X-ray structure of CuCl2 and CuBr2.67 Reproduced with the permission
of ref 67.
24
2.2.5 Nitrogen Based Ligand
For ARGET ATRP, a suitable combination of catalyst and ligand is used to
establish a dynamic equilibrium between growing radicals and dormant chains.
The ligand is crucial for the solubility of catalyst and stability of the metal complex.
It determines the activator and deactivator concentration during the reaction,
which has great influence on the polymerization rate. Interestingly, it is reported
that 2-(dimethylamino) ethyl methacrylate (DMAEMA) can be synthesized by
ARGET ATRP without adding a reducing agent. This is because DMAEMA can
reduce the catalyst to its low oxidation state to serve as an intrinsic reducing
agent.73, 74 Based on this, some nitrogen based ligands e.g.
tetramethylethylenediamine (TMEDA)
N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA) can also serve as
reducing agent.32 Matyjaszewski’s group has shown the UV-visible spectra to
detect the concentration of Cu (II) with these two ligands in figure 2.2.25
25
A
!
B
!
图 3 Figure 2.2 UV-visible spectra of Cu (II) Br2 with (A) TMEDA, (B) PMDETA
Figure 2.2 UV-visible spectra of Cu (II) Br2 with (A) TMEDA, (B) PMDETA. In
acetonitrile at 25℃ ([Cu(II)Br2]/[ligand]=5/50 mmol L-1).25 Reproduced with the
permission of ref 25.
26
The decrease of the concentration of Cu (II) is caused by the reduction by
reducing agent. A faster decrease rate is observed in TMEDA than in PMDETA.
Thus TMEDA has stronger reducing ability compared to PMDETA.25
It was previously reported that the activities of complexes with ligands increases
with the increase of nitrogen numbers.75 The equilibrium constants affected by
various complexes are measured and confirmed such observation.65 Some other
factors such as steric and electronic effects and bite angle also have a effect on
the equilibrium constants.76
2.3 Experimental Section
2.3.1 Materials
Methyl methacrylate (MMA; Aldrich, 99%) was passed through a column filled
with alumina to remove the inhibitor. Ethyl 2-bromoisobutyrate (EBiB; Aldrich,
98%), Cu (II) Br2 (Aldrich, 99%), Cu (II) Cl2 (Aldrich, 99%),
N,N,N’,N’-tetramethylethylenediamine (TMEDA; Aldrich, 99%),
N,N,N’,N’,N-pentamethyldiethylenetriamine (PMDETA; Aldrich, 99%),
polyethylene glycol 200, 600, 1000, 2000 (Alfa), anisole (Aldrich, 99.9%),
acetonitrile (Aldrich, 99.8%) , methanol (sigma), tetrahydrofuran (THF)(sigma)
were used as received.
27
2.3.2 ARGET ATRP of MMA in PEG (600) with PMDETA as Reducing Agent and
Ligand
In a typical experiment, CuBr2 (0.05 mmol, 0.011 g) and MMA (0.05 mol, 5 g)
were dissolved in 5ml of PEG (600 Da)/anisole/acetonitrile in 25ml round bottom
flask with a stirring bar. EBiB (0.05mmol, 0.00734 ml), PMDETA (0.1mmol, 0.02ml)
was added and purged with nitrogen gas for 30 min. The reaction flask was
sealed by rubber stopper and placed in a pre-heated thermal stage to avoid
temperature gradient at 80°C. Set the stirring speed as 500r/min. After a varying
reaction time ranging from 2 h to 21h, the flask was placed out of the thermal
stage and cooled down to room temperature. The product was dissolved in 10ml
THF and subsequently precipitated in 100 mL distilled water. Then the
precipitation was re-dissolved in 20ml THF and precipitated by 100ml distilled
water. After 3 times re-dissolving and precipitation the product was dried in a
vacuum oven at 85°C overnight.
2.3.3 Characterization of PMMA
The molecular weight and molecular weight dispersity was measure by gel
permeation chromatography (GPC). It was equipped with three columns in Water
Breeze system. The eluting solvent was THF at 35 °C with an elution rate of 1.0
ml/min. The GPC was calibrated by polystyrene (PS) standards. Molecular weight
-4
-1
of PMMA was calculated by Mark-Houwink parameters. (PS: K=1.1x10 dL g ,
-1
-4
a=0.716; PMMA: K=9.94x10 dL g , a=0.719)77.
28
2.4 Results and Discussion
2.4.1 Scheme of Reactants
2.1 Reaction Equation of ARGET ATRP of MMA
O
Br
O
+
O
CuBr 2/CuCl2 , PMDETA/TMEDA
anisole/acetonitrile/PEG 80°C
O
O
O
O
Br
O
n
公式 4 Scheme 2.1 Reaction equation of ARGET ATRP of MMA
Scheme 2.1 Reaction equation of ARGET ATRP of MMA.
29
A
N
B
O
H
n OH
C
O
公式 5 Scheme 2.2 Chemical structures of (A) acetonitrile, (B) PEG, and (C)
anisole
Scheme 2.2 Chemical structures of (A) acetonitrile, (B) PEG, and (C) anisole.
30
A
N
N
N
B
N
N
公式 6 Scheme 2.3 Chemical structure of (A) PMDETA, (B) TMEDA
Scheme 2.3 Chemical structure of (A) PMDETA, (B) TMEDA.
31
2.4.2 Effect of Solvents
ARGET ATRP of MMA is carried out in three different solvents: anisole,
acetonitrile and PEG-600. With a target MW of 100 kDa, the molar ratio of [MMA]0:
[EBiB]: [CuBr2]: [ligand]=1000:1:1:2 were kept constant. Within the same reaction
time, MW and Đ are measured. The polymerization results are shown in table 2.1
and 2.2. The solubility of CuBr2 test is carried out in these three solvents as
presented in Fig 2.3.
32
Table 2.1. Effect of solvents on the ARGET ATRP of MMA using PMDETA.
Solvent
Mn (kDa)
Acetonitrile
Đ
No precipitation
Anisole
20
1.30
PEG 600
96
1.21
表格 1 Table 2.1. Effect of solvents on the ARGET ATRP of MMA using PMDETA
Polymerization conditions: [MMA]0: [EBiB]: [CuBr2]: [PMDETA]=1000:1:1:2,
VMMA=VPEG-600=5ml, T=80℃, t=3h; EBiB: ethyl α-bromoisobutyrate, PMDETA:
N,N,N’,N’,N-pentamethyldiethylenetriamine.
Table 2.2 Effect of solvents on the ARGET ATRP of MMA using TMEDA.
Solvent
Mn (kDa)
Đ
Acetonitrile
81
2.12
Anisole
16
1.30
PEG 600
98
1.34
表格 2 Table 2.2 Effect of solvents on the ARGET ATRP of MMA using TMEDA
Polymerization conditions: [MMA]0: [EBiB]: [CuBr2]: [TMEDA]=1000:1:1:2,
VMMA=VPEG-600=5ml, T=80℃, t=3h; EBiB: ethyl α-bromoisobutyrate, TMEDA:
N,N,N’,N’-tetramethylethylenediamine.
A
B
33
C
图 4 Figure 2.3 Solubility test of CuBr2 in (A) acetonitrile, (B) PEG-600, and (C) anisole
Figure 2.3 Solubility test of CuBr2 in (A) acetonitrile, (B) PEG-600, and (C) anisole.
0.3mmol CuBr2 (0.06g) and 5 mL of solvent.
34
Tab 2.1 shows that a constant reaction time, polymerization in PEG-600 almost
reaches 100% conversion while Đ stays low at 1.21. Polymerization in anisole
ends up with low conversion with 1.30 Đ suggesting a low polymerization rate.
Polymerization in acetonitrile almost has no polymer produced. After the
replacement of PMDETA by TMEDA, a similar result is found. Polymerization in
PEG-600 ends up with nearly 100% conversion with 1.34 Đ. Although an 81 kDa
MW is observed in acetonitrile, the broad Đ beyond 2 indicates a poor control.
The polymerization in anisole also has low conversion as with PMDETA.
As Figure 2.3 shows, CuBr2 has better solubility in acetonitrile and PEG-600 than
anisole. The polymerization in PEG-600 and acetonitrile are homogeneous. The
polymerization in anisole is heterogeneous with some insoluble salt remaining in
the bottom. Good solubility leads to a higher concentration of deactivator in the
reaction. This provides a better control and faster reaction rate. Although the
copper catalyst is fully dissolved in acetonitrile, it is noted that the color of the
acetonitrile solution is different from the PEG-600 solution.
The polarity of acetonitrile is the strongest followed by PEG-600 and then anisole.
Although polar solvent helps to dissolve the catalyst, the catalyst poisoning by the
solvent and some solvent assisted side reactions is more pronounced in more
solvent.37 Therefore, among the three solvents tested it appears that PEG
provides a balance between fast polymerization and MW control.
Effect of PEG Molecular Weight
35
ARGET ATRP of MMA using a series of PEG with molecular weights ranging from
200 to 2000 Da were carried out. The MW, Đ, and conversion were measured as
a function of time. The MW vs. time is shown in Figure 2.4, the kinetic plots are
shown in Fig 2.5, and Ð vs. time is shown in Figure 2.6.
36
图 5 Figure 2.4 Molecular weight evolution affected by PEG molecular weight
Figure 2.4 Molecular weight evolution affected by PEG molecular weight.
Polymerization conditions: [MMA]0: [EBiB]: [CuBr2]: [PMDETA]=1000:1:1:2,
VMMA=VPEG0=5ml, T=80; EBiB: ethyl α-bromoisobutyrate, PMDETA:
N,N,N’,N’,N-pentamethyldiethylenetriamine
37
图 6 Figure 2.5 Kinetic plots affected by PEG molecular weight
Figure 2.5 Kinetic plots affected by PEG molecular weight. Polymerization
conditions are identical to those in Figure 2.2.
38
图 7Figure 2.6 Molecular weight dispersity (Đ) affected by PEG molecular weight
Figure 2.6 Molecular weight dispersity (Đ) affected by PEG molecular weight.
Polymerization conditions are identical to those in Figure 2.4.
39
Fig. 2.5 shows that with the increase of PEG MW from 200 to 600 Da, the reaction
rate increases. The highest reaction rate was observed in PEG-600. For PEG
molecular weights above 600 Da, there is decrease in the reaction rate with
increasing PEG MW. The slowest reaction rate was observed with PEG 2000.
The highest conversion was also achieved by using PEG-600. Kinetic plots exhibit
first order, which would be expected for controlled polymerizations. The Đ stays
low at ca. 1.2 except for polymerization in PEG-1000. Đ in PEG-1000 is greater
than 1.3. Optimum solvent is observed as PEG-600 which helps achieve 96 kDa
MW with Đ as low as 1.2 within 2 hours.
This is consistent with the competing effects of viscosity, complex stability
constant and end group concentration. With the increase of PEG MW, viscosity
increases. PMMA chains are not fully solvated in highly viscous media and some
stop growing at certain chain length.57 However the PEG-metal complex stability
constant increases with PEG MW. In lower MW PEG, it prefers to form single
metal crystal. While the higher MW PEG helically wrap the metal center.62, 63, 64, 65
As it is reported before the terminal hydroxyl group in PEG could potentially
enhance the polymerization rate.54 Increase of PEG MW brings a decreased
concentration of hydroxyl group. As a comprehensive result, polymerization rate
increases from PEG-200 to PEG-600 but then decreases with increasing PEG
MW.
40
2.4.3 Effect of Catalyst
ARGET ATRP of MMA using CuBr2 or CuCl2 as a catalyst were carried out with
molar ratio of [MMA]0 :[EBiB]: [CuBr2]/[CuCl2]: [TMEDA]=1000:1:1:2. The MW, Đ,
and conversion were measured as a function of time. The MW vs. time is shown
in Figure 2.7, the kinetic plots are shown in Fig 2.8, and Ð vs. time is shown in
Figure 2.9.
41
图 8 Figure 2.7 Molecular weight evolution affected by catalyst
Figure 2.7 Molecular weight evolution affected by catalyst. Polymerization
conditions: [MMA]0 : [EBiB]: [CuBr2]/[CuCl2]: [TMEDA]=1000:1:1:2,
VMMA=VPEG-=5ml, T=80; EBiB: ethyl α-bromoisobutyrate, TMEDA:
N,N,N’,N’-tetramethylethylenediamine
42
图 9 Figure 2.8 Kinetic plots affected by catalyst
Figure 2.8 Kinetic plots affected by catalyst. Polymerization conditions are
identical to those in Figure 2.7.
43
图 10 Figure 2.9 Molecular weight dispersity (Đ) affected by catalyst
Figure 2.9 Molecular weight dispersity (Đ) affected by catalyst. Polymerization
conditions are identical to those in Figure 2.7.
44
From figure 2.8, CuBr2 gives a faster reaction rate than CuCl2. The conversion
reaches 98% with CuBr2. The MW reaches 98 kDa in CuBr2, Đ in CuBr2
decreases at first than remains stable at 1.3. For the polymerization in CuCl2,
conversion can reach nearly 100%. Đ increases with the conversion. The reaction
rate is slower than CuBr2. Đ goes up to 1.7 at the end of reaction, which indicates
poor control.
This result is consistent with that the weaker C-Br bond leads to a faster
generation rate.37 Also the higher trend of halide ion loss in CuCl system leads to
a decreased concentration of deactivator and a decreased deactivation rate.
Hence side reactions and oxidation termination becomes more prominent in CuCl
system.62, 69
2.4.4 Effect of Ligand
ARGET ATRP of MMA using TMEDA/PMDETA as ligand and reducing agent
were
carried
out
with
ratio
of
[MMA]0:
[EBiB]:
[CuBr2]:
[TMEDA]/[PMDETA]=1000:1:1:2. The MW, Đ, and conversion were measured as
a function of time. The MW vs. time is shown in Figure 2.10, the kinetic plots are
shown in Fig 2.11, and Ð vs. time is shown in Figure 2.12.
45
图 11 Figure 2.10 Molecular weight evolution affected by ligand
Figure 2.10 Molecular weight evolution affected by ligand. Polymerization
conditions: [MMA]0: [EBiB]: [CuBr2]: [PMDETA]=1000:1:1:2, VMMA=VPEG-600-=5ml,
T=80; EBiB: ethyl α-bromoisobutyrate, PMDETA:
N,N,N’,N’,N-pentamethyldiethylenetriamine, TMEDA:
N,N,N’,N’-tetramethylethylenediamine
46
图 12 Figure 2.11 Kinetic plots affected by ligand
Figure 2.11 Kinetic plots affected by ligand. Polymerization conditions are
identical to those in Figure 2.10.
47
图 13 Figure 2.12 Molecular weight dispersity (Đ) affected by ligand
Figure 2.12 Molecular weight dispersity (Đ) affected by ligand. Polymerization
conditions are identical to those in Figure 2.110.
48
Figure 2.9 shows polymerization rate is faster when using PMDETA compared to
TMEDA. The MW can reach 96 kDa with Đ as low as 1.2 within 2 hours. The first
order kinetic plots shows that the concentration of growing radical keeps constant
during the reaction. Đ remains stable near 1.2 during the whole reaction. With the
good control, the equilibrium between growing radical and the dormant species is
established during both the early stage and the later process. In the TMEDA
ligand system, MW can reach 90 kDa with Đ of 1.4. Đ decreases with conversion
at the beginning and remains stable near 1.4 to the end.
This is consistent with the fact that activities of complexes with ligands increases
with the increase of nitrogen numbers.72 PMDETA as a tridentate ligand brings a
higher equilibrium constant than TMDETA as a bidentate ligand.63
2.5 Conclusions
High MW PMMA (96 kDa) with narrow Đ of 1.2 was successfully synthesized
using ARGET ATRP method using PEG as solvent and PMDETA as ligand and
reducing agent. Compared with anisole and acetonitrile, PEG can provide the
fastest polymerization rate and best control. The reason is that copper catalyst
has good solubility in PEG and PEG can stabilize the metal catalyst by
complexing. By varying the MW of PEG, there was significant change in the
reaction kinetics. As PEG MW increases, viscosity also increases which makes
poor polymer chain diffusion. The PEG-metal stabilization constant increases with
PEG MW. The polymer diffusion ability and stabilizing ability leads to competing
effects in the reaction kinetics. The optimum solvent was observed to be PEG-600.
49
The structure of the catalyst also have strong effect on the polymerization, CuBr2
provides faster reaction rate than CuCl2 due to the Cu-Br bond. For the effect of
the ligand, PMDETA has stronger coordination ability than TMEDA. The
first-order kinetic plot of PMDETA indicates a stronger coordination ability of
PMDETA with the copper catalyst.
50
CHAPTER III
SYNTHESIS OF PMMA-PS BY ARGET ATRP
3.1 Introduction
PMMA-PS block copolymers have unique properties compared to their
constituent homopolymers. The self assembly behavior of PMMA-PS block
copolymer has been comprehensively studied and put into use for lithography.78,
79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89
PMMA-PS block copolymers are synthesized through
various polymerization methods. The strict experimental conditions for anionic
polymerization make it more difficult to carry out. Controlled free radical
polymerization such as RAFT, NMP and ATRP are desired to make block
copolymers. ARGET ATRP was investigated using a less oxygen sensitive
catalyst and a reducing agent, which successfully solve the oxygen sensitive
problem in ATRP.
To synthesize a PMMA-PS block copolymer by ARGET ATRP, a PMMA
macroinitiator with an active –X halogen chain end is needed.90 To conduct a
well-controlled block copolymer polymerization by ARGET ATRP, the initiation
rate should be greater than the propagation rate. High initiator efficiency is
needed to produce narrow molecular weight distribution block copolymer. If there
51
is low initiator efficiency, the added monomer prefers to form homopolymer rather
then chain extends with the macroinitiator. This leads to a bimodal peak of
molecular weight distribution and homopolymer remaining in the block copolymer.
High MW PMMA was successfully synthesized by ARGET ATRP and reported in
the last chapter. This chapter focuses on the synthesis of PMMA-PS using PMMA
as macroinitiator. Before chain extension, some PMMA synthesized has broad Đ
and non-symmetric peak of GPC traces. To narrow down the Đ of PMMA,
fractional precipitation method was used to separate different MW PMMA. Then
bulk polymerization was used for chain extension. The high MW PMMA has long
chain, which diffused slower in the solution compared to low MW monomer. Thus,
selecting the suitable amount of styrene to obtain a low solution viscosity is
essential. Different amount of styrene were used to find the best concentration for
polymerization.
3.1.1 Fractional Precipitation
Some of the synthesized PMMA have a relatively high Đ, which is around 1.5 and
exhibit non-symmetric GPC peak. It is a potential problem for further PMMA-PS
synthesis. To separate the low MW polymer from the high MW polymer, fractional
precipitation is introduced as follows.
The basic idea of this method is that solubility of polymer has a dependency on
the chain length if the structure of the polymer is kept the same. The longer chain
52
has the less solubility than the shorter chain. In other words, the high MW polymer
is less soluble than the low MW polymer. There are several explanations for this
phenomenon. The earliest theory is proposed by Schulz. He proposed that the
solubility of the polymer in the solution depends on the relative energies in the
polymer phase and solvent phase and the Boltzmann probability.91 The
thermodynamics has been studied to further confirm this theory. To quantify the
thermodynamics, the entropies and heats of mixing are calculated in the two
phases. They consider the aggregation of precipitation is a reversible reaction.
To separate different MW polymer, firstly a good solvent is selected to dissolve
the sample. Then a poor solvent is added dropwise into the solution until the
solution becomes cloudy. High MW polymer precipitates first while low MW
polymer stays in the solvent. Then the cloudy solution allowed to settle overnight
until it phase separates into two phases. Thus, different MW polymers are
separated.
53
图 14 Figure 3.1 Process of fractional precipitation
Figure 3.1 Process of fractional precipitation.92 Reproduced with the permission of
ref 92.
54
Figure 3. 2 and table 3.1 show the result of fractionation. After fractionation, broad
peak is divided into sharp peaks with decreased Đ. Different MW polymer is
separated.
图 15 Figure 3.2 GPC traces of PMMA (a) before fractionation, (b) after fractionation
55
Figure 3.2 GPC traces of PMMA (a) before fractionation, (b) after fractionation.93
NF, F1, F2 and F3 are corresponding to the polymer table 3.1. Reproduced with
the permission of 93.
Table 3.1 Molecular weight (Mn) and Đ (PDI) before and after fractionation.93
图 16 Table 3.1 Molecular weight (Mn) and Đ (PDI) before and after fractionation
Reproduced with the permission of 93.
The fractional precipitation also can be carried out through a re-heating process.
Here, the cloudy solution is heated and then cooled down to the room
temperature and allowed to phase separate. The heating process is supposed to
get narrower Đ polymer.
56
3.2 Experimental Section
3.2.1 Materials
Styrene (St; Aldrich, 99%) was passed through a column filled with alumina to
remove the inhibitor. PMMA (15-160kDa) with Đ of 1.1-1.5 was synthesized by
ARGET ATRP. CuBr2 (Aldrich, 99%), N,N,N’,N’,N-pentamethyldiethylenetriamine
(PMDETA; Aldrich, 99%), methanol (sigma), tetrahydrofuran (THF) (sigma),
toluene (sigma) were used as received.
3.2.2 Fractional Precipitation
PMMA (5g) was dissolved in toluene (95g) in a 1000ml beaker with a stir bar to
make the 5%wt solution. Methanol was added by dropwise addition with a burette
until the solution became cloudy and the volume of added methanol was recorded
as volume V. The cloudy solution was transferred to a separation funnel, which
was left to stand overnight allowing the solution to phase separate. The polymer
rich layer was collected and dried.1/5V of methanol was added to the solvent-rich
layer to make it cloudy again and shake the separation funnel. The phase
separation and collection process was repeated. This separation process was
repeated until no polymer precipitated during methanol addition.
3.2.3 Chain Extension of PMMA Macroinitiator with Styrene
PMMA macroinitiator (Mn=96 kDa, PDI=1.23) (0.5g, 5.3µmol) prepared by
ARGET ATRP, CuBr2 (1.2mg, 5.3µmol) and PMDETA (2.2µL, 10.6mmol) were
57
dissolved in styrene monomer (20ml, 174mol) in a 25ml round bottom flask and
purged with nitrogen gas for 30 min. The reaction flask was sealed and placed in
a pre-heated reaction block at 90°C and stirred at 500r/min. After 24h, the flask
was removed from the heating block and cooled to room temperature. The
product was dissolved in 10ml THF and passed through a column filled with basic
alumina to remove the copper catalyst. The polymer was then precipitated from
solution by addition to 100 mL methanol. The precipitate was re-dissolved in 20ml
THF and precipitated by 100ml methanol. After 3 times re-dissolving and
precipitating, the product was dried in a vacuum oven at 85°C overnight.
3.2.4 Characterization of PMMA-PS
Đ is determined by gel permeation chromatography (GPC). It was equipped with
three columns in Water Breeze system. The eluting solvent was THF at 35 °C with
an elution rate of 1.0 ml/min. The GPC was calibrated by polystyrene (PS)
standards. 1H NMR spectra (Varian Mercury-300 MHz spectrometer) is used to
characterize the chemical structure of PMMA and its MW with deuterated
chloroform (CDCl3) as solvent at room temperature.
3.3 Results and Discussion
3.3.1 Fraction Precipitation of PMMA
Figure 3.3 shows that the original broad peak is divided into two sharper peaks.
Different MW PMMA is separated with Đ as low as 1.2. However some low MW
58
PMMA is lost since it is to dilute to precipitate out from the solution. The high MW
PMMA with low Đ is desired for the chain extension using styrene.
图 17 Figure 3.3 GPC traces of PMMA before and after fractionation
Figure 3.3 GPC traces of PMMA before and after fractionation.
Table 3.2 Molecular weight (Mn) and Đ before and after fractionation
Name
Mn (kDa)
59
Đ
Before Fractionation
137
1.34
Fraction 1
219
1.19
Fraction 2
180
1.20
表格 3 Table 3.2 Molecular weight (Mn) and Đ before and after fractionation
3.3.2 Effect of Styrene Amount
Figure 3.4 shows the GPC traces of PMMA-PS with different volume of styrene
added. Table 3.3 shows the MW, Đ and PS volume fraction of PMMA-PS. Figure
3.5 shows the NMR spectra of PMMA-PS.
60
图 18 Figure 3.4 GPC traces of PMMA-PS
Figure 3.4 GPC traces of PMMA-PS. Polymerization conditions: [PMMA]: [CuBr2]:
[PMDETA]=1:1:2, T=90℃,t=24h; (A) PMMMA macroinitiator (B) styrene 6ml, (C)
styrene 10ml, (D) styrene 20ml; PMDETA:
N,N,N’,N’,N-pentamethyldiethylenetriamine.
61
Table 3.3 Effect of styrene amount.
Amount
PDI
(kDa)
of Styrene
PMMA
MnNMR
PS
Volume Fraction
_
1.2
－
－
6ml
1.77
174
43%
10ml
1.82
577
82%
20ml
1.33
490
80%
（A）
PMMA-PS
（B）
PMMA-PS
（C）
PMMA-PS
（D）
表格 4 Table 3.3 Effect of styrene amount
Polymerization conditions are identical to Figure 3.4. Volume fraction of PS is calculated by
NMR with mass densities: PMMA 1.184 g/cm3; PS 1.05 g/cm3.
62
O
O
b
O
O
Br
n
a
b
a
图 19 Figure 3.5 NMR spectra of PMMA-PS
Figure 3.5 NMR spectra of PMMA-PS. Polymerization conditions are identical to
Figure 3.4 (D).
63
With decreasing styrene concentration, the GPC traces showed shoulders on the
right side of the polymer peak. This indicates that some PMMA macroinitiator was
prematurely terminated during the reaction. As more styrene was added the
shoulder disappeared. The peak also shifts to a high MW. Confirmed by NMR, the
peak appearing near 7 ppm is corresponding to the benzene group in PS.
PMMA-PS block copolymer is successfully synthesized with molecular weigh of
490 kDa and Đ of 1.33.
This can be explained by the PMMA chain diffusion. 96 kDa PMMA macroinitiator
has long chain, which is hard to diffuse in small amount of styrene. In small
amount of styrene, some PMMA chain is not fully diffused and prematurely
terminated.
3.3.2 Effect of PMMA Macroinitiator Molecular Weight
A series PMMA macroinitiators with different MW were used to synthesize
PMMA-PS. Table 3.4 shows the MW, Đ and volume fraction of PS of the block
copolymers synthesized.
64
Table 3.4 Effect of macroinitiator molecular weight.
PMMA Mn (kDa)
Mn PMMA-PS (kDa)
Đ PMMA-PS
PS Volume
Fraction
15
76
1.27
80%
40
225
1.29
82%
68
620
1.32
89%
96
490
1.33
80%
101
505
1.30
80%
161
770
1.32
79%
表格 5 Table 3.4 Effect of macroinitiator molecular weight
Polymerization conditions are identical to Figure 3.4 (D).
65
By keeping the other reaction conditions the same, as the PMMA MW increases,
the initiator concentration decreases. Within the same reaction time, volume
fraction of polystyrene is nearly the same. Đ are all under 1.35, which indicates
good control. In the synthesis of block copolymer using ARGET ATRP, the chain
extension is efficient only if the crosspropagation rate is faster than the second
block propagation rate. The propagation rates follow the order that
AN>MMA>St=MA.37 The addition of monomer to produce block copolymer obeys
this order, which enables the high initiator efficiency.
3.4 Conclusions
PMMA-PS was successfully synthesized by a PMMA macroinitiator using ARGET
ATRP method. Fractional precipitation was used to narrow down Đ of PMMA
macroinitiator. Bulk polymerization was used to chain extend PMMA with styrene.
When adding small amount of styrene, the PMMA chain cannot fully dissolve and
some PMMA macroinitiator was prematurely terminated during the reaction. As it
was observed, GPC traces of PMMA-PS block copolymer showed shoulder.
When the amount of styrene increased the shoulder disappeared and the peak
shifted in the high MW direction. 490 kDa MW PMMA-PS with 80% volume
fraction of polystyrene was obtained. Different MW of PMMA was used a
macroinitiator. By keeping the styrene amount the same, chain extension
polymerizations were all under good control. The MW of block copolymer reached
as high as 770 kDa with Đ as low as 1.32. The propagation rate of MMA was
higher than styrene. So the initiation rate is higher than the propagation rate in
66
chain extension. High initiator efficiency enabled the good control of block
copolymer synthesis.
67
CHAPTER IV
CONCLUSIONS
A facile and efficient ARGET ATRP was utilized to synthesize high MW PMMA.
PMMA with MW of 96 kDa and Đ as low as 1.2 was synthesize by this method.
Normal ARGET ATRP method does not enable the synthesize PMMA with MW
higher than 100 kDa efficiently because of the termination and side reactions. By
introducing low MW PEG as a solvent, the reaction rate was significantly
increased. The mechanism of PEG to enhance the polymerization rates arises
from the good solubility of copper catalyst in. Also PEG can stabilize catalyst by
complexing. The dependency on the polymerization results on the PEG MW was
also studied. As the PEG MW increased, the microviscosity increased which
leaded to slower diffusion of the polymer chain. At the same time, the coordination
ability of PEG to stabilize the catalyst increased with increasing MW PEG. These
two factors competitively affected the reaction kinetics and resulted in an optimum
PEG MW for polymerization. Then two copper catalysts were compared. The
shorter bond of Cu-Br leaded to a faster radical generation rate compared to
Cu-Cl. And two nitrogen-based ligand also acting as reducing agent were
discussed. PMDETA was weaker in reducing ability but stronger in coordination
ability than TMEDA.
68
Then fractional precipitation method was used to narrow down the Đ of
synthesized PMMA. 96 kDa PMMA with Đ 1.2 was used as macroinitiator to chain
extend with styrene in bulk polymerization by ARGET ATRP. The chemical
structure of PMMA-PS was characterized by NMR. Variation in the styrene
amount leaded to different Đ. Increasing amount of styrene made a environment
that enables PMMA chain diffusing better.
69
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