Methyl Methacrylate) by Activators Regenerated by Electron Transfer (ARGET) Atom Transfer Radical Polymerization (ATRP) Was Extensively Studied

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 Department Chair Dr. Kevin A. Cavicchi Dr. Sadhan C. Jana

______Advisor Dean of the College Dr. Bryan D. Vogt Dr. Eric J. Amis

______Committee Member Interim Dean of the Graduate Dr. Sadhan C. Jana Dr. Rex D. Ramsier

______Committee Member Date Dr. Alamgir Karim

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 Ka I II R-X+Cu X/L R+Cu X2/L Kda +M kt

K P II R-R+Cu X2/L

公式 2 Scheme 1. 2 Mechanism of ATRP

Scheme 1.2 Mechanism of ATRP.

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:

n+1 Đ= Mw/Mn = 1+1/DPn+ ([RX]0 kp/kdeact[Mt X/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

0 26 (EH)2), and Cu . 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 Ka I II R-X+Cu X/L R+Cu X2/L Kda +M kt

K P II R-R+Cu 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.

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

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.

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.

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

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

图 2 Figure 2.1 X-ray structure of CuCl2 and CuBr2

67 Figure 2.1 X-ray structure of CuCl2 and CuBr2. Reproduced with the permission of ref 67.

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

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

-1 25 acetonitrile at 25℃ ([Cu(II)Br2]/[ligand]=5/50 mmol L ). Reproduced with the

permission of ref 25.

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 ,

-4 -1 a=0.716; PMMA: K=9.94x10 dL g , a=0.719)77.

2.4 Results and Discussion

2.4.1 Scheme of Reactants

2.1 Reaction Equation of ARGET ATRP of MMA

O Br

O CuBr2/CuCl2 , PMDETA/TMEDA O + 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

O H OH n

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.

N N N

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.

Table 2.1. Effect of solvents on the ARGET ATRP of MMA using PMDETA.

Solvent Mn (kDa) Đ

Acetonitrile 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 C 33

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.

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.

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

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.

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.

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

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.

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.

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

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.

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.

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.

图 14 Figure 3.1 Process of fractional precipitation

Figure 3.1 Process of fractional precipitation.92 Reproduced with the permission of ref 92.

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.

93 Table 3.1 Molecular weight (Mn) and Đ (PDI) before and after fractionation.

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) 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 MnNMR PS

of Styrene (kDa) Volume Fraction

PMMA _ 1.2 －－

（A）

PMMA-PS 6ml 1.77 174 43%

（B）

PMMA-PS 10ml 1.82 577 82%

（C）

PMMA-PS 20ml 1.33 490 80%

（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 O b Br

O n

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).

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).

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.

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