Living Radical Polymerization - American

Chapter 19

Downloaded by NORTH CAROLINA STATE UNIV on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch019

Synthesis and Properties of Copolymers with Tailored Sequence Distribution by Controlled/Living Radical Polymerization 1

2

Jean-François Lutz , Tadeusz Pakula , and Krzysztof Matyjaszewski 1

1

Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213 Max-Planck-Institute for Polymer Research, Postfach 3148, 55021 Mainz, Germany

2

The control of the overall composition and the sequence distribution in copolymers with both linear and branched architectures is described. The synthesis of alternating and gradient copolymers as well as graft copolymers with statistical, gradient and block architectures was accomplished by controlled/living radical polymerization (atom transfer radical polymerization and reversible addition-fragmentation transfer polymerization). The properties of these tailored copolymers indicate that adjustment of sequence distribution at the molecular level has important effects on the macroscopic properties of the obtained polymeric materials.

268

© 2003 American Chemical Society

In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

269


Introduction Developing new materials with controlled properties is an important issue in polymer science. The macroscopic properties of polymeric materials depend on their nanoscale morphology and consequently on the macromolecular structure of the components. In that context, it is important to develop synthetic routes, which allow for precise control o f the shape and composition o f the macromolecules. Controlled/living radical polymerization (CRP) techniques (13) such as nitroxide mediated polymerization (NMP) (4), atom transfer radical polymerization (ATRP) (5-7) or reversible addition-fragmentation transfer polymerization (RAFT) (8) are powerful methods for macromolecular engineering. Several examples of well-defined macromolecules prepared by C R P with controlled chain length, polydispersities, compositions, functionalities and architectures have been reported in the literature (9,10). However, only a few studies describe the synthesis of copolymers possessing controlled comonomer sequences by C R P . In the case of both linear polymer chains and branched polymer chains, several types of sequence distribution are possible (Scheme 1). Among these various classes of polymers, some copolymers with special comonomer distributions such as gradient, periodic or alternating may exhibit unusual properties and consequently could provide a new avenue for the production of novel materials. Recent advances in controlling comonomer sequence distribution will be discussed and examples of linear (alternating or gradient) and branched

Scheme 1. Different types of comonomer sequences distribution

Linear polymers CXXXXXXXXXXXXXXO

MOOMOtOMOOtM

otototototototo*

Homopolymer

Random copolymer

Alternating copolymer

OOOOOOOOMMMM

Block copolymer

M M N O N M N O

Periodic copolymer

axaoototOMOM* Gradient copolymer

Branched polymers

Block brush copolymer

Heterogeneously distributed branches

Homogeneously distributed branches


270 copolymers with controlled comonomer sequences prepared by C R P will be presented. The thermomechanical properties of some of these copolymers will be also discussed.


Linear polymers with controlled sequence distribution Alternating copolymers: Alternating copolymers form a specific class of copolymers in which both comonomer units alternate in a regular fashion along the chain (11). The ability o f a comonomer pair to copolymerize spontaneously in an alternating fashion depends, mostly, on the polarity of the polymerizable double bonds. For example, acceptor monomers with a low electron density on the double bond preferentially react with a radical with an electron donor substituent rather than their own radical (assuming that the polarization of a radical is similar to a monomer). For example, maleic anhydride and styrene copolymerize spontaneously in an alternating fashion. However, for many comonomer pairs, the difference in polarity between acceptor and donor monomers is insufficient to result in alternation. In those cases, the tendency towards alternation may be enhanced by addition of a Lewis acid, including ahiminum halides (EtAlCl , E t A l C l , E t A l C l ) , and Z n C l , T i C l , BC1 or S n C l (12,13). The Lewis acid complexes with the acceptor monomers reduce electron density at the conjugated double bonds and favor alternating copolymerization. Only a few examples of well-defined alternating copolymers synthesized by CRP were reported in literature. Most of those studies reported copolymers from a combination of comonomers with a spontaneous tendency for alternation: a strong electron accepting monomer (maleic anhydride (14,15), iV-butylrnaleimide (16), or AZ-phenylmaleimide (17-19) ) and an electron donating monomer (styrene). Therefore, it was considered to be important to investigate the C R P synthesis of alternating copolymers from comonomer pairs without this inherent tendency. Two systems were investigated: A T R P of acrylates with isobutene, a less reactive electron rich monomer, and R A F T copolymerization of methyl methacrylate with styrene in the presence of Lewis acids. 2

2

3

2

3

2

4

3

4

ATRP Synthesis of well-defined alternating copolymers of acrylates and isobutene: Alternating copolymers based on acrylates (methyl acrylate ( M A ) or w-butyl acrylate (BA)) and isobutene (IB) are thermoplastic elastomers with interesting properties such as high tensile strength, high thermal decomposition temperature and good resistance to hydrolysis (20). When polymerized via a conventional radical process, copolymers from IB and acrylic esters exhibit a relatively low content of IB (-20%) and low molecular weights because of the degradative chain transfer of isobutene. However, alternating copolymers p(acrylate-afr-IB) containing -50% of IB were synthesized in the presence of Lewis acids such as aluminum or boron halides (21-23). No control of molecular weight and polydispersity was possible in these systems. In an earlier


271 communication, we reported that well defined copolymers p(MA-aft-IB) could be prepared by A T R P , using a large excess of IB with respect to M A in the feed (24). These copolymers exhibited controlled molecular weight (up to M„=50000 g.mol' ), relatively low polydispersities (MJM < 1.6) and high content of IB (-45%). Sen et al. also reported the synthesis of well-defined copolymers based on M A and non-polar alkenes (25). The copolymerization of B A with IB was also investigated i n the presence of several A T R P catalyst systems (Table I) (26). When the reaction was conducted in homogeneous systems; CuBr/4,4'-di(ter^butyl)-2,2'-bipyridine (dtBbpy) in 1,2-dimethoxybenzene solution, or CuBr/4,4'-di-(5-nonyl)-2,2'-bipyridine (dNbpy) in bulk, the polymerization was well controlled yielding polymers with molecular weights close to theory and low polydispersities (Table I, entry 1 and 2). 1

n


2

2

Table I: Copolymerization of B A and I B with various A T R P catalysts Conv. M„ exp FIB Mnth fcmor ) famoï ) BA(%) 1810 1530 24.1 DMB" 107 0.21 1 CuBr/dtBbpy 29.4 1870 1690 0.27 2 89 CuBr/dNbpy 9480 3380 112 0.36 3 68.9 CuBr/dtBbpy 19730 94 4 17.5 0.40* CuBr/bpy 4820 4520 0.41* 100 7 5 CuBr/PMDETA Experimental conditions: 50°C, catalyst/methyl 2-bromopropionate/BA/IB = 1/1/25/75 1,2 dimethoxybenzene ; obtained gravimetrically ; obtained by E NMR Catalyst

Solvent

Time

1

W

MJM

n

1

1.35 1.47 1.93 2.42 1.40

c

2

c

2

c

2

2

a

b

c

l

Limited B A conversions and a low fraction of isobutene in the final copolymer F were observed with these catalysts. This could be due to differences in the reactivities of the two dormant polymeric halides present in the copolymerization. Model studies confirmed this hypothesis (26), and indicated that the absence of resonance stabilizing group on the terminal carbon of pIB-Br chain end, resulted in a much slower activation by the copper catalyst o f this moiety than of the acrylate analogue. Therefore, during the copolymerization, the chains deactivated after addition of an IB unit could be considered as dead chains (at least comparatively to pMA-Br). In the presence of heterogeneous catalyst systems such as CuBr/bpy or CuBr/dtBbpy in bulk, due to the low solubility of the catalyst in the reaction mixture, polymerizations behave partially as a conventional radical process and much higher conversions and F\ were observed (Table I, entry 3 and 4). However, broad molecular weight distributions and significant discrepancies between experimental and theoretical molecular weights were observed. A compromise was found using a slightly heterogeneous and more active catalyst system C u B r / P M D E T A (Table I, entry 5). With this catalyst, well defined copolymers p(BA-co-IB) with controlled molecular weight, relatively low polydispersities and high content of IB (41%) m

2

2

B


272 were obtained. This value of F suggests that the obtained copolymers are not pure alternating copolymers, but could be represented by the sequences of alternating p(BA-afe-IB) sections randomly interrupted by short B A sequences. I B

Synthesis of well-defined alternating copolymers p(MMA-alt-S) by RAFT polymerization in the presence of Lewis acids. R A F T copolymerization of methyl methacrylate ( M M A ) and styrene (S) in the presence of a range of Lewis acids: tin (IV) chloride (SnCU), zinc chloride (ZnCl ), ethylaluminum sesquichloride (EASC) and diethylaluminum chloride, was studied at 60 °C and at 40 °C (Table II). Downloaded by NORTH CAROLINA STATE UNIV on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch019

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Table II: R A F T copolymerization of M M A and S with Lewis acids M exp Overall Lewis acid TemperatureTime (mn) Conversion fe-mot ) (°C) 2800 120 40 SnCl 10800 ZnCl 40 125 7600 60 15 ZnCl 24500 73% EASC 80 40 31500 EASC 60 9 20200 62.5% 60 100 Et AlCl n

1

a

1 2 3 4 5 6

4

2

2

2

MJM M» (zmot ) 2.50 1.80 3.74 1.22 23500 1.45 20000 1.38 b

n

th

1

Experimental conditions: Bulk, [S]o/[CDB] = [MMA]o/[CDB] =190 ; [Lewis acid]o/[M]o = 0.4 ; [CDB](/[azo-bis isobutyronitrile]o = 10 Measured by gravimetry M theoretical = ([M]o+[S]o)(cowv.)(104.15+100)/(2(2[AiBN]o+[CDB]o)) 0

0

0

h

n

R A F T was selected as the preferred C R P method, since Lewis acids interact less with the dithioester control agent than with A T R P ligands or nitroxide radicals. In the presence of strong Lewis acids (SnCL* and ZnCl ), poor control of the polymerization was observed (Table II, entry 1-3). These strong Lewis acids may form a complex with the cumyl dithiobenzoate (CDB) and may also lead to decomposition o f C D B . The alkylaluminum halide Lewis acids (EASC and Et AlCl) could also form complexes with C D B , as evidenced by the formation of an intense orange color (instead of the pink uncomplexed CDB). However, in the presence of E A S C at 40°C or E t A l C l at 60°C, the copolymerization of M M A with S exhibited a controlled/living behavior with experimental molecular weights close to the theoretical values and narrow molecular weight distribution (Table II, entry 4-6). In the presence of both aluminum halides, the kinetics of the copolymerization were much faster (about 40 times) that in the case of R A F T copolymerization without Lewis acids. This results from higher values of the cross-propagation rate constants in the presence of Lewis acid than in their absence. Lewis acid could also accelerate initiation (13,27). The comonomer sequence distribution of the synthesized copolymers was investigated by 600 2

2

2


273 !

M H z H N M R . The percentage of various methyl methacrylate centered triads was calculated from die integrations of the regions of absorption of the methoxy groups (28-30). The copolymers synthesized with E A S C or E t A l C l contain -90% alternating triads S - M M A - S , whereas the copolymers synthesized in the absence o f Lewis acid 50000 g.mol' , the copolymers had higher polydispersities (A/ /M ~1.5) and the experimental M were lower than theoretical values. In the presence of E A S C , copolymers with lower polydispersities (MJM

Living Radical Polymerization - American

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