Kinetics and Mechanism of RAFT Polymerization - American Chemical

Chapter 36

Kinetics and Mechanism of RAFT Polymerization

Downloaded by STANFORD UNIV GREEN LIBR on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch036

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Graeme Moad , Roshan T. A. Mayadunne , Ezio Rizzardo , Melissa Skidmore , and San H. Thang 2

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1CSIRO Molecular Science and C R C for Polymers, CSIRO Molecular Science Bag 10, Clayton South, Victoria 3169, Australia

RAFT polymerization has emerged as one of the more versatile methods of living radical polymerization. In this paper aspects of the kinetics and mechanism of R A F T polymerization are discussed with a view to pointing out some of the advantages and limitations of various R A F T agents and providing some guidance on how to select a R A F T agent for a particular polymerization. Factors discussed include: transfer constants (Ctr, C-tr) of R A F T agents - (measurement, substituent effects, prediction with M O calculations, reversibility), retardation (examples, dependence on R A F T agent and monomer, possible mechanisms) and formation of multimodal distributions (examples, contributing mechanisms).

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© 2003 American Chemical Society

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


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Over the last 10 years, a considerable effort has been expended to develop free radical processes that display the essential characteristics of living polymerizations (1-6). These radical polymerizations can provide molecular weights that are predetermined by reagent concentrations and conversion, yield narrow molecular weight distributions and, most importantly, polymer products that can be reactivated for chain extension or block copolymer synthesis and enable the construction of complex architectures. RAFT Polymerization (Radical Polymerization with Reversible AdditionFragmentation chain Transfer) is one of the most recent entrants and arguably one of the more effective methods in this field (7-11). Some of the advantages of RAFT polymerization, over competing technologies [atom transfer radical polymerization (ATRP) (5,6), nitroxide mediated polymerization (NMP) (4)1, stem from the fact that it is tolerant of a very wide range of functionality in monomer and solvent (e.g. -OH, -COOH, CONR , -NR , S0 Na). This means that it isAppl.icableto a vast range of monomer types and that polymerizations and copolymerizations can be successfully carried out under a wide range of reaction conditions (bulk, solution, emulsion, suspension). The RAFT process has been shown to be effective over a wide temperature range (polymerizations have been successfully performed over the range 20-150°C). Indeed, with some limitations imposed by the need to limit termination reactions, the reaction conditions employed in RAFT polymerization are typical of those used for conventionalfreeradical polymerization. The RAFT process is extremely versatile. However, it is important to recognize that not all RAFT agents work with equal efficiency in all circumstances. In this paper we consider aspects of the kinetics and mechanism of RAFT polymerization with a view to understanding how to choose RAFT agents and polymerization conditions to maximize living characteristics and minimize side reactions and retardation. 2

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Results and Discussion The mechanism originally proposed for RAFT polymerization is shown in Figure 1 (11). In RAFT polymerization, the chain equilibration process is a chain transfer reaction. Radicals are neither formed nor destroyed in this step. In principle, if the RAFT agent behaves as an ideal chain transfer agent, the reaction kinetics should be similar to those of conventional radical polymerization. The rate of polymerization should be half-order in initiator and zero order in RAFT agent. This behavior is observed with, for example, methyl methacrylate (MMA) over a wide range of RAFT agent concentrations (12). However, departures from this ideal are evident with certain RAFT agents, particularly when used at high concentrations (8), and can be pronounced for monomers with high k for example, acrylate esters (8) and vinyl acetate (9) see below. pt


522 Reaction conditions for RAFT polymerization should usually be chosen such that the fraction of initiator-derived chains is negligible. The degree of polymerization (DP) can then be estimated using the relationship (1) (12).


DP (cale) ~ [monomer consumed]/[RAFT agent]

(1)

Positive deviations from equation (1) indicate incomplete usage of RAFT agent. Negative deviations indicate that other sources of polymer chains are significant. These will include initiator-derived chains. With due attention to reaction conditions it is possible to achieve and maintain a high degree of livingness. It is possible to prepare narrow polydispersity block copolymers with undetectable levels of homopolymer impurities (13). initiation

M initiator

*·

I"

M

.

p n

**

chain transfer

Ρ·

+ X ^ X - R I L

P - X ^ X - R *-add

Τ L

\

l

P„-X^X k.

+ R*

Γ

p

Z

3

reinitiation

R

#

— -

R-M?

k\

κ

Pi ρ

chain equilibration

Pm

+

ΧγΧ-Ρ,

P -X^.X-P„ m

Pm-ΧγΧ

+

?»*

termination

P * + n

P * m

dead polymer

Figure 1. Mechanism ofRAFTpolymerization

Polydispersities also depend on the properties of both the initial (1) and the polymeric RAFT agent (3). In order to obtain narrow polydispersities, the initial RAFT agent (1) and reaction conditions need to be chosen such that 1 is rapidly consumed during the initial stages of the polymerization. We can show that the rate of consumption of 1 depends on two transfer constants as shown in equation (2) (12,14,15).


where C* =kJk =kjk = WV(*-add+Ml and k* = k.^k^ik^kp)]. Other parameters are defined in Figure 1. The value of depends on properties of the radical R* and how it partitions between adding monomer and adding to the polymeric RAFT agent. Depending on the value of C.*, therateof consumption of 1 will be slower when high RAFT agent concentrations are used and may reduce with conversion. The generic features common to all RAFT agents are summarized in Figure 2. The RAFT agent should to be chosen with attention to the particular polymerization process (the monomers and the reaction conditions). Particular design features to take into account are:


P)

•

h

The RAFT agents (1 and 3) should have a high in the monomers being polymerized. This requires a high rate of addition (k^) and a favorable partition coefficient (ViV^-add))- The value of & is determined mainly by X and Ζ while the partition coefficient depends on the relative leaving group abilities of R* and the propagating radical. For 3 the partition coefficient will be ~ 0.5. add

Weak single bond R is free radical

Ζ modifies addition and fragmentation rates

Figure 2. Generic RAFT agent structure

X-R

M

P -X^X-R n

Ζ Figure 3, Possible side reactions in RAFTpolymerization


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The intermediates (2 and 4) should fragment rapidly and give no side reactions such as coupling (where Τ · might be 2, 4, R-, an initiator derived radical, a propagating radical or oxygen - as might be present in poorly degassed samples) or propagation (Figure 3). The radical R- should efficiently reinitiate polymerization. This requires that kç>k and it is also desirable that kç>kp. The value of C. should be small. p

ti


Transfer Constants of RAFT Agents Transfer constants of RAFT agents have been estimated using the Mayo method (16,17) or by fitting the evolution of the molecular weight distribution with conversion (18,19). We have advocated determining transfer constants by (8,12,20) using the integrated form of rate equation. This avoids some of the difficulties associated with measuring high transfer constants. It is often assumed that chain transfer to the initial RAFT agent is irreversible. In this case equation (2) simplifies as follows,

d[l] •^j _

d[M]

_ β

W

[1]

*[M]

. _ which suggests C*

"

—

C=CH2 towards radical addition. Dithioesters (11) and trithiocarbonates (9,25) and certain dithiocarbamates (where the nitrogen lone pair is delocalized) (26,27) are preferred with (meth)acrylic and styrenic monomers in that their use affords narrow polydispersity polymers in a batch polymerization process. For styrene polymerization, rates of addition decrease (and rate of fragmentation increase) in the series Ζ is aryl > S-alkyl ~ alkyl - iv*-pyrrolo » OC F > JV-lactam > 2

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Ο

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% Conversion Figure 4 (14). Evolution ofpolydispersity with conversion for bulk polymerization ofstyrene at 60°C (0.0017 M AIBN initiator) in the presence of dithiobenzoates PhC(=S)SR, where R = -CH Ph (0.0093 M) (·); R = -C(Me) Ph (0.0083 M) (4); -C(Me) C0 C H (0.0083 M) (A); -C(CH ) CH C(CH ) (0.0083 M) (U). Kinetic simulation with initial RAFT agent C» = 50 and C.» = 0( = 400 and C+ = 11600 ( ), or = 2000 and C_„ = 10000 (- - - -). C». (4) = 6000 .(24) 2

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5

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3

6 R'=Ph 7 R'=CN


526 OC H > O-alkyl » N(alkyl) (8,15). Only the first four of this series provide narrow polydispersities (MJM

Kinetics and Mechanism of RAFT Polymerization - American Chemical

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