No abstract

Free-radical polymerization in the presence of suitable addition-fragmentation chain transfer agents [SdC(Z)S-R] (RAFT agents) possess the characteristics of a living polymerization (i.e., polymer products can be reactivated for chain extension and/or block synthesis, molecular weights are predetermined by RAFT agent concentration and conversion, narrow polydispersities are possible). Styrene polymerizations (110 °C, thermal initiation) were performed for two series of RAFT agents [SdC(Z)S-CH 2Ph and SdC(Z)S-C(Me)2CN]. The chain transfer coefficients decrease in the series where Z is Ph. N(alkyl)2 (only the first five in this series provide narrow polydispersity polystyrene (< 1.2) in batch polymerization). More generally, chain transfer coefficients decrease in the series dithiobenzoates > trithiocarbonates ∼ dithioalkanoates > dithiocarbonates (xanthates) > dithiocarbamates. However, electron-withdrawing substituents on Z can enhance the activity of RAFT agents to modify the above order. Thus, substituents that render the oxygen or nitrogen lone pair less available for delocalization with the CdS can substantially enhance the effectiveness of xanthates or dithiocarbamates, respectively. The trend in relative effectiveness of the RAFT agents is rationalized in terms of interaction of Z with the CdS double bond to activate or deactivate that group toward free radical addition. Molecular orbital calculations and the estimated LUMO energies of the RAFT agents can be used in a qualitative manner to predict the effect of the Z substituent on the activity of RAFT agents.

This paper describes a versatile and effective method for the control of free radical polymerization and its use in the preparation of narrow polydispersity polymers of various architectures. Living character is conferred to conventional free radical polymerization by the addition of a thiocarbonylthio compound of general structure S=C(Z)SR, for example, S=C(Ph)SC(CH3)2Ph. The mechanism involves Reversible Addition-Fragmentation chain Transfer and, for convenience of referral, we have designated it the RAFT polymerization. The process is compatible with a very wide range of monomers including functional monomers such as acrylic acid, hydroxyethyl methacrylate, and dimethy laminoethyl methacrylate. Examples of narrow polydispersity (51.2) homopolymers, copolymers, gradient copolymers, end-functional polymers, star polymers, A-B diblock and A-B-A triblock copolymers are presented.

The chain transfer activities of a series of ω-unsaturated methyl methacrylate oligomers [(dimer (1), trimer (2), tetramer (3), and a methyl methacrylate macromonomer with average chain length of 24 units (4)] have been evaluated in methyl methacrylate polymerizations over the temperature range 45−100 °C. Transfer constants were determined by analysis of the ln chain length distributions. The dimer (1) was found to be substantially less effective as a chain transfer agent than the trimer (2) or the higher macromonomers (3, 4) [C T(60 °C) 1, 0.013; 2, 0.19; 3, 0.31; 4, 0.21]. The transfer constants show only a small temperature dependence and no variation with conversion. No discernible retardation was observed in these polymerizations. A reduced yield of polymer observed at conversions >10% in bulk polymerizations of MMA carried out in the presence of 1 − 4 can be attributed to the absence of the gel (Trommsdorff) effect. The results are interpreted in terms of the addition−fragmentation mechanism for chain transfer.

The kinetics and mechanism of the initiation and reinitiation of free radical polymerization is reviewed. The importance of understanding the kinetics, specificity and efficiency of initiation and chain transfer when predicting polymerization kinetics and polymer composition is highlighted. These factors are particularly important when making low molecular weight polymers and in living or controlled polymerization processes. Examples of RAFT polymerization and catalytic chain transfer are provided.