Substituent Effects in Xanthate-Mediated Polymerization of Vinyl Acetate: Ab Initio Evidence for an Alternative Fragmentation Pathway

High-level ab initio calculations have been performed for the addition and fragmentation steps in reversible addition-fragmentation chain transfer (RAFT) polymerization of vinyl acetate. The RAFT agents considered were a series of xanthates of the form S=C(O-Z′)S-R, where Z′ = methyl, ethyl, iso-propyl, and tert-butyl and R = CH2OCOCH3 and CH3. The results indicate that increasing substitution within the Z′ group stabilizes the RAFT adduct radical, thereby reducing the rate of fragmentation of the S-R bond. For the model vinyl acetate system, there is an additional substantial reduction in rate for the bulkier iso-propyl and tert-butyl substituents (compared with methyl and ethyl) associated with a sterically induced conformational change in the transition structures. However, the calculated S-R fragmentation rate for Z′ = tert-butyl is still not low enough to explain the experimentally observed rate retardation in this system. Instead, the rate retardation appears to be the result of the preferred fragmentation of the O-C bond in the tert-butoxy group of the RAFT adduct. This fragmentation pathway is not normally favored in RAFT polymerization. However, in this particular system, the combination of the vinyl acetate radical being a poor homolytic leaving group, the tert-butyl radical being a good leaving group, and charge-transfer stabilization of the transition structure for O-C β-scission tips the balance in favor of the O-C β-scission reaction