RAFT polymerization with phthalimidomethyl trithiocarbonates or xanthates. On the origin of bimodal molecular weight distributions in living radical polymerization

Phthalimidomethyl trithiocarbonate reversible addition-fragmentation chain transfer (RAFT) agents are effective in imparting living characteristics to radical polymerizations of butyl acrylate (BA) and N-isopropylacrylamide (NIPAM) and provide a route to end-functional polymers with predictable_molecular weight and narrow molecular weight distributions (e.g., number-average molecular weight for PBA (M̄_n) = 21 300 and polydispersity (M̄_w/M̄_n) = 1.1 at 96% conversion). End group determination suggests that bimodal molecular weight distributions and long chain branches in PBA arise by copolymerization of a PBA macromonomer formed by backbiting β-scission. The S-phthalimidomethyl xanthates provide good control over polymerizations of the less-activated monomers N-vinylpyrrolidone (NVP) and vinyl acetate (VAc). In the case of PBA with a trithiocarbonate end or PVAc with a xanthate end, the C-S bond to the thiocarbonylthio end group can be homolyzed by thermolysis at > 180°C leaving the phthalimidomethyl end group and the ester side groups intact providing macromonomers, with ω end group -CH₂-C(CO₂C₄H₉)(=CH ₂) or -CH₂-C(O₂CCH₃)(=CH ₂) respectively, as the main products.