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 −CH 2 −C(CO 2 C 4 H 9 )( CH 2 ) or −CH 2 −C(O 2 CCH 3 )( CH 2 ) respectively, as the main products.