Macromonomer Synthesis Using Oligomers of ω-Unsaturated Methacrylate as Addition−Fragmentation Chain Transfer Agents:  Increased Efficiency by Manipulation of Steric Hindrance

The benzene solution polymerizations of ethyl methacrylate (EMA), cyclohexyl acrylate (CHA), and styrene (St) in the presence of ω-unsaturated methyl methacrylate oligomers (MMA-n; n = 2−4) and cyclohexyl methacrylate oligomers (CHMA-n; n = 2 and 3) as addition−fragmentation chain transfer (AFCT) agents have been studied to synthesize macromonomers efficiently by radical polymerization. The number of unsaturated end groups per chain (f) was used as an index to evaluate the effectiveness of the AFCT agents. In the EMA and St polymerizations, greater steric hindrance of the α-substituent of the AFCT agent and higher temperatures yielded polymers with higher f, and the level of retardation decreased with increasing f. CHA polymerization in the presence of MMA-n at 60 °C resulted in high-f polymer, although accompanied by marked retardation. Polymer structure analysis revealed that backward β-fragmentation of the EMA adduct radical, resulting in regeneration of the propagating radical and the AFCT agent, is a significant reaction mode of the adduct radical.