Macromonomer synthesis using α‐(2‐methyl‐2‐phenylpropyl)acrylates as addition–fragmentation chain‐transfer agents expelling the cumyl radical

α‐(2‐Methyl‐2‐phenylpropyl)acrylate (RS‐2) was examined as a C? C bond‐cleavage type addition–fragmentation chain transfer (AFCT) agent in the benzene solution polymerizations of styrene (St), ethyl methacrylate (EMA), and cyclohexyl acrylate (CHA) with the objective of achieving efficient macromonomer synthesis by radical polymerization. The AFCT efficiency was evaluated in terms of the decrease in the number‐average molecular weight (M_n) upon the addition of the AFCT agent and the number of unsaturated end groups introduced per chain (f). The AFCT efficiency was rationalized by the consideration of the relative importance of AFCT as an end‐forming event and the competition between ‐fragmentation and crosspropagation as adduct radical reaction pathways. In St and EMA polymerizations at 60 °C, RS‐2 resulted in higher f values and lower M_n values than methyl α‐(2‐methyl‐2‐carbomethoxypropyl)acrylate (MMA‐2), and this suggested the facilitation of ‐fragmentation due to the expulsion of the more stable cumyl radical from the RS‐2 adduct radical. Higher f values were observed for MMA‐2 than for RS‐2 in CHA polymerization because of unsaturated end group formation by ‐fragmentation of midchain radicals. However, RS‐2 resulted in lower M_n values for poly(CHA) than MMA‐2 because of a smaller contribution of crosspropagation. Retardation in the presence of the AFCT agents was affected by the balance between b‐fragmentation and crosspropagation and by the addition rate of the propagating radical to the AFCT agent. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6021–6030, 2004