Vinyl polymerization. 179. Effects of substituents on chain transfer reaction of substituted cumenes toward poly(methyl methacrylate) radical

In order to study the effects of substituents on the chain transfer reaction to cumenes, the polymerization of methyl methacrylate in a series of nuclear-substituted cumenes with α,α′-azobisisobutyronitrile as initiator was carried out at 60°C. and the chain transfer constants C were determined. The C values obtained for all substituted cumenes were greater than that (C₀) for unsubstituted cumene, regardless of the electronattracting or -repelling nature of the substituents. Hence the plot of log (C/C₀) against the Hammett σ constants gave no linear relationship. When the plots were made by the modified Hammett equation including resonance effect of the substituents: log (C/C) = ρσ + γE_R, however, a straight line with ρ = 0.03 and γ = 0.9 was obtained. From these results, it may be concluded that the effect of the substitutents on the chain transfer reactivities of cumenes toward attack of a poly(methyl methacrylate) radical is attributable mainly to the resonance contribution in the transition state. These results are also compared with those for polystyryl radical reported previously and discussed.