A comparative study was conducted on chain transfer mechanisms in the free radical polymerization of a viscous methacrylate, viz. 2-phenoxyethyl methacrylate. Two chain transfer agents were subjected to investigation: n-dodecanethiol (DDM) and bis[(difluoroboryl)diphenylglyoximato]cobalt (II) (COPhBF). The chain transfer constant (Cs) for DDM was found to be 0.7 (at 60 degrees C); a value comparable with the Cs value obtained for a low viscosity methacrylate, methyl methacrylate, under similar conditions. In contrast, the Cs value for COPhBF was found to be 2 x 10(3) (at 60 degrees C) which is one order of magnitude lower than the Cs value published for methyl methacrylate. This result is explained in terms of the different chain transfer mechanisms operating in the two polymerizing systems. Radical reactions with DDM involve a chemically controlled hydrogen transfer event, whereas experimental results obtained for the reaction of methacrylate radicals with COPhBF indicate a diffusion-controlled, rate determining step in the hydrogen transfer process. Thus in the reaction of COPhBF with 2-phenoxyethyl methacrylate radicals, the high viscosity reaction medium (monomer) has a significant influence on the transfer rate.