The kinetics and mechanism of the bulk free-radical polymerization of vinyl neodecanoate in the presence of polybutadiene are studied in a series of rate and molecular weight distribution measurements over the temperature range 50-90 degrees C. The polymerization of the vinyl ester is greatly retarded by the presence of polybutadiene. The dependences of the rates and molecular weight distributions on the ratio of monomer to the amount of polybutadiene are quantitatively in accord with a retardative chain transfer mechanism, whereby an alkanoate radical undergoes hydrogen transfer with polybutadiene, leaving an unreactive radical species (probably allylic in character) which cannot propagate and instead undergoes termination with another radical. The data yield Arrhenius parameters for the transfer to polybutadiene as k(tr,P) = 10(6.9) dm(3) mol(-1) s(-1) exp(-27.6 kJ mol(-1)), which, although subject to high uncertainty, are consistent with the rate-determining step being either (a) diffusion to a hindered butadiene unit of the chain end of an alkanoate macroradical or (b) the actual (chemically controlled) hydrogen transfer event, which would then have to have an unusually low activation energy.