The average propagation rate coefficient, [k(p)], has been measured in the free-radical copolymerization of styrene (STY) and cl-methylstyrene (AMS) using pulsed-laser polymerization. The value of[k(p)] was found to decrease by 2 orders of magnitude as the mole fraction of STY, f(STY), is decreased from 1.00 to 0.12. The effect of temperature on [k(p)] was measured over the range 17.9-47.4 degrees C, and the Arrhenius parameters were determined. The reduction in [k(p)] with decreasing f(STY) is primarily attributed to entropic factors associated with the a-methyl group in AMS, as revealed by a reduction in the Arrhenius preexponential factor, A. Homopolymerizations of AMS did not yield molecular weight distributions suitable for successful evaluation of k(p), which is attributed to transfer to monomer dominating the chain stopping events. Mathematical expressions relating [k(p)] to monomer composition, allowing for different depropagation kinetics, are derived. It was found that depropagation events did not play a significant role in this work and that the terminal model for [k(p)] provided an adequate description of the observed propagation kinetics. Since k(p) for pure AMS could not be measured directly, values were estimated by extrapolation of the copolymerization [k(p)] data and the resulting Arrhenius parameters are E-a = 36.7 kJ.mol(-1) and A = 10(6.17) dm(3).mol(-1).s(-1).