Average propagation rate coefficients in the free-radical copolymerization of styrene and α-methylstyrene measured by pulsed-laser polymerization

The average propagation rate coefficient, 〈k_p〉, has been measured in the free-radical copolymerization of styrene (STY) and α-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 °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 α-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·mol^-1·s^-1.