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 3 ·mol -1 ·s -1 .