A study on kinetics of bulk thermal polymerization of styrene. (2) Number-average molecular weight

In this paper, the expressions for calculating number-average molecular weight were derived based on Three Stage Polymerization Model (TSPM) and the second-order initiation mechanism for bulk thermal polymerization of styrene (St). With the use of the experimental data of the molecular weight in literatures, the general expressions for calculating number-average molecular weight at different temperature were obtained. It was found from analysis of the expressions that the propagation rate constant kp and chain transfer rate constant to monomer ktr.m or chain transfer constant to monomer Cm are unchanged and Cm is independent of the reaction temperature from lower conversion to gel effect stage for bulk free-radical polymerization. In addition, the obtained results showed that the number-average molecular weight is independent of the conversion and only relative to the reaction temperature at lower conversion stage. However, the molecular weights were varied with the conversion at the gel effect stage and the variation was more obvious as the reaction temperature rises.