Why Is the Trommsdorff Effect Weak for Styrene? An Investigation Using Small-Angle Neutron and Wide-Angle X-ray Scattering

During the bulk radical polymerization of monomers, such as methyl methacrylate (MMA) and styrene, a sudden reaction acceleration, known as the Trommsdorff effect, has long been recognized. While this effect is significant during the polymerization of MMA, it is much more moderate during the bulk polymerization of styrene. Traditionally, the Trommsdorff effect has been attributed to an increase in viscosity and a corresponding decrease in the termination rate, which, however, fails to quantitatively describe the phenomenon. Recent studies suggest that apparent phase separation during the bulk polymerization of MMA is closely related to the effect. In this study, the bulk polymerization of styrene was analyzed by using small-angle neutron scattering (SANS) and wide-angle X-ray scattering (WAXS). SANS data revealed that the correlation length for the concentration fluctuation changes rapidly during the polymerization of styrene in conjunction with the reaction acceleration. While this tendency of styrene is qualitatively the same for MMA, the change in concentration fluctuations is smaller than that observed in the polymerization of MMA. Pair distribution function (PDF) analysis and molecular dynamics (MD) simulation showed that the amorphous structures of styrene and polystyrene are similar, and the transition between them occurs relatively smoothly. These findings support the hypothesis that changes in the amorphous molecular structure are related to the rapid increase in the correlation length of concentration fluctuations at the nanometer scale in the vicinity of polymerization-induced vitrification. This nanometer-scale apparent phase separation abruptly changes the probability that reactive species will encounter each other, leading to a sudden shift in reaction kinetics.