AbstractThe rate of reaction of sulfur dioxide in styrene solutions at 60°C., with azodiisobutyronitrile as thermal catalyst, has been measured: (1) by gravimetric determination of the total rate of polymerization and analysis of the polymer formed and (2) volumetrically. An experimental arrangement for the latter method, which can be used when the partial pressure of the solvent (styrene solution) is appreciable compared to that of the solute (sulfur dioxide) is described. The kinetics of reaction conform to the mechanism previously proposed, according to which sulfur dioxide reacts in the form of a 1/1 styrene/sulfur dioxide complexC. It is shown that certain features attributable to depropagation reactions, such as theincreasein the styrene content of the copolymer withdecreasingstyrene concentration in the reaction mixture, are much more marked at 60°C than at 20°C. By combining the rate measurements with new data on the temperature dependence of copolymer composition at elevated temperatures, it is possible to assign an approximate value to the velocity constant of a depropagation reaction involving breakage of a carbon‐sulfur bond. The activation energy is estimated at 50 to 62 kJ. (12 to 15 kcals.), which is much lower than that for depropagation of a vinyl polymer; the frequency factor is about 4 X 1010to 3 X 1012sec.−1. Some data on the composition of polymers prepared at 0° and −20°C. are appended. It is shown that theC· +Cpropagation step, which seems negligible at 20°C. or above, becomes of some significance at lower temperatures. Under these conditions polymers containing less than two styrene units per sulfur dioxide can