Complex formation and reversibility in copolymerization

Barb's data4–6 on the copolymerization of styrene and sulfur dioxide are reviewed and shown to be in agreement with a scheme in which styrene‐sulfur dioxide complexes are not involved, but in which attack of a growing chain on sulfur dioxide is a rapidly reversible process at all temperatures, while, at higher temperatures, addition of a chain ending in sulfur dioxide to styrene becomes reversible as well. Eact for the loss of SO2 is estimated as lying between 4 and 14 kcal., while a chain ending in a sequence of styrene units reacts with SO2 at least 100 times as readily as with styrene. A similar interpretation is made of the ethylene‐carbon monoxide copolymerization,14 in which addition of CO appears to be a reversible process, since the amount of CO in the polymer increases with total pressure. Some other systems in which reversibility of chain propagation might be expected to affect copolymer compositions are pointed out.