Some experiments on sulfur as inhibitor for the polymerization of styrene

Abstract

AbstractThe rate of thermal polymerization of styrene in the presence of sulfur has been followed viscometrically and dilatometrically and the product has been examined as to its molecular weight, content of total and mercury‐extractable sulfur, and its effect on the polymerization of fresh styrene.Polymer isolated during the inhibition period by methanol precipitation contains close to eight atoms of sulfur per molecule of polymer, and these sulfur atoms are so located that complete desulforization by Raney nickel reduces the molecular weight of the polymer by not more than a factor of 2–3. The number of sulfur atoms per molecule decreases during the progress of the polymerization. The polysulfides formed at first are inhibitors, as in the case of vinyl acetate polymerization. As the size of the polysulfide units and the inhibitory power decrease, the activity of the polysulfides as chain transfer agents persists and there appears a mild initiatory reaction such that the eventual rate of polymerization is almost twice the normal thermal rate. This acceleration can be produced by adding the precipitated polymer from a sulfur‐inhibited polymerization to fresh styrene.Judged by the composition of the methanol‐insoluble polymer, the inhibitory constant (kt/kp) of sulfur for styrene is of the order of 52. During the first 24 hours of a sulfurinhibited polymerization at 80–89° sulfur molecules were consumed 22 times as fast as the presumed rate of thermal initiation of pairs of polymerization chains. One‐third of this sulfur was in polymer of too high molecular weight to be the result of diradical initiation intercepted by sulfur. The mechanisms involved are discussed.