Bis‐type modifiers in polymerization. I. Behavior of various disulfides in bulk styrene polymerization

Abstract

Modifiers with bis‐type structures are of interest as a means for conferring reactive groups onto the highest possible fraction of polymer chain ends. Various disulfides and other bis‐type compounds—some with reactive groups attached—were compared as to their effectiveness as modifiers in low conversion bulk styrene polymerizations and studies made of their mechanism of cleavage. Alkyl disulfides of primary thiols had transfer constants too low to be of much interest for this work, even when substituted at carbons near the sulfur‐sulfur bond. Aryl disulfides proved to have much higher transfer constants, and substitution on each ring with appropriate groups offered a means both of increasing the transfer constant and of incorporating desired groups.It has been shown quite conclusively that aryl disulfides cleave so as to confer one‐half the molecules onto each end of the polymer. This was demonstrated by analyzing the end group concentrations of styrene polymers of wide molecular weight range which contained various types of end groups—carboxyl, amino, naphthyl, and benzothiazyl—and comparing, wherever possible, with polymers modified with thiols containing similar groups.Polymerizations carried out at 25, 50, and 75°C. with certain of the bis‐type compounds and with a normal and a tertiary mercaptan showed that activation energies are very slight if the transfer constant is high, and begin to reach significant levels only at low levels of transfer constant.