AbstractPolystyrene and butadiene–styrene copolymers (SBR) were prepared in emulsion systems with a homologous series of commercial mercaptan modifiers. The molecular weight distribution (MWD) of the sets of polymers changed in a consistant manner when the regulating index of the mercaptan was relatively low. However the shape of the MWD curves appeared distorted in comparsion to theoretical curves when the modifier depleted rapidly and when divinylbenzene was present in the system. The divergence from the theoretical curve is attributed to a higher degree of branching in the high molecular weight fractions. Differences in MWD of SBR made withn‐ andtert‐ dodecyl mercaptans was marked. Notable differences were also found for SBR 1500 samples from the industry at random, but only slight differences were seen in a set of SBR 1503 samples. This study shows how the MWD of polymers prepared in emulsions can be varied simply by use of modifiers with different regulating in
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