Molecular Weight Control of Elastomers Prepared by Emulsion Polymerization

Abstract

Abstract Control of molecular weight is a prerequisite for the production of commercial polymers. This is especially true of elastomers for which satisfactory processing is either an implied or a formal specification. This review covers the theories and their applications for the control of molecular weights for the production of styrene-butadiene rubber (SBR), emulsion butadiene rubber (E-BR), nitrile-butadiene rubber (NBR), chloroprene rubber (CR), acrylic-butadiene rubber (ABR), and acrylic rubber. The complexity of the emulsion polymerization system is reflected in the large number of experimental and theoretical studies. Chronologically the theories and experiments based on the original mechanism proposed by Harkins and on the quantitative hypothesis of Smith and Ewart evolved into numerous publications. These works have been periodically reviewed, and several recent reviews give detailed descriptions of these studies. A review by Alexander and Napper, although subjective in choice of papers examined, presents a critical analysis of the major theories of emulsion polymerization systems. However, none of the reviews just cited cover in a significant way the control of the molecular weight of polymers prepared in the presence of reactive transfer agents. The theoretical and experimental studies of the control of molecular weight of elastomers prepared in emulsion systems came from a series of studies different from those concerned solely with emulsion theory and mechanism. In this review the broad features of the mechanism first proposed by Harkins and the hypothesis of Smith and Ewart are accepted. The derivation of modifier depletion and molecular weight equations and the calculation of the theoretical curves used in emulsion systems for the preparation of elastomers are presented next.