Molecular weight distribution and branching in butadiene polymers and copolymers

Abstract

AbstractButadiene polymers and copolymers prepared in emulsion and with complex coordination catalysts have been examined by use of light‐scattering, osmometric, viscometric, and fractionation techniques. These polymers are particularly prone to branching, the extent of which has been characterized from light‐scattering and viscometry data. A correlation was obtained between polydispersity and branching, and the lightscattering second virial coefficients of the polymers. The second virial coefficients obtained from osmotic pressure measurements were not greatly affected by the presence of branched polymer. Similar results were obtained on a butadiene–styrene copolymer which was branched by exposure to high energy radiation. Further branching, leading ultimately to crosslinking, occurred on storage of polybutadiene both in the solid polymer and in concentrated solution, the changes being influenced by catalyst residues in the polymer. On the other hand, in dilute solution or on milling in the presence of radical traps, degradation at the branch points occurred. No relationship was observed between branching and the physical properties of the polymer. Butadiene polymers prepared using complex coordination catalysts, possessed, in general, somewhat wider molecular weight distributions than emulsion polymers prepared at 10°C. They were characterized by substantial amounts of low molecular weight polymer and the absence of a high molecular weight tail. The molecular weight of essentially linear fractionated polybutadiene of 93% cis‐1,4‐ content is related to the intrinsic viscosity by [η] (benzene 100 ml. g.−1) = 1.00 × 10−4M0.77. After making corrections for differences in structure and chain stiffness this relationship is in close accord with relationships from other sources obtained on unbranched butadiene polymers.