Branching in Polymer Chains

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Abstract
The branching that occurs in molecular chains leads to extensive changes of the physico-mechanical and technological properties of various polymers, as compared with the corresponding linear polymers. For example, branched divinylstyrene rubbers, polybutadienes, and other branched elastomers possess a low elasticity, breaking strength, etc. The decrease of strength properties and the change of the dynamic and mechanical properties with the increase of the degree of branching is characteristic for such thermoplastic polymers as polystyrene, polyethylene, polyvinyl chloride, etc. There also exist considerable differences in the rheological properties of linear and branched polymers. For example, at low loads (shear stresses) linear polymers and their melts flow like newtonian fluids and possess a lower viscosity as compared with branched polymers, whereas at high loads the character of flow deviates from that of a newtonian flow, so that the viscosity of a branched polymer is lower than that of a linear polymer or almost equal. Similar regularities have also been observed for concentrated solutions. Hence, the branching, together with the regularity of the structure of polymer chains, the molecular weight, and the molecular weight distribution, represents one of the most important molecular parameters of polymers. The branching occurs during polymerization and is caused essentially by a transfer of active centers to the polymer chain.

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