Lloyd H. Wartman

BRANCHING IN POLYETHYLENE

The principal purpose of this paper is to present the current status, as viewed by the authors, of our knowledge of branching in polyethylene and the effects of branching on its properties. Another purpose is to present some speculative material, and some challenging observations, in an attempt to stimulate, in some measure, future work on the structure and chemistry of polyethylene. In endeavoring to achieve these aims, the authors have drawn heavily upon their own work and liberally from the literature, particularly the published results of R. B. Richards and his collaborators. Polyethylene resins are made by polymerizing ethylene at elevated temperatures and high pressures.’, The polymer molecules formed under these conditions are branched hydrocarbon chains3* 4 . 6 * and their average molecular size varies from approximately 50 carbon atoms per molecule, for the greasy products, to more than 1000 carbon atoms per molecule for rigid resins. The degree of branching in polyethylene has been shown to play an important role in determining the properties of the resin.’ In fact, it is now generally recognized that degree of branching and molecular weight are the most important molecular variables in polyethylene resins. In this paper, degree of branching is defined as the number of branches per 100 carbon atoms, and the number of branches per molecule is defined as the number of -CH3 groups per molecule more than two. Although some chains and branches are terminated by vinyl groups (-CH=CHJ, the concentration of these is very small compared to the concentration of methyl groups. Side chain methylene groups (RR’C=CHZ) may be regarded as short branches but, as such, represent not more than 15 per cent, and generally a much smaller proportion, of the total number of branches. As a result, this configuration is not considered significant for the purposes of this paper.