Investigation of polymerization mechanisms by means of molecular weight distributions of reaction products

Abstract

AbstractThe study of MWD functions gives insight into the mechanism of polymerization and polycondensation reactions. Especially all kinds of side reactions, by altering the already formed macromolecules, give characteristic changes in the MWD. Some special cases are discussed. For polycondensation products, two types of MWD are found. In the early stages of the reaction before the monomer has been consumed, broad distribution curves of the Flory shape are observed. In the later stages, after the polymer has been heated, quite narrow distributions of the Gaussian type are found. The theoretical explanation involves consideration of scission and macromolecular combination reactions, which result in random interchange of monomer units among macromolecules. For products of addition polymerization of dienes in the presence of a catalytic agent butyllithium, exceedingly narrow distribution functions are observed, the polymer being almost homogeneous. This is the result of simultaneous initiation and further growth of all the chains at the same rate during the entire reaction period without termination. For radical polymerization, sometimes simple unimodal, broad distribution curves are found and the termination constants (recombination and disproportionation rates) can be calculated from the curves. In special cases, however, the distribution functions are complicated and consist of many sharp maxima. It is suggested that secondary reactions of chain scission and macroradical recombination occur, which results in a sharpening of the MWD. Subsequent crosslinking of macromolecules from such sharpened distributions gives doubling, tripling, etc. of average molecular weights. This is the explanation for the polymodal sharpened distribution curves observed.