Molecular structure and melting behaviour of ethylene-vinyl acetate copolymers

A series of ethylene-vinyl acetate copolymers has been examined by 13C-NMR spectroscopy and DSC. These polymers are products of high pressure free radical copolymerizations and contain 1–40% vinyl acetate by weight. Such copolymers are random and almost all the vinyl acetate units are isolated between ethylene units. All copolymers were branched, but total branching decreased with increasing vinyl acetate content. This effect arises mainly because the vinyl acetate residues inhibit the formation of cyclic intermediates that are involved in the productions of short branches. A decrease of about one short branch per 1000 ethylene carbons is observed for every increase of 2 mol% vinyl acetate. About 10% of the vinyl acetate units are branched at main chain carbons. Copolymers that have been cooled slowly from the melt exhibit multiple melting peaks. The lower melting, minor peaks are believed to reflect crystallization of lower molecular weight, branched molecules. The melting temperature of the copolymers is a function of the average sequence length of crystallizable ethylene units, when allowance is made for vinyl acetate units and short branches.