C-13 NMR-SPECTROSCOPY AS A MEANS TO PROBE THE LOCAL MICROSTRUCTURES AND CONFORMATIONS OF ETHYLENE-VINYL ACETATE COPOLYMERS

A recently developed conformational description (RIS model) of ethylen e-vinyl acetate (E-VAc) copolymers, obtained by merging the RIS models of the constituent homopolymers, is used to calculate microstructural ly sensitive C-13 chemical shifts expected from the high-resolution, s olution C-13 NMR spectra recorded for the complete range of E-VAc copo lymers. Microstructurally sensitive C-13 resonance frequencies and loc al E-VAc conformations are connected via conformationally sensitive ga mma-gauche shielding effects. Comonomer sequences to the pentad level and VAc stereosequences to the pentad (methine carbons) and hexad (met hylene carbons) levels are considered. Calculated C-13 chemical shifts are found to be in close agreement with those observed and assigned p reviously for a series of E-VAc copolymers by Wu et al. and Sung and N oggle. In addition the C-13 chemical shifts calculated for the various stereosequences in atactic PVAc compare favorably to the observed spe ctrum of the atactic homopolymer and facilitate its assignment. The su ccessful comparison of calculated and observed C-13 chemical shifts in E-VAc copolymers validates the conformational RIS model recently deve loped for this copolymer. This conformational validation rests on our ability to predict the microstructurally sensitive local bond conforma tions in E-VAc copolymers rather than upon traditional global measures of polymer conformations, such as the mean-square end-to-end distance s and dipole moments.