E. Tart et al., C-13 NMR-SPECTROSCOPY AS A MEANS TO PROBE THE LOCAL MICROSTRUCTURES AND CONFORMATIONS OF ETHYLENE-VINYL ACETATE COPOLYMERS, Macromolecules, 26(16), 1993, pp. 4283-4286
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.