Carbon-13 nuclear magnetic resonance spectroscopy of polymers. Part I. High resolution carbon-13 nuclear magnetic resonance spectroscopy: tacticity studies on poly(vinyl acetate) and monomer distribution analysis in ethylene–vinyl acetate copolymers

Abstract

Noise-decoupled pulsed Fourier transform ¹³C n.m.r. spectra reveal the tacticity in poly(vinyl acetate) and the monomer sequence distribution (MSD) in ethylene (E)–vinyl acetate (V) copolymers. Assignments of carbon absorptions were made from ¹³C ‘un-decoupled’ spectra and the methylene and methine carbon atoms were found to be especially sensitive to environment. The signal due to the methine carbon atoms in poly(vinyl acetate) is resolved into two peaks, showing a preponderance of heterotactic triads. In E–VA five MSD triad environments are predicted for the five methylene peaks observed and four MSD triad environments predicted for the four methine peaks observed. In addition, multiplicity in the four methine MSD triad peaks reveals a partially resolved MSD pentad and a tactic triad for block vinyl acetate VVV. Evaluation of corrected methine and methylene absorption intensities allows direct comparison of MSD triad intensities and shows the self-consistency in assignments. The compositions of the three samples (67, 38, and 14 mole % vinyl acetate) were determined in terms of the % population of all eight MSD triads and plotted to show triad composition for a whole range of ethylene–vinyl acetate copolymers. In these commercial samples both ethylene and (especially) vinyl acetate units show a tendency to block.