Carbon-13 nuclear magnetic resonance spectroscopy of polymers. Part II. Determination of monomer sequence distribution in butadiene–acrylonitrile copolymer

Abstract

Noise-decoupled carbon-13 n.m.r. spectra of a range of butadiene (B)–acrylonitrile (A) copolymers confirm the ‘short-run’ alternation of acrylonitrile units in otherwise block butadiene. Direct measurements of individual peaks representing B-centred triads (BBB, ABB, ABA) can be made using the vinyl absorptions which, over a 10 p.p.m. range, show two peaks each for ABA, ABB, and BBA, together with one broad peak for BBB. The populations were plotted and extrapolated over the range 0–50 mole % acrylonitrile. Block acrylonitrile appears to be absent and essentially all the acrylonitrile exists in alternation (BAB). Deshielding effects on the vinyl carbon atoms due to nitrile groups and double bonds were quantitatively measured for each function at several distances; the results confirm the assignments. The methylene absorption indicates that the block butadiene units are linked predominantly in trans, trans-diads.