220-MHz NMR Spectra of Butadiene and Acrylonitrile Alternating and Random Copolymers

Abstract

A 220-MHz NMR investigation was carried out on random and alternating copolymers of butadiene (BD) with acrylonitrile (AN) and partially deuterated analogues of variable composition.An absorption by the CH protons of acrylonitrile units in the random copolymer splits into three peaks as the field strength increases. These peaks correspond to AN–AN–AN–, BD–AN–AN, and BD–AN–BD triad sequences. The amount of the alternating triad sequence reaches more than 98% in the alternating copolymer prepared with an EtAlCl2–VOCl3 catalyst. However, conventional free-radical copolymers have a 77-% alternating triad sequence (acrylonitrile content 54.2%).Olefin protons of the butadiene units in the random copolymer of BD-d4–AN appeared as pentad sequences. The pentad sequences centered at butadiene obey first-order Markov-chain statistics calculated by taking the monomer reactivity ratios as rBD=0.35 and rAN=0.05.An average conformation estimated from vicinal coupling constant of alternating BD-d4–AN copolymer shows a slightly distorted trans conformation.