The Relation between the Alternating Copolymerization and the Diels–Alder Reaction

Abstract

The relation between the alternating copolymerization of butadiene with acrylonitrile and the Diels–Alder addition reaction was investigated. It was found that the acidity of the Lewis acids as complexing agents affects not only the former reaction but also the latter one, but the selectivity towards both reactions is little affected. On the other hand, the addition of the vanadyl compound as a cocatalyst or the irradiation of UV-light enhances the selectivity towards the alternating copolymerization preventing the formation of the Diels–Alder adduct. Experiments at varied temperatures show that the Diels–Alder reaction is a thermal reaction, whereas the alternating polymerization is enhanced by UV-irradiation with the low activation energy. However, the yields of the products in both reactions attain their maxima at a 1:1-monomer composition and at the same moderate acidity of the complexing agent such as aluminum halide. It is concluded that both reactions arise from the same intermediate such as the ternary complex and the difference may consist in the energy state of the intermediate. According to the Woodward–Hoffmann rule, the energy state of butadiene is normal for the Diels–Alder reaction, whereas it is triplet transoid for the alternating copolymerization.