Crystallization-induced reactions of copolymers. II. Cis-trans isomerization of 1,4-poly-1,3-butadiene

The cis-trans photoisomerization reaction of 1,4-polybutadiene was carried out below the melting points on films of polymers containing high trans-1,4 contents. Under the proper conditions of temperature and polymer composition, the reaction was observed to undergo an anti-equilibrium behavior, which was attributed to an irreversible crystallization of repeating units after isomerization from cis to trans structure. As a result, the trans composition passed through a minimum with reaction time while crystallinity increased throughout the reaction, and unexpectedly the β crystalline form was observed well below the α–β transition temperature. The composition–time behavior observed was rationalized on the basis of incorporation of trans units into crystalline regions on the lamellar fold surfaces and discussed within the framework of the proposed requirements for crystallization-induced reactions of copolymers.