Conformational analysis of polypropylene chains bound to model catalytic sites

The models of catalytic sites for isospecific Ziegler-Natta polymerization proposed previously (edges and reliefs on lateral surfaces of violet TiCl₃) are compared in order to verify their validity in the presence of long alkyl groups and of analyzing the conformational constraints imposed by the TiCl₃ surface on the growing chain. Our results indicate that sites on edges of lateral surfaces parallel or nearly parallel to the c axis may be not suitable for the coordination of long alkyl chains; the coordination of long alkyl chains at less hindered sites, such as edges of inclined lateral surfaces or reliefs, do not involve any increase of repulsive interactions for an isobutyl group. Further, these latter sites allow the optimum spiralization of the chain at a very early stage of growth, and their isospecific behaviour is not affected by the presence of configuration defects in the growing chain.