New stereoregularity in the stereospecific polymerization of bulky strained olefins: Diheterotactic polynorbornene

A new semicrystalline polynorbornene was obtained by stereospecific polymerization with CrCl2(dppa)-MAO catalyst (dppa = bis(diphenylphosphino)amine). NMR, X-ray diffraction, and modeling techniques were used, concentrating on the soluble lower molecular weight fractions, to characterize the polymer structure. The polymer presents a 2,3-exo-diheterotactic stereochemistry never observed before in the polymerization of alkenes. High symmetry, low energy chain conformations accounting for C-13 NMR data are devised. Efficient crystalline packing models consistent with powder X-ray diffraction patterns and density requirements are proposed.