2,3-exo-Disyndiotactic Polynorbornene: a Crystalline Polymer with Tubular Helical Molecular Structure

Norbornene was polymerized using the AlEt2Cl-TiCl4 catalyst system at monomer/TiCl4 molar ratios higher than 18. Products of different molecular weight were obtained, all consisting of an amorphous fraction soluble in diethyl ether and a crystalline fraction, in part soluble in toluene. The crystalline components of all the products display closely similar X-ray powder diffraction patterns which were interpreted in terms of a 2,3-exo-disyndiotactic stereoregularity, just as found in the norbornene heptamer recently obtained by us with the same catalytic system at a monomer/TiCl4 molar ratio of about 11. Using molecular modelling methods combined with a rigid body refinement procedure based on powder X-ray diffraction patterns, we proved the 2,3-exo-disyndiotactic stereochemistry and we discovered that these stereoregular polynorbornene chains adopt a remarkable conformation in the crystalline state. Wide helices with relatively compact structures are formed, leaving an empty accessible tubular channel at the core, in which guest molecules like toluene are readily hosted. The perspective is both fundamental, as the polymer presents truly unique features, and applicative with possible use in sensing and recognition/separation technologies