Syndiospecific oligomerization and polymerization of norbornene with titanium catalysts

Norbornene (NB) has been polymerized with TiCl4–AlR2Cl (R = Et, Me, iBu) at various NB/TiCl4 molar ratio. The products obtained at low ratio (from ca. 5 to 12, depending on the mode of catalyst preparation) are a mixture of oligomers, from which it has been possible to isolate a crystalline heptamer having a 2,3-exo-disyndiotactic structure by single crystal X-ray analysis. The products obtained at higher NB/TiCl4 ratio have a higher molecular weight (up to ca. 5000 for the soluble products) and consist of amorphous and crystalline polymers. The crystalline polymers, characterized by X-ray and 13C NMR, are a new type of stereoregular polynorbornene (PNB), having a 2,3-exo-disyndiotactic structure. Mechanistic aspects are examined and a precise assignment of some resonances in the 13C NMR spectrum of vinyl-type PNBs is reported.