Synthesis, characterization and molecular conformation of syndiotactic 1,2 polypentadiene : the cis polymer

(Z)-1,3-pentadiene has been polymerized with the CpTiCl3-MAO system at -78 °C to a polymer consisting exclusively of cis-1,2 units and having a syndiotactic structure. The polymer has been characterized by NMR (13C and 1H in solution and 13C in the solid state), IR, DSC, GPC, X-ray, and molecular modeling techniques, comparing its conformational and structural properties with those of other 1,2-syndiotactic polymers. Diffraction and NMR data are consistent with the idea that a single crystalline polymorph prevails in the studied samples. The solid-state NMR results allow us to exclude the all-trans (T4) conformation for cis-1,2 syndiotactic polypentadiene and indicate a nonuniform helix of the T2G2 type as most plausible, although other possibilities such as the (T6G2T2G2) conformation cannot be completely ruled out. These conclusions follow from the fact that two conformationally distinct methylene C1 carbons are observed, which experience, respectively, zero and two ç-gauche interactions. Isolated chain molecular mechanics calculations too show that both (T2G2)2 and (T6G2T2G2)n conformations are of comparable stability, while the more extended all-trans (T4) arrangement has significantly higher internal energy, at variance with the situation found in most 1,2-syndiotactic polydienes. Simple qualitative considerations on the steric conflicts involving terminal cis methyls of the side chains of adjacent monomer units in the all-trans conformation rationalize this result