The new cobalt (II) phosphine complex CoCl2(P i PrPh2)2 was synthesized by reacting CoCl2 with isopropyldiphenylphosphine in ethyl alcohol as solvent. The molecular structure of the complex was determined by the X-ray diffraction method. CoCl2(P i PrPh2)2 was then used in combination with methylaluminoxane for the polymerization of 1,3-butadiene:  it was found to be highly active and stereospecific for the preparation of 1,2 syndiotactic polybutadiene. The same system was also able to polymerize substituted butadienes giving highly stereoregular 1,2 polymers from E-1,3-pentadiene, 1,3-hexadiene, and 3-methyl-1,3-pentadiene. Some of these polymers are completely new and were never prepared before.

No abstract

Pentadiene was polymerized with the CoCl 2(P i PrPh2)2-MAO system, obtaining a crystalline polymer with a melting point of 168 °C. The polymer was characterized by IR, NMR ( 13 C and 1 H in solution and 13 C in the solid-state), DSC, GPC, and X-ray diffraction, and it was found to have a trans-1,2 syndiotactic structure. The fiber periodicity of 5.14 ( 0.03 Å indicates a trans-planar mainchain conformation similar to the one found in 1,2-syndiotactic polybutadiene. Molecular mechanics calculations using the MM2 force field show that, for trans-1,2 syndiotactic polypentadiene (t-12 sPPD), the all-trans main-chain conformation is more stable with respect to the (T 2G2) arrangement, consistent with structural data of most 1,2-syndiotactic polydienes but at variance with the cis isomer and with 1,2-poly(4-methyl-1,3-pentadiene). The crystalline structure of t-12 sPPD has been determined on the basis of a unit cell derived from fiber data and refined using the Rietveld method on unoriented patterns. Like both 1,2-syndiotactic polybutadiene and 3,4-syndiotactic polyisoprene, it adopts the high-symmetry orthorhombic space group Pbcm with the following refined lattice parameters: a ) 6.59(1), b ) 14.69(1), and c (chain axis) ) 5.11(3) Å. The two chains in the unit cell also present, besides crystallographic intramolecular glide planes, intramolecular 2-fold axes and planes of symmetry, both orthogonal to the glide plane. The main-chain conformation is slightly distorted from planarity, while the side chains adopt the same idealized skew arrangement found in other trans-1,2 syndiotactic polydienes. The refinement, giving the quality of the available data, is satisfactory. The thermal parameters, the relatively loose packing, and some features of the refined geometry points to disorder in the structure that we were unable to detail further using simple crystallographic models.

(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 γ-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.