(PRPh2)Co(eta-4-C4H6)(eta-3-C4H7) [R = methyl, ethyl, n-propyl, i-propyl, cyclohexyl]: synthesis, characterization and behavior in the polymerization of 1,3-dienes and allene

Novel allyl cobalt phosphine complexes were synthesized by reacting CoCl2(PRPh2)2 (R = methyl, ethyl, normal-propyl, iso-propyl, cyclohexyl) with 1,3-butadiene or isoprene in presence of metallic zinc1 and characterized by 1H-NMR and X-Ray analyses. These cobalt complexes were successively used in the polymerization of 1,3-butadiene. They were found to be highly active and stereospecific giving essentially 1,2 syndiotactic polymers, with a percentage of syndiotactic diads [rr] depending on the type of phosphine ligand bonded to the cobalt atom. A correlation between catalyst structure and polymer microstructure was established, allowing to point out the influence of the type of ligand on the polymerization stereoselectivity, and to confirm some features of the diene polymerization mechanism previously proposed. The above cobalt complexes were also able to polymerize 1,3-dienes other than butadiene (e.g. isoprene, 1,3-pentadiene, 3-methyl-1,3-pentadiene). Polymers having different microstructure were obtained from different monomers by using the same catalyst, meaning that the monomer structure too plays a fundamental role in determining the polymerization stereoselectivity. Furthermore, the allyl cobalt phosphine complexes also exhibited a quite good activity in the polymerization of allene, giving highly crystalline 1,2 polymers.