Mechanistic Studies on Conjugated Diene Polymerizations Promoted by a Titanium Complex Containing a Tetradentate [OSSO]-Type Bis(phenolato) Ligand

A thorough experimental and theoretical mechanistic study on the conjugated dienes polymerization promoted by the postmetallocene complex dichloro{1,4-dithiabutanediy1-2,2'-bis(4,6-di-tert-butyl-phenoxytitanium, 1, activated by methylaluminoxane (MAO), is presented. Experimental polymerization studies, previously reported on butadiene, isoprene, and 4-methyl-1,3-pentadiene, have been extended to E- and Z-1,3-pentadiene polymerizations and to ethylene-1,3-pentadiene copolymerizations. Complex 1/MAO showed to be unactive in the Z-1,3-pentadiene polymerization, whereas it was quite performing toward E-1,3-pentadiene, leading to a polymer containing a mixture of 1,2- and 1,4-trans units, with a prevalence of 1,2 units at low temperatures. Also for ethylene-1,3-pentadiene copolymerizations, complex 1/MAO showed good activity. The copolymer microstructure can be varied by changing the ratio between the monomers in the copolymerization feed, affording copolymers with 1,3-pentadiene content up to 36%. Density functional theory (DFT) mechanistic studies on butadiene, E- and Z-1,3-pentadiene polymerizations indicate that monomer insertions proceed through an allylic mechanism involving a syn-eta(3) coordination of the growing chain and a s-trans-eta(2) monomer insertion.