Bis(phenoxyimine)zirconium and -titanium Catalysts Affording Prevailingly Syndiotactic Polypropylenes via Opposite Modes of Monomer Insertion

A zirconium(IV) complex bearing two N-(3-methylsalicylidene)-2,3,4,5,6-pentafluoroaniline and two Cl ligands has been synthesized (1). X-ray analysis indicates that 1 adopts a distorted octahedral structure with a trans-O, cis-N, and cis-Cl arrangement, similarly to previously reported complexes of this class. Polymerization of propene at room temperature in the presence of 1 activated by methylaluminoxane results in the production of poorly stereoregular, prevailingly syndiotactic polypropylene. The polymer microstructure is in agreement with a “chain-end” mechanism of steric control, with P r = 0.67 (where P r is the probability of a syndiotactic placement). End group analysis, also using deuterium labeling techniques, and NMR analysis of a copolymer of propene with trace amounts of [1-13C]-ethylene, indicate a prevailing primary insertion mode in both initiation and propagation, in contrast to the prevailingly secondary regiochemistry established for related bis(phenoxyimine) titanium catalysts. To discriminate the role played by the nature of the metal from any ligand effect, a titanium complex bearing the same phenoxyimine ligands has been also tested, resulting in the production of a polypropylene having a very similar structure, with P r = 0.71. Investigation of the regiochemistry shows that propagation prevailingly occurs via secondary insertion of propene. Thus, zirconium and titanium complexes bearing the same ligands afford prevailingly syndiotactic polypropylenes having very similar structures, but via opposite regiochemistries.