Phenoxyaldimine and Phenoxyketimine Titanium Complexes in Propene Polymerization. A Different Effect of o-Phenoxy Halide Substituents

Abstract

New fluorinated bis(phenoxyaldimine)- and bis(phenoxyketimine)titanium(IV) complexes bearing o-halide substituents on the phenolate rings were synthesized and tested as catalysts for propene polymerization. Polymerization of propene at subambient temperature in the presence of bis(N-(3,5-dibromosalicylidene)-2,3,4,5,6-pentafluoroaniline)titanium dichloride activated by methylaluminoxane resulted in the unexpected production of a prevailingly isotactic polypropylene. The observed microstructure is that expected for an “enantiomorphic site” mechanism of steric control. End group analysis, also using deuterium labeling techniques, indicated a primary insertion in the initiation step, while a secondary insertion was observed in the termination step. The microstructures of the homopolymer and of a copolymer of propene with trace amounts of ethylene are consistent with the presence of (shorter) highly isotactic primary blocks and (longer) poorly isotactic secondary blocks. Similar results were obtained with the bis(phenoxyaldimine) titanium complexes bearing chlorides or iodides on the ligand phenol rings. The isotacticity of the produced polypropylenes decreases as the size of the halide substituents is reduced. A new phenoxyketimine titanium complex bearing o-bromine substituents on the phenolate rings produced, under similar polymerization conditions, truly atactic polypropylene, at variance with the isospecific phenoxyketimine titanium complexes bearing alkyl substituents.