Living syndiospecific polymerization of propylene promoted by C 1 -symmetric titanium complexes activated by dried MAO

Abstract

A series of C-1-symmetric titanium complexes with both salicylaldiminato and beta-enaminoketonato as the ligands have been synthesized and investigated as the catalysts for propylene polymerization. In the presence of dried methylaluminoxane (dMAO), the complex with bulky substituent tert-butyl ortho to alkyl oxygen can promote living polymerization of propylene with improved catalytic activity at ambient temperature, producing high molecular weight syndiotactic polypropylenes (rrrr 90.2%) with narrow molecular weight distributions (M-w/M-n = 1.071.22), via a propagation of 1,2-insertion of monomer and chain-end control of stereoselectivity. The propagation of polymer chain is completely different from that mediated by FI catalysts (the titanium complexes with phenoxy-imine chelate ligands) which favor 2,1-insertion of monomer. The interaction between a fluorine and a beta-hydrogen of a growing polymer chain, negligible chain transfer to monomer and dMAO without any free AlMe3 were responsible for the achievement of living propylene polymerization. The substituent ortho to alkyl oxygen determined the stereo structure of the resultant polypropylene. In the case of less steric congested complexes with two nonequivalent coordination positions, the growing polymer chain might swing back to the favorite coordination position (site-epimerization), forming m dyads regioirregular units. (c) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 638-648, 2012