Living syndiospecific polymerization of propylene with sterically encumbered titanium complexes activated by MMAO

Abstract

A series of sterically encumbered (salicylaldiminato)(β-enaminoketonato)titanium complexes [3-^tBu-2-OC₆H₃CHN(C₆F₅)][RNC(CF₃)CHC(^tBu)O]TiCl₂ [1a: R = Ph, 1b: R = C₆H₄F(p), 1c: R = C₆H₄Cl(p), 1d: R = C₆H₄Br(p), 1e: R = C₆H₄Br(o)] were synthesized and tested to be efficient catalysts for syndiospecific polymerization of propylene in the presence of modified methylaluminoxane at room temperature. The introduction of a bulky bromine atom ortho to the imine nitrogen of the β-enaminoketonato ligand not only successfully improved the pentad syndiotacticity (rrrr) of the resulting polypropylenes from 88.5% to 97.2%, but also provided better protection of the active site from attack of free AlR₃ or monomers and thus contributed to the living polymerization nature, while keeping high catalytic activity. More importantly, compared with the famous pentafluorinated FI-Ti/MAO catalyst system, the sterically congested complex 1e with the bromine atom ortho to a N-aryl group displayed almost two times higher catalytic activity (14.5 vs. 28.0 kg mol⁻¹ h⁻¹), producing polypropylenes with even higher pentad syndiotacticity (rrrr = 97.2% vs. 96.0%) and similar narrow molecular weight distributions (M_w/M_n = 1.12–1.26). In addition, the polymerization proceeded with a different monomer insertion mode of 1,2-insertion and a similar chain-end control mechanism. Quantitative ¹³C NMR spectra revealed that polymers with various stereo structures ranging from highly syndiotactic and regioregular to atactic and regio-irregular polymers at different reaction temperatures were achieved, and the probable formation routes were proposed. The percentage of regio-irregularities of the monomer sequence arising from 2,1-insertion and 3,1-enchainment increased with the rise of reaction temperature.