Solution XAS Analysis for Exploring Active Species in Syndiospecific Styrene Polymerization and 1-Hexene Polymerization Using Half-Titanocene-MAO Catalysts: Significant Changes in the Oxidation State in the Presence of Styrene

Mechanistic studies through Ti K-edge XANES and EXAFS spectra of the catalyst solution for 1-hexene polymerization using Cp*TiX2(O-2,6-(Pr2C6H3)-Pr-i) [X = Cl (1), Me (2)1-MAO catalysts and for syndiospecific styrene polymerization using ((BuC5H4)-Bu-t)TiCl2 (O-2,6-(Pr2C6H3)-Pr-i) (3)-MAO catalyst have been explored. Significant changes in the XANES spectrum (low energy shift in the edge peak, in addition to decrease in intensities of two pre-edge peaks) were observed when styrene (200 equiv) was added into a toluene solution containing 3 and MAO, strongly suggesting that the complex 3 [Ti(IV)] was reduced to Ti(III) by addition of styrene under these conditions. The EXAFS analysis suggests that the Ti-O bond (corresponding to coordination of phenoxide) was preserved along with dissociation of Ti-Cl bonds by treating with MAO. These XAS analysis data thus suggest that neutral Ti(III) species, ((BuC5H4)-Bu-t)Ti(R)(OAr), play a role as the active species. The DFT-based computational analysis on the syndiospecific styrene insertion reaction also revealed that the neutral Ti(III) catalyst exhibits the lower activation energy than the others, strongly supporting the above mechanism. In contrast, no significant changes (in the oxidation state, basic structure) in the XANES spectra were observed when the toluene solution containing 1 (or 2) was added MAO and 1-hexene, whereas preservation of the Ti-O bond along with dissociation of the Ti-Cl by treating with MAO was suggested through the EXAFS analysis.