New multinuclear half-titanocene catalysts in the syndiospecific polymerization of styrene

Abstract

The syndiospecific polymerization of styrene with a new class of multinuclear transition metal catalysts in the presence of methylalumoxane and triisobutylaluminum has been investigated. The new multinuclear catalysts [(η 5-C 5Me 5)Ti] 4(μ-O) 6 and [(η 5-C 13H 17)Ti] 4(μ-O) 6 were received by reaction of the corresponding mononuclear compounds with water and characterized by X-ray crystal structure analysis. The molecular structure of both complexes is tetrameric with six bridging oxygen atoms between the four titanium atoms, forming an adamantane-like cage structure with a substituted cyclopentadienyl ligand remaining η 5-bonded to each titanium atom. The bulky [(η 5-C 13H 17)Ti] 4(μ-O) 6 shows higher polymerization conversions than [(η 5-C 5Me 5)Ti] 4(μ-O) 6. The polymerization activity is significantly increased by an enhancement of the MAO concentration after a short retardation period and levels off at MAO/[(η 5-C 13H 17)Ti] 4(μ-O) 6 molar ratios above about 600. Triisobutylaluminum increases the polymerization yield to a maximum at a TIBA/[(η 5-C 13H 17)Ti] 4(μ-O) 6 molar ratio of about 30–100, but considerably decreases it at higher molar ratios below the polymerization conversion reached without any additional aluminum alkyl. Both compounds affect molecular weight and molecular weight distribution without any influence on the stereospecificity of the different catalytic sites active in polymerization reactions. The new multinuclear transition metal catalysts reach about 30–50% of the polymerization activity of the mononuclear catalysts on a molar basis and show a remarkably high catalytic activity in complex-coordinative polymerizations even after storage in non-inert-atmosphere conditions. The active polymerization sites of the multinuclear catalysts are not as uniform as the active sites of the mononuclear catalysts are and provide polystyrenes of a slightly lower syndiospecificity, but do not significantly influence the weight average molecular weights.