Abstract
Summary: The polymerization of methyl methacrylate (MMA) in the presence of the neutral chloro zirconocene enolates, Cp~2~ZrCl[OC(OMe)ο£ΎCMe~2~] (1), Me~4~C~2~(Cp)~2~ZrCl[OC(OMe)ο£ΎCMe~2~] (2), and Me~2~C(Cp)~2~ZrCl[OC(OMe)ο£ΎCMe~2~] (3), was investigated. None of these compounds is catalytically active on its own. They could be activated by adding Ph~3~CB(C~6~F~5~)~4~ (4); however, only if the initial concentration of enolate is higher than that of the perfluorated phenyl borate. Polymers were produced with a number average molar mass of up to 100β000 g/mol and a dispersion index of 1.1β1.3 with mixtures comprising 1 and 4 or 2 and 4. The degree of polymerization depends only on the quantity of excess enolate and not on the absolute concentration of the initiator components. In contrast, a mixture of 3 and 4 was not very active. Kinetic modeling of the systems as well as NMR spectroscopic investigations indicate that a bimetallic mechanism can describe chain growth. The activity of these catalytic systems depends on the structure of the ligand. The two zirconocenes with bridged cyclopentadienyl ligands, (2) and (3), convert MMA considerably slower. In addition to the methyl zirconocene enolates with unbridged cyclopentadienyl ligands, the corresponding chloro zirconocene enolates are also suitable for the polymerization of MMA. The latter compounds offer the advantage that preparations for the production of the initiators are comparatively simple.
Structures of investigated initiators.
magnified imageStructures of investigated initiators.