Chiral metallocene/Lewis acid hybrid polymerization catalysts and heterobimetallic polypyridine substituted dioxocyclams

Abstract

The investigation of ansa-zirconocene catalysts for the polymerization of polar monomers is described, revealing striking differences in mechanism and the stereochemistry of products obtained depending on the nature of the Lewis Acid cocatalyst, M(C6Fs)3 (M=B,A1). Whereas borane-mediated processes proceed via a cationic active site, the aluminates apparently operated by a novel anionic mechanism. This observation led to the production of stereo-diblock copolymers of MMA via sequential catalyst addition. Further, stereo-multiblock copolymers were produced by judicious blending of catalysts. This represents catalysis wherein both cation and anion operate in tandem, generating distinct stereoregular segments. Model studies are presented which support the proposed mechanisms. Towards this end, the first chiral ama-metallocene (mono- and bis-) ester enolates were prepared and characterized, and demonstrated to be competent catalysts and useful model compounds. The complexation of a new class of ligands, wherein polypyridine functionality is covalently appended onto capped dioxocyclams, was investigated. Coordination linkage of these species was then achieved through Ru(II) and Co(II) of terpyridine capped cyclams. Further investigations revealed an N,C,N variant of terpyridine was more practical. This new (dipyridyl benzene, dpb) capped dioxocyclam readily cyclometallated Ru. The resultant products were then subjected to metal complexation of the dioxocyclam cavity, resulting in heterobimetallic products containing an Ru photosensitizer group covalently linked to cobalt and copper dioxocyclams.