Metallocene catalysts formation schemes

Most of transmetalation between main group metal compounds and transition metal complexes leads to the formation of a transition metal-carbon bond. The reaction which causes alkyl or aryl ligand transfer from transition metal to main group element is much less common. Olefin polymerization catalyzed by a metallocene catalyst is sometimes accompanied by chain transfer caused by the transfer of the growing polymer end from Ti or Zr to an A1 compound that is used as the cocatalyst (Scheme 5.23) [139,140]. 

The stereochemical mechanism responsible for the isoselectivity of Ci-symmetric metallocene catalysts has been a topic of considerable debate. There are two limiting mechanisms possible for the formation of isotactic polypropylene with such Ci-symmetric catalysts having diastereotopic coordination sites. These are the site epimerization mechanism and the alternating mechanism, as shown in Scheme 2.3. 

Recent advances in the development of well-defined homogeneous metallocene-type catalysts have facilitated mechanistic studies of the processes involved in initiation, propagation, and chain transfer reactions occurring in olefins coordi-native polyaddition. As a result, end-functional polyolefin chains have been made available [103].For instance, Waymouth et al.have reported about the formation of hydroxy-terminated poly(methylene-l,3-cyclopentane) (PMCP-OH) via selective chain transfer to the aluminum atoms of methylaluminoxane (MAO) in the cyclopolymerization of 1,5-hexadiene catalyzed by di(pentameth-ylcyclopentadienyl) zirconium dichloride (Scheme 37). Subsequent equimolar reaction of the hydroxyl extremity with AlEt3 afforded an aluminum alkoxide macroinitiator for the coordinative ROP of sCL and consecutively a novel po-ly(MCP-b-CL) block copolymer [104]. The diblock structure of the copolymer... 

SCHEME 1.1 Formation of an active catalyst species from a metallocene dichloride. 

When propylene is polymerized using homogeneous metallocene/methylaluminoxane (MAO) catalysts, several chain transfer mechanisms occur to release free polymer chains. Their relative frequencies are dependent on the polymerization conditions and the catalyst structure. Three chain transfer mechanisms are identified that form chain-end unsaturated polypropylene. These are (1) P-hydride transfer to metal after a primary (1,2-) propylene insertion, (2) f)-hydride transfer to monomer after a primary propylene insertion, and (3) f)-hydride transfer after a secondary (2,1-) propylene insertion. The formations of these chain ends and associated polymer head groups (chain starts) are shown in Scheme 10.1. Mechanism (2) is commonly referred to as chain transfer to monomer. 

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