Metallocene monomers, synthesis

Various strategies for the synthesis of metallocene monomers were described in [198]. Vinylmetallocenes 38 like vinylferrocene or ri -(vinylcyclopentadienyl)-dicarbonylnitro-sylmanganese 39 are prepared several decades ago by synthesizing the vinyl group in the metallo-derivatives [199]. Other 7c-type compounds such as ri -(styrene)tricarbonyl-chromium 40 are obtained by reaction of styrene with triamine-tricarbonylchromium [200]. 

A long-standing goal in polyolefins is the synthesis of polymers bearing polar functional groups such as acrylate, esters, or vinyl ethers, etc [24,40]. These copolymers might endow polyolefins with useful properties such as adhesiveness, dyeability, paintability, and print-ibility. Advances have recently been made in polymerizing polar monomers with cationic metallocene catalysts... 

Some of the drawbacks of the metallocene catalysts are their limited temperature stability and the production of lower-molecular-weight materials under commercial application conditions. It follows that they have a limited possibility for comonomer incorporation due to termination and chain-transfer reactions prohibiting the synthesis of block copolymers by sequential addition of monomers. This led to the development of half-sandwich or constrained geometry complexes, such as ansa-monocyclopentadienylamido Group IV complexes (67) 575,576... 

A single step of the polymerization is analogous to a diastereoselective synthesis. Thus, to achieve a certain level of chemical stereocontrol, chirality of the catalytically active species is necessary. In metallocene catalysis, chirality may be associated with the transition metal, the ligand, or the growing polymer chain (e.g., the terminal monomer unit). Therefore, two basic mechanisms of stereocontrol are possible (145,146) (i) catalytic site control (also referred to as enantiomorphic site control), which is associated with the chirality at the transition metal or the ligand and (ii) chain-end control, which is caused by the chirality of the last inserted monomer unit. These two mechanisms cause the formation of microstructures that may be described by different statistics in catalytic site control, errors are corrected by the (nature (chirality) of the catalytic site (Bernoullian statistics), but chain-end controlled propagation is not capable of correcting the subsequently inserted monomers after a monomer has been incorrectly inserted (Markovian statistics). 

In contrast to Group IV-based polymerization catalysts, late transition metal complexes can carry out a number of useful transformations above and beyond the polyinsertion reaction. These include isomerization reactions and the incorporation of polar monomers, which have allowed the synthesis of branched polymer chains from ethylene alone, and of functional polyolefins via direct copolymerization. The rational design of metallocene catalysts allowed, for the first time, a precise correlation between the structure of the single site catalyst and the mi-crostructure of the olefin homo- or copolymer chain. A similar relationship does not yet exist for late transition metal complexes. This goal, however, and the enormous opportunities that may result from new monomer combinations, provide the direction and the vision for future developments. 

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