Stereoselectivity, homogeneous metallocenes

In this chapter we will discuss a few topics in the area of alkene polymerisations catalysed by homogeneous complexes of early and late transition metals (ETM, LTM). One of the main research themes for the ETM catalysts has been the polymerisation of propene, while industries have also paid a lot of attention to metallocenes giving LLDPE (linear low-density polyethylene, for thinner plastic bags). In less than a decade a completely new family of catalysts has been developed which enables one to synthesise regioselective and stereoselective polymers of a wide variety of monomers. These new catalysts are starting to find application on industrial scale, but as yet only reports on commercialisation of (branched) polyethylene and polystyrene have appeared. 

Busico et al., on the other hand, came to a conclusion [299] that the stereoregularity of polypropylene produced with C2-symmetric group 4 ansa-metallocene catalysts is a result of the interplay of two competing reactions. These are isotactic monomer polyinsertion and a side process of epimerization of the polymer chain at its active end. That makes this class of homogenous catalysts different from the typical Ziegler-Natta catalyst, because with these catalysts enantioselectivity and stereoselectivity are not necessarily coincidental [96]. 

Keywords metallocene catalyst, Ziegler-Natta catalyst, olefin polymerization, polyolefins, homogeneous catalysts, supported catalysts, stereoregularity, molecular weight distribution (MWD), chemical composition distribution, Unipol , Novolen , stereoselectivity, single site catalyst, multiple site catalyst, gas phase process, slurry process, homopolymerization, copolymerization. 

It was found that polypropylene made by such complexes (cocatalyzed by MAO) tended to be isotactic, as opposed to the atactic polypropylene that had been hitherto made by metallocenes such as Cp2ZrCl2. The strong relationship between ligand structure and polymer tacticity is considered to be the most secure proof of the migratory insertion mechanism for polymerization. The lure of homogeneous stereoselective olefin polymerization is responsible for much of the diversity of ligand structures within the bis(Cp) family. 

The fluorenyl ligand has proven to be an essential component in many homogeneous, stereoselective olefin polymerization catalysts. Since the first fluorenyl-containing metallocene was employed in 1988 for syndioselective propylene polymerization, other tacticities have been accessed isotactic, hemiisotactic, isotactic-hemiisotactic, and syndiotactic-hemiisotactic. Indeed, fluorenyl-containing metallocenes are remarkably versatile, valuable for mechanistic inquiry, and functional in the synthesis of polymers with novel structures and properties. 

Homogeneous polymerization catalysts that have non-metallocene structures, so-called post-metallocene catalysts, have been extensively explored. Stereoselective polymerization using these catalysts has also been reported, although most examples describe polypropylene. 

It has been shown that a variety of metallocene complexes serve as effective homogeneous, stereoselective polymerization catalysts. They achieve imprece-dented polymers that could not be produced by traditional catalysts. Many examples of non-metallocene catalysts have also been studied. In industry, however, conventional heterogeneous catalyst systems are still major processes for the production of polyolefins. 

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