Propene, stereoselective polymerisation

Complex 18 polymerises propene to crystalline isotactic polypropene after activation with methylaluminoxane. X-ray crystallography shows that 18 has a C2 symmetry axis and that both coordination sites are equally shielded. The related complexes, bis l-(5-cholesten-3a-yl)indenyl zirconium dichloride and diraethylsilyl-his(2-methyl-4-r-butyl-cyclopentadienyl) zirconium dichloride, can also be activated by methylaluminoxane for the stereoselective polymerisation of propene. i . ... 

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. 

An analysis of polymer end groups provided insight into the mechanism of stereo-control in such catalysts. The first polymerisation step, where propene inserts into a Zr-Me bond, is in fact not stereoselective, while the insertion into a Zr-iso-butyl bond proceeds with high enantioselectivity. Ligand stereo-control operates therefore by an indirect mechanism the ligand determines the conformation of the polymery] chain, and this in turn influences the preferred orientation of the incoming alkene [127], as illustrated in structure 89 for a syndiospecific case. 

---