Homogeneous catalysis stereoselective polymerization

The symmetry of the ligand can influence the product stereoselectivity and enantio-selectivity. Enantiomerically pure chemicals are extremely important for the agrochemical, pharmaceutical, and food industries. Many of the bidentate ligands used in these processes feature C2-type symmetry, dividing the space around the metal center into two empty quadrants and two full quadrants (see Section 3.1.3 on asymmetric homogeneous catalysis) [61,62], Ligand symmetry is also important in polymerization catalysis [63], where it can influence the polymer s tacticity (Figure 3.22). 

Considerable effort has been directed to the stereoselective synthesis of aimulated cyclopentadienyl complexes of the group 4 transition metals. Ci asymmetric and C2 synunetric aimulated cyclopentadienyl ligands have been used to prepare chiral organotitanium complexes, which are now being actively studied as catalysts for asymmetric synthesis and olefin polymerization see Asymmetric Synthesis by Homogeneous Catalysis). 

W(NAr)(=CHBuO(OR)2 (R = CMe(CF3)2, 50) are highly active catalysts for the metathesis of internal alkenes (equation 16), and also effect the stereoselective olefmation of hydroxy ketones (equation 17). The reactivity of these catalysts can be tuned by varying the aUcoxide ligands for example, when R = Bn, the complex acts only upon strained cyclic alkenes and is a highly effective ring-opening metathesis polymerization (ROMP) catalyst (see Metathesis Polymerization Processes by Homogeneous Catalysis). 

It is now clear that the advent of single-site catalysts for the stereoselective polymerization of olefins has initiated a revolution in polymer synthesis. Although the vast majority of homogeneous polymerization catalysts are designed for olefin polymerization, the extension of this area of catalysis for the polymerization of other monomers, such as lactones and epoxides, is already in progress. 

Razavi, A. Peters, L. Nafpliotis, L. Geometric flexibility, ligand and transition metal electronic effects on stereoselective polymerization of propylene in homogeneous catalysis. J. Mol. Catal. A Chem. 1997,115,129-154. 

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