Ancillary stereocontrol

The Cp Ti(OR)3—[Al(Me)0]x system [52] appears to be one of the best catalysts. It has been suggested that ancillary ligands on titanium (Cl or OR) primarily influence the rate of formation of the active species but have little effect on the ability of the active site to assert stereocontrol of the propagation [81]. 

Inclusion of the agostic metal—H bond may thus be of particular importance for studies of stereocontrol since the presence of this bond forces the second carbon atom of the polymer chain into a position which ensures some repulsive interaction with one of the ancillary ligands as seen for the two nonagostic hydrogens of the methyl group at the TS of ethylene... 

In this chapter we present an overview of this increasingly active research field. The first section focuses on coordination polymerization with metal complexes, classified by the nature of their ancillary ligands. The spectacular achievements reported recently in organocatalyzed and stereocontrolled ROP are then presented. The third section concerns the macromolecular engineering of poly(a-hydroxyac-ids) by varying both their substitution pattern (with alternative monomers to lactide and glycolide) and their architecture (via block, star and dendritic copolymers). The well-established and rapidly emerging applications of these synthetic polyesters are discussed briefly in the last section. 

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