Controlling Polymer Microstructure through Catalyst Design in Ring-Opening Polymerisation of Cyclic Esters

Exploiting Stereoselectivity in Cyclic Ester Ring-Opening Polymerisation for the Preparation of Biodegradable Polymers

Rachel H. Platel
Department of Chemistry, Lancaster University, Lancaster, LA1 4YB

Aliphatic polyesters, such as poly(lactide) (PLA) prepared via metal-mediated ring-opening polymerization (ROP) of lactide, are attractive alternatives to polyolefins, since they are both biodegradable and derive from renewable materials (Fig. 1).1 These plastics are now produced on an industrial scale and have multiple applications, including in medical devices. The stereochemistry of PLA greatly affects polymer properties, in particular, crystallinity and melting temperature. Thus, developing stereoselective initiators that can control polymer stereochemistry from a racemic mixture of lactide is an importance aspect of research in this area. Whilst yttrium and scandium complexes have delivered some of the most active initiators over the past decade, those that show stereoselectivity generally produce heterotactic PLA. The highest isotacticities are delivered by group 3 complexes, in particular with aluminium salen complexes.2,3 However, high temperatures are required and the reactions typically proceed very slowly.

The combination of chiral salen ligands supporting scandium and yttrium centres is shown to give highly active complexes for the stereoselective ROP of rac-lactide. Reactions proceed rapidly at room temperature, and isotactic PLA is produced. Complex synthesis, characterization and polymerizations, as well as relevant mechanistic aspects of the polymerizations will be discussed. Additionally, the use of known rare-earth stereoselective initiators in copolymerization reactions of other cyclic ester monomers will be explored.

1. R. H. Platel, L. M. Hodgson, C. K. Williams, Polym. Rev. 2008, 48, 11.
2. N. Nomura, R. Ishii, Y. Yamamoto and T. Kondo, Chem. Eur. J. 2007, 13, 4433.
3. N. Spassky, M. Wisniewski, C. Pluta and A. LeBorgne, Macromol. Chem. Phys 1996, 197, 2627.