Stereoselective polymerization of lactones: properties of stereocomplexed PLA building blocks

Biomaterial science has become an important aspect in medical developments, and many so-called ‘biomaterials’ have emerged. These materials should be biocompatible, i.e. perform with an appropriate host response on a specific application without causing e.g. inflammatory or toxicological responses, and preferably biodegradable, i.e. removed from the body due to natural, biological processes. Important examples are poly(lactic acid)s (PLA) and poly(ε-caprolactone)s (PCL), which are conveniently prepared by ring-opening polymerization (ROP). For these chiral monomers, stereoselectivity of the ROP catalyst has become an important topic, since enrichment in stereoregular sequences in the resulting polymers can significantly affect the material properties. Seven ROP catalysts have been decribed in the thesis, that is: three phenolate-ligated zinc-based catalysts of the type [LnZnEt]2 (Chapter 3) and three thiophenolate-ligated zinc-based catalysts of the type [LnZnEt]n (Chapter 4) for the ROP of lactide, and R,R’-(salen) aluminum isopropoxide for the ROP of 4-methyl-ε-caprolactone and 6-methyl-ε-caprolactone (Chapter 5). Polymerizations of the zinc phenolates were fast and well-controlled. The stereoselectivity of 2,6-bis[(dimethylamino)methyl]-4-methylphenoxy ethylzinc was shown to depend on the polymerization medium, giving atactic polymers in dichloromethane and isotactic enrichments in THF and toluene (Pm ≈ 0.60). The polymerizations using the zinc thiophenolates resulted in heterotactic enrichment (Pr ≈ 0.60). The catalyst R,R’-(salen) aluminum isopropoxide (previously shown to preferably polymerize S,S-lactide over R,R-lactide) showed a preference for the R-enantiomer in the ROP of 6-methyl-ε-caprolactone, but no stereoselectivity upon polymerizing 4-methyl-ε-caprolactone. Chapter 6 describes the synthesis of γ-Boc-amino-ε-caprolactone and ring-opening reactions thereof. Rather than giving the functional amine-protected poly(ε-caprolactone), γ-Boc-amino-ε-caprolactone rearranged into an oxopyrrolidine carboxylate. Chapter 7 decribes the synthesis of several heterotelechelic polylactides. It was shown that PLA crystallization as well as stereocomplexation is hampered by modification of the chain ends. Chapter 8 describes the synthesis of PLA-dextran copolymers, with grafting based upon carbamate and secondary amine linkages, and the formation of hydrogels thereof. Although the hydrolytic stability of the copolymers was higher when compared to grafting by ester or carbonate linkages, their relatively poor water solubility resulted in rather weak hydrogels (200-500 Pa), which cannot bear heavy loads but may serve as vehicles for the release of e.g. medicine.