Stereocontrolled Ring-Opening Polymerization of Lactide Monomers by Lewis-Acidic Metal Complexes

The main objective of the present work is the investigation of the ring-opening polymerization (ROP) of lactide monomers initiated by complexes based on lewis acidic metal centers. In Chapter II, synthesis of new OSSO-type bis(phenols) from salt metathesis reactions of 2,4disubstituted phenols with dibromoalkanes are described.

Chapter III describes the preparation of scandium and yttrium silylamide bis(phenolate) complexes by reaction of tetradentate bis(phenols) with one equivalent of scandium or yttrium silylamide complexes. The complexes [LnL x {N(SiMe2H)2}THF] (Ln = Sc,Y] are isolated as colorless or light yellow powders that are highly sensitive towards air and moisture.

In Chapter IV the application of these complexes in the ring-opening polymerization (ROP) of rac- and meso-lactide to biodegradable poly(lactide) (PLA) is discussed. The complexes are very active and selective initiators for the ROP of lactide. rac-Lactide is polymerized heteroselectively while meso-lactide is polymerized syndioselectively. Complexes that polymerize rac-lactide highly heteroselective do also show the highest syndioselectivity. Analyses of the microstructure of poly(meso-lactides) shows the rare earth metal bis(phenolate) silylamide complexes to possibly epimerize meso-lactide into rac-lactide which results in enchainments of rac-monomers into the poly(meso-lactide) chains, potentially preventing the formation of PLA with syndiotacticity values exceeding Ps = 0.92. Chapter V describes the application of indium bis(phenolate) isopropoxide complexes [InL x (O i Pr)] in the ROP of rac- and meso-lactide. ROP of rac-lactide forms atactic PLA while meso-lactide is polymerized predominantly syndioselectively. Mechanistic studies demonstrate a coordination-insertion mechanism occuring. Determination of the activation parameters of the ROP proof the ROP of rac- and l-lactide being less favored than the ROP of meso-lactide.

Chapter VI describes the reactions of alkali and alkaline earth metal precursors with a neutral and a cationic heterocycle-containing phenol based proligand. The proligand is prepared by halide elimination reaction of Me3TACD and 2,4-disubstituted-bromomethylphenol. Treatment with silylamides of potassium, calcium and magnesium as well as n-BuLi gives the corresponding metal complexes. By treatment with Brønsted acids the proligand is transformed into a cationic intermediate. Further treatment with calcium and magnesium silylamide precursors afford the formation of cationic complexes. The complexes are applied in the ROP of rac- and meso-lactide and produce predominatly atactic polylactides.