High Molecular Weight Poly(α,α′,β-trisubstituted β-lactones) As Generated by Metal-Free Phosphazene Catalysts

The use of different phosphazene bases has been investigated as catalysts in polymerization of [R S]-4-benzyloxycarbonyl-3,3 dimethyl-2-oxetanone (dMMLABz) in presence of carboxylic acids used as initiators Whatever the catalyst considered (P-1-t Bu, P-2-t Bu, or P-4 t-Bu) a very good control over the polymerization has been obtained in terms of polyester molecular weights and end groups fidelity as attested by spectroscopic techniques such as H-1 NMR, gel permeation chromatography (GPC) and matrix assisted laser desorption ionization mass spectrometry (MALDI-MS) experiments Characterized by a complete absence of transfer reactions, the system allows for the generation of very high molecular weight PdMMLABz trough a mechanism selectively involving the O-alkyl scission of the beta-lactone monomer A clear dependence of the basicity of the phosphazene catalyst on the overall polymerization kinetics has been observed where the most basic catalytic species i e 1 tert-butyl 4 4 4-tris(dimethylamino)-2 2 bis[tris(dimethylamino)phosphoranyl-idenamino]-2 Lambda(5),4 Lambda(5)-catenadi-(phosphazene) (P-4-t Bu), shows the highest active ion pair Accordingly P-4-t-Bu allowed for the synthesis of PdMMLABz chains characterized by a number average molecular weight higher than 1 5 x 10(6) g mol(-1)