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

Abstract

The use ofdifferent 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 1 H 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 β-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]-2Λ 5 ,4Λ 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 .