New amphiphilic poly[(R,S)-β-malic acid-b-ε-caprolactone] diblock copolymers by combining anionic and coordination-insertion ring-opening polymerization

A three-step strategy to synthesize well-defined amphiphilic poly[(R,S)-β-malic acid-b-ε-caprolactone] diblock copolymers (P(MLA-b-CL)), which combines the anionic polymerization of benzyl β-malolactonate (MLABz) with the coordination-insertion ring-opening polymerization (ROP) of ε-caprolactone (CL), followed by the selective removal of benzyloxy protective groups is described. The first step involves initiation of the anionic polymerization of MLABz by potassium 11-hydroxydodecanoate in the presence of 18-crown-6 ether. This step was carried out at 0°C with an initial monomer concentration of 0.2 mol L^-1 in order to limit the occurrence of undesirable transfer and termination reactions by proton abstraction. After selective methylation of the carboxylic acid end group of the resulting α-hydroxy, ω-carboxylic acid poly(β-malolactonate), the polymerization of CL was initiated by the hydroxyl end group previously activated by AlEt₃. Finally, after catalytic hydrogenation of the benzyl ester functions, the P(MLA-b-CL) diblock copolymer was recovered and the amphiphilic character evidenced by proton NMR spectroscopy with varying the solvent polarity.