Synthesis and micellization properties of novel symmetrical poly(ε-caprolactone-b-[R,S] β-malic acid-b-ε-caprolactone) triblock copolymers

A four-step strategy to synthesize well-defined amphiphilic poly(ε-caprolactone-b-[R,S] β-malic acid-b-ε-caprolactone) triblock copolymers [P(CL-b-MLA-b-CL)], which combines the anionic polymerization of [R,S] benzyl β-malolactonate (MLABz), and the coordination-insertion ring-opening polymerization (ROP) of ε-caprolactone (CL), followed by the selective removal of benzyloxy protective groups of the central poly(malolactonate) block is described. The first step involves MLABz initiated 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 reduction of the carboxylic acid end-group of the resulting α-hydroxy, ω-carboxylic poly([R,S] benzyl β-malolactonate) leading to an α,ω-dihydroxy PMLABz, the polymerization of CL was initiated by each hydroxyl end-groups previously activated by AlEt₃. Finally, after catalytic hydrogenation of the benzyl ester functions, the P(CL-b-MLA-b-CL] triblock copolymer was recovered and the amphiphilic character evidenced by UV spectroscopy. As demonstrated, the CMC of these new P(CL-b-MLA-b-CL) triblock copolymer is higher by one order of magnitude than that of a P(MLA-b-CL) diblock copolymer of similar composition.