Metal-Free Catalyzed Ring-Opening Polymerization of β-Lactones: Synthesis of Amphiphilic Triblock Copolymers Based on Poly(dimethylmalic acid)

A polylactide- block -(dimethyl β-malic acid- co -β-butyrolactone)- block -polylactide (PLA- b -P(dMMLA- co -BL)- b -PLA) triblock copolymer has been synthesized according to a totally original three-step strategy. In a first step, the ring-opening polymerization (ROP) of dimethyl benzyl β-malolactonate (dMMLABz) and β-butyrolactone (BL) has been carried out in a toluene/ t -BuOH solvent mixture at 80 °C by using ethylene glycol as initiator and the commercially available 1,3,4-triphenyl-4,5-dihydro- 1H -1,2,4-triazol-5-ylidene carbene 1 as catalyst. The ROP mechanism selectively involves O-acyl cleavage of both dMMLABz and BL cyclic comonomers with a preferential incorporation of dMMLABz units in the growing polyester chains as attested by 1 H NMR spectroscopy. In a second step, the so-produced α,ω-dihydroxy P(dMMLABz- co -BL) copolyester have been further considered as difunctional macroinitiatior in l, l -lactide (LA) monomer ROP at 90 °C. In the third and last step, the benzylic ester functions pending along the recovered PLA- b -P(dMMLABz- co -BL)- b -PLA triblock copolymer have been reduced/deprotected by catalytic hydrogenation leading to the expected PLA- b -P(dMMLA- co -BL)- b -PLA symmetric triblock copolyester. The amphiphilic character of this novel type of A−B−A triblock copolymers has been evidenced by some preliminary dynamic light scattering experiments in aqueous solution. While “flower” type micelles are formed at low temperature (e.g., 4 °C), microgelation occurs at higher temperature (at ca. 25 °C). By a further increase in the temperature and consistent with a temperature-dependent reversible gel−sol transition, the microgel dissolves at ca. 40 °C to the benefit of primary micelles. These observations have allowed for drawing a preliminary sol−gel phase diagram in water characterized by lower and upper concentration-dependent critical gelation temperatures.