Metal-free catalyzed ring-opening polymerization of β-lactones: Synthesis of amphiphilic triblock copolymers based on poly(dimethylmalic acid)

Abstract

A polylactide-block-(dimethyl β-malic acid-co-β-butyrolactone)- block-polylactide (PLA-b-P(dM-MLA-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 1H 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. © 2006 American Chemical Society.