Injectable thermoresponsive hydrogels based on (Me)PEG–poly(menthide) amphiphilic block copolymers from bioderived lactone

Abstract

In conjunction with the rise in cancer incidence-mortality and handicaps of conventional poly(ethylene glycol)-based polylactide, poly(lactide-co-glycolide), or poly(ε-caprolactone) (PEG-based PLA, PLGA, or PCL) injectable thermoresponsive hydrogel platforms, formulating novel biomaterials exploiting sustainable resources for local drug release purposes has currently become critical. From this point of view, we synthesized MePEG–poly(menthide) (MePEG–PM) diblock and poly(menthide)–PEG–poly(menthide) (PM–PEG–PM) triblock copolymers through ring-opening polymerization of (−)-menthide (70%), acquired from (−)-menthone, a readily accessible ketone derivative of the natural product (−)-menthol, using MePEG and PEG as initiators and Sn(Oct)₂ as a catalyst with high conversions (>97%), narrow molecular weight distributions (1.12–1.22), and monomodal GPC traces. The molecular weights of MePEG–PM diblock and PM–PEG–PM triblock copolymers evaluated by GPC and calculated from ¹H NMR were close to the theoretical values and increased linearly with increasing monomer-to-initiator ratios. Structural determination of the copolymers was performed by comprehensive analyses via two-dimensional ¹H–¹H COSY and ¹H–¹³C HMQC techniques. The critical point in the thermoresponsive phase transition behavior was found to be the length of the PM component, which was meticulously tuned during the synthesis of MePEG–PM and PM–PEG–PM. Specifically, injectable thermoresponsive hydrogels based on these diblock and triblock copolymers prepared with lower (Me)PEG/PM ratios were found to be suitable copolymer formulations for local therapy applications as they showed fluid characteristics (sol form) at around 40–44 °C and turned into a gel form after cooling to body temperature. Moreover, the hydrolytic degradation of block copolymers in PBS at two different pH values (6.5 and 7.4) at 37 °C resulted in very high degradations (>50% at 30 days), indicating quite impressive results considering the copolymers to be used in local drug delivery systems.