Improved reverse thermo-responsive polymeric systems

Novel reverse thermo-responsive (RTG) polymeric systems displaying superior rheological properties were generated by polymerization of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) segments. Two basic synthetic pathways were followed: (1) The bulk polymerization of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock (PluronicRTM F127) (MW=12,600, 70 wt% PEO) with hexamethylene diisocyanate (HDI) and (2) The covalent binding of poly(ethylene glycol) and poly(propylene glycol) chains, using phosgene as the connecting molecule. While in the former, the basic amphiphilic F127 repeating unit is known for its own RTG behavior, the latter polymers consist of segments unable of exhibiting reverse thermal gelation of their own. These new materials achieved viscosities at least 15 times higher than F127, at 37°C. Dynamic light scattering measurements revealed that the microstructures formed by these novel polymers were markedly larger than those generated by PEO–PPO–PEO triblocks. While the size of Pluronic F127 micelles ranged from 15 to 20 nm, the higher molecular weight amphiphiles generated much larger nanostructures (20–400 nm). Finally, the ability of reverse thermo-sensitive gels to perform as drug delivery systems was exemplified by releasing an anti-restenosis model drug (RG-13577). A 30% P[F127]4 gel delivered the drug over 40 days, whereas a F127 gel having the same concentration released the drug over a 7 days period.