Polymers based on PNIPAm and PDEAm have great utility in injectable biomedical applications as they exhibit inverse phase solubility at transition temperatures between room temperature and physiological temperature. The aim of this study was to design polymers that would lead to physical gels at elevated temperatures.

Two polymer systems were studied, one a triblock poly(ethylene-alt-propylene)- b-poly(ethyleneoxide)-b-poly(N-isopropyl acrylamide-co-acrylic acid), PEP-PEOP(NIPAm-co-AA), or PO(N/A) triblock. This construct comprised of a hydrophobic PEP and a hydrophilic PEO mid block. The third block was based on PNIPAm, a thermoresponsive polymer exhibiting inverse phase solubility at a temperature of 32°C. PNIPAm was substituted with AA to render the block dual temperature and pH responsive. The other triblock PNIPAm-P(NIPAm-co-AA)-PDEAm [(PN(N/A)D] comprised thermoresponsive blocks with the midblock being both temperature and pH responsive. PDEAm exhibited an LCST, 30°C, close to that of PNIPAm.

The triblocks were designed with the intent that on heating, the reduced solubility of the side blocks would cause the polymer to self assemble resulting in polymer aggregation/gelation. The polymers were synthesized by RAFT polymerization and anionic polymerization. Polymer association was studied by DLS, UV spectroscopy and rheology. Greater success was found with the PO(N/A) triblocks in achieving gelation at a critical temperature at low pH, than for the PN(N/A)D system, perhaps due to differences in the self assembly mechanisms.