Chain extension as a strategy for the development of improved reverse thermo-responsive polymers

The hypothesis that chain extension can be harnessed to the generation of improved reverse thermo-responsive polymers was tested by following two basic synthetic pathways: (1) the polymerization of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) tri-blocks using hexamethylene diisocyanate (HDI) as chain extender 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 repeating unit is known for its own RTG behavior, the latter polymers consist of segments incapable of exhibiting a reverse thermal gelation (RTG) of their own. Dynamic light scattering (DLS) measurements revealed that the nanostructures formed by the chain extended 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). The chain extended polymers achieved much higher viscosities and their gels displayed enhanced long-term stability at 37 degrees C. Copyright (c) 2007 John Wiley & Sons, Ltd.