The effect of pendant group structure on the thermoresponsive properties of N-substituted polyesters

Synthetic polymers exhibiting reversible lower critical solution temperature (LCST), such as poly(N-iso-propylacrylamide) (PNIPAM), are intriguing materials with various potential applications. We recently developed a new class of biodegradable thermoresponsive polyesters (TR-PEs) based on N-substituted diol monomers. TR-PEs exhibited reversible cloud point temperatures (Tcp) between 0-50 degrees C and were shown to be non-cytotoxic. The synthesis of N-substituted diols and TR-PEs is highly modular, allowing for a wide variety of possible homo-and copolyesters. In this work, we report the synthesis and characterization of 20 homopolyesters in order to better understand the structure-property relationship of TR-PEs. UV-vis spectroscopy showed that the Tcp of TR-PEs was highly dependent on pendant group structure, such as secondary or tertiary amides, cyclic and linear groups, and substitution of oxygen atoms. Structure-Tcp analysis provides a correlation between Tcp and the number of heteroatoms relative to the number of carbon atoms in the pendant group thereby providing a rationale for the design of thermoresponsive polyesters with desired Tcp values. To demonstrate the expanded tunability of the TR-PE system, copolyesters bearing covalently attached ibuprofen were synthesized and shown to exhibit LCST behavior dependent on comonomer hydrophilicity.