Introducing reactive carboxyl side chains retains phase transition temperature sensitivity in N-isopropylacrylamide copolymer gels

Introduction of significant quantities of functional carboxyl groups into temperature-responsive poly(N-isopropylacrylamide) (PIPAAm) hydrogels without compromising their intrinsic temperature sensitivity has proven difficult. We have overcome this problem by incorporating the newly synthesized 2-carboxyisopropylacrylamide (CIPAAm) monomer, with a side chain structure similar to N-isopropylacrylamide (IPAAm). Hydrogels containing more than 10 mol % CIPAAm exhibit large and sensitive volume phase transitions in response to temperature changes. These volume phase transition temperatures were nearly identical to that seen for IPAAm homopolymer gels. This is in contrast to IPAAm-acrylic acid (AAc) copolymer gels whose phase transition temperatures increase with reduced magnitudes of phase transitions with increasing AAc content. Moreover, volume phase transition temperatures and transition magnitudes for IPAAm-CIPAAm gels were not influenced by solution pH, which significantly influences the IPAAm-AAc gel. These results indicate that IPAAm-CIPAAm gels maintain their hydrophobic aggregation forces without disruption by ionized or hydrogen-bonded carboxyl groups. Because of the common carboxyl functionality and the noted apparent differences between the structures of CIPAAm and AAc monomers, differences in respective gel behaviors were rationalized to result from the structural analogy of CIPAAm's isopropylamide side chain groups with those of IPAAm. We therefore propose that maintaining alignment of isopropylamide side chains within the copolymer facilitates introduction of large amounts of functional groups into IPAAm copolymer gels without loss of phase transition behavior. The new monomer, CIPAAm, should prove useful to introduce functional carboxyl groups into temperature-responsive IPAAm hydrogels while maintaining their intrinsic temperature-sensitive behavior.