Kinetics of N-isopropylacrylamide polymerizations in supercritical carbon dioxide fluids

Free-radical polymerization kinetics of N-isopropylacrylamide (NIPAAm) using 2,2′-azobis(isobutyronitrile) (AIBN) as an initiator in supercritical carbon dioxide (scCO2) was investigated. A high-pressure differential scanning calorimeter revealed that the melting temperatures of NIPAAm and AIBN were both decreased with increasing CO2 pressure in a linear fashion and the polymerization could occur in CO2 at 55 °C. The polymerization kinetics of NIPAAm in scCO2 was compared with that in methanol (MeOH). At 55 °C, the induction periods of polymerizations in scCO2 of 27.6 MPa were much longer (up to 6 h) than those in MeOH (about 30 min) for similar feed concentrations. The monomer conversions reached and molecular weights produced were much higher in scCO2 than in MeOH. The reaction orders for initial monomer and initiator concentrations, [NIPAAm]0 and [AIBN]0, in initial stage of polymerizations were respectively 1.48 and 0.79 in scCO2 and 1.27 and 0.51 in MeOH.