Utilization of CO2 in supercritical conditions for the synthesis of cyclic poly (N-isopropylacrylamide) via emulsion and homogeneous reactions

Abstract

In this study, cyclic poly (N-isopropylacrylamide) (cPNIPAAM) was synthesized in supercritical carbon dioxide (SC-CO2) using emulsion and homogeneous reactions for the first time. This was accomplished by applying free radical polymerization and nitroxide compounds to produce low molecular weight precursors in the SC-CO2 solvent. The cyclization reaction occurred in a homogeneous phase in the SC-CO2 solvent, with dimethylformamide (DMF) serving as a co-solvent for dissolving the linear precursor. This reaction was also conducted in SC-CO2 emulsion in water. The effects of pressure and time on the morphology, molecular weight, and yield of a difunctionalized chain were investigated, where a higher pressure led to a higher yield. The maximum yield was 64% at 23 MPa, and the chain molecular weight (Mw) was 4368 (gr/mol). Additionally, a lower pressure reduced the solubility of materials (particularly terminator) in SC-CO2 and resulted in a chain with a higher molecular weight (9326 (gr/mol), leading to a lower conversion. Furthermore, the effect of cyclization reaction types on the properties of cyclic polymers was investigated. In cyclic reactions, the addition of DMF as a co-solvent resulted in the formation of a polymer with a high viscosity average molecular weight (Mv) and a high degree of cyclization (100%), whereas the CO2/water emulsion resulted in the formation of a polymer with a lower Mv and increased porosity. Polymers were characterized by 1HNMR, FTIR, DSC, TLC, GPC, and viscometry tests. The results were presented and thoroughly discussed.