Modification of Linear Polyethylenimine with Supercritical CO2: From Fluorescent Materials to Covalent Cross-Links

Along with their DNA binding ability, the secondary amines in linear polyethylenimine (L-PEI) units can also act as a reaction platform to introduce new functionalities. This renders the possibility to make a plethora of copolymers without the burden of new monomer synthesis. In this work the modification of the secondary amines of L-PEI, in either a pure L-PEI polymer or in a PEtOx-PEI copolymer, with supercritical CO2 is reported. This reaction between a secondary amine and CO2 results in formation of a carbamate, a fluorescent zwitterionic group. This gives a facile route to fluorescent carbamate-functionalzied L-PEI and PEtOx, thereby expanding the chemical toolbox of both polymers. In depth analysis of the obtained polymers by 2D nuclear magnetic resonance spectroscopy revealed that, besides the expected carbamate groups, there were also some side reactions leading to the irreversible introduction of (cyclic) urea groups and partial cross-linking of the L-PEI. Altogether, this work introduces a new simple tool for the preparation of fluorescent L-PEI and PEtOx, while also shedding light on the occurrence of side reactions in PEI-based materials for CO2 sorption.