Clean polycondensation through mechanochemistry: Catalyst-free production of new urea-citric acid copolymer

Abstract

The copolymerization reaction of biodegradable monomers urea and citric acid was performed for the first time via mechanochemistry using a planetary mill with controlled variables analysis (time and stirring velocity). The copolymer was characterized by Infrared Spectroscopy (FTIR), Matrix-Assisted Laser Desorption Ionization Process - (MALDI-TOF), X-ray Crystallography – XRC, Nuclear Magnetic Resonance – NMR, Thermogravimetry Analysis – TGA, Differential Scanning Calorimetry– DSC and Scanning Electron Microscopy – SEM. The polymeric material was produced involving a selective reaction of the terminal units of the carboxylic acids of citric acid with urea, and the central COOH remained intact. The copolymerization reaction was also investigated under solution condition with water as solvent, to compare the polymer production performance in both conditions. The yield of urea-citric copolymer under mechanochemistry condition (82%) is superior to the solution condition (70%). The product has low molecular weight, high crystallinity, and good thermal stability, and these two characteristics are superior to the mechanochemical product in comparison to that produced in solution. In this way, it was demonstrated a technologically feasible, catalyst and solvent-free clean route to produce urea-citric copolymer using mechanochemistry, without the formation of wastewater.