Side-reactions in diisocyanate-derived bulk polyurea synthesis

Polyureas comprise a versatile family of polymers, widely applied mainly due to their outstanding mechanical properties. Nevertheless, due to the inherently very high reactivity of the isocyanate groups, polyurea synthesis is accompanied by crosslinking side-reactions, strongly affecting the polymers properties, and possible applications. The present research focused on the direct investigation and diagnostic determination in solid state, of the possible chemical structures formation in bulk polyurea synthesis, using a combination of Fourier transform infrared (FTIR) and solid-state ¹³C Nuclear magnetic resonance (NMR) analysis. Syntheses of polyureas were hereby designed to yield a very high density of the newly formed chemical structures and, consequently obtaining strong and accurate diagnostic analytical signals. The results of the present research, conclusively revealed that even in so-designed linear polyureas, there is a predominant occurrence of biuret formation and a significantly lower urea group occurrence. Polyureas of a 2:1 excess of diisocyanate:diamine, exhibited an almost exclusive biuret content. Synthesis of a new branched and crosslinked polymer using a monoamine and diisocyanate, provided strong evidence of a preferential further side-reaction of isocyanate groups with formed biurets, rather than with the very abundantly remaining urea groups to form new biurets. An energetically-favorable mechanism for the further side-reaction of formed biurets is hereby suggested. Solid-state ¹³C NMR analysis results were highly consistent with the FTIR results.