Conclusive chemical deciphering of the consistently occurring double-peak carbonyl-stretching FTIR absorbance in polyurethanes

Polyurethanes include an extremely vast and varied family of polymers, exhibiting a vast range of properties and applications. Although the urethane chemical structure consists of a single carbonyl group, the vast majority of polyurethane Fourier transform infrared spectroscopy (FTIR) spectra exhibit two distinct adjacent carbonyl-stretching absorbances. It was the purpose of the present research to investigate and determine the reason of occurrence of this consistently observed phenomenon. A polyurethane, designed and synthesized here as linear and containing only urethane and methylene groups, strongly exhibited two very distinct carbonyl-stretching FTIR absorbances. A new polyurethane, exhibiting an extremely high degree of trifunctional crosslinking, was hereby designed and synthesized to sterically inhibit diisocyanate access to already established urethanes and thus inhibit the allophanate and further tertiary oligo-uret forming side-reactions. The resulting polymer dramatically exhibited only a single, strong and sharp, carbonyl-stretching FTIR absorbance belonging only to the urethane group. Synthesis of a polymer exhibiting a lower degree of crosslinking led to the reappearance of the split double carbonyl-stretching FTIR absorbance. Solid-state ¹³ C NMR measurement results of the same polymers were highly consistent with the FTIR spectroscopy results. The experimental results of the present research conclusively prove and determine the exclusive side-reaction-related double carbonyl-stretching absorbance in the FTIR analysis of polyurethanes. These research results conclusively reveal that, in fact, the so-called linear polyurethanes synthesized from diisocyanates and diols are branched or even loosely crosslinked.