A series of poly(ethylene oxide) (PEO)/poly(epsilon-caprolactone) (PCL) containing biodegradable poly(ether ester urethane)s, covering a wide range of compositions, were synthesized and characterized. The synthesis consisted of a two-step process. During the first step, the ring-opening reaction of epsilon-caprolactone was carried out, initiated by the hydroxyl terminal groups of the PEO chain. The second step involved the chain extension of these PCL-PEO-PCL trimers with hexamethylene diisocyanate. By varying either the ethylene oxide/epsilon-caprolactone ratio or the length of both segments, we obtained a series of polymers having different morphologies and displaying a broad range of properties.
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 13C 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.
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.
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