Resolving the Regioregularity of Poly(N-n-hexyl-N'-phenylcarbodiimide) via Nitrogen-15 Labeling

Nitrogen-15 nuclear magnetic resonance (NMR) spectroscopy and infrared spectroscopy (IR) were performed on an isotope-enriched poly(N-n-hexyl-N'-phenylcarbodiimide) to determine directly the connectivity and regioregularity of a polymer. Up to this point, the imine, C═N, IR stretch at 1660-1620 cm^-1 was thought to be a sufficient handle to elucidate the presence or absence of a regioregular microstructure; however, recent findings cast some uncertainties when expanded to all polycarbodiimides. Therefore, an enriched ¹⁵N NMR study was undertaken to unambiguously resolve that a N-n-hexyl-N'-phenylcarbodiimide, when polymerized with a 2,2,2-trifluoroethoxide trichlorotitanium(IV) catalyst in chloroform, will produce a completely regioregular polymer. The only regioisomer present is the one in which the phenyl pendant group is positioned on the imino-nitrogen. The study was expanded to a chiral, (R)-BINOL-Ti(IV)-diisopropoxide catalyst which revealed no change in the regioisomer or the degree of regioregularity. In addition to ¹⁵N NMR spectra, the IR imine stretch exhibited isotope shifts for poly(N-n-hexyl-N'-phenylcarbodiimide) when labeled on both imino- and amino-nitrogen or solely the imino-nitrogen of ∼11 cm^-1; however, no shift was manifested when labeling was restricted to the amino-nitrogen.