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

Abstract

Nitrogen-15 nuclear magnetic resonance (NMR) spectroscopy and infrared spectroscopy (IR) were performed on an isotopeenriched 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 15 N 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 15 N 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.

Evidence of Entropy-Driven Bistability through 15N NMR Analysis of a Temperature- and Solvent-Induced, Chiroptical Switching Polycarbodiimide

“…15 N NMR collection of the polymers and precursor compounds was achieved using the same procedure previously reported. 48,49 Each spectrum was referenced externally to 15 Effect of Solvent on 15 N NMR Spectra of Poly-3. Taking another look at the CN imine stretch in the IR spectra of Poly-3 reveals an interesting result.…”

Novel Approaches in Selective Tryptophan Isotope Labeling by Using Escherichia coli Overexpression Media

“…Indole is converted into tryptophan in vivo by tryptophan synthase [14] and has been used in protein overexpression. [ 15 N]indole 12 was prepared via [ 15 N]aniline hydrochloride 10,which is accessible by areported procedure with ammoniumc hloride 7 as the source for 15 N. [17] Houben-Hoesch acylation with chloroacetonitrile and subsequent reductivec yclization with NaBH 4 yielded [ 15 N]indole 12. We synthesized simple [ 13 C]-and [ 15 N]indole isotopologues (Scheme 3).…”

Developments in synthesis, characterization, and self‐assembly of polycarbodiimide systems

“…To further resolve and definitively prove regioregularity or irregularity for polycarbodiimides, DeSousa and Reuther synthesized a series of N labeled polycarbodiimides containing various aryl and alkyl groups (Fig. ) . The N‐labeled polymers were synthesized via amidation of various acyl chlorides with NH 4 Cl, Hofmann rearrangement, and deprotection to afford a library of N labeled amines .…”

“…This was followed by the formation of N‐labeled urea, dehydration to carbodiimide monomers, and polymer synthesis. The first polymer synthesized, poly( N ‐phenyl‐ N ′‐hexylcarbodiimide), was N labeled at both the aromatic and alkyl nitrogens ( Poly‐1 ), the aromatic nitrogen position ( Poly‐2 ), and the alkyl nitrogen ( Poly‐3 ) . The equally isotopically labeled polymer ( Poly‐1 ) showed evidence of being completely regioregular due to the presence of two equally intense peaks at 136.5 ppm, for the imine nitrogen, and 15.5 ppm, for the amine nitrogen (Fig.…”