Direct Probing of Regioregularity for Polycarbodiimide Systems via¹⁵N NMR Analysis

Abstract

After more than a decade of ambiguity, polycarbodiimides have been discovered to be fully regioregular when containing two sterically inequivalent pendant groups. To directly probe the regioregularity, a series of nitrogen-15 isotopically enriched polycarbodiimides with various combinations of pendant groups was synthesized using a variety of catalysts. Subsequent 15 N NMR analysis was performed on each of the labeled polymers to accurately determine the preferred regioisomer(s) and any particular bias present for monomer insertion. More sterically hindered substituents, i.e., aromatics, were found to be relegated to the imine nitrogen while the less hindered aliphatic groups were, in all cases, located on the amine nitrogen. No electronic biases were observed and the use of different titanium(IV) catalysts yielded the same regioisomer. Carbodiimides bearing sterically equivalent groups were polymerized to form regioirregular polymers with a 1:1 mixture of both regioisomers.

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.…”

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

“…Therefore, FTIR analysis compared to the unlabeled version of the polymer was used in order to assign regioregularity. The unlabeled poly[ N ‐(4‐nitrophenyl)‐ N ′‐hexylcarbodiimide) displayed a strong imine stretch at 1634 cm −1 , comparable to Poly‐5 with an imine stretch at 1633 cm −1 , indicating a lack of an isotopic shift for the imine stretching mode . This provides evidence for the N‐labeled nitrogen being relegated to the amine position on the polymer.…”

“…A second series of polymers were synthesized to determine if the previous findings were consistent for symmetric polymers, asymmetric polymers containing substituents that have electronic influences, and polymers with pendant groups containing secondary carbons attached α, β, and γ to the nitrogen of the backbone . Various substituents were attached at the 4‐position of the phenyl pendant group to influence electronic effects, in order to observe any impact induced upon the polymer's microstructure.…”

“…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 .…”

“…) . 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.…”