Identification of the specific, shutter-like conformational reorientation in a chiroptical switching polycarbodiimide by VCD spectroscopy.

The specific conformational states responsible for the unique, reversible temperature- and solvent-driven chiroptical switching process experienced by poly(N-1-naphthyl-N'-octadecyl-carbodiimide) (PNOC) have been identified using VCD spectroscopy and DFT calculations. The distinct VCD spectra of PNOC corresponding to the two specific conformations were obtained for the polymer dissolved in DCM-d2 (state A) and CDCl3 (predominantly state B). To specifically assign the structures of both conformations, two simplified 7mer models were constructed and optimized using DFT calculations. The theoretical spectra associated with these model conformations show a high level of agreement when compared to the experimental VCD spectra. The two states consist of the naphthyl pendant groups aligned directionally opposing the helical rotation (model A) and aligned with the helicity of the backbone (model B). This pendant reorientation causes very large OR and ECD Cotton effect inversions upon modification to the temperature or solvent composition of dilute (+)-PNOC solutions in specific solvents. In addition, the pendant group equilibrium from state A to B causes a contraction of the helical pitch from the more expanded 5/1 pitch to the more contracted 7/2 pitch resulting in increased disorder of the solvation sphere surrounding the polymer chain.

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.

Inhibition of Acinetobacter baumannii biofilm formation on a methacrylate polymer containing a 2-aminoimidazole subunit.

A polymeric composite containing a 2-aminoimidazole derivative was synthesized. It was found that this polymer was resistant to biofilm colonization by Acinetobacter baumannii, no leaching of the 2-aminoimidazole derivative was observed after 2 weeks of treatment with deionized water, and the resulting polymer was not hemolytic.