Macromolecular Stereochemistry: The Effect of Pendant Group Structure on the Axial Dimension of Polyisocyanates

The optically active and racemic forms of the 1,2 acetone ketal of propylene glycol were converted via the hydroxyl function to the derived isocyanates. These monomers were polymerized following Shashoua, using sodium cyanide in dimethylformamide at -65 °C. Light scattering determination in chloroform of the weight-average molecular weight and the root-mean-square radius of gyration (M_w, (S²)^1/2) shows, by estimates of the persistence lengths, that the optically active polymer (730000,1600 Å) has a more extended chain than its racemic derived isomer (500000, 850 Å) and, based on the literature, is also more extended than poly(n-butyl isocyanate). The optically active polymer exhibited a circular dichroism spectrum (Δ_ϵ = -3, Δ_max = 244 nm) similar to that observed by Goodman and Chen on poly(2-methylbutyl isocyanate). The ultraviolet spectra showed an unusual sensitivity to the pendant group: the optically active and racemic derived polyisocyanates were similar (Δ_max = 238 nm, ϵ = 2.4 X 10³) and (Δ_max = 242 nm, ϵ = 2.5 X 10³) and differed from poly(n-butyl isocyanate) (Δ_max = 254 nm, ϵ = 3.7 X 10³). These data require the sense of the polyisocyanate helix to be related to the pendant group configuration and also demand the formation of a d,l copolymer in the polymerization of the racemic monomer. In addition, the results are consistent with helix reversals as important contributors to wormlike behavior in polyisocyanates. The 125.7-MHz carbon-13 NMR spectra showed broadened resonances for both backbone and side-chain nuclei in the isocyanate polymers as compared to random coil polymers. Further work is needed to separate the role of pendant group asymmetry and steric bulk in the conformational properties of the polyisocyanates.