Chain stiffness in polyisocyanates

Light‐scattering measurements of the molecular conformation of n‐hexyl polyisocyanate (PIC) in tetrahydrofuran (THF) show that the ratio of the root‐mean‐square end‐to‐end distance to the maximum extension exceeds 25% even at a molecular weight of 3 × 105 and that the effective bond length in the equivalent freely linked chain is 23 A., corresponding to about 17 links per statistical segment. These results are at least twice as high as the values of cellulose trinitrate in acetone at a comparable degree of polymerization. Although a significant variation in intrinsic viscosity occurs in different solvents, the recently reported viscosity‐ and sedimentation coefficient–molecular weight exponents of 1.18 and 0.16 for n‐butyl PIC in THF indicate that the polymer behavior lies outside the Gaussian region and must result from the short range structure rather than from strong interactions with the solvent. In fact, by using a form of the Kirkwood‐Riseman theory, modified to take account of the expansion factor, a viscosity–molecular weight exponent of 0.9 is calculated for theta conditions. It is proposed that the chain is stiffened by restrictions to rotation imposed by resonance effects in the peptide bonds that make up the polymer backbone.