RATES OF CONFORMATIONAL TRANSITIONS IN SOLUTIONS OF RANDOMLY COILED POLYMERS .I. NUCLEAR MAGNETIC RESONANCE STUDY OF TRANSITIONS IN BACKBONE OF POLYAMIDES

Ntnr spectra were recorded for polyamides derived from piperazine or N.Nʹ-dimethylethylenediamine and aliphatic aromatic dicarboxylic acids in aqueous phenol (containing 92.5 wt % phenol) over a range of temperatures. Similar measurements were carried out on acetylpiperidine and on acetyl or benzoyl derivatives of piperazine and N.Nʹ-dimethylethylenediamine used as analogs of the polymers. Hindered rotation around the amide bond was evidenced by the splitting of NCH2 and NCH5 bands at low temperatures and coalescence of the split peaks on heating. The free energy of activation for hindered rotation was obtained from the coalescence temperature and the true chemical shift of the split peaks. The enthalpy of activation was computed from the temperature dependence of the half-width of the coalesced peak. No significant difference was found for these activation parameters in the polymers and their analogs and the implication of this finding on the mechanism of conformational transitions in long-chain polymers is discussed. The nmr spectrum of poly(succinylpiperazine) gives evidence of a preferred helical conformation. This feature is less pronounced in the adipyl and absent in the sebacyl polyamide. Differences in the interactions of NCH3 groups and phenyl residues in poly(terephthaloyl-N,Nʹ-dimethyIethylenediamine) and its analog are attributed to restricted rotation of the phenyl rings in the polymer. © 1969, American Chemical Society. All rights reserved.