An inversional-rotational isomeric state (IRIS) scheme including first-order to fourth-order intramolecular interactions has been developed and applied to conformational analysis of poly(trimethylene imine) (PTMI) and poly(N-methyltrimethylene imine) (PMTMI). Bond conformations and conformational energies of PTMI and PMTMI were evaluated from ab initio molecular orbital calculations at the MP2/6-311++G(3df, 3pd)//HF/6-31G(d) level and 1H and 13C NMR experiments for the monomeric model compounds, CH3NR1CH2CH2CH2NR2CH3 (R1 = R2 = H; R1 = H and R2 = CH3; R1 = R2 = CH3). The IRIS analysis yielded the following data on the polymers at 25 °C:  the characteristic ratio for the infinite chain, 3.5 (PTMI) and 4.2 (PMTMI); trans fractions of the C−C and C−N bonds, respectively, 0.29 and 0.77 (PTMI) and 0.40 and 0.65 (PMTMI); the meso-diad probability, 0.44 (PTMI) and 0.48 (PMTMI). Intramolecular hydrogen bonds were found in the polyimines:  PTMI, N−H···N (the interaction energy, −0.83 kcal mol-1) and C−H···N (−0.15 kcal mol-1); PMTMI, C−H···N (−0.40 kcal mol-1). The chain dimension, stereochemical configuration, and bond conformations of the polyimines are sensitive to the first-order interaction energy around the C−C bond and the hydrogen-bond energies. Polar solvents weaken the hydrogen bonds to increase the chain dimension and randomize the configuration.

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