Conformational Analysis of Poly(trimethylene imine) and Poly(N-methyltrimethylene imine) by the Rotational Isomeric State Scheme with up to Fourth-Order Intramolecular Interactions

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 1 H and 13 C NMR experiments for the monomeric model compounds, CH 3 NR 1 CH 2 CH 2 CH 2 NR 2 CH 3 (R 1 = R 2 = H; R 1 = H and R 2 = CH 3; R 1 = R 2 = CH 3 ). 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.