Poly(propylenimine), PPI, was methylated using Eschweiler-Clark conditions to produce poly(N-methylpropylenimine), PMPI. Differences may be seen in the IR spectra of the PMPI (-CH2CH2CH2NCH3-) and its homolog poly(N-methylethylenimine), PMEI, (-CH2CH2NCH3-), especially in the conformation region (-1100 to -1400 cm(-1)). The addition of lithium trifluoromethanesulfonate, (LiCF3SO3), sharpens the distinctions between these systems. Comparison of IR spectra of polymer: LiCF3SO3 complexes at compositions ranging from pure polymer to 5:1 N:Li+ (molar ratio) suggests significant differences in the nature of polymer salt interactions and the complex structure present in each system. These are further evidenced by differential scanning calorimetry data in which PMPI displays less variation in glass transition temperature, T-g, with the addition of salt than seen in PMEI These observations may be interpreted in terms of local structural changes originating in cation-anion and cation-polymer interactions, particularly at mid to high salt concentrations. (C) 2009 Published by Elsevier B.V.