C=N stretching frequencies in some model compounds for acrylonitrile-styrene copolymers

Abstract

The C=N stretching frequency of some model compounds for acrylonitrile (AN)-styrene (ST) copolymers in dimethylformamide was studied in order to explain the fact that the C=N stretching band of AN units in AN-ST copolymers shifts to higher frequency as the probability for finding the AN-AN bond in AN-ST copolymers increases. The following results have been obtained. (1) The C=N stretching band in mononitriles, CH₃(CH₂)_nCN (3≤n≤9), occurs at the essentially constant frequency {(2242± 0.5)cm^-1}. (2) The C=N stretching band in dinitriles, NC(CH₂)_nCN (5≤n≤8), occurs at the same frequency (2242.5 cm^-1), but when n≤4 the band shifts to higher frequency as n decreases. (3) The C=N stretching band in NC(CH₂)_nCN occurs at higher frequency compared with the band in CH₃(CH₂)_nCN or _??_-(CH₂)_nCN, when n≤3. These results show that the C=N band shifts to higher frequency due to intramolecular inductive effects of the nitrile groups. Moreover, the noticeable results have also been obtained in other model compounds. (1) The C=N stretching band of 2, 4, 6-tricyanoheptane, the model compound for AN-AN-AN triad, occurs at 2239.5 cm^-1 and that of polyacrylonitrile at the essentially same frequency (2239.6 cm^-1). (2) The C=N stretching band of 4-cyano-2, 6-diphenylheptane, the model compound for ST-AN-ST triad, occurs at 2234.0 cm^-1 and that of AN-ST alternating copolymer at the nearly equal frequency (2234.6 cm^-1). In conclusion, the fact that the C=N stretching mode of AN units in AN-ST copolymers is sensitive to the sequence environment and shifts to higher frequency as AN content increases can be reasonably explained by the intramolecular inductive effect of the nitrile groups in the copolymers.