C=O stretching frequencies in some model compounds for methylacrylate-styrene copolymers

Abstract

The C=O stretching frequency of some model compouds of methylacrylate (MA)-styrene (ST) copolymers in benzene was studied in order to explain the fact that the C=O stretching band of MA units in MA-ST copolymers shifts to higher frequency as the MA content increases. The following results have been obtained. (1) The C=O stretching band in methyl monocarboxylate, CH₃(CH₂)_nCOOCH₃ (2≤n≤8), occurs at the essentially constant frequency {(1740.5±0.3) cm^-1}. (2) The C=O stretching band in dimethyl dicarboxylate, CH₃OOC(CH₂)_nCOOCH₃ (5≤n≤8), also occurs at the same frequency {(1740.5 ±0.3)cm^-1}, but when n≤4 the band shifts to higher frequency as n decreases. (3) The C=O stretching band in CH₃OOC(CH₂)_nCOOCH₃ occurs at higher frequency compared with the band in CH₃(CH₂)_nCOOCH₃ or _??_-(CH₂)_nCOOCH₃ when n≤3. These results show that the C=O band shifts to higher frequency due to intramolecular inductive effects of the carbonyl groups. Moreover, the noticeable results have also been obtained in other model compounds. (1) The C=O stretching band of 2, 4, 6-tricarbomethoxyheptane, the model compound for MA-MA-MA triad, occurs at 1739.4 cm^-1 and that of polymethylacrylate at the essentially same frequency (1739.6 cm^-1). (2) The C=O stretching band of 4-carbomethoxy-2, 6-diphenylheptane, the model compound for ST-MA-ST triad, occurs at 1735.0 cm^-1 and that of MA-ST alternating copolymer at just the same frequency (1735.0 cm^-1). In conclusion, the fact that the carbonyl stretching mode of MA units in MA-ST copolymers is sensitive to the sequence environment and shifts to higher frequency as MA content increases can be reasonably explained by the intramolecular inductive effect of the carbonyl groups in the copolymers.