NMR Composition Analysis of Copolymers

Abstract

Abstract High resolution NMR spectroscopy has been shown to be a most useful tool for elucidation of compositions and structures of polymers. It is a rapid, accurate method of analysis and requires no external calibration. This technique of composition analysis can be applied to any copolymer, terpolymer, or homopolymer with various structural isomers, provided a suitable number of resonance peaks can be unambiguously assigned to at least one proton of each of the structures present. Accuracy depends upon the relative isolation of these selected peaks. The method was applied to butadiene styrene and butadiene isoprene copolymers. Equations developed previously for both systems were extended and modified to give better accuracy. In the former copolymer the NMR analysis for block styrene agreed very well with the chemical oxidative degradation method over the range 0-20 per cent. It is believed that sequence lengths smaller than five styrene units can be observed as block styrene by NMR. It was also shown that there is a sequence distribution effect in the 1,2 addition butadiene olefinic resonance region of the spectrum. This effect is also observed in polybutadienes and in the butadiene isoprene copolymers. Further study of this might be fruitful. Accuracy of the NMR method was demonstrated in the butadiene isoprene case by study of four high conversion, emulsion copolymers and six blends of butadiene and isoprene homopolymers. In all samples except one the total isoprene content found by either the detailed NMR analysis or the much more simplified NMR analysis agreed within 2 per cent of that known to be present. Accuracy is greatest for those copolymers or blends which have relatively low 1,2 addition butadiene contents, such as with butyllithium-catalyzed polymers. Accuracy is poor when the copolymer or blend has an isoprene content less than 10 per cent along with a high 1,2 addition butadiene content. Composition analysis of copolymers by NMR will become increasingly important since it often provides a direct, simple fingerprint of the various structures. However, NMR will probably make its most significant contributions in stereochemical studies of tacticities, sequence distributions, and orientation, such as head-to-head or tail-to-tail linkages. In these problems deuteration techniques, double resonance techniques, and higher field strengths play an indispensable role.