Polymer conformation and NMR chemical shifts, 9. Poly(α‐methylene‐γ‐butyrolactone)

Abstract

AbstractThe conformational energies of poly(α‐methylene‐γ‐butyrolactone) are calculated and compared with those of poly(methyl methacrylate). In spite of the structural resemblance of these two polymers, the patterns of the energy contour maps are clearly distinguishable from each other; the energy barriers between rotational isomeric states are appreciably higher in the former than in the latter polymer. The calculation indicates large non‐bonded interactions between the protons in one lactone ring and those in the adjacent lactone rings. The broad NMR spectrum of poly(α‐methylene‐γ‐butyrolactone) apparently reflects its rigid conformational structure. 1H and 13C NMR chemical shifts are calculated by theoretical shielding calculations based on conformational analysis. Much lower magnetic field resonances of the OCH2 and α‐CH2 carbons in poly(α‐methylene‐γ‐butyrolactone) as compared with those of the OCH3 and α‐CH3 carbons in poly(methyl methacrylate) are well reproduced by the calculation. The shift to lower magnetic field is mainly attributed to paramagnetic shielding derived from the interaction between OCH2 carbon and α‐CH2 carbon. Tacticity‐ and conformation‐dependent 1H and 13C NMR chemical shifts of poly(α‐methylene‐γ‐butyrolactone) are well interpreted on the basis of the conformational analysis.