STEREOSEQUENCE-DEPENDENT C-13-NMR CHEMICAL-SHIFTS OF POLYSTYRENE OLIGOMERS

Stereosequence-dependent 13C-NMR chemical shifts are calculated for the polystyrene oligomers 2, 4-diphenylpentane (2, 4-DPP), 2, 4, 6-triphenylheptane (2, 4, 6-TPH), and 2, 4, 6, 8-tetraphenylnonane (2, 4, 6, 8-TPN). Calculated chemical shifts are obtained by quantitatively accounting for the number of γ interactions, or gauche arrangements, between carbon atoms separated by three bonds, i.e., carbons γ to each other. In addition, the effect of the magnetic shielding produced by phenyl groups that are first and second neighbors along the chain in either direction from a given carbon atom is considered. Agreement between calculated and observed (Jasse et al.) 13C-NMR chemical shifts is good for each of the polystyrene model compounds. For all but the methine carbons, phenyl ring current contributions to the calculated chemical shifts are found to be small in comparison to the dominant γ effects. Comparison of the 13C chemical shifts calculated for the central methylene carbons in 2, 4-DPP, 2, 4, 6-TPH, and 2, 4, 6, 8-TPN with those calculated for the 20 different stereoisomers of 2, 4, 6, 8, 10, 12-hexaphenyltridecane (2, 4, 6, 8, 10, 12-HPTD) indicate the extreme long-range nature of the stereosequence dependence of 13C-NMR chemical shifts in polystyrene. Only in 2, 4, 6, 8, 10, 12-HPTD do the conformational characteristics, and therefore the magnetic environment and resulting 13C chemical shifts, closely approximate the environment of a methylene carbon in the various steroisomeric sequences of polystyrene. © 1979, American Chemical Society. All rights reserved.