ASSIGNMENT OF 13 C-NMR SPECTRA OF POLYMER WITH SEMIEMPIRICAL METHOD——A STUDY OF VINYL TERTIARY CARBON IN 1,2-POLYBUTADIENE (1,2-PBD)

Abstract

Matrix multiplication technique has been employed to calculate the probabilities of different rotational isomeric states of the bonds in the main chains of 1, 2-PBD. Using the relation that relative chemical shift is in proportion to the sum of bond probabilities which make contribution to the chemical shift of a certain carbon, a new assignment has been obtained directly from the sum of bond probabilities. According to the assignment, these pentads are mmmm, mmmr, rmmr, mmrr, mmrm, rmrr, rmrm, rrrr, mrrr and mrrm from downfield to upfield. In order to prove experimentally the result mentioned above, isotactic 1,2-PBD was successfully synthesized in our lab. The polymer is identified by the infrared spectroscopy, X-ray diffraction and differential scanning calorimetry. Comparison between the 13 C-NMR spectra of heterotactic and isotactic 1,2-PBD reveals that the first peak downfield in the former represents the pentad mmmm. Consequently, the assignment is in agreement with neither that of Elgert nor the result of Kumar. By means of regression analysis, the γ values based on the experimental data of relative chemical shifts (relative to mmmm) are —4.3 and —4.2 ppm. Fairly good is the agreement between the experimental values of relative chemical shifts and the calculated ones at 40 and 120℃, respectively.