Direct Determination of Molecular Weight Distribution by Size Exclusion Chromatography with 750 MHz 1H NMR Detection (On-Line SEC-NMR)

A size exclusion chromatograph coupled to a 750 MHz 1H NMR spectrometer (on-line SEC-NMR) was used to analyze molecular weight distributions of isotactic PMMAs with M̄n ranging from 3.27×103 to 2.22×104. The molecular weight of the polymer in flowing eluate was determined directly (without a conventional calibration procedure) from the relative intensity of NMR signals due to the end-group and repeating units. The molecular weight distribution determined in this manner was proved to be accurate by SEC-NMR experiments on uniform isotactic PMMAs consisting of exactly 23, 40, and 80 repeating units. Investigation was also undertaken to optimize flow rate and NMR parameters for quantitative SEC-NMR measurements. The signal-to-noise ratio of 750 MHz SEC-NMR spectra was 9.2 times that obtained previously with 500 MHz SEC-NMR operated under similar conditions.