DIRECT DETERMINATION OF MOLECULAR-WEIGHT DISTRIBUTION BY SIZE-EXCLUSION CHROMATOGRAPHY WITH 750 MHZ H-1-NMR DETECTION (ONLINE SEC-NMR)

A size exclusion chromatograph coupled to a 750 MHz H-1 NMR spectromet er (on-line SEC-NMR) was used to analyze molecular weight distribution s of isotactic PMMAs with (M) over bar(n) ranging from 3.27 x 10(3) to 2.22 x 10(4). The molecular weight of the polymer in flowing eluate w as determined directly (without a conventional calibration procedure) from the relative intensity of NMR signals due to the end-group and re peating units. The molecular weight distribution determined in this ma nner was proved to be accurate by SEC-NMR experiments on uniform isota ctic PMMAs consisting of exactly 23, 40, and 80 repeating units. Inves tigation was also undertaken to optimize flow rate and NMR parameters for quantitative SEC-NMR measurements. The signal-to-noise ratio of 75 0 MHz SEC-NMR spectra was 9.2 times that obtained previously with 500 MHz SEC-NMR operated under similar conditions.