Liquid chromatography at the critical adsorption point (LC CAP) with on-line NMR detection (on-line LC CAP NMR) was utilized for analysis of tacticity distribution of stereo-regular poly(ethyl methacrylate)s (PEMAs). The separation of a model PEMA sample composed of four constituents with similar molar masses (Mw = (14-16) x 10(3) g mol-1) differing in their tacticity (rr triad content = 0, 33, 68, and 89%) was achieved by LC CAP with a mixed eluent composed of acetone, acetone-d6, and cyclohexane. The tacticity composition within each peak eluted from the LC CAP column was directly determined by a 750-MHz 1H NMR spectrometer that was used as a real-time detector in the continuous-flow mode. Tacticity distribution in a particular PEMA sample with mm/mr/rr = 2/45/53 and narrow molar mass distribution of Mw/Mn = 1.05 has been revealed by the LC CAP NMR technique.
ABSTRACT:A size exclusion chromatograph coupled to a 750 MHz 1 H NMR spectrometer (on-line SEC-NMR) was used to analyze molecular weight distributions of isotactic PMMAs with M. ranging from 3.27 x 10 3 to 2.22 x 10 4 . 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 500MHz SEC-NMR operated under similar conditions.
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