Gpcnmr Coupling

One of the most impressive advantages of continuous-flow 1H NMR spectroscopy is the direct monitoring of the change in the microstructure of polymers and in the chemical composition of copolymers during GPC. In the case of synthetic polymers, the amount of

available sample is not limited and GPC is not sensitive to peak dispersion effects. A NMR flow cell with a detection volume of 120 |L with a 400 MHz spectrometer yields adequate signal-to-noise values within a reasonable resolution time of 8 s.

One example shows the possibilities of GPC-NMR coupling and is typical of a multitude of similar prob lems in the chemical industry. Two styrene-butylac-rylate copolymers were synthesized under similar conditions, but the physical properties of the copolymers differed. Conventional polymer analysis failed to distinguish between the samples. In both cases the chemical composition and microstructure were identical.

Figure 9 1H NMR spectrum of ibuprofen extracted from the SPE-HPLC-NMR run depicted in Figure 8.

Figure 10 Stacked plot (400 MHz) of the GPC-NMR separation of a styrene-butylacrylate copolymer.

Figure 10 Stacked plot (400 MHz) of the GPC-NMR separation of a styrene-butylacrylate copolymer.

A GPC separation of 100 |L of a 7.5% copolymer solution was performed with a 250 x40 mm GPC column using dichloromethane as eluent at a flow rate of 0.4mLmin~1. Sixteen transients were co-

added, defining a time resolution of 8.4 s. The Fourier-transformed spectrum results in a row in the two-dimensional plot of 1H chemical shifts versus retention times (Figure 10).

Figure 11 Selected rows of the GPC-NMR separation of a styrene-butylacrylate copolymer (Figure 10) showing signals from the aliphatic and aromatic spectral region.
Figure 12 Styrene-butylacrylate copolymer composition versus GPC elution time. (A) Latex A; (B) Latex B.

The methyl group and oxymethylene signals of the acrylate (A) and the aromatic resonances of the styrene (S) units can be used in an online GPC-NMR run to derive information about the molecular weight dependence of the chemical composition.

Within one separation run, up to 128 rows were accumulated, resulting in an overall acquisition time of 42 min. Three selected rows are depicted in Figure 11, showing the varying intensities of the CH3 signals of butylacrylate at 0.85 p.p.m. versus the aromatic signal of styrene at 7 p.p.m. for one copolymer sample.

Thus, the copolymer composition can be directly determined from the elution curves of both signals at any row of the chromatogram. The results from the GPC-NMR coupling for both samples are shown in Figure 12. The copolymers show a completely different behaviour in their dependence of the chemical composition on molecular weight.

This example demonstrates the great time-saving nature of the hyphenation of chromatography with NMR spectroscopy. To yield the same information as in the online GPC-NMR run, 128 fractions of the GPC separation would have to be collected and 128

routine JH NMR spectra recorded. Whereas the GPC-NMR data were obtained within less than 1 h, offline separation and NMR examination would take at least 3 h.

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