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Optimum Cr(acac)3 Concentration for NMR Quantitative Analysis of Polyolefins



Olefin copolymers are major industry products. 13C NMR is a critical analytical tool to characterize the composition as well as the monomer sequence distribution. Due to the low natural abundance, small magnetogyric ratio and long spin lattice relaxation time (T1) of 13C, a large number of scans are needed to obtain reasonable signal-to-noise (S/N) for quantitative analyses. This needs a significant amount of NMR time. A relaxation reagent, such as Cr(acac)3, can reduce the T1 and abolish unwanted NOE. Based on the effect of Cr(acac)3 on 13C T1 and line width of HDPE and an ethylene-octene copolymer, 0.025M Cr(acac)3 is recommended for routine 13C NMR experiments and 0.05 M Cr(acac)3 can be used with less stringent resolution requirements. The end-group and unsaturation analysis of polyolefins by 1H NMR is complicated by three factors: 1) Low signal level; 2) Significant and anomalous molecular weight and concentration dependences for the 1H T1; 3) Sample degradation induced by long acquisition time. To solve these problems, Cr(acac)3 can be added to reduce the 1H T1, hence NMR instrument time. By the systematic study of the effect of Cr(acac)3 on the 1H T1 and spectrum quality, the optimum Cr(acac)3 concentration was found to be 0.001 M, at which concentration, the longest 1H T1 of polyolefins is reduced by about a factor of 4, while retaining almost the same spectral quality. Sample degradation is also avoided due to much shorter acquisition time. This provides a robust and efficient procedure for characterizing parts-per-million level unsaturated structures and end-groups in polyolefins.