Optimum Cr(acac)3 Concentration for NMR Quantitative Analysis of Polyole?ns

摘要

Ole?n copolymers are major industry products. 13 C 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 13 C, a large number of scans are needed to obtain reasonable signal-to-noise (S/N) for quantitative analyses. This needs a signi?cant 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 13 CT1 and line width of HDPE and an ethylene-octene copolymer, 0.025M Cr(acac)3 is recommended for routine ~(13) C NMR experiments and 0.05M Cr(acac)3 can be used with less stringent resolution requirements. The end-group and unsaturation analysis of polyole?ns by ~1 H NMR is complicated by three factors: 1) Low signal level; 2) Signi?cant and anomalousmolecular weight and concentration dependences for the ~1 HT1; 3) Sample degradation induced by long acquisition time. To solve these problems, Cr(acac)3 can be added to reduce the ~1 HT1, hence NMR instrument time. By the systematic study of the effect of Cr(acac)3 on the ~1 HT1 and spectrum quality, the optimum Cr(acac)3 concentration was found to be 0.001M, at which concentration, the longest ~1 HT1 of polyole?ns 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 ef?cient procedure for characterizing parts-per- million level unsaturated structures and end-groups in polyole?ns.