Summary: 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 (T 1 ) of 13 C, 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 T 1 and abolish unwanted NOE. Based on the effect of Cr(acac) 3 on 13 C T 1 and line width of HDPE and an ethylene-octene copolymer, 0.025M Cr(acac) 3 is recommended for routine 13 C NMR experiments and 0.05 M Cr(acac) 3 can be used with less stringent resolution requirements. The end-group and unsaturation analysis of polyolefins by 1 H NMR is complicated by three factors: 1) Low signal level; 2) Significant and anomalous molecular weight and concentration dependences for the 1 H T 1 ; 3) Sample degradation induced by long acquisition time. To solve these problems, Cr(acac) 3 can be added to reduce the 1 H T 1 , hence NMR instrument time. By the systematic study of the effect of Cr(acac) 3 on the 1 H T 1 and spectrum quality, the optimum Cr(acac) 3 concentration was found to be 0.001 M, at which concentration, the longest 1 H T 1 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-permillion level unsaturated structures and end-groups in polyolefins.
1Summary: Detection and quantification of long-chain branches in some polyethylene copolymers is challenging due to the near coincidence of the chemical shifts for the carbons at the short-chain and long-chain branches present in these copolymers. The small chemical shift difference can be enhanced by changes in solvent and temperature. This allows one to use lower field magnets for some copolymers. Results are presented comparing several solvents and blends at a variety of temperatures using 500, 600 and 750 MHz spectrometers.
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