The effect of an unsymmetric branched branch on the C-13 NMR chemical shifts of main chain carbons in polyethylene

The C-13 NMR spectra of copolymers of ethylene with 4-methyl-1-hexene and 4 -methyl-1-pentene, respectively, were compared. The 4-methyl-1-hexene/ethyl ene copolymer, which contains an unsymmetric 2-methylbutyl branch, exhibits two distinct C-13 NMR peaks for each of the pairwise methylenes spaced one , two, and three carbons from the backbone methine. The chemical shift diff erences for these pairwise methylenes are 57.4 Hz, 18.7 Hz, and 4.3 Hz, res pectively, with chemical shift differences decreasing with increasing dista nce from the asymmetric carbon. The frequency differences for carbons farth er from the branch were not distinguishable. The magnitude of the chemical shift difference also varies with temperature, with the first and second me thylene carbon chemical shift differences decreasing with increasing temper ature. The third carbon is almost unaffected by temperature variations. In contrast, the 4-methyl-1-pentene/ethylene copolymer exhibits a single peak for each of the pairs of methylenes in the branch's vicinity. This is the f irst reported observation of a branched branch affecting the chemical shift s: of main chain carbons in polyethylene containing short chain branches. ( C) 2000 John Wiley & Sons, Inc.