¹³C NMR Characterization of Short Chain Branches of Nickel Catalyzed Polyethylene

Abstract

All peaks in the 13C NMR spectrum of a sample of nickel-catalyzed polyethylene can be assigned to linear short chain branches having specific spatial arrangements along the polymer backbone. The chain walking mechanism of branch formation controls the specific spacing and conformational arrangements between these linear short chains. Certain unusual short chain branching sequences, involving two branches separated either by two or four methylene backbone carbons, were observed. Invariably one of the two branches in these structures was found to be methyl. The observed frequency of the second branch, whether it be methyl, ethyl, or a longer short chain branch, correlated roughly to the relative overall abundance of each branch in the bulk polymer sample. The existence of “branches on branches” was not confirmed, based on the peaks present in this spectrum. No clusters of short chain branches, i.e., branches separated by one methylene carbon, were found in this polymer sample.

A Comprehensive Review on Controlled Synthesis of Long‐Chain Branched Polyolefins: Part 3, Characterization of Long‐Chain Branched Polymers

“…[21] The position information of methine peaks could also be used in this analysis. [22][23][24][25] Rinaldi et al [26] provided an indepth study and observed chemical shift differences of 0.003-0.007 ppm between the peak positions of CH(B 6 ) (38.283 ppm), CH(B 8 ) (38.286 ppm), and CH(B 10 ) (38.293 ppm) in 188.6 MHz 13 C NMR spectra. Such differences could be enhanced by an order of magnitude through adjusting temperature and sample concentration.…”

The effect of an unsymmetric branched branch on the13C NMR chemical shifts of main chain carbons in polyethylene

“…The 13 C NMR spectrum of the 4-methyl-1-hexene/ethylene copolymer is presented in Figure 2. The polymer was found to contain approximately 0.8 mole percent 4-methyl-1-hexene by 13 C NMR analysis. The most interesting peaks in this polymer sample are those assigned to the backbone methylene carbons S ␣␦ϩ , S ␤␦ϩ , and S ␥␦ϩ .…”

“…1,2,[11][12][13] However, the spectra of polymers are often very complex and the exact structural relationship between branches is unknown, making assignment of resonances difficult. A great help in the assignment of chemical shifts to structural forms comes from the ability to theoretically predict chemical shift values 14 -17 or through the synthesis of model compounds.…”

“…In order to unambiguously assign 13 C NMR chemical shifts to specific structures, we prepared model polymers containing branched branches through the copolymerization of ethylene and branched ␣-olefins. Our analysis of these samples revealed that unsymmetrical branches cause adCorrespondence to: N. W. Eilerts (E-mail: nweiler@ppco.…”

Quantification of branching in disordered materials

“…[34][35][36][37][38] High-resolution 13 C NMR has ben used for this application. With this technique, shifts in the radio-frequency vibrations of up to five carbon atoms from a branch point have been estimated 39 and can be used to calculate the number density of branches.…”