Copolymerization of Ethylene and 1,4-Pentadiene: Structure Characterization and Reaction Mechanism

Copolymerization of ethylene and 1,4-pentadiene was conducted by using a homogeneous catalyst to understand the insertion mode of 1,4-pentadiene. The polymer microstructure of the synthesized material was subjected to structural characterization using quantitative NMR spectroscopy, diffusometry, two-dimensional correlation spectroscopy, and selectively refocused insensitive nuclei enhanced by polarization transfer (Sel-RINEPT). The multimodal NMR characterization allows effective differentiation of chain ends and backbone signals, filters irrelevant spectral information, and provides site-specific information to achieve unambiguous structural elucidation. The experimental results reveal that pentadiene has been primarily converted to cis- and trans-dialkyl-substituted cyclohexane moieties along the polymer backbone over the course of polymerization. The analysis also confirms that unreacted pendant allyl groups from pentadiene remain at a concentration that is significantly lower than the cyclic structures. In comparison to the literature results, some discrepancies in structural assignment have been identified and the results will be discussed. Detailed molecular structure elucidation provides critical insights into advancing the understanding of the reaction mechanism.