Characterization of polyethylenes produced in supercritical carbon dioxide by a late-transition-metal catalyst

The coordination polymerization of ethylene with a palladium-based catalyst is studied using supercritical carbon dioxide (scCO(2)) as a reaction medium at different temperatures, pressures, and ethylene concentrations. Additionally, the polymerization is performed in dichloromethane as a reference. The polymers are analyzed with gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and hydrogen and carbon nuclear magnetic resonance (H-1 and C-13 NMR). The polymerizations in scCO(2) and dichloromethane result in highly branched amorphous polymers of high molecular weight and narrow molecular weight distributions. Although the polymerization in scCO(2) is a precipitation polymerization, the obtained results are similar to the polymerization in dichloromethane, which is a solution polymerization. Moreover, the polymers produced in scCO(2) show a higher degree of short chain branching (SCB), which likely originates from the nonpolar environment compared to the polar dichloromethane. Within the range investigated, temperature and ethylene concentration have an effect on the molecular weight of the polymer, however, not on the SCB of the polymers. Furthermore, analysis of the C-13 NMR spectra of the polyethylenes gives a strong indication of a new branch-on-branch structure, which has not been assigned before.