The separation properties of two novel triethoxy bearing polynorbornenes (PNBs)-both a ROMP and addition type-are compared to unmodified polydimethylsiloxane (PDMS) using multicomponent hydrocarbon mixtures up to 800 psi. Partial pressures of C2H6, C3H8, C4H10 and CO2 as high as 120, 80, 32 and 80, respectively, were used in order to expose the membranes to simulated natural gas mixtures. All three materials displayed strong increasing permeability coefficients with pressure, which is indicative of swelling and plasticization. Despite this behavior, the PNB5 showed stable selectivity for C3H8/CH4 and C4H10/CH4 up to 500 psi. ROMP PNB's selectivity for both aforementioned gas pairs was effectively constant at 3 and 4 for C3H8/CH4 and C4H10/CH4, respectively, over the entire feed pressure range (200-800 psi). Selectivity in Addition PNB fell by similar to 27% between 500-800 psi. In the 5-component mixture at 800 psi, Addition PNB displayed C3H8/CH4 and Claw/CHI selectivity of 4 and 6, respectively, which was the highest out of the three materials. PDMS selectivity declined significantly with feed pressure. The largest reduction was observed in the 5-component compared to the 3-component feed, which is consistent with penetrant-induced plasticization. These results demonstrate the strong dependence of selectivity on feed pressure for PDMS. Selectivity in the PNB5, despite apparent plasticization, showed much lower dependence on feed pressure, suggesting plasticization may not be as detrimental to these reverse selectivity glassy polymers compared to traditional diffusion selective glasses. (C) 2016 Elsevier B.V. All rights reserved.