Next-generation membranes use highly engineered polymeric structures with enhanced chain rigidity, yet difficulties in polymerization often limit molecular weights required for film formation. Addition-type polynorbornenes are promising materials for industrial gas separations, but suffer from these limitations owing to endo-exo monomeric mixtures that restrict polymerization sites. In this work, a synthetic approach employing the reductive Mizoroki-Heck reaction resulted in exo-selective products that polymerized up to >99% yields for ROMP and addition-type polymers, achieving molecular weights an order of magnitude higher than addition-type polymers from endo-exo mixtures and impressive side group stereoregularity. Due to this increased macromolecular control, these polynorbornenes demonstrate unique solubility-selective permeation with mixed gas selectivities that exceed commercially used PDMS. In addition to thermal and structural characterization, XRD and computational studies confirmed the results of pure and mixed-gas transport testing, which show highly rigid membranes with favorably disrupted chain packing.