A new family of poly(ethylene glycol) (PEG) based membranes for CO2 separation was developed using thiol-ene photopolymerization. Compared to photopolymerized PEG-containing acrylate membranes, these new thiol-ene based membranes offer improved mechanical properties and processing advantages. The starting material, a combination of a trithiol cross-linker and a PEG diene, was gradually modified with a PEG dithiol while maintaining 1:1 thiol:ene stoichiometry. This approach made it possible to decrease the network cross-link density, resulting in simultaneous increases in free volume and PEG content. Materials with high concentrations of dithiol were very stretchable, with largely, up to 500%, improved elongation at break, yet they exhibited commendable CO2/N-2, O-2, H-2, and CH4 permeability-selectivity performance. The average molecular weight of polymer chains between cross-links, Me, was determined experimentally by fitting the classic network affine model to stress strain data obtained via tensile testing. Me was also calculated assuming an ideal, lattice-like, network structure based on monomer stoichiometry. The effect of Me on glass transition temperature and gas permeation behavior was studied. A free volume based model was employed to describe experimental gas permeability (diffusivity) trends as a function of M-c.