Thiol-ene chemistry draws much attention nowadays in the construction of functional polymer materials due to its versatility and fast reaction kinetics, though only a few studies have been reported on its utilization in the fabrication of elastic polymer materials. Herein, a series of elastic, poly(dimethylsiloxane)-poly(ethylene glycol) methyl ether acrylate (PDMS-PEGMEA)-based co-polymer membranes are synthesized via a one-pot thiol-ene reaction. These membranes are highly stable and exhibit tunable thermal/mechanical properties by tailoring the cross-linker and side- chain functionality. When used for gas separation application, all grafted elastomer membranes show excellent gas permeability and selectivity, and the membrane with an optimal composition (PDMS-PEGMEA(30)-EOPDMS10) has reached the Robeson upper bound (CO2 permeability similar to 800 barrer and alpha[CO2/N-2]similar to 39). The high permeability originates from the extremely fast chain mobility of PDMS molecules at the ambient temperature. Tailoring the PEGMEA content allows control of the alpha[CO2/N-2] ranging from 21 to 39 by enhancing gas solubility within the membrane matrix. This study provides a promising strategy to be utilized for the gutter layer, selective layer, or their combination in the industrial gas separation modules.