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In this study, the preparation of robust and transparent poly(butenyl sorbate) (PBS) membranes (PBS-R) was studied in terms of their CO2/CH4 separation performance. The organocatalytic group transfer polymerization of butenyl sorbate was used for the synthesis of PBS with a designed molecular weight and a moderate molecular weight distribution. For the first time, facile fabrication of PBS-R was achieved through thiol-ene addition between the butenyl groups of PBS and dithiols under 365-nm ultraviolet irradiation. The mechanical properties and CO2/CH4 separation performance of the PBS-R membranes were controlled by introducing an amino-triazole functional group and regulating the length and bulkiness of the dithiol spacers during thiol-ene addition. The amino and triazole groups of the PBS-R membrane showed high affinity to CO2, thereby increasing the CO2/CH4 permselectivity from 9.18 to 10.8 and the CO2 permeability from 127 to 142 barrer. The effect of tensile strength on CO2 permeability was evaluated by preparing PBS-R membranes using dithiols with two rigid benzene rings as spacers and flexible spacers. In addition, the effect of the spacer structure on the permselectivity of PBS-R was investigated using 1,4-butanedithiol, 2,2′-oxydiethanethiol, 1,8-octanedithiol, 1,4-benzenedithiol, and 4,4′-thiobisbenzenethiol. [ABSTRACT FROM AUTHOR]