Effect of Pendant Dioxolane Rings in Polymers on Gas Transport Characteristics

Polymers with pendant ring structures often exhibit unique gas transport characteristics. For example, poly(perfluoro-4-methyl-2-methylene-1,3-dioxolane) (poly(PFMMD)) exhibits superior separation properties of He over other gases such as H2, CH4, and N2. However, it is unclear whether the superior gas separation properties are derived from the fluorinated nature, pendant dioxolane, or both. Herein we synthesized poly(4-methyl-2-methylene-1,3-dioxolane) (poly(MMD), a hydrocarbon analogue of poly(PFMMD)) by cationic ring-retained polymerization of MMD. The chemical structure of the polymer is verified by FTIR and 1H NMR, and its physical properties were characterized such as glass transition temperature, d-spacing, and fractional free volume. Gas transport properties of poly(MMD) are compared with those in poly(PFMMD) and polymers containing an ether oxygen such as cellulose triacetate (CTA) and poly(ethylene oxide) (PEO) to derive the structure/property relationship. The polar pendant dioxolane groups improve polymer chain packing efficiency and thus size-sieving ability, resulting in a good separation performance of He/N2 and H2/CO2, while the bulky −CF3 groups in poly(PFMMD) increase the FFV and gas permeability without decreasing the gas selectivity.