Polymer membranes with ultrahigh CO 2 permeabilities and high selectivities are needed to address some of the critical separation challenges related to energy and the environment, especially in natural gas purification and post-combustion carbon capture. However, very few solution-processable, linear polymers are known today that access these types of characteristics, and all of the known structures achieve their separation performance through the design of rigid backbone chemistries that concomitantly increase chain stiffness and interchain spacing, thereby resulting in ultramicroporosity in solid-state chain-entangled films. Herein we report the separation performance of a porous polymer obtained via Ring-Opening Metathesis Polymerization (ROMP), which possesses a flexible backbone with rigid, fluorinated side chains. This polymer exhibits ultrahigh CO 2 permeability (> 21000 Barrer) and exceptional plasticization resistance (CO 2 plasticization pressure > 51 bar). Compared to traditional polymers of intrinsic microporosity (PIMs), the rate of physical aging is slower, especially for gases with small effective diameters (i.e., He, H 2 , and O 2 ). This structural design strategy, coupled with studies on fluorination, demonstrates a generalizable approach to create new polymers with flexible backbones and pore-forming side chains that have unexplored promise for small molecule separations.Membranes are a promising platform technology for energy-efficient chemical separations.Unlike other separation processes, membranes do not require thermal regeneration, phase changes, or moving parts. [1] Increasing the permeability of polymer membranes used for gas separations is essential for enhancing productivity and reducing membrane areas required for large-scale gas and vapor separations. [2] Specific membrane applications include natural gas purification, hydrogen separations, air separation, and CO 2 capture from flue gas. [3,4] Over the past decade, polymers of intrinsic microporosity (PIMs) have defined the state-ofthe-art for gas separations. [5,6] Their rigid and contorted backbone structures lead to excellent separation performance for a variety of challenging binary separations (e.g., CO 2 /N 2 , CO 2 /CH 4 , O 2 /N 2 ,

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Highly porous and solution processable organic polymers that can be structurally tailored for various applications are in great demand. Previously reported strategies to prepare porous polymers usually rely on a high level of crosslinking or structurally rigid polymer backbones. We now demonstrate that one-dimensional linear polymers with flexible backbones having rigid non-complementary sidechains prepared through ring opening metathesis polymerization can be highly porous. This new strategy allows facile access to diversified porous organic polymers having tunable mechanical/chemical properties bearing different functionalities.

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