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Polymers of intrinsic microporosity (PIMs) are attractive materials for gas adsorption due to their high surface area and interconnected microporosity. However, the low CO2affinity of PIM-1 results in only a small amount of physisorbed CO2, making the addition of higher affinity species, such as basic amines, a requirement for use in dilute CO2applications. This has been previously accomplished by trapping added amines in the polymer pores, with deleterious consequences for gas transport, regeneration energy, and amine loss. To address these disadvantages, we have explored functionalization of the PIM-1 backbone, comparing simple primary amine to guanidine groups. While both performed similarly at high partial pressures of CO2, the addition of guanidine groups to the PIM-1 polymer provided enhanced CO2affinity relative to the parent and amine-functionalized materials at low CO2concentrations. Evaluated by breakthrough and gravimetric methods, PIM-guanidine achieved a CO2uptake of 1.3 mmol/g (dry) and 2.0 mmol/g (humid) from a 40 mbar CO2feed, among the highest values reported for all-polymer sorbents in humid natural gas combined cycle (NGCC) flue gas conditions. Detailed 13CO2adsorption experiments coupled with quantitative NMR spectroscopy showed that guanidine and water combine to produce carbonate/bicarbonate species. PIM-guanidine was shown to undergo slow temperature-dependent degradation over multiple humid CO2cycles (40 mbar and 1 bar) when regenerated at higher temperatures (150 °C). Excellent performance and stability could be achieved by cycling at lower temperatures (40–70 and 30–90 °C), establishing PIM-guanidine as a promising candidate for scale-up in all-polymer contactors for NGCC CO2capture.