Mixtures of the ionic liquid butyltrimethylammonium bis(trifluoromethylsulfonyl)imide, [N4111][NTf2], with poly(ethylene oxide) polymers, PEO, of different molecular weights were investigated for CO2 capture. The liquid mixtures are close to ideal in terms of their volumetric properties but their enthalpies of mixing are negative, decreasing when the size of the polymer chain increases and being close to those observed for associative compounds mixed with ionic liquids (ΔmixH = −7 kJ mol−1 for [N4111][NTf2] + PEO1000 at 318 K). The favorable interactions between the polymer and the ionic liquid could be related to the dynamic properties of the mixtures, especially for mixtures of polymers of longer chain sizes which have a higher viscosity than that of the pure mixture components. Raman spectroscopy confirmed an association between the cation of the ionic liquid and the polymers that probably surround the cation and thus contribute to slow the dynamics of the mixtures. The increase of the carbon dioxide absorption with the increase of the polymer mole fraction composition in the liquid mixture is explained by the more favorable interactions between the gas and the polymer as evidenced by the analysis of the thermodynamic properties of solvation.