Development of novel polymeric materials capable of efficient CO2 capture and separation under ambient conditions is crucial for cost-effective and practical industrial applications. Here we report the facile synthesis of a new CO2-responsive polymer through postpolymerization modification of poly(2-vinyl-4,4-dimethylazlactone) (PVDMA). The reactive pendant azlactone groups of PVDMA are easily modified with 4-(N-methyltetrahydropyrimidine)benzyl alcohol (PBA) without any byproduct formation. FTIR and TGA experiments show the new PBA-functionalized polymer powder can reversibly capture CO2 at room temperature and under atmospheric pressure. CO2capture was selective, showing a high fixing efficiency even with a mixed gas system (20% CO2, 80% N2) similar to flue gas. CO2 release occurred at room temperature, and release profiles were investigated as a function of temperature. Density functional theory (DFT) calculations coupled with modeling and simulation reveal the presence of two CO2 binding sites in the PBA-functionalized polymer resulting in a two-step CO2 release at room temperature. The ease of material preparation, high fixing efficiency, and robust release characteristics suggest that postpolymerization modification may be a useful route to designing new materials for CO2 capture.