Imidazolium-based ionic liquid 1,n-dialkyl-3,3′-bis-1-vinyl imidazolium dibromide (n = 2, 4 and 6) monomers were synthesized and further polymerized inside a MIL-101 framework to afford poly(ionic liquid)@MIL-101 composites (PIL-n@MIL-101, n = 2, 4 and 6). Furthermore, the ratio of poly(ionic liquid) to MIL-101 affected the structures of the composites. Different molar ratios of PIL-4/MIL-101 were prepared and characterized using elemental analysis, powder X-ray diffractometry, thermogravimetric measurements, N₂ adsorption/desorption and CO₂ adsorption measurements. Thanks to the high Brunauer–Emmett–Teller surface area and porous structure of MIL-101 together with the incorporated poly(ionic liquid), PIL-4@MIL-101-0.5 exhibited good CO₂ uptake capacity (42.7 cm³ g⁻¹ at 100 kPa, 273 K). Additionally, due to the synergistic effects of the unsaturated coordinated Lewis acidic Cr sites and nucleophilic Br⁻ in the PIL-4@MIL-101 composites, the developed composites exhibited efficient catalytic performance in the CO₂ and epichlorohydrin cycloaddition reaction, with 99.6% conversion obtained at 90 °C and 1.0 MPa for 2.5 h without an additional cocatalyst or solvent. The composites also showed easy separation and better recycling stability. Combining the advantages of facile synthesis, excellent performance for CO₂ adsorption and conversion, good thermal stability and recycling stabilities, the poly(ionic liquid)@MIL-101 composites are appealing candidates for application in CO₂ capture and utilization.