Preparation of well-defined hyper-branched polymers and the CO₂ separation performance

Polyoxyethylene (POE) exhibits great potential for selective CO₂ permeation and is tethered onto poly(methyl methacrylate) (PMMA) or poly(styrene-r-acrylonitrile) (PSt-r-PAN) by atom transfer radical polymerization (ATRP) of POE methacrylate (POEM) to endow preferential CO₂ permeability. The polymer backbones are first prepared from corresponding monomers and 4-(chloromethyl)styrene (Cl-St) with various mixture ratios by free radical polymerization. The POE grafting is then carried out by chain propagation of POEM from the Cl-St moiety by ATRP. The introduction of Cl-St to a polymer backbone is readily controlled when the monomer ratio was less than 2mol%, and thus the average distance of POE graft chains can be tuned along the polymer backbone. ATRP also defines the length of the POE graft chains. CO₂ separation properties over N₂ of the well-defined hyper-branched polymers are investigated by comparing permeability of CO₂ and N₂ pure gases. The gas permeabilities increase with increase of POE content, and the resulting CO₂/N₂ selectivity is also enhanced in comparison to the pristine polymers and 35 and 40 for PMMA-based and PSt-r-PAN-based hyper-branched copolymers, respectively. Higher POE content indeed gives higher diffusion coefficient for both of CO₂ and N₂, but the CO₂ and N₂ diffusivities are almost the same. Thus the gas permeation progresses under solubility control. The CO₂ permeability is 135 Barrer with the CO₂/N₂ selectivity 42, when the POE content is 76wt% of PSt-r-PAN-based hyper-branched copolymer.