## 2 Crosslinking of PEI and Reaction Mechanisms
The materials prepared were used to investigate the adsorption capacity of CO\({}_{2}\) and the impact of material exposure to SO\({}_{2}\) on the adsorbed CO\({}_{2}\). The highly porous crosslinked material showed a high efficiency in CO\({}_{2}\) capture, with PEI 196 showing better performance, which could be due to the presence of higher concentrations of primary and secondary amines, among other factors. More discussion on the adsorption of CO\({}_{2}\) and effect of SO\({}_{2}\) is available in Section 3.4. Similar to the crosslinking reaction discussed at \(-78\) and \(-196\)\({}^{\circ}\)C (ice templating method), [PERSON] et al. crosslinked PEI with glycerol diglycidyl ether (GDE) in excess water at \(-18\)\({}^{\circ}\)C [49], which resulted a porous structure. The material was used for the direct removal of organic dyes (discussed in Section 3.3). The crosslinking reaction involves epoxide ring opening and the formation of secondary amines, tertiary amines, and esters, as discussed by [PERSON] et al. [47] and [PERSON] et al. [50] and shown in Scheme 6. [PERSON] et al. reported the crosslinking of PEI with PEGDGE (n = 9 and n = 23) (PEGDGE-400 and PEGDGE-1000) to yield a PEI-PEGDGE gel in a reaction referred to be a ring opening addition reaction [51; 52]. The reaction was conducted in DMSO (30 wt% of the monomers) as a solvent in the presence of PPh\({}_{3}\) as a catalyst at 90 \({}^{\circ}\)C or just in water without the use of the PPh\({}_{3}\) catalyst at room temperature to yield the gels. The materials were characterized  
Figure 6: S\({}_{\text{N}}\)2 ring opening of the epoxide between polyethylene imine and poly(ethylene glycol)diglycidyl ether-crosslinked polyethylene imine [47; 50]. with different techniques. In the IR study, the characteristic peaks of the epoxy ring at 800 cm\({}^{-1}\) and amine group at 1500 and 1600 cm\({}^{-1}\) disappeared, indicating the progression of the crosslinking reactions under the reaction conditions used. The gel formation times of PEGDGE-400 and PEGDGE-1000 were monitored using a viscometer at room temperature, revealing a shorter time for PEGDE400 as compared to its counterpart crosslinked material. The synthesis of gels and porous polymers was achieved by an aza-Michael addition reaction of PEI with polyethylene glycol diacrylate (PEGDA) in DMSO, EtOH, or H\({}_{2}\)O, and reaction is shown in Scheme 7[51]. Different molecular weight PEGDA samples were used (PEGDA200, PEGDA400, PEGDA600, and PEGDA1000). The crosslinked gel prepared in water was not gelling, probably due to the hydrolytic degradation of the ester functional group. However, the reaction in DMSO gave a gel. Interestingly, gel formation in ethanol was found to depend on the molecular weight of the cross linker (PEGDA). It was found that relatively higher molecular weight crosslinkers gave a gel, while for a lower molecular weight crosslinker (PEGDA200), gel formation was found to depend on the reaction conditions. One can infer from this that not only the solvent type used will affect the crosslinking reaction but also the molecular weight of the crosslinker should be kept in the picture to fully understand the process. Like before, in this case, IR spectroscopy was also used to monitor the extent of the crosslinking reaction. In the PEI-PEDA400 reaction system in 30 wt% in DMSO, the IR peaks at 700 cm\({}^{-1}\) (from acrylate), 1500 cm\({}^{-1}\), and 1600 cm\({}^{-1}\) (from amine) almost vanished, indicating the occurrence of the reaction between the amine and the crosslinker. The mechanical properties and the speed of gel formation have been also studied. PEI-PEDA obtained in EtOH with 20 wt% monomers were also tested for its solvent absorption capacity. The polymer absorbed solvents (hexane, ethanol, acetone, toluene, water, DMSO, DCM, and chloroform) and gained 100-350% in volume based on the original size. A crosslinking reaction of PEI is also possible using crosslinkers that have a leaving group, such as an sp\({}^{3}\) C-Br bond.