Poly(ethylene imine) (PEI) is a cationic polymer that is commercially available in linear PEI (LPEI) and branched PEI (BPEI) architectures for both biological and environmental applications. When the LPEI or BPEI is coupled with divinyl sulfone, cross-linked PEI is formed, and it has been researched for its same robust properties as traditional PEI, but it has a rigid, insoluble structure. Herein, we present the first direct comparison of cross-linked linear PEI microgels and branched PEI microgels (LPM and BPM, respectively) for both their intrinsic characteristics, such as morphology, surface charge, and surface chemistry, and their applications in antimicrobial activity, copper chelation, and CO2 capture. The Cu(II) adsorption capacity of the LPMs is larger than their branched counterparts, and their performance is comparable to that of similar materials with a maximum adsorption capacity of 86.8 mg/g. LPM and BPM shows no significant inhibition in bacterial growth compared to a positive control (culture inoculated with bacteria and no PEI treatment) while the PEI precursors all show complete inhibition of growth. The LPMs were found to be good CO2 adsorbents compared to the BPMs, which adsorbed and desorbed CO2 immediately. The best performing LPM has an adsorption capacity of 4.34 mmol/g at 1 bar CO2. In summary, we have described the first pro-environmental PEI gels, which function as a well-performing dry CO2 adsorbent and copper-chelating agent that is benign to bacteria.