Notably, the increase in the secondary amine portion was much faster than the increase in the tertiary amine portion, and this effect correlates with an increasing functional chain length as well. This indicates that the functionalization is selective towards converting primary amines to secondary amines rather than the alkylation of secondary amines to tertiary amines. This effect is even more pronounced when the chain length of the adduct is increased. For instance, in the case of methyl-modified PEI, 70% of the methyl reagent (CH3I) reacted was attributed to the conversion of primary to secondary amines. In comparison, the other 30% was used for the conversion of secondary to tertiary amines. In the case of propylmodified PEI, 90% of the adduct went towards converting primary amines to secondary. We suggest that this selectivity is due to steric hindrance of the secondary amine sites, which provides less accessible reaction sites compared to the primary amines. In the case of reaction with 1,2-epoxypropane, producing a hydrophilic adduct, a similar reaction trend in primary amine sites is observed; however, the conversion of primary to secondary amines was not as pronounced as propyl-modified PEIs. The selective alkylation of primary to secondary amines is desirable to limit reductions in CO2 adsorption capacities, as tertiary amines capture CO2 much less efficiently than primary and secondary amines. Similar degrees of functionalization were observed for the three different adducts. For example, with a 0.25 molar ratio of adduct to nitrogen in the solution, regardless of the adduct, the resulting samples have 20-22% primary amines, 44-48% secondary amines, and 30-35% tertiary amines. Despite the similar extent of functionalization, differences are observed in performance between the different functional groups. For the 0.25 molar ratio functionalization solutions, the CO2 uptake capacity is more significantly reduced when the hydrophobic alkyl groups such as methyl and propyl were used for modification, and the reduction is even more pronounced for the larger propyl group. In contrast, the 2hydroxypropyl modified PEI retains approximately 0.6 mmol/g CO2 capacity, with a heat of adsorption of -45 kJ/mol, while the methyl-and propyl-functionalized PEI materials have CO2 uptakes of less than 0.4 mmol/g and heats of adsorption between -45 and -35 kJ/mol. The trends in CO2 uptake are consistent with the heat of CO2 adsorption measured in a in a TGA/DSC system under 400 ppm. Unmodified PEI showed the largest heat of adsorption for CO2 (-80 kJ/mol). The methyl-modified-PEI samples showed a substantially lower heat of CO2 adsorption (-20 to -50 kJ/mol), which decreased gradually as the side chains became bulkier to propyl, or when more alkyl groups are added. This can be attributed to longer chains reducing the accessibility of amines due to steric hindrance, weakening carbamate species formation. In the case of 2-hydroxypropyl functionalized samples, the heat of adsorption is determined by steric factors as well as the electron-withdrawing nature of the adduct, which reduces the basicity of the amine sites and results in weaker amine -CO2 interactions. Notably, the heat of adsorption is higher in similarly titrated 2-hydroxypropyl modified PEI versus methyl modified PEI, suggesting more accessible amines, forming more enthalpically favored alkylammonium carbamate groups, in 2-hydropxypropyl PEI. Because of its greater retention of capacity, the 2hydroxypropyl-modified PEI materials (HP-PEIs) were identified as the best candidates for further study.