The cationic ring-opening polymerization of azetidine to form branched poly(propylenimine) (PPI) is investigated for the purpose of evaluating the utility of PPI/silica composite adsorbents for CO2 capture. The polymerization kinetics and primary:secondary:tertiary amine distribution are monitored with H-1 NMR during reaction with varied synthesis conditions (i.e., reaction time 20-150 h), temperature (343-353 K), and monomer to acid initiator (here; HClO4) ratio. It is found that primary amines are converted to tertiary amines with increased polymerization time, while the addition of monomer over the first 6 h of polymerization increases the primary amine content. This suggests a monomer is rapidly consumed, leaving dimers or small oligomers that still contain rings as key reaction centers. The synthesized polymer is neutralized with either NH4OH or a basic resin and impregnated into mesoporous silica (SBA-15). The CO2 capture properties of these composite adsorbents are investigated, giving information about the effectiveness of the acid neutralization processes.