Insights into Azetidine Polymerization for the Preparation of Poly(propylenimine)-Based CO₂ Adsorbents

Abstract

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 1H 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 mechanism where the 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.