Insight into the synthesis of branched polyethylenimines from 2-haloethylamine via a one-pot two-stage process

Polyethylenimine (PEI) polymers have found applications as varied as heavy-metal adsorbents, gene transfection agents, and even in CO2 capture to mitigate global warming. The commercial synthesis involves the acid-catalyzed ring-opening polymerization of highly toxic aziridine monomer, at a great cost in equipment and management. Herein we report a one-pot synthesis of PEIs from 2-haloethylamine in aqueous NaOH solution without separation of the aziridine intermediate. The most suitable raw material is 2-chloroethylamine, and the relatively optimized synthesis involves a two-stage process at different temperatures. Stage I at 25 °C for 24 h is sufficient to transform 2-chloroethylamine into aziridine (major product, yield 77.5%) and PEI oligomer, via intramolecular and intermolecular dehydrochlorination, respectively. Stage II involves hydrothermal reaction at 100 °C for 24 h, where the aziridine molecules can be ring-opened by amino groups from the PEI oligomer to maintain chain growth until the depletion of aziridine monomer. Meanwhile, aziridine molecules themselves can undergo ring-opening polymerization forming aziridine-ring-terminated PEI chains. A terminal aziridine ring can be attacked by a secondary amino group in the same PEI chain, forming a terminal piperazine or larger ring, and terminating chain growth. Two side products, ethanolamine and piperazine, can be incorporated into the final PEI chains as terminal hydroxyethyl groups, ether linkages, or in-chain piperazine rings. All these chain impurities, together with the terminal piperazine rings, account for as few as 4% of the total carbons in the final polymers. This investigation provides an insight into the mechanisms of aziridine polymerization in basic conditions and might pave the way toward a safer and more economical manufacture of PEI polymers.