Anionic polymerization of activated aziridines
Polyethyleneimine, also referred to as polyaziridine, is a polymer that has a repeat unit of (-CH2CH2NH-). Due to its high amine density, PEI finds application in several domains including gene transfection, antimicrobial/antibacterial medicine, CO2 capture, thin film metal deposition, industrial wastewater treatments, and more. Commercially, PEI is obtained via the uncontrolled polymerization of aziridine and the cationic ring opening polymerization (CROP) of oxazolines. However, the available cationic polymerization techniques of aziridines are poorly controlled. In order to synthesize well-defined PEIs, recent literature approaches have used the anionic ring opening polymerization (AROP) of various N-substituted aziridines. The present research, done at The University of Alabama, has focused on the synthesis and AROP of N-substituted aziridines, without the substitution at the 2-position of the aziridine ring. The studied monomers include three tert-butylcarboxy-protected aziridine monomers, specifically tert-butyl aziridine-1-carboxylate, tert-butyl 2-methylaziridine-1-carboxylate, tert-butyl 2-decylaziridine, and six 1-(alkylsulfonylsulfonyl) aziridine monomers, i.e. 1-(methylsulfonyl)aziridine, 1-(toluenesulfonyl)aziridine, 1-(octylsulfonyl)aziridine, 1-(sec-butylsulfonyl)aziridine, 1-((2-nitrophenyl)sulfonyl)aziridine, and 1-((4-nitrophenyl)sulfonyl)aziridine. After characterization, the synthesized monomers were homopolymerized by AROP using various nucleophilic initiators. Except for BOCDecAz, all the synthesized monomers were successfully converted into the corresponding polymers. However, only short oligomers of the resulting homopolymers were formed, as high molecular weight polymer chains were insoluble. The synthesized polymers can potentially be used as precursor to pure linear polyamines A single sulfonyl aziridine, namely1-((ortho-nitrophenyl)sulfonyl)aziridine (oNsAz) was found to produce high molecular weight poly(oNsAz) that was soluble in DMF and DMSO. This is significant as it is the first example of a soluble poly(1-sulfonylaziridine) homopolymer; prior examples were limited to random copolymer. The deprotection of poly(oNsAz) was also attempted in effort to synthesize linear PEI. Although evidence was found for the formation of linear PEI, satisfactory purification of the linear PEI was not achievable.