Cationic ring-opening polymerization aziridines

The cationic ring opening polymerization of oxolane (THF) or of N-substituted aziridines can be initiated by oxocarbenium salts [42]. The methacrylic ester unsaturation is insensitive to cationic sites, and polyoxolanes (poly-THF) macromonomers are obtained in good yields. 

Polyamines can also be synthesized by cationic ring-opening polymerization of ethyleneimines (aziridines), trimethyleneimines (azetidines), and 2-oxazolines. 

In cationic ring-opening polymerization, there are not too many examples of the systems in which ratios of kplk, are known. In the polymerization of substituted aziridines and substituted thietanes the ratios of rate constants of chain transfer to polymer to the rate constants of propagation have been measured and at least the value obtained for polymerization of N-/-butylaziridine (1.2-104) [260], indeed indicates the living character... 

The earliest reported PHAI was hyperbranched PEI (B-PEI), synthesized by the cationic ring-opening polymerization (CROP) of aziridine in water. The formation of a hyperbranched structure is due to the high nucleophilicity of the amino groups in both the monomer and the polymer causing significant chain transfer of cationically growing chains to already formed polymer chains, followed by reinitiation of a new chain by so-called proton transfer (Scheme 2.2). 

The more crown-like tetra-aza-molecule (447), which had previously been prepared by the action of acid on N benzyl aziridine, is now reported as the product of an anodic oxidation. The mechanism postulated proceeds via a ring-opened radical cation (446), which polymerizes, cyclizes, and then initiates another sequence. 

On the basis of ring opening reactions of 2 substituted aziridines described in the literature [17,18,19], the polymerization initiated with dimethylsulfate or boron trifluoride etherate was expected to propagate mainly by nucleophilic attack of the free aziridine on the methylene carbon of the aziridinium cation following the scheme ... 

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