Ring opening polymerization of aziridines

In principle, one can obtain an unbranched polyethyleneimine by saponifying the amide groups that are located on the polymer (Example 5-4). It is important to note that the ring-opening polymerization of aziridine does not yield linear polyethyleneimines but rather highly branched polymer structmes. 

Scheme I Synthesis of hyperbranched poly(ethylene imine) (PEI) via acid catalyzed ring-opening polymerization of aziridine. Each color represents a different branching unit blue for linear units (L), black for dendritic units (D), red for terminal units (T). PEI has a degree of branching (DB) of 62-73%, the depicted structure represents only a small idealized fragment. Reproduced with permission from [91]...

Problem 10.5 Like cyclic ethers, cyclic amines can be polymerized by ionic ring-opening method. Thus poly(ethyleneimine) can be prepared by the ring-opening polymerization of aziridine (see Table 10.1) with initiation by protonic acids followed by nucleophilic attack of the monomer. Account for the fact that the process gives rise to a branched polyamine and suggest a method by which the branching could be avoided to obtain a linear polyamine. 

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. 

Polymerization of racemic aziridines afforded soluble polymers, whereas the use of enantiopure aziridines provided sparingly soluble polymers. KHMDS serves to deprotonate the primary sulfonamide initiator, providing an amide anion, which then functions as a nucleophile to promote the ring-opening polymerization of the aziridine. The polymerization kinetics were shown to feature a first-order dependence on both the aziridine substrate and the anionic initiator. 

The living anionic ring-opening polymerization of NSAz s has been reported by Stewart et al., revealing that the solubility of the formed polymers is very poor. Only the poly(N-sulfonylaziridine)s (PNSAz s) with long allaziridine rings were soluble in common solvents. 

Polyethyleneimlnes.—Commercial polyethyleneimine (14) is prepared by ring opening polymerization of ethyleneimine (aziridine). This polymer is a highly... 

Other stereoregular polymers with asymmetric carbon atoms in the main chain as stereoisomerism sites have been obtained by ring-opening polymerization of optically active cyclic monomers. These monomers include epoxides (100-104), episulfides (105,106), aziridines (107-109), lactides(110), lactones (111-116), thiolactones (117), lactcunes (118-120), cyclic acetals (121), and N-carboxy cuihydrides (122). Some stereoregular polymers containing atropoisomeric units as stereoisomerism sites (VIII (123) and IX (124)) have also been synthesized. 

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... 

Polyethylenimine (PEI) is an integral polyelectrolyte that is available commercially, e.g., Polymin (BASF). It is formed by the ring-opening polymerization [reaction (28)] of ethyleneimine (aziridine). The resulting polyamine has about 50% of the expected secondary-amine functionality and about 50% primary and tertiary due to branching ... 

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). 

These reactions are useful as processes to form building blocks for pol)mier s)mthesis and organic s5mthesis. The p-lactones and p-lactams formed by the carbonylation of epoxides and aziridines are monomers that can be used in ring-opening polymerization. The... 

The ring-opening copolymerization of propylene oxide and carbon monoxide forms the polyester poly(p-hydroxybutyrate) (PHA, Equation 17.52). The physical and mechanical properties of some PHAs are similar to those of isotactic polypropylene. This polymerization was first reported by Furukawa and co-workers in 1965, and more recent studies have been reported by Osakada, Rieger, and Alper. These polymerizations have been conducted with Co2(CO)j and additives. The combination of Co2(CO)j, a 1,10-phenan-throline derivative, and benzyl bromide afforded polyester with an value of 19.4 kg/ mol and M /M of 1.41. In addition to propylene oxide, 1,2-epoxybutane was successfully copolymerized with CO to yield the corresponding poly(3-hydrox5 entanoate) with an value of 16.7 kg/mol and a M /M of 1.28. The role of benzyl bromide is unclear. Related copolymerizations of aziridines and CO to form polyamides have also been reported. Polymer values as high as 27.5 kg/mol have been reported, and tjqjical M /M values varied from 1.11 to 1.64. 

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 ... 

Cyclic ethers are not the only heterocycles that can be polymerized. Poly(eth-ylene imine) can be prepared by the ring opening of aziridine, which can be initiated by protonic acids, followed by nucleophilic attack of the monomer to give the dimer. Further addition of the monomer can lead to linear polymer formation, but the dimer... 

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