Azetidine polymerization

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

The only example known for the formation of azetidine 82 by direct intramolecular aza-Wittig reaction is the reaction of the /3-azidoketone 81 with triphenylphosphane (Scheme 41). Attempts to transfer this reaction to 83 and 84 were not successful (87NKK1250). This failure can be attributed to the formation of intermediates with highly energetic transition states, where the rate of intramolecular attack on the carbonyl function is so slow that oligo- and polymeric compounds are preferentially formed. 

Cationic polymerization of 4-membered imines (IUPAC azetidines) generally follows the same patterns as the aziridines [Matyjaszewski, 1984a,b Muhlbach and Schulz, 1988]. Imines are generally unreactive toward anionic polymerization presumably because of the instability of an amine anion (which would constitute the propagating species). The exception occurs with V-acylaziridines as a result of the electron deficiency of the nitrogen coupled with the highly strained 3-membered ring. 

Thietene is a liquid that polymerizes within an hour at room temperature. However, thietane, also a liquid, is more stable it exists as a puckered structure, similar to that of azetidine (note the sulfur atom and hence its lone-pair electrons occupy more space than those of oxygen). 

Higher homologs of PEI can be obtained by cationic ring-opening polymerization of azetidine monomers21). Polymers of this kind can also be N-methylated as previously described in the case of PEI. The cationic polymerization of N-substituted azetidines may give rise to poly(tertiary amine)s. Linear poly(tertiary amine)s have been obtained by selective dealkylation of poly(quaternary ammonium salt)s22). 

Goethals, E. J., Schacht, E. H. Polyamines and Polyammonium salts derived from azetidine monomers, in Polymeric amines and ammonium salts (ed.) Goethals, E. J., p. 67, New York, Pergamon Press 1980, and references therein... 

Also the polymerization of bicyclic monomer containing azetidine ring, l-azabicyclo[4.2.0]octane (conidine), proceeds without appreciable transfer and/or termination [178]. 

The simplest systems involve copolymerization of structurally related pairs of comonomers, polymerizing irreversibly. Copolymerization of different oxetanes [294], thietanes [295], azetidines [296], and oxazolines [297] was studied, the results were interpreted in terms of simple four-parameter copolymerization scheme and the corresponding reactivity ratios for some systems were determined. 

Low activity or inactivity of azetidine D-ring 94a and 94b, very poor activity of 4-carbonate-thia-paclitaxel 95 in tubulin polymerization and inactivity in cytotoxicity tests, together with inactivity of 4-deacetyl-selena-paclitaxel 96 in both... 

In the cationic ring-opening polymerization of unsubstitued azetidine (i 78ai a proton is tranrferred from the ffimeric km (i 78c) to the monomer molecule. This wocess leads to unexpected structure of the polymeric badcbone ) ... 

Polymerization of cyclic sulfides and amines can be initiated not only by the usual cationic and cationogenic compounds like Lewis and protonic acids, carbenium and oxonium salts and esters of strong acids, but also by alkyl halides, which are active enough to induce polymerization of some azetidines 9 10). 

Mixed cyclic anhydrides (e g. 3-hydroxy-l-propanesulfonic acid sulfone 2,)) provide the zwitterionic polymerization of l-(2-cyanoethyl)azetidine 22) and l-(2-cyanoethyl)-aziridine 231. The sulfonate anions are sufficiently stable for the formation of high-molecular-weight polyamines. 

These species were directly observed by H-NMR spectroscopy in the polymerization of thietanes41), azetidines 10,42) and aziridines 19 For a few monomers polymerization is nearly living (many are slowly dying as described by Goethals 18)). These processes were applied in the preparation of block, graft, star-shaped, etc., polymers. 

Polymerization of cyclic amines proceeds usually with high activation energies, due to the high strength of the C—N bond. To attain significant rates (k 10-3 mol-1 1 s 1) requires temperatures from 30 to 100 °C for azetidines and about 200 °C for the less strained bicyclic six-membered amines quinuclidine and triethylenediamine 9). Cyclic sulfides polymerize more rapidly than amines but much slower than ethers of a comparable structure (e.g. for 3,3-dimethylthietane k, = 2 10-2 mol-1 1 s at 35 °C, which is at least 104 times lower than for 3,3-dimethyloxetane (cf. Adv. Polymer Sci. 37 (1980)). 

In contrast to the polymerization of four-membered cyclic ethers that yield a large proportion of cycles (predominantly tetramers), macrocycles were not detected in the polymerization of azetidines or thietanes. 

---