Oxetane cationic polymerisation

With appropriately substituted oxetanes, aluminum-based initiators (321) impose a degree of microstmctural control on the substituted polyoxetane stmcture that is not obtainable with a pure cationic system. A polymer having largely the stmcture of poly(3-hydroxyoxetane) has been obtained from an anionic rearrangement polymerisation of glycidol or its trimethylsilyl ether, both oxirane monomers (322). Polymerisation-induced epitaxy can produce ultrathin films of highly oriented POX molecules on, for instance, graphite (323). Theoretical studies on the cationic polymerisation mechanism of oxetanes have been made (324—326). 

Oxetane, a four-membered cyclic ether, is highly susceptible to cationic polymerisation [83]. However, this monomer also undergoes coordination polymerisation in the presence of catalysts such as zinc dimethoxide [84], triethylaluminium water acetylacetone [85-87], aluminium isopropoxide zinc chloride and di-ethylzinc water [87,88], as well as tetraphenylporphinatoaluminium chloride methylaluminium di(2,6-di-/-butyl-4-methylphcnoxidc) [89]. Studies of the microstructure of the polymer derived from the polymerisation of 2-methylox-etane with the triethylaluminium-water-acetylacetone (2 1 2) catalyst showed that the polyether obtained consisted of regioregular monomer unit sequences, fairly rich in isotactic triads [87] ... 

Poly-AMMO is synthesized via cationic polymerisation from the monomer 3-azidomethyl-methyl-oxetane (AMMO). The polymerisation reaction is quenched with water to get polymer chains with hydroxyl endgroups which enable to react these pre-polymers later with isocyanate for curing reaction. Poly-AMMO is suggested as - energetic binder component in -< composite propellants and is in the scope of actual research. 

It is also possible to combine the cationic polymerisation of epoxides and oxetanes with free radical polymerisation of acrylates. This results in the formation of interpenetrating polymer networks (IPNs). 

Cyclobutane has not been polymerised cationically (or by any other mechanism). Thermochemistry tells us that the reason is not thermodynamic it is attributable to the fact that the compound does not possess a point of attack for the initiating species, the ring being too big for the formation of a non-classical carbonium ion analogous to the cyclopropyl ion, so that there is no reaction path for initiation. The oxetans in which the oxygen atom provides a basic site for protonation, are readily polymerizable. Methylenecyclobutane polymerises without opening of the cyclobutane ring [72, 73]. 

RUD 12] Rudati P.S., Mueller D.C., Meerholz K., Preparation of insoluble hole-injection layers by cationic ring-opening polymerisation of oxetane-derivatized tiiphenylamine dimer for otganic electronics devices , Procedia Chemistry, vol. 4, pp. 216-223, 2012. 

The cationic curing systems can impart requisite barrier and electrical properties unlike the free radical initiated polymerisation of acrylates. In the cationic photopolymerisation, the process involves the ring opening of oxiranes and/or oxetanes and is initiated by strong protonic acids. The use of photosensitisers like thioxanthone and anthracene derivatives and photoinitiators further enhances the polymerisation process. 

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