Polymerizability oxetanes

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]. 

Lewis Acid Driven Anionic Polymerization of a Monomer with High Cationic Polymerizability Ring-Opening Polymerization of Oxetane with Aluminum Porphyrin in the Presence of a Lewis Acid [66]... 

Polymerization of some cationically polymerizable monomers like cyclohexene oxide [49], 3,3-bis(chloromethyl)oxetane [50], or 1,3,5-trioxane [51] may be initiated by irradiation with y-rays both in liquid or solid state. 

The enthalpy of polymerization of 3- and 4-membered rings is so much higher than the entropy factor that substitution does not significantly reduce their polymerizability. Disubstituted oxiranes (e.g., isobutylene oxide) or oxetanes (3,3-dimethyloxetane) still polymerize practically irreversibly. Substitution may prohibit polymerization of 5-membered monomers, however. 

The thermodynamic polymerizability of oxetanes, in regard of their ring strain, is high for unsubstituted oxetane AHp -80kJ/mol ( -20 kcal/mol) 6). Thus oxetanes like oxiranes may be polymerized to complete conversion ([M]e is very low) and in this respect differ considerably from the next group in the homologous series oxolanes (5-membered cyclic ethers), for which an equilibrium character of polymerization is clearly noticeable. 

T. Matsumoto and H. Mazaki. Liquid crystalline oxetane compound, polymerizable liquid crystalline composition, method for producing liquid crystal film, optical film, and liquid crystal display. US Patent 7 087 273, assigned to Nippon Oil Corporation (Tokyo, JP), August 8, 2006. 

The kinetic polymerizability of monomers belonging to the same class of compounds and studied at similar conditions could be compared using thermodynamic activation parameters. Actually, these parameters are determined from the dependence of the rate constants of elementary reactions (In fep) on 1/T in several instances comparison of fep could be sufficient. Comparison of and A S is more subtle since it provides information on the genuine source of differences in kp and therefore on kinetic polymerizabilities. A good example of such a comparison for CROP of oxetane, 3-methyloxetane, and 3,3-dimethyloxetane is given in a classical work by Saegusa and Kobayashi. ... 

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