Oxetanes, polymerization

Because of the high ring strain of the four-membered ring, even substituted oxetanes polymerize readily, ia contrast to substituted tetrahydrofurans, which have tittle tendency to undergo ring-opening homopolymerization (5). 

Some cationic ring-opening polymerizations take place without termination and are reversible. Oxirane and oxetane polymerizations are seldom reversible, but polymerizations of larger-sized rings such as tetrahydrofuran are often reversible. The description of reversible ROP is presented below [Afshar-Taromi et al., 1978 Beste and Hall, 1964 Kobayashi et al., 1974 Szwarc, 1979]. It is also applicable to other reversible polymerizations such as those of alkene and carbonyl monomers. The propagation-depropagation equilibrium can be expressed by... 

Due to the high ring strain of 3- and 4-membered rings, oxiranes and oxetanes polymerize practically irreversibly. Thermodynamic polymeri-zability of these groups of monomers is not significantly affected by substi-... 

Rose [50] carried out one of the earliest, really thorough investigations of the kinetics of polymerization of a cyclic ether polymerization. He studied oxetane polymerizations initiated by BF3. Rose was aware from the work of Farthing and Reynolds [51] that polymerization does not occur when BF3 comes into contact with pure, dry monomer. However, simultaneous addition of water, ethanol or hydroxy terminated polymer is sufficient to initiate polymerization. Since Rose [50] observed polymerization in his sytem, he assumed that it was not completely dry and discussed his results assuming water is a co-catalyst. As the concentration of water increased, the rate of polymerization at first increased. The rate... 

In their 1971 work Saegusa et al. [52] measured the change of [P ] in the course of oxetane polymerization at —27.8°C as shown in Fig. 3. [P ] remains almost constant during the polymerization and is equal to about 80% of the initial concentration of BF3. THF. Similar [P ]—time relationships were observed at other temperatures. A propagation scheme... 

Propagation rate coefficients for oxetane polymerized by BF3. THE catalyst [52, 54]... 

Depending on such factors as the relative nucleophilicity of heteroatoms in the monomer and in the polymer, the flexibility of the chain (leading to the larger or smaller conformational assistance-neighbouring group participation) the relative rates of both processes may be different. If the rate of cyclization is comparable to the rate of propagation (like in oxetane polymerization) considerable amounts of cyclic oligomers (compared to final equilibrium concentration) are formed within the time needed to attain complete monomer conversion (cf. Sect. 5.3.5). If, however, the rate of cyclization is low (e.g. THF), macrocycles will still form slowly after the monomer-polymer equilibrium has been established. 

The fact, that the difference between these two systems is quantitative rather than qualitative (i.e. the same mechanism but different kinetics) is not always appreciated. For example in Ref.26) it is claimed, that cyclization in oxetane ind THF polymerization proceed by different mechanisms. This is based on the observation that in oxetane polymerization cyclization is concurrent with propagation but in THF polymerization macrocycles appear after the monomer-polymer equilibrium has already been reached. This assumption of two different mechanisms is not necessary, however as we have shown the difference comes only from the differences in kp/kb in Scheme 3.6, conforming to the polymerization of both monomers. 

The scope of applications of cationic oxetane polymerization is rather limited, with one exception [3,3-bis(chloromethyl)oxetane, BCMO] polyoxetanes have not found any practical application. BCMO, is not as easily available as some of the 3-or 5-membered cyclic ethers (ethylene oxide, propylene oxide, epichlorohydrin, tetrahydrofuran) which are made from simple petrochemical products. 

Two initiating systems were most often used for the initiation of oxetane polymerization, namely BF3 and its complexes with ethers, and the R3A1/H20 system. 

Theoretically, any Lewis acid can catalyze oxetane polymerizations. However, these acids differ considerably in their effectiveness. Boron trifluoride and its etherates are the most widely reported catalysts. Moisture must be excluded as it tends to be detrimental to the reaction. " ... 

It was reported that when oxetane polymerizations are carried out with boron trifluoride catalyst in methylene chloride at temperatures between 0 °C to -27.8 °C, a cocatalyst is not required. The product, however, is a mixture of a linear polymer and a small amount of a cyclic tetramer. This is in agreement with an earlier observation that the polymerizations of oxetane are complicated by formations of small amounts of cyclic tetramers. Other catalysts, protonic acids, capable of generating oxonium ions, will polymerize oxetane. These acids are sulfuric, trifluoracetic, and fluorosulfuric. The initiation reaction can be illustrated as follows ... 

In contrast to three-membered cydic ethers, oxiranes, and four-membered cydic ethers, oxetanes polymerize only by cationic mechanism (although anionic polymerization of oxetanes has been occasionally reported ). Pol5mierization of oxetanes, due to the rdativdy high ring strain is practically irreversible. 

It should be noted that although generally oxetanes polymerize only by a cationic mechanism, there are a few reports on the anionic polymerization of (hydroxymethyl)oxetanes. 

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