Oxetanes cyclic oligomers formation

The oligomer formation during the polymerization of oxetanes has been reininves-tigated by two groups recently. Drey fuss and Dreyfuss (21) found that with oxonium salts or with ethyl trifluoromethanesulfonate as initiator, oxetane forms not only cyclic tetramer and polymer as reported by Rose, but also cyclic trimer. The amount and type of oligomer obtained depends on counter ion, temperature and solvent. Polymeri-... 

Other evidence for the formation of macrocycles by similar medhanisins comes from the studies of the polymerization of oxetane and 3,3-dimethyloxetane. It was demonstrated by Rose h and confirmed by Dreyfuss, Wotsfold and Goethals ) that cyclic trimers and tetramers and higher cyclic oligomers are formed from these two monomers in polymerization. 

Formation of cyclic oligomers in the polymerization of oxetane has already been noted by Rose, who found that in addition to polymer, cyclic tetramer was also formed in quantities depending on the temperature 13). Polymerization of OX with BF3 initiator at —80 °C gave 4% tetramer while at 100 °C as much as 50% cyclic tetramer was isolated. This work was later reinvestigated, and cyclic trimer in addition to cyclic tetramer was isolated from the polymerization products the quantities of both oligomers were dependent on the temperature and on the initiator used 32). The effect of the initiator structure is illustrated by the data in Table 5.6. 

Table 5.6 Formation of cyclic oligomers in the polymerization of oxetane 1,2-dichloroethane, 70 °C 32)...

Formation of cyclic oligomers in cationic polymerization of highly strained four-membered cyclic ethers, oxetanes, seems to proceed in parallel with propagation and virtually stops when monomer is consumed. The equilibrium is not attained probably because the active species of propagation are not stable and their terminated forms, like protonated polymer, formed after finished polymerization, are not effective in starting any cyclization reactions, unless a new portion of monomer is added. 

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