Cyclic Oligomers in the Cationic Polymerization of Heterocycles

The cationic polymerization of heterocyclic monomers may lead to polymers containing substantial, yet Variable amounts of cyclic oligomers. This review gives a survey of the systems where such oligomers have been found. Oligomer formation is generally observed in the polymerization of oxiranes and thiiranes. The most frequently observed oligomers are the cyclic dimers and cyclic tetramers but other structures are also possible. It is not clear at the present which factors determine the structure of the oligomer. During the polymerization of aziridines also, cyclic oligomers may be formed. With these monomers, the nature of the catalyst, solvent and temperature are important factors. Of the four-membered heterocyclics, only oxetanes have been found to produce cyclic oligomers. From studies with oxiranes, thiiranes and some aziridines it follows that the oligomers are produced by degradation of first formed polymer and not directly from the monomer. The active species for degradation are the corresponding onium ions. From a thermodynamic point of view it is expected that polymers obtained from three-membered ring heterocycles degrade to form oligomers but it is not clear why in some cases dimers, and in other cases higher oligomers are formed.