Cyclic Oligomers in the Cationic Ring Opening Polymerization

Macrocyclics (starting with dimers) have been observed in every cationic ringopening system, provided the authors specifically searched for cyclic entities. In the polymerization of THF the first report indicated that macrocyclics are not formed 11 but shortly thereafter Pruckmayr corrected this view and described a series of cyclic oligomers identified by gas chromatography coupled with mass spectroscopy2,3). 

The content and distribution of macrocyclics depend very much on monomer structure. Ethylene oxide exclusively gives a dimer (1,4-dioxane), while in other systems (e.g. 1,2-propylene oxide, epichlorohydrin, tert-butyl ethylene oxide) cyclic tetramers are predominant products 40. 1,3-Dioxolane or hexamethyltrisiloxane yield a complete sequence of cyclic oligomers, without any preference of a specific product51. 

This Section gives a uniform treatment of the mechanism of macrocyclizations which may be applied to all systems mentioned above and listed in Table 3.1. 

Dimers, trimers and tetramers form either separately or as members of a continuous spectrum of cyclic oligomers. These products are formed either by end-to-end cyclization (end-biting) or by back-biting reactions involving tertiary onium ions as discussed further in this section  

To form a given macrocycle, the chain must assume a conformation enabling ring closure and then the chemical reaction may occur. Thus, the probability of cyclization depends on the conformational probability which is governed by such factors 

---

Cyclic Oligomers in the Cationic Ring Opening Polymerization... 

---