Active Species in the Polymerization of Cyclic Acetals

It has been shown in the previous sections that at least the polymerization of cyclic ethers, sulfides and amines proceeds via onium ions. The large majority of authors have agreed on this point (the mechanism of propagation of disubstituted cyclic ethers like isobutylene oxide is, however, still in dispute) .  

In the polymerization of cyclic acetals there has been a long lasting discussion about the nature of growing species and it seems to be at present only partially resolved. 

Another group 152-156) propagation of 1,3-dioxolane in mote conventional terms assuming that not cyclic but linear macromolecules grow, having active centers alkoxycarbenium or oxonium at their ends  

For a moment, we make only this distinction, namely between the ring-expansion and the linear growth mechanisms later, the carbenium-oxonium controvert will be tested. 

First of all, it will be shown that the recent direct NMR observation of the growing species has allowed a discrimination between these two concepts indeed, propagation involves linear macromol ul. Then, we shall proceed with the discussion of the more act structure of the growing species in these linear macromolecules. 

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Active species in the polymerization of cyclic acetals undergo fast isomerization. This results in chain transfer to polymer, that is, formation (by intramolecular reaction) of cyclic structures or formation (by intermolecular reaction) of branched oxonium ions, followed by exchange of the linear fragments of the chain (transacetaiization) (cf. Scheme 18). 

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