Bicyclic orthoester polymerization

Monomers listed above polymerize by the cationic mechanism. For some groups of monomers (lactones, carbonates) anionic or coordinate mechanism also operates and, from a synthetic point of view, this is the preferred method of converting cyclic esters into linear polyesters. The cationic polymerization of lactones, glycolide and it substituted analog, lactide, as well as spiroorthoesters and bicyclic orthoesters has been studied in some detail. 

This is similar to polymerization of bicyclic orthoesters reported earlier where, depending on reaction conditions, polymers resulting from the opening of one or both rings could be prepared [210]. 

Thermodynamical studies of the polymerization of bicyclic orthoesters has not yet been done. 

This review summarizes all the data we obtained on the synthesis and cationic ring-opening polymerization of bicyclic acetals and orthoesters, and discusses the relationship between ring-strain and poly-merlzablllty. This ties In with earlier work on the polymerlzabll-Ity of monocyclic and bicyclic lactams ( - ). A new mechanism for the propagation step In the polymerization of bicyclic orthoesters Is supported. 

Reactivity. The reactivities of the bicyclic orthoesters can be compared examining the conditions necessary to form polymer. Although the hydrolytic reactivities were not accelerated, these monomers were Indeed very reactive In polymerization. In contrast to the behavior of the bicyclic acetals, no correlation Is found between the hydrolytic reactivity and the reactivity towards cationic Initiators for the bicyclic orthoesters. The following order can be proposed [2.2.1] > [2.2.2] > [3.2.1] > [3.3.1] which Is the expected order from the ring strains (18-19). 

Taking all these data Into account, the following A( 2 mechanism (blmolecular addition on a carbenlum Ion) Is supported for the polymerization of the [2.2.1]blcycllc orthoester. It presumably Is valid for all bicyclic orthoesters, at least at room temperature or higher temperatures. ... 

Yokoyama, Y. and Hall, H. K. Ring-Opening Polymerization of Atom-Bridged and Bond-Bridged Bicyclic Ethers, Acetals and Orthoesters. Vol. 42, pp. 107—138. 

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