Cyclic acetals polymerization

Transfer and termination occur by the modes described previously for cyclic ether polymerizations. Chain transfer to polymer (both inter- and intramolecular) is facilitated in cyclic acetal polymerizations compared to cyclic ethers because acetal oxygens in the polymer chain are more basic than the corresponding ether oxygens [Penczek and Kubisa, 1989a,b]. Working at high monomer concentrations, especially bulk polymerizations, is used to depress cyclic oligomer formation. 

Cyclic acetals polymerize exclusively by cationic mechanism it should be noted, however, that polyoxymethylene may be obtained also by anionic polymerization of formaldehyde. Both processes are used in industry. 

With further shift into the direction of still more advanced breaking of the bond within active species this borderline Sn2 mechanism could eventually convert into the Sjql mechanism. This should be promoted by the presence of the stabilizing group located closely to the carbe-nium ion (like in cyclic acetals polymerization) and/or high ring strain (like in the three membered rings). Indeed, contribution of Sj l mechanism in both cases has been postulated for polymerization of 1,3-dioxolane and isobutylene oxide but there is still no clear-cut evidence for its operation. 

In papers53,54 an attempt has been made to investigate the mechanism of cationic cyclic acetal polymerization of 1,3-dioxolane. The presence of an acetal bond in the monomer molecule decreases the stability of active centers which are subjected to various reversible and irreversible chemical transformations. 

Both intermolecular and intramolecular reactions can be either reversible or irreversible (termination). In reversible reactions true chain transfer takes place when the rate constant of the backward reaction (k ) becomes comparable with the rate constant of propagation. This is valid in the case of cyclic acetal polymerization in which the product of chain transfer is equally active in propagation. 

It should be remembered, that cyclic acetal polymerization is always accompanied by cyclization. These aspects of polymerization are discussed in detail in Sect. 3 of this volume. Here, we wish only to stress that high-molecular-weight linear polymers are not the only products of polymerization carried out under conditions listed in Table 7-2, but some cyclic oligo- and polymers also form. 

Equilibrium constants for the model reaction between methoxymethylium cation and dimethoxymethane (simple linear model of acetal) have been determined by dynamic NMR studies and were found to be Kga = fea/ d = 3x10 mol (SO2, -70 °C). This value indicates that active species in cyclic acetal polymerization exist predominantly in the form of oxonium ions, although a small proportion exist in the form of alkoxycarbenium ions. 

Chain transfer to polymer in cyclic acetal polymerization is a special case of transacetaiization reaction which is well known in organic chemistry. By studying the model system it was found that in a mixture of DXL with alcohols in the presence of an acid, fast equilibration occurs as shown in Scheme 22. ... 

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