Cyclic acetal polymerization reaction

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. 

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. ... 

The above reaction is reversible, and the monomer competes with ylid 5 for the reaction with HX. Those monomers which are more nucleophilic than 5 can be polymerized, i.e. they are epoxides, vinyl ethers and cyclic acetals. 

The rate constant of the ion-trapping reaction, kz, is much larger than the propagation rate constant, at least in cyclic ether and acetal polymerizations. The ratio kz [pRii/yw] is large and termination is instantaneous compared with growth [136], In this way, the number of centres can be determined at any moment during polymerization. The chemical shifts of some quaternary phosphonium ions (of structure corresponding to terminated polyheterocyclic chains) are summarized in Table 2. 

Formally similar scheme may also operate for other initiating systems. Thus, for polymerization of cyclic acetals initiated with triphenyl-methylium (trityl) salts, the cationation of the monomer is a fast, reversible reaction [7], The next reaction, however, due to the steric hindrance, is very slow and consequently active species are formed by a parallel path involving hydride transfer (cf., Section II.A.2) [8,9],... 

Thus, chain transfer to polymer does not influence the number average DP , it may however alter the molecular weight distribution. If the reversible chain transfer to polymer described in Eq. (78) occurred frequently, it would lead to statistical distribution, i.e., MJM = 2. The other consequence is that if the two originally present chains are different, the repetition of reaction sequence will lead to segmental exchange (so called scrambling ). Both effects are clearly detectable, for example, in the cationic polymerization of cyclic acetals as it will be discussed in Section III.B. 

Termination by Reactions of More Reactive Species Existing in Equilibrium with Stable Onium Species As already discussed, in the systems, in which unimolecular ring-opening of cyclic onium ion leads to highly stabilized carbocationic species, a concentration of the latter species in equilibrium with onium ions may be significant. This is, for example, the case of cationic polymerization of cyclic acetals, where carboxonium ions exist in equilibrium with their oxonium counterpart ... 

In contrast to the previously discussed case of THF polymerization, where chain transfer to polymer is slow as compared to propagation, in the polymerization of cyclic acetals, chain transfer to polymer is fast as compared to propagation and the polymerization is dominated by reactions involving polymer chains. Polymerization of the two best studied monomers of this group, 1,3-dioxolane and 1,3,5-trioxane, shows certain specific features. Thus both systems will be discussed separately in the following sections, with special emphasis on the consequences of the chain transfer to polymer. 

If R = R (bifunctional polymers), reaction (119) does not affect the functionality but leads to the broadening of the molecular weight distribution, which is occurring anyway, due to the reversibility of propagation. Thus, several bifunctional polymers of 1,3-dioxolane were prepared and used, for example, to form the networks containing degradable and hydrolyzable polyacetal blocks (cf., Section IV.B). Reaction (119), however, may effectively prohibit the preparation of monofunctional polymers, e.g., macromonomers. Indeed, two recent attempts to prepare macromonomers by cationic polymerization of cyclic acetals led to nearly statistical... 

In many instances in cationic ring-opening polymerization, all the reaction steps, however, are reversible. The final composition of copolymer (in equilibrium) is governed then by thermodynamics. Thermodynamic approaches have been developed [305] and recently reviewed [306]. Such thermodynamic approach has been used to analyze the copolymerization of pairs of cyclic acetals (1,3-dioxolane with 1,3-dioxepane and... 

Unlike anionic initiators or anionically growing alkoxide chains which can only grow (or terminate), cationic initiators (Lewis, Bronsted acids or preformed initiators) or the cationically growing chain may cause acetal-interchange reactions. These reactions are also called transacetalization and cause rearrangement in the molecular weight distribution in homopolymers. The rates of transacetalization are relatively slow compared to that of polymerization except at high temperatures. In the presence of cyclic ethers or cyclic formals, for example, dioxolane, polyformaldehyde can incorporate randomly the co-monomer polyoxyethylene units into the polymer under transacetalization conditions. 

The polymer has a relatively low ceiling temperature (119 °C), but this is the highest of all the formaldehyde polymers. The lack of success at polymerizing other monomers was due to the low ceiling temperatures (e.g. —39 C for acetaldehyde) (Odian, 1991). Aldol condensation can be a side reaction competing with polymerization for these monomers. There are other routes, such as cationic ring-opening polymerization of cyclic acetals, to achieve the same polymer (Penczek and Kubisa, 1989). 

The cationic polymerization of monomeric formaldehyde with cyclic acetals and ethers, which bypasses the stage of cyclic formaldehyde oligomer formation, has been known some time60,61. It was, considered difficult, however, to introduce a process of this kind into industry because of the high sensitivity of the reaction system to polar impurities. 

However, unlike the triaryIsulfonium salts, these compounds undergo reversible photoinduced ylid formation rather than homolytic carbon-sulfur bond cleavage. Because the rate of the thermal back reaction is appreciable at room temperature, only those monomers that are more nucleophilic than the ylid will polymerize. Epoxides, vinyl ethers, and cyclic acetals undergo facile cationic polymerization when irradiated in the presence of dialkylphenacylsulfonium salts as photoinitiators. 

Cyclic acetals of ketoses are prepared most commonly from acetone or benzaldehyde formaldehyde, acetaldehyde, butanone, and cyclohexanone have been used occasionally. These carbonyl reagents are frequently used directly, although such derivatives as 2,2-di-methoxy- or 2,2-diethoxy -propane (acetone dialkyl acetals), or l,l-dimethoxyethane (acetaldehyde diethyl acetal), are often employed in experiments in which intermediate acetals are of interest,or in which the presence of water in the reaction mixture adversely affects the yield of products. A polymeric form of an aldehyde is the reagent to be preferred whenever the monomer is volatile for example, acetaldehyde is often used in the form of a trimer, paraldehyde, and formaldehyde is employed as formalin solution, as paraformaldehyde, or as polyoxymethylene. An excess of the carbonyl reagent is generally used as the solvent, and the condensation is usually effected at room temperature. 

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