Trioxane cationic polymerization

Another monomer that belongs to this group is cyclic trimer of formaldehyde, 1,3,5-trioxane. Cationic polymerization of 1,3,5-trioxane leading to polyoxymethylene (polyformaldehyde, polyacetal) is one of the few industrially important processes in cationic ring-opening polymerization. 

Commercial polymers of formaldehyde are also produced using cationic polymerization. The polymer is produced by ring opening of trioxane. Since the polyacetal, POM, is not thermally stable, the hydroxyl groups are esterified (capped) by acetic anhydride (structure 5.22). These polymers are also called poly(methylene oxides). The commercial polymer is a... 

Medium-size members of homologous polymeric series such as dimers, trimers, etc. are called oligomers. They can be linear or cyclic and are often found as byproducts of polymer syntheses, e.g., in cationic polymerizations of trioxane or in polycondensations of e-aminocaproic acid (see Example 4-9). For the preparation of linear oligomers with two generally reactive end groups, the so-called telechelics, special methods, i.e., oligomerizations, were developed. 

In ionic polymerizations, the molecular weight can be regulated by temperature, type of catalyst and nature of solvent. In some cases also regulators can be used which, as in the case of cationic polymerization of trioxane, lead to the incorporation of special endgroups. 

Formaldehyde polymerizes by both anionic and cationic mechanisms. Strong acids are needed to initiate cationic polymerization and anionic polymerization is initiated by relatively weak bases (e.g., pyridine). Boron trifluoride (BF3) or other Lewis acids are used to promote polymerization where trioxane is the raw material. 

Polyformaldehyde. Polyformaldehyde or polyacetal is made by two different processes. Delrin is made from formaldehyde by anionic polymerization catalyzed by a tertiary amine. The homopolymer is end-capped with acetic anhydride. Celcon is made from trioxane cationic copolymerization using boron trifluoride catalyst and ethylene oxide (2-3%) as the comonomer. Boron trifluoride is a Lewis acid that associates with trioxane and opens up the six-membered ring. Ethylene oxide provides the end capping. Without an end cap, polyformaldehyde is thermally unstable and loses formaldehyde units. 

Tn the cationic polymerization and copolymerization of trioxane in the - melt or in solution, an induction period usually exists, during which no solid polymer is formed and the reaction medium remains clear. Nevertheless, reactions are known to occur during this period. By using BF3 or an ether ate as catalyst, in homopolymerization, Kern and Jaacks (I) reported the formation of formaldehyde via depolymerization of polyoxymethylene cations. 

Fig. 9. Examples of conversion curves. Effect of water [at concentrations linearly increasing from case (1) to (8)] on cationic polymerization of trioxane [72],...

Rakova and Korotkov compared the rates of homopolymerization and copolymerization of styrene and butadiene [226], Styrene polymerizes very rapidly and butadiene slowly. Their copolymerization is slow at first, with preferential consumption of butadiene. When most of the butadiene is consumed, the reaction gradually accelerates yielding a product with a high styrene content. In the authors opinion, this is caused by selective solvation of the active centres by butadiene only after butadiene has polymerized, does styrene gain access to the centres [227], A similar behaviour was observed by Medvedev and his co-workes [228] and by many others. In our laboratory we observed this kind of behaviour in the cationic polymerization of trioxane with dioxolane. Although trioxane is polymerized much more rapidly than dioxolane, their copolymerization starts slowly, and is accelerated with progressing depletion of dioxolane from the monomer mixture [229],... 

This type of transfer is of some importance in the cationic polymerization of 1,3,5-trioxane, where it has been shown that some macromolecules are terminated with —OCH3 groups [101]. 

The process based on cationic polymerization of 1,3,5-trioxane employs a different principle for stabilization of polymer. Trioxane is copolymerized with a few percent of 1,3-dioxolane (or ethylene oxide). The sequence of —OCH2— units is then separated from time to time by —OCH2CH2— units. The product of copolymerization is subsequently heated to eliminate the terminal units (unstable fraction). Depropagation proceeds until the stable —CH2CH2OH group is reached ... 

Table 11 Polymerization Conditions, Amounts of Alkaline-Stable Fraction, and Low Molecular Weight Fraction in the Cationic Polymerization of 1,3,5-Trioxane...

Cationic polymerization of 1,3,5-trioxane, however, is different from other typical ring-opening polymerizations, because polymer is insoluble in its own monomer or in any typical organic solvent (it may be dissolved however in DMF at —150° C and perfluorinated alcohols at room temperature). Thus, at any polymerization system, already at the early stage of polymerization, polymer precipitates out of solution and further reaction involves active species in crystalline phase. 

These developments in cationic polymerization of 1,3,5-trioxane are discussed in more detail, because in this system the problems related to the mechanism of cyclization are now well understood. Cyclic oligomers were identified, isolated, their molecular weight distribution was determined, and the plausible explanation for observed distribution was given. From the synthetic point of view, the cationic polymerization of 1,3,5-trioxane offers the possibility of preparing macrocyclic polymers with relatively narrow molecular weight distribution and predictable (within discussed limits) molecular weights. The cyclic polymers can be prepared easily in relatively large quantities and conveniently separated from linear polymer by alkaline hydrolysis of the latter. 

The cationic polymerization of trithiane is assumed to proceed by similar mechanism on the polymerization of trioxane. Polymer is insoluble in common organic solvents and, like unstabilized polyoxymethylene, is thermally unstable [155]. 

Kern and Jaacks51 investigated the cationic polymerization of 1,3,5-trioxane by boron trifluoride in dichloromethane at 30 °C. Unlike cationic vinyl polymerizations initiated by metal halides, traces of water rather than increasing the polymerization rate, slightly reduced it. Boron trifluoride induced ionisation of water is thereby ruled out as a step in the initiation mechanism. 

Polyformaldehyde can also be prepared by polymerization of trioxane, the cyclic trimer of formaldehyde. Trioxane polymerizes by ring opening polymerization and cationic initiators are the only effective initiators. Formaldehyde is always present when trioxane is polymerized because the growing polyoxymethylene chains by depropagation may lose one monomer unit, which is formaldehyde not trioxane. In spite of the fact that formaldehyde plays an (as yet incompletely understood) role in trioxane polymerization, which is a cyclic ether polymerization like dioxolane or tetrahydrofurane [5], trioxane will not be discussed in this review. 

Many papers have been published concerning the structure of the active centers in anionic and cationic ring-opening polymerization reactions of oxacyclic monomers. Recently, attention has been paid in our laboratory to the influence of the structure of complex carbonium salt initiators, especially of the dioxolanyllum salts used for initiating the cationic polymerization reactions of trioxane, tetrahydrofuran and dioxolane, on the course of the polymerization ( ). 

When studying the cationic polymerization of 1,3,5-trioxane (the short form trioxane )31, 32), investigators ran into peculiarities of formal kinetics, such as apparently high orders with respect to the monomer and their relationship to the nature of the solvent, temperature, etc., induction periods that could not in any way be ex-... 

Table 7.7 Growth of polyoxymethylene crystals during cationic polymerization of 1,3,5-trioxane hi nitrobenzene at 35 °C I22>...

Formaldehyde, trioxane Cationic or anionic chain polymerization Appliances, plumbing and hardware, transportation... 

Trioxane is unique among the cyclic acetals because it is used commercially to form polyoxymethylene, a polymer that is very much like the one obtained by cationic polymerization of formaldehyde. Sc ne questions still exist about the exact mechanism of initiation in trioxane polymerizations. It is uncertain, for instance, whether a cocatalyst is required with strong Lewis acids like BF3 or TICU. 

The cationic polymerization of trioxane can be initiated by protonic acids, complexes of organic acids with inorganic salts, and compounds that form cations. These initiators differ from each other in activity and in the influence on terminations and on side reactions. Trioxane can also be polymerized by high-energy radiation. In addition, polymerizations of trioxane can be carried out in the solid phase, in the melt, in the gas phase, in suspension, and in solution. Some of these... 

Typical cationic polymerizations of trioxane are characterized by an induction period. During that period only oligomers and monomeric formaldehyde form. This formaldehyde apparently results from splitting the carbon cations that form in the primary steps of polymerization. The reaction starts after a temperature-dependent equilibrium concentration of formaldehyde is reached. ... 

An oxonium ion is formed in the proton-initiated cationic polymerization of trioxane. The ring opens because of resonance stabilization of the open-chained species ... 

Chloral, CChCHO, can be anionically or cationically polymerized. The polymerization is initiated above the ceiling temperature of 58° C and then allowed to proceed well below the ceiling temperature. Phosphines and lithium /-butoxide are especially suitable as anionic polymerization initiators, whereas tertiary amines only produce poly (chlorals) of low thermal stability. Anionic copolymerization of chloral with excess isocyanates produces alternating polymers, as is also the case for the cationic copolymerization of chloral with trioxan. 

In the cationic polymerization of styrene, the attack on the j8-C atom has been established by copolymerizing with trioxane, the cyclic trimer of formaldehyde. Trioxane eliminates formaldehyde during polymerization (see Section 26.1.2) See equation (16-66) on p. 570. 

Formaldehyde can be polymerized cationically to high-molecular-weight poly(oxymethylene) (catalysts BF3, HCIO4, etc. cf. the polymerization of trioxane), or anionically (tributylamine, triphenyl phosphine, diphenyl zinc, etc.), or by an insertion polymerization mechanism (aluminum isopropylate). Some anionic initiators, such as dimethylamine, induce a high proportion of the Cannizzaro reaction by-product, and thus there is a sharp decrease in the polymerization rate as the initiator concentration is increased. Extensive transfer reactions are occasionally observed in the cationic polymerizations. The transfer constant = KJkp has the value of 0.5-2.0 for low-molecular-weight acetals, 0.026 for methyl formate, and 0.(X)06 for halogen derivatives. 

Trioxane can be polymerized cationically (catalyst BF3, HCIO4, etc.) or anionically (R3N, etc.). In the cationic polymerization, the hydrogen ion from the HCIO4, for example, protonates the acetal oxygen and forms an oxonium ion. The ring opens because the newly formed open-chain species is resonance-stabilized. The trimer eliminates formaldehyde up to an equilibrium concentration of about 0.07 mol of formaldehyde/liter. The actual chain growth probably involves the addition of formaldehyde, not trioxane. Thus, if the reaction is not too fast, an induction period is observed. The formaldehyde consumed in polymerization is replenished via the depolymerization of the trioxane ... 

Epoxy compound is a typical molecule for cationic polymerization. However, trioxanes, aziridines, vinylethers, and oxetanes show cationic polymerization by Lewis acid, as shown in Figure 1.8. 

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