Polymerization of Trioxane

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 catimiic polymerization of formaldehyde. Some 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 TiCLt- 

The cationic polymerization of trioxane can be initiated by protonic acids, complexes of organic acids with inorganic salts, and compounds that form cations [70]. 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 [70]. 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 procedures lead to different products, however, because variations in polymerization conditions can cause different side reactions. 

Polymerizations in the melt above 62°C are very rapid. They come within a few minutes to completion at 70°C when catalyzed by ten moles of boron trifluoride. This procedure, however is only useful for preparation of small quantities of the polymer, because the exothermic heat of the reaction is hard to control. 

Typical cationic polymerizatiOTis of trioxane are characterized by an inductimi 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 equiUbrium concentration of formaldehyde is reached [70]. 

Several reaction mechanisms were proposed. One of them is based on the concept that Lewis acids, like BF3 coordinate directly with an oxygen of an acetal. This results in ring opening that is induced to form a resonance stabilized zwiter ion [71]  

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  

Small quantities of formaldehyde initiate the spontaneous polymerization of trioxane seen, for example, on sublimation, which presumably occurs as an orientation polymerization, via formaldehyde, on the surfaces of the trioxane crystals. These traces of formaldehyde can be removed with Ag20, after which no polymerization occurs. 

Small amounts of water, which acts as a chain transfer agent, sharply decrease the molecular weight. Water reacts with formaldehyde to form methylene glycol (and higher condensation products)  

Both the methylene glycol and the water can interfere with the polymerization, and they form unstable hemiacetals as end groups  

These stabilizers are not just transfer agents, however, but also copolymerization components. In the copolymerization of trioxane with, for example, ethylene oxide, the ethylene oxide is first incorporated into the copolymer quantitatively at low yields. Later, because of the simultaneously occurring transacetalization, a random distribution of the ethylene oxide residues in the copolymer results. 

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The enthalpy of the copolymerization of trioxane is such that bulk polymerization is feasible. For production, molten trioxane, initiator, and comonomer are fed to the reactor a chain-transfer agent is in eluded if desired. Polymerization proceeds in bulk with precipitation of polymer and the reactor must supply enough shearing to continually break up the polymer bed, reduce particle size, and provide good heat transfer. The mixing requirements for the bulk polymerization of trioxane have been reviewed (22). Raw copolymer is obtained as fine emmb or flake containing imbibed formaldehyde and trioxane which are substantially removed in subsequent treatments which may be combined with removal of unstable end groups. 

Cyclic ether and acetal polymerizations are also important commercially. Polymerization of tetrahydrofuran is used to produce polyether diol, and polyoxymethylene, an excellent engineering plastic, is obtained by the ring-opening polymerization of trioxane with a small amount of cycHc ether or acetal comonomer to prevent depolymerization (see Acetal resins Polyethers, tetrahydrofuran). 

The same considerations apply to the explanation which Kern and Jaacks [65] put forward to account for their observation that the polymerization of trioxane by boron fluoride in methylene dichloride not only appears not to require a co-catalyst, but is retarded by water. They write the initiation reaction as... 

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. 

Several reviews of early work on topotactic polymerizations and isomeriza-tions are available, and the reader is referred to the summaries of Morawetz [88] and Gougoutas [8] for a more complete account. The earliest study of a topotactic reaction appears to have been the observation, in 1932, of the polymerization of trioxane to poly-oxy-methylene [89]. Similar polymerizations of tetraoxane [90] and of trithiane [91 ] have also been reported to show retention of crystallographic axes from the monomer lattice. Other examples are discussed below. The topo-tacticity of a reaction can be determined solely by x-ray crystallographic analysis at the reactant and product endpoints. Thus a simple classification of a reaction as topotactic tells very little about how the structure of the crystal lattice changed in the course of reaction. 

The polymerization of trioxane in solution has been studied by Okamura (26) and his co-workers and by Kern and Jaacks (56, 58, 63). The initiators were borontrifluo-ride or its complexes and anhydrous perchloric acid. During the polymerization the eight-membered ring, tetroxane, is formed rapidly but this compound takes part in the polyoxymethylene formation. This results in an equilibrium concentration of tetroxane when the rate of formation becomes equal to the rate of consumption (27). Minor amounts of the ten-membered ring, pentoxane, are also formed (28). The authors conclude that tetroxane and pentoxane are formed by a back-biting mechanism. It is assumed that the active species in the reaction is a carbenium ion. Although this ion... 

During the initial polymerization of trioxane with (C4H9)2OBF3 in melt or solution, no solid polymer is formed, and the reaction medium remains clear. Using a high resolution NMR spectroscope, C. S. H. Chen and A. Di Edwardo observed the appearance of soluble linear polyoxy-methylene chains. In the cationic copolymerization of trioxane with 1,3-dioxolane, V. Jaacks found also that a soluble copolymer forms first and turns later into a crystalline copolymer of different composition. Crystallization and polymerization proceed simultaneously in the solid phase. 

On the other hand copolymer with a trioxane unit at the cationic chain end (Pi+) may be converted intp P2+ by cleavage of several formaldehyde units. These side reactions change the nature of the active chain ends without participation of the actual monomers trioxane and dioxo-lane. Such reactions are not provided for in the kinetic scheme of Mayo and Lewis. In their conventional scheme, conversion of Pi+ to P2+ is assumed to take place exclusively by addition of monomer M2. Polymerization of trioxane with dioxolane actually is a ternary copolymerization after the induction period one of the three monomers—formaldehyde— is present in its equilibrium concentration. Being the most reactive monomer it still exerts a strong influence on the course of copolymerization (9). This makes it impossible to apply the conventional copolymerization equation and complicates the process considerably. 

A great majority of polymerizations are simultaneously affected by many physical and chemical factors, and their course is the result of a superposition of these effects. Only in rare cases does one of these factors dominate and the polymerization is formally simplified. In topochemical polymerizations, the growth of macromolecules is governed by forces in the crystal lattice of the monomer. Solid-state polymerization of trioxane (trioxacyclohexane) is a typical example of topochemical polymerization. 

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],... 

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]. 

Crystalline poly(oxymethylenes) with peculiar morphologies were produced via y-initiated topochemical polymerization of trioxane. Depending on the crystal modification of the trioxane, porous poly(oxymethylene) crystals, showing cylind-... 

In some recent studies [145] of the kinetics of the polymerization of trioxane initiated by BF3. Et2 0 and by FeCla. PhjCCl, the course of the polymerization appeared to be quite complex. Rate and equilibrium parameters were evaluated and their significance was discussed. The number of active centres was not determined independently, so again these results must be considered preliminary and will not be quoted here. 

In contrast to the polymerization of trioxane which has been studied a great deal, few studies of the polymerization of trithietane have been reported [36]. It has been polymerized using many of the same initiators as trioxane and the mechanism is probably just as complex. As far as we know kinetic studies have not been published. 

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. 

This oxonium ion may in some cases react in its resonance form, the carbonium ion. The propagation step could be by electrophilic attack of the electrophilic carbonyl atom of the methylene group on the oxygen atom of the carbonyl group of the highly polar formaldehyde. This mechanism is similar to that advanced for the polymerization of trioxane or tetrahydrofuran. A further refinement of the mechanism takes into account the likely possibility that this oxonium ion... 

Miyama, H., and M. Kamachi Solid state polymerization of trioxane by the... 

Similar careful control of nucleation conditions is necessary for the polymerization of trioxane to high crystallinity and high molecular weight polyoxymethylene from solution (78). This reaction is cationic and may be started by a large number of initiators (79). For the polymerization step of the reaction in solution, catalyzed by the etherate of boron trifluoride, the following mechanism has been proposed (80) ... 

Table I. Solid State Polymerizations of Trioxane and Tetraoxane by y-Ray or Plasma Initiation...

Such complicated dependencies are actually observed in the polymerization of trioxane in cyclohexane and heptane34. Similar catalytic phenomena have also been observed in reversible heterogeneous polymerization of l,4-diazabicyclo[2.2.2J-octane another cyclic monomer38. ... 

Among other subjects the influence of the conditions, in which the polymerization process is conducted, on the supramolecular structure of the polymer formed is discussed in studies40-42. Using these data the researchers analyzed the impact of polymerization kinetics on the polymers supramolecular structure and formulated the basic principles for controlling the structure of the polymer in the course of its synthesis. They also proposed a new thermodynamic approach for controlling supramolecular structures. The possible uses of this method are demonstrated in the polymerization of trioxane and triethylamine in different solvents and at different monomer concentrations. The purpose of this approach and the manner in which it differs from the conventional kinetic approach are roughly illustrated by the scheme in Fig. 5. 

Polyoxymethylene polymers, POM, commonly known as polyacetals or Acetal resins are linear thermoplastic polymers containing predominantly the -CH -O- repeat unit in their backbone. There are two types of acetal resins available commercially (1) homopolymers made by the polymerization of formaldehyde, followed by endcapping, (2) copolymers derived from the ring opening polymerization of trioxane (a cyclic trimer of formaldehyde), and a small amount of a comonomer such as ethylene oxide. Acetal resins are... 

Polymerization of trioxane witii BF3 etiierate as catalyst to produce polyoxymetiiylene Continuous manufacture of polym-etiiene foam blocks... 

Polyoxymethylene (polyacetal) — sometimes known as polyformaldehyde — is the polymer of formaldehyde. It is obtained either by anionic or cationic solution polymerization of formaldehyde or cationic ring-opening bulk polymerization of trioxane. Highly purified formaldehyde is polymerized in the presence of an inert solvent such as hexane at atmospheric pressure and a temperature usually in the range of -50 to 70°C. The cationic bulk polymerization of trioxane is the preferred method of production of polyoxymethylene. 

Polyoxymethylene (polyacetal) is the polymer of formaldehyde and is obtained by polymerization of aqueous formaldehyde or ring-opening polymerization of trioxane (cyclic trimer of formaldehyde, melting point 60-60°C), the latter being the preferred method [52]. This polymerization of trioxane is conducted in bulk with cationic initiators. In contrast, highly purified formaldehyde is polymerized in solution using using either cationic or anionic initiators. 

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