Cyclic formaldehyde trimer

Polymers with the repeat units -fCH - Of, known as polyacetals, have been known since the early 920 s. It was not until the late 1950 s, however, that research efforts resulted in high molecular weight linear, stable thermoplastic materials. Whereas these developments were directed towards homopolymers of formaldehyde at DuPont, the research at Celanese Corporation was aimed towards copolymers using trioxane, a cyclic formaldehyde trimer, as the major monomer. 

Dramatic effects of the number of participating water molecules on the reaction rates are predicted. At 300 K, enhancements of nearly four orders of magnitude and over nine overs of magnitude are obtained for the addition of a water molecule to the cyclic water trimer and the formaldehyde water dimer, respectively. Tunneling is also predicted to be important for these systems but the effect diminishes as additional water molecules participate actively in the proton transfer. 

Cyclic acetals (trioxane cyclic trimer of formaldehyde) O—CHo... 

The cyclic trimer (trioxane) and tetramer are obtained by a trace of sulphuric acid acting on hot formaldehyde vapour (i) Figure 19.1). Linear polymers with degrees of polymerisation of about 50 and a terminal hydroxyl group are obtained by evaporation of aqueous solutions of formaldehyde (ii). In the presence of strong acid the average chain length may be doubled. Evaporation of methanol solution leads to products of type (iii). 

When catalyzed by acids, low molecular weight aldehydes add to each other to give cyclic acetals, the most common product being the trimer. The cyclic trimer of formaldehyde is called trioxane, and that of acetaldehyde is known as paraldehyde. Under certain conditions, it is possible to get tetramers or dimers. Aldehydes can also polymerize to linear polymers, but here a small amount of water is required to form hemiacetal groups at the ends of the chains. The linear polymer formed from formaldehyde is called paraformaldehyde. Since trimers and polymers of aldehydes are acetals, they are stable to bases but can be hydrolyzed by acids. Because formaldehyde and acetaldehyde have low boiling points, it is often convenient to use them in the form of their trimers or polymers. 

By organic chemistry formalism, polyacetals are reaction products of aldehydes with polyhydric alcohols. Polymers generated from aldehydes, however, either via cationic or anionic polymerization are generally known as polyacetals because of repeating acetal linkages. Formaldehyde polymers, which are commercially known as acetal resins, are produced by the cationic ring opening polymerization of the cyclic trimer of formaldehyde, viz., trioxane [29-30] (Fig. 1.5). 

Figure 1.5 Cationic ring opening polymerization of a cyclic trimer of formaldehyde (viz., trioxane)...

Starting ingredients may be formaldehyde or the cyclic trimer rrioxane, CH2OCH2OCH2O. Both form polymers of similar properties. Boron trifluoride of other Lewis acids are used to promote polymerization where trioxane is the raw material. 

Polymerization of 1,3-trioxane, a cyclic trimer of formaldehyde, is catalyzed by H3PM012O40 (177). The polymerization is very fast at concentrations of H3PM012O40 as low as 10-6 mol dm-3. To obtain comparable rates using BF3 catalyst, a BF3 concentration of 10 3 mol m 3 is required. 

Acetal resins are those homopolymers (melting point ca. 175°C, density ca. 1.41) and copolymers (melting point ca. 165°C, density ca. 1.42) where the backbone or main structural chain is completely or essentially composed of repeating oxymethylene units (-CH20-)n. The polymers are derived chiefly from formaldehyde (methanal, CH2=0), either directly or through its cyclic trimer, trioxane or 1,3,5-trioxacyclohexane. 

Cyclotriveratrylenes are made by the condensation of veratrole with formaldehyde and were a curiosity when they were first prepared by Robinson in 1915 [47], At the time she believed the compound to be a dimer, 2,3,6,7-tetramethoxy-9,10-dihydroxyanthracene. The reason for this was that the composition of the original compound was determined by elemental analysis. This technique determines the percentage of hydrogen and carbon in the sample which would be identical for a dimer, trimer or any other cyclic product with the same proportion of carbon to... 

The polymer F of formaldehyde is called polyoxymethylene or paraformaldehyde. It contains two OH groups, one at each end of the molecule, but these are omitted for clarity in the structural formula shown in Figure 7.14. This shows that one molecule of water, which is present everywhere even in the most careful work, has been incorporated. If these terminal OH groups are functionalized, paraformaldehyde can be used as plastic. When paraformaldehyde is heated with acid, it is transformed (entropy gain) into the cyclic trimer B from Figure 7.14, which is called 1,3,5-trioxane. 

Masella M, Flament J-P (1999) A theoretical study of five water/ammonia/formaldehyde cyclic trimers Influence of cooperative effects. J Chem Phys 110 7245-7255... 

Formaldehyde and acetaldehyde are the most common aldehydes. Formaldehyde is a gas at room temperature, so it is often stored and used as a 40% aqueous solution called formalin. When dry formaldehyde is needed, it can be generated by heating one of its solid derivatives, usually trioxane or paraformaldehyde. Trioxane is a cyclic trimer, containing three formaldehyde units. Paraformaldehyde is a linear polymer,... 

Trioxane is a stable, cyclic trimer of formaldehyde. It has chloroform like odor, and is a crystalline solid with a melting point of 64 Celsius, and a boiling point of 114.5 Celsius. It sublimes readily and is very soluble in water, acetone, alcohol, ether, and chlorinated hydrocarbon solvents. Trioxane forms an azeotrope when distilled with water, boiling at 91 Celsius, and containing 70% trioxane by weight. Trioxane slowly depoly merizes when treated with acids, and in the absence of water, it breaks down to monomeric formaldehyde when treated with acids. Trioxane is inert to alkalies. It is commercially available. 

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. 

Monomeric units split off from the chain as formaldehyde. 1,3,5-Trioxane is of course simply the cyclic trimer of formaldehyde. 

Trioxane is the cyclic trimer of formaldehyde and it can be polymerized to yield polyoxymethylene having the same structure as polyformaldehyde. Polymerization has been carried out with or without catalyst in the liquid, solid, and sublimed states. All polymerizations appear to proceed by a cationic mechanism and the usual type of cationic initiators are effective [122,138—141]. The structure of the cationic chain ends is not clear and two types of props ating centres have been proposed [142], namely, tertiary oxonium ions and carbenium ions. Their propagation reactions are... 

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. 

In the case of 1,3-dicarbonyl compounds, the solvent frequently interferes with the coupling reaction. So with diethyl sodio malonate in ethanol [212b], methanol [212b], dimethylacetamide [212b], or HMPTA [216], besides the dimer and the trimer, the compounds LIXa-c are obtained. They are presumably formed by oxidation of the solvent to the aldehyde and its condensation with the active methylene compound. No dimer was detected in the oxidation of sodio acetoacetate in ethanol, with the major product being LX [217]. Anodic oxidation of cyclic 1,3-diketones in aqueous methanolic sodium hydroxide does not yield the dimer but product LXI, formed by condensation of the starting compound with formaldehyde [218]. 

Metaformaldehyde The cyclic trimer of formaldehyde is usually called trioxane, but is sometimes called metaformaldehyde or trioxymethylene. The hydrated linear polymer form of formaldehyde, (CH20)n H20, is called paraformaldehyde. The trimer of acetaldehyde is called paraldehyde,... 

Little is known about the overall mechanism of cyclic oligomer formation, although the mechanism of the initial stages of the sequence seems fairly clear. The first chemical event is the reaction of formaldehyde (formed in the Petrolite procedures by depolymerization of paraformaldehyde) with phenol to form 2-hydroxy-methyl- and 2,6-6w(hydroxymethyl)phenols in a base-catalyzed process, as shown in Fig. 3. Such compounds were characterized many years ago50), obtained from the action of aqueous formaldehyde on phenol in basic solution at room temperature. Subsequent condensation between the hydroxymethylphenols and the starting phenol occurs to form linear dimers, trimers, tetramers, etc. via a pathway that might involve o-quinonemethide intermediates which react with phenolate ions in a Michael-like reaction, as portrayed in Fig. 4. The condensation of hydroxymethyl-... 

---