Trioxane, formaldehyde polymer Production

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). 

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. 

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. 

The formaldehyde process is an air oxidation of methanol, CH3OH, which has water as a by-product. Formaldehyde is a gas at room temperature, but is usually handled either as a water solution called formalin or as polymers called paraformaldehyde and trioxane. Both are readily converted back ro formaldehyde. Some uses of formaldehyde are the manufacture of polymer resins and as a germicide. 

Valuable information on the mechanism of the process and on the confirmation of the formulated assumptions was obtained by analyzing the low-molecular-weight by-products of trioxane polymerization reaction 1,3,5,7-tetraoxane and formaldehyde. Theoretical analysis has shown that, depending on the state of active centers (surface or dissolved) and the length of the dissolved portion of the polymer chain, the steady-state concentration of 1,3,5,7-tetraoxane and formaldehyde changes. A comparison between experimental and theoretical data has shown that at monomer... 

Formaldehyde (mp = —I18 C, bp1-01J => 19 C, dl201131 = 0.8153) is normally a gas. It is chiefly marketed in the form of aqueous solutions of formalin containing 37 to 60 per cent weight of the pure product. These solutions tend to polymerize, even more easily in high concentrations. Hence if they are not used immediately, they must be stabilized by the addition of methanol (7 to 15 per cent weight). A small amount of formaldehyde is sold in the form of solid polymers (paraldehyde, trioxane). 

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. 

The Celanese route for the production of polyacetal yields a more stable copolymer product via the reaction of trioxane, a cyclic trimer of formaldehyde, and a cyclic ether, such as ethylene oxide or 1,3 dioxolane. The structures of these monomers are shown in Figure 3.25. The polymer structure is represented in Figure 3.26. 

Formaldehyde f6r- mal-d9- hld, fer- [ISV form- + aldehyde] (1872) (formic aldehyde, methanal, oxymethylene) n. HCHO. A colorless gas with a pungent, suffocating odor, obtained most commonly by the oxidation of methanol or low-boiling petroleum gases such as methane, ethane, etc. The gas is difficult to handle, so it is sold commercially in the form of aqueous solutions (formalin), solvent solutions, as its oligomer, paraformaldehyde, and as the cyclic trimer, 1,3,5-trioxane (a-trioxym-ethyl-ene). High-molecular-weight, commercial polymers of formaldehyde are called poly-oxymethylene or acetal resin. Formaldehyde is also used in the production of other resins such as phenolic resin (phenol-formaldehyde) and amino resin (urea formaldehyde). Syn methylene oxide, methanal. See formalin. 

Commercial polyacetal copolymers contain 0.1 to 15 mole percent of a cyclic ether, commonly ethylene oxide or 1,3-dioxolane. Typical catalysts for this reaction are BF, or its ether complexes. In 1964, Weissermel and coworkers[5] showed that in the copolymerization of trioxane with ethylene oxide, the latter was almost completely consumed before any visible polymer was observed. During this stage of the polymerization, soluble prepolymers of ethylene oxide could be isolated [6], These prepolymers consisted primarily of oligomers with mono-, di-, and tri-ethylene oxide units. Celanese workers in 1980[7] verified also the presence of cyclic ethers, predominately 1,3-dioxolane and 1,3,5-tri-oxepane, as part of the reaction mixture. These are likely formed as reaction products of ethylene oxide and monomeric formaldehyde generated from the opening of the trioxane ring. 

The trimer of formaldehyde, 1,3,5-trioxane, has been converted to a polyoxymethylene under anhydrous conditions in ethylene dichloride solution using boron trifluoride etherate or stannic chloride as the initiator. The product needs to be stabilized to prevent thermal degradation back to formaldehyde. The capping of this polymer was carried out in dimethyl-formamide solution with an excess of acetic anhydride and a small amount of a high-boiling tertiary amine such as A, A-dimethylcyclohexylamine or... 

Details of the procedures used in the preparation of commercial formaldehyde copolymers have not been fully disclosed. The principal monomer is trioxan and the second monomer is a cyclic ether such as ethylene oxide, 1,3-dioxolane or an oxetane ethylene oxide appears to be the preferred comonomer and is used at a level of about 2%. Boron trifluoride (or its etherate) is apparently the most satisfactory initiator, although many cationic initiators are effective anionic and free radical initiators are not effective. The reaction is carried out in bulk. The rapid solidification of the polymer requires a reactor fitted with a powerful stirrer to reduce particle size and permit adequate temperature control. The copolymer is then heated at 100°C with aqueous ammonia in this step, chain-ends are depolymerized to the copolymer units to give a thermally-stable product. The polymer is filtered off and dried prior to stabilizer incorporation, extrusion and granulation. 

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