Monomeric formaldehyde

Formaldehyde (Monomeric Formaldehyde, Methanol, Formic Aldehyde, Formalin or FOrmol h H.CH 0 or CHgQi 30. 0 3s ... 

The details of the commercial preparation of acetal homo- and copolymers are discussed later. One aspect of the polymerisation so pervades the chemistry of the resulting polymers that familiarity with it is a prerequisite for understanding the chemistry of the polymers, the often subde differences between homo- and copolymers, and the difficulties which had to be overcome to make the polymers commercially useful. The ionic polymerisations of formaldehyde and trioxane are equiUbrium reactions. Unless suitable measures are taken, polymer will begin to revert to monomeric formaldehyde at processing temperatures by depolymerisation (called unsipping) which begins at chain ends. 

Anhydrous, monomeric formaldehyde is not available commercially. The pure, dry gas is relatively stable at 80—100°C but slowly polymerizes at lower temperatures. Traces of polar impurities such as acids, alkahes, and water greatly accelerate the polymerization. When Hquid formaldehyde is warmed to room temperature in a sealed ampul, it polymerizes rapidly with evolution of heat (63 kj /mol or 15.05 kcal/mol). Uncatalyzed decomposition is very slow below 300°C extrapolation of kinetic data (32) to 400°C indicates that the rate of decomposition is ca 0.44%/min at 101 kPa (1 atm). The main products ate CO and H2. Metals such as platinum (33), copper (34), and chromia and alumina (35) also catalyze the formation of methanol, methyl formate, formic acid, carbon dioxide, and methane. Trace levels of formaldehyde found in urban atmospheres are readily photo-oxidized to carbon dioxide the half-life ranges from 35—50 minutes (36). 

Trioxane and Tetraoxane. The cycHc symmetrical trimer of formaldehyde, trioxane [110-88-3] is prepared by acid-catalyzed Hquid- or vapor-phase processes (147—151). It is a colorless crystalline soHd that bods at 114.5°C and melts at 61—62°C (17,152). The heats of formation are — 176.9 kJ/mol (—42.28 kcal/mol) from monomeric formaldehyde and —88.7 kJ/mol (—21.19 kcal/mol) from 60% aqueous formaldehyde. It can be produced by continuous distillation of 60% aqueous formaldehyde containing 2—5% sulfuric acid. Trioxane is extracted from the distillate with benzene or methylene chloride and recovered by distillation (153) or crystallization (154). It is mainly used for the production of acetal resins (qv). 

Monomeric methylols, which have been formed mainly by the reaction of the post-added urea with the high eontent of free formaldehyde at the still high molar ratio of the acid condensation step... 

Monomer emulsion preparation, 76 286 Monomeric formaldehyde, properties of, 72 108t... 

Amino-substituted (Mannich base) polymers can be prepared by reacting amide-containing polymers with formaldehyde and a suitable amine. Sugiyama and Kamogawa (16) treated PAM in aqueous solution with excess paraformaldehyde (50°C, lh) followed by excess dimethylamine (50°C, lh). This procedure gave 68% conversion to amine. Schiller and Suen (9) used a similar procedure with monomeric formaldehyde and various amines, but with excess PAM. Muller et al. (17) prepared monomeric amines from... 

Burmester, A. (1967). Tests for wood treatment with monomeric gas of formaldehyde using gamma rays. Holzforschung, 21(1), 13-20. 

The two key isocyanates that are used in the greatest volumes for polyurethane polymers are toluene diisocyanate (TDl) and methylene diphenyl diisocyanate (MDl). Both isocyanates are produced first by nitration of aromatics (toluene and benzene, respectively), followed by hydrogenation of the nitro aromatics to provide aromatic amines. In the case of MDl, the aniline intermediate is then condensed with formaldehyde to produce methylene dianiline (MDA), which is a mixture of monomeric MDA and an oligomeric form that is typical of aniline/formaldehyde condensation products [2]. The subsequent reaction of phosgene with the aromatic amines provides the isocyanate products. Isocyanates can also be prepared by the reaction of aromatic amines with dimethylcarbonate [3, 4]. This technology has been tested at the industrial pilot scale, but is not believed to be practiced commercially at this time. 

Polymers of formaldehyde with semiacetal end groups are thermally unstable they decompose at temperatures as low as 150 °C, splitting off monomeric formaldehyde. Upon acetylation of the hydroxy end groups, thermal stability up to 220 °C is achieved alkylation also provides stability against alkali, but not against acids since these are capable of splitting the acetal bonds in the polymer chains (see Example 5-13). 

The industrial synthesis of polyformaldehyde [poly(oxymethylene)] occurs by anionic polymerization of formaldehyde in suspension. For this the purification and handling of monomeric formaldehyde is of special importance since it tends to form solid paraformaldehyde. After the polymerization the semiacetal end groups have to be protected in order to avoid thermal depolymerization (Example 5-13). This is achieved by esterfication with acetic anhydride (see Example 5-7). As in the case of trioxane copolymers (see Sect. 3.2.3.2) the homopolymers of formaldehyde find application as engineering plastics. 

Both liquid and gaseous formaldehyde polymerize readily at low temperatures and can be kept in the pure monomeric state only for a limited time. Because of these facts, formaldehyde is sold and transferred either in solution or in polymerized form, such as paraformaldehyde and trioxane, described here under Formaldehyde polymers Commercial, 37% solution of formaldehyde (So-called Methanol-free)... 

All of these polymers are linear with the exception of sym-trioxane [frtrioxymethylene) (HCHO)3] and tetraoxymelhylene HCHO)4, both of which are true polyoxymethylenes, although they are not prepd by the polymerization of monomeric formaldehyde... 

The concept of a (bound) formaldehyde intermediate in CO hydrogenation is supported by the work of Feder and Rathke (36) and Fahey (43). Experiments under H2/CO pressure at 182-220°C showed that paraformaldehyde and trioxane (which depolymerize to formaldehyde at reaction temperatures) are converted by the cobalt catalyst to the same products as those formed from H2/CO alone. The rate of product formation is faster than in comparable H2/CO-only experiments, and product distributions are different, apparently because secondary reactions are now less competitive. However, Rathke and Feder note that the formate/alcohol ratio is similar to that found in H2/CO-only reactions (36). Roth and Orchin have reported that monomeric formaldehyde reacts with HCo(CO)4 under 1 atm of CO at 0°C to form glycolaldehyde, an ethylene glycol precursor (75). The postulated steps in this process are shown in (19)—(21), in which complexes not observed but... 

Indium-promoted organometallic reactions are greatly accelerated in water, especially when the coreactant carbonyl compound also has good water solubility. Otherwise, aqueous tetrahydrofuran can be used. To date, indium is the most effective metal for promoting Barbier-type reactions under aqueous conditions. As illustrated here, this is of particular value where formaldehyde is concerned, since the need to generate monomeric formaldehyde by thermal cracking is avoided. 

Notice that as in the reaction between phenol and formaldehyde shown earlier, a molecule of water is split out when the two compounds react with each other, resulting in a new molecule capable of reacting with another adipic acid molecule at one end and another hexamethylenediamine molecule at the opposite end. Over a period of time, a long chain builds up consisting of alternate monomeric units of adipic acid and hexamethylenediamine. (The presence of six carbons in each compound accounts for the 66 in the commercial name of the final product nylon 66.)... 

R. W. Jansen, G. Molema, G. Harms, J. K. Kruijt, T. J. C. Van Berkel, M. J. Hardonk, and D. K. F. Meijer, Formaldehyde treated albumin contains monomeric and polymeric forms that are differently cleared by endothelial and Kupffer cells of the liver evidence for scavenger receptor heterogeneity, Biochem. Biophys. Res. Commun. 180 23-32 (1991). 

Resorcinol differs from other phenols in that it reacts readily with formaldehyde under neutral conditions at ambient temperature. To make stable adhesives, which can be cured at the point of use, they are prepared with less than a stoichiometric amount of formaldehyde. About two thirds of a mole of formaldehyde for each mole of resorcinol will give a stable resinous condensation product. The resin is formed into a liquid of convenient solids content and viscosity. Such solutions have infinite stability when stored in closed containers. Glue mixes formed at the point of use from these solutions, on addition of paraformaldehyde-containing hardeners, will have a useful life of several hours due to two principal factors (1) the paraformaldehyde depolymerizes to supply monomeric formaldehyde at a slow rate, as determined by the pH (2) the availability of the formaldehyde is also controlled by the kind and amount of alcohol in the solvent. Formaldehyde reacts with the alcohol to form a hemiacetal. This reaction is reversible and forms an equilibrium which exerts further control on the availability of the formaldehyde. 

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