Polymerization of formaldehyde

Formaldehyde can be polymerized to poly(oxymethylene) [poly-(formaldehyde)] (I) or poly(hydroxymethylene) (II)  

Generally, poly(hydroxymethylene) occurs in small yields, although yields of 90% can be achieved with TlOH as catalyst. Since Cannizzaro reactions also occur, only low molecular weights, up to hexoses, are obtained. 

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. 

The ceiling temperature of poly(oxymethylene) is 2TC. Under processing conditions, therefore, the monomer can depolymerize by an unzipping mechanism beginning at the end groups. For this reason, the polymer is stabilized by reacting with acetic anhydride and pyridine as catalyst in order to esterify the end groups. 

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Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

The chain polymerization of formaldehyde CH2O was the first example of a chemical conversion for which the low-temperature limit of the rate constant was discovered (see reviews by Goldanskii [1976, 1979]). As found by Mansueto et al. [1989] and Mansueto and Wight [1989], the chain growth is driven by proton transfer at each step of adding a new link... 

Uses of Formaldehyde. Formaldehyde is the simplest and most reactive aldehyde. Condensation polymerization of formaldehyde with phenol, urea, or melamine produces phenol-formaldehyde, urea formaldehyde, and melamine formaldehyde resins, respectively. These are important glues used in producing particle hoard and plywood. 

Polyacetals are among the aliphatic polyether family and are produced by the polymerization of formaldehyde. They are termed polyacetals to distinguish them from polyethers produced by polymerizing ethylene oxide, which has two -CH2- groups between the ether group. The polymerization reaction occurs in the presence of a Lewis acid and a small amount of water at room temperature. It could also be catalyzed with amines ... 

Formaldehyde is a colorless gas that is soluble in water (3). Commercial aqueous preparations of formalin contain 37 0% w/w solubilized gas. They also contain formic acid (<0.05%) and 10-15% methanol, which is added to prevent the polymerization of formaldehyde into paraformaldehyde (3,11). Methanol and formic acid make these solutions an unacceptable fixative for fine structures (9). Paraformaldehyde is a polymerized form of formaldehyde that dissociates at 60°C and neutral pH. Freshly prepared solutions of paraformaldehyde are preferred for most immunochemical procedures because they provide a fixative free of extraneous additives and are usually the conservative fixatives of choice when beginning the development of a fixation protocol (3,5). 

In far too many instances trade-name polymer nomenclature conveys very little meaning regarding the structure of a polymer. Many condensation polymers, in fact, seem not to have names. Thus the polymer obtained by the step polymerization of formaldehyde and phenol is variously referred to a phenol-formaldehyde polymer, phenol-formaldehyde resin, phenolic, phenolic resin, and phenoplast. Polymers of formaldehyde or other aldehydes with urea or melamine are generally referred to as amino resins or aminoplasts without any more specific names. It is often extremely difficult to determine which aldehyde and which amino monomers have been used to synthesize a particular polymer being referred to as an amino resin. More specific nomenclature, if it can be called that, is afforded by indicating the two reactants as in names such as urea-formaldehyde resin or melamine-formaldehyde resin. 

The amino resins or plastics, closely related to the phenolics in both synthesis and applications, are obtained by the polymerization of formaldehyde with urea (XXXVII) (/ = 4) or melamine (XXXVIII) (f — 6). Synthesis of the amino plastics can be carried out either in alkaline or acidic conditions [Drumm and LeBlanc, 1972 Nair and Francis, 1983 Updegraff, 1985]. Control of the extent of reaction is achieved by pH and temperature control. The prepolymer can be made at various pH levels depending on the reaction temperature chosen. Polymerization is stopped by cooling and bringing the pH close to neutral. Curing of the prepolymer involves heating, usually in the presence of an added acid catalyst. 

Besides the carbon-carbon double bond, this is the only other unsaturated linkage whose polymerization has been successfully carried out to an appreciable extent [Furukawa and Saegusa, 1963 Kubisa et al., 1980 Vogl, 1967, 1976, 2000], The polymerization of formaldehyde has been studied much more intensely than that of other aldehydes. Although formaldehyde was successfully polymerized over a hundred years ago, it was not until much later that high-molecular-weight polymers of other aldehydes were obtained. 

Polyoxymethylene, also referred to as acetal resin or POM, is obtained either by anionic polymerization of formaldehyde or cationic ring-opening copolymerization of trioxane with a small amount of a cyclic ether or acetal (e.g., ethylene oxide or 1,3-dioxolane) [Cherdron et al., 1988 Dolce and Grates, 1985 Yamasaki et al., 2001]. The properties and uses of POM have been discussed in Sec. 5-6d. 

Cationic polymerization of formaldehyde (which should be carried out under the driest possible conditions to avoid transfer reactions) can be initiated with protic acids, Lewis acids (see Sect. 3.2.1.1), or other compounds that yield cations such as acetyl perchlorate or iodine ... 

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. 

Cosmic ray particles can work as a trigger of the chain of polymerization of formaldehyde adsorbed (e.g., at the surface of interstellar silicate dust). However, the triggering of the polymerization chain is a necessary but still not sufficient condition for obtaining the interstellar polymers. If the addition of any new link of the chain would require a hit of adsorbed monomer layer by another cosmic ray particle (i.e., if there is no spontaneous growth of chains after they are started by some external factor), the formation of polymers in interstellar space would be highly improbable. Therefore the spontaneous growth of polymer chains near absolute... 

The polymerization of formaldehyde involves the complete reorganization of molecules, including the changes in both lengths and angles of valence bonds. Contrary to the simple transfer of an electron, it can be called a chemical reaction in the full sense of these words. 

RESINS (Acetal). These are thermoplastic resins, obtainable both as homopolymers and copolymers, and produced principally from formaldehyde or formaldehyde derivative. Acetal resins have the highest fatigue endurance of commercial thermoplastics. A variety of ionic initiators, such as tertiary amines and quaternary ammonium salts, are used to effect polymerization of formaldehyde. Chain transfer, shown by the following reactions, controls the molecular weight of resulting resins ... 

Acetal Resins. Acetal resins (qv) are poly (methylene oxide) or polyformaldehyde homopolymers and formaldehyde [50-00-0] copolymerized with aliphatic oxides such as ethylene oxide (42). The homopolymer resin polyoxymethylene [9002-81-7] (POM) is produced by the anionic catalytic polymerization of formaldehyde. For thermal stability, the resin is endcapped with an acyl or alkyl function. 

Diethyl Ether. Decomposes vigorously in ether, especially if peroxides are present.3 Dimethyl Sulfoxide. Reacts violently or explosively in dimethyl sulfoxide,4 probably due to the formation and polymerization of formaldehyde.5 Dinitrogen Pentoxide. Reacts explosively.6 Lead Dioxide. Reacts explosively.7 Phosphorus. Red phosphorus reacts vigorously on warming.8... 

Dimethyl sulfoxide. Violent or explosive reaction occurs due to the formation and exothermic polymerization of formaldehyde.5 6... 

Some aldehydes oligomerize in the presence of acids. The polymerization of formaldehyde (formation of H see Figure 9.10) as well as that of aldehydes with strong electronegative sub-... 

Figure 9.10 presents the mechanism of the polymerization of formaldehyde starting from anhydrous formaldehyde and formaldehyde hydrate. In addition, a reaction path is shown that also connects trimeric formaldehyde ( trioxane, F) with paraformaldehyde (H). In practice, though, this reaction path is only taken in the reverse direction, upon heating (entropy gain ) of paraformaldehyde in aqueous acid as a depolymerization of H —> F. 

The carboxonium ions of Figure 9.10 act as electrophiles in the polymerization of formaldehyde and formaldehyde hydrate. The most simple of them has the structural formula A, i.e., it is protonated formaldehyde from which the carboxonium ions B, C, E and so on are formed successively. The nucleophile causing these conversions is formaldehyde, which reacts with the cited electrophiles via its carbonyl oxygen and thus acts as a heteroatom nucleophile. 

Polyoxymethylene (POM) plastics are highly crystalline thermoplastics that are obtained by polymerization of formaldehyde and can also be in the form of trioxy-methylene oligomers (trioxane). The world-wide consumption in 1997 was 0.5 x 106 t for car parts and other articles processed by injection moulding. Polyacetals are primarily engineering materials being used to replace metals. 

Cleavage of formaldehyde from the active centers and polymerization of formaldehyde at the same cationic chain ends (polymerization-depolymerization equilibrium of formaldehyde) (9). 

On the other hand, no difference in equilibrium concentration is expected between Models C and B. In the latter only the "dead chain segments are crystallized while the cationic active centers, at which depolymerization and polymerization of formaldehyde takes place, are in solution. 

You saw a carbonyl addition reaction forming a polymer right at the beginning of the chapter—the polymerization of formaldehyde. If an amine is added to formaldehyde, condensation to form imines and imine salts occurs readily. These intermediates are themselves electrophilic so we have the basis for ionic polymerization—electrophilic and nucleophilic molecules present in the same mixture. Reaction with a second molecule of amine gives an aminal, the nitrogen equivalent of an acetal. 

Paraformaldehyde is a white crystalline powder with an odor of formaldehyde. It is slowly soluble in cold water, and more readily soluble in hot water with evolution of formaldehyde. It is insoluble in alcohol, and ether. Keep paraformaldehyde in tightly sealed bottles. Paraformaldehyde is prepared by the polymerization of formaldehyde, and is commercially available. 

Summary Trioxane is prepared by the polymerization of formaldehyde with a catalytic amount of sulfuric acid. Paraffin oil is used to stabilize the reaction, forming an emulsion. Commercial Industrial note Part or parts of this laboratory process may be protected by international, and/or commercial/industrial processes. Before using this process to legally manufacture the mentioned compound, with intent to sell, consult any protected commercial or industrial processes related to, similar to, or additional to, the process discussed in this procedure. This process may be used to legally prepare the mentioned compound for laboratory, educational, or research purposes. 

Cyclic acetals polymerize exclusively by cationic mechanism it should be noted, however, that polyoxymethylene may be obtained also by anionic polymerization of formaldehyde. Both processes are used in industry. 

Cationic polymerization of 1,3,5-trioxane provides one of a few examples of industrial application of cationic ring-opening polymerization. Polymerization leads to polyoxymethylene (polyformaldehyde, polyacetal), important engineering plastic. Polyformaldehyde may also be obtained by anionic polymerization of formaldehyde and this process is also used in industry. 

The polyoxymethylenes are presently widely used in different areas. Approximatively one-third of the market is represented by homopolymers and two-thirds by copolymers. Homopolymers are produced by anionic polymerization of formaldehyde using amines, alkoxides, and other types of anionic initiators. The details of these polymerizations will not be discussed in this paper, although some of their properties will be compared to those of copolymers which are obtained by cationic copolymerization of trioxane with cyclic ethers or cyclic esters. Comprehensive reviews on general aspects of synthesis and properties of acetal resins are available [158-162],... 

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