Polymers, formaldehyde Cyclic

Ethylene glycol in the presence of an acid catalyst readily reacts with aldehydes and ketones to form cyclic acetals and ketals (60). 1,3-Dioxolane [646-06-0] is the product of condensing formaldehyde and ethylene glycol. Applications for 1,3-dioxolane are as a solvent replacement for methylene chloride, 1,2-dichloroethane, 1,1,1-trichloroethane, and methyl ethyl ketone as a solvent for polymers as an inhibitor in 1,1,1-trichloroethane as a polymer or matrix interaction product for metal working and electroplating in lithium batteries and in the electronics industry (61). 1,3-Dioxolane can also be used in the formation of polyacetals, both for homopolymerization and as a comonomer with formaldehyde. Cyclic acetals and ketals are used as protecting groups for reaction-sensitive aldehydes and ketones in natural product synthesis and pharmaceuticals (62). 

Acetal homopolymer has a T oi about 175°C (347°F) and specific gravity of 1.41 g/cc acetal copolymer has a T oi about 165°C (329°F) and specific gravity of 1.42 [5]. Acetal polymers are synthesized entirely from formaldehyde directly, as noted earlier, or by copolymerization of the formaldehyde cyclic trimer (cyclic oligomer) trioxane and cyclic ethers. 

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 reversible acid-catalyzed transacetalation of the cyclophane formal 3 has been shown to undergo a ring-fusion/ring-fission process to generate a mixture of polymer cyclic formaldehyde acetals by means of oxonium ion intermediates <06CEJ8566>. The stepwise... 

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). 

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. 

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. 

The copolymerization of trioxane with cyclic ethers or formals is accomplished with cationic initiators such as boron trifluoride dibutyl etherate. Polymerization by ring opening of the six-membered ring to form high molecular weight polymer does not commence immediately upon mixing monomer and initiator. Usually, an induction period is observed during which an equilibrium concentration of formaldehyde is produced. 

Alkyd and polyester resins, epoxy compounds, phenol-formaldehyde resin, urea and/or melamine-aldehyde resin, cyclic urea resin, carbamide acid ester formaldehyde resin, ketone formaldehyde resin, polyurethane, polyvinylester, polyvinyl acetate, polyvinyl chloride and polymer mixtures, polyethylene, polystryrene, styrene mixtures and graft copolymers, polyamide, polycarbonate, polyvinyl ether, polyacrylic and methacrylic acid esters, polyvinyl flouride, polyvinylidene chloride copolymers, UV and/or electron irradiated lacquers. 

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. 

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,... 

Formaldehyde homopolymer is composed exclusively of repeating oxymethylene units and is described by the term polyoxymethylene (POM) [9002-81-7]. Commercially significant copolymers, for example [95327-43-8], have a minor fraction (typically less than 5 mol %) of alkylidene or other units, derived from cyclic ethers or cyclic foimals, distributed along the polymer chain. The occasional break in the oxymethylene sequences has significant ramifications for polymer stabilization. 

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,... 

The polymer has a relatively low ceiling temperature (119 °C), but this is the highest of all the formaldehyde polymers. The lack of success at polymerizing other monomers was due to the low ceiling temperatures (e.g. —39 C for acetaldehyde) (Odian, 1991). Aldol condensation can be a side reaction competing with polymerization for these monomers. There are other routes, such as cationic ring-opening polymerization of cyclic acetals, to achieve the same polymer (Penczek and Kubisa, 1989). 

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