Linear polymers Paraformaldehyde

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. 

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,... 

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,... 

When formaldehyde is heated, a white crystalline polymer that melts at 123 °C is obtained. This polymer, known as paraformaldehyde, is formed from between 50 to 100 monomers of formaldehyde. It is a linear polymer and decays into formaldehyde when heated. 

When formaldehyde is heated with concentrated sulphuric acid, it forms the polymer polyoxymethylene, which contains at least 100 monomers. Polyoxymethylene is linear like paraformaldehyde. The main difference between the two of them is the number of monomers in the polymer. When heated, it also decays into formaldehyde. It is widely used in the textile industry. 

Paraformaldehyde ipar-9-f6r- mal-do- hid (1894) n. A low-molecular-weight, linear polymer of formaldehyde HOCH2-... 

A reaction closely related to acetal formation is the polymerization of aldehydes. Both linear and cyclic polymers are obtained. For example, methanal in water solution polymerizes to a solid long-chain polymer called paraformaldehyde or polyoxymethylene ... 

In some reports, linear polyanilines and polypyrroles have been used as polymer precursors and diiodomethane/paraformaldehyde has been used as a cross-linker, forming networks. " However, this is different from the previously mentioned lithiation route as it does not utilize any lithiating agents or Lewis acids thus, this strategy not only skips the lithiation step but also prevents release of copious amounts of HCl gas produced in other cases (Figure 3.1). 

By reacting 1,4-butane diol with paraformaldehyde in the presence of sulfuric acid at 150-180" , the seven-membered 1,3-dioxepane is prepared. With other aldehydes, homologous 2-alkyl-substituted 1,3-dioxepanes have also been prepared using a cationic ion exchange resin instead of sulfuric acid. This latter technique was introduced by Astle [42]. The dioxepanes have been converted to polymers in methylene dichloride or in 1,2-dichloro-ethylene. The initiator used was boron trifluoride etherate the reaction temperatures ranged from -10° to +10°. The reactions were carried out under anhydrous conditions by techniques suitable for reaction kinetics studies. The work indicated that, at least for this class of compounds, the polymerization propagation step involves linear alkoxycarbenium ions [47]. 

The addition of water to the carbonyl group in aqueous solutions can lead to the formation of hydrates. The reactions and stability of hydrates depend primarily on the inductive effect of substituents. Hydration of formaldehyde readily yields methylene glycol (methanediol), which polymerises to linear oligomers and also to polymers known as paraformaldehyde. Hydrates of a-dicarbonyl and a-hydroxycarbonyl compounds spontaneously dimerise into various cyclic 1,4-dioxanes (see Figure 4.62). These hydrates are intermediates of other a-dicarbonyl and a-hydroxycarbonyl compounds in the oxidation-reduction reactions and precursors of carboxylic acids. Dialkyl ketones do not form hydrates. 

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