Polyacetal formaldehyde

Although there is a substantial body of information in the pubHc domain concerning the preparation of polyacetals, the details of processes for manufacturiag acetal resins are kept highly confidential by the companies that practice them. Nevertheless, enough information is available that reasonably accurate overviews can be surmised. Manufacture of both homopolymer and copolymer involves critical monomer purification operations, discussion of which is outside the scope of this article (see Formaldehyde). Homopolymer and copolymer are manufactured by substantially different processes for accomplishing substantially different polymerisation chemistries. 

In the above examples the polymerisation takes place by the opening of a carbon-carbon double bond. It is also possible to open carbonyl carbon-oxygen double bonds and nitrile carbon-nitrogen triple bonds. An example of the former is the polymerisation of formaldehyde to give polyformaldehyde (also known as polyoxymethylene and polyacetal) (Figure 2.3). 

From the time that formaldehyde was first isolated by Butlerov in 1859 polymeric forms have been encountered by those handling the material. Nevertheless it is only since the late 1950s that polymers have been available with the requisite stability and toughness to make them useful plastics. In this period these materials (referred to by the manufacturers as acetal resins or polyacetals) have achieved rapid acceptance as engineering materials competitive not only with the nylons but also with metals and ceramics. 

Cationic initiators can also polymerize aldehydes. For example, BF3 helps produce commercial polymers of formaldehyde. The resulting polymer, a polyacetal, is an important thermoplastic (Chapter 12) ... 

Ring opening polymerization may also occur by an addition chain reaction. For example, a ring opening reaction polymerizes trioxane to a polyacetal in the presence of an acid catalyst. Formaldehyde also produces the same polymer ... 

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 ... 

Polyacetal resins have a repeating unit of -O-CH2-. They are strong, stiff polymers for valves, hoses, and tube connectors. Pentaerythritol finds end-uses in alkyd resins and explosives (pentaerythritol tetranitrate). To appreciate this synthesis, the student should review two condensation reactions, the crossed aldol and the crossed Cannizzaro. Acetaldehyde reacts with 3 mol of formaldehyde in three successive aldol condensations. This product then undergoes a Cannizzaro reaction with formaldehyde. 

Commercial polymers of formaldehyde are also produced using cationic polymerization. The polymer is produced by ring opening of trioxane. Since the polyacetal, POM, is not thermally stable, the hydroxyl groups are esterified (capped) by acetic anhydride (structure 5.22). These polymers are also called poly(methylene oxides). The commercial polymer is a... 

Polyoxymethylene (POM), which is called polyacetal, is a crystalline polymer of formaldehyde which has the following repeating unit ... 

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). 

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). 

Ethers are formed in the usual way the te-chloromethyl ether is obtained by using formaldehyde and hydrogen chloride. With aldehydes or their derivatives, butanediol forms acetals, either 7-membered rings (1,3-dioxepanes) or linear polyacetals the rings and chains are easily intraconverted. 

Polyformaldehyde. Polyformaldehyde or polyacetal is made by two different processes. Delrin is made from formaldehyde by anionic polymerization catalyzed by a tertiary amine. The homopolymer is end-capped with acetic anhydride. Celcon is made from trioxane cationic copolymerization using boron trifluoride catalyst and ethylene oxide (2-3%) as the comonomer. Boron trifluoride is a Lewis acid that associates with trioxane and opens up the six-membered ring. Ethylene oxide provides the end capping. Without an end cap, polyformaldehyde is thermally unstable and loses formaldehyde units. 

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. 

PB PBI PBMA PBO PBT(H) PBTP PC PCHMA PCTFE PDAP PDMS PE PEHD PELD PEMD PEC PEEK PEG PEI PEK PEN PEO PES PET PF PI PIB PMA PMMA PMI PMP POB POM PP PPE PPP PPPE PPQ PPS PPSU PS PSU PTFE PTMT PU PUR Poly(n.butylene) Poly(benzimidazole) Poly(n.butyl methacrylate) Poly(benzoxazole) Poly(benzthiazole) Poly(butylene glycol terephthalate) Polycarbonate Poly(cyclohexyl methacrylate) Poly(chloro-trifluoro ethylene) Poly(diallyl phthalate) Poly(dimethyl siloxane) Polyethylene High density polyethylene Low density polyethylene Medium density polyethylene Chlorinated polyethylene Poly-ether-ether ketone poly(ethylene glycol) Poly-ether-imide Poly-ether ketone Poly(ethylene-2,6-naphthalene dicarboxylate) Poly(ethylene oxide) Poly-ether sulfone Poly(ethylene terephthalate) Phenol formaldehyde resin Polyimide Polyisobutylene Poly(methyl acrylate) Poly(methyl methacrylate) Poly(methacryl imide) Poly(methylpentene) Poly(hydroxy-benzoate) Polyoxymethylene = polyacetal = polyformaldehyde Polypropylene Poly (2,6-dimethyl-l,4-phenylene ether) = Poly(phenylene oxide) Polyp araphenylene Poly(2,6-diphenyl-l,4-phenylene ether) Poly(phenyl quinoxaline) Polyphenylene sulfide, polysulfide Polyphenylene sulfone Polystyrene Polysulfone Poly(tetrafluoroethylene) Poly(tetramethylene terephthalate) Polyurethane Polyurethane rubber... 

Disinfection by-product) production of urea-formaldehyde, phenolic, melamine, pentaerythritol and polyacetal resins industrial synthesis of a number of organic compounds cosmetics fungicides textiles embalming fluids 3210,322, 351, 3529,354... 

Another monomer that belongs to this group is cyclic trimer of formaldehyde, 1,3,5-trioxane. Cationic polymerization of 1,3,5-trioxane leading to polyoxymethylene (polyformaldehyde, polyacetal) is one of the few industrially important processes in cationic ring-opening polymerization. 

Polyacetals form a different subclass of compounds with oxygen in the backbone chain. In this group are included polymers that contain the group -0-C(R2)-0- and can be formed from the polymerization of aldehydes or ketones. A typical example of a polymer from this class is paraformaldehyde or polyformaldehyde or polyoxymethylene (CH20)n. Polyoxymethylene can be prepared by anionic catalysis from formaldehyde in an inert solvent. Acetylation of the -OH end groups of the polymeric chain is common since it improves the thermal stability of the polymer. Some results reported in literature regarding thermal decomposition of these polymers are indicated in Table 9.2.1 [1]. 

The polymerization of aldehydes to give polyacetal is readily undertaken anionically in the presence of base, due to the susceptibility of formaldehyde to nucleophilic attack (Odian, 1991) as shown in Scheme 1.25. It should be noted that the resulting polymer is thermally unstable and stabilization is achieved by an esterification reaction of the unstable hemiacetal end groups after polymerization. 

Scheme 1.25. Elementary reactions in the anionic polymerization of formaldehyde to form polyacetal.

Figure 1.42. Weight loss from polyacetal during processing due to liberation of formaldehyde and inhibition by an acid scavenger combined with a hindered phenol antioxidant. Adapted from Zweifel (1998).

Chain growth polymerizations very often contain a double bond however, cyclic ethers will polymerize in this manner [5], POM (polyoxymethylene) made by the Celanese method shown in Figure 3.6 is an example of a cyclic ether with this method. 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 (e.g., ethylene oxide or 1,3 dioxalane). 

Several papers57"59 were devoted to investigating a complex process such as the cationic copolymerization of monomeric formaldehyde with dioxolane in the gas, liquid, and gas-liquid phases. It is known that polyacetal resins are industrially produced by copolymerizing cyclic acetals (trioxane, 1,3,5,7-tetraoxane), or by anionic homopolymerization of monomeric formaldehyde with subsequent modification of end groups. 

New investigations into the kinetics of monomeric formaldehyde copolymerization were subsequently conducted. In papers63,64 a degradation kinetics method for the analysis of polyacetal microstructure is proposed in paper6 5 is outlined a pyrolitie method based on a chromatographic determination of 1,3,5-trioxepane at the junctions of oxymethylene and dioxolane blocks. 

Independent determinations of microstructure and composition heterogeneity of polyacetals, which were obtained using various methods and based on different monomers (formaldehyde and trioxane to produce oxymethylene blocks, and methylene oxide and dioxolane to produce stabilized oxymethylene blocks), made it possible to reveal differences in the mechanism of the synthesis of such products. 

Hydrolysis has been the main method used for the chemical recycling of other condensation polymers, such as polyacetals and polycarbonates. Hydrolysis of polyacetals leads back to the starting monomers, formaldehyde or trioxane. Polycarbonates are polymers synthesized by the reaction of phosgene and a dihydric phenol, commonly bisphenol A. Chemical recycling of polycarbonate... 

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