Acetal resins formaldehyde polymerization

Acetal formation, microwaves in, 76 557 Acetalization, of PVA, 25 602-603 Acetal polymerization, 74 271 Acetal resins, 70 183-185 Acetal resins, formaldehyde in, 72 122 Acetals, 2 64 70 529 aroma chemicals, 3 253 inorganic pigment applications, 7 372t organic pigment applications, 7 368t typical soluble dye applications, 7 376t Acetaminophen, 4 701. See also AT-Acetyl-p-aminophenol (acetaminophen)... 

Acetals. Acetal resins (qv) are polymers of formaldehyde and are usually called polyoxymethylene [9002-81-7]. Acetal homopolymer was developed at Du Pont (8). The commercial development of acetal resins required a pure monomer. The monomer is rigorously purified to remove water, formic acid, metals, and methanol, which act as chain-transfer or reaction-terminating agents. The purified formaldehyde is polymerized to form the acetal homopolymer the polymer end groups are stabilized by reaction with acetic anhydride to form acetate end groups (9). 

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. 

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. 

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). 

Polymerized formaldehyde (trioxane) is a ring compound of anhydrous formaldehyde with the formula (HCHO)j. See also Acetal Resins. Trioxane is a colorless crystalline solid with a pleasant odor, mp 62°C, bp II5°C. sp gr 1.17. This compound is used as a tanning agent and solvent and as a source of dry HCHO gas. Because trioxane ignites readily at I I3°C and bums with an odorless, hot flame, it has been furnished in tablet form as a replacement for solidified alcohol in portable heating applications. 

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. 

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],... 

Although there is a substantial body of information in the public domain concerning the preparation of polyacetals, the details of processes for manufacturing 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 polymerization chemistries. 

Polyoxymethylene polymers, POM, commonly known as polyacetals or Acetal resins are linear thermoplastic polymers containing predominantly the -CH -O- repeat unit in their backbone. There are two types of acetal resins available commercially (1) homopolymers made by the polymerization of formaldehyde, followed by endcapping, (2) copolymers derived from the ring opening polymerization of trioxane (a cyclic trimer of formaldehyde), and a small amount of a comonomer such as ethylene oxide. Acetal resins are... 

Acetal polymers are formed from the polymerization of formaldehyde. They are also given the name polyoxymethylenes (POMs). Polymers prepared from formaldehyde were studied by Staudinger in the 1920s, but thermally stable materials were not introduced until the 1950s, when DuPont developed Dehin. Hompolymers are prepared from very pure formaldehyde by anionic polymerization as shown in Fig. 2.1. Amines and the soluble salts of alkali metals catalyze the reaction. The polymer formed is insoluble and is removed as the reaction proceeds. Thermal degradation of the acetal resin occurs by unzipping with the release of formaldehyde. The thermal stability of the polymer is increased by esterification of the hydroxyl ends with acetic anhydride. An alternative method to improve the thermal stabihty is copolymerization with a second monomer, such as ethylene oxide. The copolymer is prepared by cationic methods developed by Celanese and mar-... 

An example of the importance of end-group control is in acetal resin or polyoxymethylene (POM). As normally prepared from cationic polymerization of sym-trioxane, this polymer has —OH end groups, which readily initiate depropagation to formaldehyde (2) by a molecular mechanism (9) ... 

The Du Pont Company offered the first commercial acetal homopolymer, a high molecular weight polymer with a molecular structure of repeating carbon>oxygen links, under the trademark Delrln acetal resin In January 1960. The commercialization of Delrln followed 10 years of chemical research and development stimulated by work with pure monomeric formaldehyde and an Improved polyformaldehyde made by polymerization In a nonsolvent. 

These materials form yet another class of polymer which may be prepared by more than one polymerization method. For example polyoxymethylene (polyformaldehyde, polymethanal, acetal resin) may be prepared by double-bond polymerization from formaldehyde or by ring-opening of trioxane ... 

Acetal resin is either a homopolymer or copolymer of formaldehyde. The homopolymer is sometimes called polyoxymethylene or POM. It is made by polymerizing either anhydrous formaldehyde or its cyclic trimer triox-ane. The polymerization can he done using either acidic or basic catalysts. At least some of the end groups are hydroxyl groups and with heat, the resin... 

Trioxane n (sym-trioxane, triformol, trioxin) CH2OCH2OCH2O. The stable, cyclic trimer of formaldehyde, a colorless, crystalline solid. It is easily depolymerized in the presence of acids to its monomer, or may be further polymerized to form ACETAL RESINS. This trimer should not be confused with Paraformaldehyde. 

Uses ndReactions. The Prins reaction of 3-carene with formaldehyde in acetic acid gives mainly 2-carene-4-methanol acetate, which when saponified produces the 2-carene-4-methanol, both of which are commercial products of modest usage (60). 3-Carene (28) also reacts with acetic anhydride with a catalyst (ZnCl2) to give 4-acetyl-2-carene (29) (61), which is also a commercial product. Although 3-carene does not polymerize to produce terpene resins, copolymerization with phenol has been successfully commercialized by DRT in France (62). 

Various polymeric materials were tested statically with both gaseous and liquefied mixtures of fluorine and oxygen containing from 50 to 100% of the former. The materials which burned or reacted violently were phenol-formaldehyde resins (Bakelite) polyacrylonitrile-butadiene (Buna N) polyamides (Nylon) polychloroprene (Neoprene) polyethylene polytriflu-oropropylmethylsiloxane (LS63) polyvinyl chloride-vinyl acetate (Tygan) polyvinylidene fluoride-hexafluoropropylene (Viton) polyurethane foam. Under dynamic conditions of flow and pressure, the more resistant materials which binned were chlorinated polyethylenes, polymethyl methacrylate (Perspex) polytetraflu-oroethylene (Teflon). 

When two polymeric systems are mixed together in a solvent and are spin-coated onto a substrate, phase separation sometimes occurs, as described for the application of poly (2-methyl-1-pentene sulfone) as a dissolution inhibitor for a Novolak resin (4). There are two ways to improve the compatibility of polymer mixtures in addition to using a proper solvent modification of one or both components. The miscibility of poly(olefin sulfones) with Novolak resins is reported to be marginal. To improve miscibility, Fahrenholtz and Kwei prepared several alkyl-substituted phenol-formaldehyde Novolak resins (including 2-n-propylphenol, 2-r-butylphenol, 2-sec-butylphenol, and 2-phenylphenol). They discussed the compatibility in terms of increased specific interactions such as formation of hydrogen bonds between unlike polymers and decreased specific interactions by a bulky substituent, and also in terms of "polarity matches" (18). In these studies, 2-ethoxyethyl acetate was used as a solvent (4,18). Formation of charge transfer complexes between the Novolak resins and the poly (olefin sulfones) is also reported (6). 

The mechanical degradation and production of macroradicals can also be performed by mastication of polymers brought into a rubbery state by admixture with monomer several monomer-polymer systems were examined (10, 11). This technique was for instance studied for the cold mastication of natural rubber or butadiene copolymers in the presence of a vinyl monomer (13, 31, 52). The polymerization of methyl methacrylate or styrene during the mastication of natural rubber has yielded copolymers which remain soluble up to complete polymerization vinyl acetate, which could not produce graft copolymers by the chain transfer technique, failed also in this mastication procedure. Block and graft copolymers were also prepared by cross-addition of the macroradicals generated by the cold milling and mastication of mixtures of various elastomers and polymers, such as natural rubber/polymethyl methacrylate (74), natural rubber/butadiene-styrene rubbers (76) and even phenol-formaldehyde resin/nitrile rubber (125). 

Suspension polymerization is the most widely used process for making plastic resins both in terms of the number of polymer products and in tonnage production. Practically all of the common thermoplastic resins, including some of the newer polymers, are made by this method. Styrene, methyl methacrylate, vinyl chloride, vinylidene chloride, vinyl acetate, the fluorocarbons, and some gaseous monomers, including ethylene, propylene and formaldehyde, may be polymerized by the suspension polymerization process. 

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