Polymers, formaldehyde Depolymerization

Paraformaldehyde [30525-89-4] is a mixture of polyoxymethylene glycols, H0(CH20) H, with n from 8 to as much as 100. It is commercially available as a powder (95%) and as flake (91%). The remainder is a mixture of water and methanol. Paraformaldehyde is an unstable polymer that easily regenerates formaldehyde in solution. Under alkaline conditions, the chains depolymerize from the ends, whereas in acid solution the chains are randomly cleaved (17). Paraformaldehyde is often used when the presence of a large amount of water should be avoided as in the preparation of alkylated amino resins for coatings. Formaldehyde may also exist in the form of the cycHc trimer trioxane [110-88-3]. This is a fairly stable compound that does not easily release formaldehyde, hence it is not used as a source of formaldehyde for making amino resins. 

The polymer also can be made from trioxane (the trimer of formaldehyde), usually as a copolymer with ethylene oxide. The —CH2CH2— fragments in the copolymer chain prevent depolymerization acetal copolymer was developed by Celanese (10). 

Polymers of formaldehyde were found recently in interstellar space by N. Wickramasinghe [Nature, 252, 462 (1974)]. It is well known that polyformaldehyde is thermodynamically unstable already at not very high temperatures (close to room temperature), but it should be stable versus depolymerization near absolute zero. Therefore the formation of poly-oxymethylene near absolute zero is not a thermodynamic but a kinetic problem. 

Tn the cationic polymerization and copolymerization of trioxane in the - melt or in solution, an induction period usually exists, during which no solid polymer is formed and the reaction medium remains clear. Nevertheless, reactions are known to occur during this period. By using BF3 or an ether ate as catalyst, in homopolymerization, Kern and Jaacks (I) reported the formation of formaldehyde via depolymerization of polyoxymethylene cations. 

The details of the commercial preparation of acetal homo- and copolymers are discussed later. One aspect of the polymerization so pervades the chemistry7 of the resulting polymers that familiarity with it is a prerequisite for understanding the chemistry of the polymers, the often subtle differences between homo- and copolymers, and the difficulties which had to be overcome to make the polymers commercially useful. The ionic polymerizations of formaldehyde and trioxane are equilibrium reactions. Unless suitable measures are taken, polymer will begin to revert to monomeric formaldehyde at processing temperatures by depolymerization (called unzipping) which begins at chain ends. 

It is quite often not realized that not only can raw materials or products be hazardous substances, but so also can by-products formed during the process. This can hapjten a result of side reactions or decomposition reactions that occur either intentionally as a result of an intrinsic property of a chemical process or unexpectedly due to deviations of the process. Examples are the possible formation of nitro-samines in nitrite-containing cooling fluids or the release of volatile monomers, e. g., styrene or formaldehyde during processing of polymers and plastics because of depolymerization at elevated temperatures. Disproportionation reactions of tri-valent organic phosphorus esters with formation of volatile toxic phosphines or decomposition reactions of unstable compounds like hydroxylamine, metal carbonyls, nitro-compounds or peroxides are further examples. 

Formaldehyde is a versatile reagent [9]. It has some disadvantages, however, because it must be generated before use from solid polymer paraformaldehyde by way of thermal depolymerization and it self-polymerizes easily [10]. On the other hand, commercial formaldehyde solution, which is an aqueous solution containing 37% formaldehyde and 8-10% methanol, is cheap, easy to handle, and stable even at room temperature [11, 12]. 

Acrylic adhesives are essentially acrylic monomers which achieve excellent bonding upon polymerization. Typical examples are cyanoacrylates and ethylene glycol dimethacrylates. Cyanoacrylates [28] are obtained by depolymerization of a condensation polymer derived from a malonic acid derivative and formaldehyde. 

When monomeric formaldehyde is needed, to react with a Grignard reagent, for example, it is prepared as needed by heating the polymer in order to depolymerize it. 

The commercial polymers are stabilized directly by additives in the polymerizing mixture, and this is in contrast to polymerization from formaldehyde, where the polymer is first produced and then subsequently stabilized. Here, a distinction is made between thermal degradation stabilization and stabilization against degradation induced by alkali. Cyclic ethers such as ethylene oxide are thermal stabilizers, that is, they stabilize against a depolymerization starting from the chain ends. They are quantitatively incorporated into the chain as comonomers at small yields ... 

Polyoxymethylene depolymerizes into formaldehyde at 220°C. This was found to be a first-order reaction with the rate varying from 0.42 to 5.8%/min, depending upon conditions of polymer preparation and the molecular weight of the polymer [457]. 

Polyacetal can be divided into two basic types, acetal homoploymer and acetal copolymer. Both homopolymer and copolymer are available in a range of molecular weights (M = 20 000-100 000). The homopolymer is a polymer of formaldehyde with a molecular structure of repeated oxymethylene units (Staudinger, 1932). Large-scale production of polyformaldehyde, i.e. polyacetal, commenced in 1958 in the USA (US Patent 2 768 994,1956) (British patent 770 717,1957). Delrin (1959) was the first trade mark for this polymer by Du Pont Company. The copolymers were introduced by the Celanese Corporation of America, and the first commercial product named Celcon (1960). One of the major advantages of copolymerization is to stabilize polyacetal because the homopolymer tends to depolymerize and eliminate formaldehyde. The most important stabilization method is structural modification of the polymer by, for example, copolymerization with cyclic ether. 

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