Depolymerization of polyoxymethylene

The thermal depolymerization of polyoxymethylene starts from the unstable hydroxy end groups, but the oxidative and acid-catalyzed hydrolytic degradation takes place within the main chain. Flence, if polyoxymethylene is heated in air or in the presence of strong acids samples with blocked end groups will also degrade. 

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. 

Figure 5.197 Depolymerization of polyoxymethylene reverse cleavage of formaldehyde ( zipper...

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. 

Two-shot techniques for acyclic diene metathesis, 435-445 for polyamides, 149-164 for polyimides, 287-300 for polyurethanes, 241-246 for transition metal coupling, 483-490 Anionic deactivation, 360 Anionic polymerization, 149, 174 of lactam, 177-178 Apolar solvents, 90 Aprotic polar solvents, 185, 338 Aprotic solvents, low-temperature condensation in, 302 Aqueous coating formulations, 235 Aqueous polyoxymethylene glycol, depolymerization of, 377 Aqueous systems, 206 Ardel, 20, 22... 

Figure 3.14 Interruption of depolymerization of copolymerized polyoxymethylene at a comonomer component...

Cyclic ether and acetal polymerizations are also important commercially. Polymerization of tetrahydrofuran is used to produce polyether diol, and polyoxymethylene, an excellent engineering plastic, is obtained by the ring-opening polymerization of trioxane with a small amount of cycHc ether or acetal comonomer to prevent depolymerization (see Acetal resins Polyethers, tetrahydrofuran). 

Polyoxymethylene is susceptible to depolymerization, or unzipping, under molding conditions. To improve thermal stability, end capping is essential. The capping of the hydroxyl end groups is achieved by etherification or, preferably, by esterification using acetic anhydride ... 

Polyoxymethylenes have a marked tendency to undergo thermal depolymerization with loss of formaldehyde. To prevent thermal depolymerization, polyoxymethylenes are structurally modified, the two possibilities being acetylation to block the reactivity of the end groups of co-polymerization with cyclic ethers, e.g., ethylene oxide. Polyacetals are also sensitive towards autoxidation, which invariably leads to depolymerization as a result of chain scission. The formaldehyde released by depolymerization is very likely to be oxidized to formic acid, which can catalyze further depolymerization. 

Polyoxymethylene (POM) is commonly used as a direct replacement for metals due to its stiffness, dimensional stability and corrosion resistance. Copolymers including ethylene oxide are quite common, primarily because they reduce the propensity for depolymerization at normal processing temperatures. Some typical properties are Glass Transition Temperature, Tg = -30 C Melting Temperature, Tm = 183 C Amorphous density at 25 C = 1.25 g/cc Crystalline density at 25 C = 1.54 g/cc Molecular weight of repeat unit = 30.03 g/mole. There are a number of grade variations of this material commercially available. 

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]. 

Most of the basic commercial polymers are flammable [1]. Burning of plastics is a complicated phenomenon depending mainly on the chemical structure of polymers and on some physical factors [2]. The flammability of plastics is particularly severe when the basic polymers undergo depolymerization to form flammable monomers or active products. Such is the case for PS, PMMA, polyoxymethylene (POM), NR, etc. 

The main difference between homo- and copolymers is the fact that copolymers exhibit higher thermal-oxidative resistance. Figure 5.196. Thermal-oxidative degradation in polyoxymethylene leads to the formation of unstable end groups that initiate depolymerization under the formation of formaldehyde. In homopolymers, this results in total decomposition. In copolymers, depolymerization proceeds only to the next comonomer unit (mostly polyethylene, comonomer content approx. 0.5 to 5 wt.%). That widens the processing window for copolymers and reduces the risk of mold fouling [771]. 

Polyoxymethylene (6) as prepared has OH ends the polymer is of low stability and degrades to formaldehyde in 100% yield from about 100 °C. The stability is improved by acetylating the chain ends, which indicates that end initiation of depolymerization plays an important part, but the product of degradation of the end-capped polymer is the same. The formation of formaldehyde has been explained by a nonradical mechanism as shown in Scheme 36. A similar six-centre transition state may be envisaged, leading to the release of two molecules of HCHO at each step. 

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