Trioxane, depolymerization

If the substrate is predominantly unprotonated in the acidity range covered, the protonation correction term (second on the left in equation (35)) will be zero, and if the activity coefficient term on the right cancels to zero, log values will be linear in —Ho with unit slope (the Zucker-Hammett hypothesis).146 In practice, linearity is usually observed,23 e.g. for the trioxane depolymerization in equation (31),144 although the slopes are seldom exactly unity. For more strongly basic substrates that are predominantly protonated in the acidity range covered, equation (36) is easily derived this is acidity independent and should have zero slope against — H0 if the substrate is fully protonated and the last term cancels.145... 

Due to the difference in basicity between EO (pKi, 7.3) and trioxane (pi b 10), ° the cyclic oxonium resulting from protonation of EO was considered to be largely predominant. It has been assumed that its reaction with the formaldehyde from trioxane depolymerization should lead to the ring-expanded cyclic oxonium and then to DXL by proton exchange with EO. Further insertion of... 

Polymerization. Paraldehyde, 2,4,6-trimethyl-1,3-5-trioxane [123-63-7] a cycHc trimer of acetaldehyde, is formed when a mineral acid, such as sulfuric, phosphoric, or hydrochloric acid, is added to acetaldehyde (45). Paraldehyde can also be formed continuously by feeding Hquid acetaldehyde at 15—20°C over an acid ion-exchange resin (46). Depolymerization of paraldehyde occurs in the presence of acid catalysts (47) after neutralization with sodium acetate, acetaldehyde and paraldehyde are recovered by distillation. Paraldehyde is a colorless Hquid, boiling at 125.35°C at 101 kPa (1 atm). 

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). 

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). 

The effect of propagation-depropagation equilibrium on the copolymer composition is important in some cases. In extreme cases, depolymerization and equilibration of the heterochain copolymers become so important that the copolymer composition is no longer determined by the propagation reactions. Transacetalization, for example, cannot be neglected in the later stages of trioxane and DOL copolymerization111, 173. This reaction is used in the commercial production of polyacetal in which redistribution of acetal sequences increases the thermal stability of the copolymers. 

This consists of protonation in a fast pre-equilibrium process followed by ratedetermining reaction of the protonated substrate SH +, equation (30) a typical example is the much-studied acid-catalyzed depolymerization of trioxane, shown in equation (31).144... 

The industrial synthesis of polyformaldehyde [poly(oxymethylene)] occurs by anionic polymerization of formaldehyde in suspension. For this the purification and handling of monomeric formaldehyde is of special importance since it tends to form solid paraformaldehyde. After the polymerization the semiacetal end groups have to be protected in order to avoid thermal depolymerization (Example 5-13). This is achieved by esterfication with acetic anhydride (see Example 5-7). As in the case of trioxane copolymers (see Sect. 3.2.3.2) the homopolymers of formaldehyde find application as engineering plastics. 

The concept of a (bound) formaldehyde intermediate in CO hydrogenation is supported by the work of Feder and Rathke (36) and Fahey (43). Experiments under H2/CO pressure at 182-220°C showed that paraformaldehyde and trioxane (which depolymerize to formaldehyde at reaction temperatures) are converted by the cobalt catalyst to the same products as those formed from H2/CO alone. The rate of product formation is faster than in comparable H2/CO-only experiments, and product distributions are different, apparently because secondary reactions are now less competitive. However, Rathke and Feder note that the formate/alcohol ratio is similar to that found in H2/CO-only reactions (36). Roth and Orchin have reported that monomeric formaldehyde reacts with HCo(CO)4 under 1 atm of CO at 0°C to form glycolaldehyde, an ethylene glycol precursor (75). The postulated steps in this process are shown in (19)—(21), in which complexes not observed but... 

Figure 9.10 presents the mechanism of the polymerization of formaldehyde starting from anhydrous formaldehyde and formaldehyde hydrate. In addition, a reaction path is shown that also connects trimeric formaldehyde ( trioxane, F) with paraformaldehyde (H). In practice, though, this reaction path is only taken in the reverse direction, upon heating (entropy gain ) of paraformaldehyde in aqueous acid as a depolymerization of H —> F. 

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. 

Dioxolane was also formed in the absence of trioxane when the soluble copolymer was simply dissolved in a 0.03M solution of SnCl4 in methylene dichloride at 30°C. (conditions similar to those of the copolymerization in Figure 3). Within one hour half of the soluble copolymer was depolymerized under formation of dioxolane monomer and formaldehyde. 

One of the most prominent features in the heterogeneous copolymerization of trioxane is the occurrence of two different kinds of active centers—dissolved and crystalline copolymer cations. They have different copolymer reactivity ratios and different tendencies to depolymerize, i.e., different formaldehyde equilibrium concentrations. At first the formation of soluble copolymer with high dioxolane content did not raise much hope for obtaining a crystalline copolymer of good thermal stability from trioxane and dioxolane but the gradual depolymerization of the soluble copolymer proved to be a useful side reaction which greatly improved the situation. Eventually, the entire complicated process turned out to be quite favorable for the formation of a stable crystalline copolymer with the desired random distribution. 

In the copolymerization of trioxane with dioxolane, however, depolymerization and regeneration of dioxolane monomer is a faster and more effective way of converting soluble into crystalline copolymer with random distribution. A similar mechanism may hold true for trioxane polymerization with similar comonomers such as 1,3-dioxane, 1,3-dioxacyclo-heptane and in part even for copolymerization of trioxane with ethylene oxide which also involves formation of some dioxolane and soluble copolymer. 

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. 

The chromatogram in Fig. 2.4 was obtained on a column packed with 15% of polyphenyl ester on Gas-Chrome RZ for the depolymerization of trioxane to formaldehyde. The depolymerization rate measured on the basis of the derived data at 124°C was 2.65-10 sec [59]. 

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