1.3.5- Trioxane solid-state polymerization

A great majority of polymerizations are simultaneously affected by many physical and chemical factors, and their course is the result of a superposition of these effects. Only in rare cases does one of these factors dominate and the polymerization is formally simplified. In topochemical polymerizations, the growth of macromolecules is governed by forces in the crystal lattice of the monomer. Solid-state polymerization of trioxane (trioxacyclohexane) is a typical example of topochemical polymerization. 

Reactivity in the solid-state is always connected with specific motions which allow the necessary contact between the reacting groups. In most cases solid-state reactions proceed by diffusion of reactions to centers of reactivity or by nucleation of the product phase at certain centers of disorder. This leads to the total destruction of the parent lattice. If the product is able to crystallize it is highly probable that nucleation of the crystalline product phase at the surface of the parent lattice will lead to oriented growth under the influence of surface tension. In such topotactic reactions certain crystallographic directions of parent and daughter phases will coincide. Typical examples for this behaviour are the solid-state polymerizations of oxacyclic compounds such as trioxane, tetroxane or 3-propiolactone... 

Miyama, H., and M. Kamachi Solid state polymerization of trioxane by the... 

Crystalline trithiane, the sulfur containing analog of trioxane polymerizes in a topotactic reaction after irradiation on subsequent heating to 180° C 93). Again the crystal structure is twinned and differential thermal analysis has shown a higher melting point for polymers produced by solid state polymerization than for solution polymerized trithiane... 

Table I. Solid State Polymerizations of Trioxane and Tetraoxane by y-Ray or Plasma Initiation...

The Kohlschutter report of solid-state polymerization (1) involved gas solid reactions of trioxane, and there is continuing interest in reactions of trioxane and other cyclic ethers (13). 

The conformation of polymers produced by solid state polymerization often depends strongly on the crystal structure of the monomers. For example, the polymerization of tetroxane, (CH20)4, leads to helix structures, whereas the polymerization of trioxane, (CH20)3, gives a zigzag poly-(oxymethylene). 

The polymerization of trioxane in the crystalline state is a well-known example of radiation-induced solid-state polymerization In the polymerization of crystalline monomers, the topotactical conditions are very important and polymerization is possible only for a very few monomers. The situation is completely different for... 

The Polymerization-Crystallization Stage in the (Co)polymerizations of 1,3,5-Trioxane Radiation-Initiated Solid-State Polymerization of 1,3,5-Trioxane Properties of Poly(oxymethylene)... 

A unique feature of high-energy-radiation apphcations relates to solid-state polymerizations [27-31]. In this case, a catalyst is not required and the irradiations can be performed at deliberately chosen temperatures, because the absorption of radiation is temperature-independent. Of particular interest in this context are compounds such as trioxane, hexamethylcyclotrisiloxane, P-propiolactone and diketene, all of which polymerize readily in the solid state but slowly, or not at ah, in the hquid state. Other compounds such as styrene, acrylonitrile and formaldehyde polymerize faster in the crystalline state than in the liquid state just above the melting point Compounds that undergo solid-state polymerization upon y-hradiation are listed in Table 5.1. In contrast, vinyl chloride, vinyl acetate and methyl methacrylate do not polymerize in the solid state. 

The solid-state polymerizations of trioxane [32,33] and of diacetylenes [34], proceed as topochemical reactions - that is, the polymerization is crystal-lattice controlled and proceeds with a minimum of atomic and molecular movement. Upon ring-opening, trioxane molecules form helical polyoxyraethylene chains lying in the direction of the c-axis of the trioxane crystals. Such polymerization affords only a very small volume change. 

There are certain monomers which do not polymerize in the liquid state but will undergo solid-state polymerization. One such monomer that has now been extensively studied is trioxane where single crystals of the monomer will polymerize to give polymer crystals when they are irradiated. 

Besides in the liquid phase, some polyreactions are also performed in the solid state, for example, the polymerization of acrylamide or trioxane (see Example 3-24). The so-called post condensation, for example, in the case of polyesters (see Example 4-3), also proceeds in the solid phase. Finally, ring closure reactions on polymers with reactive heterocyclic rings in the main chain (e.g., poly-imides, see Example 4-20) are also performed in the solid state. 

The solubility of polyoxymethylene is very poor so that the ring-opening polymerization of 1,3,5-trioxane proceeds heterogeneously both in bulk (melt) and in solution. 1,3,5-Trioxane can also be readily polymerized in the solid state this polymerization can be initiated both by high-energy radiation and by cationic initiators (see Example 3-24). 

Polymerization of some cationically polymerizable monomers like cyclohexene oxide [49], 3,3-bis(chloromethyl)oxetane [50], or 1,3,5-trioxane [51] may be initiated by irradiation with y-rays both in liquid or solid state. 

Radiation-induced Solid-state (RISS) Polymerization of 1,3,5-Trioxane... 

Crystalline polymers were obtained by the radiation-induced polymerization of various cyclic compounds in the solid state " . Especially the cyclic oligomers of formaldehyde (CHjO), i.e., trioxane (TOX) (n = 3), tetroxocane (TeOX) (n = 4), pentoxaiK (P OX) (n = 5), and hexoxane (HeOX) (n = 6), easily produce polyfoxy-methylene (POM) by radiation-induced polymerization in the solid state... 

Okamura, S., Hayashi, K., and Nishii, N. (1962) Polymer crystals obtained by radiation polymerization of trioxane in solid state./. Polymer Sci., 60, —26. 

A number of ethylenic monomers and cyclic monomers undergo chain polymerization in the solid state when exposed to high-energy radiation (e.g. y-radiation). Examples include acrylamide, acrylic acid, acrylonitrile, vinyl acetate, styrene, 1,3,5-trioxane, propriolactone and hexamethylcyclo-trisiloxane. These polymerizations may involve free-radical and/or ionic species, but in most cases the precise mechanisms operating are not known. 

Deviations from these assumptions lead, in certain polymerization systems, to interesting phenomena, an instructive example of which is provided by the solid-state 1,3,5-trioxane polymerization reviewed extensively in Ref. [3b], Section 7.3. Some other pertinent examples are briefly discussed below. 

Properties and handling. Formaldehyde is a colorless, toxic gas at room temperature, with a pungent, irritating odor. It is flammable and explosive in presence of air. Both gaseous and liquid forms of formaldehyde polymerize at room temperature, and because of this, it can only be maintained in the pure state for a very short period. Because of these unhandy conditions, there are two ways formaldehyde gets into commerce, as a water solution called formalin and as a solid called paraformaldehyde or trioxane. 

Trioxane is the cyclic trimer of formaldehyde and it can be polymerized to yield polyoxymethylene having the same structure as polyformaldehyde. Polymerization has been carried out with or without catalyst in the liquid, solid, and sublimed states. All polymerizations appear to proceed by a cationic mechanism and the usual type of cationic initiators are effective [122,138—141]. The structure of the cationic chain ends is not clear and two types of props ating centres have been proposed [142], namely, tertiary oxonium ions and carbenium ions. Their propagation reactions are... 

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