1.3.5- Trioxane polymerization

In addition to the usual polymer-monomer propagation-depropagation equilibrium that may he present, trioxane polymerization proceeds with the occurrence of a polymer-formaldehyde equilibrium ... 

Trioxane polymerizations proceed with induction periods, which correspond to the buildup of the equilibrium concentration of formaldehyde [Lu et al., 1990]. This also corresponds to a buildup in 1,3,5,7-tetroxocane, apparently by insertion of formaldehyde into... 

An additional termination in the trioxane polymerization is chain transfer to monomer hy hydride ion transfer, which results in terminating the propagating chain with a methoxyl group while carbocation XXII reinitiates polymerization [Kern et al., 1966 Weissermel et al., 1967]. 

The combined techniques of NMR, gas chromatography, and mass spectrometry, together with polymerization kinetics, should be sufficient to understand the complicated mechanism of trioxane polymerization. 

In the homopolymerization of dioxolane below 30°C. tertiary oxonium ions exist exclusively (2, 5). Otherwise hydride transfer would occur (carbonium ions abstract hydride from monomeric cyclic formats) (II, 16). In trioxane polymerization, however, at least some of the active chain ends are carbonium ions they cause hydride transfer and elimination of formaldehyde (9, II, 13). Thus, in copolymerization we must expect two different kinds of structures for cationic chains with terminal trioxane unit. Oxonium ions (I) and carbonium ions (II) may have different reactivity ratios in the copolymerization, but hopefully this does not cause severe disturbance since I and II seem to be in a fast kinetic equilibrium with each other (3). Hence, we expect [I]/[II] to be constant under similar reaction conditions. 

In the experimental investigations the soluble copolymer from trioxane and 25 mole % of dioxolane was used as Model C. This is permissible because dioxolane does not cleave off formaldehyde during polymerization. The experimental determination of equilibrium concentrations of formaldehyde during trioxane polymerization is somewhat problematic. Up to now the only available method is extraction of... 

The occurrence and a limited importance of chain transfer by transacetalization cannot be doubted. We proposed this type of reaction for trioxane polymerization as early as 1959 (6) and assumed that intramolecular transacetalization produces some thermally stable macrocyclic polyoxymethylene (10). We have utilized bimolecular chain transfer by polymers to produce thermally stable block copolymers at temperatures over 100°C. [—e.g., with polyesters, polypropylene oxide, or with polyvinyl butyral)] (12). 

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. 

We have observed a reduction in the rate of trioxane polymerization initiated by the siloxonium dication (di-ion pair)... 

Fig. 6 Dependence of the degree of polymerization of polyoxymethylene on conversion during trioxane polymerization. Contents of water as transfer agent [10 3 mol kg-1] (1) 2.8, (2) 5.6, (3) 8.3, (4) 11.1. The points correspond to experimentally measured values the full curves were calculated by means of eqn. (74) [82],...

The polymerizability of a monomer is also influenced by the physical state of the polymerization. For example, crystallization of poly(oxy methylene) provides the driving force for trioxane polymerization. In this case, propagation occurs at active sites on the crystal lattice rather than in solution, and AGP includes the change in free energy of the phase transition as well as that of the solution polymerization [Eq. (19)]. 

Although the corresponding quantitative measurements of the carbenium-oxoniumion equilibrium in the polymerization of 1,3,5-ttioxane have not yet been carried out, one can assume that the proportion of oxonium ions relative to this monomer will be lower than with 1,3-dioxolane which is much more nucleophilic. Moreover, the importance of hydride anion transfer may be higher for 1,3,5-trioxane polymerization. Thus, in spite of 1,3,5-trioxane itself beitig a much weaker hydride donating agent than 1,3-dioxolane, the hydride anion transfer processes play an important role in the polymerization of the former. 

Polyformaldehyde can also be prepared by polymerization of trioxane, the cyclic trimer of formaldehyde. Trioxane polymerizes by ring opening polymerization and cationic initiators are the only effective initiators. Formaldehyde is always present when trioxane is polymerized because the growing polyoxymethylene chains by depropagation may lose one monomer unit, which is formaldehyde not trioxane. In spite of the fact that formaldehyde plays an (as yet incompletely understood) role in trioxane polymerization, which is a cyclic ether polymerization like dioxolane or tetrahydrofurane [5], trioxane will not be discussed in this review. 

A long induction period and an accelleiation of the reaction as time goes on characterize the trioxane polymerization in solution 80). The sequence of reactions leading to actively growing polymer crystals can be described by the following steps 83) ... 

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... 

Valuable information on the mechanism of the process and on the confirmation of the formulated assumptions was obtained by analyzing the low-molecular-weight by-products of trioxane polymerization reaction 1,3,5,7-tetraoxane and formaldehyde. Theoretical analysis has shown that, depending on the state of active centers (surface or dissolved) and the length of the dissolved portion of the polymer chain, the steady-state concentration of 1,3,5,7-tetraoxane and formaldehyde changes. A comparison between experimental and theoretical data has shown that at monomer... 

The ideas presented above also made it possible to explain some other features of trioxane polymerization kinetics (the dependence of induction period on monomer concentration, the limitedness of kinetic curve, etc.). Moreover, in accordance with the same ideas, the rate vs. monomer concentration curve can pass through a maximum (at [M] < if the active centers on the surface of polymer crystal... 

Fig. 5 a-d. Energy diagrams for irreversible (a) and reversible (b) processes of polymer formation in trioxane, polymerization in methylene dichloride (c) and nitrobenzene (d) (1, 2, 3 different polymer structures)... 

To confirm the correctness of the hypotheses formulated above, an investigation was conducted on poly(oxymethylene) supramolecular structures obtained as a result of trioxane polymerization in different solvents and in the presence of BF3 0(C2Hs)2 and SnCl4. In all cases the thermodynamic conditions of polymerization were varied by changing the monomer concentration. 

It will be shown in Chapter 7 that the low [M]e (and therefore high yields) in 1,3,5-trioxane polymerization is due to the polymerization proceeding simultaneously with crystallization. Otherwise, only a small fraction of monomer could polymerize at moderate temperatures. 

Table 2.14. Thermodynamic parameters of 1,3,5-trioxane polymerization in C6H6 solvent82)...

Mechanistic Aspects of 1,3,5-Trioxane Polymerization Pertinent to Polymer Synthesis... 

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