Poly-1,3,5-trioxan

Poly-1,3,5-trioxan is known to contain hemiacetal -OH groups some of these are formed because of the chain transfer to water 32, Acetylation of the -OH groups greatly enhances tKermal stability of the poly-oxymethylene polymers,and is at the base of the com -mercialization of the first polyacetal (a homopolymer of CH-O) 132a I known, in fact many years ago from the classical works of Staudinger and Kern,... 

In the domain of the cationic ring-opening polymerization in dispersion, until now only one system has been investigated. In 1968, Penczek et al published results of the studies of the cationic copolymerization of 1,3-dioxolane and 1,3,5-trioxane initiated with BF3 and carried out in cyclohexane in the presence or the absence of poly(ethylene oxide). Hie initial concentration of 1,3-dioxolane in these studies was 20 times lower than the initial concentration of 1,3,5-trioxane. The former monomer was used with the purpose of protecting poly (1,3,5-trioxane) from depolymerization. It was found that depolymerization stops when 1,3-dioxolane monomeric unit is the terminal one. 

Characterization of Crystalline Poly(trioxane) and Poly(tetraoxane) Obtained through Plasma-Initiated Polymerization... 

Poly( trioxane) consbts of two kinds of crystab, i.e., main- and sub-crystab, irre ctive to the polymerization temperature. 

The fibrillar crystallites of both samples, poly (trioxane) and poly (etroxocane), reveal the mean crystallite lengths of500 A and 300 A, respectively. 

Fig. 2.8. Wide-angle X-ray scattering profiles of the 009 and 0018 reflections of poly(trioxane) as-polymerized and Delrin-500 as-drown...

Table 2.3. Relative difference in lattice constants (Ac/c) inPTEOX-62,-8I,and-105fromc = 17.29 Adeteimined from 009 and 17.32 A from 0018 of poly(trioxane)...

Fig. 4.13a- . Small-angle X-ray scattering patterns of the melt-OTstallized poly(oxymethylene), the acetylated poly(trioxane) a original (melt-crystallized) and annealed, b at 156 °C fw 30 min c 165 °C, 20min d 168 °C, 20min e sample d annealed at 172 °C for 10min. Intensities are not comparable...

Table 4.1. Lattice constants c (A) of poly(tetroxocane) obtained at 80 °C (PTeOX-80) and at 105 °C (PTgOX-lOS), poly(trioxane) (PTOX), and Delrin-500...

Fig. 5.1a and b. Heating curves of poly(trioxane) irradiated with various doses a 6% polymer yield b 80% polymer yield heating rate, 160 °C/min. The exposure dose is indicated in the figure... 

Fig. 5.3. Thermogravimetric curve and heating curve of poly(trioxane) (80% yield) transformed into powder by mixing mill Heating rate, 10 C/min in TG, and 16 °C/min in DSC heating curve...

Fig. 5.5. Relationship between the ratio (S/So,tg) of the area per w unit weight under the endothermic peak of the irradiated (S) to the unirradiated (S ,) and dose (D) O, poly(trioxanc) O, Delrin A, Takafest, acetylated poly(trioxane)... 

Fig. 5.4. Heating curves of the melt-crystallized poly(trioxane) Heating rate, 16 C/min. The values in the figure indicate tte exposure dose...

Table 5.1. Reductions of fractional amounte of main crystals, sub-crystals, amorphous contents, and randomly oriented crystals, and weight loss of poly(trioxane) irradiated in air at room temperature...

Fig. 5Jla-c. Electron micrographs a of the free surface of as-polymerized poly(trioxane) cry als b irradiated with O.S MGy c 1.0 MGy... 

Fig. 5.14. Wide-angle X-ray scattering profiles of 009 reflections of poly(tetroxocane), poly(trioxane), and Delrin...

Radiolysis or etdiii by irradiation may take place at a region which is strained or entropy restricted, quite similar as in the case of poly(trioxane) mentioned above. 

Radiolysis reveals the characteristic features of polymers, where chain scission by irradiation occurs at a selective point. Poly(trioxane) forms a layer-like void, suggesting periodical characteristics of about 1000 A along the c-axis of the polymer crystals. In the case of poly(tetroxocane) similar results are observed. The mean crystallite lengths of fibrillar crystals are obtained as about 500 A in poly(trioxane) and 300 A in poly(tetroxocane). 

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Poly oxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of trioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 175°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are Hsted in Table 1. See also reference 13. 

Commercial polymers of formaldehyde are also produced using cationic polymerization. The polymer is produced by ring opening of trioxane. Since the polyacetal, POM, is not thermally stable, the hydroxyl groups are esterified (capped) by acetic anhydride (structure 5.22). These polymers are also called poly(methylene oxides). The commercial polymer is a... 

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 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. 

Both end groups can be determined quantitatively. A second side reaction is the transacetalization. Here a poly(oxymethylene) cation attacks an oxygen of a poly(oxymethylene) chain with formation of an oxonium ion that decomposes. Through continued cleavage and recombination of poly(oxymethylene) chains one obtains polymers which are chemically and molecularly largely homogeneous. For the case of a trioxane/ethylene oxide copolymer the following reaction scheme can be formulated ... 

Several reviews of early work on topotactic polymerizations and isomeriza-tions are available, and the reader is referred to the summaries of Morawetz [88] and Gougoutas [8] for a more complete account. The earliest study of a topotactic reaction appears to have been the observation, in 1932, of the polymerization of trioxane to poly-oxy-methylene [89]. Similar polymerizations of tetraoxane [90] and of trithiane [91 ] have also been reported to show retention of crystallographic axes from the monomer lattice. Other examples are discussed below. The topo-tacticity of a reaction can be determined solely by x-ray crystallographic analysis at the reactant and product endpoints. Thus a simple classification of a reaction as topotactic tells very little about how the structure of the crystal lattice changed in the course of reaction. 

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)]. 

Crystalline poly(oxymethylenes) with peculiar morphologies were produced via y-initiated topochemical polymerization of trioxane. Depending on the crystal modification of the trioxane, porous poly(oxymethylene) crystals, showing cylind-... 

When BF3 is added to 1,3,5-trioxane, the rate at which formaldehyde is split off the poly(oxymethylene) chain seems to be much higher than the rate at which new chains are formed. At the start of polymerization when the formaldehyde concentration is low, drain growth is not possible hence, the induction period. 

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