Ethylene copolymerization with 1,3,5-trioxane

The less nucleophilic acetals copolymerize with vinyl compounds more readily. Perhaps, in these systems the alkoxycarbenium ions (... —OCH ) that coexist in equilibrium with oxonium ions facilitate copolymerization with vinyl compounds. Styrene copolymerizes with trioxane 51,52) and tetraoxane53). The latter system yields polytrioxane and trioxane-styrene copolymer together with 1,4-phenyl-1,3-dioxane. It was formed in 25% yield in ethylene dichloride at 30 °C after 1 hr using [BF3 OEtj] = 10-2 mol l-1, [styrene], = [tetraoxane = 0.5 mol l-1. The proposed mechanism of 4-phenyl- 1,3-dioxane formation is shown below (cf. also Chap. 7) ... 

Polyoxymethylene, also referred to as acetal resin or POM, is obtained either by anionic polymerization of formaldehyde or cationic ring-opening copolymerization of trioxane with a small amount of a cyclic ether or acetal (e.g., ethylene oxide or 1,3-dioxolane) [Cherdron et al., 1988 Dolce and Grates, 1985 Yamasaki et al., 2001]. The properties and uses of POM have been discussed in Sec. 5-6d. 

Polyformaldehyde. Polyformaldehyde or polyacetal is made by two different processes. Delrin is made from formaldehyde by anionic polymerization catalyzed by a tertiary amine. The homopolymer is end-capped with acetic anhydride. Celcon is made from trioxane cationic copolymerization using boron trifluoride catalyst and ethylene oxide (2-3%) as the comonomer. Boron trifluoride is a Lewis acid that associates with trioxane and opens up the six-membered ring. Ethylene oxide provides the end capping. Without an end cap, polyformaldehyde is thermally unstable and loses formaldehyde units. 

Figure 2. Copolymerization of trioxane with 1.5 mole % ethylene oxide in bulk at 65°C. BFS Bu2Q 1.0 X 10 s mole %...

Copolymerization with Ethylene Oxide. Figures 2-6 show the NMR spectra of copolymerization of trioxane and ethylene oxide where the... 

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 process based on cationic polymerization of 1,3,5-trioxane employs a different principle for stabilization of polymer. Trioxane is copolymerized with a few percent of 1,3-dioxolane (or ethylene oxide). The sequence of —OCH2— units is then separated from time to time by —OCH2CH2— units. The product of copolymerization is subsequently heated to eliminate the terminal units (unstable fraction). Depropagation proceeds until the stable —CH2CH2OH group is reached ... 

In contrast to acetal homopolymer, acetal copolymers have built-in heat stabilization. They are prepared by copolymerization of trioxane with small amounts of comonomer, usually cyclic ethers like ethylene oxide or 1,3-diozolane. 

These stabilizers are not just transfer agents, however, but also copolymerization components. In the copolymerization of trioxane with, for example, ethylene oxide, the ethylene oxide is first incorporated into the copolymer quantitatively at low yields. Later, because of the simultaneously occurring transacetalization, a random distribution of the ethylene oxide residues in the copolymer results. 

Commercial polyacetal copolymers contain 0.1 to 15 mole percent of a cyclic ether, commonly ethylene oxide or 1,3-dioxolane. Typical catalysts for this reaction are BF, or its ether complexes. In 1964, Weissermel and coworkers[5] showed that in the copolymerization of trioxane with ethylene oxide, the latter was almost completely consumed before any visible polymer was observed. During this stage of the polymerization, soluble prepolymers of ethylene oxide could be isolated [6], These prepolymers consisted primarily of oligomers with mono-, di-, and tri-ethylene oxide units. Celanese workers in 1980[7] verified also the presence of cyclic ethers, predominately 1,3-dioxolane and 1,3,5-tri-oxepane, as part of the reaction mixture. These are likely formed as reaction products of ethylene oxide and monomeric formaldehyde generated from the opening of the trioxane ring. 

Acetal (polyformaldehyde or polyoxymethylene, POM) is obtained by anionic or cationic polymerization of HCHO, or cationic ring-opening of trioxane. The ceiling temperature of POM is 127 °C and therefore the unzipping that would occur at the processing temperature (T 180 °C) has to be inhibited by end-capping with acetic anhydride or copolymerizing with 5-10% ethylene oxide (section 1.7.1). 

In particular the polymerization of 1.2.- and 1.3-epoxides (l)-(5) (18) (19) tetrahydrofurane (1)-(4) (6) (2o) Hioxolane (2177(22) and trioxane (11) (23)-X26) was thoroughly TnvestTgated. For reviews see (7T (8) (27) (28) (3o). It should be emphasized, that different oxacyclic monomers can also be copolymerized by cationic catalysts. Of great practical importance is e.g. the copolymerization of trioxane with ethylene oxide or dioxolane (31). Macromolecules with a statistic distribution of oxymethylene- and oxy-ethylene-units are formed in this way. On the other hand, however, the homopolymerization of dioxolane yields a polymer consisting of strictly alternating oxymethylene- and oxyethylene units (21) (32) therefore it can formally be considered as an alternating copolymer (eq.i). 

In contrast, transfer rates for trioxane to polymer are extremely fast. This can be seen from the observation that during copolymerization of trioxane with 1,3-dioxolane, ethylene oxide, and similar comonomers, the comonomer is nearly completely consumed at an early stage of reaction [229, 234-236]. One would therefore expect longer sequences of these comonomers. However, it has been shown, from... 

The cationic copolymerization of trioxane with ethylene oxide, 1,3-dioxolane, and suchlike is initiated either with strong protonic acids or Lewis acids, for example BF3. Molecular weight is controlled by the catalyst concentration and monomer purity, and also by chain transfer agents such as methylal [248], which may lead to more stable end groups. Most processes are run below the melting temperature of the polymer (164-167°C) in precipitating agents or in bulk, and are carried out in kneaders or double-screw reactors [249, 250], but there are also some descriptions of melt processes [251]. 

It has to be remembered, however, that this process may be complemented by the reversibility of the propagation step. In the copolymerization of 1,3,5-trioxane with 1,3-dioxolane (or ethylene oxide), the complex set of equilibria is established, involving comonomers but also formalde-... 

Polymerization of 1,3,5-trioxane (TXN) gives linear polyoxymethylene (POM), a homopolymer of formaldehyde 39). This is the only polyacetal made on the technical scale. Two methods are used for the industrial production of stable, high-molecular-weight POMs. This is either the anionic polymerization of formaldehyde or the cationic copolymerization of the cyclic trimer of formaldehyde TXN with ethylene oxide or 1,3-dioxolane (DXL) ... 

Polyacetal can also be stabilized against degradative conditions by copolymerizing trioxane with small amounts of ethylene oxide. This introduces a random distribution of -C-C- bonds in the polymer chain. Hydrolysis of the copolymer with aqueous alkali gives a product with stable hydroxyethyl end groups. The presence of these stable end groups coupled with the randomly distributed C-C bonds prevents polymer depolymerization at high temperature. 

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. 

Figure 3 Influence of the average diameter of particles on concentration of poly(ethylene oxide) in the copolymerizing mixture of 1,3,5-trioxane ([1,3,5-trioxane] = 7.0 mol kg" ) and 1,3-dioxolane ([1,3-dioxolane] = 0.35 mol kg" ). Concentration of BF3OBU2 initiator was equal to 1.75 x 10 mol kg " (o), 3.50 X 10 mol kg"" ( ), and 4.50 x 10" mol kg"" (A). Reproduced with permission from Penczek, S. Fejgin, J. Sadowska, A. Tomaszewicz, M. Makromol. Chem. 1968, 116, 203." ...

The most important polyacetal is obtained from the polymerization of either formaldehyde or trioxane, its cyclic trimer. This polyacetal is called poly(oxy-methylene), and its acronym is POM. Trioxane can possibly be copolymerized by cationic process with other heterocycles, particularly ethylene oxide, leading to polymer chains having structural regularity less than that of the conventional POM and yielding less crystalline and less cohesive materials. 

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