Copolymerization of trioxane with 1,3-dioxolane

Some peculiarities of this system have already been described in connection with transacetalization reaction. As indicated by Jaacks 53), TXN-DXL copolymerization cannot be analysed by using the conventional Mayo plot because all propagation reactions are reversible and propagation is accompanied by chain transfer to polymer (transacetalization). Jaacks developed a method which, although based on some simplifying assumptions, gives the reactivity ratio r4. The final equation 

1 Analysis of the Sequence Distribution in 1,3,5-Trioxane-1,3-Dioxolane Copolymers 

Thermal stability of the TXN-DXL copolymers depends critically on the distribution of the DXL units (cf. Sect. 7.3.5.3.1). Thus, methods for the determination of copolymer microstructure based on NMR were developed, mostly by Schulz 134), and 

Monomer Repeating unit of polymer Chemical shifts ppm 5 Conditions Ref. 

Studying the NMR spectra of polymers and copolymers of TXN, DXL and 1,3,5-trioxocane, modeling the various distributions of oxymethylene (M) and oxyethylene (E) units in the resulting polymers, these authors were able to determine the concentrations of MMM, MME and EME triads. The respective chemical shifts are given in Table 7.14. Thus, the chemical shifts of M in triads is not constant, and depends on the neighbouring units. The difference, however, between MMM and EMM triads is only 0.05 ppm 6 for the same TXN-DXL copolymer, which is insufficient to distinguish between these triads with a lower resolution apparatus. 

One of the most complex copolymerization systems in the field of cyclic ethers is that of the cationic copolymerization of trioxane with 

Yamashita et al. [157] have derived a copolymer composition equation that includes the depropj ation reaction such as might be expected in the cationic copolymerization of BCMO and THF. They consider two models. For the first one it is assumed that monomer M2 adds reversibly to both active chain ends mf and m and that depropagation by detachment of an M1 unit is neglected. The elementary reactions are then 

The second model was similar, except that both Mi and M2 monomer add to the active chain end (mf reversibly) and depropagation of the active end m is neglected. That is, elementary reactions (1) and (2) are considered to be irreversible and elementary reactions (3) and (4) are considered to be reversible. Two additional parameters are defined 

The symbols m2 and M2 always refer to the THF, polymer and monomer, respectively. Positive subscript fe s refer to the forward reactions and negative subscript fe s to reverse reactions as usual. The scheme is based on several facts and assumptions  

Some elements of the scheme were previously presented by Yamashita et al. [157]. The general kinetic solution of the scheme is given by the equation 

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Cationic Copolymerization of 1,3>5-Trioxane with 1,3-Dioxolane (Ring-Opening Copolymerization)... 

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-... 

Table 7.6 Copolymerization of 1,3,5-trioxane (7mol-l ) with 1,3-dioxolane (0.35 mol-1 ) m cyclohexane at 60 °C 501...

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 ... 

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) ... 

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." ...

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. 

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. 

Side Reactions 2 and 3 may have similar effects (9). Tetroxane was found to be produced by a fast backbiting reaction during homo- and copolymerizations of trioxane (9). At 30°C. tetroxane reaches an equilibrium concentration of 0.1M. Furthermore, in the copolymerization of trioxane with dioxolane chain ends of the type P2 + cleave off 1,3,5-trioxe-pane (1,3,5-trioxacycloheptane) (18) to yield Pi+ (Reaction 2). Transformation of P2 + into Pi+ without monomer participation can also occur by transfer by polymer (transacetalization as in Reaction 3) ... 

Several papers57"59 were devoted to investigating a complex process such as the cationic copolymerization of monomeric formaldehyde with dioxolane in the gas, liquid, and gas-liquid phases. It is known that polyacetal resins are industrially produced by copolymerizing cyclic acetals (trioxane, 1,3,5,7-tetraoxane), or by anionic homopolymerization of monomeric formaldehyde with subsequent modification of end groups. 

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. 

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). 

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