Boron trifluoride trioxane

Starting ingredients may be formaldehyde or the cyclic trimer rrioxane, CH2OCH2OCH2O. Both form polymers of similar properties. Boron trifluoride of other Lewis acids are used to promote polymerization where trioxane is the raw material. 

The copolymerization of trioxane with cyclic ethers or formals is accomplished with cationic initiators such as boron trifluoride dibutyl etherate. Polymerization by ring opening of the six-membered ring to form high molecular weight polymer does not commence immediately upon mixing monomer and initiator. Usually, an induction period is observed during which an equilibrium concentration of formaldehyde is produced. 

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. 

Materials. Trioxane (Celanese) was purified by refluxing over metallic sodium followed by distillation (b.p., 114.3°C.). Ampoules of ethylene oxide (Eastman white label) were opened immediately before use. The purity was established by mass spectrometry. Boron trifluoride dibutyl etherate (Eastman white label) was used without further purification. 

Kern and Jaacks51 investigated the cationic polymerization of 1,3,5-trioxane by boron trifluoride in dichloromethane at 30 °C. Unlike cationic vinyl polymerizations initiated by metal halides, traces of water rather than increasing the polymerization rate, slightly reduced it. Boron trifluoride induced ionisation of water is thereby ruled out as a step in the initiation mechanism. 

Similar careful control of nucleation conditions is necessary for the polymerization of trioxane to high crystallinity and high molecular weight polyoxymethylene from solution (78). This reaction is cationic and may be started by a large number of initiators (79). For the polymerization step of the reaction in solution, catalyzed by the etherate of boron trifluoride, the following mechanism has been proposed (80) ... 

Trioxane, 1,3-dioxepane, and boron trifluoride diethyl etherate were obtained from Aldrich Chemical Co. TOX was refluxed continuously in the presence of sodium for at least 48 hours to remove trace amounts of water in a Fuchs style reflux apparatus under dry argon at 120 °C. Dioxepane was purified using a similar procedure. Initiator BF3-OEt2 was added with dried 1,4-dioxane as carrier. 

The trimer of formaldehyde, 1,3,5-trioxane, has been converted to a polyoxymethylene under anhydrous conditions in ethylene dichloride solution using boron trifluoride etherate or stannic chloride as the initiator. The product needs to be stabilized to prevent thermal degradation back to formaldehyde. The capping of this polymer was carried out in dimethyl-formamide solution with an excess of acetic anhydride and a small amount of a high-boiling tertiary amine such as A, A-dimethylcyclohexylamine or... 

Details of the procedures used in the preparation of commercial formaldehyde copolymers have not been fully disclosed. The principal monomer is trioxan and the second monomer is a cyclic ether such as ethylene oxide, 1,3-dioxolane or an oxetane ethylene oxide appears to be the preferred comonomer and is used at a level of about 2%. Boron trifluoride (or its etherate) is apparently the most satisfactory initiator, although many cationic initiators are effective anionic and free radical initiators are not effective. The reaction is carried out in bulk. The rapid solidification of the polymer requires a reactor fitted with a powerful stirrer to reduce particle size and permit adequate temperature control. The copolymer is then heated at 100°C with aqueous ammonia in this step, chain-ends are depolymerized to the copolymer units to give a thermally-stable product. The polymer is filtered off and dried prior to stabilizer incorporation, extrusion and granulation. 

The mechanism of polymerization of trioxan has not been completely elucidated. A possible scheme, in which boron trifluoride-water is the initiator is as follows ... 

In the first step, trioxan is protonated by the complex protic acid formed by interaction of boron trifluoride and water. (It has been shown that no reaction occurs in the complete absence of water [3]). The resulting oxonium ion undergoes ring-opening to give a resonance-stabilized species. This then depolymerizes to build up an equilibrium concentration of formaldehyde, which remains constant during the polymerization. The actual propagation step then involves the addition of formaldehyde rather than trioxan. This scheme accounts for the observation that the polymerization of pure trioxan involves an induction period which may be reduced or even eliminated by the addition of formaldehyde. 

---