PARAFORMALDEHYDE

Chemical properties

The homopolymers and copolymers of formaldehyde, prepared as described above, are rigid materials with broadly similar properties. They are particularly noted for their stiffness, fatigue resistance and creep resistance and are counted as one of the 'engineering plastics'. They find application principally in injection moulded mechanical parts such as gears, cams and plumbing components. The copolymers are somewhat less crystalline and therefore have lower density, melting point, hardness, tensile strength and flexural modulus. The main advantage claimed for the copolymers is improved processability, with less degradation at processing temperatures.
As is characteristic of crystalline polymers which do not interact with any liquids, there are no effective solvents at room temperature for the commercial formaldehyde polymers. At temperatures above 70°C, solution occurs in a few solvents such as the chlorophenols. The resistance of the polymers to inorganic reagents is not, however, outstanding. Strong acids, strong alkalis and oxidizing agents cause a deterioration in mechanical properties. (The copolymers are significantly superior to the homopolymers in alkali resistance.)
Oxidation of polyformaldehyde occurs in air on prolonged exposure to ultraviolet light and/or elevated temperature. Antioxidants are therefore commonly added to the polymers.

Preparation

(a) Homopolymers In the preparation of high molecular weight polyformaldehyde the initial operation consists of the production of pure formaldehyde, free from low molecular weight polymers and other hydroxy compounds which cause chain transfer. In a typical process potassium hydroxide-precipitated paraformaldehyde (degree of polymerization approximately 200) is carefully washed with water and dried for several hours in vacuo at 80??C. The dried polymer is then decomposed in nitrogen at 150-160??C; the product is passed through several traps at -15??C to remove water, glycols, and other impurities. The resulting formaldehyde has a water content (free and combined) of less than 0.1 %.
The formaldehyde is then introduced into a reactor where it passes over the surface of a rapidly stirred solution of initiator (either a Lewis acid or base; triphenylphosphine appears to be favoured) in a carefully dried inert medium such as heptane at about 40??C. The process is designed to give a very low concentration of formaldehyde to minimize transfer from polymer to monomer. To the initiator solution may be added a polymer stabilizer (e.g. diphenylamine) and transfer agents (e.g. traces of water or methanol). Polymerization is continued until the concentration of polymer in the slurry is about 20% and then the polymer is collected by filtration.
In the final stage the polymer is subjected to an esterification reaction to improve its thermal stability. The esterification may be effected with a number of anhydrides, but acetic anhydride is generally preferred. Typically, the polyformaldehyde is heated under slight pressure to about 160??C with acetic anhydride and a small amount of sodium acetate (catalyst). The polymer is soluble in acetic anhydride at this temperature but is precipitated when the solution is cooled. The acetylated polymer is collected by filtration, washed with water (to remove the anhydride and catalyst) and then acetone (containing di-fi-naphthyl-p-phenylenediamine as antioxidant), and dried in vacuo at 70??C. The product is then extruded and chopped into granules. The average molecular weight (Mn) of the polyformaldehyde produced by these methods is generally in the range 30000-100000.
The polymerization of formaldehyde by Lewis bases such as triaryl amines (R3N), arsines, and phosphines proceeds by the following anionic mechanism:

The polymerization of formaldehyde by Lewis acids such as boron trifluoride proceeds according to following cationic mechanism:

The hydroxy-terminated polymers have poor thermal stability. Loss of a proton, possibly to an initiator residue, from a chain end gives an anion capable of decomposing to formaldehyde by the reverse of the propagation process. The stability of the polymer is therefore improved if the hydroxy endgroups are removed by esterification:

It may be noted here that the polymerization of formaldehyde cannot be effected with free radical initiators.
(b) Copolymers
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 lOoDe 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). 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.