PARAFORMALDEHYDE

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 80C. The dried polymer is then decomposed in nitrogen at 150-160C; the product is passed through several traps at -15C 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 40C. 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 160C 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 70C. 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