Trioxane, formaldehyde polymer Preparation

An example of the importance of end-group control is in acetal resin or polyoxymethylene (POM). As normally prepared from cationic polymerization of sym-trioxane, this polymer has —OH end groups, which readily initiate depropagation to formaldehyde (2) by a molecular mechanism (9) ... 

Formaldehyde will polymerize in a number of ways and although commercially useful polymers have been prepared only relatively recently, several different types of formaldehyde polymers have been recognized for over a century. For example, distillation of aqueous solutions of formaldehyde containing 2% of sulphuric acid yields the trimer, trioxan ... 

Trioxane oi alpha-trioxymethylene, (CHi>0)3, the cyclic trimer of formaldehyde, is a stable chemical indiiridual possessing unique, well-defined properties. Because of preparational difficulties, it has in the past been studied by veiy few iQ, estigators. Its obscurity has also been further enhanced by the incoiTect use of the term ririoxymethylene as a designation for linear formaldehyde polymers. 

The details of the commercial preparation of acetal homo- and copolymers are discussed later. One aspect of the polymerisation so pervades the chemistry of the resulting polymers that familiarity with it is a prerequisite for understanding the chemistry of the polymers, the often subde differences between homo- and copolymers, and the difficulties which had to be overcome to make the polymers commercially useful. The ionic polymerisations of formaldehyde and trioxane are equiUbrium reactions. Unless suitable measures are taken, polymer will begin to revert to monomeric formaldehyde at processing temperatures by depolymerisation (called unsipping) which begins at chain ends. 

Paraformaldehyde [30525-89-4] is a mixture of polyoxymethylene glycols, H0(CH20) H, with n from 8 to as much as 100. It is commercially available as a powder (95%) and as flake (91%). The remainder is a mixture of water and methanol. Paraformaldehyde is an unstable polymer that easily regenerates formaldehyde in solution. Under alkaline conditions, the chains depolymerize from the ends, whereas in acid solution the chains are randomly cleaved (17). Paraformaldehyde is often used when the presence of a large amount of water should be avoided as in the preparation of alkylated amino resins for coatings. Formaldehyde may also exist in the form of the cycHc trimer trioxane [110-88-3]. This is a fairly stable compound that does not easily release formaldehyde, hence it is not used as a source of formaldehyde for making amino resins. 

These materials form yet another class of polymer which may be prepared by more than one polymerization method. For example polyoxymethylene (polyformaldehyde, polymethanal, acetal resin) may be prepared by double-bond polymerization from formaldehyde or by ring-opening of trioxane ... 

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. 

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