Polymerisation of formaldehyde

In the presence of lime water more complex reactions occur, leading to the formation of aldoses and hexoses (iv). This particular reaction is of interest to the biochemist as it is now generally held that optically active plant carbohydrates are obtained from carbon dioxide and water via formaldehyde. 

In the early 1940s an intensive research programme on the polymerisation of formaldehyde was initiated by the Du Pont Company. As a consequence of this work polymers, both tough and adequately stable to processing conditions, were prepared and eventually marketed (Delrin). 

In order to manufacture such polymers, it is first necessary to produce a very pure form of formaldehyde. This is typieally produced from an alkali-precipitated low molecular weight polyformaldehyde which has been carefuly washed with distilled water and dried for several hours under vacuum at about 80°C. The dried polymer is then pyrolysed by heating at 150-160°C, and the resultant formaldehyde passed through a number of cold traps (typically four) at -15°C. Some prepolymerisation occurs in these traps and removes undesirable 

Staudinger also found that diacetates of polyoxymethylenes with a degree of polymerisation of about 50 were less stable. Truly high molecular weight polyoxymethylenes (degree of polymerisation -1000) were not esterified by Staudinger this was effected by the Du Pont research team and was found to improve the thermal stability of the polymer substantially. 

The esterification reaction may be carried out with a number of different anhydrides but the literature indicates that acetic anhydride is preferred. The reaction is catalysed by amines and the soluble salts of the alkali metals. The presence of free acid has an adverse effect on the esterification reaction, the presence of hydrogen ions causing depolymerisation by an unzipping mechanism. Reaction temperatures may be in the range of 130-200°C. Sodium acetate is a particularly effective catalyst. Esterification at 139°C, the boiling point of acetic anhydride, in the presence of 0.01% sodium acetate (based on the anhydride) is substantially complete within 5 minutes. In the absence of such a catalyst the percentage esterification is of the order of only 35% after 15 minutes. 

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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. 

In the above examples the polymerisation takes place by the opening of a carbon-carbon double bond. It is also possible to open carbonyl carbon-oxygen double bonds and nitrile carbon-nitrogen triple bonds. An example of the former is the polymerisation of formaldehyde to give polyformaldehyde (also known as polyoxymethylene and polyacetal) (Figure 2.3). 

In absence of diluent or other effective control of reaction rate, the sulfoxide reacts violently or explosively with the following acetyl chloride, benzenesul-fonyl chloride, cyanuric chloride, phosphorus trichloride, phosphoryl chloride, tetrachlorosilane, sulfur dichloride, disulfur dichloride, sulfuryl chloride or thionyl chloride [1], These violent reactions are explained in terms of exothermic polymerisation of formaldehyde produced under a variety of conditions by interaction of the sulfoxide with reactive halides, acidic or basic reagents [2], Oxalyl chloride reacts explosively with DMSO at ambient temperature, but controllably in dichloromethane at -60°C [3]. 

Two violent pressure-explosions occurred during preparations of dimethylsulfinyl anion on 3—4 g mol scale by reaction of sodium hydride with excess solvent. In each case, the explosion occurred soon after separation of a solid. The first reaction involved addition of 4.5 g mol of hydride to 18.4 g mol of sulfoxide, heated to 70°C [1], and the second 3.27 and 19.5 g mol respectively, heated to 50°C [2]. A smaller scale reaction at the original lower hydride concentration [3], did not explode, but methylation was incomplete. Explosions and fire occurred when the reaction mixture was overheated (above 70°C) [4]. Reaction of 1 g mol of hydride with 0.5 1 of sulfoxide at 80°C led to an exotherm to 90°C with explosive decomposition [5]. These and similar incidents are explicable in terms of exothermic polymerisation of formaldehyde produced from sulfoxide by reaction with the hydride base [6]. The heat of reaction was calculated and determined experimentally. Thermal decomposition of the solution of hydride is not very violent, but begins at low temperatures, with gas evolution [7]. 

The simplest type of polyether, polyoxymethylene, is obtained by the similar polymerisation of formaldehyde in the presence of water ... 

Phenolic resins are prepared by a step-growth polymerisation of formaldehyde and phenol or phenol derivative using an acid or a base catalyst. The product type and the quality largely depend on the ratio of the reactants used and the nature of the catalyst. Phenolic resins are available in two varieties 1) novolac, which is a thermoplastic type and can be used as it is or can be cured with hexamethylene tetramine (HMTA) to get a crosslinked structure. This can also be viewed as a reactive intermediate, and can be transformed into other groups so different types of structures can be generated and 2) resole, which is a multifunctional reactive compound and can be cured thermally without a catalyst or an acid catalyst. 

Linear, hard, tough synthetic resins produced by the polymerisation of formaldehyde (for acetal homopolymers) or of formaldehyde with trioxane (for acetal copolymers). Acetal resins are also called as polyacetals and are used as substitutes for metals. 

Acetal thermoplastics material produced by polymerisation of formaldehyde and possessing high softening point and numerous good physical properties resulting in its use for bearing, gears, bushes, etc. 

Acetal copolymers thermoplastic materials produced by polymerisation of formaldehyde with other monomers, as opposed to polyacetal (qv). 

Polyacetal thermoplastic material produced by polymerisation of formaldehyde alone (see acetal). 

There are two different methods for producing polyacetals. Anionic polymerisation of formaldehyde produces homopolymers that crystallize particularly well and therefore have high stiffness and strength. The other method is cationic polymerisation of trioxane. Here the addition of small amounts of comonomers lowers the crystallinity to increase toughness. The stiffness and strength are however somewhat lower than for homopolymers. 

Obtd. by polymerisation of formaldehyde soln. with H2SO4. Mp 170-172°. Polyoxymethylene is also an old name for Paraformaldehyde. 

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