Polymers, formaldehyde Thermodynamic

Polymers of formaldehyde were found recently in interstellar space by N. Wickramasinghe [Nature, 252, 462 (1974)]. It is well known that polyformaldehyde is thermodynamically unstable already at not very high temperatures (close to room temperature), but it should be stable versus depolymerization near absolute zero. Therefore the formation of poly-oxymethylene near absolute zero is not a thermodynamic but a kinetic problem. 

Important in combustion is not so much the thermal stability of the material itself but rather the amount and nature of the decomposition products. It is sufficient to compare the LOI of poly(vinyl chloride), whose thermal decomposition begins at 160-175 °C with that of heat resistant phenol-formaldehyde fibers (Kynol). The thermodynamic approach to the problem seems to be most reasonable. It allows to consider the polymer structure to explain the details of the combustion reactions and to estimate the heat of combustion of polymers. 

Polyoxymethylene (POM) can be synthesized cationically not only from TXN but also from formaldehyde (FA) and 1,3,5,7-tetraoxocane (TTXN). In fact, when TXN polymerizes all these products coexist in multiple equilibria. The polymer is crystalline and precipitates out from the reaction mixture. Thus, the concentration of the components of the system depends on the thermodynamics of the equilibria ... 

The general thermodynamics of polymerization of cyclic acetals and the influence of substitution are discussed in Chapt. 2 of this volume (Thermodynamics). It may suffice to state here, that the monomers used to date for polymer synthesis are mostly derivatives of 1,2-glycols or 1,4-glycols and formaldehyde (i.e. 5- and 7-membered formals). 6-membered formals (1,3-dioxane and its derivatives) are nonpolymeri-zable due to the thermodynamic restrictions. 

The thermodynamics of TXN polymerization also influences the polymerization kinetics. Kern 92) has proposed that induction periods, frequently observed in the cationic polymerization of TXN, are due to the build up of the equilibrium formaldehyde concentration, which has to occur before polymer can be formed. According to other authors, it is not formaldehyde but TTXN that must be formed prior to polymerization 93). We conclude that formation of polymer may require that the equilibrium concentration of any monomeric species which is in equilibrium with polymer is reached first. 

Polymer formed initially from TXN, is thermodynamically unstable with respect to formaldehyde and TTXN because their concentrations (zero at the beginning of the reaction) are below [M]e. If the rates of reactions (2) and (3) are comparable with that of reaction (1), then, polymer formed from TXN will depropagate to formaldehyde and TTXN and this process will continue until the equilibrium concentrations of formaldehyde and TTXN are reached. Consequently, at this stage TXN will be consumed, however, polymer will not be formed. 

In addition, models for special substances like carboxylic acids, hydrogen fluoride, formaldehyde, electrolytes, and polymers are introduced and the capabilities of high-precision equations of state and various predictive methods are explained. Recommendations for the parameter fitting procedure and numerous hints to avoid pitfalls during process simulation are given. Because of the space limitation in the book we were not able to cover the whole range of thermodynamics, for example, adsorption has been left out completely as it cannot be presented within a short chapter. 

In discussing formaldehyde chemistry, we shall first gi e attention to ] the methods by which it is normally produced. Following this, we shall deal with the physical and thermodynamic properties of the various formaldehyde substances the simple monomer (Chapter II), formaldehyde solutions (Chapters III-VI), and polymers (Chapter VII). Chapters TII-X are de oted to the chemical properties of formaldehyde and its reactions with arious types of inorganic and organic chemicals. Chapters XM and X TI deal mth formaldehyde detection and analysis. 

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