Polymerization

POLYMERIZATION, the process of forming polymers (see Polymeria) as a transition from a less stable form of a substance to a more stable one, is an exothermic process. The heat released during P. is usually significant; for example, when three molecules of acetaldehyde polymerize into paraldehyde, 24.1 cal are released. The process of P. is reversible, and with an increase in temperature, the equilibrium shifts toward the formation of the monomer, since according to the isochore equation for exothermic compounds, an increase in temperature promotes their decomposition. If polymers are so strong that it is not possible to break them down without deep destruction upon heating, then to avoid this and conduct the reaction at a lower temperature, catalysts are used for the depolymerization reaction. - During the process of P., changes in the properties of the substance occur. Changes in physical properties: melting points increase, solubility decreases (for example, formaldehyde is easily soluble in water, while paraformaldehyde is almost insoluble in water), specific gravity increases, the refractive index increases, light absorption changes (polymers, in contrast to monomers, are colorless), and viscosity increases significantly (for example, acrolein, a mobile liquid, turns into viscous diallyl); for high-molecular-weight polymers, a colloidal state is characteristic. All these changes in physical properties allow for the quantitative measurement of P. Depending on whether a given P. process approaches condensation or association processes (see Polymeria), the chemical properties of the substance change to a greater or lesser extent. An example for the first case can be the polymerization of acetylene in benzene, in which the chemical character of the substance changes sharply; another example is the polymerization of aldehydes into paraldehydes, which no longer give the typical reactions of aldehydes. As an example for the second case, one can point to the polymers of formaldehyde, which behave differently than the products of polymerization of other aldehydes. Specifically, in paraformaldehyde and polyoxymethylene, individual molecules of formaldehyde are linked so weakly that depolymerization occurs not only upon heating but even upon dissolution; this is a case of labile polymerization, close to association. Similar behavior is exhibited by 1,2-glycosides and alpha- and beta-oxialdehydes and oxiketones. From the above, it is evident that among unsaturated compounds there exist representatives of the most diverse degrees of instability; often a small change in the structure of the molecule is sufficient to strongly increase the ability to polymerize or change the course of polymerization. For each class of unsaturated compounds, there are their own types of polymerization, and within the same type, the behavior of individual representatives is very individual. The greatest tendency to polymerize is shown by the initial members of unsaturated classes. Asymmetrically constructed compounds polymerize more easily than symmetric ones. Some substances in the monomeric state are extremely unstable. On the other hand, there are substances that are capable of polymerization but can be preserved as monomers for a long time, and external influence is required to accelerate their transition to a more stable state through polymerization (heating, illumination, addition of a catalyst). An example of the influence of heating: the sharp acceleration of the conversion of butadienes into synthetic rubber occurs upon heating to 90–100°. Heating not only accelerates the polymerization process but also influences the course of the process: at a higher temperature, less high-molecular-weight polymers are obtained. - The influence of light on P. is even more significant than heat. Since a molecule must absorb light energy to react photochemically, it is far from indifferent what wavelength of light illuminates the substance; for example, due to the fact that the monomer of styrene absorbs only light of long wavelength, while polymers absorb only short wavelength, the longer the wavelength of light, the greater the yield of polystyrenes; therefore, polymerization is favored by sunlight illumination, but not ultraviolet. It is especially important that after the illumination is stopped, the polymerization reaction continues in the dark. Excited molecules of the monomer, having absorbed light, can enter into successive reactions for which the action of light is no longer required (chain reactions). According to the basic law of photochemistry, in a photochemical reaction each molecule absorbs 1 quantum of energy, i.e., hv (h is Planck's constant, v is the frequency of the incident light, equal to c/lambda, where c is the speed of light and lambda is the wavelength). If experience shows that a large number of reacted molecules correspond to each quantum, this indicates the presence of a chain reaction. However, the quantum yield in the case of polymerization has not yet been experimentally determined. - Catalysts can extremely increase the speed of polymerization in many cases; for example, polymerization of aldehydes is accelerated in the presence of HCl, ZnCl₂, etc.; olefins in the presence of H₂SO₄. Both the use of various heating or illumination conditions and the use of various catalysts can direct polymerization in the desired direction. An example: butadiene forms, upon heating with a small amount of ozonides or peroxides, ozonide-rubbers close to normal rubber; upon heating with metallic sodium, butadiene gives anomalous sodium-rubber, and upon heating with acids, butadiene gives dimeric terpene hydrocarbons. According to the opinion of Moureu and Dufraine, oxygen plays an extremely large role in the process of P. They advanced a theory according to which the so-called autooxidation, i.e., the addition of gaseous oxygen to a substance, occurs through the intermediate formation of unstable peroxide forms. These same peroxide forms act as catalysts in the process of P. The addition of substances that slow down oxidation suppresses P. The most studied case is the formation of diallyl from acrolein. Before the investigation by Moureu and Dufraine, the preparation of acrolein in quantities exceeding a few liters was impossible due to its polymerization into diallyl; the addition of 0.025% of phenols to acrolein stops the ability for autooxidation and polymerization, which allowed the preparation of acrolein in tons in France during the war. These chemicians extended their theory to other cases of polymerization, for example, to the aging of rubber.