Effects of Changes in Solubility
Changes in solubility can occur during the courses of various reactions. Such changes are observed, for instance, during the chlorination of polyethylene in aromatic and chlorinated solvents [29]. There is an increase in the solubility until 30% conversion is reached. After that, solubility decreases and reaches a minimumat 50–60% chlorine content. Following that it increases again. This, however, is not typical of many reactions of polymers in solutions. More common is that the starting material is soluble, but not the product or the opposite is true. Higher conversions are, usually, expected when the polymers are solvated and the chains are fully extended. In such situations, the reagents have ready access to the reactive sites [29]. If the products are insoluble in the reaction medium and tend to precipitate as the reaction progresses, the sites become increasingly less accessible. This can result in low conversions and premature terminations. If the opposite is true and the product is more soluble than the starting material, homogeneous limited reactions can be controlled. When the starting material is incompatible with the product, mutual precipitation or coiling of the chains can take place. This can also result in limited reactions. In addition, only minor differences in the constitutions of two polymers can cause incompatibility. For instance, among methacrylate polymers, there are incompatibilities in benzene solutions that result from differences only in the amount of branching of the alkyl groups [29]. Problems with solvent incompatibility can sometimes be overcome by using mixtures of solvents. Those that are good for the starting materials can be combined with those that are good for the products. With careful experimentation, it may be possible to develop a mixture of solvents that will keep all components in solution [30]. In some instances, however, insolubility of the products might be an advantage. This is the case in alcoholysis reactions of poly(vinyl acetate), where the polymer precipitates during the reactions and in doing so absorbs the catalyst with it. The phenomenon permits complete alcoholysis, particularly with the higher molecular weight species that precipitate first. Secondary reactions, like cross-linking and gelation, can result in precipitations from solution. The extent of the reactions, however, is not necessarily limited, because diffusions of low molecular weight species are still possible. Isolation of useful products, however, often becomes very difficult.
