Ring-Chain Equilibria

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Abstract
Ring-chain equilibration in polymer chemistry is a more widespread phenomenon than was once supposed. Theoretical treatments of the subject were developed in the 1950's [1–11], but the practical consequences have been recognized fully only in recent years. Much of the current interest in ring-chain interconversions has grown from attempts to synthesize new high polymers by the polymerization of cyclic compounds. An additional factor has been the realization that the high-temperature depolymerization reactions of some high polymers are a further manifestation of ring-chain equilibration [12]. Thus, the subject has become intimately associated with the practical problems of thermal stability in high polymers, and particularly with the high-temperature behavior of inorganic and heteroatom backbone polymers. Because some of the most important examples of ring-chain equilibrations are found among heteroatom systems, much of the following discussion will be concerned with these derivatives. Even a brief examination of the literature in this field raises a number of important questions. For example, why do certain cyclic compounds polymerize to linear high polymers whereas others do not? What factors influence the depolymerization of high polymers to small ring species? What are the mechanisms which are involved? Do monomeric compounds participate in ring-chain equilibria? Which molecular structural features shift the equilibria to favor rings or chains? In order to attempt to answer these questions, let us first consider some examples of systems which undergo ring-chain equilibration and some which do not. We can then formulate a number of general observations.

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