Linear siloxane polymers equilibrium reaction

The polymerization reaction leads to an equilibrium mixture of linear polysiloxanes and cyclic siloxanes, wheras the equilibrium of the polycondensation reaction can be shifted to the polymer side by removing the condensation water. 

Various reactions, both of polymerization and of polymer degradation, can produce cyclic polymer molecules. A well-known process is the ring-chain equilibration reaction, which may be used to produce cyclic siloxanes and o er cyclic polymers. The linear chain reacts intramolecularly and yields a cyclic and a linear chain. In the initial stages, the molar fraction of cyclics increases at the expense of the linear chains. After some time, equilibrium conditions are achieved and the molar fraction of cyclics remains constant. In some cases, all the sites in the macromolecular backbone are equivalent and no peculiar bond exists which is preferentially attacked. This case is referred to as thermodynamically controlled cyclization. 

This state is, by definition, independent of the starting siloxane substrates and of the initiator used (either anionic or cationic). The polymer yield and its characteristics are not related to the polymerization kinetics. Instead, the knowledge of the thermodynamics of the process is essential. The final state of the reaction involves complex equilibria between the polymeric species of two homologous series, cydic and linear polysilox-anes. The equilibrium state may be described by general eqn [5]. 

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