A THEORETICAL STUDY OF RING CLOSURE: APPLICATION TO THE CYCLIC–LINEAR DISTRIBUTION IN POLYDIMETHYLSILOXANE

Abstract

The purpose of this work was to provide insight into the formation of cyclics from fundamental considerations of chain geometry. The structural variables which must be specified are the length(s) of the valence bond in the skeletal structure, bond angle(s) in the skeletal structure, and the energetics and positions of the allowed conformations in the system. A mathematical model involving only these structural variables has been developed which is valid for chains with any number of bonds.This model is applied to the polydimethylsiloxane system in which cyclic molecules containing from six to eighteen atoms in their skeletal structures make up approximately 12 to 20% by weight of the equilibrated system. A three-rotational-state model is assumed for the polydimethylsiloxane chain. The relationship between cyclic formation and other features of polymer behavior and structure will be interpreted in terms of the model. A good correlation is shown between the experimental equilibrium constant for octamethylcylcotetrasiloxane and the theoretical equilibrium constant from the model calculation.