Cyclosiloxanes structure

Cyclosiloxanes, cationic ring-opening polymerization of 85MI16. Cyclosiloxanes, structures of 80UK518. 

Thermomechanical studies have indicated that the glass transition temperature of synthesized copo-lymers is decreased as the volume of cyclosiloxane ring in the chain is increased. The results of X-ray structural analysis indicate that the copolymers represent amorphous systems, and increase of cyclic fragment volume leads to an insignificant increase of the interchain distance. 

Base-Initiated Polymerization. Although the base-initiated mechanism for reaction 2 was once thought to proceed via free ionic intermediates 31), little evidence supported this mechanism. Conductivities of solutions of alkali metal silanolates in moderately polar solvents are essentially nil (32), and most workers now agree that reactive intermediates consist of ion pairs or charge-separated ion pairs 4-5). The key intermediate is believed to involve coordination of the countercation of the ion pair at the chain end, for example potassium, with the cyclosiloxane in a manner analogous to the crown ethers or cryptates (33), as shown by structure 1. 

The structure of the active propagation center in the cationic ring-opening polymerization (ROP) of cyclosiloxanes is still controversial. Trisilyloxonium ions generated from hexamethyl-cyclotrisiloxane, D3, and octamethylcyclotetrasiloxane, D4, were observed at low temperature by Olah et al. [I] and their participation as intermediates in cationic ROP of cyclosiloxanes was postulated. However, the main objection against this mechanistic concept is a relatively low rate of polymerization when an initiator able to form persistent tertiary oxonium ions is used [2]. 

The structural chemistry of organocyclosiloxanes have been a field of intensive study for the past 30 years. As a comprehensive review288 of this topic appeared in 1980 only a brief account will be presented in the present article with emphasis being placed on recent developments. For cyclosiloxanes (RR SiO) (n=2-4) the following trends may be ascertained ... 

Knowledge of the polymerization and copolymerization of cyclosiloxanes and the structure of the polymers so formed is fundamental to understanding the structure-property relationships of polysiloxanes. Such knowledge in the areas of polymer stability, reactivity and surface activity is of prime importance in the industrial application of polysiloxanes. 

Steric effects of this type are noted in the ring-chain equilibria of orthosilicates and organosili-conates derived from dihydroxyalkanes. Although not dealing specifically with cyclosiloxanes, the study did involve closely related O—Si—O structural elements and is hence pertinent to this subject. For details, consult C. L. Frye, J. Org. Chem., 34, 2496 (1969). 

Ring-ring interconversions can interconvert specific pairs of cyclosiloxanes selectively without causing unwanted wholesale structural proliferation e.g., conditions are defined that interconvert cis- and trans-2,6-diphenylhexamethylcyclotetrasiloxane with-... 

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