Interconversion of siloxanes

The acid catalysed interconversion of 2,6-diphenylhexamethylcyclotetrasiloxane in ..~ the ·presence of water was studied. Lewis and Bronsted acids were evaluated. Protic and aprotic solvents were used, as well as polar and nonpolar solvents. The interconversion 'of one stereoisomer of2,6-diphenylhexamethylcyclotetrasiloxane (either 2,6-cis or 2,6-trans) to the equilibrium mixture of the two isomers occurs under the mild conditions of low temperature and weakly nucleophilic catalyst. Polymerisation becomes predominant under harsher reaction conditions (high temperature, high concentration of catalyst, strong nucleophile source). Several findings were observed with the reactions of2,6-diphenylhexamethylcyclotetrasiloxane. Interconversion is shown to be catalysed by both Lewis (e.g. iron chloride) and Bronsted (e.g. trifluoromethanesulphonic) acids in a polar aprotic solvent (e.g. nitromethane). The balance between interconversion and polymerisation can be controlled by temperature~ndcatalyst acidity. Hydrochloric and nitric acid catalysts favour polymerisation, where interconversion conditions were found for trifluoromethanesulphonic and methanesulphonic acids. Only polar aprotic solvents were found to favour interconversion, which would support a separation of charge. No interconversion was observed with the nonpolar solvent pentane, while polymerisation was observed in the protic solvent n-butanol. The study of this system has provided mechanistic insights for the acid catalysed interconversion of cyclosiloxanes, with implications for the mechanism of polymerisation. Common mechanistic steps are proposed for the acid catalysed interconversion and polymerisation reactions. A relatively low activation energy (approximately 16 kJ morl) was determined for the interconversion of2,6-cis-diphenylhexamethylcyclotetrasiloxaneEThOS - Electronic Theses Online ServiceGBUnited Kingdo