Interconversion of siloxanes

Abstract

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 of 2,6-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 of 2,6-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 and catalyst 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 mol-l) was determined for the interconversion of 2,6-cis-diphenylhexamethylcyclotetrasiloxane catalysed by methanesulphonic acid. The activation energy for polymerisation of the same system was three times that of interconversion, approximately 40 kJ mol-l. Protonation of the cyclosiloxane oxygen, or possibly a metal complex coordination with the oxygen in the case of Lewis acid catalysis, is proposed as the first step of both reactions. Protonation is proposed as the essential requirement for the reaction, because the use of a proton sponge prevented both interconversion and polymerisation reactions. A large negative entropy of activation (in the order of -200 J mol-l K-l) was observed for all of the acid catalysed interconversion and polymerisation reactions studied o f 2,6-diphenylhexamethylcyclotetrasiloxane. Nucleophilic attack at the silicon forms a pentacoordinate intermediate, and is consistent with a negative entropy of activation. The stability/ lifetime of this intermediate dictates the balance between interconversion and polymerisation products. Berry pseudorotations about the pentacoordinate silicon can occur if the intermediate is stable, or long-lived. Three consecutive rotations of the pentacoordinate silicon intermediate could result in stereoisomer interconversion after loss of the nucleophile, reforming a tetracoordinate silicon. Polymerisation ensues by siloxane bond breaking when the pentacoordinate silicon intermediate is less stable, or short-lived. This work emphasises the mechanism for interconversion of asymmetric cyclosiloxanes in the presence of water, catalysed by acids. The control of this reaction might offer the explanation for the difficulty researchers have observed with acid catalysis of cyclosiloxanes to produce stereoregular siloxane-based polymers.