Kinetics and mechanism of oligosiloxanol condensation and oligosiloxane rearrangement catalysed with model phosphonitrile chloride catalysts

Article abstract of DOI:10.1016/S0022-328X(96)06883-0

The condensation kinetics of 1,1,3,3,3-pentamethyldisiloxanol (MDH) in n-heptane solution were compared for two types of phosphonitrilic catalyst, hexachloro-1 λ-diphosphaza-1-enium hexachloroantimonate salt, [Cl₃PNPCl₃]⁺[SbCl₆]^-, 1, and P-trichloro-N- dichlorophosphoryle phosphazene, [Cl₃PNP(O)Cl₂], 2. The kinetic law of reaction is not changed when 1 is replaced by 2. The process is selective leading to linear decamethyltetrasiloxane (MD₂M, where D denotes the dimethylsiloxane unit, and M denotes the trimethylsiloxane unit) as almost the exclusive primary product. Other oligomers of the MD_nM series are formed as a result of the MD₂M rearrangement. The MD₂M rearrangement was studied in separate experiments in the absence of the siloxanol and water. Both catalysts 1 and 2 gave similar rate-concentration behaviour. Some of the kinetic features of the process resemble those of chain reactions and mechanisms of the MDH condensation, and the MD₂M rearrangement are discussed.