Cross-linking of functionalised siloxanes with alumatrane: Reaction mechanisms and kinetics

Cross-linking of functionalised polysiloxanes is an important tool to adjust their viscoelastic properties and can be achieved by the reaction with alumatrane. The cross-linking reaction has been found to proceed only with hydrolyzed alumatrane species. Siloxane model compounds with different functional groups such as alkoxy, siloxy, and hydroxy groups were considered in order to optimise the rheological properties of the polymer. The activation energy barriers of the related reactions were analysed using the density functional theory under the assumption of the presence of Al–OH groups formed by the hydrolysis of alumatrane. The cross-linking involving hydroxy groups of siloxane and hydrolyzed alumatrane has been found to have the lowest activation energy (−14 kJ/mol). As the reaction of the Si–O–Si-polymer backbone with the hydroxy groups of the hydrolyzed alumatrane turned out to have the very low activation energy of +2 kJ/mol, this type of reaction is predicted to play a key role for the cross-linking of polysiloxanes with hydrolyzed alumatrane. The involved water molecules are formed back in the course of subsequent polycondensation reactions, therefore H2O can be considered as a cross-linking catalyst in these systems.

Cross-linking of functionalised polysiloxanes with hydrolyzed alumatrane species has been studied both experimentally and theoretically. The reaction of the Si–O–Si-polymer backbone with the hydroxy groups of the hydrolyzed alumatrane turned out to have very low activation energy (+2 kJ/mol), presenting an alternative reaction mechanism along with the ordinary condensation of the hydroxy groups and playing a key role for the controlled modification of the rheological properties of polysiloxanes.Figure optionsDownload as PowerPoint slide