Generic, Metal-Free Cross-Linking and Modification of Silicone Elastomers Using Click Ligation

Abstract

Silicone elastomers are widely used in a variety of biomaterials applications. Their high hydrophobicity can, in some cases, compromise their utility. There exist few convenient, efficient and metal-free processes to introduce hydrophilic groups onto the elastomer surface. We describe the utilization of the metal-free click reaction between azido- and alkynyl-modified silicones to both cross-link and functionalize silicone elastomers. Initial cross-linking at, optionally, reduced or elevated temperatures depending on the alkyne used, results in elastomers whose moduli and tackiness can be controlled by manipulation of cross-link density through use of different chain length constituents: the systematic capping of azides on the azide-rich partner provides a level of control of both cross-linking and residual functionality after cross-linking. The residual azides or alkynes resulting from nonstoichiometric mixing of the starting materials can be used for sequential functionalization of the elastomer as shown through the improved wettability that results from the grafting of alkyne-functional PEG to azide-rich silicone elastomers: contact angles decrease from ∼105° for the pure silicone elastomers to 85° for the azide-rich silicone elastomer, and to as low as 41° for the PEG-modified product. It is possible to modify the entire elastomer body or only the surface by judicious choice of the solvent used for the hydrophilic modification.