Surface-Initiated Thermal Radical Polymerization on Gold

We report surface-initiated thermal radical polymerization from cross-linked monolayers of azo-initiators on Au. Initial attempts to graft polymer layers from initiators attached to alkanethiol monolayers yielded polymer films with thicknesses less than 5 nm. Efficient grafting from such surfaces is not possible because initiator-containing monolayers are somewhat unstable under thermal radical polymerization conditions, and desorbed thiols may serve as efficient chain-transfer reagents that inhibit radical polymerization. In addition, reactive radicals can attack the Au−S bonds that link the initiator monolayer to the surface. To overcome these problems, we employed mercaptopropyltrimethoxysilanes to form an adhesion layer for initiator attachment. Cross-linked poly(siloxane) layers apparently stabilize the initiator layer, allowing well-defined surface radical polymerization to occur from Au substrates. We characterized the grafted polymer layers with ellipsometry, reflectance Fourier transform infrared spectroscopy, and atomic force microscopy.