Catalytic Amplification of Patterning via Surface-Confined Ring-Opening Metathesis Polymerization on Mixed Primer Layers Formed by Contact Printing

Abstract

Thin films of polymers synthesized from strained olefins were formed on Si(100) via ring-opening metathesis polymerization (ROMP) initiated by a Ru−alkylidene catalyst bound to the surface by means of the alkylidene ligands. Specifically, a mixture of 7-octenyltrichlorosilane (3) and octyltrichlorosilane (4) was deposited onto Si/SiO2 surfaces via contact printing, and this assembly was subsequently treated with RuCl2(PCy3)2(CHPh) (1). The resulting surface-tethered Ru alkylidene complex reacts with norbornene derivatives to give thin polymer films. The growth of the polymer film, as monitored by ellipsometry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM), is strongly influenced by the initial concentration of the catalyst at the surface and by the monomer reactivity. The catalyst concentration at the Si/SiO2 surface was controlled by adjusting the relative ratio of 3 and 4 in the “primer” used in the contact-printing step. These studies revealed that the success of the catalytic amplification step required that the linker be present on the surface in very low coverage. The optimal concentration of 3 in the mixed primer was 20−40% with a total mass coverage of the mixed self-assembled monolayer (SAM) of approximately one-third that of a full monolayer coverage. Possible causes of the inefficient polymerization at high coverages of 3 are discussed; a leading candidate is that the catalytic Ru centers are deactivated by reactions with neighboring olefinic linker sites or with other Ru centers. In a representative class of polymerization, reticulated films of poly(2,2,2-trifluoroethyl bicyclo[2.2.1]hept-2-ene-5-carboxylate) (5) with an average thickness of ∼100 Å were obtained after ∼3 h of reaction at room temperature. The polymerization can be completely described by a rate law involving coupled step-growth and competing unimolecular termination reactions. The polymerization process can be used to amplify catalytically (that is, to increase the mass coverages of) latent images present in SAMs that have been patterned via microcontact printing (μCP). To do so, a patterned resist SAM of octadecyltrichlorosilane (OTS) was deposited first via μCP. A second SAM based on the mixed phase of 3 and 4 was then orthogonally assembled in a self-registering printing step carried out using an unpatterned poly(dimethylsiloxane) (PDMS) stamp. Activation of this composite SAM via a treatment first with 1 and then with 5 resulted in an additive deposition of the polymer pattern.