Synthesis of Crystalline, Nanometer-Scale, −(CH₂)_x− Clusters and Films on Gold Surfaces

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Abstract

Stable, nanometer-scale thickness films of −(CH2)x− have been observed to form by the surface-catalyzed decomposition of CH2N2 on evaporated Au film substrates. In the early stages, growth occurs in the form of isolated clusters at defect regions in the {111} textured surfaces. As the average thickness increases beyond ∼20 nm, growth spills out onto the {111} terraces with eventual coverage of the entire surface. At all coverages, the dominant structure is highly trans, extended polymethylene chains packed in an orthorhombic lattice, similar to the typical structure of crystalline, bulk-phase polyethylene but containing more conformational defects than well-formed bulk crystals. Chain melting occurs at ∼135 °C, and cooling to room temperature results in differing extents of ordering as a function of the total film coverage, an indication that the structures of the growing films are constrained in metastable forms by the presence of adjacent gold surface defect features. The polymerization mechanism appears to involve surface-catalyzed decomposition of the diazomethane at gold defect sites to produce methylidene adsorbate species, which subsequently initiate the formation of linear polymers via a free radical propagation process. This process provides a useful limiting case of the surface-catalyzed formation of linear hydrocarbons from C1 intermediates on transition metals.

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