Growth and structure of surface-initiated poly(n-alkylnorbornene) films

We report the surface-initiated growth of poly(alkylnorbornene) films via ring-opening metathesis polymerization (ROMP). The films are grown by exposure of a vinyl-terminated self-assembled monolayer (SAM) on gold to Grubbs first-generation catalyst and the subsequent exposure to an alkylnorbornene monomer. We investigate the influence of alkyl side chains on the structure, barrier, surface properties, and the growth kinetics of surface-initiated ROMP-type poly(norbornene) films. Rate constants for film growth are estimated for the comparison of monomer reactivity. The rate constant for film growth decreases by 3 orders of magnitude from norbornene to decylnorbornene, indicating a strong effect of chain length on initiation and/or propagation rates. Reflectance-absorption infrared spectroscopy is used to show the molecular level packing within the poly(alkylnorbornene) films is disrupted by the alkyl side chains. Tapping-mode atomic force microscopy is used to show that norbornene, butylnorbornene, and hexylnorbornene polymerize from the surface to form dense coatings, whereas decylnorbornene polymerizes to form isolated polymer clusters. The methyl terminus of the alkyl side chains increases the hydrophobicity of the poly(alkylnorbornene) films (thetaA(H2O) = 109-114 degrees) beyond that of a typical poly(norbornene) film (thetaA(H2O) approximately 106 degrees). The additional hydrophobicity throughout the film correlates with superior resistances against redox probes (Rf approximately 105 Omega.cm2) for poly(hexylnorbornene) when compared to polynorbornene (Rf approximately 104 Omega.cm2). The resistance of the poly(decylnorbornene) film (Rf approximately 102 Omega.cm2) is consistent with its nonuniform, cluster-like morphology.