We report the surface-initiated ring-opening metathesis polymerization (SI-ROMP) of 5-(perfluoro-n-alkyl)norbornenes (NBFn) to grow partially fluorinated polymer films of tunable thicknesses ranging from tens of nanometers to micrometers on gold substrates. The perfluoroalkyl chains range in length (n) from 4 to 10 carbons, representing 67-83% of the molecular weight of the repeat unit. The growth rate of the film depends on the perfluoroalkyl chain length, with longer chains enabling faster surface-initiated growth and greater ultimate thicknesses. These films exhibit hydrophobic and oleophobic surface properties and provide effective barriers to the diffusion of aqueous ions. The critical surface tensions of the films exhibit a minimum for a fluorocarbon chain of 8 (pNBF8) and range from 9 to 19 mN/m. The morphologies of the pNBFn films consist of densely packed 20-30 nm clusters that differ in the concentration of fluorocarbon chains in the outer few nanometers of the film.

The ability to chemically wire ionomer films to electrode surfaces can promote transport near interfaces and impact a host of energy-related applications. Here, we demonstrate proof-of-concept principles for the surface-initiated ring-opening metathesis polymerization (SI-ROMP) of norbornene (NB), 5-butylnorbornene (NBH4), and 5-perfluorobutylnorbornene (NBF4) from Pt-modified gold substrates and the subsequent sulfonation of olefins along the polymer backbones to produce ultrathin sulfonated polymer films. Prior to sulfonation, the films are hydrophobic and exhibit large barriers against ion transport, but sulfonation dramatically reduces the resistance of the films by providing pathways for proton diffusion. Sulfonated films derived from NBF4 and NBH4 yield more anodic potentials for oxygen reduction than those derived from NB or unfunctionalized electrodes. These improvements are consistent with hydrophobic structuring by the fluorocarbon or hydrocarbon side groups to minimize interfacial flooding and generate pathways for enhanced O(2) permeation near the interface. Importantly, we demonstrate that the sulfonated polymer chains remain anchored to the surface during voltammetry for oxygen reduction whereas short-chain thiolates that do not tether polymer are removed from the substrate. This approach, which we extend to unmodified gold electrodes at neutral pH, presents a method of cleaning the ionomer/electrode interface to remove molecular components that may hamper the performance of the electrode.