Understanding the Redox-Induced Polymer Grafting Process: A Dual Surface-Solution Analysis

As recently reported, the Graftfast process is a grafting method that provides covalently grafted polymer films. It relies on the chemical reduction of diazonium salts by reducing agents in absence or presence of a vinylic monomer. Contrary to electroinduced methods delivering strongly grafted and stable polymer films such as cathodic electrografting (CE) of vinylic monomers (which requires drastic experimental conditions) or surface electroinitiated emulsion polymerization (SEEP), the Graftfast process provides strongly grafted polymer films on any type of materials (conductors, semiconductors, and insulators). Moreover, it is a fast one-step reaction occurring at atmospheric pressure, ambient air and room temperature in water, which makes it more suitable for applications than the slower ATRP-based methods. This article aims to complete the first paper on this process by giving preliminary answers to the question: How does the Graftfast process work? To achieve this mechanistic study, dual surface-solution analyses were performed. Both spontaneous and redox-induced grafting of polynitrophenylene-like (PNP) films and poly(hydroxyethyl) methacrylate (PHEMA) films were analyzed by infrared-attenuated total reflection (IR-ATR) and X-ray photoelectron spectroscopy (XPS) while the corresponding reactive solutions were studied by electronic paramagnetic resonance, EPR (spin-trapping method using 2-methyl-2-nitrosopropane MNP as spin-trapping agent). The EPR spectra and hyperfine structure of MNP adducts provide evidence of aryl radicals production, growing polymer chains radicals formation, and the existence of critical concentration values, leading to favorable grafting kinetics.