Highly Confined Surface-Initiated Polymerizations for Polymer Brush Structuring

A central objective in the synthesis of polymer brushes has been focusing on the development of surface grafting methods enabling the accessible and precise tuning of brush characteristics, i.e. film thickness and chain density. In order to meet this need, we concentrated on surface-initiated atom transfer radical polymerizations (SIATRPs), demonstrating that when these are applied within spatially confined systems, the reaction mechanisms deviate from the ones observed for “open” polymerizations, giving access to brush structure’s modulation. This is the case of SI-ATRP performed in a highly confined reaction volume, where the initiating surface is placed at a distance ranging from few micrometers to hundreds of nanometers from an inert plane (a). Within such a restricted reaction space, the increase in solution viscosity alters the relative diffusion of activators/deactivators and, depending on the degree of confinement, leads different polymerization rates. This enables the synthesis of ultra-thick brushes, and the easy fabrication of brush-thickness gradients. When the confining plane is replaced by a Cu-coated surface the polymerization mechanism is dominated by the supplemental activator reducing agent (SARA) process, where Cu species migrate to the initiating/propagating surface and regulate the brush growth. Within this confined environment, the distance between the metallic and the initiating surfaces regulate the local concentration of Cu(I)-based activators, determining the extent of radical termination and thus the grafting density of the formed brush (b). Finally, when a confined reaction interface is generated between a physisorbed layer of catalytically active biomolecules and a growing brush with thermoresponsive swelling properties, the grafting process can be governed by tuning the physical affinity of the brush towards the biocatalyst. Within sequential protein adsorption/desorption cycles, triggered by narrow temperature shifts, the desired brush thickness can be “dialed-in”, and precisely reproduced (c). All these strategies suggest that interpreting ATRP processes within reaction media characterized by a modular confinement opens up previously undisclosed and readily accessible possibilities for brush design.