Nitroxide-Mediated Photopolymerization: When Surface Grafting on Cross-Linked Networks Goes up to Several Hundred Microns

Chain extension with multiple blocks is one of the major advantages of reversible deactivation radical polymerization (RDRP), and as such, it is widely applied to the synthesis of (block)copolymers. Mainly exploited for the synthesis of linear and branched structures, this characteristic is less recognized in highly cross-linked networks by RDRP, possibly due to the limited thickness usually achieved, which generally sits below a few tens of nanometers. In this study, we demonstrated how, under optimized conditions, chain extension on highly cross-linked networks is possible up to several hundred micrometers, with obvious opportunities for microfabrication and structuring of complex polymer surfaces. By using nitroxide-mediated photopolymerization (NMP2) and patterning by digital light processing (DLP) photolithography, we successfully grew multiple layers on model glass substrates which extended up to more than 300 μm by simply depositing bare monomer on top of every polymerized layer. A systematic study on irradiation conditions, UV–vis, Fourier transform infrared, and profiling measurements revealed that the surface grafting process was more efficient when the supporting underlayer was not fully grown. We speculated that its “incomplete polymerization” grants the polymer a gel phase allowing easy diffusion of both the fresh monomer and reactive nitroxide ends. A similar behavior, although much weaker, was also observed on reference, free-radical photopolymerized resins, which possibly relates to the slow diffusion and activation of the unreacted photoinitiator. Thus, besides the obvious advantage of using NMP2 in DLP photolithography, this study also illustrates how interlayer properties critically affect the efficiency of surface grafting by surface-(re)initiated radical polymerization, opening unique possibilities for highly cross-linked networks by NMP2.