Modulation of Surface-Initiated ATRP by Confinement: Mechanism and Applications

Abstract

The mechanism of surface-initiated atom transfer polymerization (SI-ATRP) of methacrylates in confined volumes is systematically investigated by finely tuning the distance between a grafting surface and an inert plane by means of nanosized patterns and micrometer thick foils. The polymers were synthesized from monolayers of photocleavable initiators, which allow the analysis of detached brushes by size-exclusion chromatography (SEC). Compared to brushes synthesized under “open” polymerization mixtures, nearly a 4-fold increase in brush molar mass was recorded when SI-ATRP was performed within highly confined reaction volumes. Correlating the SI-ATRP of methyl methacrylate (MMA), with and without “sacrificial” initiator, to that of lauryl methacrylate (LMA) and analyzing the brush growth rates within differently confined volumes, we demonstrate faster grafting kinetics with increasing confinement due to the progressive hindering of CuII-based deactivators from the brush propagating front. This effect is especially noticeable when viscous polymerization mixtures are generated and enables the synthesis of several hundred nanometer thick brushes within relatively short polymerization times. The faster rates of confined SI-ATRP can be additionally used to fabricate, in one pot, precisely structured brush gradients, when volume confinement is continuously varied across a single substrate by spatially tuning the vertical distance between the grafting and the confining surfaces.

Optimisation of Surface-Initiated Photoiniferter-Mediated Polymerisation under Confinement, and the Formation of Block Copolymers in Mesoporous Films

“…Polymerisation in spatially confined mesoporous materials is a key technology that has been widely investigated over the last decade with advances in both soft chemistry and controlled/“living” radical polymerisation (CLRP) techniques [ 7 , 8 , 17 , 18 ]. A number of CLRP techniques have been successfully applied to the functionalisation of mesoporous silica including atom transfer radical polymerisation (ATRP) [ 19 , 20 , 21 ], reversible addition‒fragmentation chain transfer (RAFT) polymerisation [ 22 ], nitroxide-mediated radical polymerisation (NMP) [ 23 ], and photoiniferter-mediated polymerisation (PIMP) [ 23 , 24 ] through a grafting from or surface-initiated approach. While the successful grafting of pre-formed polymer to the interior and exterior surface has been reported in large-pore (15 nm) membranes [ 25 ], this approach is preferred for gated membranes with sterics limiting the molecular weight, grafting density, and pore filling achievable [ 8 , 17 , 22 , 26 ].…”

Tuning and in situ monitoring of surface-initiated, atom-transfer radical polymerization of acrylamide derivatives in water-based solvents

“…CuCl 2 was chosen rather than sacrificial initiator, in order to avoid the formation of free polymer chains and the resulting effects associated with increasing viscosity in the medium-an increase in viscosity not only affects the coupling between the QCM-D-sensor surface and the reaction medium 27 but also dramatically influences the SI-ATRP kinetics. [28][29][30] Although the overall kinetics slowed down with increasing CuCl 2 concentration, as represented by the diminishing slopes of the ΔF-t plots ( Fig. 2a), the ΔF-t plots also exhibited a saturation of the resonance frequency within a short period of polymerization, suggesting a transient SI-ATRP lifetime, albeit convoluted with the hearing-loss effect of QCM-D, similar to that observed in the absence of additional CuCl 2 .…”

Quasi‐3D‐Structured Interfaces by Polymer Brushes

“…By simply positioning an inert surface at variable distances from the initiator‐functionalized substrate, and stimulating the formation of viscous polymerization mixtures by adding sacrificial initiator in solution, the diffusion of bulky Cu II ‐based deactivators to the growing brush front could be locally controlled. In particular, deactivation of growing radicals was markedly hindered in areas where the distance between the inert and initiating substrate was reduced . By applying this simple fabrication, a continuous increase of brush molar mass and polydispersity index (PDI) could be obtained across the surface ( Figure ).…”

“…This effect allowed us to generate polymer brush‐thickness gradients by simply positioning an inert plane at a variable distance from the initiating surface. Reproduced with permission . Copyright 2017, American Chemical Society.…”

“…For instance, a precise determination of molecular weight and polydispersity of polymer brushes synthesized by SI‐ATRP became a feasible task through the application of photocleavable initiator adsorbates, which allowed detached‐brush analysis via size exclusion chromatography (SEC) …”