The generation of polymer brushes by surface-initiated controlled radical polymerization (SI-CRP) techniques has become a powerful approach to tailor the chemical and physical properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Additionally, polymer brushes prepared via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chemical and physical characterization of polymer brushes, as well as an ever increasing set of computational and simulation tools that allow understanding and predictions of these surface-grafted polymer architectures. The aim of this contribution is to provide a comprehensive review that critically assesses recent advances in the field and highlights the opportunities and challenges for future work.

Densely grafted, hydrophilic polymer brushes produced via surface-initiated controlled radical polymerization have been shown to undergo degrafting upon exposure to aqueous media. This degrafting process has been proposed to involve swelling-induced, mechanochemically facilitated hydrolysis of bonds located at the brush−substrate interface. While a number of reports have described degrafting of hydrophilic polymer brushes, only little is known about the key structural parameters of these thin films that dictate this process. Using a series of PPEGMA and PPEGMEMA brushes produced by surface-initiated atom transfer radical polymerization (SI-ATRP), this report investigates the influence of three parameters: (i) the chemical structure of the ATRP initiator, (ii) the molecular weight of the surface-grafted polymer chains, and (iii) surface curvature. Studies performed with PPEGMA and PPEGMEMA brushes grown from substrates modified with different ATRP initiators indicated that hydrolysis of both siloxane as well as ester/amide bonds contributes to degrafting. Finally, experiments with PPEGMEMA-modified silica nanoparticles revealed the influence of surface curvature and suggested that degrafting is more pronounced as surface curvature decreases.

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Rapid adhesion between tissue and synthetic materials is relevant to accelerate wound healing and to facilitate the integration of implantable medical devices. Most frequently, tissue adhesives are applied as a gel or a liquid formulation. This manuscript presents an alternative approach to mediate adhesion between synthetic surfaces and tissue. The strategy presented here is based on the modification of the surface of interest with a thin polymer film that can be transformed on-demand, using UV-light as a trigger, from a nonadhesive into a reactive and tissue adhesive state. As a first proof-of-concept, the feasibility of two photoreactive, thin polymer film platforms has been explored. Both of these films, colloquially referred to as polymer brushes, have been prepared using surface-initiated atom transfer radical polymerization (SI-ATRP) of 2hydroxyethyl methacrylate (HEMA). In the first part of this study, it is shown that direct UV-light irradiation of PHEMA brushes generates tissue-reactive aldehyde groups and facilitates adhesion to meniscus tissue. While this strategy is very straightforward from an experimental point of view, a main drawback is that the generation of the tissue reactive aldehyde groups uses the 250 nm wavelength region of the UV spectrum, which simultaneously leads to extensive photodegradation of the polymer brush. The second part of this report outlines the synthesis of PHEMA brushes that are modified with 4-[3-(trifluoromethyl)-3H-diazirin-3-yl]benzoic acid (TFMDA) moieties. UV-irradiation of the TFMDA containing brushes transforms the diazirine moieties into reactive carbenes that can insert into C−H, N−H, and O−H bonds and mediate the formation of covalent bonds between the brush surface and meniscus tissue. The advantage of the TFMDA-modified polymer brushes is that these can be activated with 365 nm wavelength UV light, which does not cause photodegradation of the polymer films. While the work presented in this manuscript has used silicon wafers and fused silica substrates as a first proof-of-concept, the versatility of SI-ATRP should enable the application of this strategy to a broad range of biomedically relevant surfaces.

Densely grafted polymer brushes prepared by surface-initiated polymerization (SIP) represent a versatile platform to engineer surface and interface properties. At high grafting densities, surface tethered polymer chains are forced into an extended chain conformation, which can impart these films with appealing materials properties such as low friction or efficient non-biofouling properties. The extended chain conformation of densely grafted polymer chains can also influence their chemical reactivity, in particular in close proximity to the polymer brush–substrate interface. This chapter provides an overview of recent efforts aiming at investigating detachment or degrafting of densely grafted hydrophilic polymer brushes from substrates upon exposure to aqueous media. The collective results from these reports suggest that swelling-induced stretching of such surface-attached polymers activates mechanochemical bonds near the brush–substrate interface and facilitates chain cleavage. Understanding the mechanochemistry of polymer brushes may provide novel opportunities to design robust, durable polymer brush films or, alternatively, systems that are designed to degraft on-demand.

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