Abstract
Straightforward and versatile surface modification, functionalization and coating have become a significant topic in material sciences. While physical modification suffers from severe drawbacks, such as insufficient stability, chemical induced grafting processes efficiently modify organic and inorganic materials and surfaces due to covalent linkage. These processes include the “grafting from” method, where polymer chains are directly grown from the surface in terms of a surface-initiated polymerization and the “grafting to” method where a preformed (macro)-molecule is introduced to a preliminary treated surface via a coupling reaction. Both methods require an initiating species that is immobilized at the surface and can be triggered either by heat or light, whereas light induced processes have recently received increasing interest. Therefore, a major challenge is the ongoing search for suitable anchor moieties that provide covalent linkage to the surface and include initiators for surface-initiated polymerization and coupling reactions, respectively. This review containing 205 references provides an overview on photoinitiators which are covalently coupled to different surfaces, and are utilized for subsequent photopolymerizations and photocoupling reactions. An emphasis is placed on the coupling strategies for different surfaces, including oxides, metals, and cellulosic materials, with a focus on surface coupled free radical photoinitiators (type I and type II). Furthermore, the concept of surface initiation mediated by photoiniferters (PIMP) is reviewed. Regarding controlled radical polymerization from surfaces, a large section of the paper reviews surface-tethered co-initiators, ATRP initiators, and RAFT agents. In combination with photoinitiators or photoredox catalysts, these compounds are employed for surface initiated photopolymerizations. Moreover, examples for coupled photoacids and photoacid generators are presented. Another large section of the article reviews photocoupling and photoclick techniques. Here, the focus is set on light sensitive groups, such as organic azides, tetrazoles and diazirines, which have proven useful in biochemistry, composite technology and many other fields.
Highlights
Surface modification, functionalization and coating is of great interest both in academia as well as in the industrial sector
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An elemental analysis of the extracts showed that the content of extractable photoinitiator is below detection limit when the BAPO initiator is coupled onto particle surfaces. Another example of a type I photoinitiator, which was attached to silica nanoparticles via silanization, was given by Moehrke et al The silane functionalized initiator (Figure 5I) was synthesized starting from the commercially available photoinitiator 2,2dimethoxy-2-phenylacetophenon and was used to graft n-butyl methacrylate from silica surfaces to investigate the suitability of the SP-PLP-ESR technique for examination of the termination kinetics of surface-tethered macroradicals
Summary
Functionalization and coating is of great interest both in academia as well as in the industrial sector. Surface modification can be done by both, physical and chemical modification, whereas physical modification methods, such as spin casting or dip coating, have numerous disadvantages. These include, low thermal stability of the prepared films, their inability to withstand high shear forces and the fact that these layers can be removed by chemicals since they are only physically adsorbed to the substrate [1,3]. Details about well-known coupling strategies, such as silanization of (pretreated) oxidized surfaces and the formation of selfassembled-monolayers (SAM) from thiol compounds on gold surfaces, can be found in another comprehensive review (see ref. [23])
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