Photochemical Surface Grafting

Over the past decades a large variety of procedures has been developed to covalently attach many different polymers to solid surfaces [1]. Most approaches toward this direction use polymers that were functionalized with reactive groups, either at one end or in pendant side groups, that can be used for surface attachment. Following these so-called grafting-to procedures it is possible to chemisorb ultrathin layers of the respective polymers with film thicknesses of typically 3-5 nm by simply exposing the substrates to solutions of these polymers. The major disadvantage of these strategies, however, must be seen in the sometimes extensive synthetic effort that is necessary, to synthesize polymers that are decorated with anchor groups for surface attachment. To overcome this disadvantage, we were looking for a fast and simple way to covalently attach physisorbed films of polymers that are not specifically modified for this purpose by a subsequent chemical reaction. This reaction should ideally be independent of the structure of the polymer that is to be attached. Therefore, procedures that make use of functional groups are not applicable for this goal as these would always be limited to only a small number of polymers. Consequently, we have chosen a reaction pathway for attachment that is based on a light-induced process between a photoreactive moiety on the surface and the polymeric overcoat. The system is schematically shown in Figure 1. As the photoreactive group we have chosen the benzophenone moiety because the photochemistry of this compound is well documented [2] and it is known that benzophenone attaches to C-H bonds in very different chemical environments.

Figure 1: Schematic representation of the photochemical attachment of polymer chains to solid surfaces via illumination of polymer-covered monolayers of a benzophenone derivative.

Using this approach we attached a variety of different polymers with very different properties, such as hydrophobic (polystyrene, polymethyl methacrylate) and hydrophilic polymers (e.g. polyethyloxazoline), very flexible (polydimethylsiloxane) and stiff polymers (poly-p-phenylenes) as well as polyolefine derivatives (polyethylene-co-norbornene). In addition, the approach can also be used to synthesize surface attached polymer networks by simultaneous crosslinking of photoreactive groups within the polymer (e.g. pendant anthracene units) and surface coupling via benzophenone monolayers [3].

References
[1] a) J. Rühe, Nachr. Chem. Tech. Lab. 1994, 42, 1237; b) "Polymers at Interfaces", G.J. Fleer, M.A. Cohn-Stuart, J.M.H.M. Scheutjens, T. Cosgrove and B. Vincent, Chapman & Hall, London, 1993.
[2] a) G. Dormán, G.D. Prestwich, Biochemistry 1994, 33, 5661; b) N.J. Turro, Modern Molecular Photochemistry; University Science Books: Mill Valley, 1991.
[3] Y. Ederle, K. Müller, O. Prucker, J. Rühe, in preparation.