Dithioesters and Trithiocarbonates as Anchoring Groups for the “Grafting-To” Approach Anne-Sophie Duwez, ² Pierre Guillet, ‡ Catheline Colard, ‡ Jean-Franc ¸ ois Gohy, ‡ and Charles-Andre ´ Fustin* ,‡ Unite ´ POLY and CeRMiN, UniVersite ´ catholique de LouVain, Place Croix du Sud 1, 1348 LouVain-la-NeuVe, Belgium, and Unite ´ CMAT and CeRMiN, UniVersite ´ catholique de LouVain, Place L. Pasteur 1, 1348 LouVain-la-NeuVe, Belgium ReceiVed February 7, 2006 ReVised Manuscript ReceiVed March 9, 2006 Reversible addition fragmentation chain transfer (RAFT) polymerization is an extremely versatile, controlled radical polymerization technique that is becoming increasingly popular due to its tolerance toward functional monomers, enabling the synthesis of a vast range of (co)polymers. RAFT operates via a degenerative transfer mechanism in which a thiocarbonylthio compound acts as a chain transfer agent (CTA). 1,2 As a consequence of the mechanism, polymers prepared by this technique bear a dithioester end group or a trithiocarbonate group in the middle of the chain according to the type of CTA used. Polymers produced by RAFT are thus highly interesting for the functionalization of metal, and in particular gold, surfaces via the “grafting-to” approach. Up to now, the reported procedures to modify gold surfaces with RAFT polymers involve the prior transformation of the sulfur-containing group coming from the CTA into thiols by reaction with nucleophiles such as primary amines. 3-6 However, this step is incompatible with some polymers due to the reactivity of the main chain (e.g., poly(acrylic acid) or polymers bearing activated esters) and restricts thus the range of application. Moreover, it has been shown that it is rather difficult to selectively obtain thiols by this procedure and that disulfides are often formed, resulting from the coupling between two thiol-functionalized polymer chains. 7-9 In this paper we show for the first time that the prior transformation of dithioesters and trithiocarbonates into thiols is not mandatory for the functionalization of metal surfaces, these two species being able to chemisorb onto gold. Preparation of monolayers based on a sulfur-Au bond has been extensively investigated, 10 including the use of thiols, 11 disulfides, 12 sulfides, 13 thiophene, 14 thiocarboxylic acids, 15 dithiocarboxylic acids, 16 xanthates, 17 thiocarbamates, 18 and dithiocarbamates. 19 However, to the best of our knowledge, the chemisorption of dithioesters and trithiocarbonates onto gold has never been reported. We have prepared monolayers of two different CTAsbenzyl dithiobenzoate (BDTB) and dibenzyl trithiocarbonate (DBTTC)s by immersing gold substrates into dilute solutions of these CTA for 4 h, followed by copious rinsing with pure solvent (see Supporting Information for more details). The monolayers were then characterized by X-ray photoelectron spectroscopy (XPS). Figure 1 shows the S 2p spectra recorded on monolayers of BDTB and DBTTC. Only one peak is observed on the spectrum of BDTB, indicating that both sulfur atoms experience the same chemical environment. Moreover, the location of this peak, around 162 eV, is in very good agreement with the value reported in the literature for chemisorbed dithiocarboxylic acid on gold. 16 These two observations demonstrate that BDTB is chemisorbed with both sulfurs attached to the gold substrate, as represented in Scheme 1. For DBTTC, the peak is clearly broader (much broader than the resolution of 1.5 eV), and the reconstruction of the spectrum evidences the presence of two doublets in a 2:1 ratio located around 162 and 163.5 eV. The binding energy and the width of the most intense doublet are identical to those observed for BDTB and is therefore associated with chemisorbed sulfurs. The minor doublet, located at 163.5 eV, is associated with the sulfur of the C-S bond not interacting with the gold substrate. Its binding energy is indeed in very good agreement with the values reported for a C-S bond in bulk compounds. 20 These observations evidence the chemisorp- tion of DBTTC in a configuration where two of three sulfur atoms interact with the gold substrate, as shown in Scheme 1. This bridged configuration is reminiscent of the one found for xanthates and dithiocarbamates where only the two sulfur atoms are chelating gold. 17,19 This is likely due to geometrical constraints. To further evidence the chemisorption of dithioesters and trithiocarbonates and their utility as anchoring groups in the “grafting-to” approach, we have prepared polymer brushes on gold substrates, using polystyrene made by RAFT with the two CTA. The brushes were prepared by immersing gold substrates into PS solutions in toluene for 24 h followed by careful rinsing with toluene. These samples were then characterized by AFM- based force spectroscopy, steric forces being a well-known characteristic of polymer brushes. 21-24 Figure 2 shows the approach profile obtained in toluene between a bare silicon nitride tip and a layer of the PS prepared from BDTB. The monotonically increasing repulsive forces, which are typical of ² Unite ´ POLY and CeRMiN. ‡ Unite ´ CMAT and CeRMiN. * Corresponding author. E-mail: fustin@chim.ucl.ac.be. Figure 1. S 2p XPS spectra recorded on layers of BDTB and of DBTTC on gold. The red and green lines are the envelopes of doublets used to fit the spectra (see Supporting Information for details). Scheme 1. Chemisorption Configuration of the BDTB (Left) and of the DBTTC (Right) 2729 Macromolecules 2006, 39, 2729-2731 10.1021/ma0602829 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/28/2006