Reversible addition-fragmentation chain transfer (RAFT) polymerization1 is a very versatile controlled radical polymerization technique. The use of thiocarbonylthio chain transfer agents (CTAs) yields polymers terminated with a dithioester, which is sensitive toward radicals, nucleophiles, temperature, and reducing agents. Thus, many postpolymerization methods may be employed to modify this dithioester end group. By thermolysis or reduction2, 3 the dithioester group may be removed, whereas treatment with an excess of a diazo compound allows for the introduction of functional end groups. 4, 5 Very popular is the aminolysis of the dithioester end group; however, the products of this procedure depend very much on the chemical structure of the polymer. Polystyrene derivatives and poly [(meth) acrylamides] yield terminal thiols (or symmetrical disulfides if oxygen is present). 6-12 These end groups are particularly interesting for producing self-assembled monolayers (SAMs) on gold surfaces13, 14 which find applications in biochemistry, 15-17 catalysis, 18, 19 nanotechnology, 20 and microelectronics. 21-24 Further, thiol-terminated polystyrene, 25 polystyrene derivatives, 6 and several poly [(meth) acrylamides] 6-8 have been used to encapsulate gold nanocrystals, and thiol-terminated poly [(meth) acrylamides] have been used for end-group modifications9, 10 and fluorescent labeling. 11, 12 Poly [(meth) acrylates], however, suffer from the great drawback that the aminolysis affords the desired terminal thiol only in low yields. 26-28 It has been reported that reduced polymers with an H end group, a double molecular weight material even in the absence of oxygen, 26, 27 and other ill-defined products26 are produced. Quantitative analysis of a thiol end group such as with Ellman’s reagent9 is only rarely presented. Xu et al. 28 investigated the aminolysis of polymethacrylates in the absence of oxygen in greater detail, providing evidence that the resulting polymer bears a terminal thiolactone, which is obtained by backbiting of an initially formed thiol into the penultimate monomer unit. Although being a defined polymer end group, this thiolactone cannot be employed for conjugation or for surface modifications, very much in contrast to a thiol end group. Also, the native dithioester end groups have been investigated for adsorbing onto a gold surface by Duwez et al.; 29 however, they could only obtain a grafting density of 0.013 chains/nm2, which is very low compared to the literature values for terminal thiols grafted to gold nanoparticles such as 0.94 chains/nm2 for polystyrene30 or 1.8-3.6 chains/nm2 for poly [N-isopropylacrylamide]. 8 In this Communication we investigate the effect of methyl methanethiosulfonate (MTS, 2) during the aminolysis reaction of polymethacrylates that have been prepared by RAFT polymerization with the goal of producing a defined polymethacrylate end group capable of producing a dense SAM on a gold surface. MTS reacts quickly and selectively with thiols to yield the respective methyl disulfides. Amines increase the hydrolyzation rate but do not attack the sulfur atoms themselves. Thus, methanethiosulfonates have found intensive use in selective31 labeling of cysteine groups for the determination of structure and function of ion channel, 32 receptor, 33 and membrane34 proteins. We expect thiols that are released by aminolysis from the polymer dithioester end group also to react with MTS and to prefer this reaction over the competing reactions of thiolactone formation and oxidation to symmetrical polymer-polymer disulfides by oxygen. As a model compound we synthesized a short poly (methyl methacrylate)(PMMA)(1)(Mn) 3900 …