Covalent carbon-carbon bonds are hard to break. Their strength is evident in the hardness of diamonds 1,2 and tensile strength of polymeric fibres [3][4][5][6] ; on the single-molecule level, it manifests itself in the need for forces of several nanonewtons to extend and mechanically rupture one bond. Such forces have been generated using extensional flow [7][8][9] , ultrasonic irradiation 10 , receding meniscus 11 and by directly stretching a single molecule with nanoprobes [12][13][14][15][16] . Here we show that simple adsorption of brush-like macromolecules with long side chains on a substrate can induce not only conformational deformations 17 , but also spontaneous rupture of covalent bonds in the macromolecular backbone. We attribute this behaviour to the fact that the attractive interaction between the side chains and the substrate is maximized by the spreading of the side chains, which in turn induces tension along the polymer backbone. Provided the side-chain densities and substrate interaction are sufficiently high, the tension generated will be strong enough to rupture covalent carbon-carbon bonds. We expect similar adsorption-induced backbone scission to occur for all macromolecules with highly branched architectures, such as brushes and dendrimers. This behaviour needs to be considered when designing surface-targeted macromolecules of this typeeither to avoid undesired degradation, or to ensure rupture at predetermined macromolecular sites.A series of brush-like macromolecules with the same number average degree of polymerization of a poly(2-hydroxyethyl methacrylate) backbone, N n ¼ 2,150^100, and different degrees of polymerization of poly(n-butyl acrylate) (pBA) side chains ranging from n ¼ 12^1 to n ¼ 140^12 were synthesized by atom transfer radical polymerization (see 'Polymer Characterization' in the Methods) 18 . Owing to the high grafting density, the side chains repel each other and thereby stretch the backbone into an extended conformation. Placing these macromolecules on a surface enhances the steric repulsion between the side chains, which results in both an extension of the polymer backbone and an increase of the persistence length.The effect is illustrated in Fig. 1, which shows atomic force microscopy (AFM) micrographs of monolayers of pBA brushes with short ( Fig. 1a) and long side chains (Fig. 1b). Measurements on both types of molecules yielded a number average contour length per monomeric unit of the backbone of l ¼ L n =N n ¼ 0:23^0:02 nm (see 'Atomic Force Microscopy' in Methods), which is close to l 0 ¼ 0.25 nm, the length of the tetrahedral C-C-C section. This means that even for short side chains (n ¼ 12), the backbone is already fully extended and adopts an all-trans conformation. As the side chains become longer, we observe global straightening of the backbone reflected in the increase of the persistence length (Fig. 1c).Chain extension requires a substantial amount of force, which we estimate using simple spreading arguments (Fig. 2). Just as in normal liquids, the polymer...

A new class of water-soluble, amphiphilic star block copolymers with a large number of arms was prepared by sequential atom transfer radical polymerization (ATRP) of n-butyl methacrylate (BMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA). As the macroinitiator for the ATRP, a 2-bromoisobutyric acid functionalized fourth-generation hyperbranched polyester (Boltorn H40) was used, which allowed the preparation of star polymers that contained on average 20 diblock copolymer arms. The synthetic concept was validated by AFM experiments, which allowed direct visualization of single molecules of the multiarm star block copolymers. DSC and SAXS experiments on bulk samples suggested a microphase-separated structure, in agreement with the core−shell architecture of the polymers. SAXS experiments on aqueous solutions indicated that the star block copolymers can be regarded as unimolecular micelles composed of a PBMA core and a diffuse PPEGMA corona. The ability of the polymers to encapsulate and release hydrophobic guests was evaluated using 1H NMR spectroscopy. In dilute aqueous solution, these polymers act as unimolecular containers that can be loaded with up to 27 wt % hydrophobic guest molecules.

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Cylindrical brush molecules adsorbed on a surface change their contour length and then switch conformation from rodlike to globular upon decrease of the surface energy of the substrate. The conformational changes result from partial desorption of poly(n-butyl acrylate) side chains as the surface pressure drops from 23.7 to 3.1 mN/m and the energy of interaction between the side chains and the substrate decreases from 89.7 mJ/m2 to 69.1 mJ/m2. At the lowest value of the interaction energy, one observes a coexistence of rodlike and globular molecules. This result is in agreement with the theoretical prediction of the rod-globule transition of surface confined brush molecules as a conformational phase transition of the first order.