Adsorption-induced scission of carbon–carbon bonds

Abstract

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...

Thermal Stability of Polymers Confined in Graphite Oxide

“…These results suggest that either the thermal reduction of GO initiates the decomposition of the PEO chains intercalated in the GO interlayer, or PEO promotes the degradation of GO. In addition, the fact that PEO chains in PEO/GO are under considerable internal strain as a result of their extended planar zig-zag conformation, distinct from that in the bulk, 9,10 and that they are strongly interacting with GO sheets, it is expected that the increase of the molecular motion upon heating facilitates the rupture of the polymer backbone more than in the bulk polymer, 20 where the chains are free to slide one along another. However, ascribing the origin of the thermal instability to PEO or GO in the intercalate is not straightforward.…”

A Complete NCI Perspective: From New Bonds to Reactivity

“…Chemical interactions between a protein and a drug, or a catalyst and its substrate, self-assembly of nanomaterials, [29,30], and even some chemical reactions, [31,32] are dominated by non-covalent interactions. This class of interactions spans a wide range of binding energies, and encompasses hydrogen bonding, dipole-dipole interactions and London dispersion [33] as well as more up to date interactions such as halogen bonds, CH· · · π and π · · · π interactions.…”

Anti-Arrhenius cleavage of covalent bonds in bottlebrush macromolecules on substrate

“…mechanochemistry | molecular brushes | molecular imaging T he phenomenon of molecular "fatal adsorption," previously reported by us in 2006 (1), is a unique mechanochemical process attributed to spontaneous scission of covalent bonds in brush-like macromolecules upon their adsorption onto a substrate ( Fig. 1).…”

“…First, strong covalent bonds rupture spontaneously without applying any external force. Significant bond tension is generated within adsorbed macromolecules as they opt to rearrange their conformations in order to maximize the number of contacts between the side chains and the substrate (1,2). Second, the rate constant of the bond-scission reaction exhibits extraordinary sensitivity to minute variations of the surface energy (γ) of the underlying substrate (3).…”