Molecular Bottlebrushes

The amplified tension caused by adsorption of side chains can lead to the scission of covalent bonds in the backbone once it exceeds their strength. We have shown that the C-C bonds in poly(2-hydroxyethyl methacrylate) backbone with long poly(n-butyl acrylate) (PBA) side chains mptured spontaneously uprm adsorption onto an aqueous substrate (Fig. 9) [126]. The kinetics of backbone scission was followed by monitoring the average contour length by AFM as a 

10 Site-specific activation, (a) Mid-chain scission of molecular bottlebrushes occurs selectively at the disulfide linkage, (b) In a short bottlebrush, tension is focused to the middle of the backbone, resulting in its mid-chain scission, (c) By focusing txmd tension at the branching center, molecular stars with bottlebrush arms undergo sequential scissimi of covalent bonds upon adsorption onto an aqueous substrate 

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As in externally induced deformation, we should distinguish between strain-controlled and stress-controlled molecular systems. For example, deformation induced by a well-defined conformational transition (Fig. lb, bottom) can be considered as strain-controlled, where the strain is encoded by the new molecular configuration. In contrast, extension of the backbone in molecular bottlebrushes (Fig. Ic, middle) occurs at a constant tensile force controlled by steric repulsion of the densely grafted side chains. In addition to the strain distribution, it is important... 

Li Y, Nese A, Hu X, Lebedeva NV, LaJoie TW, Burdyhska J, Stefan MC, You W, Yang W, Matyjaszewski K, Sheiko SS (2014) Shifting electronic structure by inherent tension in molecular bottlebrushes with polythiophene backbones. ACS Marco Lett 3 738... 

S-K A, Pickel DL, Kochemba WM, Chen J, Uhrig D, Hinestrosa JP, Carrillo J-M, Shao M, Do C, Messman JM, Brown WM, Sumpter BG, Kilbey SM n (2013) Poly(3-hexylthiophene) molecular bottlebrushes via ring-opening metathesis polymerization macromolecular architecture enhanced aggregation. ACS Macro Lett 2 761-765... 

Shi Y, Zhu W, Yao D, Long M, Peng B, Zhang K, Chen Y (2014) Disk-like micelles with a highly ordered pattern from molecular bottlebrushes. ACS Macro Lett 3 70-73... 

When a linear polymer is grafted with a large number of much shorter side chains, cylindrical polymer brushes are formed [33, 108-111]. They are also denoted as bottlebrushes or molecular brushes. Although most cylindrical polymer brushes contain linear side chains, dendritic or even hyperbranched space demanding grafts can also render cylindrical shapes, which leads to the so-called dendronized [112-116] andhypergrafted[117, 118] polymers, respectively. In this review, we will focus on cylindrical brushes with linear side chains. Due to their anisotropic nature in topology, they have attracted more and more research interest in their synthesis, bulk, or solution properties, as well as applications. 

Fig. 8 (a) The backbone tension in molecular bottlebmshes depends on side chain length and solvent quality, (b) The backbone of an adsorbed bottlebrush is pulled by side chains with a force of/sc, resulting in a linear increase of force along the backbone at the end-cups of the bottlebrush as / = Sx, where S is the spreading coefficient. In the transmission zone, the force along the backbone is constant f = Sd... 

Third, molecular branched architecture can be tuned to ensure maximum concentration of mechanical tension at a specific chemical bond. For example, star-like [134] and pom-pom structures [125] as well as short bottlebrushes [135] may be used to introduce site-specific activation at the branching center. Figure 10b displays the concentration of tension in the middle of the backbone resulting in its mid-chain scission. In a similar fashion (Fig. 10c), molecular stars with a spoke wheel core and bottlebrush arms exhibit preferential dissociation of the arms followed by the scission of covalent bonds in the bottlebrush backbones... 

Burdynska J, Li Y, Aggarwal AV, Hoger S, Sheiko SS, Matyjaszewski K (2014) Synthesis and arm dissociation in molecular stars with a spoked wheel core and bottlebrush arms. J Am Chem Soc 136 12762... 

Bottlebrush polymers contain a long flexible macromolecule as a backbone to which side chains, which may also be flexible, are grafted [59, 60]. The idea then is that via suitable choices of parameters such as the grafting density, solvent quality, and side chain molecular weight the local stiffness of this cylindrical molecular brush can be controlled. The qualitative pictine one draws is that of a wormlike... 

Russano, D., Carrillo, J.-M.Y., Dobrynin, A.V. Interaction between brush layers of bottlebrush polyelectrolytes molecular dynamics simulations. Langmuir 27, 11044—11051 (2011). doi 10.1021/la2018067... 

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