A. Dieter Schlüter

Two‐Dimensional Polymers: Just a Dream of Synthetic Chemists?

In light of the considerable impact synthetic 2D polymers are expected to have on many fundamental and applied aspects of the natural and engineering sciences, it is surprising that little research has been carried out on these intriguing macromolecules. Although numerous approaches have been reported over the last several decades, the synthesis of a one monomer unit thick, covalently bonded molecular sheet with a long-range ordered (periodic) internal structure has yet to be achieved. This Review provides an overview of these approaches and an analysis of how to synthesize 2D polymers. This analysis compares polymerizations in (initially) a homogeneous phase with those at interfaces and considers structural aspects of monomers as well as possibly preferred connection modes. It also addresses issues such as shrinkage as well as domain and crack formation, and briefly touches upon how the chances for a successful structural analysis of the final product can possibly be increased.

A two-dimensional polymer prepared by organic synthesis

Synthetic polymers are widely used materials, as attested by a production of more than 200 millions of tons per year, and are typically composed of linear repeat units. They may also be branched or irregularly crosslinked. Here, we introduce a two-dimensional polymer with internal periodicity composed of areal repeat units. This is an extension of Staudingers polymerization concept (to form macromolecules by covalently linking repeat units together), but in two dimensions. A well-known example of such a two-dimensional polymer is graphene, but its thermolytic synthesis precludes molecular design on demand. Here, we have rationally synthesized an ordered, non-equilibrium two-dimensional polymer far beyond molecular dimensions. The procedure includes the crystallization of a specifically designed photoreactive monomer into a layered structure, a photo-polymerization step within the crystal and a solvent-induced delamination step that isolates individual two-dimensional polymers as free-standing, monolayered molecular sheets.

Shape-Persistent, Nano-Sized Macrocycles

This Microreview concentrates on synthetic issues regarding shape-persistent cycles whose diameters range from a bit more than 1 nm up to the presently largest representative with approximately 5 nm. Its goal is to provide an overall feasibility picture and give those, who want to enter this field, some guidelines. An unambiguous structural characterization of these huge molecules is not always an easy task and some related problems will therefore also be discussed. The Microreview is mainly concerned with cycle formation itself and not so much precursor synthesis. Additionally, it places some emphasis on what has been achieved with these intriguing compounds applicationwise and what future they may have. Because of the wealth of material available, a few, clearly stated, yet to some extent artifical restrictions in the selection of cycles had to be applied. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Gram-scale synthesis of two-dimensional polymer crystals and their structure analysis by X-ray diffraction

The rise of graphene, a natural two-dimensional polymer (2DP) with topologically planar repeat units, has challenged synthetic chemistry, and has highlighted that accessing equivalent covalently bonded sheet-like macromolecules has, until recently, not been achieved. Here we show that non-centrosymmetric, enantiomorphic single crystals of a simple-to-make monomer can be photochemically converted into chiral 2DP crystals and cleanly reversed back to the monomer. X-ray diffraction established unequivocal structural proof for this synthetic 2DP, which has an all-carbon scaffold and can be synthesized on the gram scale. The monomer crystals are highly robust, can be easily grown to sizes greater than 1 mm and the resulting 2DP crystals exfoliated into nanometre-thin sheets. This unique combination of features suggests that these 2DPs could find use in membranes and nonlinear optics.

Synthesis of Free‐Standing, Monolayered Organometallic Sheets at the Air/Water Interface

The recent discovery of how to obtain and utilize individual layers of graphite, that is, graphene, and other inorganic monolayered sheets has turned the spotlight brighter than ever on a basically ignored field of chemistry, the rational synthesis of two-dimensional polymers. While there have been numerous reports on the synthesis of monolayered polymer films with irregularly networked internal structures since the pioneering work by Gee in 1935, little is known to date about the synthesis of a free-standing, 2D network with an ordered internal structure. This paucity is contrasted by the richness of fragments of such networks that were obtained by approaches such as iterative organic synthesis, selfassembly, or on-surface polymerization. As of now, the lateral dimensions of these fragments are too small to expect sheet-like properties; furthermore, they cannot yet be isolated and manipulated. Considering the huge application potential for structurally well-defined, free-standing 2D networks, which ranges from ultrasensitive membranes, molecular sieves, and devices based on high charge carrier mobility to materials with outstanding mechanical strength and the like, we felt the need to establish a synthesis program aiming at filling the gap between the one-dimensional (linear synthetic and biological polymers, carbon nanotubes, etc.) and the three-dimensional structures (hyperbranched and cross-linked bulk polymers, laminar crystals such as graphite, diamond, etc.) by providing access to structurally defined 2D polymers. Herein we report the synthesis of a free-standing monolayer sheet consisting of the hexafunctional terpyridine (tpy)-based D6h-symmetric monomer 1 (Figure 1) which was designed for the present purpose 12] and is to be held together by metal ion complexes between ideally all six tpy units of one monomer with one of the tpy units of each of the six neighboring monomers. This mode of polymerization is in principle reversible and could allow for dynamic bond formation. It has often been used for construction of complex but well-defined compounds as well as supramolecular assemblies. 15, 16] Figure 1 shows a targeted network. To avoid any three-dimensional growth of the coordination network during polymerization, monomer 1 was confined to two dimensions prior to polymerization by spreading it at the air/water interface on a Langmuir–Blodgett (LB) trough. The main advantages of using the air/water interface for the present synthesis instead of solid substrates include 1) the flat and uniform surface on a large length scale, 2) the availability of the water subphase as a pool of reagents and catalysts, 3) the straightforward preparation and facile isolation of single sheets by transfer onto solid substrates and supports of all sorts, 20] 4) the possibility to preset the lateral surface pressure and lateral concentration of monomers prior to the polymerization, and 5) the ease in performing polymerization under ambient conditions. A sub-monolayer of monomer 1 was spread at the air/ water interface from chloroform solution and compressed to a pressure of 30 mNm . The compression process was monitored by Brewster angle microscopy up to 10 mNm 1 and found to be fully reversible (Figure S1 in the Supporting Information) and to provide thin layers that are homogenous at the resolution of micrometers (Figure S2 in the Supporting Information). The point of inflection of the corresponding surface pressure–area isotherm (Figure 2a) was observed at a pressure of approximately 10 mNm , from which a mean molecular area of approximately 520 2 is estimated. This preliminary value is in good agreement with the formation of a dense monolayer in which the monomers lie flat on the interface. This arrangement was supported by AFM contactmode scratching and tapping-mode imaging experiments after vertical transfer of the compressed monolayer (at 10 mNm ) onto a mica substrate. Figure 2 b shows the scratched area and the corresponding height profile, providing the apparent height happ 0.8 nm. While happ values obtained by ambient-condition AFM are known to not accurately reflect real heights, the value of approximately 0.8 nm nevertheless suggests a monolayer. Not only is it in a reasonable range for a conjugated structure, parts of which may significantly deviate from coplanarity, but also a [*] T. Bauer, Dr. Z. Zheng, Dr. J. Sakamoto, Prof. A. D. Schl ter Department of Materials, Institute of Polymers Swiss Federal Institute of Technology, ETH Z rich HCI J 541, 8093 Z rich (Switzerland) E-mail: schlueter@mat.ethz.ch sakamoto@mat.ethz.ch

Tuning Polymer Thickness: Synthesis and Scaling Theory of Homologous Series of Dendronized Polymers

The thickness of dendronized polymers can be tuned by varying their generation g and the dendron functionality X. Systematic studies of this effect require (i) synthetic ability to produce large samples of high quality polymers with systematic variation of g, X and of the backbone polymerization degree N, (ii) a theoretical model relating the solvent swollen polymer diameter, r, and persistence length, lambda, to g and X. This article presents an optimized synthetic method and a simple theoretical model. Our theory approach, based on the Boris-Rubinstein model of dendrimers predicts r approximately n(1/4)g(1/2) and lambda approximately n(2) where n = [(X - 1)(g) - 1]/(X - 2) is the number of monomers in a dendron. The average monomer concentration in the branched side chains of a dendronized polymer increases with g in qualitative contrast to bottle brushes whose side chains are linear. The stepwise, attach-to, synthesis of X = 3 dendronized polymers yielded gram amounts of g = 1-4 polymers with N approximately = 1000 and N approximately = 7000 as compared to earlier maxima of 0.1 g amounts and of N approximately = 1000. The method can be modified to dendrons of different X. The conversion fraction at each attach-to step, as quantified by converting unreacted groups with UV labels, was 99.3% to 99.8%. Atomic force microscopy on mixed polymer samples allows to distinguish between chains of different g and suggests an apparent height difference of 0.85 nm per generation as well as an increase of persistence length with g. We suggest synthetic directions to allow confrontation with theory.

A Two-Dimensional Polymer from the Anthracene Dimer and Triptycene Motifs

A two-dimensional polymer (2DP) based on the dimerization of anthraceno groups arranged in a triptycene motif is reported. A photoinduced polymerization is performed in the crystalline state and gives a lamellar 2DP via a crystal-to-crystal (but not single-crystal to single-crystal) transformation. Solvent-induced exfoliation provides monolayer sheets of the 2DP. The 2DP is considered to be a tiling, a mathematical approach that facilitates structural elucidation.

The Largest Synthetic Structure with Molecular Precision: Towards a Molecular Object

Pushing the limits: A 200A - 10 Da structurally defined, linear macromolecule (PG5) has a molar mass, cross-section dimension, and cylindrical shape that are comparable to some naturally occurring objects, such as amyloid fibrils or certain plant viruses. The macromolecule is resistant against flattening out on a surface; the picture shows PG5 embracing the tobacco mosaic virus (TMV).

Synthesis of a covalent monolayer sheet by photochemical anthracene dimerization at the air/water interface and its mechanical characterization by AFM indentation

Covalent monolayer sheets in 2 hours: spreading of threefold anthracene-equipped shape-persistent and amphiphilic monomers at the air/water interface followed by a short photochemical treatment provides access to infinitely sized, strictly monolayered, covalent sheets with in-plane elastic modulus in the range of 19 N/m.

Thermoresponsive dendronized polymers with tunable lower critical solution temperatures

A series of first (PG 1) and second generation (PG 2) dendronized polymers were synthesized which exhibit fast and sharp phase transitions with negligible hystereses in aqueous solutions and apparent lower critical solution temperatures (LCSTs) in the range of 33-49 degrees C.