Exploring the Reversibility of the Ring-Closing Metathesis Mediated Cross-linking of Dendrimers

Intramolecularly Cross-Linked Polymers: From Structure to Function with Applications as Artificial Antibodies and Artificial Enzymes

Cross-linking of polymers significantly alters their physical properties, greatly expanding their everyday utility. Indeed, the polymeric networks resulting from linkages between polymer chains are found in everyday materials from soft contact lenses and automobile tires to enamel coatings and high-performance adhesives. In contrast, intramolecularly cross-linked polymers have received far less attention until recent years, in large part because they are synthetically more challenging to prepare. In this Account, we trace our own efforts to develop the chemistry of intramolecularly cross-linked macromolecules, starting with dendrimers. Dendrimers provided an excellent starting point for investigating intramolecular cross-linking because they are single molecular entities. We showed that the end groups of dendrimers can be extensively cross-linked using the ring-closing metathesis reaction and that the discrete structure of the dendrimer provides unique opportunities for characterizing the number and location of the cross-links as well as some physical properties of the macromolecule such as its size and rigidity. Increasing the number of ring-closing metathesis reactions correlated with a reduction in size and an increase in rigidity. The general strategy applied to dendrimers was extended to star polymers and hyperbranched polyglycerols. Each of these macromolecules has a core or an initiating group from which the branches emanate. Linking the end groups or branches of these polymers presents a unique opportunity to chemically remove the core of the cross-linked macromolecule in a process that is reminiscent of that used to produce covalent molecular imprinted polymers. Recognizing this analogy, we sought a compelling application for cross-linked dendrimers, the first example of unimolecular imprinting, where a single polymer contains a single molecular imprint. The quality of the imprinting was mixed but pointed to an alternative general strategy for molecular imprinting in polymers. The effort also focused attention on synthetic antibodies and the general biomimicry provided by this class of macromolecules. Indeed, cross-linking of polymers either covalently or non-covalently bears a loose resemblance to folding of proteins into defined three-dimensional shapes. The synthesis and study of cross-linked linear polymers, often called single-chain nanoparticles (SCNPs), has emerged as a very active area of research in the past few years. Our experience with the cross-linking of branched polymers combined with an interest in performing organic synthesis within living cells led us to develop copper-containing SCNPs as artificial clickases. These polymeric clickases exhibit all of the hallmarks of enzymatic catalysis. One clickase containing a polyacrylamide backbone performs low-concentration copper-assisted alkyne-azide click reactions at unprecedented rates. Another performs click reactions within living cells. Other organic transformations can be performed intracellularly, and some of the most advanced SCNPs engage in concurrent and tandem catalysis with a naturally occurring biocatalyst. By tracing our own efforts, this Account provides a few entry points into the broader literature and also points to both the remaining challenges and overall promising future envisioned for this unique class of functional macromolecules.

Cross-linked ROMP polymers based on odourless dicyclopentadiene derivatives

Hydroxydicyclopentadiene (DCPD-OH) and some ester and ether derivatives were synthesized and used for the first time as ring-opening metathesis polymerization (ROMP) monomers to create cross-linked thermoset polymers with Ru-catalysts.

Photo-crosslinking of a self-assembled coumarin-dipeptide hydrogel

The photo-crosslinking of a coumarin-functionalized dipeptide hydrogel enhances the stability of the self-assembled nanofibers that comprise the hydrogel.

Molecular imprinting science and technology: a survey of the literature for the years 2004-2011

Herein, we present a survey of the literature covering the development of molecular imprinting science and technology over the years 2004-2011. In total, 3779 references to the original papers, reviews, edited volumes and monographs from this period are included, along with recently identified uncited materials from prior to 2004, which were omitted in the first instalment of this series covering the years 1930-2003. In the presentation of the assembled references, a section presenting reviews and monographs covering the area is followed by sections describing fundamental aspects of molecular imprinting including the development of novel polymer formats. Thereafter, literature describing efforts to apply these polymeric materials to a range of application areas is presented. Current trends and areas of rapid development are discussed.

Molecular Chemistry to the Fore: New Insights into the Fascinating World of Photoactive Colloidal Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals possess unique properties that are unmatched by other chromophores such as organic dyes or transition-metal complexes. These versatile building blocks have generated much scientific interest and found applications in bioimaging, tracking, lighting, lasing, photovoltaics, photocatalysis, thermoelectrics, and spintronics. Despite these advances, important challenges remain, notably how to produce semiconductor nanostructures with predetermined architecture, how to produce metastable semiconductor nanostructures that are hard to isolate by conventional syntheses, and how to control the degree of surface loading or valence per nanocrystal. Molecular chemists are very familiar with these issues and can use their expertise to help solve these challenges. In this Perspective, we present our group's recent work on bottom-up molecular control of nanoscale composition and morphology, low-temperature photochemical routes to semiconductor heterostructures and metastable phases, solar-to-chemical energy conversion with semiconductor-based photocatalysts, and controlled surface modification of colloidal semiconductors that bypasses ligand exchange.

Self-assembly and electrochemical oxidation of polyamidoamine-carbazole dendron surfmer complexes: nanoring formation

We report a detailed and quantitative study on the supramolecular complexation of amine-functionalized polyamidoamine (PAMAM) dendrimer G(4)-NH(2) with carboxylic acid terminal dendrons containing peripheral electroactive carbazole groups of different generations (G(0)COOH, G(1)COOH, and G(2)COOH). While the focus is on a detailed understanding and mechanism of complex formation, subsequent electrochemical oxidation of the dendron surfmers resulted in the formation of nanoring structures electrodeposited on the conducting substrate. Complexation was confirmed by NMR, UV-vis, and IR measurements. Critical micelle concentration (CMC), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) studies revealed that the ringlike structures were formed during the equilibrium-decomplexation stage and that the electrochemical process did not destroy the complex but rather stabilized it. The different generations of the dendrons provided various structures and complex formation efficacy. This type of template polymerization combined with electrochemically anodic oxidation has not been previously reported.