Band 89, Heft 11 - 2016 IUPAC-SOLVAY International Award for Young Chemists

The synthesis of carbon nanotube (CNT) fragments has long captivated organic chemists, despite the simplistic, symmetric nature of the requisite achiral targets. Such molecules hold the potential to allow for the synthesis of homogeneous CNTs, rendering their properties more suitable for advanced applications in electronics and sensing. The [ n ]cycloparaphenylene family, comprised of molecules with para-linked phenyl rings in a contiguous macrocycles, represents a major landmark towards achieving absolute control of CNT architecture from the bottom-up. Attempts towards accessing the [ n ]cyclacene and [ n ]cyclophenacene families, both of which are comprised of double-stranded macrocyclic belts, have only recently been successful, however. These targets have been plagued by unstable, strained intermediates and stereochemical pitfalls that have largely thwarted accessing these fascinating structures. Herein, we disclose our synthetic strategy toward overcoming several stereochemical challenges en route to [ n ]cyclophenacenes via highly substituted [ n ]cycloparaphenylene precursors.

This review summarizes the history and recent developments of the field of organometallic chemical biology with a particular emphasis on the development of novel bioconjugation approaches. Over the years, numerous transformations have emerged for biomolecule modification with the use of organometallic reagents; these include [3+2] cycloadditions, C–C, C–S, C–N, and C–O bond forming processes, as well as metal-mediated deprotection (“decaging”) reactions. These conceptually new additions to the chemical biology toolkit highlight the potential of organometallic chemistry to make a significant impact in the field of chemical biology by providing further opportunities for the development of chemoselective, site-specific and spatially resolved methods for biomolecule structure and function manipulation. Examples of these transformations, as well as existing challenges and future prospects of this rapidly developing field are highlighted in this review.

Well-defined amphiphilic triblock poly(sodium methacrylate)-polystyrene-poly(sodium methacrylate) (PMAA- b -PS- b -PMAA) copolymers characterized by a different length of either the hydrophilic or the hydrophobic block have been synthesized by ATRP. In solution the micelle-like aggregates consist of a collapsed PS core surrounded by stretched charged PMAA chains. The micelles are kinetically ‘frozen’ and as a consequence the triblock copolymers do not show a significant surface activity. The hydrophilic block length has a major influence on the rheology, the shortest PMAA blocks yielding the strongest gels (at the same total weight concentration). The hydrophobic block length has only a minor influence until a certain threshold, below which the hydrophobic interactions are too weak resulting in weak gels. A mathematical model is used to describe the micelle radius and the results were in good agreement with the experimentally found radius in transmission electron microscopy. The influences of the ionic strength, pH and temperature on the rheology has also been investigated, showing the potential of these polymers as smart hydrogels. The change in conformation of the hydrophilic corona from the collapsed state to the stretched state by changing the pH was quantified with zeta-potential measurements. To the best of our knowledge, this is the first systematic investigation of this kind of triblock copolymers in terms of their rheological behavior in water.

Synthetic resins were introduced in paintings conservation during the 1930s, as an alternative to natural resins, due to their superior resistance to degradation. Their composition usually includes a small amount of additives, such as titanium dioxide. The objective of this work is to study the effect of TiO 2 additive on the durability of Paraloid B72 (acrylic resin) and Laropal A81 (urea-aldehyde condensation polymer), both used in art conservation, against photochemical degradation. A methodology involving separating particulate TiO 2 from the organic fraction of the resins has been applied, followed by accelerated ageing of the resins in their commercial (C) and modified (M, i.e. after TiO 2 removal) has been implemented. The morphological characteristics of resin films were examined through scanning electron microscopy (SEM). Chemical changes, colour properties and photo-chemical stability of the resins were studied with FTIR, UV-Vis absorption spectroscopy and spectro-colorimetry. The results showed a considerably different behaviour between the C and M states of both materials. In particular, C-Paraloid B72 collapses after prolonged irradiation, but within a certain time frame it appears to be relatively stable; on the other hand, C-Laropal A81 is considerably destabilized in comparison to its M state. It can be suggested that TiO 2 acts as a UV-blocker for the underlying pigment layers, at the expense of resins’ stability.

This work studies the polymerization process of four different commercially available silicon-based polymers and their consequent effect on surface tension and water transport properties of natural stones used in architectural monuments, essential for modeling and predicting the durability of natural stones against weathering action of aqueous solutions. The four products studied consisted of two ethyl-silicate based materials, an alkyl-alkoxy siloxane oligomer with hydrophobic agents and finally, a silane/siloxane emulsion. In all cases, the morphology of the amorphous material deposited into the pore network of stones was examined by electron microscopy (SEM). The polymerization process was studied by infrared spectroscopy (FTIR) on both inert surface and porous stone substrate. The treated stone specimens were further tested after polymerization in terms of determination and comparison of water absorption coefficient, open porosity, pore size distribution (mercury intrusion porosimetry) and surface tension (contact angle measurements) properties. According to the analysis, the modification of the wetting properties of the stone surface should be related rather with the chemistry than the microstructure of the xerogels alone.

The relative gas-phase stabilities of ring-substituted phenylboranylmethyl anions were computationally determined using isodesmic reactions. The energies of species included in the reactions were calculated at the B3LYP/6-311+G(2d,p) level of theory. The obtained substituent effects were analyzed by the extended Yukawa-Tsuno equation, and unexpectedly substantial r − (0.59) and s (0.65) values were found for the fully-optimized planar anion. The substantial through-resonance effect quantified by the r − value was observed, although it is not possible to draw a canonical form in which the negative charge is delocalized on the benzene ring. Substituent effects were also analyzed for the anions in which the dihedral angle (φ) between the side chain plane and the benzene ring was fixed. The r − value decreased significantly by changing the φ from 0° to 90°, while the s value changed little. NBO analyses revealed that the r − value is proportional to the sum of the π–π* and σ–π* orbital interactions between the side chain and the benzene ring. This fact shows that the through-resonance effect quantified by the r − value is present at all φ, and therefore, the anion cannot become an ideal σ 0 -reference system. The constant saturation effect quantified by the s value can be explained by the constant charge distributed to the benzene ring. The combination of substituent-effect analysis and NBO analysis successfully revealed the nature of the anion.

A list of constitutional units (CU) used for naming polymers according to IUPAC nomenclature rules is provided. This list contains CUs of commercial and well-known polymers, as well as CUs which have been used in polymer-nomenclature-related IUPAC documents. A preferred name is provided for each constitutional unit. Other acceptable names and names which must not be used because they are outdated or incorrect, although they may have been correct in the past or in a different context, are also listed. A second table contains names of common polymers: structure-based, source-based, and retained traditional polymer names, as well as names that are not acceptable, again because they are outdated or incorrect. These tables supersede similar tables and names in previous documents. The rules given in those documents are still valid, but names of constitutional units should be checked for agreement with the present document. The preferred names of constitutional units should be used in structure-based names of regular and irregular polymers.