373 research outputs found
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Catechol-based macrocyclic aromatic ether-sulfones: Synthesis, characterization and ring-opening polymerization
No full textCyclocondensation between 4,4`-bis(4-chlorobenzenesulfonyl)biphenyl and catechol, with subsequent chromatographic separation of the reaction products, led to the isolation of four novel ether-sulfone macrocycles (cyclic dimer, -trimer, -tetramer and -pentamer). Similarly, cyclocondensation of catechol with a novel seven-ring diketone/disulfone monomer allowed the isolation of the two new aromatic ether-ketone-sulfone macrocycles, a cyclic monomer and a cyclic dimer. Transannular shielding and deshielding effects in the cyclic monomer produce substantial chemical shift differences for chemically equivalent protons in the 1H NMR spectra of the cyclic monomer and -dimer. Fluoride-initiated ring-opening polymerization of the ether-sulfone cyclic trimer affords a novel, high-molecular weight poly(ether-sulfone)
Synthesis and solution-state dynamics of donor-acceptor oligorotaxane foldamers
No full textWe describe in detail a strategy for creating foldamers in which interactions between mechanically interlocked components dictate the single-molecule assembly of a folded secondary structure. This unique folding motif is based on a flexible polyether dumbbell bearing 1,5-dioxynaphthalene (DNP) donors, which folds its way through a series of cyclobis(paraquat-p-phenylene) (CBPQT(4+)) acceptor rings in a serpentine fashion to enable extended donor-acceptor (D-A) stacking between DNP and the electron-poor 4,4'-bipyridinium (BIPY2+) units in CBPQT(4+). These oligorotaxanes can be prepared in a wide range of sizes, with molecular weights up to >15 000 Da, on account of novel one-pot reactions we developed to generate the necessary oligo-DNP precursors. The product distributions from the final kinetically controlled stoppering reactions are highly biased towards oligorotaxanes in which approximately half of the DNP units are encircled by rings, a fact which can be rationalized if the dominant solution-state structures of the pseudorotaxane precursors reflect the solid-state superstructures of analogous compounds, which express 50% recognition site occupancy because of their proclivity to pack into continuous D-A-D-A stacks. The presence of well-defined folded structures in solution have been confirmed by H-1 NMR spectroscopy in CD3CN. Moreover, we discovered an empirical selection rule forbidding CBPQT(4+) rings to occupy adjacent DNP sites, which elegantly explains both the product distributions and the H-1 NMR spectra. Depending on their adherence to this selection rule, all of the oligorotaxanes belong to one of three families: whereas 'Confused' oligorotaxanes adopt multiple translational isomers that satisfy the rule and 'Frustrated' species cannot obey it at all, members of the 'Happy' family each express only one rule-compliant 'Goldilocks' isomer. The NMR spectra of these oligorotaxanes also shed light on their dynamics; rapid 180 degrees rotations of DNP units cause pairs of heterotopic BIPY2+ protons in the accompanying CBPQT(4+) rings to exchange sites, giving rise to time-averaged signals. This process, which we term 'superrotation', will apply much more generally to other mechanically interlocked systems
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Sequence-modification in copoly(ester-imide)s: a catalytic/supramolecular approach to the evolution and reading of copolymer sequence-information
No full textCatalytic ester-interchange reactions, analogous to mutation and recombination, allow new sequence-information to be written, statistically, into NDI-based poly(ester-imide) chains. Thus, both insertion of the cyclic ester cyclopentadecanolide ("exaltolide") into an NDI-based homopolymer, and quantitative sequence-exchange between two different homopoly(ester-imide)s, are catalysed by di-n-butyl tin(IV) oxide. Emerging sequences are identified at the triplet and quintet levels using supramolecular complexation of pyrene-d10 at the NDI residues to amplify the separation of 1H NMR resonances associated with different sequences. In such systems, pyrene is able to act as a "reader-molecule" by generating different levels of ring-current shielding from the different patterns of supramolecular binding to all the NDI-centred sequences of a given length
Jüri Okas’ ‘specific objects’: diverging discourses in Estonian Art in the 1970s.
No full textPreviously in the University eprints HAIRST pilot service at http://eprints.st-andrews.ac.uk/archive/00000367/Article 3 of 6 in issue devoted to the visual culture of the Scandinavian and Baltic region.This article will look at the early works of Estonian architect and artist Jüri Okas and will try to work between diverging languages and interpretations, reading works by Okas against the background of Anglo-american conceptualism and minimalism of the same period. The first part of the paper will analyse a print by Jüri Okas that paraphrases works by the American artist Donald Judd and will try to show how Okas’ concept of minimalism differed from the Western one and the reasons behind it. The second part of the paper will focus on a conceptual book by Jüri Okas, consisting of a series of photographs of everyday and banal architectural objects, and compare it to Rober Venturi’s book on Complexity and Contradiction in Architecture. Finally, a comparison will be made with works of Robert Smithson in the context of concepts of waste, excess and the remainders of industrial civilisationPostprin
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Big and little Meccano
No full textThe emergence of the mechanical bond during the past 25 years is giving chemistry a fillip in more ways than one. While its arrival on the scene is already impacting materials science and molecular nanotechnology, it is providing a new lease of life to chemical synthesis where mechanical bond formation Occurs as a consequence of the all-important templation Orchestrated by molecular recognition and self-assembly. The way in which covalent bond formation activates noncovalent bonding interactions, switching on molecular recognition that leads to self-assembly, and the template-directed synthesis of mechanically interlocked molecules-of which the so-called catenanes and rotaxanes may be regarded as the prototypes-has introduced a level of integration into chemical synthesis that has not previously been attained jointly at the supramolecular and molecular levels. The challenge now is to carry this I vel of integration during molecular synthesis beyond relatively small molecules into the realms of precisely functionalized extended molecular Structures and superstructures that perform functions in a collective manner as the key sources of instruction, activation, and performance in multi-component integrated Circuits and devices. These forays into organic chemistry by a scientific nomad are traced through thick and thin from the Athens of the North to the Windy City by Lake Michigan with interludes on the edge of the Canadian Shield beside Lake Ontario, in the Socialist Republic of South Yorkshire, on the Plains of Cheshire beside the Wirral, in the Midlands in the Heartland of Albion, and in the City of Angels beside the Peaceful Sea. (C) 2008 Elsevier Ltd. All rights reserved
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Recognition of polyimide sequence information by a molecular tweezer
No full textSpecific monomer sequences in aromatic copolyimides are recognized through their -stacking and hydrogen-bonding interactions with a sterically and electronically complementary molecular tweezer. These interactions enable the tweezer molecule to read monomer sequences comprising up to 27 aromatic rings by multiple adjacent binding to neighboring sites on the polymer chain
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Recent work on entropically-driven ring-opening polymerizations: some potential applications
No full textThe entropically-driven ring-opening polymerization of macrocyclic monomers (> ca. 14 ring atoms per repeat unit) and/or macrocyclic oligomers is a relatively new method of polymer synthesis that exploits the well-known phenomenon of ring-chain equilibria. It attracts interest because of its novel features. For example, these ring-opening polymerizations emit no volatiles and little or no heat. This review considers the principles of entropically-driven ring-opening polymerizations, gives selected examples and discusses potential applications. The latter include micromolding, high throughput syntheses and the synthesis of supramolecular polymers. Copyright (c) 2005 John Wiley T Sons, Ltd
Second-sphere co-ordination of neutral and cationic transition metal complexes by crown ethers
No full textThe [ammine(η5-cyclopentadienyl)dicarbonyliron(II)] cation and the neutral complex, amminepentacarbonyltungsten(0), form 1:1 adducts in solution with 18-crown-6 and dibenzo-18-crown-6 by virtue of hydrogen bonding between the ammine ligand in the transition metal complex and oxygen atoms of the crown ether; a crystalline adduct, [W(CO)5NH3.dibenzo- 18-crown-6], has been isolated as a benzene solvate.link_to_subscribed_fulltex
Bentham on Preventive Police: The Calendar of Delinquency in Evaluation of Policy and the Police Gazette in Manipulation of Opinion
No full textIn 1798–1799 Bentham lent his services to Patrick Colquhoun in drafting Bills to regularize the new Thames Police Office and to establish a Central Board of Police. While recognizing Colquhoun as the ‘author of the system’, Bentham brought his own utilitarian philosophy to bear on the task, and his arguments shed light on the twin role of licensing in both providing the finance necessary for an expansion of police, and in generating a flow of information for use in deterring, detecting and apprehending criminals. The Police Gazette and the Calendar of Delinquency were to be official publications of the proposed Board of Police, which combined the promulgation of information (thus increasing public knowledge through the understanding) with the effort to mould public opinion (thus influencing the will). Dissemination of facts provided grist to the existing moral sanction’s mill, and facilitated co-operation between the people and the agents of penal law. In reaction to anxiety about contagion from revolutionary France, Bentham also explicitly seeks to guide and direct public opinion, thus connecting his police writings directly with the esoteric elements of indirect legislation
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