Organic synthesis--art or science?

The LHASA rules for finding strategic bonds in polycyclic target structures are analyzed with respect to the following question: Do the strategic bonds tend to give the greatest simplification upon disconnection, as measured by recently introduced indices of molecular complexity? The answer is yes, at least for the more general rules. This result implies that the bonds most useful for retrosynthetic disconnection can now be identified by a simple calculation rather than by application of a body of rules. It is concluded that organic synthesis, as far as described by these rules, has a mathematical basis and consequently can be considered a science as well as an art.     

Polymerization with Fischer-Tropsch olefins – a new challenge in polymer science

Co- and ter-polymerization of ethylene with Fischer-Tropsch derived olefins having odd carbon number and branched olefins are investigated. A method of preparation of a robust, high productivity Ziegler-Natta catalyst suitable for ethylene co-polymerization with Fischer Tropsch olefin is reviewed. In supporting titanium tetrachloride, an attempt was made to control the titanium oxidation state. The resulting experimental data were fitted to a sub-sites model that associates titanium oxidation state with catalyst activity.     

The distribution of degree of crystallinity—New concept in polymer science

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Can glycoprofiling be helpful in detecting prostate cancer?

Glycans are chains of carbohydrates attached to proteins (glycoproteins and proteoglycans) or lipids (glycolipids). Glycosylation is a post-translational modification and glycans have a wide range of functions in the human body including involvement in oncological diseases. Change in a glycan structure can not only indicate the presence of a pathological process but, more importantly, in some cases also its stage. Thus, a glycan analysis has the potential to be an effective and reliable tool in cancer diagnostics. Lectins are proteins responsible for natural biorecognition of glycans; even carbohydrate moieties still attached to proteins or whole cells can be recognised by lectins, which makes them an ideal candidate for designing label-free biosensors for glycan analysis. This review seeks to summarise evidence that the glycoprofiling of biomarkers by lectin-based biosensors can be of significant help in detecting prostate cancer.     

Coarsening of the phase structure in immiscible polymer blends. Coalescence or ostwald ripening?

The results of theories of the Ostwald ripening and coalescence applied to molten quiescent polymer blends with dispersed phase structure were analyzed. From a comparison of predictions of the theories with available experimental results follows that coarsening of the phase structure in quiescent polymer blends with medium or high interfacial tension is induced by the coalescence. Both the mechanisms play a role in coarsening of the phase structure in blends with low interfacial tensions. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 181–187, 1999     

A metallopolymer case-history: polymer, ring or ligand reaction?

Multinuclear cobalt(II) complexes of ditopic ligands with alpha,omega-dioxy spacers linking two tpy metal-binding domains react with alcohols to generate mononuclear 4'-alkoxy-species.     

What is the unperturbed state in polymer solutions?

We report theoretical results about amphiphilic random copolymers in a quasi‐ideal conformation with an overall size very close to that of the analogue homopolymers. We found that a few states may coexist with about the same free energy and a similar radius of gyration, but with different intramolecular conformations. We also argue that, in most cases, amphiphilic copolymers may never achieve the unperturbed Θ state, defined thermodynamically by a vanishing second virial coefficient. Thus, we suggest that such copolymers usually show neither an unperturbed conformation nor an unperturbed state from the thermodynamic viewpoint. We also briefly discuss star homopolymers, which show a depression of the Θ temperature with respect to linear chains and a significant, though finite, Θ swelling, as well as linear chains in the Θ state and in the melt. The main general conclusion is that interactions between chain segments do not cancel each other and are non‐negligible. Accordingly, we suggest that the word "unperturbed" be used only with reference to solution thermodynamics and not for the chain size or conformation.     

Plant oils: The perfect renewable resource for polymer science?!

Already for a long time, plant oils and their derivatives have been used by polymer chemists due to their renewable nature, world wide availability, relatively low price, and their rich application possibilities. Although many different synthetic approaches have been used, more recent examples are pointing in the direction of catalytic transformations and other efficient reactions to achieve a more sustainable production of polymers from these renewable resources. In this context, olefin metathesis, thiol–ene additions, and other processes can contribute not only to a more efficient synthesis of plant oil based polymers, but also to broaden the application possibilities of plant oils. This feature article provides an overview of the present situation with special attention to the use of olefin metathesis and thiol–ene chemistry as synthetic methods and as polymerization techniques.     

Is there a higher-order mode coupling transition in polymer blends?

We present simulations on a binary blend of bead-spring polymer chains. The introduction of monomer size disparity yields very different relaxation times for each component of the blend. Competition between two different arrest mechanisms, namely, bulklike dynamics and confinement, leads to an anomalous relaxation scenario for the fast component, characterized by sublinear time dependence for mean squared displacements, or logarithmic decay and convex-to-concave crossover for density-density correlators. These anomalous dynamic features, which are observed over time intervals extending up to 4 decades, strongly resemble predictions of mode coupling theory for nearby higher-order transitions. Chain connectivity extends anomalous relaxation over a wide range of blend compositions.     

Anions or cations: who is in charge of inhibiting the nickel(II) promoted B- to Z-DNA transition?

Various weakly binding cations and anions were studied at a concentration of 10 mM to ascertain their interaction with the nickel(II) promoted B- to Z-DNA transition of poly d(GC). These salts were ranked according to the decreasing amounts of nickel needed for the B- to Z-DNA transition and provided the following order: NaCl approximately Me4NCl > LiCl > MgCl2 > no salt > NaBF4 approximately NaNO3 approximately NaClO4. Remarkably, it was found that going from sodium nitrate to sodium chloride increased the necessary amount of nickel to induce the transition to the left-handed helix of poly d(GC) by a factor of 10. This dramatic effect cannot be explained by the binding constant of nickel(II) to chloride to form the monocationic complex. We believe that this is the first reported example of the role of chloride anions, which appear to modulate the interaction of nickel(II) ions with the polyanionic DNA.     

Welding dynamics in an atomistic model of an amorphous polymer blend with polymer–polymer interface

We consider an atomistic model of thermal welding at the polymer-polymer interface of a polyetherimide/polycarbonate blend, motivated by applications to 3D manufacturing in space. We follow diffusion of semiflexible chains at the interface and analyze strengthening of the samples as a function of the welding time t_w by simulating the strain–stress and shear viscosity curves. The time scales for initial wetting, and for fast and slow diffusion, are revealed. It is shown that each component of the polymer blend has its own characteristic time of slow diffusion at the interface. Analysis of strain–stress demonstrates saturation of the Young's modulus at t_w = 240 ns, while the tensile strength continues to increase. The shear viscosity is found to have a very weak dependence on the welding time for t_w > 60 ns. It is shown that both strain–stress and shear viscosity curves agree with experimental data.     

Enhancement of the photoluminescence of CdSe quantum dots during long-term UV-irradiation: privilege or fault in life science research?

The present study describes an impressive enhancement of the photoluminescence (PL) intensity of low-temperature synthesized CdSe nanocrystals (75 degrees C) during long-term UV-irradiation. The integrated PL-intensity of CdSe core and CdSe/ZnS core/shell nanocrystals, dispersed in chloroform, enhanced about 3 and 6 times, respectively, during 9 h exposure to UV-light, without any significant changes in the characteristic absorbance spectra and shifting of PL-spectra. After termination of the irradiation a comparatively slow photobleaching was detected with tau(1/2) = 6 h for CdSe core and tau(1/2) = 14 h for CdSe/ZnS core/shell nanocrystals. The most impressive was the effect of UV-irradiation on the photoluminescence of water-soluble CdSe nanocrystals. The integrated PL-intensity enhanced about 10 times during 11 h exposure to UV-light and the improved PL-intensity was preserved during 3 days after termination of the irradiation without any significant photobleaching. The results are discussed in the context of application of CdSe nanocrystals as novel fluorophores in life science experiments.     

Carbene or zwitterion? Competition in organoplatinum complexes

The reaction of a known dimeric dicarbene complex of platinum with a number of ligands results in four new platinum complexes. The structure of the new complexes is described: one complex must exist as a neutral complex with no charge separation, and the other three are assigned a charge-separated (zwitterionic) structure, rather than a carbene form, on the basis of comparative 13C NMR shifts.     

How accurately do mechanophores report on bond scission in soft polymer materials?

Mechanophores are molecules that are incorporated into a host material and react to the local mechanical condition—the state of stress or strain—of that host material. Among their many purposes is that of a reporter: Mechanophores whose optical activity changes in response to mechanical cues can reveal bulk material processes that are ordinarily hidden, such as fatigue and fracture. Moreover, they may do so well before a material is fully fractured. To extract quantitative information from the optical signals from embedded mechanophores it is important that the mechanophores, which are generally a minority component of the material, report proportionally and unambiguously on the mechanical condition of the bulk. This is particularly important for early reporting of damage and wear, for which the optical signal from the mechanophore should accurately reflect bulk bond scission. In this article, we develop and analyze a general theory for the quality of optical mechanoreporting by mechanophores in soft materials, based on the Bell-Evans theory of bond breaking. We find, that at the typical low fractions in which mechanophores are incorporated the overall change in strength is limited, but that the proportionality of the reporting can be off by significant amounts, particularly at short times after loading but, for non-scissile bonds, at long times as well.     

Percolation phenomena in diffusion-controlled polymer matrix systems

The controlled release of two kinds of drugs,5-fluorouracil (5-FU) and hydrocortisonum (Hydro.) loaded in poly(ethylene-vinylalcohol) (EVAL) was dealt with,of which 5-FU/EVAL and Hydro /EVAL matrix systems are composed.The results were analyzed using the pseudo-steady-diffusion models coupled with the fundamental concepts of percolation theory.The percolation thresholds for the two systems were calculated,which could indicate the contributions of pore diffusion and matrix diffusion.     

How does solvent molecular size affect the microscopic structure in polymer solutions?

Monte Carlo simulation has been used to investigate the effects of linear solvent molecular size on polymer chain conformation in solutions. Increasing the solvent molecular size leads to shrinkage of the polymer chains and increase of the critical overlap concentrations. The root-mean-square radius of gyration of polymer chains (R(g)) is less sensitive to the variation of polymer concentration in solutions of larger solvent molecules. In addition, the dependency of R(g) on polymer concentration under normal solvent conditions and solvent molecular size is in good agreement with scaling laws. When the solvent molecular size approaches the ideal end-to-end distance of the polymer chain, an extra aggregation of polymer chains occurs, and the solvent becomes the so-called medium-sized solvent. When the size of solvent molecules is smaller than the medium size, the polymer chains are swollen or partially swollen. However, when the size of solvent molecules is larger than the medium size, the polymer coils shrink and segregate, enwrapped by the large solvent molecules.     

Which molecularly imprinted polymer is better?

Molecularly imprinted polymers (MIP) have been successfully synthesized toward many different compounds in the last decades. The mechanistic details of selective binding at binding sites are not yet well understood. For this reason the characterization of MIP binding has been mostly phenomenological and this makes the transfer of results between different laboratories or between different types of applications difficult. In this paper we analyze the relationship between different types of characterization like isotherms, binding site models, chromatographic k and α values, etc. as they relate to different applications like HPLC, solid phase extraction (SPE), binding assays, batch extraction and sensors. It is shown that α values determined by elution chromatography depend on seemingly irrelevant factors as the length and diameter of the column, respectively. The determination of distribution ratios or partition coefficients is proposed as an easily understandable and useful quantity in the characterization of novel MIPs. Data used for the characterization of a MIP should be transferable between different applications but the qualification of MIPs as better or worse will depend on the application in case.