Gelation-crystallization in isotactic polystyrene solutions and its implications to crystal morphology,to the origin and structure of gels,and to the chemical homogeneity of polyolefins

As part of a wider study on the crystallization of isotactic polystyrene solutions it was observed that at sufficient concentrations (> 3–5%) gelation sets in below a certain (very high) supercooling in competition with the usual single crystal formation which in itself produces turbid suspensions. It was established that gelation is a form of crystallization (mode A) which must be of fringed micellar type to provide the connectedness as opposed to the chain folded lamellae (mode B) which gives rise to discrete particles. The gel crystals (A) display sharp melt endotherms and produce distinct x-ray diffraction patterns both of which, however, differ decisively from those provided by crystals B, a distinction which can be preserved even after removal of the solvent. The melting points of A are significantly lower than those of B and the x-ray diffraction patterns of A are incompatible with the recognized structure of polystyrene (3₁ helix) possessed by B; they point to a broadly planar zig-zag arrangement of the chain. This strongly suggests that we have blocks of chemically distinct sequences which could be syndiotactic or head-to-head tail-to-tail (presently with substantial support for the latter) which is responsible for the gel forming crystallization. However, so far the C¹³ nuclear magnetic resonance (NMR) results do not provide the evidence for these distinct species but explanations for our observations on any other basis seem to lead to unsuperable difficulties from other points of view. Consequently, the paper is left open ended with the possibilities discussed. Amongst these the existence of a very few but long, chemically distinct sequences seems most attractive. The wider implications of the facts as they stand for crystal morphology (fringed micelles versus lamellae), for the origin and structure of gels in general, for the crystallization of block copolymers and for issues relating to chemical homogeneity (tacticity, head-to-head tail-to-tail) are discussed and preliminary effects are quoted which indicate that these issues may also be relevant to the usual atactic polymers.     

Pyrrolo[3,4‐e][1,2,3]triazolo[1,5‐a]pyrimidine and pyrrolo[3,4‐d] [1,2,3]triazolo[1,5‐a]pyrimidine. New tricyclic ring systems of biological interest

Derivatives of the new ring system pyrrolo[3,4‐e][1,2,3] triazolo[1,5‐a]pyrimidine 6 were prepared in high yields in one step by reaction of 3‐azidopyrrole 3 and substituted acetonitriles. Compound 6b rearranged, upon heating in dimethyl sulfoxide in the presence of water, to pyrrolo[3,4‐d][1,2,3]triazolo‐[1,5‐a]pyrimidine 7.     

Dicyanoperylene-diimide thin-film growth: a combined optical and morphological study

N,N′-bis (n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI8-CN₂) molecules represent an important example of novel n-type organic materials for realization of air-stable n-channel organic thin film transistors. In this work, the growth by evaporation of PDI8-CN₂ thin films deposited on silicon/silicon dioxide substrates has been investigated as a function of the film thickness through the combination of optical and morphological analyses. A continuous transition from two-dimensional to three-dimensional growth is observed at increasing thickness, accompanied by a modification of the photoluminescence spectrum of the films. Correlations between morphology and optical emission (photoluminescence) of the films are evidenced: namely, an emission band, red shifted with respect to the excitonic transition, emerges at increasing thickness. Time-resolved photoluminescence analysis demonstrates that the decay kinetics of such a band is different from the one associated with exciton recombination. Such a feature may thus represent a useful signature of defect-related trap states.     

Synthesis of 3-Triazenopyrroles

The 3-triazenopyrroles, a new class of pyrrole derivatives, were synthesized in quantitative yield by coupling 3-diazopyrroles with secondary amines.     

Violin Varnishes: Microstructure and Nanomechanical Analysis

The aim of the current work is twofold: to demonstrate the application of in situ non-invasive imaging by portable atomic force microscopy (AFM) on the surfaces of a violin and to integrate compositional and mechanical analysis at the nano scale level on model samples of varnished wood. These samples were prepared according to traditional recipes by an Italian lute-maker family well practised in the art. Samples of oil and spirit-based varnishes on maple wood, naturally and accelerated light aged, were studied. AFM was used to measure the nanomechanical properties of the model samples and established that the spirit-based varnish was stiffer than the oil-based. Synchrotron radiation micro- Fourier Transform Infra-red analysis of the layer structure revealed that stiffer spirit-based varnish showed less penetration into the wood than the oil-based. Further PeakForce Quantitative Nanomechanical Mapping (QNM) demonstrated a difference in adhesion values between the oil- and spirit-based samples.     

2‐Diazo‐2H‐indoles

2‐Diazo‐2H‐indoles were prepared by diazotization of the corresponding 1H‐indol‐2‐amines and subsequent neutralization. On the basis of NMR data and ab initio and semiempirical calculations, we suggest that the zwitterionic form A is the most representative structure for 2‐diazo‐2H‐indoles. In fact, spectral data are compatible with a 1H‐indole structure, and the fully optimized molecules gave distances in agreement with those reported for the anion obtained from 1H‐indole. The calculated charges are compatible with a zwitterionic structure in which the negative charge is mainly located at the ring N‐atom at variance with the case of diazopyrroles and 3‐diazo‐3H‐indoles where the negative charge is essentially located on the ipso C‐atom.     

Polycondensed nitrogen heterocycles. Part 21. Complete 13C NMR assignment of annelated phenanthridines

Two-dimensional nmr techniques were used for the complete assignment of ¹³C nmr spectra of pyrrolo-[1,2-f]-, pyrazolo[1,5-f]-, and 1,2,4-triazolo[1,5-f]phenanthridines.     

Tin(IV) catalyzed D-galacturonic acid anomerization

A comparative kinetic study of anomerization between the four main forms (alpha/beta-pyranosides and alpha/beta-furanosides) of D-galacturonic acid (GalAH(2), 1) and tin(IV) complexed D-galacturonate ([Sn(CH(3))(2)(GalA)(H(2)O)(2)], 2) in D(2)O, is reported. Important differences are shown by pD vs. mutarotation rate profiles of 1 and 2. Coordination, in fact, causes: i) a spectacular increase of the rate due to kinetic template assistance of the carboxylic tail holding the metal in close proximity to the reactive centre and ii) a change in the dependence of the rate on the pD. Entropic and enthalpic factors contribute to the huge activation energy decrease in 2. Anomerization rates of the complexed ligand are determined by 2D-EXSY cross-peak integration. Analysis of up to twelve direct and reverse rate constants in the pD range 7.0-2.5 evidences ring size memory, and at least one reaction path which bypasses the aldehydo open chain intermediate. Solvent oxygen atom incorporation into the ligand is, at pH 4.5, about ten fold faster when catalyzed by the metal, and proves that the metal-assisted nucleophilic attack of water on the anomeric carbon atom cannot significantly contribute to mutarotation rates that are thousands of times faster.     

Sensing of multimode-fiber strain by a dynamic photorefractive hologram

We present a strain sensor in which a multimode fiber is used as a sensitive element. High sensitivity to dynamic strains is achieved by means of vectorial wave mixing in a photorefractive CdTe:V crystal. It was found that the largest source of noise in our sensor is related to the instability of the polarization state of speckles emerging from the fiber. This noise is significantly diminished in fiber with a core of large diameter (550 microm).     

"Proteomineering" serum biomarkers. A study in scarlet

The performance of sera pre-treatment for biomarker searching via combinatorial peptide ligand libraries (CPLL) has recently been challenged (Proteomics 2010, 10, 1416-1425) and stated to allow discovery of only medium to high-abundance proteins. We have thus investigated four elution protocols, as published in recent reports: (i) in 4?M urea+1% CHAPS; (ii) in 4?M urea+1% CHAPS+5% acetic acid; (iii) in 8?M urea+2% CHAPS+5% acetic acid; (iv) in boiling 4% SDS+25?mM DTT. One milliliter of serum, in all cases, was captured with 50?μL of CPLL beads, which were then eluted with the four eluants described above. In the first three cases, after the first elution, the beads were re-eluted with cocktail (iv), known to offer maximal release of proteins adsorbed by the CPLL ligands. Eluant (i) released only ca. 20% of the species adsorbed, eluant (ii) ca. 60%, eluant (iii) ca. 80%. Thus, the poor performance of the CPLL methodology, as reported in (i) is not due to any fault of the capture technique, but simply to the adoption of a very poor elution protocol. Even those using eluants (ii) and (iii) should know that a substantial fraction of the captured species still remains bound to the beads and is thus not available to biomarker discovery. Once more, eluant (iv) is recognized as the only one able to offer optimal recovery from the CPLL baits.