The influence of side branches on the structure of crystalline phase in ethylene-1-alkene copolymers

The influence of side branches on the unit cell dimensions in ethylene-1-alkene copolymers was investigated both at room temperature as well as during heating of the samples up to melting. Time resolved X-ray diffraction investigations revealed that in the case of ethylene-1-octene copolymers with low and moderate comonomer content, the transversal expansion of the unit cell occurs due to the limited thickness of lamellae, however the samples with higher comonomer content (above 5 mol %) exhibit distinctly larger expansion resulting from the stresses imparted on the basal surfaces of crystals by the excluded branches. At high temperatures, independently on the thermal expansion, an additional increase of the unit cell volume occurs, caused probably by a penetration of the end-parts of side branches, residing near the crystals surface, into the surface cells. Similar phenomenon is observed for ethylene-1-butene copolymer. On contrary, the ratio of the average unit cell volume in ethylene-1-propene copolymer to the unit cell volume in linear polyethylene at the same temperature, decreases starting from about 50 °C. Such a behavior is a new confirmation of a partial inclusion of methyl branches into the crystalline phase. Most probably, when the temperature increases, the volume of included branch becomes a decreasing fraction of the thermally expanding unit cell volume.     

Mechanism of plasma polymerization ofN-silyl-substituted cyclodisilazane: Structure and properties of polymer film

NN-Bis(dimethylsilyl)tetramethylcyclodisilazane (NSCDSN) was selected for plasma polymerization as a model monomer representing N-silyl-substituted cyclodisilazanes. Owing to the aromatic character resulting from the strong (d-p) coupling between silicon and nitrogen atoms, these compounds manifest the highest thermal stability among the organosilicones. GC/MS examination of a low-molecular-weight fraction evaporated from the plasma polymer revealed the presence of various monomer derivatives having mostly the general structure of N-silyl-substituted cyclodisilazane (Si₄N₂) units. GC/MS and ATR/IR studies have shown that the Si₄N₂ skeleton displays a high resistance to plasma fragmentation and that it was incorporated as such into the polymer film. A radical mechanism for plasma polymerization was postulated assuming the formation of and propagation precursors. The low value found for the polar component of the surface free energy confirmed the contribution of Si₄N₂ units to the polymer film. TGA investigation showed the rate and degree of thermal decomposition of plasma-polymerized (PP) NSCDSN to be lower than those of plasma polymers from N-hydrogen-substituted silazanes. Vacuum pyrolysis, at 800°C, converted the polymer film into a glassy, dense, and almost inorganic material with a strong adhesion to the metal substrates.     

Structure of the nitrogen-centered radical formed during inactivation of E. coli ribonucleotide reductase by 2'-azido-2'-deoxyuridine-5'-diphosphate: trapping of the 3'-ketonucleotide

Ribonucleotide reductases (RNRs) catalyze the conversion of nucleotides to deoxynucleotides providing the monomeric precursors required for DNA replication and repair. The class I RNRs are composed of two homodimeric subunits: R1 and R2. R1 has the active site where nucleotide reduction occurs, and R2 contains the diiron tyrosyl radical (Y*) cofactor essential for radical initiation on R1. Mechanism-based inhibitors, such as 2'-azido-2'-deoxyuridine-5'-diphosphate (N(3)UDP), have provided much insight into the reduction mechanism. N(3)UDP is a stoichiometric inactivator that, upon interaction with RNR, results in loss of the Y* in R2 and formation of a nitrogen-centered radical (N*) covalently attached to C225 (R-S-N*-X) in the active site of R1. N(2) is lost prior to N* formation, and after its formation, stoichiometric amounts of 2-methylene-3-furanone, pyrophosphate, and uracil are also generated. On the basis of the hyperfine interactions associated with N*, it was proposed that N* is also covalently attached to the nucleotide through either the oxygen of the 3'-OH (R-S-N*-O-R') or the 3'-C (R-S-N*-C-OH). To distinguish between the proposed structures, the inactivation was carried out with 3'-[(17)O]-N(3)UDP and N* was examined by 9 and 140 GHz EPR spectroscopy. Broadening of the N* signal was detected and the spectrum simulated to obtain the [(17)O] hyperfine tensor. DFT calculations were employed to determine which structures are in best agreement with the simulated hyperfine tensor and our previous ESEEM data. The results are most consistent with the R-S-N*-C-OH structure and provide evidence for the trapping of a 3'-ketonucleotide in the reduction process.     

Coulometric titrations in microscale study of ionic equilibria

Summary Coulometric titration, a well established technique in research and in routine analysis, can successfully be applied to the study of ionic equilibria in solution. Processes, where either lyate or lyonium ions serve as titrants, for determination of acids and bases, respectively, were investigated. Among the systems studied are: autoprotolytic equilibria of various amphiprotic solvents, acid-base equilibria in water, organic and mixed solvents and complexation reactions. In all these examples the titration was followed by a potentiometric pH-indicator electrode. In nonaqueous solvents coulometry overcame the limited solubility of electrolytes, the difficulty of standardization on solutions and the necessity of working in isolated systems to avoid contamination of the sample with water vapour or with carbon dioxide.Because volumes below 1 ml and concentrations in the order of 0.001 mol/l can be used, this technique can be successfully applied to the investigation of micromole amounts of compounds, when protolytic properties of substances available in minute quantities should be evaluated. Furthermore this technique was applied to the evaluation of protonation constants of biologically important buffers and the stability constants of their calcium complexes.     

Carbon-mineral adsorbents prepared by pyrolysis of waste materials in the presence of tetrachloromethane

Natural bentonite spent in the process of plant oil bleaching was used as an initial material for preparation of carbon-mineral adsorbents. The spent bleaching earth was treated using four procedures: T (thermal treatment); H (hydrothermal treatment); C (thermal treatment with addition of CCl4 vapor); M (modification of porous structure). Raw bentonite, RB (raw bleaching earth), and carbon materials prepared using plant oil were compared. The physicochemical characteristics of the adsorbents were determined using different methods: nitrogen adsorption/desorption, XRD, TEM, and MS-TPD. Carbon-mineral adsorbents contain from 5.23 to 19.92% C (w/w) and carbon adsorbents include from 84.2 to 91.18% C (w/w). Parallel processes of organic substance carbonization, porous structure modification, sublimation or evaporation of metal chlorides, and removal of hydrogen chloride take place during pyrolysis of waste mineral materials in the CCl4 atmosphere.     

Controlled polymerization of cyclic esters. structure of initiators and of active species related to the selectivity of initiation and propagation

This paper reviews mostly the already published work of our own, complemented with some new data and related to the control (i.e. selectivity) in the anionic and covalent (pseudoanionic) polymerization of cyclic esters. Ionic and covalent growing species are briefly compared in terms of their susceptibility to undergo side reactions. Then, structures of covalent initiators (mostly multivalent metal alkoxides) are discussed from the view point of their behavior in initiation and formation of active species. R₂AlOR', (RO)₃Al, and other metal alkoxides are discussed.     

On the difference of reactivities of various aggregated forms of aluminium triisopropoxide in initiating ring-opening polymerizations

The polymerization of ε-caprolactone (εCL) initiated with aluminium triisopropoxide (Al(OⁱPr)₃) trimer ( A ₃) and/or tetramer ( A ₄) was studied. The rate of A ₃ ⇔︁ A ₄ interconversions in the diluted (≤0,1 mol · L^-1) C₆D₆, C₆D₆/εCL, and THF/εCL solutions were found to be slow, when compared with the rate of propagation. Comparison of the ¹H NMR spectra of the initiators with those of the polymerization mixtures revealed that A ₃ is much more reactive than A ₄ in their reactions with εCL. From the initiator reacted with εCL all threeOⁱPr groups from Al(OⁱPr)₃ are transferred into the poly(εCL) as end groups. Kinetic studies of polymerization confirmed the large reactivity difference between A ₃ and A ₄.     

Thermodynamic and Kinetic Polymerizability of Cyclic Esters

Recent advances in the controlled ring-opening polymerization (ROP) of cyclic aliphatic esters are reviewed with a particular attention to thermodynamics and kinetics of the polymerization of industrially important monomers, namely: lactide (LA), 1,4-dioxan-2-one (DX), and ϵ-caprolactone (CL). For comparison, polymerization of β-lactones is discussed. Progress in stereocontrolled polymerization of monomers bearing centers of chirality [LA and β-butyrolactone (BL)] is also briefly reviewed.     

Living polymerization of cyclic esters – a route to (bio)degradable polymers. Influence of chain transfer to polymer on livingness

Polymerization of cyclic esters leads to (bio)degradable polymers of the increasing industrial importance. These polymerizations are of the living nature, although chain transfer to polymer with chain scission may cause deviations from the livingness and introduce structural differences (e.g. in end-groups), important for physical properties. Two different systems are discussed. In the first one two living macromolecules react one with another and reproduce two living macromolecules, retaining the same reactivities and the same end-groups. Polymerizations of ϵ-caprolactone and lactide belong to this category. On the other hand, polymerization of cyclic carbonates proceeds with chain transfer, in which disproportionation of the living chains takes place: from two living macromolecules one “dead” and one “doubly active” can be formed. Conditions of retaining the livingness in terms of the ratios of the rate constants of transfer, reinitiation, and propagation are discussed.     

Reliability of Deterioration System

Mathematical models of the reliability assessment of a structure treated as the time to the first passage of the capacity which is degrading in the time are presented. Two stochastic models of the structure degradation caused by different factors and two stochastic models of loading are considered. The relationships for calculating the probability of failure for presented models of the structure load and the capacity degradation, are given. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)