Analysis of the molecular weight distribution (MWD) in polymerization processes: contribution of different families of active sites to the MWD of polyethylene prepared using supported transition metal catalysts

The broad molecular weight distribution (MWD) curves obtained by gel-permeation chromatography (GPC) of polyethylenes prepared in the presence of MgCl₂ supported monometallic (Ti or Hf) and bimetallic (Ti/Hf) catalysts were analyzed by deconvolution into component curves. The MWD curves of the monometallic catalysts can be accounted for with only three component curves associated with three distinct families of active sites. The term family is used to indicate a group of relatively similar active sites producing a polymer with M _w/M _n < 3. This approach seems valid for both Ti and Hf catalysts in spite of the different characteristic values; indeed we have for the former system 1 < M _w/10⁵ < 2,5 with 6 < M _w/M _n < 10, whereas for the latter M _w ≈ 7 × 10⁵ and M _w/M _n ≈ 18. In the case of the bimetallic Ti/Hf system with 1,9 < M _w/10⁵ < 2,6 and M _w/M _n ≈ 17, the MWD distribution can be better accounted for on the basis of 6 families of active sites, corresponding to the linear combination of the balanced contributions from both Ti and Hf sites.     

Number of active centers and propagation rate constants in the propene polymerization on supported Ti–Mg catalysts in the presence of hydrogen

Polymerization inhibition with radioactive carbon monoxide was used to determine the number of active centers (C_p) and the propagation rate constant (K_p) in the isospecific polymerization of propene with the catalytic system TiCl₄/diisobutyl phthalate/MgCl₂—AlEt₃—PhSi(OEt)₃. The presence of hydrogen was found to decrease C_p (by a factor of ≈ 2) and to increase k_p (by a factor of ≈ 4). In the absence of hydrogen a considerable part of the active centers (titanium-polymer bonds) reacting with ¹⁴CO seems to be inactive in the propene polymerization for steric reasons. Hydrogen is able to transform such centers into an active state. Thus, k_p ≈ 2,5 · 10³ L/(mol · s) at 70°C obtained in propene polymerization in the presence of the hydrogen seems to be correct, since the portion of centers in the active state is maximum. The influence of polymerization time, external and internal donors on C_p and k_p was studied.     

The structure of influenza virus. II. A model based on the morphology and composition of subviral particles

Electron micrographs of subviral particles of the WSN strain of influenza virus A are presented. After removal of the glycoprotein spike layer, the internal structure of virtually every particle in a preparation is revealed. Negatively stained preparations and thin sections through pellets of purified particles show structures that are consistent with the ribonucleoprotein (RNP) being a long strand which is turned back on itself to form twisted, double-stranded loops. The RNP, which is composed of the viral RNA and a protein of 65,000 daltons, is enclosed in a double-walled sac. Fixed particles from which the spike layer and the lipid layer have been removed are also shown. Evidence is presented that the RNP is enclosed in a sac of protein made up of carbohydrate-free molecules of 26,000 daltons. A 92,000 dalton protein of unknown function is probably also inside this layer of protein. In the intact virion, this protein sac is surrounded by a layer of lipid and beyond that by a layer of spikes. Although the spikes can be removed without disrupting the internal component of the virion, damage to the spike layer permits distortion of the virus particle. A model for influenza viruses A and B is proposed which relates their polypeptide composition to their structure. The model requires genetic information for the synthesis of about 3.0 × 10⁵ daltons of protein.     

On structure and temperature transformations of nascent polyethylene,produced by polymerization of ethylene in presence of a modified Ziegler-Natta catalyst

Nascent polyethylene with high molecular mass (M_n ≈ 10⁵, M_w ≈ 10⁶) produced in hydrocarbon medium in presence of TiCl₄/(C₂H₅)₂AlCl/(C₆H₅)₂Mg catalyst was studied by means of X-ray diffraction, electron and light microscopy, and low-angle light scattering. It consists of porous spherulite-fibril aggregates indicating a not very high degree of crystallinity α(α≈). Data, concerning the dependence of spherulite dimensions, degree of crystallinity and other structure parameters of nascent polyethylene on polymerization time and temperature are discussed. Using the same structure-analytical methods some samples of nascent polyethylene were studied after annealing them in nitrogen atmosphere at different temperatures between 110–250°C with successive cooling to room temperature. Furthermore both the endo- and exothermal transitions occurring in the nascent and annealed structures after heating to 250°C were studied by means of DTA and DSC. The experimental information leads to the conclusion that nascent polyethylene structures consist of crystalline folded chain lamellae with an increased amount of tie molecules between them as well as of extended chain crystals.     

Study of the state of titanium ions and the composition of the active component in titanium-magnesium catalysts for ethylene polymerization

The state of Ti³⁺ ions formed at various steps of preparation and activation of highly active titanium-magnesium catalysts of various composition (dry milling of TiCl₃ with MgCl₂; reduction of TiCl₄ by nonsolvated magnesium dialkyl; interaction of the TiCl₄/MgCl₂ system with an organoaluminium compound) was studied by the ESR method. It was found that in all systems studied the same three types of Ti³⁺ ions in the chlorine environment are formed, differing in coordination and localization sites in MgCl₂. The content of these ions depends on the composition and preparation conditions of the catalysts, and does not exceed 20% of the total content of titanium. Most of the Ti³⁺ ions formed in all systems enter the composition of particles or surface aggregates of TiCl₃ and appear in the ESR spectra only after dissolution of the sample in pyridine. The active centers of titanium-magnesium catalysts are supposed to be constituents of these particles or of surface aggregates.     

The effect of polyethylene glycol structure on paclitaxel drug release and mechanical properties of PLGA thin films

Thin films of poly(lactic acid-co-glycolic acid) (PLGA) incorporating paclitaxel typically have slow release rates of paclitaxel of the order of 1 μg day(-1) cm(-2). For implementation as medical devices a range of zero order release rates (i.e. 1-15 μg day(-1) cm(-2)) is desirable for different tissues and pathologies. Eight and 35 kDa molecular weight polyethylene glycol (PEG) was incorporated at 15%, 25% and 50% weight ratios into PLGA containing 10 wt.% paclitaxel. The mechanical properties were assessed for potential use as medical implants and the rates of release of paclitaxel were quantified as per cent release and the more clinically useful rate of release in μg day(-1) cm(-2). Paclitaxel quantitation was correlated with the release of PEG from PLGA, to further understand its role in paclitaxel/PLGA release modulation. PEG release was found to correlate with paclitaxel release and the level of crystallinity of the PEG in the PLGA film, as measured by Raman spectrometry. This supports the concept of using a phase separating, partitioning compound to increase the release rates of hydrophobic drugs such as paclitaxel from PLGA films, where paclitaxel is normally homogeneously distributed/dissolved. Two formulations are promising for medical device thin films, when optimized for tensile strength, elongation, and drug release. For slow rates of paclitaxel release an average of 3.8 μg day(-1) cm(-2) using 15% 35k PEG for >30 days was achieved, while a high rate of drug release of 12 μg day(-1) cm(-2) was maintained using 25% 8 kDa PEG for up to 12 days.     

Study of the molecular structure of ethylene-propylene copolymers obtained with catalysts of different composition

The monomer composition and sequence distribution in ethylene-propylene copolymers (EPCs) obtained with supported titanium/magnesium (TMC) and vanadium/magnesium (VMC) catalysts as well as with titanium trichloride was studied by means of ¹³C NMR spectroscopy. Based on these data ethylene-propylene reactivity ratios were determined. The EPCs obtained on these catalysts are inhomogeneous in composition; the soluble fractions contain larger amounts of propylene. The microstructures of copolymers obtained on TMCs and TiCl₃ are similar. For EPCs obtained on VMCs, propylene “head-to-head” or “tail-to-tail” inversions and a more homogeneous distribution of propylene are observed.     

Determination of the number of active centers and of the propagation rate constant for ethylene polymerization on supported Ni-containing catalysts by using 14CO

Ethylene polymerization on supported Ni-containing catalysts was studied using ¹⁴CO as the inhibitor and as a label for determining the number of active centers (AC). The polymerization rate decreases sharply after introducing a mole ratio ¹⁴CO/Ni = 0.5 ÷ 1 and recovers after the removal of ¹⁴CO. The number of labels in the polymer and in the liquid phase (hexane) increases with the amount of ¹⁴CO introduced and with ¹⁴CO contact time (τ_co). In the absence of ethylene the number of labels in the polymer does not increase with τ_co and the number of labels in the liquid phase is essentially less than in the presence of ethylene. The rate of the label accumulation in the polymer during short contact times (τ_co = 5 min) is essentially higher than that during the following periods of time (τ_co = 5–135 min). Fast label accumulation occurs through first ¹⁴CO molecule insertion into the Ni-polymer bond, and the slower one through copolymerization of ¹⁴CO with ethylene, i.e. when the second and more ¹⁴CO molecules insert into the polymer chain. Catalyst activity and the number of AC (in the range of 1.5 to 6% of Ni content) decrease with polymerization time. For both catalysts the values of the propagation rate constant are maximum at short polymerization time (5 to 15 min) and equal to (3 ± 0.8) · 10³ L/(mol · s) at 65°C.     

The effect of simvastatin on polyethylene particle-induced osteolysis

This study aimed to investigate the effects of the HMG-CoA reductase inhibitor simvastatin on ultra-high molecular weight polyethylene (UHMWPE) particle-induced osteolysis. The murine calvarial osteolysis model was used in 21 C57BL/J6 mice randomized to three groups. Group I underwent sham surgery only, group II received UHMWPE particles, and group III, particles and simvastatin treatment. After two weeks, calvaria were processed for histomorphometry. Bone resorption was measured as resorption within the midline suture using Giemsa staining. Osteoclast numbers were determined per high-power field using TRAP-staining. Statistical analysis was performed using one-way ANOVA and Student's t-test. Bone resorption in midline suture was 0.094+/-0.007 mm(2) in sham controls (group I), 0.25+/-0.025 mm(2) after particle implantation without further intervention (group II), and 0.131+/-0.02 mm(2) with particle implantation and additional simvastatin treatment (group III) (p=0.00003). Osteoclast numbers were 15.3+/-3.6 in group I, 48.7+/-7.1 in group II and 6.2+/-3.1 in group III (p=0.00002). In conclusion, simvastatin treatment markedly decreased UHMWPE particle-induced osteolysis in a murine calvarial model. This finding suggests that simvastatin may have a role for noninvasive prevention and treatment of wear debris-mediated periprosthetic osteolysis after total joint arthroplasty.     

The effect of immunoglobulin G1 structure on macrophage binding to supported planar lipid monolayers.

We have studied the antibody-dependent binding of macrophages to supported planar lipid monolayers containing haptenated phospholipids (Tnp-Cap-DPPE). Eight monoclonal anti-TNP IgG1s, which had similar affinities to the TNP residues in solution and in the membranes, were used in the experiment. The results showed that mouse macrophages (P388D1 and J774.1) bound with different affinities to these IgG1-coated lipid monolayers. The monoclonal antibody shown to be deficient in macrophage binding was also relatively ineffective in activating complement. These results indicated that individual monoclonal antibodies of a given subclass may prove deficient in terms of the biological activities associated with the group as a whole.     

季节因素对精子形态的影响

目的了解季节因素对精子形态的影响;方法259例浙江省计划生育科学技术研究所人类精子库的供精者精液标本,按采集时间分为春、冬季与夏、秋季两组,其中春、冬季组141例;夏、秋季组118例,对所有精液标本进行精子形态学分析;结果春、冬季组精子正常形态率高于夏、秋季组（P〈0.05）;春、冬季组精子头部异常率、颈部及中段异常率、胞浆小滴、畸形精子指数（TZI）及精子畸形指数（SDI）低于夏、秋季组（P〈0.05）;尾部异常率两组无明显差异（P〉0.05）。结论不同季节对精子的形态有影响,春、冬季精子形态正常率高。     

The structure and time evolution of a cellulose sheet in the nascent fibril produced by acetobacter xylinum

Structure and time evolution of a nascent cellulose sheet, produced by Acetobacter xylinum, were examined regarding the resistance of the nascent fibrils to alkali. It became clear that the never-air-dried cellulose, obtained after an incubation period of 5 min, changes its morphology after an alkali treatment at very low concentrations (ranging from 1,0 · 10^?3 to 1,0 · 10^?2 wt.-%). It is subsequently transformed into cellulose II. On the other hand, although the fibril after an incubation period of 30 min seems to be in a stainable state toward sodium tungstophosphate when it was treated with 4,6 wt.-% alkali, 70–80% of it maintains the fibrous shape. Electron diffraction experiments, involving the fibrils, indicate a cellulose I structure. Even when the sample after an incubation period of 30 min was treated with 9,6 wt.-% alkali, about 30% of it still maintains the fibrous shape. Electron diffraction experiments, involving the fibrils, also indicate a cellulose I structure. The shape of a cellulose sheet, just after its extrusion from a cell, seems to be maintained by hydrophobic bonds. The enhancement of the resistance of cellulose toward alkali with time must be due to the development of inter-and intramolecular hydrogen bonds in each cellulose sheet.     

On the morphology and orgin of the fibres observed in Nascent Ziegler polyethylene

Fibrils in nascent ZIEGLER polyethylene have been compared to those crystallised from stirred solutions, and similarities between the two types have been observed. It is concluded that both the ZIEGLER polyethylene and the stirring-induced fibres have a composite extended chain and chain-folded structure. The origin of the nascent polyethylene fibrils is discussed, and the possibility that they are flow-induced is considered. An alternative mechanism is also envisaged, in which the proposed structure is accounted for in terms of unequal growth rates of molecules from clusters of active sites.     

Recent results on the active site of amine oxidases

A preparation of bovine plasma amine oxidase, containing only one copper ion per molecule, was obtained by treatment with diethyldithiocarbamate. The enzyme preparation still retained full activity. Thus, only one of the two copper ions contained in the enzyme molecule appears to be involved in the catalytic process.     

Studies on the active site of human leukocytic pyrogen

Leukocytic pyrogen, a polypeptide produced by phagocytic mononuclear cells, is thought to be the endogenous mediator of fever. In addition to its effects on thermoregulation, leukocytic pyrogen has been shown to induce synthesis of acute-phase proteins, increase lymphocyte blastogenesis to mitogens, and cause release of neutrophil-specific granule contents. Despite its important role in biologic responses, little is known concerning the structure-function relationship of the molecule. In the present studies several protein-modifying conditions were used in order to examine specific amino acid participation at the active site. Because the state of purity of leukocytic pyrogen may be critical during certain reaction conditions, highly purified preparations were used. Experiments suggest that the active site requires the gamma-carboxyl group of glutamic acid and that blocking arginine reduces both the pyrogenic and neutrophil releasing properties of the molecule. Other studies demonstrate that the pyrogenicity of human leukocytic pyrogen is not due to serine esterase or carboxypeptidase B activity and that the 15,000-dalton molecule may be a glycoprotein. These experiments provide further evidence that the lymphocyte-activating and neutrophil-granule-releasing properties of human leukocytic pyrogen require the same active site which produces fever.     

The effect of cinnamaldehyde on the growth and the morphology of Candida albicans

We examined the viability and morphology of Candida albicans under experimental conditions after treatment with varying concentrations of cinnamaldehyde, the major component of cassia (Cinnamomum cassia), using XTT assay, fluorescent microscopy, scanning electron microscopy, and thin-section electron microscopy. At 10 μg/ml level, cinnamaldehyde inhibited mycelial growth, but did not affect the growth of yeast cells, metabolic activity, cell shape, or the ultrastructure of the cells. At 40 μg/ml level, cinnamaldehyde showed fungicidal activity accompanied by alteration of the membrane and interior of Candida cells. These findings indicate that cinnamaldehyde has both fungistatic and fungicidal activities against C. albicans and affects the structure of the cells.