Prepolymerization for the gas phase polymerization of propylene

Since the development of Ziegler-Natta catalysts in the 1950s, continuous improvements were performed with regard to higher catalyst activity and higher stereo-selectivity. However, especially at the polymerization start, when the pure catalyst is injected under high rate conditions, the high activity of the catalyst may cause particle overheating and/or lead to an uncontrolled catalyst fragmentation resulting in lower catalyst activity and bad particle morphology. One possibility to solve the problem is to apply a prepolymerization step before the main polymerization. In the prepolymerization, the reaction starts at low rate conditions (mild reaction temperatures, low monomer concentration) in order to realize a controlled catalyst fragmentation and to improve heat removal conditions for the main polymerization by creating a higher heat transfer area. The current contribution deals with the prepolymerization of propylene in lab-scale under industrially relevant conditions. The main gas phase polymerization is conducted in a 5 l horizontal stirred tank reactor. Prepolymerization is performed in-situ, meaning in the same reactor by injecting the catalyst at a low initial temperature, or externally in a 250 ml calorimeter. The latter case offers the benefit of controlled prepolymerization conditions with access to prepolymerization kinetics and defined starting conditions for the main polymerization. Experiments were performed for varying reaction conditions. The two prepolymerization methods are compared and the effect of prepolymerization on the activity of the main polymerization is discussed

Effect of hydrogen partioning on homo-propylene polymerization kinetics

Despite the intense amount of research to improve the performance, efficiency and costs of polypropylene production, scale up of the scientific results from laboratory scale batch reactors to industrial scale continuous production plants remains still a challenge. In order to illustrate effects of scale-up in terms of kinetics, bulk phase syntheses of homo-polypropylene are presented for two grades with different melt flow rate (MFR) targets (50, 7.5 g/10mins) in different reactor sizes (0.25 and 5l) with calorimetric monitoring of the heat of reaction. In coordinative polymerization of propylene, molecular weight is mainly controlled by addition of hydrogen as chain transfer agent [1]. In laboratory scale, bulk phase polymerization reactions are typically carried out in partially filled reactors. In such partially filled reactors, partitioning of hydrogen between gas phase and liquid phase takes place. In the used 5 liter reactor, for all MFR grades studied, uni-modal molecular weight distributions have been observed. In contrast in the 0.25 liters reactor, it was surprisingly observed, that high MFR grades show a bi-modality in molecular weight. In order to study this phenomenon, hydrogen concentration in liquid phase ( ) has been investigated before and during the polymerization reaction by a combination of gas chromatography measurements and mass balance calculations. It could be shown, that in the 0.25 liter reactor, a drift in hydrogen concentration in the liquid phase is responsible for the observed bimodalities in molecular weight. Due to consumption of propylene, the hydrogen concentration is increasing during the course of reaction. A two-step feeding procedure for the 0.25 liter reactor has been developed in order to minimize these drifts in concentration. The developed procedure allows to produce unimodal product also at high MFR grades. In addition, a correlation between the composition of materials in the reactor ( ) and the concentration of hydrogen in the liquid phase has been established. This correlation has been used to design bimodal reactions in the 5 liter reactor. The experimental MFR results were in agreement with the targeted values. Please click Additional Files below to see the full abstract

Disruption of Growth Hormone Receptor Prevents Calorie Restriction from Improving Insulin Action and Longevity

Most mutations that delay aging and prolong lifespan in the mouse are related to somatotropic and/or insulin signaling. Calorie restriction (CR) is the only intervention that reliably increases mouse longevity. There is considerable phenotypic overlap between long-lived mutant mice and normal mice on chronic CR. Therefore, we investigated the interactive effects of CR and targeted disruption or knock out of the growth hormone receptor (GHRKO) in mice on longevity and the insulin signaling cascade. Every other day feeding corresponds to a mild (i.e. 15%) CR which increased median lifespan in normal mice but not in GHRKO mice corroborating our previous findings on the effects of moderate (30%) CR on the longevity of these animals. To determine why insulin sensitivity improves in normal but not GHRKO mice in response to 30% CR, we conducted insulin stimulation experiments after one year of CR. In normal mice, CR increased the insulin stimulated activation of the insulin signaling cascade (IR/IRS/PI3K/AKT) in liver and muscle. Livers of GHRKO mice responded to insulin by increased activation of the early steps of insulin signaling, which was dissipated by altered PI3K subunit abundance which putatively inhibited AKT activation. In the muscle of GHRKO mice, there was elevated downstream activation of the insulin signaling cascade (IRS/PI3K/AKT) in the absence of elevated IR activation. Further, we found a major reduction of inhibitory Ser phosphorylation of IRS-1 seen exclusively in GHRKO muscle which may underpin their elevated insulin sensitivity. Chronic CR failed to further modify the alterations in insulin signaling in GHRKO mice as compared to normal mice, likely explaining or contributing to the absence of CR effects on insulin sensitivity and longevity in these long-lived mice

Gene expression of key regulators of mitochondrial biogenesis is sex dependent in mice with growth hormone receptor deletion in liver

Mitochondrial biogenesis is an essential process for cell viability. Mice with disruption of the growth hormone receptor (GHR) gene (Ghr gene) in the liver (LiGHRKO), in contrast to long-lived mice with global deletion of the Ghr gene (GHRKO), are characterized by lack of improved insulin sensitivity and severe hepatic steatosis. Tissue-specific disruption of the GHR in liver results in a mouse model with dramatically altered GH/IGF1 axis. We have previously shown increased levels of key regulators of mitochondrial biogenesis in insulin-sensitive GHRKO mice. The aim of the present study is to assess, using real-time PCR, the gene expression of key regulators of mitochondrial biogenesis (Pgc1 alpha, Ampk, Sirt1, Nrf2 and Mfn2) and a marker of mitochondrial activity (CoxIV) in brains, kidneys and livers of male and female LiGHRKO and wild-type (WT) mice. There were significant differences between males and females. In the brain, expression of Pgc1 alpha, Ampk, Sirt1, Nrf2 and Mfn2 was lower in pooled females compared to pooled males. In the kidneys, expression of Ampk and Sirt1 was also lower in female mice. In the liver, no differences between males and females were observed. Sexual dimorphism may play an important role in regulating the biogenesis of mitochondria

Growth hormone action predicts age-related white adipose tissue dysfunction and senescent cell burden in mice

The aging process is associated with the development of several chronic diseases. White adipose tissue (WAT) may play a central role in age-related disease onset and progression due to declines in adipogenesis with advancing age. Recent reports indicate that the accumulation of senescent progenitor cells may be involved in age-related WAT dysfunction. Growth hormone (GH) action has profound effects on adiposity and metabolism and is known to influence lifespan. In the present study we tested the hypothesis that GH activity would predict age-related WAT dysfunction and accumulation of senescent cells. We found that long-lived GH-deficient and -resistant mice have reduced age-related lipid redistribution. Primary preadipocytes from GH-resistant mice also were found to have greater differentiation capacity at 20 months of age when compared to controls. GH activity was also found to be positively associated with senescent cell accumulation in WAT. Our results demonstrate an association between GH activity, age-related WAT dysfunction, and WAT senescent cell accumulation in mice. Further studies are needed to determine if GH is directly inducing cellular senescence in WAT or if GH actions on other target organs or alternative downstream alterations in insulin-like growth factor-1, insulin or glucose levels are responsible

Resummation in Hot Field Theories

There has been significant progress in our understanding of finite-temperature field theory over the past decade. In this paper, we review the progress in perturbative thermal field theory focusing on thermodynamic quantities. We first discuss the breakdown of naive perturbation theory at finite temperature and the need for an effective expansion that resums an infinite class of diagrams in the perturbative expansion. This effective expansion which is due to Braaten and Pisarski, can be used to systematically calculate various static and dynamical quantities as a weak-coupling expansion in powers of g. However, it turns that the weak-coupling expansion for thermodynamic quantities are useless unless the coupling constant is very small. We critically discuss various ways of reorganizing the perturbative series for thermal field theories in order to improve its convergence. These include screened perturbation theory (SPT), hard-thermal-loop perturbation theory (HTLPT), the Phi-derivable approach, dimensionally reduced (DR) SPT, and the DR Phi-derivable approach.Comment: 82 pages, 20 figures; v2 - typos corrected, references adde

System size and centrality dependence of the balance function in A+A collisions at sqrt[sNN]=17.2 GeV

Electric charge correlations were studied for p+p, C+C, Si+Si, and centrality selected Pb+Pb collisions at sqrt[sNN]=17.2 GeV with the NA49 large acceptance detector at the CERN SPS. In particular, long-range pseudorapidity correlations of oppositely charged particles were measured using the balance function method. The width of the balance function decreases with increasing system size and centrality of the reactions. This decrease could be related to an increasing delay of hadronization in central Pb+Pb collisions

System size and centrality dependence of the balance function in A + A collisions at sqrt s NN = 17.2 GeV

Electric charge correlations were studied for p+p, C+C, Si+Si and centrality selected Pb+Pb collisions at sqrt s_NN = 17.2$ GeV with the NA49 large acceptance detector at the CERN-SPS. In particular, long range pseudo-rapidity correlations of oppositely charged particles were measured using the Balance Function method. The width of the Balance Function decreases with increasing system size and centrality of the reactions. This decrease could be related to an increasing delay of hadronization in central Pb+Pb collisions

Strangeness production at SPS energies

We present a summary of measurements of strange particles performed by the experiment NA49 in central and minimum bias Pb+Pb collisions in the beam energy range 20A - 158A GeV. New results on Xi production in central Pb+Pb collisions and on Lambda, Xi production in minimum bias collisions are shown. Transverse mass spectra and rapidity distributions of strange particles at different energies are compared. The energy dependence of the particle yields and ratios is discussed. NA49 measurements of the Lambda and Xi enhancement factors are shown for the first time.Comment: Submitted to J. Phys. G (Proceedings of the 9th International Conference on Strangeness in Quark Matter, Los Angeles, USA, March 26-31, 2006). 9 pages, 9 figure

Delayed and Accelerated Aging Share Common Longevity Assurance Mechanisms

Mutant dwarf and calorie-restricted mice benefit from healthy aging and unusually long lifespan. In contrast, mouse models for DNA repair-deficient progeroid syndromes age and die prematurely. To identify mechanisms that regulate mammalian longevity, we quantified the parallels between the genome-wide liver expression profiles of mice with those two extremes of lifespan. Contrary to expectation, we find significant, genome-wide expression associations between the progeroid and long-lived mice. Subsequent analysis of significantly over-represented biological processes revealed suppression of the endocrine and energy pathways with increased stress responses in both delayed and premature aging. To test the relevance of these processes in natural aging, we compared the transcriptomes of liver, lung, kidney, and spleen over the entire murine adult lifespan and subsequently confirmed these findings on an independent aging cohort. The majority of genes showed similar expression changes in all four organs, indicating a systemic transcriptional response with aging. This systemic response included the same biological processes that are triggered in progeroid and long-lived mice. However, on a genome-wide scale, transcriptomes of naturally aged mice showed a strong association to progeroid but not to long-lived mice. Thus, endocrine and metabolic changes are indicative of “survival” responses to genotoxic stress or starvation, whereas genome-wide associations in gene expression with natural aging are indicative of biological age, which may thus delineate pro- and anti-aging effects of treatments aimed at health-span extension