Browsing by Subject "Polycarbonates"
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item Genotoxicity in Xenopus laevis exposed to sodium perchlorate and UV radiation(Texas Tech University, 2004-08) McDaniel, Les NThe purpose ofthis work was to determine if simultaneous exposure of amphibian larvae to sodium perchlorate (SP) and UV-B radiation would affect mortality, final stage of development, single strand DNA breaks (SSB), pyrimidine dimer formation, and oxidative DNA damage. Two trials were performed where 450 Xenopus laevis larvae were exposed to combinations of UV-B and SP. Nominal SP exposure concentrations were 0.0 mg/L, 0.05 mg/L, or 100.0 mg/L. Perchlorate exposure groups were further subdivided into UV-B treatments where half the animals received 0.0mJ/cm 2/d and the remaining half received 100 mj/cm2 /d. Fifteen Nieuwkoop-Faber (N-F) stage 51 (trial 1) larvae and N-F stage 50 (trial 2) larvae were maintained in FETAX media in each of the 30 l.0L glass beakers. Exposures were carried out in a Precision Scientific model 815 microprocessor controlled low temperature incubator adapted to uniformly deliver UV-B radiation. A 12:12 light:dark cycle was maintained with digital timer-controlled cool white fluorescent bulbs. Ultraviolet light was similarly produced by digital timer controlled medical grade 20W UV-B fluorescent bulbs with an on:off cycle set appropriately to deliver daily dosage and to approximate a natural midday peak of UV. Completion of exposures was defined as when 80% of the larvae exposed to 0.0 mg/L SP plus 0.0 mJ/cm^/d UV-B reached N-F stage 60. Upon exposure completion, tadpoles were staged and tails were removed 0.5cm from the point of attachment and stored in liquid nitrogen until DNA extraction. Tail tissue from two animals from each replicate were combined for DNA extraction, purification, and analysis. Single strand break analyses were performed using agarose gel electrophoresis. Pyrimidine dimers were assessed using a combination of T4 endonuclease V digestion and agarose gel electrophoresis. Oxidative DNA damage was similarly assessed using simultaneous endonuclease III plus Formamidopyrimidine DNA-Glycosylase (fpg) digestion and agarose gel electrophoresis. Sodium perchlorate and UV-B interaction had a significant effect on mortality in the second trial. Perchlorate also had a significant effect on rates of metamorphosis in both trials in a dose-dependant manner. Average molecular length of DNA fragments (as a measure of SSB) was lower for all UV-B exposed groups, although not at significant levels. The incidence of cyclobutane pyrimidine dimers within the DNA was increased by UV-B treatment below significant levels. Perchlorate, UV-B, and their interaction had no apparent effects on oxidative damage within DNA samples. We believe that, while SP does pose a threat to normal development and growth to natural amphibian populations, SP exposure does not enhance the susceptibility of these populations to the DNA-damaging effects of UV-B. Likewise, UV-B exposure does not enhance the toxicity of SP to amphibian larvae.Item Mechanistic studies of the metal catalyzed formation of polycarbonates and their thermoplastic elastomers(2009-05-15) Choi, WonsookStudies concerning the formation of industrially useful polycarbonates are the focus of this dissertation. Of particular importance is the biodegradable polymer, poly(trimethylene carbonate) which has a wide range of medical applications. The production of polycarbonates can be achieved by the ring-opening polymerization of cyclic carbonate, or the copolymerization of carbon dioxide and oxiranes or oxetanes. For the production of polycarbonates from these monomers, Schiff base metal complexes have been designed, synthesized, and optimized as catalysts. Detailed kinetic and mechanistic studies have been performed for the ring-opening polymerization of cyclic carbonates, as well as the copolymerization of carbon dioxide and oxiranes or oxetane. In addition, the copolymerization of cyclic carbonates and cyclic esters to modify the mechanical and biodegradable properties of materials used for medical devices has been studied using biocompatible metal complexes. In the process for ring-opening polymerizations of trimethylene carbonate or lactides, Schiff base metal complexes (metal = Ca(II), Mg(II) and Zn(II)) have been shown to be very effective catalysts to produce high molecular weight polymers with narrow polydispersities. Kinetic studies demonstrated the polymerization reactions to proceed via a mechanism first order in [monomer], [catalyst], and [cocatalyst] if an external cocatalyst is applied, and to involve ring-opening by way of acyl-oxygen bond cleavage. The activation parameters (?H?, ?S? and ?G?) were determined for ringopening polymerization of trimethylene carbonate, ring-opening polymerization of lactides, and copolymerization of trimethylene carbonate and lactide. In the process for copolymerization of carbon dioxide and oxetane, metal salen derivatives of Cr(III) and Al(III) along with cocatalyst such as n-Bu4NX or PPNX (PPN = bis(triphenylphosphine)iminium, and X = Br, Cl and N3) have been shown to be effective catalysts to provide poly(trimethylene carbonate) with only trace amount of ether linkages. The formation of copolymer is proposed not to proceed via the intermediacy of trimethylene carbonate, which was observed as a minor product of the coupling reaction. To support this conclusion, ring-opening polymerization of trimethylene carbonate has been performed and kinetic parameters have been compared with those from the copolymerization of carbon dioxide and oxetane.Item The Copolymerization of CO_(2) and Cyclic Ethers and Their Degradation Pathways(2013-07-24) Wei, Sheng-HsuanPolycarbonates are found in a variety of common products in daily life due to their favorable mechanical and electrical properties. In addition, they are widely used in biomedical areas due to their stability and biological inertness. Therefore, the production of polycarbonates became an important industrial process in the past decades. However, the current industrial process usually requires toxic phosgene gas as a starting material. Thus, the environmentally benign route by using metal catalyzed couplings of epoxides and CO_(2) to produce polycarbonates has received attention from researchers. In this dissertation, metal catalyzed CO_(2)/cyclic ether copolymerization, depolymerization of polycarbonates, and the equilibria between polycarbonate and corresponding six-membered cyclic carbonate will be investigated. First, the Co(III) catalyzed copolymerizations of CO_(2) and various epoxides with electron-withdrawing substituents to afford polycarbonates are examined. Comparative kinetic studies were performed via in situ infrared measurements as a function of temperature to assess the activation barriers for the production of cyclic carbonate versus copolymer involving electronically different epoxides: styrene oxide, epichlorohydrin, and propylene oxide. Thermodynamically stable cyclic carbonate byproducts are produced during the course of the reaction from the degradations of propagating polymer chains. The depolymerization reactions of several polycarbonates produced from the completely alternating copolymerization of styrene oxide, epichlorohydrin, propylene oxide, cyclohexene oxide, indene oxide, and cyclopentene oxide with carbon dioxide have been investigated. Various reaction pathways can be found under different reaction conditions, including process involving chain-end backbiting and radical intermediates. Temperature-dependent kinetic studies have provided energy of activation barriers for cyclic carbonate formation. In addition, the generated monomeric materials from the degradation of select polycarbonates show the possibility of chemical recycling of plastic waste. For the copolymers made from CO_(2) and oxetane derivatives, this study focuses on the influence of steric hindrance in the 3-position of the monomer oxetane. The (salen)CrCl/onium salt catalyzed coupling reactions of these oxetane derivatives and carbon dioxide are reported. Depolymerizations of copolymers to their corresponding cyclic carbonates were also studied. In addition, several six-membered cyclic carbonates were synthesized to examine their equilibria between monomeric cyclic carbonates and their corresponding polycarbonates.Item Toughening of polyester-based polymers via reactive compatibilization(2006) Khamwichit, Attaso; Paul, Donald R.