Kinetics degradation rates

Recently, Brich and coworkers (40) reported the synthesis of lactide/glycolide polymers branched with different polyols. Polyvinyl-alcohol and dextran acetate were used to afford polymers exhibiting degradation profiles significantly different from that of linear poly-lactides. The biphasic release profile often observed with the linear polyesters was smoothened somewhat to a monophasic profile. Further, the overall degradation rate is accelerated. It was speculated that these polymers can potentially afford more uniform drug release kinetics. This potential has not yet been fully demonstrated. 

In disc form, when prepared by compression molding, the more hydrophobic polymers, PCPP and PCPP-SA, 85 15, displayed constant erosion kinetics over 8 months. By extrapolation, 1-mm-thick discs of PCPP will completely degrade in over 3 years. The degradation rates were increased by copolymerization with sebacic acid. An increase of 800 times was observed when the sebacic acid concentration reached 80%. By altering the CPP-SA ratio, nearly any degradation rate between 1 day and 3 years can be achieved (4). 

Studies of chlorophyll degradation in heated broccoli juices over the 80 to 120°C range revealed that chlorophylls degrade first to their respective pheophytins and then to other degradation products in what can therefore be described as a two-step process. Both chlorophyll and pheophytin conversions followed a first-order kinetics, but chlorophyll a was more heat sensitive and degraded at a rate approximately twice that of chlorophyll This feature had been observed by other authors. Temperature dependence of the degradation rate could adequately be described by the Arrhenius equation. ... 

Since it is easier to control and change the conditions of carotenoid studies carried out in model systems, information on degradation kinetics (reaction order model, degradation rate, and activation energy) and products formed are often derived from such studies. 

Degradation rates were determined for the reported data using a nonlinear regression of conventional first-order kinetic equations. The software used for this fitting procedure was Model Manager, Version 1.0 (Cherwell Scientific, 1999). 

The simplest scenario to simulate is a homopolymerization during which the monomer concentration is held constant. We assume a constant reaction volume in order to simplify the system of equations. Conversion of monomer to polymer, Xp defined as the mass ratio of polymer to free monomer, is used as an independent variable. Use of this variable simplifies the model by combining several variables, such as catalyst load, turnover frequency, and degradation rate, into a single value. Also, by using conversion instead of time as an independent variable, the model only requires three dimensionless kinetics parameters. 

The mechanisms for the NMHCs (except DMS) required to fully characterise OH chemistry were extracted from a recently updated version of the Master Chemical Mechanism (MCM 3.0, available at http //mcm.leeds.ac.uk/MCM/). The MCM treats the degradation of 125 volatile organic compounds (VOCs) and considers oxidation by OH, NO3, and O3, as well as the chemistry of the subsequent oxidation products. These steps continue until CO2 and H2O are formed as final products of the oxidation. The MCM has been constructed using chemical kinetics data (rate coefficients, branching ratios, reaction products, absorption cross sections and quantum yields) taken from several recent evaluations and reviews or estimated according to the MCM protocol (Jenkin et al., 1997, 2003 Saunders et al., 2003). The MCM is an explicit mechanism and, as such, does not suffer from the limitations of a lumped scheme or one containing surrogate species to represent the chemistry of many species. 

Deng et al. (1997) studied the reaction of metallic iron powder (5 g 40 mesh) and vinyl chloride (15.0 mL) under anaerobic conditions at various temperatures. In the experiments, the vials containing the iron and vinyl chloride were placed on a roller drum set at 8 rpm. Separate reactions were performed at 4, 20, 32, and 45 °C. The major degradate produced was ethylene. Degradation followed pseudo-first-order kinetics. The rate of degradation increased as the temperature increased. Based on the estimated activation energy for vinyl chloride reduction of 40 kilojoules/mol, the investigators concluded that the overall rate of reaction was controlled at the surface rather than the solution. 

Fig. 16.2 Degradation rate constant for methyl parathion as a function of pH, in aqueous 5.0 mM hydrogen sulfide with and without natural organic matter (NOM), at 25°C. Reprinted with permission from Guo XF, Jans U (2006) Kinetics and mechanism of the degradation of methyl parathion in aqueous hydrogen sulfide solution Investigation of natural organic matter effects. Environ Sci Technol 40 900-906. Copyright 2006 American Chemical Society...

Nguyen, C. Zahir, K.O. UV induced degradation of herbicide methyl viologen kinetics and mechanism and effect of ionic media on degradation rates. J. Environ. Sci. Health, 1999, B34, 1. 

The rate of ethanol degradation in the liver is limited by alcohol dehydrogenase activity. The amount of NAD" available is the limiting factor. As the maximum degradation rate is already reached at low concentrations of ethanol, the ethanol level therefore declines at a constant rate (zero-order kinetics). The calorific value of ethanol is 29.4 kj g Alcoholic drinks—particularly in alcoholics—can therefore represent a substantial proportion of dietary energy intake. 

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