Chemical structure, hydrolytic

Chemical structure of monomers and intermediates was confirmed by FT-IR and FT-NMR. Molecular weight distribution of polymers was assessed by GPC and intrinsic viscosity. The thermal property was examined by differential scanning calorimetry. The hydrolytic stability of the polymers was studied under in vitro conditions. With controlled drug delivery as one of the biomedical applications in mind, release studies of 5-fluorouracil and methotrexate from two of these polymers were also conducted. 

Challinor, Haworth and Hirst101 determined the chemical structure of the levan produced by the action of B. mesentericus on sucrose. Methylated levan appeared homogeneous when fractionally precipitated from mixed solvents. Fractional distillation of the hydrolytic products of methylated levan yielded tetramethyl-D-fructofuranose in an amount corresponding to a levan chain length of from ten to twelve fructofuranose units, joined as previously940 shown through the 2- and 6-positions. 

Its chemical structure does not allow elimination of HNO, thus supporting the oxidative pathway of activation to NO, a mechanism still possible in this blocked SIN-1A derivative. The final product of the NO-release was found to be l-amino-2-cyanomorpholine (110) [106]. Its formation can be rationalized assuming that, after the oxidative NO-release, deprotonation occurs at the a-position of the morpholine, followed by migration of the cyano group and hydrolytic cleavage of the hydrazone moiety. 

Wholly aromatic polymers are thought to be one of the more promising routes to high performance PEMs because of their availability, processability, wide variety of chemical compositions, and anticipated stability in the fuel cell environment. Specifically, poly(arylene ether) materials such as poly-(arylene ether ether ketone) (PEEK), poly(arylene ether sulfone), and their derivatives are the focus of many investigations, and the synthesis of these materials has been widely reported.This family of copolymers is attractive for use in PEMs because of their well-known oxidative and hydrolytic stability under harsh conditions and because many different chemical structures, including partially fluorinated materials, are possible, as shown in Figure 8. Introduction of active proton exchange sites to poly-(arylene ether) s has been accomplished by both a polymer postmodification approach and direct co-... 

Amorphous nylons are transparent. Heat-deflection temperatures are lower than those of filled crystalline nylon resins, and melt flow is stiffer hence, they are more difficult to process. Mold shrinkage is lower and they absorb less water. Warpage is reduced and dimensional stability less of a problem than with crystalline products. Chemical and hydrolytic stability are excellent. Amorphous nylons can be made by using monomer combinations that result in highly asymmetric structures which crystallize with difficulty or by adding crystallization inhibitors to crystalline resins such as nylon-6 (61). 

In addition to chemical substrate concentration, chemical structure and physical/chemical properties have considerable impact on the rate and pathways of biodegradation. The chemical structure determines the possible pathways that a substrate may undergo, generally classified as oxidative, reductive, hydrolytic, or conjugative. 

The chemical structure of the prepolymer influences its chemical resistance. As a result of their structure, polyesters have inherently better oil resistance but lower hydrolytic stability. The ether groups in the polyether urethanes... 

The rate of reversion, or hydrolytic instability, depends on the chemical structure of the base polymer, its degree of crosslinking, and the permeability of the adhesive or sealant. Certain chemical linkages such as ester, urethane, amide, and urea can be hydrolyzed. The rate of attack is fastest for ester-based linkages. Ester linkages are present in certain types of polyurethanes and anhydride cured epoxies. Generally, amine cured epoxies offer better hydrolytic stability than anhydride cured types. 

Chemical, or abiotic, transformations are an important fate of many pesticides. Such transformations are ubiquitous, occurring in either aqueous solution or sorbed to surfaces. Rates can vary dramatically depending on the reaction mechanism, chemical structure, and relative concentrations of such catalysts as protons, hydroxyl ions, transition metals, and clay particles. Chemical transformations can be genetically classified as hydrolytic, photolytic, or redox reactions (transfer of electrons). 

The hydrolytic reactions are of great importance from the viewpoint of chemical degradation of some water pollutants [14-17]. Both inorganic and organic water pollutants frequently have a chemical structure which renders them liable to hydrolysis. 

It is evident from the scheme shown in Fig. 3.21 that the chemical structures of pesticides are quite diverse they undergo various physico-chemical effects in the environment after application (solar radiation, heat, air, soil, water) as well as being subjected to various metabolic transformations in plants, microorganisms, insect and animals. Common metabolic transformations are schematically surveyed in Table 3.20, which shows that primarily oxidation, hydrogenation, reduction and hydrolytic reactions are concerned. Also among the individual chemical compounds mutual chemical reactions take place. Some examples of photochemical reactions of pesticides in water are presented in Figs 3.22 to 3.26. Biochemical reactions of DDT and DDE are shown in Fig. 3.27. The number of individual chemical species is hence significantly multiplied in the hydrosphere due... 

The chemical structures of hydrolyzable functional groups that are prevalent in environmental chemicals, and the reaction products resulting from their hydrolysis, are listed in Table 2.1. For these classes of chemicals, hydrolysis may be the dominant pathway for their transformation in aquatic ecosystems. Hydrolytic processes are not limited to the bodies of water such as rivers, streams, lakes, and oceans usually associated with the term aquatic ecosystems. Hydrolysis of organic chemi-... 

ViUuendas I., Molina I., Regano C., Bueno M., Martinez de Ilarduya A., Galbis J.A., Munoz-Guerra S., Hydrolytic degradation of poly(ester amidejs made from tartaric and succinic acids. Influence of the chemical structure and microstructure on degradation rate. Macromolecules, 32, 1999, 8033-8040. 

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