This chapter deals with the biodegradability of vinyl ester-based polymers with a special emphasis on poly(vinyl acetate) and poly(vinyl alcohol). A proper discussion of the importance of the biodegradability of a certain polymer class cannot be made without understanding the impact that polymer class has on the environment. Therefore, apart from discussing the actual biodegradation mechanisms, other issues will be addressed. These include, but are not limited to, how long the class of vinyl ester-based polymers has been known and produced on an industrial scale, what quantities are produced and released into the environment each year, and what applications are addressed with this polymer class. We will also look at the general physical and chemical properties of this polymer class and how these properties can influence biodegradability. After a discussion of what "biodegradability" actually means -and what not -the mechanisms for the biodegradation of poly(vinyl ester)s will be discussed in more detail and a summary given of the biological systems able to process poly(vinyl ester)s.
Human milk oligosaccharides (HMOs) are structurally versatile sugar molecules constituting the third major group of soluble components in human breast milk. Based on the disaccharide lactose, the mammary glands of future and lactating mothers produce a few hundreds of different HMOs implicating that their overall anabolism utilizes rather high amounts of energy. At first sight, it therefore seems contradictory that these sugars are indigestible for infants raising the question of why such an energy-intensive molecular class evolved. However, in-depth analysis of their molecular modes of action reveals that Mother Nature created HMOs for neonatal development, protection and promotion of health. This is not solely facilitated by HMOs in their indigestible form but also by catabolites that are generated by microbial metabolism in the neonatal gut additionally qualifying HMOs as natural prebiotics. This narrative review elucidates factors influencing the HMO composition as well as physiological roles of HMOs on their way through the infant body and within the gut, where a major portion of HMOs faces microbial catabolism. Concurrently, this work summarizes in vitro, preclinical and observational as well as interventional clinical studies that analyzed potential health effects that have been demonstrated by or were related to either human milk-derived or synthetic HMOs or HMO fractions.
Triethoxysilane HSi(OEt)3, tetraethoxysilane Si(OEt)4 and hexaethoxydisiloxane Si2O(OEt)6 have been probed as reagents for the synthesis of hydrogen-rich silyl-arenes Ar(SiH3)n. A large set of new silyl-arenes, varying in their substitution patterns and grades, have been prepared. The results establish the two new silylating agents HSi(OEt)3 and Si2O(OEt)6 as particularly useful alternatives to Si(OEt)4. The products, which include trihydrosilyl-substituted methylbenzenes, naphthalenes and ferrocenes, have been characterized by NMR and IR spectroscopy, mass spectrometry and single crystal X-ray diffraction.
The use of tri(alkoxy)silanes (RO)3SiH, which have recently become commercially available
in greater than research scale quantities (R = Me, Et), has been probed for the preparation
of hydrogen-rich arylsilanes ArSiH3. It was found that the silylation of aryl-lithium or (in
situ) aryl-Grignard reagents is followed by RO/H ligand redistribution and can lead to fully
hydrogenated products in a one-pot reaction without employment of any additional metal
hydride. After hydrolytic workup, the overall yields are between 20 and 30%. Silane gas
and tetra(alkoxy)silanes are the main byproducts. At an early stage of the reactions, the
whole set of mixed-ligand silanes (RO)3
-
n
H
n
SiAr can be detected by GLC/MS techniques.
Induced by the organometallic base, the reaction also includes aryl scrambling to give silanes
Ar
n
SiH4
-
n
, Ar
n
Si(OR)4
-
n
, and Ar
n
Si(H/OR)4
-
n
. A reaction scheme is proposed that accounts
for the product distribution. Examples are given for Ar = phenyl, 4-biphenylyl, 4,4‘-biphenyldiyl, 1-naphthyl, and 2-anisyl. The reaction gives only very poor yields of di(silyl)arenes. Silanes of this type were therefore prepared from (RO)3SiH compounds and the
corresponding di(halo)arenes by the in situ Grignard procedure followed by LiAlH4 reduction.
Representative cases are 1,2- and 1,4-di(silyl)benzene and 1,4-di(silyl)-2,5-dimethylbenzene.
The primary reaction products 1,4-(EtO)3SiC6H4Si(OEt)3 and 1,4-[(EtO)3Si]2-2,5-(CH3)2C6H2
have been isolated, and the crystal structure of the latter was determined.
The anodic oxidation potentials of a series of 16 silylarenes have been determined by cyclic voltammetry in acetonitrile. All oxidation steps have been found to be irreversible. The effect of substituents and substitution patterns on the oxidation potentials can be presented as a linear correlation between the experimental oxidation potentials and the calculated vertical ionization energies of the silylarenes, using correlated ab initio methods at the MP2 level of theory. This correlation is useful to predict oxidation and ionization potentials of other silylarenes.
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