Synthetic polymers stereochemistry

The cooperativity of amplification, switching, and memory in synthetic helical polymers might thus be shared with ideas of a scenario for the biomolec-ular homochirality, autocatalytic mechanism in chiral chemical synthesis, and bifurcation equilibrium mechanisms in crystallization of chiral crystals. Indeed, amplification phenomena in several optical activity and helicity of synthetic polymers in isotropic solution appears to be common and are now established as sergeants and soldiers experiment and majority rules in polymer stereochemistry [17,18]. Any minute chiral forces caused by intramolecular and intermolecular systems can be detectable, when a proper model polymer system is chosen to elucidate the cooperativity of amplification, switching, and memory. 

High-resolution and NMR, and recently, multidimensional methods have revealed the microstructures of complex polymers. In particular, multidimensional (2D- and 3D-) NMR have proven to be useful techniques to identify small amounts of irregular structures in synthetic polymers. In this entry, specific topics to be covered include the use of solution NMR methods to study polymer stereochemistry/ tacticity, monomer composition and sequence distribution, short-chain branches, and chain-end structure, as these parameters influence the material s mechanical, thermal, optical, and electrical properties. 

The economic impact of synthetic polymers is too great to send them to the end of the book as a separate chapter or to group them with biopolymers. We regard polymers as a natural part of organic chemistry and pay attention to them throughout the text. The preparation of vinyl polymers is described in Chapter 6, polymer stereochemistry in Chapter 7, diene polymers in Chapter 10, Ziegler-Natta catalysis in Chapter 14, and condensation polymers in Chapter 20. 

A wide variety of chemical catalysts is nowadays available to polymerize monomers into well-defined polymers and polymer architectures that are applicable in advanced materials for example, as biomedical applications and nanotechnology. However, synthetic polymers rarely possess well-defined stereochemistries in their backbones. This sharply contrasts with the polymers made by nature where perfect stereocontrol is the norm. An interesting exception is poly-L-lactide, a polyester that is used in a variety of biomedical applications [1]. By simply playing with the stereochemistry of the backbone, properties ranging from a semicrystalline, high melting polymer (poly-L-lactide) to an amorphous high Tg polymer (poly-meso-lactide) have been achieved [2]. 

Staudinger predicted a correlation between the physical properties of a polymer and its main-chain stereochemistry as early as 1929. Flowever it was not imtil 1947 that Schildknecht reported the first stereoregular synthetic polymer. - Amidst considerable controversy, he attributed the crystalline properties of a polyfisobutyl vinyl ether) to an ordered stereochemistry, or tacticity, of the polymer backbone. In 1954, research in the field of stereoregular polymers gained a tremendous amormt of momentum when Natta discovered the synthesis of a crystalline isotactic polypropylene using a heterogeneous or-ganometallic catalyst. Since these initial discoveries, the synthesis of polymers of defined stereochemistry... 

Owing to the well-defined stereochemistry, the diversities in choosing hydropbobic/hydrophilic amino acids, and specific secondary structures, polypeptides have been intensively investigated as a biomaterial.Contrary to the random hydrophobically driven self-assembly of the most synthetic polymer, the secondary structures of the polypeptides such as a-helix, /3-sheet, and random coil significantly affect the gelation behavior. 

In order to prepare synthetic polymers showing enzyme-Uke catalytic activity, a number of prerequisites have to be fulfilled. First, a cavity or a cleft has to be made with a defined shape corresponding to the shape of the substrate or, even better, to the shape of the transition state of the reaction. At the same time, functional groups have to be introduced that act as binding sites, coenzyme analogues, or catalytic sites within the cavity and are in a defined stereochemistry. Since binding and catalysis in enzymes are rather complex procedures, simplified structures have to be found that can be handled more easily. 

In addition, stereochemistry is highly relevant to unnatural systems. As we will describe herein, the properties of synthetic polymers are extremely dependent upon the stereochemistry of the repeating units. Finally, the study of stereochemistry can be used to probe reaction mechanisms, and we will explore the stereochemical outcome of reactions throughout the chapters in parts II and III of this text. Hence, understanding stereochemistry is necessary for most fields of chemistry, making this chapter one of paramount importance. 

The focus of this section is the polymerization stereocontrol made possible by ligand modification of metallocenes. Thus, propylene will be the monomer of focus since the stereochemistry of polypropylene is the best tmderstood of any polyolefin," if not of any synthetic polymer ever studied. The observed correlation between a catalyst s stmcture/symme-try and a catalyst s stereoselectivity is often referred to as Ewen s Symmetry Rules.Metallocenes have been manipulated to a remarkable degree to direct the enantiomorphic site control mechanism for polymerization stereoselectivity. ... 

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