Macromolecular stereochemistry

The discovery of the polymerization processes promoted by transition metal catalysts and the foundation of macromolecular stereochemistry represents a major breakthrough in chemistry in the second half of this century. Since the first discoveries by Ziegler in Miilheim and Natta in Milan there has been an enormous development of fundamental and applied research that still continues... 

Given the vastness of the subject matter I have limited myself to dealing with the structural (or static) aspects of macromolecular stereochemistry. An adequate treatment of the stereochemistry of polymerization, with specific regard to the polymerization of olefins and conjugated diolefins, would have occupied so much space and called for such a variety of additional information as to make this article excessively long and complex. I trust that others will successfully dedicate themselves to this task. However, the connection between polymer structure and polymerization mechanism is so important that the fundamentals of dyruunic macromolecular stereochemistry cannot be completely ignored in this chapter. 

Macromolecular stereochemistry has been treated in this series only once, more than IS years ago, in a chapter by Goodman (6). Reference may be made to this review and to a contemporary treatise on the same subject, edited by Ketley (7), to know the state of the art at the end of the 1960s, at the end, that is, of the most creative era in the development of this field. 

The butadiene polymers represent another cornerstone of macromolecular stereochemistry. Butadiene gives rise to four different types of stereoregular polymers two with 1,2 linkage and two with 1,4. The first two, isotactic (62) and syndiotactic (25), conform to the definitions given for vinyl polymers, while the latter have, for eveiy monomer unit, a disubstituted double bond that can exist in the two different, cis and trans, configurations (these terms are defined with reference to the polymer chain). If the monomer units all have the same cis or trans configuration the polymers are called cis- or trans-tactic (30 and 31). The first examples of these stereoisomers were cited in the patent literature as early as 1955-1956 (63). Structural and mechanistic studies in the field have been made by Natta, Porri, Corradini, and associates (65-68). 

In the following pages we shall treat, in detail, the NMR analysis of two polymers, poly(methyl methacrylate) and polypropylene, which have epitomized, in consecutive epochs, the most classical and fhiitful examples of interaction between NMR spectroscopy and macromolecular stereochemistry. 

With regard to fundamental principles I have shown the relation as well as the differences between the stereochemical treatment of low molecular weight and that of macromolecular compounds. If some confusion has resulted in the past, it is due to the improper use of concepts and of methods outside their proper held. Macromolecular stereochemistry can be subjected to physico-math-ematical approaches based on concepts of system and structure and on topological principles. Interesting developments in this regard were recently published by Danusso and co-workers (411-413). 

To conclude this review I must pay a tribute to the work and figure of Giulio Natta who pioneered macromolecular stereochemistry and was an active protagonist in die field for many years. The most important aspects of his discoveries and the present significance of the research derived therefrom have been illustrated by his students and by scientists from all over the world (16, 18). As may be seen finom the present article, many of the fundamental discoveries were derived from the work of his research group at the Milan Polytechnic. A large part of the later development, also, is the fruit of a cultural tradition that has influenced the whole Italian school of polymer chemistry. 

Green MM, Reidy MP, Johnson RJ, Darling G, Oleary DJ, Willson G (1989) Macromolecular stereochemistry the out-of-proportion influence of optically active comonomers on the conformational characteristics of polyisocyanates. The sergeants and soldiers experiment. J Am Chem Soc 111 6452-6454... 

Keywords Macromolecular stereochemistry. Main-chain chiral polymer, Atropisomeric polymer, Kinetic resolution. Anionic polymerization. Cationic polymerization. Insertion polymerization... 

The field of stereochemistry serves as a unifying theme for the expanded definition and diversification of chemistry. The consequences of molecular and macromolecular shape and topology are central to issues of chemical reactivity, physical properties, and biological function. With that view, the importance of stereochemistry had never been greater, and it is hoped that this series will provide a forum for documentation of significant advances in all of these subdisciplines of chemistry. 

Protein polymers based on Lys-25 were prepared by recombinant DNA (rDNA) technology and bacterial protein expression. The main advantage of this approach is the ability to directly produce high molecular weight polypeptides of exact amino acid sequence with high fidelity as required for this investigation. In contrast to conventional polymer synthesis, protein biosynthesis proceeds with near-absolute control of macromolecular architecture, i.e., size, composition, sequence, topology, and stereochemistry. Biosynthetic polyfa-amino acids) can be considered as model uniform polymers and may possess unique structures and, hence, materials properties, as a consequence of their sequence specificity [11]. Protein biosynthesis affords an opportunity to completely specify the primary structure of the polypeptide repeat and analyze the effect of sequence and structural uniformity on the properties of the protein network. 

In brief, we can say that the study of macromolecular compounds has introduced a new dimension into organic stereochemistry. This is true not only in the spatial sense if one considers the shape of the macromolecule, but also in the time sense if one examines the process of polymerization and the transmission of stereoregularity and chirality within each macromolecule. Finally, the study of macromolecules has necessitated the introduction of concepts and methods (e.g., the statistical approach), which are usually not pertinent to the stereochemistry of low molecular weight compounds (4). 

Macromolecular diffraction data are rarely of sufficient quality and quantity to allow construction of atomic models that would obey basic stereochemistry just based on this optimization of parameters to data. The observation-to-parameter ratio is a key factor in optimization procedures. For the optimization of a crystallographic model. 

The system of conjugated double bonds responsible for carotenoid colors also helps to impart specific shapes to these largely hydrophobic molecules and ensures that they occupy the appropriate niches in the macromolecular complexes with which they associate. Information on stereochemistry is provided in a short review by Britton.138... 

Adrian Zajgc was born in Krzepice, Poland, in 1981 he studied at Jan Dlugosz Academy of Czestochowa, where he obtained an M.Sc. in 2005. Presently he is employed at the Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Lodz. His research interests include chemistry and stereochemistry of hetero-organic compounds and asymmetric synthesis. 

In subsequent chapters we analyze how the macromolecular components that are found in tissue give rise to the complex biological structures that make up the intracellular and extracellular materials. To do this we need to consider how stereochemistry of the units contained in macromolecules... 

At a relative humidity of 80%, the sodium salt of pig-mucosal heparin shows a fiber repeat of 1.65 nm, with a triclinic unit-cell containing one chain. The sodium salt of macromolecular heparin from rat skin, at 78% r.h., showed a repeat of 1.73 nm, and crystallized in an orthorhombic unit-cell. After 24 hours at 84% r.h., the repeat changed to 1.65 nm, the pattern corresponding to that of pig-mucosal heparin. The stereochemistry of the chain in terms of the chain repeat and , i/r maps was discussed. It was suggested that the chemical formula for heparin is poly[(l— 4)-/3-D-GlcpA-(l— 4)- -d-GlcNAcp], and a scheme of 5-epimerization of /3-D-glucuronic acid to a-L-iduronic acid was discussed. 

The revolutionary discoveries by Ziegler and Natta, relating to the low pressure polymerization, respectively, of ethylene and of propylene and other a-olefins onto the previously unknown crystalline polymers, opened a new era in science and technology. Since then, remarkable progress has been made in the fields of coordination catalysis, macromolecular science and stereochemistry. With the discovery and development of the new generation catalytic systems for polyethylene in the late 1960 s, and more recently for polypropylene, enormous progress was made in terms of polymerization process as to economics and product quality Further process simplification and, above all, ever more accurate product quality control by taylor made catalytic systems is the aim of the 1980 s. 

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