Chemistry
Stereochemistry of Polymers
Stereochemistry of polymers refers to the spatial arrangement of atoms in polymer molecules. It involves the study of how the arrangement of substituents around the polymer backbone affects its properties and behavior. Understanding stereochemistry is crucial for designing polymers with specific characteristics, such as strength, flexibility, and thermal stability.
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5 Key excerpts on "Stereochemistry of Polymers"
eBook - PDF- Robert J. Young, Peter A. Lovell(Authors)
- 2011(Publication Date)
- CRC Press(Publisher)
147 6 Stereochemistry and Coordination Polymerization 6.1 INTRODUCTION TO STEREOCHEMISTRY OF POLYMERIZATION In addition to the effects of skeletal structure and of the chemical composition of the repeat units, the properties of a polymer are strongly influenced by its molecular microstructure. Variations in the geometric and configurational arrangements of the atoms in the repeat unit, and the distribution of these different spatial arrangements for the repeat units along the chain, are of particular importance. Different molecular microstructures arise from there being several possible stereochemical modes of propagation. The possibility of head-to-tail and head-to-head placements of the repeat units has been considered already, with the observation that for both steric and energetics reasons the placement is regioselective , giving almost exclusively head-to-tail placements for most poly-mers. Therefore, only head-to-tail placements will be considered in this chapter. The factors which influence the stereochemistry of propagation for mono-olefins and 1,3-dienes will be described in generic terms before considering the extent to which they can be controlled in radical and ionic polymerizations. More specialized methods of polymerization which involve strong coordination of monomer during propagation will then be introduced since they provide much greater constraints on the stereochemistry and are capable of producing highly stereoregular polymers. 6.2 TACTICITY OF POLYMERS Chiral is the term used to describe objects which are non-superimposable on their mirror image (e.g. human hands and feet). Molecular chirality is of great importance and is used, for example, by nature to control biochemistry. The simplest chiral molecules have an sp 3 -hybridized carbon atom to which four different groups are attached, which makes the carbon atom asymmetric .
eBook - PDFPolymers
Chemistry and Physics of Modern Materials, Third Edition
- J.M.G. Cowie, Valeria Arrighi(Authors)
- 2007(Publication Date)
- CRC Press(Publisher)
157 6 Polymer Stereochemistry The physical behavior of a polymer depends not only on the general chemical composition but also on the more subtle differences in microstructure. As it is now possible to exercise a larger degree of control over the synthesis of speci fi c structures, it is prudent at this point to elaborate on the types of microstructural variations encountered before discussing how each can be produced. Several kinds of isomerism or microstructural variations can be identi fi ed, and these are grouped under four main headings: architectural, orientational, con fi gurational, and geometric. 6.1 ARCHITECTURE Differences here include branching, network formation, and polymers derived from isomeric monomers, for example, poly(ethylene oxide), I, poly(vinyl alcohol), II, and polyacetaldehyde, III, in which the chemical composition of the monomer units is the same, but the atomic arrangement is different in each case. This makes a considerable difference to the physical properties of the polymers, e.g., the glass transition temper-ature T g of structure I is 206 K, for II T g = 358 K, and for III T g = 243 K. 6.2 ORIENTATION When a radical attacks an asymmetric vinyl monomer, two modes of addition are possible: This leads to the con fi guration of the monomer unit in the chain being either head-to-tail if route I is favored, or a chain containing a proportion of head-to-head, tail-to-tail structure IV if route II is followed. CH 2 CH 2 O CH 2 CH OH CH O CH 3 n I n II III n R H 2 C CH X RCH 2 CH X RCHCH 2 X + I II III CH 2 CH CH 2 X CH CH 2 X CH X CH 2 CH X 158 Polymers: Chemistry and Physics of Modern Materials The actual mode of addition depends on two factors: the stability of the product and the possible steric hindrance to the approach of R • caused by a large group X in the molecule.
eBook - PDF- Muralisrinivasan Natamai Subramanian(Author)
- 2017(Publication Date)
- River Publishers(Publisher)
Polymer builds structurally from a repeating radical or structural unit. The general formula of one of the polymer is Where R = H, Cl, C 6 H 5 , etc. Polymer structure indicates flexibility and rigid nature. The hydrogen-bonding present in the polymer such as nylon, etc., enhances rigidity and makes solvent resistance. The long backbone chain provides mechanical properties. The structural features affect the accessibility of various chain conformations and thus are pivotal for a control of the mechanical behaviour of polymers and for the morphology of the polymers. 4.3 Stereochemistry In polymer, the regularity of the molecular backbone determines its control such as stereochemistry. The polymer structure distinguishes itself from atomic materials. The constitution and configuration are directly reflecting the mechanical properties. Polymer having identical constitutional repeating units can nevertheless differ because of isomerism. Linear, branched, and cross-linked polymers of the same monomer are considered as structural isomers. The number of conformation and morphology control the mechan-ical properties. Stereoisomerism occurs from differences in configuration of asymmetric carbon atoms in the chain. Properties of polymers have asso-ciated with their structure with the configuration of units in the polymeric molecules. Structural isomerism formation is due to branches in homopolymer and variation in the monomer distribution in copolymers. Structural variations in polymer occur, they are 1. Sequence isomerism 2. Stereoisomerism 3. Geometric isomerism 58 Polymer Chemistry 4.3.1 Sequence Isomerism Sequence isomerism arises from the variations in orientation of asymmetric monomer units. Most monomers used chain growth polymerizations are non-symmetrical results in three possible sequences of the monomer units in the chain.
eBook - PDFStructural Order in Polymers
Lectures Presented at the International Symposium on Macromolecules, Florence, Italy, 7-12 September 1980
- Francesco Ciardelli, Paolo Giusti, Francesco Ciardelli, Paolo Giusti(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
Finally, (4) if the units possess loci of asymmetry, stereochemical isomerism may introduce disorder into the structure of the chain. Each of these categories of irregularities, in the character of successive units of the molecule may obstruct crystallization. The structures of biopolymers are remarkably regular and uniform from one macromolecule of a given kind to another. The sequential arrangement of the structural units is precisely defined in proteins composed of some twenty different amino acid residues and in polynucleotides compounded from four nucleotides. The chiral senses of asymmetric centers, occurring in profusion in these macromolecules, are uniquely specified. The asymmetric centers in the structural units of cellulose and other polysaccharides are likewise fully specified. Synthesis of copolymers from a plurality of monomers with enchainment according to a prescribed sequence as in native proteins and polynucleotides is a feat beyond reach of present artifical synthetic methods. Similarly, the precise regulation of stereochemical structure in polymers of biological origin, and the replication thereof, are beyond the present capabilities of synthetic chemistry. The achievements of Professor Giulio Natta and his collaborators in their monumental investigation of stereoregular polymerization of vinyl monomers represent a major thrust in this direction, however. The results of these far-reaching investigations vividly illustrate the essential importance of structural and stereochemical regularity for the occurrence of crystallinity in polymers. Crystallinity, in turn, provides the key to a range of mechanical properties that is not otherwise attainable. STEREOREGULAR VINYL POLYMERS The prevalence of head-to-tail concatenation of monomers in vinyl polymers, so that the substitutent R groups occur on alternate chain atoms, -CH 2 -CHR-CH 2 -CHR- etc., 1 2 was demonstrated by Staudinger and Steinhofer and especially by Marvel and co-workers.
eBook - PDF- Norman L. Allinger, Ernest L. Eliel(Authors)
- 2009(Publication Date)
- Wiley-Interscience(Publisher)
-CH2-CHR-CHR-CHa-CHZ-CHR-CHR-CHz-CH-R- arrangement (Fig. 1). It is also possible for the same monomeric units to be arranged in a head-to-head and tail-to-tail fashion (Fig. 1) although these structures are rare. We will omit any discussion of these materials and branched or network polymers. It was as a result of the discoveries of Ziegler (1) and Natta and his co-workers (2-6) that the era of stereochemistry of linear synthetic polymers achieved maturity. Earlier workers such as Staudinger, Huggins, and Schildknecht discussed and even prepared stereoregular macromolecules. It remained for Natta to provide convenient routes for the preparation of stereoregular polymers. 11. STEREOCHEMISTRY OF A POLYMER CHAIN A. Tacticity Natta’s work created a whole new area of polymer chemistry (7). In order to define the structures of these new materials, he suggested the terminology which remains basic to the area of polymer stereochemistry (8). Figure 2 shows the designation of stereoregular placements of the monomeric residues in vinyl and related polymers. When all of the relative configurations* at the tertiary carbons are identical, the material is called isotactic; when the relative configurations at these tertiary carbons alternate along the polymer chain, the polymer is called syndiotactic. Random stereochemistry for the tertiary carbon leads to an atactic polymer structure (Fig. 2). *The assignment of “configuration” (D or L) for vinyl stereoregular polymers is open to serious question. Since the vinyl polymers really contain pseudo- asymmetric units (ignoring the end groups) it is definitely not proper to use D and L symbols. Bovey (9) suggested, and this author heartily concurs, that the terms D and L be used with low molecular weight compounds only, or with those polymers such as polypeptides, polypropylene oxide, or others where there is a genuine asymmetry within the polymer chain or side chains.
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