Polymer chemistry linear chains

Following their introduction in 1953, Ziegler-Natta catalysts revolutionized the field of polymer chemistry because of two advantages the resultant polymers are linear, with practically no chain branching, and they are stereochemical ly controllable. Isotactic, syndiotactic, and atactic forms can all be produced, depending on the catalyst system used. 

From the viewpoint of synthetic polymer chemistry, although the formation of stereospecific polymers (isotactic and syndiotactic) is very popular, the present polymer is the first example having a double syndiotactic structure. In ad tion, the polymer consists of an alternating zigzag-linear main chain structure. 

The history and development of polysilane chemistry is described. The polysilanes (polysilylenes) are linear polymers based on chains of silicon atoms, which show unique properties resulting from easy delocalization of sigma electrons in the silicon-silicon bonds. Polysilanes may be useful as precursors to silicon carbide ceramics, as photoresists in microelectronics, as photoinitiators for radical reactions, and as photoconductors. 

The connection between polymer chemistry and ceramic science is found in the ways in which linear macromolecules can be converted into giant ultrastructure systems, in which the whole solid material comprises one giant molecule. This transformation can be accomplished in two ways—first by the formation of covalent, ionic, or coordinate crosslinks between polymer chains, and second, by the introduction of crystalline order. In the second approach, strong van der Waals forces within the crystalline domains confer rigidity and strength not unlike that found when covalent crosslinks are present. 

Ribbed helices (costal helices) are important in organic chemistry because linear polymers contain side chains as well as backbones. We may, then, discern not only the catenal helix of the backbone, but the intercostal helix formed by all of the ribs and the infracostal helicesof the individual side chains. The intercostal helix may be iterative (as in an isotactic head-to-tail vinyl polymer or homogeneous poly-a-amino acid) or non-iterative (as in a random copolymer, an atactic polymer or typical protein). The intracostal helices can best be analyzed as short-chain crooked lines, as in Section III. Important as costal helicity is, it is secondary to catenal helicity and we therefore limit our attention to the primary helicity, that of long chains. Indeed, we limit our attention to catenal helices having chain motifs of two atoms and two bonds as found in head-to-tail vinyl homopolymers ... 

Polymer solutions represent the most convinient systems for studying the properties of the macromolecules. In effect, almost the all information that we have now about the properties of macromolecules comes from the characterization realized in solution. This is the state in which linear chains are characterized. Osmotic pressure measurements in polymer solutions revealed for the first time the existence of high molecular masses and this result confirmed the macromolecular hypothesis. The development of our knowledge of the polymer solutions reflects to some extention the development of the Polymer Chemistry itself. 

As in so many things in this field, if you want to work through the arguments yourself, you cannot do better than go to Flory— see Principles of Polymer Chemistry, Chapter EX. Stockmayer s equation illustrates the point we wish to make with dazzling simplicity as f the number of branches, increases, the polydispersity decreases. Thus for values of/equal to 4, 5 and 10, the polydispersity values are 1.25, 1.20 and 1.10, respectively. Note also that for / = 2, where two independent chains are combined to form one linear molecule (Figure 5-28), the polydispersity is predicted to be 1.5. Incidentally, an analogous situation occurs in free radical polymerization when chain termination is exclusively by combination. 

A class of related copolymers (8 to 13 in Table 5.2) interesting from the view point of chemistry and properties are the copolymers (produced as Technora by Teijin Ltd.) based on 1,4-phenylenediamine, tereph-thaloyl chloride and the third monomer 3,1 -d i a min od i p he ny I e t he r (Ozawa et al, 1978). The polymerization can be carried out at 0°C-80°C in an amide solvent (NMP or DMAc) with a small amount of salt (CaCL or LiCl). The incorporation of 3,4 -diaminodiphenylether moieties in the chain makes the polymer less rigid than PPD-T and it has a slightly lower decomposition temperature (500 °C as compared to 550 °C for PPD-T). However, as shown in Figure 5.1, except for side-steps the polymer can take a fully extended linear chain conformation because of the unique 3,4 -disubstitution of the diphenylether unit. If the two substitutions are in... 

Guar Polymer Chemistry. Cuar is a high molecular weight polysaccharide composed of a linear chain of D-mannose residues with randomly pendant D-galactosyl units see structure). The ratio of anhydromannose to... 

Depending on their coordination in the backbone network, group IV elements can form extended, covalently bonded structures of different dimensionality. As the most commonly known example, the fourfold coordination of the sp hybridized atoms leads to the three-dimensional (3D) crystalline solids diamond, c-Si, c-Ge, and a-Sn with their well-known semiconducting properties. On the other hand, linear (ID) polymer chains (XR2) with X = C, Si, Ge, Sn are based on a twofold coordination of the backbone atoms and are of great importance in organic and inorganic polymer chemistry. ... 

In the physical chemistry of polymers, the degree of branching is described by the g-factor, which is defined as the ratio between the mean radius of gyration of the branched chain and that of the linear chain of the same molecular mass... 

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