Chain structures molecules

The high molecular weight of a polymer is one of the most immediate consequences of the chain structure of these molecules. As indicated in Sec. 1.2, it is also the basis for describing the size of the polymer molecule, either directly or through the degree of polymerization. Most methods for the determination... 

In this chapter we examine the flow behavior of bulk polymers in the liquid state. Such substances are characterized by very high viscosities, a property which is directly traceable to the chain structure of the molecules. All substances are viscous, even low molecular weight gases. The enhancement of this property due to the molecular structure of polymers is one of the most striking features of these materials. 

In this chapter we examine the elastic behavior of polymers. We shall see that this behavior is quite different from the elasticity displayed by metals and substances composed of small molecules. This is a direct consequence of the chain structure of the polymer molecules. In many polymers elasticity does not occur alone, but coupled with viscous phenomena. The combination of these effects is called viscoelasticity. We shall examine this behavior as well. 

This will lead initially to branched chain structures such as indicated schematically in Figure 2.10, G indicating a glycerol residue and P a phthalic acid residue. In due course these branched molecules will join up, leading to a cross-linked three-dimensional product. 

In the first case, that is with dipoles integral with the main chain, in the absence of an electric field the dipoles will be randomly disposed but will be fixed by the disposition of the main chain atoms. On application of an electric field complete dipole orientation is not possible because of spatial requirements imposed by the chain structure. Furthermore in the polymeric system the different molecules are coiled in different ways and the time for orientation will be dependent on the particular disposition. Thus whereas simple polar molecules have a sharply defined power loss maxima the power loss-frequency curve of polar polymers is broad, due to the dispersion of orientation times. 

It is usual to think that plastics are a relatively recent development but in fact, as part of the larger family called polymers, they are a basic ingredient of animal and plant life. Polymers are different from metals in the sense that their structure consists of very long chain-like molecules. Natural materials such as silk, shellac, bitumen, rubber and cellulose have this type of structure. However, it was not until the 19th century that attempts were made to develop a synthetic... 

In a thermoplastic material the very long chain-like molecules are held together by relatively weak Van der Waals forces. A useful image of the structure is a mass of randomly distributed long strands of sticky wool. When the material is heated the intermolecular forces are weakened so that it becomes soft and flexible and eventually, at high temperatures, it is a viscous melt. 

Both of these structures are open-chained compounds corresponding to crown ethers in function if not exactly in structure (see Chap. 7). They have repeating ethyleneoxy side-chains generally terminated in a methyl group. Montanari and co-workers introduced the polypodes 22 as phase transfer catalysts . These compounds were based on the triazine nucleus as illustrated below. The first octopus molecule (23) was prepared by Vogtle and Weber and is shown below. The implication of the name is that the compound is multiarmed and not specifically that it has eight such side-chains. Related molecules have recently been prepared by Hyatt and the name octopus adopted. For further information on this group of compounds and for examples of structures, refer to the discussion and tables in Chap. 7. 

Look up the structure of human insulin (Section 26.7), and indicate w here in each chain the molecule is cleaved by trypsin and chymotrypsin. 

If all the oxygen containing groups are reduced, n-hexane results. This test helps establish that the glucose molecule has a chain structure. One representation of the structural formula of glucose, C6Hi206, is... 

We shall consider now the various degrees of order which characterize polymeric molecules. The addition of a monomeric unit to a growing chain may take place in more than one way. In the case of a vinyl or vinylidine monomer, i.e., CH2—CHA or CH2—CAB, head-to-head or head-to-tail addition may occur. In most cases the head-to-tail addition has a vastly greater probability than the head-to-head or tail-to-tail addition, and thus the latter is responsible only for small imperfections in the chain structure. Studies of head-to-tail and head-to-head additions were vigorously pursued in the 30 s and 40 s, and a good account of this work is available, for example, in Flory s recent monograph.15... 

In addition to the size of the molecules and their distribution, the shapes or structures of individual polymer molecules also play an important role in determining the properties and processability of plastics. There are those that are formed by aligning themselves into long chains of molecules and others with branches or lateral connections to form complex structures. All these forms exist in either two or three dimensions. 

Dimethyltin dichloride has a similar chain structure (375). In diethyltin diiodide (374), dimethyltin diisothiocyanate (376, 377), and di-chloro bis(chloromethyl)stannane (378), however, the distorted, trans-RgSnX geometry of each tin atom is completed by two bridging bonds involving the halogen or pseudohalogen atoms on the same, neighboring molecule. 

By ab initio MO and density functional theoretical (DPT) calculations it has been shown that the branched isomers of the sulfanes are local minima on the particular potential energy hypersurface. In the case of disulfane the thiosulfoxide isomer H2S=S of Cg symmetry is by 138 kj mol less stable than the chain-like molecule of C2 symmetry at the QCISD(T)/6-31+G // MP2/6-31G level of theory at 0 K [49]. At the MP2/6-311G //MP2/6-3110 level the energy difference is 143 kJ mol" and the activation energy for the isomerization is 210 kJ mol at 0 K [50]. Somewhat smaller values (117/195 kJ mor ) have been calculated with the more elaborate CCSD(T)/ ANO-L method [50]. The high barrier of ca. 80 kJ mol" for the isomerization of the pyramidal H2S=S back to the screw-like disulfane structure means that the thiosulfoxide, once it has been formed, will not decompose in an unimolecular reaction at low temperature, e.g., in a matrix-isolation experiment. The transition state structure is characterized by a hydrogen atom bridging the two sulfur atoms. 

A molecule of composition S4O has not been observed yet. Ab initio MO calculations indicate that a cyclic and an open-chain structure have almost the same energy but no details have been released yet [58]. 

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