Chemical repeating unit

Coarse-grained models have a longstanding history in polymer science. Long-chain molecules share many common mesoscopic characteristics which are independent of the atomistic stmcture of the chemical repeat units [4, 5 and 6]. The self-similar stmcture [7, 8, 9 and 10] on large length scales is only characterized by a single length scale, the chain extension R. 

Z = number of chemical repeat units per unit cell. 

Cartesian and cylindrical polar atomic coordinates of the structural repeating unit of 31 polysaccharide helices are provided in Tables A1 to A31. Errors, if any, in the original publications have been corrected. The coordinates of hydrogen atoms are given in a majority of structures. If missing, they are not available in the references cited in Table I. Each table caption contains the structure number and polymer name assigned in Table I. Refer to Table II for its chemical repeating unit. Cartesian (x, y, z) and cylindrical (r, , z) coordinates are related by x r cost ), y = r sin<(> and z is the same in both systems. 

Pneumococcus, polysaccharides from, 6-7 Poly(a-L-guluronic acid), 353, 355-356,415 Poly(P-D-mannuronic acid), 353-354,414 Polysaccharides, 311 -439 amino sugar derivatives, 166 chemical repeating units, 321, 324-325... 

At this point, we will comment on how this procedure generalizes to other polymers. The other case that was considered by us [28,30,32,175,176] was concerned with bisphenol-A-polycarbonate (BPA-PC) (cf. Fig. 5.1). While for PE we had a correspondence that five chemical repeat units correspond to one effective bond of the bond fluctuation model, for BPA-PC the mapping ratio was inverse - one chemical repeat unit was mapped onto three effective bonds One must consider, however, the very different sizes of the chemical repeat units while for PE this is a single CH2 group, in BPA-PC the repeat unit involves 12 C-C or C-0 bonds along the backbone, and the end-to-end distance of the repeat unit is of the order of 10 A. Thus in this case also one effective bond corresponds to a group of four successive covalent bonds along the backbone of the chain, and a lattice unit corresponds to about 2.03 A [175],... 

Crystallization of polymers in chiral crystals, even in the case of achiral polymers, is quite frequent and strictly related to the occurrence of helical conformations of the chains. The crystallizable polymer consists of a regular sequence of a chemical repeating unit which can be chiral if it presents an asymmetric center or achiral. On the contrary, helical conformations assumed by the polymer chains in the crystalline state are intrinsically chiral, even though the chemical repeat is achiral. Three possible cases can be distinguished ... 

At the center of the dendrimer is the core that represents the first synthetic step in divergent synthesis, or the last step in convergent synthesis. Each concentric shell of branched units is a generation (X), designated (GX), of identical repeat units. Hundreds of chemical repeat units have been reported [3, 4], but our characterization has been focused primarily on poly(amidoamine) (PAMAM), -(CH2CH2CONHCH2CH2N) < [1], or poly(propyleneimine)... 

Figure 5. Chemical repeating units of (a) agarose, (b) iota-carrageenan, and (c) kappa-carrageenan.

As pointed out above, the occurrence of a pronounced piezoelectric effect in synthetic polymers can be mainly attributed to the presence of a non-eentro-symmetric unit cell and a net polarization in the material. To achieve this in polymeric materials one requires [28] (a) a large dipole moment in the chemical repeat unit (b) feasibility of crystallizing in a non-centrosymmetric unit cell (c) alignment of molecular dipoles. A dipole moment arises from a... 

A major impetus was given to work, both academic and industrial, in the field of lyotropic systems by the development by duPont of commercial fibres having exceptionally high tensile strength and modulus through use of nematic anisotropic solutions of relatively rigid-chain aromatic polyamides. The earliest product to appear, Fibre B, was based upon poly (p-benzamide) (I)10), but was replaced by the fully commercial product, Kevlar, based upon poly (p-phenylene terephthalamide) (II) U). Arenka, from Akzo, also has the latter chemical repeating unit. 

Tetra, octa, and dodecasaccharides have been prepared from Salmonella typhimurium lipopolysaccharide by specific degradation of the O chain with phage enzyme. These oligosaccharides contain 1,2, or 3 chemical repeating units (Svenson and Lindberg, 1981). 

Figure 1. Synthesis of a chemical repeating unit of the Shigella flexneri variant Y O-antigen, employing a sequential chain extension strategy.

The primary structure of the extracellular polysaccharide from X. aampestris was recently reinvestigated by Jansson et al. (2), and by Melton et al. (8). The chemical repeat unit is a pentasaccharide. The backbone is a 8-1,4 glucan like cellulose and chitin (9,10) with a trisaccharide side-chain on each cellobiose unit (I). 

FIG. 11.12 Chemical repeat units of pristine forms of several families of conducting polymers. 

Strictly speaking, the foregoing discussion of segmental correlation has assumed that the polymers are isotactic. When the chemical repeat units along the polymeric chains are not all stereo-identical, and syndiot ctic or even atactic isomers are present, the simple molecular concept of a segmental correlation factor no longer applies. A more elaborate treatment is then required (Volkenstein, 1963). Riande and Saiz (1992) discuss examples in more detail. 

Nylon 6 and nylon 6,6 are two commercially produced polyamides. How are they synthesized and how do their chemical repeat units differ (Note you won t find a description of the synthesis of nylon 6 in the book. Check other sources )... 

It might seem that we have a problem, because there is no isolated NMR line in the spectrum that is solely characteristic of one or the other repeat unit. Fear not, this information can be obtained in an indirect manner. The relatively broad lines appearing between 6.2 and 7.2 ppm in spectrum (A) correspond to the aromatic protons that occur in both St and f-BSOS. So, if we measure this total area (which reflects the contributions from the 5 aromatic protons of the styrene repeat and the 4 aromatic protons of the r-BSOS repeat), subtract out the contribution from the f-BSOS repeat (i.e., 4 times the normalized area per proton calculated from Equation 7-37) and then divide by 5 (the number of aromatic protons in the styrene repeat), we will have the normalized area per proton corresponding to the St chemical repeat unit. In summary Equation 7-38) ... 

Like polyethylene, polytetrafluoroethylene (PTFE) has a very simple chemical repeat unit, (-CFz-CF2-)n, and its discovery is a great story that in large part depended on Lady Luck, as opposed to design. Dr. Roy Plunkett, who obtained his Ph.D. from The Ohio State University, was hired by DuPont in 1936 and assigned to work on a new Freon -type refrigerant CClF2CHFr In order to synthesize this compound, he needed to prepare about 100 pounds of tetrafluoroethylene gas (TFE or CF2=CF2) as an intermediate. This he did with the aid of his technician,... 

The model used (independently) by Floiy and Huggins (following an earlier suggestion by Fowler and Guggenheim) assumes a large molecule such as a polymer can be treated as a set of linked segments. Each of these segments is not necessarily equal in size to the chemical repeat unit, but defined to have a molar volume equal to that of the solvent. 

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