Repeating unit size

Exceptions to this repeat-unit-size rule (see Section 2.3) were found, however, even when the parental mitochondrial genomes carried the same ori sequence (compare the supppressivities of petites bl7/b20, bll/b7, a 1/7/8. f in/20 of Fig. 2.16 and the predominance in crosses of b7 over bll, of b over hp5, of b 13/1 over a-23/3 see also Rayko et al., 1988). This indicates that noncoding, intergenic sequences flanking ori sequences also play a role in the modulation of replication efficiency. Since in different petites such sequences differ in primary structure, size, and position relative to ori sequences, this modulation is likely to take place through an indirect effect on DNA and nucleoid structure (Rayko et al., 1988 see Fig. 2.21). 

In writing Eqs. (7.1)-(7.4) we make the customary assumption that the kinetic constants are independent of the size of the radical and we indicate the concentration of all radicals, whatever their chain length, ending with the Mj repeat unit by the notation [Mj ], This formalism therefore assumes that only the nature of the radical chain end influences the rate constant for propagation. We refer to this as the terminal control mechanism. If we wished to consider the effect of the next-to-last repeat unit in the radical, each of these reactions and the associated rate laws would be replaced by two alternatives. Thus reaction (7. A) becomes... 

In the concluding chapters we again consider assemblies of molecules—this time, polymers surrounded by solvent molecules which are comparable in size to the repeat units of the polymer. Generally speaking, our efforts are directed toward solutions which are relatively dilute with respect to the polymeric solute. The reason for this is the same reason that dilute solutions are widely considered in discussions of ionic or low molecular weight solutes, namely, solute-solute interactions are either negligible or at least minimal under these conditions. 

Eor some uses, higher molecular weight polymer consisting of 150—200 repeat units is required. Such polymer usually is prepared by soHd-state polymerization in which pellets are heated under an inert atmosphere to 200—240°C. The 2G is removed continuously. The rate of polymerization depends on particle size, end group composition, and crystallinity (65). 

The size of a polymer molecule may be defined either by its mass (see Chapter 6) or by the number of repeat units in the molecule. This latter indicator of size is called the degree of polymerisation, DP. The relative molar mass of the polymer is thus the product of the relative molar mass of the repeat unit and the DP. 

Crystalline solids are built up of regular arrangements of atoms in three dimensions these arrangements can be represented by a repeat unit or motif called a unit cell. A unit cell is defined as the smallest repeating unit that shows the fuU symmetry of the crystal structure. A perfect crystal may be defined as one in which all the atoms are at rest on their correct lattice positions in the crystal structure. Such a perfect crystal can be obtained, hypothetically, only at absolute zero. At all real temperatures, crystalline solids generally depart from perfect order and contain several types of defects, which are responsible for many important solid-state phenomena, such as diffusion, electrical conduction, electrochemical reactions, and so on. Various schemes have been proposed for the classification of defects. Here the size and shape of the defect are used as a basis for classification. 

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],... 

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