Repeat unit sequence distribution

These equations show that the composition of the copolymer formed from a specific comonomer mixture is controlled by the monomer reactivity ratios for the copolymerization. Additionally, they control the sequence distribution of the different repeat units in the copolymer. If ta > 1 then "> A prefers to add monomer A (i.e., it prefers to homopropagate) and extended sequences of A-type repeat units are introduced, whereas if ta < 1 A prefers to add monomer B, i.e., to cross-propagate. In a similar way, ra describes the behaviour of monomer B. The effects of some specific combinations of ta and re values upon copolymer composition and repeat unit sequence distribution are considered in the next section. 

Effects of Repeat Unit Sequence Distribution and Specific Interactions. Most theories and quantitative structure-property relationships for Tg only consider the case of a random distribution of repeat units along the polymer chains in treating copolymers. They give equations which predict a monotonic change of Tg between the Tg values of the homopolymers of the constituent repeat... 

Macromolecules are very much like the crystalline powder just described. A few polymers, usually biologically-active natural products like enzymes or proteins, have very specific structure, mass, repeat-unit sequence, and conformational architecture. These biopolymers are the exceptions in polymer chemistry, however. Most synthetic polymers or storage biopolymers are collections of molecules with different numbers of repeat units in the molecule. The individual molecules of a polymer sample thus differ in chain length, mass, and size. The molecular weight of a polymer sample is thus a distributed quantity. This variation in molecular weight amongst molecules in a sample has important implications, since, just as in the crystal dimension example, physical and chemical properties of the polymer sample depend on different measures of the molecular weight distribution. 

The final class of polymers are copolymers containing one or more of the repeat units of classes 2 and 3 (15-18). Copolymer effectiveness would presumably be a function of the chemical structures of each comonomer, comonomer sequence distribution, and polymer molecular weight. The comonomer could be a relatively... 

The metathesis reaction of the imine bond was also able to dynamically select for oligomers that form the most stable host-guest complex [88]. It has been shown that rodlike and dumbbell-shaped guests have a higher affinity and specificity for oligomers of 20-22 repeat units [58,61 ]. Imine starter sequences 60,61a, and 61b, which can potentially form 16- (63a), 22- (63b), and 28-mers (63c) as well as smaller molecular weight (MW) materials (62a and 62b), were chosen as starter sequences for an imine ligation experiment (Fig. 37). When equiUbrated in chloroform the product distribution of 63a-c was close to 1 1 1,... 

Consider the description of the sequence distribution of isotactic and syndiotactic placements in the polymerization of a monosubstituted ethylene. The approach is general and can be applied with appropriate modification to the 1,4-polymerization of a 1,3-diene. Dyad tac-ticity is defined as the fractions of pairs of adjacent repeating units that are isotactic or syndiotactic to one another. The isotactic and syndiotactic dyads (XV) are usually referred to as meso and racemic dyads. The horizontal line in XV represents a segment of the polymer... 

The presence of a second type of repeat unit causes the dilute solution behavior to be more complex than that of homopolymers [1], Copyolymer composition and sequence distribution directly effect the intrinsic viscosity. Interactions between unlike chain segments and preferential interaction of solvent molecules with one of the comonomers are also of considerable importance. 

Initially, the protein-like HP sequences were generated in [18] for the lattice chains of N = 512 monomeric units (statistical segments), using for simulations a Monte Carlo method and the lattice bond-fluctuation model [34], When the chain is a random (quasirandom) heteropolymer, an average over many different sequence distributions must be carried out explicitly to produce the final properties. Therefore, the sequence design scheme was repeated many times, and the results were averaged over different initial configurations. 

Moreover, different molecules of the same biopolymer may vary in the proportion and distribution of these units, and the whole substance should be considered as a continuum of molecules transitional between two or more extreme structures.95 Hence, for complete elucidation of the primary structure, it is necessary to identify the repeating units present in a galactan molecule and to determine their distribution or sequence. It is clear that the procedure of structural analysis becomes much more difficult for less regular polysaccharides.16... 

When one looks at a polymer chain, one will soon realize that there are a number of structural factors that should or may be controlled in syntheses (Fig. 1). These factors may include (1) molecular weight (chain length) (2) molecular weight distribution (MWD) (3) end group (4) pendant group (5) sequence (the arrangement of constitutional repeat units... 

Yet another type of classification of polymers is based on the type of repeating unit. A homopolymer has one type of repeat unit. Copolymers are polymers that have more than one type of monomers or repeat units. If the monomers in a copolymer are distributed randomly along the chain, it is called a regular or random copolymer. If, on the other hand, a sequence of one type of monomer is followed by a sequence of another type of monomer, it is called a block copolymer. If the main chain is one type of monomer and the branch chains are of another monomer, it is called a graft copolymer. 

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