Polymeric molecules

Chromium(III) Chemistry. The most characteristic reactions of Cr(III) in aqueous solution at >4 pH, eg, in the intestine and blood, and hydrolysis and olation (147). As a consequence, inorganic polymeric molecules form that probably are not able to diffuse through membranes. This may be prevented by ligands capable of competing for coordination sites on Cr(III) (see Coordination compounds) (147). Thus any large fraction of ingested Cr(III) should be absorbed. Chromium (ITT) in the form of GTF may be more efficiendy absorbed. 

Most reactions of nucleic acid hydrolysis break bonds in the polynucleotide backbone. Such reactions are important because they can be used to manipulate these polymeric molecules. For example, hydrolysis of polynucleotides generates smaller fragments whose nucleotide sequence can be more easily determined. 

Through direct excitation of a monomeric or polymeric molecule or of a molecular complex (A) followed by a reaction producing an initiating species ... 

Using the same method a block copolymer of polypeptides and vinyl monomers was also prepared. As mentioned in Section II, Bamford and Mullik [22] introduced an interesting method of photoinitiation of vinyl monomers by the Mn2(CO)io or the Re2(CO)io/C2F4 system. By these methods polymeric molecule with (CO)sMn—CF2CF2—terminals is produced (see Scheme [12]). If a polymer of this kind is heated to 100°C in the presence of vinyl monomer, a block copolymer AB or ABA with Cp2- F2 linkage is produced [ ] ... 

The propagation step of polymerization involves an addition of monomeric units to the growing centers followed by regeneration of these centers. A series of consecutive propagation steps yields eventually a long polymeric molecule. 

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... 

A different situation arises when one considers a stereospecific catalyst which is endowed with optical activity and which favors therefore a specific configuration. Such a catalyst, if highly stereospecific, should form polymers, for example of all d configuration with an occasional inclusion here and there of l units. Of course if a racemic mixture of such a catalyst is used, then formation of a racemic mixture of polymers is expected, each polymeric molecule having an all d configuration incrusted with l units or an all l configuration incrusted with d units. 

The termination step of a polymerization involves a reaction which destroys the activity of the growing end and thus leads to cessation of its growth. The resulting polymeric molecule is frequently referred to as a dead polymer. If the activity of a growing chain is transferred to another molecule, the process is referred to as chain transfer if it is lost entirely, a normal termination step is involved. 

While 1,2- and 1,3-alkanediols form only monomeric 1,3,2-dioxaborolanes and 1,3,2-dioxaborinanes with triethylborane, 1,4- and 1,5-alkanediols give mainly dimeric macrocyclic boronates 41 and 42, which are obtained in yields of 70-75% after purification by vacuum distillation. As lateral products oligo-and polymeric molecules are formed. The main products have a 14- and 16-membered heterocyclic ring, respectively (Fig. 15) [84]. 

Still another, and chains, long or short, may be built up. This is the mechanism of free-radical polymerization. Short polymeric molecules (called telomers), formed in this manner, are often troublesome side products in free-radical addition reactions. 

At all temperatures liquid sulfur consists of a complex mixture of all homocycles from Ss to at least S35 and of larger polymeric molecules of cyclic and chain-like structure (collectively termed as Sqo) [34]. At temperatures above 250 °C smaller molecules such as S5, S4, S3, and S2 are also likely components of the liquid as the composition of the equilibrium vapor demonstrates [9] (see above). In addition, branched rings and chains are probably minor components at temperatures near the boiling point of 445 °C [35] (see below). 

Yamagishi, A., Takeuchi, T., Higashi, T. and Date, M. (1989) Diamagnetic orientation of polymerized molecules under high magnetic field. J. Phys. Soc. Jpn., 58, 2280-2283. 

Polymers are examples of organic compounds. However, the main difference between polymers and other organic compounds is the size of the polymer molecules. The molecular mass of most organic compounds is only a few hundred atomic mass units (for reference, atomic hydrogen has a mass of one atomic mass unit). The molecular masses of polymeric molecules range from thousands to millions of atomic mass units. Synthetic polymers include plastics and synthetic fibers, such as nylon and polyesters. Naturally occurring polymers include proteins, nucleic acids, polysaccharides, and rubber. The large size of a polymer molecule is attained by the repeated attachment of smaller molecules called monomers. 

A two-dimensional disorder results when rod-like polymeric molecules are mutually shifted with statistical frequency. Translational symmetry then only exists in the direction of the molecules, and not in the transverse directions. The rod is a three-dimensional object with one-dimensional translational symmetry. Its symmetry is that of a rod group. Layer groups and rod groups are subperiodic groups. They are listed in detail in International Tables for Crystallography, Volume E. 

One approach is to synthesise polymeric molecules containing crown ether rings which stack on top of each other to produce linear canals through which small cations could migrate. The poly(iminomethylene) crown ether 36 of van Beijnen et al. 241 ... 

There are five prime factors that determine the properties of starches 1. starch is a polymer of glucose (dextrose) 2. the starch polymer is of two types linear and branched 3 the linear polymeric molecules can associate with each other giving insolubility in water 4. the polymeric molecules are organized and packed into granules which are insoluble in water and 5 disruption of the granule structure is required to render the starch polymer dispersible in water. The modification of starch takes into account these factors. 

Figure 8.9 Examples of four-coordinated molecules of beryllium (a) BeCl2 2Et20 and (b) the polymeric molecule BeCl2.

Polymeric molecules contain reactive hydrogen sites in the form of alcohols and carboxylic acids. The reaction sequence, alcohol plus anhydride and acid plus oxirane, results in an increase of one in the degree of polymerization if the oxirane is DGEBA. Polymeric species also supply oxiranes via the pendant R. These reactions are generalized by the notation ... 

The several kinetic rates are a consequence of Reactions 2-5 The first two represent monomeric additions the third describes all rates by which two polymeric molecules smaller than j form a molecule of size j. The molecule A AC contains a carboxylic anion the second molecule P. supplies aS oxirane and, in general, is any polymeric molecule of iSze j-n. 

Monomeric additions result in the first and second rate expressions. Polymeric molecules react with the activated intermediate at a rate that is proportional to their cumulative molar concentration P q. ... 

The cumulative molar concentration of polymeric species P 0 may be evaluated from the population density distribution, Equation 16. The first two rate expressions represent monomeric additions which do not change the molar concentration of polymeric molecules. A rate constant that describes functionality as a separable function of the molecule s degree of polymerization satisfies this constraint. The simplest, realistic function is the linear expression... 

At this point it is necessary to make a distinction between the meanings of template and primer. The word template refers to the structural sequence of the polymerized monomeric units of a macromolecule that provides the pattern for the synthesis of another macromolecule with a complementary or characteristic sequence. The word primer, on the other hand, refers to a polymeric molecule that contains the growing point for the further addition of monomeric units. Glycogen is an example of a primer to which glucose units are added however, glycogen has no template activity. 

Figure 12.6.1 Ester linkages that connect monomers to form polymeric molecules known as polyesters.

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