Chemical Conversion of Macromolecules

Numerous chemical conversions of macromoiecuiar substances are also of technical interest, like reactions that attach or alter small parts of a polar group. 

Examples are the introduction of carboxyl or hydroxy groups to increase hy-drophilicity, crosslinking (vulcanization) of polydienes with sulfur, or conversions that proceed only at the terminal groups under retention of the molecular backbone (chain-analogous conversion). 

Amongst the important chemical conversions of macromolecular substances are the various reactions of cellulose. The three hydroxy groups per CRU can be partially or completely esterified or etherified. The number of hydroxy groups acetylated per CRU are indicated by the names, i.e., cellulose triacetate, cellulose 2-acetate, etc. Another commercially important reaction of cellulose is its conversion to dithiocarboxylic acid derivatives (xanthates). Aqueous solutions of the sodium salt are known as viscose they are spun into baths containing mineral acid, thereby regenerating the cellulose in the form of an insoluble fiber known as viscose rayon. 

Likewise, in the preparation of many ion-exchange resins, suitable functional groups are introduced by secondary reactions of macromolecular substances (that are generally crosslinked see Sect. 5.2). In this context the utilization of crosslinked polystyrene resins or poly(acrylamide) gel in the solid-phase synthesis of polypeptides (Merrifield technique) or even oligonucleotides should be mentioned. After complete preparation of the desired products they are cleaved from the crosslinked substrate and can be isolated. 

Crosslinked polymers with functional groups have recently been used even more frequently as reagents for the synthesis of low-molecular-weight organic compounds since they are easily separated after conversion and sometimes can easily be regenerated. The immobilization of enzymes by attaching them to crosslinked polymers should also be mentioned. This technique has already found industrial applications. 

Under suitable conditions many of the known reactions in organic chemistry can, in principle, be applied to the corresponding functional groups of macromolecular substances, but there are differences in some respects between the conversions of macromolecular and low-molecular-weight substances. This is also tme for the experimental implementation. 

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At low-conversion copolymerization in classical systems, the composition of macromolecules X whose value enters in expression (Eq. 69) does not depend on their length l, and thus the weight composition distribution / ( ) (Eq. 1) equals 5(f -X°) where X° = jt(x°). Hence, according to the theory, copolymers prepared in classical systems will be in asymptotic limit (/) -> oo monodisperse in composition. In the next approximation in small parameter 1/(1), where (/) denotes the average chemical size of macromolecules, the weight composition distribution will have a finite width. However, its dispersion specified by formula (Eq. 13) upon the replacement in it of l by (l) will be substantially less than the dispersion of distribution (Eq. 69)... 

The development of a quantitative theory of a free-radical copolymerization implies the derivation of equations for the rate of the monomers depletion and the statistical characteristics of the chemical structure of macromolecules present in the reaction system at the given conversion p of monomers. Elaborating such a theory one should take into account a highly important peculiarity inherent to any free-radical copolymerization. This peculiarity is that the characteristic time of a macroradical life is appreciably less than the time of the process duration. Consequently, its products represent definitely... 

Tribochemical conversions of macromolecules take place in the pol3mier layers immediately in contact with the metal counterbody. Since the counterbody surface commonly shows certain chemical and catalytic activity, this contributes to the friction process of the contact chemical reactions. 

Once the particular branching process that specifies the probability measure on the set of macromolecules of a polymer specimen has been identified, the statistical method provides the possibility to determine any statistical characteristic of the chemical structure of this specimen. In particular, the dependence of the weight fraction of a sol on conversion can be calculated by formulas [extending those (55)] which are obtainable from (61) provided the value of dummy variable s is put unity ... 

Macromolecules may be classified according to different criteria. One criterion is whether the material is natural or synthetic in origin. Cellulose, lignin, starch, silk, wool, chitin, natural rubber, polypeptides (proteins), polyesters (polyhydroxybutyrate), and nucleic acids (DNA, RNA) are examples of naturally occurring polymers while polyethylene, polystyrene, polyurethanes, or polyamides are representatives of their synthetic counterparts. When natural polymers are modified by chemical conversions (cellulose —> cellulose acetate, for example), the products are called modified natural polymers. 

The characteristic feature of these reactions is the direct conversion of the mechanical work, A, used in deforming the macromolecules, into the chemical energy of the activated chains and radicals formed. 

Mechanosensing is used to describe the process by which cells sense mechanical forces. Mechanochemical transduction is the phrase that is used to try to describe the biological processes by which external forces such as gravity influence the biochemical and genetic responses of cells and tissues. Specifically, these responses include stimulation of cell proliferation or apoptosis (death) and synthesis or catabolism of components of the extracellular matrix. These processes cause either increases in chemical energy (conversion of amino acids or other small molecules into macromolecules) or decreases in chemical energy (depolymerization of macromolecules). 

The polymer electrolytes are obviously the most suitable materials for the investigation of chemical interactions between macromolecules. By changing the ionic power, concentration and other parameters, the degree of conversion in reactions between oppositely charged chains can be regulated. 

It will be shown in a forthcoming review that a reversible change of conformation of linear macromolecules in solution may result in an appredable change of solution properties, and particularly of their viscosity. Such conformational changes and the resulting effects can be displayed by chemical means, and were actually described several years ago by Kuhn, Katchalsky and coworkers " and more recently by Osada and Saito. They are responsible for the mechanochemical behavior of polymer systems in the solid state, i.e. the conversion of chemical into mechanical energy . ... 

The chemical transformations that lead to the conversion of cellulose to mixed polysaccharides differing from cellulose in the conformation of the pyranose ring and the number and configuration of the hydroxyl groups of the repeating unit of the macromolecule, may exert a considerable effect on the structure of the material as well as on its important chemical properties (rate of acetylation and O-alkylation of OH groups, stability of the acetal linkage) and physicochemical indices (solubility of modified preparations of cellulose and cellulose ethers and esters). 

Photoinitiated polymerization uses the energy of light for the rapid conversion of monomeric liquids to solid polymeric products. The term photopolymerization implies that the initiation step of a radical, cationic, or anionic chain reaction producing a macromolecule requires the absorption of a photon. Since the absorption of one photon may start the reaction of up to 10 monomeric units, photoinitiated polymerization is, in practice, one of the most powerful chemical amplification techniques. 

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