Macromolecular active centers

Transfer of aqua electrolytes in polymers has characteristics [22] that distinguish it from the transfer of other low-molecular-weight matter. The presence of water in the medium sorbed may result in formation of aqua associates and electrolyte in the polymer. Depending on the amount of sorbed water, the polymers are subdivided into three groups (i) hydrophobic, i.e. slightly swelling in water (water concentration in the polymer below 0.5%), (ii) mildly hydrophilic (water concentration 0.5-10%), (iii) hydrophilic (water concentration above 10%). This subdivision is very conventional since the permeability mechanism depends not only on water concentration in the polymer but on the nature of their physical-chemical interactions with electrolyte ions and macromolecular active centers as well [23]. 

The number of commercially available crosslinkers for sulfhydryl and photoreactive conjugations provides enough variety to design successful experiments in photolabeling, such as studying active centers and macromolecular interactions. 

The characteristics of the active centers in free-radical polymerizations depend only on the nature of the monomer and are generally independent of the reaction medium. This is not the case in ionic polymerizations because these reactions involve successive insertions of monomers between a macromolecular ion and a more or less tightly attached counterion of opposite charge. The macroion and counterion form an organic salt which may exist in several forms in the reaction medium. The degree and nature of the interaction between the cation and anion of the salt and the solvent (or monomer) can vary considerably. 

Anionic polymerizations are chain-growth processes in which the active center to which successive monomers are added is a negative ion that is associated with a positive counterion. The degree of interaction between the macromolecular anion and its counterion depends on the nature of the respective ions and the medium in which the polymerization is proceeding. 

The probability of the existence of active centers in the form of ion pairs or free ions is determined by the free energy variation due to (a) a change in the entropy and transition from Gaussian distribution to cyclic conformation and (b) a change in the energy of Coulomb interaction at this transition. This probability evidently depends on the length of the macromolecular chain. 

Tait, P.. 1. T. "Studies on Active Center Determination in Ziegler-Natta Polymerization" Paper presented at Midland Macromolecular Symposium Midland, Mich., August 17-21, 1981. Gaunt, A. D. "Specialist Periodical Report, Catalysis" Chemical Society London, 1977 Vol. 1. 

The increase of phosphatides removal degree Q up to some polymer flocculator concentration at which the greatest value Q (Q ) is reached, is due to a simple increase of active centers (capture sites [183]) number of flocculation owing to a macromolecular coils (polymer concentration) number growth in solution. At the concentration macromolecular coils touching occurs, that corresponds to At growth coils interpenetration is realized and the indicated transition scale is determined by the criterion [131] ... 

Therefore immobilization of active centers on the supports is perhaps one possibility of diminishing the prevailing role of side reactions 4 and 5, and thereby of enhancing the efficiency of metal complex catalysts for polymerization of olefins. It was expected that spatial isolation of MX (as immobilization of enzymes prevented their deactivation) would lead to a decrease in bimolecular deactivation of active centers and in turn, to a cooperative stabilization preventing monomolecular termination. Instead, as earlier studies have shown (Fig. 12-6) [69] polymer-immobilized complexes are stable over time. Macromolecular metal complexes for polymerization processes can be used as powders, films, fiber... 

By following the pH influence on the hydrolytic efficiency, a relative stabilization was found between pH=5.8-8.2 (Figure 7), that is a much wider range than that observed for the free a-amylase activated by Ca ions Among the possible explanations of this behaviour that referring to the possible stabilization of the active center and proteic chain by the free ionizable carboxy groups remaining on the CMC macromolecular chain,deserves mention. 

Note / An active center may be located anywhere along a macromolecular chain. 

The persistence of active centers even after consumption of all the monomer allows one to trigger further chain growth by incremental addition of monomer and/or to synthesize complex macromolecular architectures that would be inaccessible by conventional polymerizations. 

Such a good definition of the molecular dimensions associated with the persistence of the active centers was extensively applied for the purpose of so-called macromolecular engineering, to design and construct precision macromolecular architectures. An account of the various possibilities is described in Chapter 9, including those based on other living polymerizations. 

Because of the small number of truly active centers, the molar masses of the polymers obtained are very high to control the sample molar mass, polymerizations are carried out in the presence of transfer agents and mostly H2 is used upon reaction with a macromolecular alkyltitanium, titanium hydride is produced that is able to reinitiate polymerization ... 

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