Macroion reactions

Polymer molecules with just one or a few ionic groups, in most cases terminal and anionic, are called macroions. They are encountered primarily in living polymers, polymer molecules present in a polymerizing reaction system that will grow as long as monomers (e.g., esters or nitriles of methacrylic acid) continue to be supplied. The ionic charge of the macroion always transfers to the last monomer added, keeping the macroion ready for the next such addition. 

We observe the a-methylene carbons of the methyl tetrahydro-furanium ion, the a-carbons of the two types of propagating chain heads, the macroion and the macroester (17). The observation of the a-methylene carbon resonances of the acyclic tertiary oxonium ion provides a direct proof of chain transfer reaction in THF polymerization. 

Again, in contrast to radical polymerization, there is no chain termination by combination, since the growing chains (macroions) repel each other electrostatically because of their like charges. Chain termination occurs only by reaction of... 

We have recently shown, that the living conditions can be achieved in the polymerization of gPL by using as initiators compounds providing higher proportion of macroions. This favourably decreases the importance of the parasitic side reactions by relatively increasing the overall rate of propagation processes [2.J. The following initiators were used ... 

Oxidation of Anionic Polymers In the Solid State The ability of the macroradical and of the macroions to diffuse In the mixture, and to interreact Is responsible for the secondary products formation coupling reaction and alcoholate synthesis. To prevent the diffusion phenomenon, we have carried out the deactivation In the solid state. The living polymers have been prepared In benzene, with or without a solvating agent (THF or TMEDA) and the solution has been freeze dried before the oxygen introduction. The experimental results are collected in Table VII. 

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. 

Rates of ionic polymerizations are by and large much faster than in free-radical processes. This is mainly because termination by mutual destruction of active centers occurs only in free-radical systems (Section 6.3.3). Macroions with the same charge will repel each other and concentrations of active centers can be much higher in ionic than in free-radical systems. Rate constants for ionic propagation reactions vary but some are higher than those in free-radical systems. This is particularly true in media where the ionic active center is free of its counterion. 

In the absence of side reactions the number average degree of polymerization will be c/[M]/r/[I] if initiation is by nucleophilic attack on the monomer or 2 d[M]/c/[I] if initiation is by electron transfer followed by dimerization of the monomeric radical anions (r/fM] and d[I] are the reacted concentrations of monomer and initiator, respectively). If the rale of initiation is very rapid compared to the propagation rate and the initiator is mixed very rapidly and efficiently into the reaction mixture, then all macroions should start growing at almost the same time and should add monomer at equal rates. The active centers can be terminated deliberately and simultaneously since there are no spontaneous termination reactions under appropriate experimental conditions. Polymers made in such reactions have molecular weight distributions which approximate the Poisson... 

The reaction medium in cationic polymerizations is usually a moderately polar chlorinated hydrocarbon like CHjCI (dielectric constant = 12.6 at —2(TC). A greater proportion of the macroions are free of their counterions in cationic than in anionic polymerizations in the usual solvents for the latter processes. Cationic polymerizations are characterized by extremely fast propagation rates. 

There is another question that has to be discussed the direction of an attack in the S), 2 reaction vs. the position of the anion in the onium salts. Indeed, the simple and attractive picture proposed by Szwarc in order to explain the differences between the reactivities of maaoanions and macroion pairs in the anionic homopropagation of styrene is based on the assumption that an ion pair has to dissociate partially when the transition state of propagation is reached ). Szwarc, after observing a similar reactivity of the polystyryl anion and polystyryl cesium ion pair, also assumed that no partial dissociation was needed for the large Cs cation. The... 

As discussed earlier (Sect. 4.2), the multiplicity of the forms of ionic species makes measurements of kp of elementary reactions more complicated nevertheless, these few cases in which kp for macrocations and macroion pairs have been determined separately are also discussed in this section. It has been shown that in the majority of the systems studied >= kp and, therefore, in Tables 10-12 listing the values of kp, we have tentatively assumed that kp represents the rate constants of propagation of maaoions and macroion pairs which coincide unless stated otherwise. 

In the polymerization of cyclic ethers, the equilibria of a number of sterns have been studied. However, oxirane and oxetane give exclusively macroesters apparently because the conversion of the macroester into the macroion, which is (as it will be shown in this section) a unimolecular reaction within a last polymer unit, is energetically unfavorable ... 

The macroester macroion pair interconversion has been analyzed as an uni-molecular opposed reaction ... 

These interactions are frequently ionic in character. The coulombic forces of interaction between macroions and lower molecular weight ionic species are central to the life processes of the cell. For example, intermolecular interactions of nucleic acids with proteins and small ions, of proteins with anionic lipids and surfactants and with the ionic substrates of enzyme catalyzed reactions, and of ionic polysaccharides with a variety of inorganic cations are all improtant natural processes. Intramolecular coulombic interactions are also important for determining the shape and stability of biopolymer structures, the biological function of which frequently depends intimately on the conformational features of the molecule. 

The propagation can take place with covalent or raacroester species, but in these cases the rate constants are lower than with macroions. The unimolecular equilibrium reaction is controlled by the ring strain. When the parent ring is not energetically favorable, the back attack involves the penultimate or other monomer unit. The classical examples are ethylene oxide and In Applied Polymer Science Tess, R., et al. ... 

Ionic polymerizations, as we shall see later, involve successive insertion of monomer molecules between an ionic chain end (positive in cationic and negative in anionic polymerization) and a counterion of opposite charge. The macroion and the counterion form an organic salt which may, however, exist in several forms depending on the nature and degree of interaction between the cation and anion of the salt and the reaction medium (solvent/monomer). Considering, for example, an organic salt a continuous spectrum of ionicities ( Winstein... 

Loo, R.R.O. Udseth, H.R. Smith, R.D. Evidence of charge inversion in the reaction of singly charged anions with multiply charged macroions. J. Phys. Chem. 1991, 95, 6412-6415. 

An Siv2 attack requires that the reaction occur at the oxygen-carbon bond. In such an attack, steric requirements are less restricted than they are in an anionic polymerization. In addition, positive and negative charges in the macroion pairs that contain the oxonium ions are dispersed and the anions are large. This means that the electrostatic interactions are less important in cationic polymerizations of this type than they are in anionic ones. 

Such contradictory behavior suggests that the copolymerization parameters are not only determined by the relative reactivities of the free macroions, but also by other parameters such as, on the one hand, the equilibria between free ions and the various ion pairs, and, on the other hand, the equilibria between one of the monomers and one of the growing macroions. Conventionally determined copolymerization parameters only provide mean reactivities if ion pairs are present. If a macroion-monomer equilibrium occurs, then the rate-determining step is the further reaction of the intermediary product, i.e.,... 

Thus, the copolymerization parameters give the ratio of the further reaction rates and not the relative reactivities of the two actual active species. It is even more complicated when only one macroion and/or one monomer forms an intermediary product. 

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