Counterions, polymeric

FIG. 3 Condensed DNA particle assembly using template polymerization of cations, (a) Principle of DNA condensation by counterion polymerization on DNA template (b) chemistry of polycation formation (c) e microscopy of condensed DNA particles, bar =100 nm. [(c) was reprinted from Ref. 97, copyright 1998 Oxford University Press.]... 

Counterion Polymerization temperature (V Reaction time (h) Conv. (%) M theor. M exp. M /M ... 

The reaction medium plays a very important role in all ionic polymerizations. Likewise, the nature of the ionic partner to the active center-called the counterion or gegenion-has a large effect also. This is true because the nature of the counterion, the polarity of the solvent, and the possibility of specific solvent-ion interactions determines the average distance of separation between the ions in solution. It is not difficult to visualize a whole spectrum of possibilities, from completely separated ions to an ion pair of partially solvated ions to an ion pair of unsolvated ions. The distance between the centers of the ions is different in... 

Friedel-Crafts (Lewis) acids have been shown to be much more effective in the initiation of cationic polymerization when in the presence of a cocatalyst such as water, alkyl haUdes, and protic acids. Virtually all feedstocks used in the synthesis of hydrocarbon resins contain at least traces of water, which serves as a cocatalyst. The accepted mechanism for the activation of boron trifluoride in the presence of water is shown in equation 1 (10). Other Lewis acids are activated by similar mechanisms. In a more general sense, water may be replaced by any appropriate electron-donating species (eg, ether, alcohol, alkyl haUde) to generate a cationic intermediate and a Lewis acid complex counterion. 

Cationic ring-opening polymerization is the only polymerization mechanism available to tetrahydrofuran (5,6,8). The propagating species is a tertiary oxonium ion associated with a negatively charged counterion ... 

For continuing polymerization to occur, the ion pair must display reasonable stabiUty. Strongly nucleophilic anions, such as C/ , are not suitable, because the ion pair is unstable with respect to THE and the alkyl haUde. A counterion of relatively low nucleophilicity is required to achieve a controlled and continuing polymerization. Examples of anions of suitably low nucleophilicity are complex ions such as SbE , AsF , PF , SbCf, BE 4, or other anions that can reversibly coUapse to a covalent ester species CF SO, FSO, and CIO . In order to achieve reproducible and predictable results in the cationic polymerization of THE, it is necessary to use pure, dry reagents and dry conditions. High vacuum techniques are required for theoretical studies. Careful work in an inert atmosphere, such as dry nitrogen, is satisfactory for many purposes, including commercial synthesis. 

Initia.tlon. The basic requirement for polymerization is that a THF tertiary oxonium ion must be formed by some mechanism. If a suitable counterion is present, polymerization follows. The requisite tertiary oxonium ion can be formed in any of several ways. 

The rate of ion propagation, is independent of the counterion and has been found to be about 46 X 10 in all cases for CF SO", AsF, SbF, SbCFg, PF g, and BF/ counterions. Conditions were the same for all counterions, ie, 8.0 M of monomer in CCI4 solvent and 25°C polymerization temperature. With less stable counterions such as SbCF and BF at most temperatures, the influence of transfer and termination reactions must be taken into account (71). 

Polyelectrolyte complex membranes are phase-inversion membranes where polymeric anions and cations react during the gelation. The reaction is suppressed before gelation by incorporating low molecular weight electrolytes or counterions in the solvent system. Both neutral and charged membranes are formed in this manner (14,15). These membranes have not been exploited commercially because of then lack of resistance to chemicals. 

D. M. Anderson. A new technique for studying microstructures NMR band-shapes of polymerized surfactants and counterions in microstructures described by minimal surfaces. J Physique Colloque 57 1-18, 1990. 

Initiator efficiency in terms of conversions and molecular weights were similar for model compounds and polymerizations. The influence of chlorine and bromine-containing counterions on polymerization was similar to that found in model study. 

Typical initiators for living anionic polymerization of siloxanes include conventional organoalkali compounds and lithium siloxanolates22). Initiators containing lithium counterions are preferable to sodium or potassium counterions due to the lower catalytic activity of lithium in siloxane redistribution reactions. Living anionic polymeriza-... 

Fig. 2. Arrhenius plots of the rate constants of the anionic polymerization of methyl methacrylate in THF as the solvent and with Na+ orCs+ as the counterion. (R. Kraft, A. H. E. Muller, V. Warzelhan, H. Hocker, G. V. Schulz, Ref.35>)...

The competing reactions are isomerization of the cationic chain end, transfer reactions to monomer, counterion and solvent, and also termination reactions. The actual process of propagation depends on the concrete interactions between the reactants present in the polymerizing system. A synopsis of interactions expected is given in Table 7. For the most important of them quantum chemical model calculations were carried out. 

Because fluoride ion, a strong nucleophile, would recombine immediately with the cationic chain end, it is not a suitable counterion for the cationic polymerization. In spite of this, the process... 

The above indicates the importance of the equilibrium in Eq. (18) for the termination of a polymeric chain. The halide transfer from the complex counterion to the cationic chain end was experimentally investigated using the model system Ph3C+MtX +1... 

The anions belonging to groups one and three as well as N and CN- possess a nucleophilicity which is too strong and therefore form an ester bond which is too stable. For this reason, their activity as counterion during reactive cationic polymerization systems is insignificant. 

Alkyl aluminium halides are used in many ways as coinitiators for the cationic polymerization. Due to presence of alkyl groups, which have the characteristics of potential carbanions, the alkyl aluminium halides and the counterions formed from them cause the following irreversible competing reactions whereby hydrocarbons are released — Termination by interaction of the cation with the alkyl group of the counterion, e-g-... 

On the basis of experimental findings Heinze et al. propose the formation of a particularly stable, previously unknown tertiary structure between the charged chain segments and the solvated counterions in the polymer during galvanostatic or potentiostatic polymerization. During the discharging scan this structure is irreversibly altered. The absence of typical capacitive currents for the oxidized polymer film leads them to surmise that the postulated double layer effects are considerably smaller than previously assumed and that the broad current plateau is caused at least in part by faradaic redox processes. 

Condensed DNA Particle Assembly Using Template Polymerization of Counterions... 

---