Combination with Counterion

Termination by combination of the propagating center with the counterion 

Alternately, the propagating ion may combine with an anionic fragment from the counterion, 

Termination by combination differs from the other modes of termination in that the kinetic chain is usually terminated, since the concentration of the initiator-coinitiator complex decreases. 

Combination with counterion is also important when aluminum alkyl-alkyl halide-initiating systems are used [DiMaina et al., 1977 Kennedy, 1976 Reibel et al., 1979]. 

Alkylation involves transfer of an alkyl anion to the propagating center. Hydridation involves transfer of a hydride ion from the alkyl anion to the propagating center. Hydridation occurs in preference to alkylation when the trialkylaluminum contains P-hydrogens. 

H-E CH2C(CH3)2 CH2C(CH3)2(BCl30H) — H-ECH2C(CH3)2 9irCH2C(CH3)2Cl + BCljOH 

This process, unlike the above mentioned processes, terminates the Icinetic chain. In general terms, the reaction can be represented by 

This type of termination occurs, for example, in the trifluoroacetic acid initiated polymerization of styrene (Throssell et al., 195 Odian, 1991 Billmeyer, Jr., 1994)  

The propagating ion may also combine with an anionic fragment from the, counterion (Kennedy and Feinberg, 1978 Odian, 1991) as in BX3.OH2 (X = halogen) initiated polymerization or terminate by alkylation or hydridation (Kennedy, 1976 Reibel et al., 1979 Odian, 1991) when aluminium aUq l-alkyl halide initiating sytems are used. 

Problem 8.17 Write equations to describe plausible termination reactions in cationic polymerization of isobutylene initiated by (a) BF3.OH2, (b) BCI3.OH2, (c) Al(CH3)3/r-butyl chloride, and (d) Al(C2H5)3/t-butyl chloride. 

The termination of growth of individual chains can occur by chain transfer to solvent or impurity present in the system, or to a transfer agent (denoted here by TA or S) deliberately added to the system. In general terms, the reaction can be 

This type of termination occurs, for example, in the tri uoroacetic acid initiated polymerization of 

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Since 1985, many different transition metal compounds have been examined for their ability to produce syndiotactic polystyrene in combination with counterions based on methylalumoxane, borane, borate and other chemicals. 

The orange-red [SsN] anion Xm2.x 465 nm) is obtained by the addition of triphenylphosphine to a solution of a [S4N] salt in acetonitrile.It can be isolated as a salt in combination with large counterions, e.g., [Ph4As] or [N(PPh3)2] , but it is unstable with respect to the formation of the blue [S4N] anion in solution or in the solid state under the influence of heat or pressure. 

The simplest possible coordination compounds of Cd and Hg with chalcogen donors are the anions MCh22. Multinuclear NMR studies of these species (Ch = Se, Te) in NH3 or en solutions with [K(2.2.2-cryptand)]+ counterions have been reported.314 Structural studies31, also combined with new multinuclear NMR measurements, have been performed.316... 

Co/pH and V o/pH results are sensitive to different aspects of the surface chemistry of oxides. Surface charge data allow the determination of the parameters which describe counterion complexation. Surface potential data allow the determination of the ratio /3 —< slaDL- Given assumptions about the magnitude of the site density Ns and the Stern capacitance C t, this quantity can be combined with the pHp2C to yield values of Ka and Ka2. Surface charge/pH data contain direct information about the counterion adsorption capacitances in their slope. To find the equilibrium constants for adsorption, a plot such as those in Figures 7 and 8 can be used, provided that Ka and Kai are independently known from V o/pH curves. 

Other molecular thermodynamic models for protein-reverse micelle complexes have also emerged. Bratko et al. [171] presented a model for phase transfer of proteins in RMs. The shell and core model was combined with the Poisson-Boltzmann approximation for the protein-RM complex and for the protein-free RM. The increase in entropy of counterions released from RMs on solubilization of a protein was the main contribution to the decrease in free energy of com-plexation. Good agreement was found with SANS results of Sheu et al. [151] for cytochrome C solubilization and the effect of electrolytes on it. However, this model assumes that filled and empty RMs are of the same size, independent of salt strength and pH, which is not true according to experimental evidence available since then. 

The same thermodynamic quantities needed for mixed micelle formation (already discussed) are also needed for mixed admicelle formation. Luckily, the monomer-admicelle equilibrium data can be fairly easily and unambiguously obtained (e.g., see Chapter 15). This should be combined with calorimetric data for a more complete thermodynamic picture of the mixed admicelle. As with micelles, counterion bindings on mixed admicelles also need to be obtained in order to account for electrostatic forces properly. Only one study has measured counterion binding on single-component admicelles (3 .), with none reported for mixed admicelles. 

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