Combination of macroions

A special kind of termination in ionic polymerizations is the mutual combination of anionic and cationic living chains (see Chap. 5, Sect. 5.8). When the two polymers consist of different monomers, block copolymers are formed. The two macroions can also consist of the same monomer. 

Kucera et al. combined anionic and cationic polydimethylsiloxane. With the ratio of active centres 1 1, a perfectly stable polymer was produced which did not depolymerize even under conditions where a trace of acid or base would lead to a rapid decomposition of all polymer chains [105]. This was the first combination of macroions described in the literature. 

As far as termination is concerned, combination of polymeric ions is unimportant because most other termination methods are cheaper. Rather, it represents an elegant way to utilize termination for producing otherwise inaccessible block or graft copolymers. 

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New types of synthesis are based on the combination of macroions (or polymeric di-ions) of opposite charge [278]... 

It appears that a combination of polymeric ions with difunctional initiators should be of some advantage over the transformation processes described above [241]. We have found that the combination of a macroanion with a dicationic initiator and of a macrocation with a dianionic initiator proceeds with high efficiency [242]. Macroions can therefore be transformed by difunctional initiators... 

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... 

The combined effects of electroneutrality and the Donnan equilibrium permits us to evaluate the distribution of simple ions across a semipermeable membrane. If electrodes reversible to either the M+ or the X ions were introduced to both sides of the membrane, there would be no potential difference between them the system is at equilibrium and the ion activity is the same in both compartments. However, if calomel reference electrodes are also introduced into each compartment in addition to the reversible electrodes, then a potential difference will be observed between the two reference electrodes. This potential, called the membrane potential, reflects the fact that the membrane must be polarized because of the macroions on one side. It might be noted that polarized membranes abound in living systems, but the polarization there is thought to be primarily due to differences in ionic mobilities for different solutes rather than the sort of mechanism that we have been discussing. We return to a more detailed discussion of the electrochemistry of colloidal systems in Chapter 11. 

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 the growing chain ends with substances such as water, alcohols, acids, and amines. The ions produced by reaction of these substances can sometimes initiate new chains (chain transfer). Under certain conditions the ionic propagation species retain their ability to grow over extended periods of time, even after complete consumption of monomer ( living polymers , see Sect. 3.2.1). 

Fig. 2 Overview of the (2r, rn)-parameter plane containing the location of typical charged colloids with monovalent counterions in aqueous solution at room temperature and the location of Systems I-IV (squares). The directions of increased value in (i) counterion charge Z, (ii) macroion charge Zm, and (iii) the combined parameter are shown by arrows. Also shown are other simulated systems (dots) and the gas-liquid coexistence curve (dashed curve with triangles) [22]...

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