Polymerizations with Zwitterions

The polymerization of zwitterions with sufficient stability to be isolated and purified has been demonstrated. These reactions of stable zwitterions provide well-defined examples of the involvement of zwitterionic propagating species. The first example of the polymerization of a stable zwitter-ion used a tetrahydrothiophenium arene oxide zwitterion 555 [Eq. (95)] [340-342]. 

A greater degree of certainty exists with zwitterion monomers that the growing chains are also zwitterions. Hence, they seem to be the most suitable class of compounds with which to study ionic reactions of this type. The concentration of growing zwitterions is high and any peculiarities associated with their growth are likely to be readily apparent. A closer examination of the polymerization of these monomers is warranted. 

Unequivocal demonstration of the formation of macrozwitterions is confined to anionic polymerization. Ironically zwitterions were first postulated as intermediates in cationic vinyl polymerizations initiated by Friedel-Crafts halides4. Friedel-Crafts halides, probably the most widely used cationic initiators, are molecules, not ions. However, formation of an anion with a metal-carbon bond seems to be energetically unfavourable and initiation is thought to occur by self ionization or involve a co-catalyst. 

As for any other functional polymer, polymeric betaines are accessible by two different synthetic routes (1) the polymerization of zwitterionic monomers or (2) the zwitterionic functionalization of reactive precursor polymers. Both routes have inherent advantages and disadvantages. The polymerization of the zwitterionic monomers leads to polymers with 100% betaine functionality, but their molecular characterization is difficult for several reasons. For instance, the conformation of the polymers in aqueous solution is very sensitive, not only to the ionic strength but also to the type of added salt, and in the case of polycarbobetaines also to the pH. Furthermore, polymeric zwitterions often exhibit strong interactions with other matter, e.g., chromatographic colmnns. Hence, reliable GPC or HPLC measurements are very difficult to perform, if at all. 

The iodometrical analysis of active oxygen in the ozonized Denka M40 solutions shows that the amount of 0-0 groups is 43 per cent. It is of interest to note that the HI reaction with ozonized polychloroprene solutions occurs quantitatively for 3 h, while in SKD the same proceeds only to 20 per cent after 24 h. The aforementioned data, however, provide insufficient information for the preferable route of the zwitterions deactivation (via dimerization, polymerization of zwitterions, or secondary processes). The DSC analysis of the products of Denka M40 ozonolyis reveals that the chloroprene rubber ozonolyis yields polyperoxide as the enthalpy of its decomposition is found to be very close to that of dicumeneperoxide (DCP), The higher value of (approximately two times of that of DCP)... 

Polymerization via zwitterions and the actual polymerization of zwitterions, themselves must be distinguished. In the polymerization via zwitterions, a zwitterion is first formed in a kind of Michael addition by an initiator molecule adding onto a monomer molecule, as, for example, in the reaction of tertiary amines with j3-lactones ... 

The classic anionic polymerization is an anionic polymerization with growth via macroanions. A new monomer molecule is added onto the growing polymer chain end in every propagation step, as the general form of (18-1) shows. It is immaterial how the macroanion has been produced, that is, if the start reaction was a two-electron reaction with formation of a monomer anion or zwitterion or whether it was a one-electron transfer. 

The polymerization via zwitterions represents a special case of a polymerization with participation by ion pairs. The ion pairs can occur either as intramolecular macrocyclics or as chainlike associates ... 

The synthesis and dilnte and hulk solution properties of end-functionalized polymers with zwitterionic end groups have recently been reviewed (187). Reportedly, such polymers with different architectures (linear, block, and star copolymers) can be prepared by means of anionic polymerization. First, dimethylamine groups were introduced by using 3-dimethylaminopropyllithium as functional initiator. This group was then switched to sulfozwitterionic one by reaction with propanesultone. 

Polymerization with phosphines as initiators was proposed to proceed via zwitterions [758], Figure 8. 

Protonation of the TMM complexes with [PhNMe2H][B(C6Fs)4] in chlorobenzene at —10 °C provided cationic methallyl complexes which are thermally robust in solution at elevated temperatures as determined by NMR spectroscopy. In contrast, addition of BfCgFsls to the neutral TMM precursors provided zwitterionic allyl complexes (Scheme 98). Surprisingly, it was found that neither the cationic nor the zwitterionic complexes are active initiators for the Ziegler-Natta polymerization of ethylene and a-olefins. °°... 

Hydrophilic interaction chromatography (HB IC) or aqueous normal-phase chromatography (ANP) refers to the use of polar stationary phases (e.g., bare silica, silica, or polymeric phases with bonded zwitterionic ligands, diol phases) in combination with a mobile phase rich in organic solvent but containing a significant amount of water (typically at least 3%). Bell [97] summarized the advantages of this technique as follows ... 

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