Ion pairs, contact

LOOSE ION PAIR------CONTACT ION PAIR------MONOMER--------SOLVATED------TRAPPED... 

A covalent compound dissociates into free ions in stages involving the formation of at least two types of ion pairs, contact and solvent-separated ion pairs cf. Section 2.6 and Eqs. (2-19) to (2-21) [289, 333]. 

Interestingly, the mesophase dependence on the counteranion was found to correlate with the chemical shift of the acidic H2 proton revealing the importance of the ion-pair contacts in the formation of the mesophase. 

In real ionic polymerization systems, however, more than one kind of ion occurs, especially in the case of anionic polymerizations. A distinction is made between free ions, solvated ion pairs (solvent separated or loose ion pairs), contact ion pairs (intimate ion pairs or tight ion pairs), polarized molecules, and ionic associates of three or more ions. A rapid dynamic equilibrium often occurs between these ionic forms ... 

Free ions, solvent-separated ion pairs, contact ion pairs, and ionic associates can often be experimentally distinguished from each other with the aid of uv, ir, Raman, or nuclear magnetic resonance spectroscopy. According to nmr measurements, poly(styryl anions), I, and poly(dienyl anions), II, are strongly charge delocalized consequently, they exhibit a strong absorption band, sometimes several bands, in the near-ultraviolet region ... 

How can such an unusual effect be explained The currently accepted answer is that there are two kinds of ion pairs contact ion pairs and solvent-separated ion pairs. A solvent-separated ion pair includes one (or perhaps several) solvent molecule between the cation and anion. It is distinct from a completely dissociated ion pair... 

So the kinetic scheme and parameters not only depend on M, but they may differ considerably according to the initiator used. Furthermore, the active centers are not uniform as in free-radical polymerization, but because of their ionic nature there usually exists a complex chemical equilibrium between different species, even if the initiator has a unique stmcture. This equilibrium [Eq. (3)] between free ions, solvent-separated ion pairs, contact ion pairs, covalent polarized bonds, and between associated and non-associated species, as well as the concentration of these species strongly depend on the polarity or solvating power of the solvent system, the solvent itself, and the presence of other salts. 

The polarity of the metal-carbon bond increases upon going down in the periodic table the lithium alkyls have some covalent character and form tetrameric clusters, whereas cesium alkyls are purely ionic. The degree of clusterification of lithium alkyls varies with the nature of the solvent between dimer (LiCHs TMEDA) and hexamer (Li-n-C4H9 cyclohexane), as can be checked by osmometry, NMR and EPR. Li NMR (I =3/2 abundance 92.6%) and Li NMR (I = 1/2 abundance 7.4%) allow to also show the dynamic fluxionahty phenomena around the Li4 tetrahedron, reversible dissociation of tetramers to dimers and ion pairs (contact ion... 

As already discussed, propagation in cationic polymerization of cyclic ethers by the ACE mechanism proceeds on tertiary 0x0-nium ion active species. Ionic species in general may exist in the form of ion-pairs (contact or solvent separated) and free ions. The fraction of each form is governed by a corresponding equilibrium constant that depends on the polarity of the medium. The knowledge of the fraction of different ionic forms, which is essential for the proper analysis of kinetics of anionic vinyl polymerization in which different forms show different reactivity, is less crucial in analyzing the kinetics of cationic polymerization of cyclic ethers because available data point out to equal reactivity of ion-pairs and free ions in propagation. 

Hogen-Esch and Smid in studies of UV absorption spectra of fluorenyl sodium in THF solvent observed two absorption peaks of intensities changing with temperamre (Scheme 4). After establishing that free ions are not responsible for these changes, they concluded that there are two kinds of ion pairs contact and solvent separated. This classical dependence that was the first spectroscopic evidence of two kinds of ion pairs is shown in Figure 2. 

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