Polymerization ionic chain

The ionic chain polymerization of unsaturated linkages is considered in this chapter, primarily the polymerization of the carbon-carbon double bond by cationic and anionic initiators (Secs. 5-2 and 5-3). The last part of the chapter considers the polymerization of other unsaturated linkages. Polymerizations initiated by coordination and metal oxide initiators are usually also ionic in nature. These are called coordination polymerizations and are considered separately in Chap. 8. Ionic polymerizations of cyclic monomers is discussed in Chap. 7. The polymerization of conjugated dienes is considered in Chap. 8. Cyclopolymerization of nonconjugated dienes is discussed in Chap. 6. 

Unlike free-radical reactions which are not selective (as most olefinic monomers undergo radical polymerization), ionic polymerizations are largely selective and are restricted to monomers whose structures enhance the stability of the ionic species involved in the process. Cationic polymerization is essentially limited to those monomers with electron-releasing substituents and anionic polymerization takes place with monomers possessing electron-withdrawing groups. These are elaborated in a later section. 

The commercial utilization of cationic and anionic polymerizations is rather limited because of the high selectivity of ionic polymerizations compared to radical polymerization, as mentioned above. Ionic polymerizations are also most difficult to carry out and require stringent reaction conditions. Thus, unlike in free-radical polymerizations in which the characteris- 

Ionic polymerizations, as we shall see later, involve successive insertions of monomers between a macromolecular ion and a counterion of opposite charge. The macroion and the counterion form an organic salt which may exist in several forms in the reaction medium. The degree and nature of the interaction between the cation and anion of the salt and the solvent (or monomer) can vary considerably. Considering an organic salt a 

Covalent Contact Solvent Solvated Free solvated 

One can visualize a range of behavior from one extreme of a completely covalent species (I) to the other of completely free (and highly solvated) ions (V). The intermediate species include the tight or contact ion pair (II) and the solvent-separated or loose ion pair (III). The contact ion pair has a counterion (or gegenion) of opposite charge close to the propagating center (unseparated by solvent). The solvent-separated ion pair involves ions that are partially separated by solvent molecules. In cationic polymerization the chain end is cationic and has a negative counterion, while in anionic polymerization the chain end is anionic and has a positive counterion. 

Besides free radical mechanisms, discussed in Chapter 6, there are several other mechanisms by which chain or addition polymerization can take place. Prominent among these are ionic mechanisms in which the growing ehain end carbon bears a negative charge (carbanion) or a positive eharge (carbonium ion). In the former ease, the polymerization is known as anionic polymerization and in the latter case as cationic polymerization. 

Ionic polymerization can, in general, be initiated by acidic or basic compounds. For cationic polymerization, complexes of BF3, AICI3, TiCU, and SnCU with water, or alcohols, or tertiary oxonium salts are particularly active initiators, the positive ions in them causing chain initiation. One can also initiate cationic polymerization with HCl, H2SO4, and KHSO4. Important initiators for anionic polymerization are alkali metals and their organic compounds, such as phenyllithium, butyllithium, phenyl sodium, sodium naphthalene, and triphenyl methyl potassium. 

Ionic polymerizations commonly involve two types of propagating species—an ion pair (II-IV) and a free ion (V)—coexisting in equilibrium with each other. The relative concentrations of these two types of species, as also the identity of the ion pair (that is, whether of type n, HI, or IV), depend on the particular reaction conditions and especially the solvent or reaction medium, which has a large effect in ionic polymerizations. Loose ion pairs are more reactive than tight ion pairs, while free ions are signi candy more reactive than ion pairs. In general, more polar media favor solvent-separated ion pairs or free solvated ions. In hydrocarbon media, free solvated ions do not exist, though other equilibria may occur between ion pairs and clusters of ions (Rudin, 1982). 

Solvents of high polarity are desirable for solvation of ions. However they cannot be employed for ionic polymerizations. Thus highly polar hydroxyhc solvents, such as water and alcohols, react with and destroy most ionic initiators and propagating species. Other polar solvents such as ketones form highly stable complexes with initiators, thus preventing initiation reactions. Most ionic polymerizations are, therefore, carried out in low or moderately polar solvents such as methyl chloride, efliylene dichloride, and pentane. 

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In contrast to ionic chain polymerizations, free radical polymerizations offer a facile route to copolymers ([9] p. 459). The ability of monomers to undergo copolymerization is described by the reactivity ratios, which have been tabulated for many monomer systems for a tabulation of reactivity ratios, see Section 11/154 in Brandrup and Immergut [14]. These tabulations must be used with care, however, as reactivity ratios are not always calculated in an optimum manner [15]. Systems in which one reactivity ratio is much greater than one (1) and the other is much less than one indicate poor copolymerization. Such systems form a mixture of homopolymers rather than a copolymer. Uncontrolled phase separation may take place, and mechanical properties can suffer. An important ramification of the ease of forming copolymers will be discussed in Section 3.1. 

The synthesis of elastomers by step, chain, and ring-opening polymerizations is reviewed. These reactions are characterized as to the process variables which must be controlled to achieve the synthesis and crosslinking of an elastomer of the required structure. Both radical and ionic chain polymerizations are discussed as well as the structural variations possible through copolymerization and s tereoregularity. 

It is important to note that regardless of how termination occurs, the molecular weight is independent of the concentration of the initiator. However, the rate of ionic chain polymerization is dependent on the dielectric constant of the solvent, the resonance stability of the carbonium ion, the stability of the gegenion, and the electropositivity of the initiator. 

Chain reactions, including ionic chain polymerization reactions, consist of at least three steps initiation, propagation, and termination. Termination generally occurs through chain transfer producing a new ion and the dead polymer. 

Is the usual configuration of polymers produced by ionic chain polymerization (a) head-to-tail or (b) head-to-head ... 

Which condition would be more apt to produce stereoregular polymers in ionic chain polymerizations (a) high temperature or (b) low temperatures ... 

Name (a) a thermoplastic, (b) an elastomer, and (c) a fiber that is produced commercially by ionic chain polymerization. 

What is the relationship between the rate of initiation to the monomer concentration in ionic chain polymerization ... 

Thus, almost all substituents are able to stabilize the propagating radical by delocalization of the radical over two or more atoms. The remainder of this chapter will be concerned with the detailed characteristics of radical chain polymerization. Ionic chain polymerizations will be... 

It is appropriate at this point to briefly discuss the experimental procedures used to determine polymerization rates for both step and radical chain polymerizations. Rp can be experimentally followed by measuring the change in any property that differs for the monomer(s) and polymer, for example, solubility, density, refractive index, and spectral absorption [Collins et al., 1973 Giz et al., 2001 McCaffery, 1970 Stickler, 1987 Yamazoe et al., 2001]. Some techniques are equally useful for step and chain polymerizations, while others are more appropriate for only one or the other. Techniques useful for radical chain polymerizations are generally applicable to ionic chain polymerizations. The utility of any particular technique also depends on its precision and accuracy at low, medium, and high percentages of conversion. Some of the techniques have the inherent advantage of not needing to stop the polymerization to determine the percent conversion, that is, conversion can be followed versus time on the same reaction sample. 

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