Solvent dependency, ionic polymerizations

Both the initiation step and the propagation step are dependent on the stability of the carbocations. Isobutylene (the first monomer to be commercially polymerized by ionic initiators), vinyl ethers, and styrene have been polymerized by this technique. The order of activity for olefins is Me2C=CH2 > MeCH=CH2 > CH2=CH2, and for para-substituted styrenes the order for the substituents is Me—O > Me > H > Cl. The mechanism is also dependent on the solvent as well as the electrophilicity of the monomer and the nucleophi-licity of the gegenion. Rearrangements may occur in ionic polymerizations. 

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. 

The species present in cationic ring-opening polymerizations are covalent ester (IX), ion pair (X), and free ion (XI) in equilibrium. The relative amounts of the different species depend on the monomer, solvent, temperature, and other reaction conditions, similar to the situation described for ionic polymerization of C=C monomers (Chap. 5). 

Greater differences between the optical activity of monomers and of polymers have been recently observed by Schulz and Hartmann 133) when investigating the free-radical and ionic polymerization of a number of variously substituted N-vinyl compounds. The above authors also observed in one case a large dependence of the optical activity of the polymers on the type of solvent used. 

The characteristics of the active centers in free-radical polymerizations depend only on the nature of the monomer and are generally independent of the reaction medium. This is not the case in ionic polymerizations because these reactions involve successive insertions of monomers between a macromolecular ion and a more or less tightly attached counterion of opposite charge. The macroion and 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. 

Better results were obtained in the methyl methacrylate polymerization reactions (Scheme 12). 153-156 showed high catalytic activity with a strong dependence on the ionic radius of the center metal. The lanthanum complex 154 was the most active catalyst and initiated the polymerization without any cocatalyst. Addition of small amounts of AlEts as cocatalyst increased the yield significantly. Polymerization initiated by 154 depended on the temperature and a low temperature (—78°C) was required to afford almost quantitative yields. The resulting polymers were basically syndiotactic and exhibited high molar masses and narrow polydispersities. The catalytic reaction with the lanthanum compound 157 showed no increase of catalytic activity but led to a larger fraction of atactic poly(methyl methacrylate). Moreover, the catalytic activity of all utilized initiators was solvent dependent. 153, 155, and 156 only showed catalytic activity by the addition of a cocatalyst. 153 afforded lower yield after changing the solvent from toluene into THF. 

Ionic polymerizations, as we shall see later, involve successive insertion of monomer molecules between an ionic chain end (positive in cationic and negative in anionic polymerization) and a counterion of opposite charge. The macroion and the counterion form an organic salt which may, however, exist in several forms depending on the nature and degree of interaction between the cation and anion of the salt and the reaction medium (solvent/monomer). Considering, for example, an organic salt a continuous spectrum of ionicities ( Winstein... 

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 II, ID, 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 significantly more reactive than ion pairs. In general, more polar media favor solvent-separated ion pairs or free solvated ions. In hydrocarbon media, jffee solvated ions do not exist, though other equilibria may occur between ion pairs and clusters of ions (Rudin, 1982). 

In addition to the radical type, there is also ionic polymerization. It is initiated by ions (cations or anions), dissociation of which is naturally heavily dependent on electrostatic effects, in particular solvation by the solvent. As in radical polymerization, the ionic process consists of a chain reaction. In the start reaction, a Lewis acid or Lewis base attaches to one C atom of the double bond of a monomer. This produces a charge at the other C atom. Whether anionic or cationic polymerization takes place depends on the nature of this charge. Chain growth involves repeated attachment to a double bond, whereby the charge jumps two C atoms further. In ionic polymerization there is no chain breakage due to recombination. Termination has to be induced by adding water, alcohols, acids, or amines. If this is not done, the reaction comes to a halt when all of the monomer is used up, whereby the reactivity is maintained for some time. 

In these equations, the exact nature of the initiating and chain-carrying species can vary from essentially covalent for transition-metal organometallic species in coordination polymerization to ion pairs or free ions in ionic polymerizations, depending on the structure of the chain-carrying species, the counterion, the solvent, and the temperature. 

For a given monomer, the stereocontrol depends primarily on the polarity of the medium. If polymerization is carried out in highly polar solvents, those that encourage dissociation, the result will be polymers with relatively high proportions of syndiotactic triads (see Table 18-6). However, the proportion of heterotactic triads is likewise very significant. Changing to apolar solvents, the proportion of isotactic triads Xa is found to increase, while the proportion of heterotactic triads remains approximately constant. It is obviously impossible to achieve holotactic polymers by ionic polymerizations with free ions. The influence of the polarity of the solvent shows that the increase in the proportion of isotactic triads probably corresponds to the increase in the concentration of contact ion pairs or ion associates. 

The reactions with chlorine ended polymers suggest that in toluene solution the condensation of these dichlorides with sodium contains no element of a condensation polymerization. Chain growth seems to be restricted to the addition of monomer units to the chain end. From the observation that the 27 ppm peak in the Si spectrum remained unchanged at the end of the reaction of chlorine ended polymers with sodium, it must be concluded that sodium does not react rapidly with a chlorine ended chain to form a sodium ended active chain. This was suggested earlier (11c) as part of the polymerization mechanism to account for the low dependence of the rate on the sodium surface area. This low dependence was first shown for the hexylmethyl monomer (1 la), but has been confirmed with the propylmethyldichlorosilane. Low order dependences of the rate of monomer consumption have been found in ionic polymerizations, and attributed to the aggregation of ionic chain ends in non-polar solvents (17). The reactive species is then a small concentration of monomeric chain ends in equilibrium with the aggregate. These conditions could well exist here in toluene solution. However, reaction with the sodium surface is an essential part of the reaction. 

X 10 exp(—13407/T) s for n-BuLi can be evaluated. As with all rate coefficients of ionic polymerization, this elimination also depends on the reaction system. The lithium hydride (LiH) elimination from butyllithium in the presence of polar additives such as lithium butoxide is reported to be several times faster than from pure lithium alkyl [90, 91], and in ethereal solvents proton abstraction or ether cleavage may occur [92]. 

The severity of the chemical heterogeneity strongly depends on the copolymerization parameters. In free-radical polymerization there is just one pair of parameters, which may depend somewhat on temperature, for one pair of monomers whereas in ionic polymerization these parameters for every pair of monomers strongly depend on the counter ion and solvent polarity (see Table 7.6). 

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