Polymerization in Solvating Media

It has been reported that when employing isoprene enriched with 13C- at the 1 -position, reaction with f-butyllithium in benzene resulted in moderately strong signals from the 1,4-chain end 233). 

The tacticity of anionically prepared polystyrenes has been the subject of extensive study by a number of groups of workers, mostly by means of 13C-NMR spectroscopy. From a study of the aromatic Cl resonances, Matsuzaki and coworkers found 234) that there is a tendency towards syndiotacticity when using -butyl-Iithium in toluene as initiator. From the sensitivity of the CMR spectrum to the nature of the solvent employed it was concluded that the polymerization did not conform to Bernoullian statistics. Randall examined the methylene resonances in the CMR spectrum and concluded that butyllithium initiated polystyrene is essentially atactic 235) and that propagation is Bernouillian. Uryu et al.236) examined polystyrene 

Wicke and Elgert 238) have concluded that the tight ion pair, the loose ion pair and the free carbanion of poly-a-methylstyrene generate the same tacticity. With n-butyllithium as the initiator in THF, the activation energy for meso propagation is a little greater than that for racemic propagation (15.6 and 14.9 kcal/mole respectively) the pre-exponential factors are the same 239). 

Under such conditions propagation does not effectively compete with cyclization similar results are obtained with DPE 

It has been reported that when employing isoprene enriched with at the 1 -position, 

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Such "living" polymer systems, however, are not limited to polymerizations in solvating media, such as ethers. Thus, the lithium-catalyzed polymerizations, which can lead to the synthesis of cis-1,4-pol yisoprene, also demonstrate the virtual absence of... 

Furthermore, polymerizations in solvating media, like ethylene glycol dimethyl ether, tetrahydrofuran, or pyridine using biphenylsodium or biphenyllithium, yield virtually monodisperse syndiotactic poly(methyl methacrylate). ... 

The stereochemistry of diene polymerization is somewhat dependent upon temperature note however the results of Morton and Rupert209 (Table 19). In general, linear Arrhenius plots are obtained. Some pertinent data are summarized in Table 21 for convenience results are presented for both solvating and non-solvating media. The structures obtained in solvating media are very different from those obtained in hydrocarbon solvents (cf Tables 17 and 22). The sensitivities to tempera-... 

The dramatic effect of even traces of ethers on the microstructure of polybutadiene and polyisoprene in organolithium polymerizations in hydrocarbon media was demonstrated very effectively by Tobolsky et al. (16. 17) They showed that highly solvating ethers, such as H -furan, when present in approximately... 

Ionic Chain Polymerization. Ionic chain polymerizations take place at relatively low or moderate temperatures and in solvating media so that the ionic centers propagate to polymeric size prior to termination. Only solvents of low or moderate polarity, e.g., alkanes, chlorinated hydrocarbons, toluene, nitrobenzene and tetrahydrofuran, are employed. Highly polar solvents such as alcohols or ketones cannot be used since they inactivate ionic initiators and propagating centers by reaction or strong complexation. 

Generally, these equilibrium constants are very solvent dependent. In particular, the values of KEA are expected to be relatively high in protic solvents because halide anions can be stabilized through solvation in such media [143], Also, on the other hand, Kx values will be likewise affected with changes in solvent polarity [50], Low values of Kx in polar and protic media have direct implications on the degree of polymerization control because of the decreased amounts of deactivator (Mt"+1X/L), as a result of halide anion dissociation from the metal center. 

Many of the considerations discussed above in connection with organic chemistry are, of course, also valid for polymer technology. Solvents for polymers, used in such materials as paints and lacquers, and as media for the polymerization reaction, provide examples of industrial applications of solvating media. 

Reacting gases may be in excess if they react with solids and do not condense in liquid phases, but supercritical media are clearly not the subject of solvent-free chemistry and deserve their own treatment. For practical reasons, this book does not deal with homogeneous or contact-catalyzed gas-phase reactions. Furthermore, very common polymerizations (except for solid-state polymerizations), protonations, solvations, complexations, racemizations, and other stereo-isomerizations are not covered, to concentrate on more complex chemical con-... 

When lithium alkyl catalysts are used in non-solvating media such as aliphatic hydrocarbons, the polymer-lithium bond is not sufficiently ionic to initiate anionic polymerization so that the monomer must first complex with vacant orbitals in the lithium. A partial positive charge is induced on the monomer in the complex, and this facilitates migration of the polymer anion to the most electrophilic carbon of the complexed monomer. This type of polymerization is more appropriately termed coordinated anionic and will be discussed in the next section. There does not appear to be any evidence that alkyl derivatives of metals which are less electropositive than lithium and magnesium can initiate simple anionic polymerization. 

The alkyl derivatives of the most electropositive elements (Cs, Rb, K, Na) are highly ionic in the solid state and require only weakly solvating media (solvent plus monomer) to exist in form (III) which initiates non-stereospecific anionic polymerization. However, in hydrocarbon solvents, alkyl sodium (Alfin1) (217, 225—227), alkyl potassium (210, 217, 228) and alkyl rubidium (217) all can produce isotactic polystyrene. 

The forms which are actually important in a given polymerization will depend on the natures of the species U and V, the solvating ability of the medium, and the temperature. It is not unusual to find two of these forms coexisting in significant quantities in a given polymerization. In general, more polar media favor solvent-separated ion pairs or free solvated ions. Free solvated ions will not exist in hydrocarbon media, where other equilibria may occur between ion pairs and clusters of ions. 

The propagating anion and its counterion exist in relatively nonpolar solvents mainly in the form of associated ion pairs. Different kinds of ion pairs can be envisaged, depending on the extent of solvation of the ions. As a minimum, an equilibrium can be conceived between intimate (contact) ion pairs, solvent-separated ion pairs, and solvated unassociated ions. The nature of the reaction medium and counterion strongly influences the intimacy of ion association and the course of the polymerization. In some cases the niicrostructure of the polymer that is produced from a given monomer is also influenced by these variables. In hydrocarbon solvents, ion pairs are not solvated but they may exist as aggregates. Such inter-molecular association is not important in more polar media where the ion pairs can be solvated and perhaps even dissociated to some extent. 

Solvating Solvents. In these media the rate of polymerization is found to be directly proportional to the monomer concentration as would be expected for a propagation reaction like... 

Hydrocarbon Solvents. Hydrocarbon solvents do not solvate the metal counterions. The rate of polymerization depends directly on the monomerconcentration as in the case of solvating media, but the rate dependence on initiator concentration is complicated and variable. Some of this complexity is due to the association of growing polymer chains and their counterions into dimers and larger aggregates. The detailed mechanisms of these reactions still require clarification. 

All early actinides from thorium to plutonium possess a stable +4 ion in aqueous solution this is the most stable oxidation state for thorium and generally for plutonium. The high charge on tetravalent actinide ions renders them susceptible to solvation, hydrolysis, and polymerization reactions. The ions are readily hydrolyzed, and therefore act as Bronsted acids in aqueous media, and as potent Lewis acids in much of their coordination chemistry (both aqueous and nonaqu-eous). Ionic radii are in general smaller than that for comparable trivalent metal cations (effective ionic radii = 0.96-1.06 A in eight-coordinate metal complexes), but are still sufficiently large to routinely support high coordination numbers. 

A major difference between the two methods of initiation is that the solvent in y-ray studies is almost inevitably the monomer itself, and these generally have lower dielectric constants than the chlorocarbon solvents most often used in the chemically initiated systems. As a result, it is not possible to compare the values of kp +) obtained from each technique without accounting for this difference in solvation. Classically, propagation involves charge dispersion in forming the transition-state complex and hence a reduction in the polarity of the system. Thus media of lower solvation power should favourably influence the process. (See reference 114 for more detailed discussion.) Experimentally the values of kp(+) from radiation-induced polymerizations are consistently higher than those obtained using stable salts as initiators, and this simplistic picture therefore seems to be confirmed. Dunn has recently carried out a detailed compilation of the available data on / p(+) and readers will find this an excellent distillation of the current position. 

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