Anionic polymerization polar monomers

Catalysts of the Ziegler type have been used widely in the anionic polymerization of 1-olefins, diolefins, and a few polar monomers which can proceed by an anionic mechanism. Polar monomers normally deactivate the system and cannot be copolymerized with olefins. However, it has been found that the living chains from an anionic polymerization can be converted to free radicals in the presence of peroxides to form block polymers with vinyl and acrylic monomers. Vinylpyridines, acrylic esters, acrylonitrile, and styrene are converted to block polymers in good yield. Binary and ternary mixtures of 4-vinylpyridine, acrylonitrile, and styrene, are particularly effective. Peroxides are effective at temperatures well below those normally required for free radical polymerizations. A tentative mechanism for the reaction is given. 

The authors reached the conclusion that, in the absence of strongly solvating solvents (e.g. ethers), a characteristic feature of the anionic polymerization is monomer complexation with the positive ion or the polarization of the monomer double bond by this ion, especially by Li+. 

In the anionic polymerization, the monomer stmcture may have indirect efferts as well, influencing, for example, the dielectric properties of the reaaion medium and, thus, the course of the polymerization. Since lactam monomers and the resultant polyamides have polar amide bonds, their bulk polymerization rate is strongly affected by the changes in medium permittivity during polymerization. In anionic polymerization, high dielectric permittivities strongly favor the dissociation of the lactam salt (initiator) releasing the free lactam anion (Section 4.14.7.1). 

Anionic polymerization of vinyl monomers can be effected with a variety of organometaUic compounds alkyllithium compounds are the most useful class (1,33—35). A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds are soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. Methyllithium [917-54-4] and phenyllithium [591-51-5] are available in diethyl ether and cyclohexane—ether solutions, respectively, because they are not soluble in hydrocarbon solvents vinyllithium [917-57-7] and allyllithium [3052-45-7] are also insoluble in hydrocarbon solutions and can only be prepared in ether solutions (38,39). Hydrocarbon-soluble alkyllithium initiators are used directiy to initiate polymerization of styrene and diene monomers quantitatively one unique aspect of hthium-based initiators in hydrocarbon solution is that elastomeric polydienes with high 1,4-microstmcture are obtained (1,24,33—37). Certain alkyllithium compounds can be purified by recrystallization (ethyllithium), sublimation (ethyllithium, /-butyUithium [594-19-4] isopropyllithium [2417-93-8] or distillation (j -butyUithium) (40,41). Unfortunately, / -butyUithium is noncrystaUine and too high boiling to be purified by distiUation (38). Since methyllithium and phenyllithium are crystalline soUds which are insoluble in hydrocarbon solution, they can be precipitated into these solutions and then redissolved in appropriate polar solvents (42,43). OrganometaUic compounds of other alkaU metals are insoluble in hydrocarbon solution and possess negligible vapor pressures as expected for salt-like compounds. 

Commercially, anionic polymerization is limited to three monomers styrene, butadiene, and isoprene [78-79-5], therefore only two useful A—B—A block copolymers, S—B—S and S—I—S, can be produced direcdy. In both cases, the elastomer segments contain double bonds which are reactive and limit the stabhity of the product. To improve stabhity, the polybutadiene mid-segment can be polymerized as a random mixture of two stmctural forms, the 1,4 and 1,2 isomers, by addition of an inert polar material to the polymerization solvent ethers and amines have been suggested for this purpose (46). Upon hydrogenation, these isomers give a copolymer of ethylene and butylene. 

To be eligible to living anionic polymerization a vinylic monomer should carry an electron attracting substituent to induce polarization of the unsaturation. But it should contain neither acidic hydrogen, nor strongly electrophilic function which could induce deactivation or side reactions. Typical examples of such monomers are p-aminostyrene, acrylic esters, chloroprene, hydroxyethyl methacrylate (HEMA), phenylacetylene, and many others. 

The situation changes dramatically when anionic polymerizations are carried out under reaction conditions such that there is strong coordination among the counter-ion, propagating center, and monomer. Thus, when lithium is the counter-ion and polymerization takes place in a solvent of low polarity, the... 

A bulky methacrylate, triphenylmethyl methacrylate (TrMA), is a unique monomer which gives an almost 100% isotactic polymer in anionic polymerization with n-butyllithium both in nonpolar and polar solvents. Moreover, even free-radical polymerization affords a highly isotactic polymer from this monomer.23 The isotactic specificity of TrMA polymerization is ascribed to the helical formation of the main chain. When TrMA is polymerized in toluene at —78°C... 

Alkyl derivatives of the alkaline-earth metals have also been used to initiate anionic polymerization. Organomagnesium compounds are considerably less active than organolithiums, as a result of the much less polarized metal-carbon bond. They can only initiate polymerization of monomers more reactive than styrene and 1,3-dienes, such as 2- and 4-vinylpyridines, and acrylic and methacrylic esters. Organostrontium and organobarium compounds, possessing more polar metal-carbon bonds, are able to polymerize styrene and 1,3-dienes as well as the more reactive monomers. 

Polar monomers, such as methyl (meth)acrylate, methyl vinyl ketone, and acrylonitrile, are more reactive than styrene and 1,3-dienes because the polar substituent stabilizes the carba-nion propagating center by resonance interaction to form the enolate anion. However, the polymerizations are more complicated than those of the nonpolar monomers because the polar... 

The need for solvation in anionic polymerization manifests itself in some instances by other deviations from the normal reaction rate expressions. Thus the butyllithium polymerization of methyl methacrylate in toluene at — 60°C shows a second-order dependence of Rp on monomer concentration [L Abbe and Smets, 1967]. In the nonpolar toulene, monomer is involved in solvating the propagating species [Busson and Van Beylen, 1978]. When polymerization is carried out in the mixed solvent dioxane-toluene (a more polar solvent than toluene), the normal first-order dependence of Rp on [M] is observed. The lithium diethylamide, LiN(C2H5)2, polymerization of styrene at 25°C in THF-benzene similarly shows an increased order of dependence of Rp on [M] as the amount of tetrahydrofuran is decreased [Hurley and Tait, 1976]. 

Copolymerizations of nonpolar monomers with polar monomers such as methyl methacrylate and acrylonitrile are especially comphcated. The effects of solvent and counterion may be unimportant compared to the side reactions characteristic of anionic polymerization of polar monomers (Sec. 5-3b-4). In addition, copolymerization is often hindered by the very low tendency of one of the cross-propagation reactions. For example, polystyryl anions easily add methyl methacrylate but there is little tendency for poly(methyl methacrylate) anions to add styrene. Many reports of styrene-methyl methacrylate (and similar comonomer pairs) copolymerizations are not copolymerizations in the sense discussed in this chapter. 

The stereoselective polymerization of various acrylates and methacrylates has been studied using initiators such as atkyllithium [Bywater, 1989 Pasquon et al., 1989 Quirk, 1995, 2002]. Table 8-12 illustrates the effects of counterion, solvent, and temperature on the stereochemistry of the anionic polymerization of methyl methacrylate (MMA). In polar solvents (pyridine and THF versus toluene), the counterion is removed from the vicinity of the propagating center and does not exert an influence on entry of the next monomer unit. The tendency is toward syndiotactic placement via chain end control. The extent of syndiotacticity... 

Monomers devoid of polar groups generally undergo anionic polymerization in a predictable manner. With polar monomers sometimes side reactions occur during the process transfer reactions in the case of acrylonitrile, or propylene oxide, and even more so with alkylacrylates deactivations (or "killing") reactions in the case of halogen substituted styrene or dienes. 

In conclusion, it has been shown that use of cryptates for the anionic polymerization of heterocyclic monomers leatis to a tremendous increase of the rates of polymerization. There are two main causes to the higher reaction rates observed with cryptates. The first one is a suppression of the association between ion pairs in the non polar media, and the second one is the possibility of ion pairs dissociation into free ions in ethereal solvents like THP or THF. By this way, it has been possible to make detailed studies of the propagation reaction for propylene sulfide, ethylene oxide, and cycloslloxanes. 

With Na+, as a cation, the activation energies for the anionic polymerization of acrolein and propylene sulfide (11) are approximately the same. On the other hand, with Li+, it is impossible to compare the acrolein activation energy with the same monomer or another polar monomer because no result is found in the literature. Moreover, for the acrolein polymerization, (Raj u+) lower than ( > +). 

The nature of the active species in the anionic polymerization of non-polar monomers, e. g. styrene, has been disclosed to a high degree. The kinetic measurements showed, that the polymerization proceeds in an ideal way, without side-reactions, and that the active species exist in the form of free ions, solvent-sparated and contact ion pairs, which are in a dynamic equilibrium (l -4). For these three species the rate constants and activation parameters (including the activation volumes), as well as the rate constants and equilibrium constants of interconversion have been determined (4-7.) Moreover, it could be shown by many different methods (e. g. conductivity and spectroscopic methods) that the concept of solvent-separated ion pairs can be applied to many ionic compounds in non-aqueous polar solvents (8). 

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