Anionic Polymerization of Polar Vinyl Monomers

An increasing number of puzzling and often contradictory features of this polymerization was revealed by the subsequent studies. These were reviewed in 1965 by Bywater364), who demonstrated that none of the mechanisms proposed by the various investigators is capable of accounting for all the observations. Although many of the reported complexities are still not eluddated, some of them are at least partially understood. 

Much of the difficulties arising in the early investigations resulted from the unfortunate choices of experimental conditions. Use of organolithium compounds as the initiators is troublesome, especially in mechanistic studies, because they react with methyl methacrylate in more than one fashion. Denothing the organo-lithium reagent by RLi, we may visualize three modes of interactions  

Some difficulties could be avoided by using initiators such as e.g. 1,1-diphenyl hexyl lithium 

The second mode is especially damaging. It causes not only a loss of the initiator but yields lithium alkoxide which, in turn, interferes with the propagation. In this respect butyl lithium is the most undesired initiator as it produces large amounts of the alkoxide in the first few seconds even at - 30 °C. Similar side reactions take place in the propagation of lithium polymethyl methacrylate, causing, e.g. ring formation or a false addition and branching371,372 . 

Formation of polymers with relatively narrow molecular weight distribution provides a fair evidence for the cleaness of the system. This was achieved in polymerization of meticulously purified methyl methacrylate initiated by biphenylide salts of alkali metals and carried out in polar solvents, THF or DME, at a low temperature, - 78°C360). Under 

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Polar Vinyl Monomers The anionic polymerization of polar vinyl monomers is often complicated by side reactions of the monomer with both anionic initiators and growing carbanionic chain ends, as well as chain termination and chain transfer reactions. However, synthesis of polymers with well-defined structures can be effected under carefully controlled conditions. The anionic polymerizations of alkyl methacrylates and 2-vinylpyridine exhibit the characteristics of living polymerizations under carefully controlled reaction conditions and low polymerization temperatures to minimize or eliminate chain termination and transfer reactions [118, 119]. Proper choice of initiator for anionic polymerization of polar vinyl monomers is of critical importance to obtain polymers with predictable, well-defined structures. As an example of an initiator that is too reactive, the reaction of methyl methacrylate (MMA)... 

Anionic Polymerization of Polar Vinyl Monomers D Baskaran and AHE Muller 623... 

It may be tentatively assumed that curve 1 in Fig. 3 does not contradict the very scarce experimental data on the anionic polymerization of symmetric vinyl monomers. For instance, it is known (bibliography see Ref. [593) that ethylene is anionically polymerized on the polarized carbon-lithium bond or the corresponding contact ion pair. However, additional experimental investigations are needed for drawing a more definite conclusion about the validity of curve 1 in Fig. 3. 

In homogeneous anionic polymerizations of simple vinyl monomers, steric placement is also temperature-dependent, just as it is in cationic polymerizations. Syndiotactic placement is favored in polar solvents at low temperature. In nonpolar solvents, however, isotactic placement predominates at the same temperatures. Here, too, this results mainly from the degree of association with the counterion. 

The importance of the electrophilic character of the cation in organo-alkali compounds has been discussed by Morton (793,194) for a variety of reactions. Roha (195) reviewed the polymerization of diolefins with emphasis on the electrophilic metal component of the catalyst. In essence, this review willattempt to treat coordination polymerization with a wide variety of organometallic catalysts in a similar manner irrespective of the initiation and propagation mechanisms. The discussion will be restricted to the polymerization of olefins, vinyl monomers and diolefins, although it is evident that coordinated anionic and cationic mechanisms apply equally well to alkyl metal catalyzed polymerizations of polar monomers such as aldehydes and ketones. 

Monomer reactivity is a broad concept, and it can not always be limited only to reactions of the double or triple bonds of a vinyl or acetylene group. Weakly polar monomers, such as styrene or butadiene, react almost exclusively by their double bonds. The anionic polymerization of polar monomers, such as a, /S-unsaturated esters and nitriles, is accompanied by many side reactions. A fairly large amount of oligomers and side products are formed, and these may affect the active centres, thus indirectly modifying propagation. 

The reactivity of the initiator and the stabilization of the resulting propagating enolate anions significantly affect the overall kinetics of the polymerization. Therefore, the reactivity of anionic initiators toward polar vinyl monomers and the control of the polymerization are strongly dependent on the choice of initiator and the polarity of the reaction medium. ... 

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. 

Catalysts of the Ziegler-Natta type are applied widely to the anionic polymerization of olefins and dienes. Polar monomers deactivate the system and cannot be copolymerized with olefins. J. L. Jezl and coworkers discovered that the living chains from an anionic polymerization can be converted to free radicals by the reaction with organic peroxides and thus permit the formation of block copolymers with polar vinyl monomers. In this novel technique of combined anionic-free radical polymerization, they are able to produce block copolymers of most olefins, such as alkylene, propylene, styrene, or butadiene with polar vinyl monomers, such as acrylonitrile or vinyl pyridine. 

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. 

In anionic polymerization of vinyl monomers (nondiene), low temperatures and polar solvents favor the preparation of syndiotactic polymers.21 Nonpolar solvents tend to favor isotactic polymerization. In the case of diene monomers such as butadiene and isoprene, the use of lithium based initiators in nonpolar... 

The restricted adsorption of anion-active emulsifier during polymerization of polar monomers (vinyl acetate) has also been reported in other work. Breitenbadi eX ul. (1970) established that in the case of polymerization initiated hy a,forming interface. The polymerization rate was found to depend on emulsifier concentration to the power of 0.1. 

In the polymerization of vinyl monomers on free anions or solvent separated ion pairs, ring opening in a cyclic intermediate occurs in the conrotatory manner [39]. Hence, anionic polymerization in polar media should tend to form syndiotactic polymers and should be slightly similar in this respect to free-... 

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