Polymerization by Anionic Initiators

Cyclosilanes can be used as monomers in a ROP to afford linear chains. Although, as mentioned earlier the three-membered cyclic compound 

Matyjasjewski s procedure can be applied to other types of cyclosilane rings. Thus, the ROP of the five-membered cyclosilane [(PhMeSi) (Me2Si)4] can be accomplished by silylanion initiators such as Mc2PhSiK (see Eq. 7.6) [48]. 

We have seen in the beginning of this chapter that methods of polymerizing stable disilenes have not yet been foimd primarily because the very process of stabilizing these compoimds by the use of sterically hindered substituents prevents their controlled polymerization to high polymers. 

On the other hand, compounds that can be described as trapped or masked disilenes can be viewed as potential monomers for polymer synthesis. The trapped or masked disilene can be liberated from its adduct by chemical or photochemical process. However, even if moderately buUq substituents are present on silicon in such compounds, the liberated disilene combines to afford the thermodynamically favorable cyclized products. Thus, the masked disilene containing isopropyl substituents on silicon affords the cyclotetrasilane [zPr2Si]4 (see Eq. 7.7) [49]. 

Sakurai and coworkers have found that disilenes such as l-phenyl-7,8-disilabicylo[2.2.2]octa-2,5-dienes can be polymerized by the use of anionic initiators such as n-butyllithium [18, 50, 51]. High-molecular-weight polymers with a narrow PDI (about 1.5-1.9) were obtained from this method. As mentioned above, the choice of substituents on silicon is quite 

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Anionic polymerization is better for vinyl monomers with electron withdrawing groups that stabilize the intermediates. Typical monomers best polymerized by anionic initiators include acrylonitrile, styrene, and butadiene. As with cationic polymerization, a counter ion is present with the propagating chain. The propagation and the termination steps are similar to cationic polymerization. 

Dimethylketene has been polymerized by anionic initiators. Three different repeat units are formed (XLVII, XLVIII, XLIX) corresponding to polymerization through the alkene... 

Isobutene is polymerized commercially by a cationic mechanism initiated by strong acids like AICI3. It is not polymerized by free-radicals or anionic initiators. Acrylonitrile is polymerized commercially by free-radical means. It can also be polymerized by anionic initiators like potassium amide but does not respond to cationic initiators. Account for the difference in behavior of isobutene and acrylonitrile in terms of monomer structure. 

In addition, with these monomers the substituent not only preferentially complexes the electrophile but may even reduce the nucleophilicity of the double bond by electron attraction. Acrylates (and similarly vinyl acetate) thus do not polymerize cationically. (It may be noted that vinyl acetate is also not polymerized by anionic initiators as they attack the acetate linkage. Vinyl acetate is polymerized only by free radicals.)... 

Isocyanates can be polymerized by anionic initiators to give polymers that exhibit a rigid, helical conformation in solution. Elegant studies by Green and coworkers have revealed that the copolymerization of achiral isocyanates with... 

Since the first preparation of stereoregular poly(methyl methacrylate) by Fox et al. and Miller et al. in 1958, a large number of papers have been published on the steieospecific polymerization of methyl methacrylate, while the NMR technique for the determination of microstructure developed by Bovey and Tiers and Nishioka et al. enabled us to accumulate the extensive information on this polymerization. Mostly anionic initiators have been used for the pdymerization. A review on the polymerization by lithium compounds was presented by Bywater In a recent review by Pino and Suter were discussed some of the factors which can influence the stereoregulation in the polymerization of vinyl monomers including a-substituted acrylate. A variety of magnesium and aluminum compounds can be utilized as stereospecific initiators. Besides methyl methacrylate, not only methacrylates with various ester groups, but also a-substituted acrylates, such as a-ethyl- or o-phenyl-acrylate, were also subjected to the stereospecific polymerization by anionic initiator. The stereospecificity in the copolymerization between the monomers described above is also a matter of interest. 

Many ferrocenylsilanes can be polymerized by anionic initiators such as n-butyllithium, phenyllithium or ferrocenyllithium (Fig. 8.21). The reaction occurs at ambient temperature and affords living polymers. The utility of anionic polymerization is that the molecular weights can be controlled and also that block copolymers can be prepared. The main disadvantage of the anionic polymerization is that the monomer and the solvent should be rigorously purified and should be free of acidic impurities including water. Even traces of impurities can be detrimental [53]. 

Alkyl cyanoacrylate monomers have been copolymerized with a variety of monomers, both by radical and anionic initiation. The radical-initiated copolymerization with acrylic monomers was performed with a sufficient amount of an acid stabilizer present to suppress polymerization by anionic means [19]. This investigation has been covered extensively elsewhere. 

Generally speaking, a monomer with electron-releasing groups will be more rapidly polymerized by cationic initiators. Anionic initiators polymerize olefins with electron-withdrawing groups more rapidly. A more sensitive test of the nature of the reaction is the behavior of a mixture of two such monomers in copolymerization in which they compete for the intermediate. This will be discussed in more detail in Chapter XII on polar versus radical mechanisms. 

Carbonyl monomers can be polymerized by acidic initiators, although their reactivity is lower than in anionic polymerization. Protonic acids such as hydrochloric and acetic acids and Lewis acids of the metal halide type are effective in initiating the cationic polymerization of carbonyl monomers. The initiation and propagation steps in polymerizations initiated with protonic acids can be pictured as... 

Isocyanates are polymerized through the carbon-nitrogen double bond to 1-nylons by anionic initiators such as metal alkyls, sodium naphthalene, and sodium cyanide [Bur and Fetters,... 

Polymerization of four-membered cyclic ethers (oxetanes) is also brought about by cationic initiators (e.g., Lewis acids) and by anionic initiators (e.g., or-ganometallic compounds). The polymer of 3,3-bis(chloromethyl)oxetane is distinguished by its very high softening point and by its unusual chemical stability. 

MW = 1034), biphenyl tetramer hexaol (MW = 782), /3-0-4 tetramer hep-taacetate (C55H66O22, MW = 1078) and /J-O-4 tetramer heptaol (MW = 784). The Igepal (GAF Corp., sold through Aldrich) standards F.W. = 749 and 1982 were also examined. Two synthetic polymers prepared by anion-initiated polymerizations of a quinonemethide according to the procedure of Chum et al. (20) were treated as intermediate MW lignin model polymers. 

Anionic Catalysis Several bulky methacrylates afford highly isotactic, optically active polymers having a single-handed helical structure by asymmetric polymerization. The effective polymerization mechanism is mainly anionic but free-radical catalysis can also lead to helix-sense-selective polymerization. The anionic initiator systems can also be applied for the polymerization of bulky acrylates and acrylamides. The one-handed helical polymethacrylates show an excellent chiral recognition ability when used as a chiral stationary phase for high-performance liquid chromatography (HPLC) [97,98]. 

Ring-opening polymerization is one of the most important applications of SCBs in organic chemistry. Polymerization of SCBs, which gives rise to carbosilane polymers, has been carried out thermally, by transition metal catalysis, or, most commonly, by anionic initiation. Thermal polymerization is rare, however, and is not covered in this chapter. For leading references into thermal polymerization of SCBs, refer to <1996CF1EC-II(1B)867> and <1995COMC-II(2)50>. 

Of a large number of possible fluorinated acrylates, the homopolymers and copolymers of fluoroalkyl acrylates and methacrylates are the most suitable for practical applications. They are used in the manufacture of plastic lightguides (optical fibers) resists water-, oil-, and dirt-repellent coatings and other advanced applications [14]. Several rather complex methods to prepare the a-fluoroalkyl monomers (e.g., a-phenyl fluoroacrylates, a-(trifluoromethyl) acrylic and its esters, esters of perfluoromethacrylic acid) exist and are discussed in some detail in [14]. Generally, a-fluoroacrylates polymerize more readily than corresponding nonfluorinated acrylates and methacrylates, mostly by free radical mechanism [15], Copolymerization of fluoroacrylates has been carried out in bulk, solution, or emulsion initiated with peroxides, azobisisobutyronitrile, or y-irradiation [16]. Fluoroalkyl methacrylates and acrylates also polymerize by anionic mechanism, but the polymerization rates are considerably slower than those of radical polymerization [17]. 

The equations developed for contain no reference to the mode of polymerization. Although they are presented here in a chapter devoted to radical polymerization, they are characteristic of the polymer and not of its method of synthesis. The same Tc applies to all polystyrenes of given molecular weight and tacticily, for example, regardless of whether they were polymerized by anionic, cationic, or free-radical initiation. 

For analogous reasons to the monomer requirements that favor cationic initiators, vinylic monomers with electron-withdrawing substituents on the carbon-carbon double bond are amenable to polymerization by anionic catalysts, since under these conditions the electron-withdrawing substituent assists in stabilization of the propagating carbon ion as it forms. However, this class of monomer is usually still sensitive to free radical-initiated polymerization because of electronic back donation from the electron-withdrawing group to the carbon-carbon double bond (Table 22.4 [11, 12]). 

It should be noted that ethylene oxide can also be polymerized by cationic initiating systems and a wide range of other anionic initiators. 

Chain transfer to monomer is much less prevalent for polymerizations with most of the anionic coordination initiators. Much higher molecular weights are thus possible in these polymerizations. For example, molecular weights of the order of 10 are reported for propylene polymerization by an initiator derived from diphenyltin sulfide and bis(3-dimethylaminopropyl)zinc. 

The on-line GPC/NMR technique is also useful for studying the molecular weight dependence of polymer properties such as tacticity and copolymer composition. This type of information is very important for an understanding of the mechanism of polymerization. The polymerization of MMA by anionic initiators often involves multiple active species with different reactivities and stereospecificities. For example, Grignard reagent exists in the Schlenk equilibrium ... 

Among the classical (summarized in [11]) reactions of cyanoacetylenes (salt and n-complex formation, nucleophilic addition of, inter alia, amines and alcohols, Diels-Alder additions with 2 (see Section 2.22.2), addition of halogens and hydrogen halides, etc.) their polymerization might deserve a second look since the products formed - polyacetylenes - have attracted much attention during the last two decades. Thus 1, several of its derivatives, and 2 have been polymerized with anionic initiators (triethylamine, sodiiun cyanide, butyllithium) to give black, low-molecular-weight polymers claimed to have structures like 33 [38-40], which is obtained from 2 by treatment with butyllithium. 

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