Polymerizations alkyl lithium initiated

Yamamoto and collaborators have pioneered efforts to prepare polymers of methacrylates containing very bulky ester substructures. For the most part, the approaches reported involve anionic polymerization utilizing chiral amines and alkyl lithium initiators. Polymers prepared in this way are highly helical and have many useful properties, including their use in chiral chromatography. 

The alkyl-lithium initiated, living anionic polymerization of elastomers was described in 1928 by Ziegler. To polymerize styrene-isoprene block copolymers Szwarc et al., [1956] used sodium naphthalene as an anion-radical di-initiator, while Shell used an organolithium initiator. The polymerization mechanism was described by By water [1965]. 

The preparations by anionic mechanism of A——A type block copolymers of styrene and butadiene can be carried out with the styrene being polymerized first. Use of alkyl lithium initiators in hydrocarbon solvents is usually a good choice, if one seeks to form the greatest amount of c/s-1,4 microstructure [346]. This is discussed in Chap. 4. It is more difficult, however, to form block copolymers from methyl methacrylate and styrene, because living methyl methacrylate polymers fail to initiate polymerizations of styrene [347]. The poly(methyl methacrylate) anions may not be sufficiently basic to initiate styrene polymerizations [345]. 

In anionic solution systems the feed stocks are typically dried over various types of dessicants because the systems are sensitive to water contamination. When using continuous anionic solution polymerization systems, it is necessary to employ low (ppm) concentrations of a chain-transfer agent in order to discourage gelation and fouling 1,2-butadiene is often used for this purpose in commercial applications. Alkyl-lithium-initiated polybutadiene is less prone to contain gel and does not contain the heavy metal catalyst residues associated with Ziegler-Natta catalyzed products. 

In anionic polymerization, the initiator, typically alkyl lithium, initiates the polymerization process and rapidly reacts with the available monomer molecules. Then the active polymer chains remain active for the duration of the pol5mier-ization process. They will continue to polymerize any additional monomer that may be added to the system. The growing polymer chains must be deactivated specifically by the addition of a deactivator at the end of the process. 

A convincing mechanism, capable of explaining all the features of the stereochemistry of the polymerization of conjugated dienes by alkyl-lithium initiators has yet to be proposed. The suggestion that a six-membered, cyclic activated complex is formed between cis diene and the carbon-lithium bond (analogous to... 

The "equibinary" l,4-cis/l,4-trans polybutadiene prepared using a n-allyl nickel trifluoroacetate catalyst [70,71,96,97] and the polybutadiene obtained by polymerization of cyclooctadiene using an olefin metathesis catalyst system where shown by nmr to have random distributions of cis- and trans-ianits, although there is some indication that "equibinary" copolymers with non-Bernoullian structures are obtained in some cases [96]. Polybutadienes prepared using alkyl lithium initiators in hydrocarbon solvents have also been shown to have random distributions of 1,4-cis and 1,4-trans units [20,23,71,90]. 

Polyisoprenes prepared by alkyl lithium initiated polymerizations conducted in hydrocarbon solvents contain predominantly cis and trans units, but these may be enchained in a 1,4- or 4,1- manner. Cmr studies indicate that the cis and trans units are distributed randomly and that negligible amounts of 4,1-1,4 and... 

Several different nucleophilic substitution reactions have been observed in the polymerization of methyl methacrylate. Attack of initiator on monomer converts the active alkyl-lithium to the less active alkoxide initiator (Eq. 5-75). Further, methyl methacrylate (MMA) is converted to isopropenyl alkyl ketone to the extent that this reaction occurs. 

The reverse reaction of oxidation of the metal or reduction of the moiety is also known to occur in stereospecific catalyst systems. It has been long known that the polymerization of olefine materials can be accomplished in non-alkyl systems. Diem, Tucker and Gibbs (43) have shown that the lithium metal polymerization of isoprene proceeds with the initial reduction by the electron seeking lithium of the nucleophilic diene to produce the corresponding alkyl lithium. Fukui, Schimidzu, Yagi, Fukumoto, Kagiya and Yuosa (127) have studied the polymeriza-... 

The methodology to synthesize polymer hybrids by living anionic polymerization is shown in Fig. 3 [30]. Polyolefins containing p-tolyl groups have been used to initiate anionic polymerization by the lithiation of the methyl moiety using alkyl lithium and amine compounds system. 

Lithium and magnesium alkyl catalysts yield metal-polymer bonds with appreciable covalent character and their cations coordinate strongly with nucleophiles. Therefore, these catalysts will initiate simple anionic polymerization only under the most favorable conditions, e. g., in basic solvents and with monomers which produce resonance stabilized polymer anions. As examples of stereoregular anionic polymerization, a-methyl-methacrylate yields syndiotactic polymer with an alkyl lithium catalyst in 1,2-dimethoxyethane at — 60° C. (211, 212) or with a Grignard catalyst at -40° C. (213). 

In lithium alkyl-initiated polymerizations only chain initiation and propagation steps need be considered in hydrocarbon solvents. Both reactions are strongly influenced by extensive association of all lithium compounds. The reactive species in chain propagation is the small amount of dissociated material which probably exists as an ion pair. Association phenomena disappear on adding small amounts of polar additives, and the aggregates are replaced by solvated ion pairs. In polar solvents of relatively high dielectric constant (e.g. tetrahydrofuran), some dissociation of the ion pairs to free ions occurs, and both species contribute to the propagation step. The polymerizations are often complicated in tetrahydrofuran by two side reactions, namely carbanion isomerization and reaction with the solvent. 

During the initiation of methyl methacrylate polymerization by alkyl-lithium, lithium alkoxide is formed [see eqn. (37)].This compound directly affects the subsequent course of the reaction. It has aroused the interest of scientists who started to used various lithium alkoxides directly as initiators... 

The syntheses of siloxanes with vinyl groups at the silicon atoms of the chain was performed by anionic ring-opening polymerization of vinyl group-containing cyclotri- or cyclotetrasiloxanes (03, 04 ) with alkyl lithium compounds as initiators. The polymerizations were terminated by chlorosilanes with additional silico- or carbofunctional groups or octyl groups. In the block copolymerizations we used 03 or 04 and hexamethylcyclotrisiloxane (D3) (Scheme 1 and 2). 

The dependence of the propagation rate on initiator concentration is more complex, however, and can be explained as reflecting the existence of more than one kind of active center in media that can solvate the counterion. The simplest situation, which is used here for illustration, corresponds to an equilibrium between free ions and ion pairs. [It is likely that various kinds of ion pairs exist (cf. Eq. 9-1) but these ramifications can be neglected in this simple treatment.] The reactions involved in the actual propagation steps in the polymerization of a monomer M by an alkyl lithium compound RLi can then be represented as... 

Solution-polymerized SBR is made by termination-free, anionic/live polymerization initiated by alkyl lithium compounds. Other lithium compounds are suitable (such as aryl, alkaryl, aralkyl, tolyl, xylyl lithium, and ot/p-naphtyl lithium as well as their blends), but alkyl lithium compounds are the most commonly used in industry. The absence of a spontaneous termination step enables the synthesis of polymers possessing a very narrow molecular weight distribution and less branching. Carbon dioxide, water, oxygen, ethanol, mercaptans, and primary/secondary amines interfere with the activity of alkyl lithium catalysts, so the polymerization must be carried out in clean, near-anhydrous conditions. Stirred bed or agitated stainless steel reactors are widely used commercially. 

This review article is concerned with chemical behavior of organo-lithium, -aluminum and -zinc compounds in initiation reactions of diolefins, polar vinyls and oxirane compounds. Discussions are given with respect to the following five topics 1) lithium alkylamide as initiator for polymerizations of isoprene and 1,4-divinylbenzene 2) initiation of N-carboxy-a-aminoacid anhydride(NCA) by a primary amino group 3) activated aluminum alkyl and zinc alkyl 4) initiation of stereospecific polymerization of methyloxirane and 5) comparison of stereospecific polymerization of methyloxirane with Ziegler-Natta polymerization. A comprehensive interpretation is proposed for chemistry of reactivity and/or stereospecificity of organometallic compounds in ionic polymerizations. 

The dissociation of the tetramers into a monomer and a trimer seems unlikely because fragments involving an odd number of Li atoms are hardly seen in the mass-spectra of the aggregated alkyl lithiums. Nevertheless, it is probable that monomeric lithium alkyls are present in solutions of the aggregates and these could be the direct initiators of polymerization of vinyl and diene monomers. Their low concentration might be offset by their very high reactivity. ... 

Mechanisms of Initiation of Anionic Polymerization by Alkyl Lithiums... 

The fragmentation or complexation of alkyl lithium aggregates has a profound effect upon their ability to initiate anionic polymerization and it greatly affects their rate of initiation. Examples of such effects are discussed in the next section. 

Initiation of anionic polymerization of styrene, dienes and their derivatives by alkyl lithium in hydrocarbon solvents was extensively studied by Ziegler163) and thereafter by many other workers. Since the rates of initiation are often comparable to those of propagation, both processes occur simultaneously and then, while the monomer is quantitatively polymerized, an appreciable fraction of the initiator remains unutilized in the system. Hence, it is advantageous to use fast alkyl lithiums as initiators, especially when a polymer of a narrow molecular weight distribution is the desired product. 

Different mechanisms govern the initiation of polymerization by alkyl lithiums depending upon whether the reaction takes place in aromatic hydrocarbons, like benzene or toluene, or whether it proceeds in aliphatic ones, like cydo-hexane or n-hexane. The following discussion is restricted to polymerization of styrene and the dienes, and the initiation in aromatic hydrocarbons is considered first. 

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