Lithium bonded initiator

Polymerization and Copolymerization with Lithium—Nitrogen-Bonded Initiator... 

From the above studies, it is clear that a copolymer of butadiene and styrene can be prepared from lithium-nitrogen bond initiators. The styrene content of the copolymer is highly dependent on the type of initiator and the polymerization conversion. These lithium-nitrogen bond initiators do not yield a randomized copolymer even with the presence of built-in polar modifier. This may be due to the heterogeneous nature of the initiator. In order to understand the mechanism of copolymerization with lithium-nitrogen bonded initiator. More work along these lines is needed. 

This review is limited to the polymerization of hydrocaibon dienes and olefins by means of organolithium initiators. It is not intended to include activated olefins or dienes that can be polymerized by bases of far lower reactivity or that do not involve direct caibon-lithium bonding. 

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 character of the counterion and the solvent both affect the microstruclure of polymers made anionically from dienes. In general, the proportion of 1,4 chains is highest for Li and decreases with decreasing clecironegativity and increasing size of the alkali metals in the order Li > Na > K > Rb > Cs. A very high (>90%) 1,4 content is achieved only with lithium alkyl or lithium metal initiation in hydrocarbon solvents. The properties of polymers of conjugated diolefins tend to be like those of thermoplastics if the monomer enchainment is 1,2 or 3,4 [reactions (4-3) and (4-4)]. Elastomeric behavior is realized from 1,4 polymerization and particularly if the polymer structure is cis about ihe residual double bond. 

Conjugated Dienes and Other Monomers. Alkyllithiums such as n-butyllithium—and even the growing polyethylene carbon-lithium bond complexed with chelating diamines such as TMEDA—are effective initiators for the polymerization of conjugated dienes such as 1,3-butadiene and isoprene. A polybutadiene of high 1,2-content can be produced from butadiene in hydrocarbon solvents using these N-chelated organolithium catalysts. 

Schleyer and his co-workers initially argued that the carbon-lithium bond was primarily covalent. - The Li 2p orbitals could overlap in either a a or IT fashion with the carbon orbitals to form the unusual bridging bonds. For example, in allyllithium, the HOMO of the allyl anion will interact with the lithium 2p orbital as shown in 5. 

Lithium alkyls initiate polymerization of polar monomers like methyl-methacrylate, vinyl pyridine, acrylo-nitrile, etc. However, these reactions are more complex. The desired addition to the C=C bond is accompanied by other processes, e.g., attack on the -COOEt group with the formation of ketones and lithium methoxide, or in vinyl pyridine polymerization by the metalation of the pyridine moiety. 

In conclusion, FMC has developed a viable, commercial synthesis of a family of omega-(/-butyldimethylsilyloxy)-l-alkyllithiums that are valuable anionic initiators. A variety of chain lengths are available between the protected hydroxyl ftmction and the carbon-lithium bond. These hydrocarbon soluble initiators afford very high 1,4-microstructure in the polymerization of polydienes, such as... 

The possibilities inherent in the anionic copolymerization of butadiene and styrene by means of organolithium initiators, as might have been expected, have led to many new developments. The first of these would naturally be the synthesis of a butadiene-styrene copolymer to match (or improve upon) emulsion-prepared SBR, in view of the superior molecular weight control possible in anionic polymerization. The copolymerization behavior of butadiene (or isoprene) and styrene is shown in Table 2.15 (Ohlinger and Bandermann, 1980 Morton and Huang, 1979 Ells, 1963 Hill et al., 1983 Spirin et al., 1962). As indicated earlier, unlike the free radical type of polymerization, these anionic systems show a marked sensitivity of the reactivity ratios to solvent type (a similar effect is noted for different alkali metal counterions). Thus, in nonpolar solvents, butadiene (or isoprene) is preferentially polymerized initially, to the virtual exclusion of the styrene, while the reverse is true in polar solvents. This has been ascribed (Morton, 1983) to the profound effect of solvation on the structure of the carbon-lithium bond, which becomes much more ionic in such media, as discussed previously. The resulting polymer formed by copolymerization in hydrocarbon media is described as a tapered block copolymer it consists of a block of polybutadiene with little incorporated styrene comonomer followed by a segment with both butadiene and styrene and then a block of polystyrene. The structure is schematically represented below ... 

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... 

Similar results were obtained with butadiene, except that the cis-1,1 contents were not as high, the meocimum attainable with undiluted monomer and 10 M initiator being about 85 . These were the first results to relate the chain structure to such subtle factors in the reaction, and are an indication of the marked sensitivity of the carbon-lithium bond to the polarity of the medium. 

The reduction of the double bond of an enamine is normally carried out either by catalytic hydrogenation (MS) or by reduction with formic acid (see Section V.H) or sodium borohydride 146,147), both of which involve initial protonation to form the iminium ion followed by hydride addition. Lithium aluminum hydride reduces iminium salts (see Chapter 5), but it does not react with free enamines except when unusual enamines are involved 148). 

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