Polymerization lithium bonds

In their papers Rodionov and coworkers described the polymerization of organolithium compounds in terms of the formation of lithium bonds (Scheme 1), analogous to hydrogen bonds, which brought about cyclic or linear association of these compounds in solution . However, the strong association of alkyllithium compounds, persisting even in the vapour phase, indicates that their association takes place through the formation of... 

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. 

Alkyllithium compounds LiR react stoichiometrically with butadiene and isoprene in hydrocarbons to form the corresponding alkyl-substituted butenyllithium compounds. If the diene is applied in excess, the polymerization can be catalyzed by further diene insertion into the allyl-lithium bond. Both steps have been proved directly but the mechanism of the selectivity remains an open question [41, 42]. 

The present state of knowledge about the true mechanism of these polymerization reactions is not sufficiently advanced to permit a satisfactory rationalization of these effects. It should be remembered that the growing chains in these systems have been convincingly demonstrated (]J) to be associated in pairs at the site of the carbon-lithium bond. Hence it appears that the incoming monomer must react with the associated complex, which apparently can affect the mode of entry. This undoubtedly can explain the greater extent of cis-1,4 addition in the case of isoprene compared to butadiene. Furthermore, such factors as lithium concentration and presence of different solvents can be assumed to have an effect on the structure and reactivity of the associated carbon-lithium bond at the active chain end. This would certainly be expected for the highly polar carbon-lithium... 

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. 

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 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 ... 

Two other phosphorus compounds reported to have short O—H—O bonds, believed to be symmetrical, are Me3NH+ PhP(0)(0H)0P(0 )(0H)Ph (13.36a), and the polymeric lithium salt, Li C2HAP2]-U13.36b). 

Because of the complicating effects of counterion and solvent associated with anionic polymerization, relatively few reactivity ratios have been determined for anionic systems. Typical reactivity ratios for the anionic copolymerization of styrene and a few other monomers are shown in Table 8.3. Most of the values were determined from the copolymer composition equation [Eq. (7.11) or (7.18)]. A dramatic effect of solvent is seen with styrene-butadiene copolymerization, where a change from the nonpolar hexane to the highly solvating THF reverses the order of reactivity. Again in the case of hydrocarbon solvent, the reaction temperature shows a minimal in uence on reactivity ratios, while in the case of polar solvents, such as THF, the reactivity ratios vary considerably, which has been rationalized by considering the solvation of carbon-lithium bond. Thus as the temperature is increased (from -78°C to 25°C), the extent of solvation by THF is expected to decrease, resulting in more covalent carbon-lithium bond. 

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 acceleration of the rates of organolithium reactions resulting from low concentrations of bases such as amines or ethers is very great (54, 60, 72-75), The effect no doubt results from complex formation which labilizes the carbon-lithium bond. Coordination of base also apparently results, as expected, in a more ionic transition state. The stereospecificity of olefin polymerization changes very markedly upon addition of base, and varies with base concentration in a manner which suggests that complex formation is responsible 31, 68, 73). 

Oxalamidinate anions represent the most simple type of bis(amidinate) ligands in which two amidinate units are directly connected via a central C-C bond. Oxalamidinate complexes of d-transition metals have recently received increasing attention for their efficient catalytic activity in olefin polymerization reactions. Almost all the oxalamidinate ligands have been synthesized by deprotonation of the corresponding oxalic amidines [pathway (a) in Scheme 190]. More recently, it was found that carbodiimides, RN = C=NR, can be reductively coupled with metallic lithium into the oxalamidinate dianions [(RN)2C-C(NR)2] [route (c)J which are clearly useful for the preparation of dinuclear oxalamidinate complexes. The lithium complex obtained this way from N,N -di(p-tolyl)carbodiimide was crystallized from pyridine/pentane and... 

On the other hand polysilylalkynes with phenyl or allyl substituents are converted with triflic acid into polymeric alkynylsilyltriflates. These polymers react with many acidic element hydrogen compounds or lithium element compounds with formation of silicon element bonds. Thus we found an easy approach to numerous new functional substituted alkynes [12], Eq.(9) shows selected examples of this reaction type. 

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