Alkyl lithium compounds 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. 

It should be noted that the preparation of n-type (reduced) polyacetylene using strong organic bases (e.g., alkyl lithium compounds) or more commonly electron transfer reagents (e.g., sodium naphthalide radical anion) employs the two major classes of initiators used in anionic polymerization of monomers such as styrene and butadiene. Reductive doping can also be accomplished by deprotonation of, for example, acetylene/butadiene copolymers and related phenylenepentadienylenes." ... 

Some new initiators soluble in hydrocarbons were described during the last few years. Organo-lithium compounds form 1 1 complexes with alkyls of Mg 134,135), Zn 136) or Cd l36), and their usefulness as initiators of anionic polymerization of styrene and the dienes was established 137). 

Some information is available on other acrylates. N,N-disubstituted acrylamides form isotactic polymers with lithium alkyls in hydrocarbons (12). t-Butylacrylate forms crystallizable polymers with lithium-based catalysts in non-polar solvents (65) whereas the methyl, n-butyl, sec-butyl and isobutyl esters do not. Isopropylacrylate also gives isotactic polymer with lithium compounds in non-polar solvents (34). The inability of n-alkylacrylates to form crystallizable polymers may result from a requirement for a branched alkyl group for stereospecific polymerization. On the other hand lack of crystallizability cannot be taken as definite evidence of a lack of stereoregulating influence, as sometimes quite highly regular polymer fails to crystallize. The butyllithium-initiated polymers of methylmethacrylate for instance cannot be crystallized. The presence of a small amount of more random structure appears to inhibit the crystallization process1. 

Hard nucleophiles such as alkyl lithium and alky lmagnesi urn compounds transfer their alkyl residues initially via a transmetallation reaction to the palladium atom in 22 Reaction product 24 is then released from 23 through a reductive elimination. Because this reductive elimination occurs with retention of configuration, the re suit here is an overall inversion of configuration. Nevertheless, re actions of this type have little preparative significance. 

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. 

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. 

The initiation of methyl methacrylate polymerization by alkyl- and aryl-lithium compounds is a very complex process which has not been completely elucidated so far. Even less information is available on the efects of or-ganometals not containing lithium. Recently, the interest of some authors has centred on the application of Grignard reagents for the initiation of anionic polymerizations, especially of MMA [171, 172],... 

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... 

With lithium alkyl or aryl initiation in tetrahydrofuran, information is available only on the addition reaction to 1,1-diphenylethylene. Preliminary assessment of the rates showed a wide variation in initiator efficiency [100]. Direct comparison of initiation rates is difficult, however, since the orders in initiator vary between one quarter and unity [101] (Fig. 13) although the order in diphenylethylene is, as expected, unity. The reaction order in lithium compound is 0.27 0.03 (methyl-Li) 0.34 0.1 (vinyl-Li) 0.66 0.04 (phenyl-Li) 1.1 0.2 (benzyl-Li). 

Lithium Alkyls. Organolithium compounds have been widely used as initiators, being readily available and experimentally very convenient. Their aggregated form in hydrocarbon solvents is readily broken down by addition of donor molecules, and initiation becomes fast and efficient. The presence of common impurities such as alkoxides can have a strong influence, almost certainly through cross-association, and may increase the rate of initiation in some solvents while depressing it in others. ... 

As stated above, CIDNP denotes the transient occurrence of anomalous line intensities in NMR spectra recorded during chemical reactions or shortly after their completion. The phenomenon was first observed in 1967 by Bargon, Fischer and Johnsen [35a] in thermal decompositions of peroxides and azo compounds, and, independently, by Ward and Lawler [35b] in the reactions of alkyl lithium with alkyl halides. It was immediately realized that the line anomalities are caused by populations of the nuclear spin states in the reaction products that deviate from the Boltzmann populations. After initial attempts of interpreting CIDNP by electron-nuclear cross-relaxation, the radical pair mechanism was developed in 1969 by Kaptein and Oosterhoff [36a], and independently by Closs [36b],... 

The chemistry of pyrylium and thiopyrylium ions is largely based on the sensitivity to nucleophiles. In this sense, they resemble pyridinium ions. The pyrylium ion is more reactive, because oxygen is more electronegative than nitrogen. The addition of nucleophiles such as alkyl lithiums, NaCN, NaOH, and so on, occurs readily and leads initially to pyran derivatives. Usually, these adducts undergo ring opening to dienals (Scheme 6.29), which are valuable in the synthesis of other compounds. 

To this end, we have recently found that low-coordinate magnesium alkyl complexes can be stabilized by bulky A,A -diisopropylphenyl P-diketiminate ligands, and that these compounds initiate a rapid polymerization of MMA. The alkyl complexes are prepared either by reaction of the diketimine with dialkyl precursors such as MgMe or by treatment of RMgX (R = Me, i-Pr, t-Bu, Ph X = Cl, Br) with the lithium salt of the ligand (Scheme 23.15). ... 

Subsequent work by D. Braun and collaborators dealt with the effect of solvent polarity and countercation on the tacticity of poly(methyl methacrylate) (PMMA) initiated with various organo alkali metal compounds. By using proton NMR analysis they concluded that polar solvents and alkyl lithium initiators favored syndiotactic configurate on( ) while non-polar solvents favored isotactic piacement( )." ... 

A notable exception to the emphasis on free-radical polymerization studies was provided by Karl Ziegler and his co-workers who extended the study of the alkali metal polymerization of dienes to include metals other than sodium and various metal alkyls. Of particular interest were the results obtained with the simplest Group I alkali metal, lithium. It was found that when lithium metal was used as a polymerization initiator 1,4- structures predominated over 1,2-polymers. It was also found that polymerization in hydrocarbon solvents further favoured production 1,4- structures whilst polymerization in polar liquids such as ethers and amines often favoured the formation of 1,2- units. It was also found that reaction of lithium with monomer led to the production of an organo-lithium compound which made feasible homogeneous polymerization—a discovery which eventually led to commercial exploitation. 

Reaction scheme (f) has been most widely subscribed to by polymer chemists. In this writer s opinion it is not a likely pathway for reaction. The enthalpy change for dissociation of hexamer into monomer molecules, Eq. (7), is not known, but is probably on the order of 100 kcal/mole of hexamer, at a minimum. This, coupled with an unfavorable entropy effect, should make the free energy change for Eq. (7) so large (positive) that no reasonable assumption about the kinetic behavior of the various candidate species will suffice to make the concentration of monomer rate-determining. It must be recalled that the mass spectral data for ethyllithium vapor at 87° C failed to show the presence of any monomer. Intermolecular exchange of alkyl groups, observed in NMR studies of ethyllithium (47), almost certainly proceeds by dissociation of hexamer into tetramer and dimer. The enthalpy of dissociation of polyisoprenyllithium dimer has been estimated to be about 37 kcal/mole 31b). If a similar value obtains for short chain n-alkyllithium compounds (and there is reason to believe it does), the rate of formation of monomer could not possibly be as fast as the observed rates of initiation. Finally, it is difficult to see how a monomeric alkyl-lithium species could confer the required stereospecificity in reaction with an olefin 73). 

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. 

While the initial surface species formed on lithium in alkyl carbonates consist of ROC02Li compounds, these species react with water to form Li,CO, C02, and ROH. This reaction gradually changes the composition of the surface films formed on... 

---