N-Butyllithium initiator

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

Alkyllithium initiators are primarily used as initiators for polymerizations of styrenes and dienes. They effect quantitative living polymerization of styrenes and dienes in hydrocarbon solution. In general, these alkyllithium initiators are too reactive for alkyl methacrylates and vinylpyridines. n-Butyllithium is used commercially to initiate anionic homopolymerization and copolymerization of butadiene, isoprene, and styrene with linear and branched structures. Because of its high degree of association (hexameric), n-butyllithium-initiated polymerizations are often effected at elevated temperatures (>50 °C) and in the presence of small amounts of Lewis base to increase the rate of initiation relative to propagation and thus obtain polymers with narrower molecular weight distributions [55, 57]. 

The time-dependent evolution of the polymerisation and copolymerisation reactions of styrene/unsaturated polyester resin was characterised using optical levitation with Raman spectroscopy (135). The n-butyllithium-initiated anionic polymerisation of styrene in ethylbenzene was measured using Raman spectroscopy (214). 

The frequent coincidence of the fractional order with the degree of association supports the postulate that the initiating species is a small amount of reactive monomeric alkyllithium in equilibrium with the much larger concentration of the unreactive aggregated species. However, the correctness of this interpretation, ie, direct dissociation to monomeric, imassociated species, has been questioned (33). The experimentally observed energies of activation, eg, 75 kJ/mol (18 kcal/mol) for n-butyllithium initiation of styrene polymerization (51), appear to be too low to include the enthalpy of complete dissociation of the aggregates, which is estimated to require approximately 452 kJ/mol (108 kcal/mol) (52). An alternative is the incomplete or stepwise dissociation of the aggregate, for example, as shown in equations 22-25 for hexamers equation 25 plus equation 24 would apply for tetramers. 

The effects of lithium alkoxides on the rates of alkyllithium-initiation reactions depend on the solvent, the monomer, the alkoxide structure, the alkyllithium initiator, and the ratio of [RLi]/[LiOR l (49,50). For n-butyllithium initiation of styrene in cyclohexane, the rate of initiation is increased at low relative concentrations of added lithium alkoxide [ -C4H90Li]/[C4H9Li]< 0.5). At a ratio of 1/1, the rate is essentially the same as the control without alkoxide beyond this ratio, the rate decreases continuously with increasing relative concentration of lithium alkoxide. In aromatic solvents, the initiation rate decreases with increasing relative concentrations of lithium alkoxide. Lithium alkoxides generally accelerate the rate of initiation by alkyllithiums (n-butyllithium and sec-butyllithium) for isoprene in hexane. 

We controlled the structure of SBR s in our work with the use of a number of polymerization variables, described in Table 3. Polybutadienes with vinyl contents ranging from 9 to 81% were synthesized in n-hexane with n-butyllithium initiator and TMEDA modifier. The desired vinyl content... 

Figure L 40 MHz proton spectra of poly (methyl methacrylate) in chloroform (a) polymer prepared using free radical initiator (b) polymer prepared using n-butyllithium initiator. (Bovey,...

Hawkridge AM, Gardella Jr. JA. Evaluation of matrix-assisted laser desorption ionization mass spectrometry for studying the sec-butyllithium and n-butyllithium initiated ring-opening polymerization of hexamethylcy-clotrisiloxane (D3). J Am Soc Mass Spectrom 2003 14 95-101. 

The result of electron donation by the six oxygen atoms from crown ether is an enhancement of the ionic character of thie carbon-sodium bond. An increase in ionic character in the copolyberization results in an increased amount of vinyl structure. Such ah explanation has been suggested by Antkowiak in the n-butyllithium initiated polymerization. 

Sivola A (1977) The n-butyllithium-initiated polymerization of myrcene and its copolymerization with styrene. Acta Polytechnica Scand, Chem Incl Metall Ser No 134... 

Methylthiophene is metallated in the 5-position whereas 3-methoxy-, 3-methylthio-, 3-carboxy- and 3-bromo-thiophenes are metallated in the 2-position (80TL5051). Lithiation of tricarbonyl(i7 -N-protected indole)chromium complexes occurs initially at C-2. If this position is trimethylsilylated, subsequent lithiation is at C-7 with minor amounts at C-4 (81CC1260). Tricarbonyl(Tj -l-triisopropylsilylindole)chromium(0) is selectively lithiated at C-4 by n-butyllithium-TMEDA. This offers an attractive intermediate for the preparation of 4-substituted indoles by reaction with electrophiles and deprotection by irradiation (82CC467). 

Most other studies have indicated considerably more complex behavior. The rate data for reaction of 3-methyl-l-phenylbutanone with 5-butyllithium or n-butyllithium in cyclohexane can be fit to a mechanism involving product formation both through a complex of the ketone with alkyllithium aggregate and by reaction with dissociated alkyllithium. Evidence for the initial formation of a complex can be observed in the form of a shift in the carbonyl absorption band in the IR spectrum. Complex formation presumably involves a Lewis acid-Lewis base interaction between the carbonyl oxygen and lithium ions in the alkyllithium cluster. 

Many initiators, such as alkyl and aryllithium and sodium and lithium suspensions in liquid ammonia, effect the polymerization. For example, acrylonitrile combined with n-butyllithium forms a carbanion intermediate ... 

Treatment of diallenyl sulfone 354 with n-butyllithium resulted in a cyclodimerization to afford 2,6-dithiaadamantane derivative 356. This dimerization is considered to be initiated by formation of the a-sulfonyl carbanion 355 and to proceed through a carbanion walk or carbanion tour process426. 

The effect of media viscosity on polymerization rates and polymer properties is well known. Analysis of kinetic rate data generally is constrained to propagation rate constant invarient of media viscosity. The current research developes an experimental design that allows for the evaluation of viscosity dependence on uncoupled rate constants including initiation, propagation and macromolecular association. The system styrene, toluene n-butyllithium is utilized. 

The readily accessible oxalamidine derivative PhN = C(NHBu )C(NHBu ) = NPh provides another useful entry into the coordination chemistry of oxalamidinato ligands. Scheme 195 summarizes the results of an initial study. Mono- and dinuclear complexes of Ti, Zr, and Ta have been isolated and fully characterized. Silylation of both N-H functions was achieved by subsequent treatment with 2 equivalents of n-butyllithium and MesSiCl. The preparation of a nickel(II) complex failed and gave a hydrobromide salt instead. ... 

Figure 26 Overall anionic chain polymerisation mechanism of styrene initiated by n-butyllithium.

Polydimethylsiloxane was prepared by the anionic polymerisation of hexamethylcyclotrisiloxane, in a solvent mixture of cyclohexane and THF (4 1 volume ratio), using n-butyllithium as the initiator. After a reaction period of 48 hr termination was effected by adding a small quantity of acetic acid. 

The synthesis and characterization of a series of dendrigraft polymers based on polybutadiene segments was reported by Hempenius et al. [15], The synthesis begins with a linear-poly(butadiene) (PB) core obtained by the sec-butyllithium-initiated anionic polymerization of 1,3-butadiene in n-hexane, to give a microstructure containing approximately 6% 1,2-units (Scheme 3). The pendant vinyl moities are converted into electrophilic grafting sites by hydrosilylation with... 

Auguste S, Edwards HGM, Johnson AF et al. (1996) Anionic polymerization of styrene and butadiene initiated by n-butyllithium in ethylbenzene determination of the propagation rate constants using Raman spectroscopy and gel permeation chromatography. Polymer 37 3665-3673... 

The initiation and propagation reactions typically show fractional orders of dependence of rate on alkyllithium. The situation is quite complex. The fractional orders for initiation and propagation are seldom the same and often vary depending on the monomer, solvent, and initiator and their absolute as well as relative concentrations. For styrene polymerization by n-butyllithium in aromatic solvents, the initiation and propagation rates are proportional to only the and -powers of n-butyllithium concentration, respectively. These results have been interpreted in terms of the following association equilibria... 

The situation is similar qualitatively but differs quantitatively for isoprene and 1,3-buta-diene. The dependence of Rp on initiator varies from g- to -order depending on the specific reaction system. The reaction orders for all monomers are affected hy the relative as well as absolute concentrations of initiator and monomer. Thus the dependence of Rp on initiator for the n-butyllithium polymerization of isoprene in benzene at 30°C is -order at initiator concentrations above 10-4 M but -order at initiator concentrations below 10 4 M [Van Beylen et al., 1988]. Higher initiator concentrations yield higher degrees of aggregation and lower kinetic orders. The excess of monomer over initiator is also important. Higher kinetic orders are often observed as the monomer initiator ratio increases, apparently as a result of breakup of initiator and propagating ion-pair associations by monomer. 

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