Sec-Butyllithium initiator

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... 

Synthesis ofm-silyl hydride-functionalized polymers The first critical step in this methodology is the preparation of well-defined, -silyl hydride-ftmaionalized polymers. For this purpose, the reaction of sec-butyllithium-initiated PSLi (Mn = 2200gmor M /Mn=l.02) with 2.3 molar equivalents of chlorodimethylsilane was effected in benzene at room temperature (see Scheme 21). A color change from red to colorless occurred within a few minutes, indicating a very fast reaction rate. The sUyl hydride end-funrtionalized polystyrene. 

The kinetic order for sec-butyllithium-initiated polymerization of styrene is close to 0.25 in benzene solution this result is also consistent with initiation by unassociated sec-butyllithium, since sec-butyllitbium is associated predominantly into tetramers in benzene solution. 

Further evidence for the quantitative addition of poly(styryl)lithium with a stoichiometric amount of 1,1-diphenylethylene can be deduced by HNMR analysis of the adduct formed after methanol termination. A characteristic peak (multiplet) at 5 3.5 ppm is observed by HNMR for the terminal methine hydrogen at the chain end in the PS-DPE and no corresponding peak is observed in the base polystyrene. Integration of the area of this peak relative to the area of the resonances corresponding to the methyl protons of the sec-butyllithium initiator fragment at 5 0.5-0.78 ppm gave a value of 1 5.9, in close... 

In nonpolar solvents, in spite of many attempts31), the problem of preparing efficient bifunctional initiators has not been entirely solved. Association between metal-organic sites causes insolubility (or even instability) of many bifunctional initiators. Satisfactory results 32,33) have been obtained with the diadducts of sec-butyllithium and diisopropenylbenzene (DIB). These adducts are made at a temperature chosen... 

Parts A and B of the procedure correspond to preparation of lithium tetramethylpiperidide, and its use in the in situ preparation and addition of dibromomethyllithium to the ester 1 producing tetrahedral intermediate 2. In Part C a mixture of lithium hexamethyldisilazide and lithium ethoxide is prepared for addition in Part D to the solution of 2. The silazide base serves to deprotonate the mono and dibromo ketones that are formed on initial warming of the reaction to -20°C, thus protecting them as the enolate anions 4 and 3. Addition of the sec-butyllithium in Part... 

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. 

The stoichiometric reaction of / -diisopropenylbenzene [3748-13-8] with two moles of sec-butyllithium in the presence of triethylamine has been reported to produce a useful, hydrocarbon-soluble dilithinm initiator because of the low ceiling temperature of the monomer (78,79) which is analogous in structure to a-methylstyrene however, other studies suggest that oligomerization occurs to form initiators with functionalities higher than two (80). 

A change from an aliphatic or aromatic hydrocarbon solvent (cyclohexane, benzene) to a polar solvent (THF) leads to a large increase in trans-1,4 and 3,4 microstructure (58). Organolithium compounds are highly associated sec-butyllithium in benzene or cyclohexane exists as a tetramer, and -butyllithium as a hexamer (64,65). This association in hydrocarbon solvents results pardy in the slow initiation observed between some organolitbiirms and isoprene (66). At low initiator concentrations, the polymerization rate of isoprene in alkyUithium polymerization is proportional to monomer and alkyUithium concentrations (67). 3,4-Polyisoprenes are obtained by modification of the lithium polymerization with ethers, such as the dialky] ethers of ethylene glycol or tertiary amines (68,69). 

In (a) there is a 4,1 4,3 tautomeric equilibrium in which the proposed minute amount of 4,3-chain end is of sufficient reactivity to account for the in-chain 3,4-units amounting to some 10 % of the chains. In alternative (b) a covalent a ionic jt equilibrium exists which is heavily in favor of the covalent structure again the re-form is supposedly highly reactive, leading to in-chain 1,4 and 3,4 placement. A fundamental difference between these proposals is that whereas (a) leads to trans 4,1 cis 4,1 carbanion isomerization, proposal (b) does not. Morton et al.142) found that on storing their samples at 50 °C for a few days, some decomposition took place and the cis trans ratio of the living ends increased if the original initiator was sec-butyllithium,but decreased if it was wo-propyllithium. This difference in behavior... 

Foss and co-workers (88) reported ABA copolymers obtained from a new dilithium reagent this organolithium initiator was formed by the addition of sec-butyllithium to m-diisopropenylbenzene in the presence of a small proportion of triethylamine, followed by reaction with isoprene to improve the hydrocarbon solubility. Unfortunately, the starting ma-... 

Viswanathan et al. introduced carbanions onto SWCNT surfaces by treating the tubes with sec-butyllithium, providing initiating sites for the polymerization of styrene [182]. Recently, Chen et al. reported that on treating SWCNTs with sec-butyllithium and reacting the carbanions formed with C02 under oxygen-free and anhydrous conditions, the SWCNTs were alkylated and carboxylated [183] (Scheme 1.30). The so-derivatized SWCNTs could be dispersed in water to give a nearly transparent solution of 0.5 mg mL4 zeta potentiometric titrations indicated a feature similar to a zwitterionic polyelectrolyte [183]. 

Copolymers SI and SIS of a wide rai e of molecular wei ts and compositions have been synthetized by anionic polymerization, under vacuum, at room temperature, in toluene solution, usir sec.-butyllithium as initiator... 

Figure 5 shows the rate of reaction of n- and sec.-butyllithium with styrene in benzene as measured spectroscopically from initial rates of... 

Fig. 5. The initial rate of reaction with styrene of sec.-butyllithium, ( ) and n-butyl-lithium, (O) in benzene at 30 C as a function of formal concentration of butyllithium [17, 36].

It can be confirmed that it is the polyisoprenyllithium formed in the initiation step which provides the autocatalytic mechanism [51], If isoprene and sec.-butyllithium are mixed in presence of polyisoprenyllithium the latter having been formed earlier in a separate reaction), then the initial rate of reaction of the sec.-butyllithium and isoprene is much faster than normal and no induction period occurs. The actual rate is then roughly that expected had the polyisoprenyllithium been formed normally as part of the initiation step (Fig. 8). These results also imply rapid mixing of sec.-BuLi and polyisoprenyllithium aggregates because if exchange was not rapid, the added polymer species could have little effect on the initiation process. 

A novel hydrocarbon-soluble trifunctional initiator was proposed by Quirk et al.25 It was prepared by the reaction of 3 mol of sec-butyllithium (s-BuLi) with l,3,5-tris(l-phenylethenyl)benzene (tri-DPE), as presented in Scheme 5. This initiator was found to be efficient for the polymerization of styrene only when THF was also added in the reaction mixture ([THF] [s-BuLi] = 20). The polymerization reaction was monitored by UV—vis spectroscopy. The limitations of the method include the extreme care that should be exersized over the stoichiometry of the reaction between s-BuLi and tri-DPE and the fact that a minimum arm molecular weight around 6 x 103 is required for a successful synthesis. For arm molecular weights lower than this limit, incomplete initiation was observed. If these requirements are fulfilled, well-defined three-arm polystyrene stars can be prepared. 

A blocked, functional initiator is made by reacting p-(N,A-bis(trimethylsilyl)amino)styrene with sec-butyllithium in benzene, and is used for anionically ring-open polymerizing hexamethylcyclotrisilox-ane (D3) in the presence of promoters such as hexamethylphos-phoroamide (HMPA), tetrahydrofuran (THF), or dimethyl sulfoxide (DMSO). The resulting living , monodisperse poly (dimethyl siloxane)... 

The products obtained from the reaction of (chloromethyl)trimethylsilane with organolithium reagents depend very much on the structure of the lithium compound. While lithium 2,2,6,6-tetramethylpiperidide initiates an a-elimination as described above, the treatment with sec-butyllithium leads to the formation of chloro(trimethylsilyl)methyllithium (11). This reagent cyclopropanates an electron-deficient alkene through sequential Michael addition and intramolecular ring closure (MIRC reaction), for example, the formation of cyclopropane 12. 

The required amount of initiator (sec-butyllithium 0.05 mL, 0.65 mmol) is then added to the polymerization tube whilst stirring vigorously. The solution should turn red in colour. 

All spectra were obtained with the Varian HR-300 NMR Spectrometer, using H in normal mode, with occasional use of Fourier transform for very high molecular weight samples. Hexachlorobuta-diene was used as solvent, with 1%> hexamethyldisiloxane as reference. The temperatures used were 110 C. for polyisoprene and 125 C. for polybutadiene. The polymer samples were prepared with sec-butyllithium as initiator, using the high vacuum techniques described elsewhere (13). 

A number of conclusions can be drawn. The effect of excess tertiary diamine above a 1-mole equivalent is not as significant for rapid metalation as is the initial amount of TMEDA. Practical metalation could only be obtained with a 1 1 molar ratio at 40°C and a 2 1 ratio at 60°C the 4 1 ratio required reflux. Side reactions appeared to cause a problem only with the 4 1 ratio at the higher temperatures. Benzene as the solvent accelerated the reaction sixfold over hexane with 10% mole excess as the solvent. Some of this acceleration may be solvent effect. The use of sec-butyllithium increased the metalation rate eight times. 

In either method alkyllithiums are the standard initiators employed. Sec-Butyllithium (s-BuLi) is frequently used because its rate of addition to styrene or conjugated diolefins is very fast. In Reaction la there are essentially three initiation steps, or more precisely one initiation and two crossover steps. 

The authors are grateful to FMC, Lithium Division, for support of this research and for providing samples of functionalized initiators and sec-butyllithium. 

The polymerization of 4-vinylbenzocyclobutene in benzene using sec-butyllithium as the initiator at room temperature has recently been observed [82]. The number of average molecular weights at different times has been measured using size exclusion chromatography (SEC). These results are shown in the Table 10.3. 

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