Alkyllithium polymerizations

In alkyllithium initiated, solution polymerization of dienes, some polymerization conditions affect the configurations more than others. In general, the stereochemistry of polybutadiene and polyisoprene respond to the same variables Thus, solvent has a profound influence on the stereochemistry of polydienes when initiated with alkyllithium. Polymerization of isoprene in nonpolar solvents results largely in cis-unsaturation (70-90 percent) whereas in the case of butadiene, the polymer exhibits about equal amounts of cis- and trans-unsaturation. Aromatic solvents such as toluene tend to increase the 1,2 or 3,4 linkages. Polymers prepared in the presence of active polar compounds such as ethers, tertiary amines or sulfides show increased 1,2 (or 3,4 in the case of isoprene) and trans unsaturation.4. 1P U It appears that the solvent influences the ionic character of the propagating ion pair which in turn determines the stereochemistry. 

The same phenomena was reported by Diem, Tucker and Gibbs (43) who found that the alkyllithium polymerization of isoprene produced... 

Alkyllithium initiation can tolerate very high temperatures. As refrigeration is not needed, alkyllithium polymerization can proceed at high reaction rates with low investment and operating costs. 

The addition of ethers to the reaction brought about a reduction in the isotactidty of the polymers. Goode, Owens, and Myers (20) studied the polymerization of methyl methacrylate with Grignard reagents and found similar results to the alkyllithium polymerizations. This group of workers efficiently utilized infrared as a means of determining the tactidty of the polymers. 

Antkowiak, A.E., Tate, D.P., Oberster, A.E., et al., 1972. Temperature and concentration effects on polar-modified alkyllithium polymerizations and copolymerizations. J. Polym. Sci. Part A ... 

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. 

Commercially, the poly(styrene-Aelastomer-Astyrene) materials are made by anionic polymerization (7,45—47). An alkyllithium initiator (RLi) first reacts with styrene [100-42-5] monomer ... 

Polymerization of butadiene using anionic initiators (alkyllithium) in a nonpolar solvent produces a polymer with a high cis configuration. A high cis-polybutadiene is also obtained when coordination catalysts are used. 

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. 

Anionic polymerization of 1,3-disilacyclobutanes also is possible. Solid KOH and alkali metal silanolates were mentioned as being effective by Russian authors [18, 19. 20]. However, alkyllithiums, which can initiate polymerization of silacyclobutanes (eq. 8) [21], do not initiate polymerization of 1,3-disilacyclobutanes [18, 22]. The problem is one of steric hindrance. 

The polymerization was carried out in THF under the conditions of high vacuum or argon atmosphere with a catalytic amount of alkyllithium as an initiator. Anionic polymerization of 3a with n-BuLi in THF followed by quenching with ethanol afforded polymer 6 in 56 % yield. The molecular weight distribution of the polymer was determined by gel permeation chromatography (GPC), calibrated by polystyrene standards, with chlorofrom as eluent Mn = 6.1xl0"4, Mw/Mn = 1.3. 

In order to avoid the SET process, we chose diphenylmethylsilyl anions (PI MeSiM 8a, M = K 8b, M = Na 8c, M = Li) as initiators for 7 instead of alkyllithium and benzene as a solvent. The polymerization did not take place in benzene with silyl anions alone. However, in the presence of an equimolar amount of suitable cryptands, the silyl anions initiated the polymerization. The results are summarized in Table 2. The molecular weights of polysilylenes thus obtained were in good agreement with the calculated values within experimental error. 

Anionic polymerization can be initiated by a variety of anionic sources such as metal alkoxides, aryls, and alkyls. Alkyllithium initiators are among the most useful, being employed commercially in the polymerization of 1,3-butadiene and isoprene, due to their solubility in hydrocarbon solvents. Initiation involves addition of alkyl anion to monomer... 

The most studied catalyst family of this type are lithium alkyls. With relatively non-bulky substituents, for example nBuLi, the polymerization of MMA is complicated by side reactions.4 0 These may be suppressed if bulkier initiators such as 1,1-diphenylhexyllithium are used,431 especially at low temperature (typically —78 °C), allowing the synthesis of block copolymers.432,433 The addition of bulky lithium alkoxides to alkyllithium initiators also retards the rate of intramolecular cyclization, thus allowing the polymerization temperature to be raised.427 LiCl has been used to similar effect, allowing monodisperse PMMA (Mw/Mn = 1.2) to be prepared at —20 °C.434 Sterically hindered lithium aluminum alkyls have been used at ambient (or higher) temperature to polymerize MMA in a controlled way.435 This process has been termed screened anionic polymerization since the bulky alkyl substituents screen the propagating terminus from side reactions. 

Polymerization inhibitors miscellaneous, 23 383 in styrene manufacture, 23 338 Polymerization initiators alkyllithiums as, 74 251 cerium application, 5 687 peroxydicarbonates as, 74 290 Polymerization kinetics, in PVC polymerization, 25 666-667 Polymerization mechanism, for low density polyethylene, 20 218 Polymerization methods, choice of,... 

The alkyllithium-initiated, anionic polymerization of vinyl and diene monomers can often be performed without the incursion of spontaneous termination or chain transfer reactions (1). The non-terminating nature of these reactions has provided methods for the synthesis of polymers with predictable molecular weights and narrow molecular weight distributions (2). In addition, these polymerizations generate polymer chains with stable, carbanionic chain ends which, in principle, can be converted into a diverse array of functional end groups using the rich and varied chemistry of organolithium compounds (3). 

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... 

In order to avoid polymerization and to achieve better stereocontrol by quasi-intramolecular addition, a carbanion-stabilizing group and a complexing substituent for capturing alkyllithium/(—)-sparteine in the substrate are useful. This carbolithiation protocol was realized with great success by Marek, Normant and coworkers (equation 125) Addition of n-BuLi/(—)-sparteine (11) onto the lithium alcoholate derived from ( )-cinnamyl alcohol (457) in cumene at 0°C afforded the addition product with 82% yield and 80% ee. 

Alkyllithium compounds are probably the most useful of these initiators, employed com-merically in the polymerizations of 1,3-butadiene and isoprene. Initiation proceeds by addition of the metal alkyl to monomer... 

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 association phenomena occurring with alkyllithium initiators in nonpolar solvents results in very low polymerization rates. A typical styrene or isoprene polymerization by... 

Some early polymerizations reported as Ziegler-Natta polymerizations were conventional free-radical, cationic, or anionic polymerizations proceeding with low stereoselectivity. Some Ziegler-Natta initiators contain components that are capable of initiating conventional ionic polymerizations of certain monomers, such as anionic polymerization of methacrylates by alkyllithium and cationic polymerization of vinyl ethers by TiCLt-... 

Nucleophilic substitution of the chlorine atom present in 2-chlorothiepane (132) using Grignard reagents has provided a synthetic route to 2-methyl- (133) or 2-phenyl- (134) thiepanes (equation 27) (69JHC115). An a-sulfinyl carbanion (114), generated by alkyllithium attack on thiepane 1-oxide (115), was found to act as nucleophile in the synthesis of cis and trans sulfoxides of thiepane (133 equation 28) (78TL5239). Polymerization of 2-thiepanone (135) has been initiated by attack of the f-butoxide anion and concomitant liberation of a thiolate anion (Scheme 26) (64MI51700). 

Lewis bases effect dramatic changes in microstructure, initiation rates, propagation rates, and monomer reactivity ratios for alkyllithium—initiated polymerizations of vinyl monomers (1-6). Some insight into the molecular basis for these observations has been provided by a variety of NMR, colligative property, and light-scattering measurements of simple and polymeric organolithium compounds in hydrocarbon and basic solvents... 

In general, simple alkyllithiums exist predominantly as either hexamers (for sterically unhindered RLi) or tetramers (for sterically hindered RLi) in hydrocarbon solvents and as tetramers in basic solvents (9-12). Polymeric organolithium compounds such as poly(styryl)lithium exist as dimers in hydrocarbon solution and are unassociated in basic solvents such as tetrahydrofuran (13-15). The state of association of poly-(dienyl)lithiums in hydrocarbon solution is a subject of current... 

We have measured the enthalpies of interaction of tetra-hydrofurans with polymeric organolithiums to characterize the specific nature of base-alkyllithium interactions and these results are reported herein. 

These efforts coupled with the much earlier work on sodium and lithium initiated polymerizations led to an appreciation of the stereospecificity of the alkyllithium initiators for diene polymerization both industrially and academically. Polymerization of isoprene to a high cis polyisoprene with butyllithium is well known and the details have been well documented 2 Control over polybutadiene structure has also been demonstrated. This report attempts to survey the unique features of anionic polymerization with an emphasis on the chemistry and its commercial applications and is not intended as a comprehensive review. 

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