Butadiene with alkyllithium initiator

Copolymerization. The copolymerization of butadiene-styrene with alkyllithium initiator has drawn considerable attention in the last decade because of the inversion phenomenon (12) and commercial importance (13). It has been known that the rate of styrene homopolymerization with alkyllithium is more rapid than butadiene homopolymerization in hydrocarbon solvent. However, the story is different when a mixture of butadiene and styrene is used. The propagating polymer chains are rich in butadiene until late in reaction when styrene content suddenly increases. This phenomenon is called inversion because of the rate of butadiene polymerization is now faster than the styrene. As a result, a block copolymer is obtained in this system. However, the copolymerization characteristic is changed if a small amount of polar solvent... 

However, the kinetics of polymerization of butadiene, isoprene, and styrene with alkyllithium initiators has been studied extensively by Hsieh (39c), and the polymerization rates broken down into initiation and propagation steps. The effects of BuLi concentration, the structure of the butyl group, and the solvent type were studied. The rates of initiation (i i) for the olefins mentioned were determined. For dienes, the order was ec-Bu > iso-Pr > iso-Bu > n-Bu > tert-Bu. With n-BuLi the order was styrene > butadiene >... 

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. 

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. 

Since the reversal of activity of butadiene with respect to styrene in alkyllithium system has been observed (12), it would be of interest to find out whether the inversion phenomenon still holds in the case of the lithium morgholinide system. Four temperatures, namely 30, 40, 50 and 60 C were chosen for this study. At 30°C polymerization temperature the curve is characteristic of block copolymerization when one plots percent bound styrene vs percent conversion (Fig. 1). Initially, a small amount (/>/3%) of styrene is polymerized. This is followed by a block of butadiene. The remaining styrene is then polymerized after all the butadiene is consumed. This result is identical to the alkyllithium initiated copolymerization. 

The use of aliphatic solvents causes profound changes in the observed kinetic behavior for the alkyllithium initiation reactions with styrene, butadiene, and isoprenc. i.e.. Ihe inverse correspondence between the reaction order dependence for alkyllithium and degree of organolithium aggregation is generally not observed. Also, initial rales of initiation in aliphatic solvents are several orders of magnitude less lhan those observed, under equivalent conditions, in aromatic solvents. Furthermore, pronounced induction periods are observed in aliphatic hydrocarbon solvents,... 

The copolymerization with alkyllithium to produce uniformly random copolymers is more complex for the solution process than for emulsion because of the tendency for the styrene to form blocks. Because of the extremely high rate of reaction of the styryl-lithium anion with butadiene, the polymerization very heavily favors the incorporation of butadiene units as long as reasonable concentrations of butadiene are present. This observation initially was somewhat confusing because the homopolymerization rate of styrene is seven times that for butadiene. However, the cross-propagation rate is orders of magnitude faster than either, and it therefore dominates the system. For a 30 mole percent styrene charge the initial polymer will be almost pure butadiene until most of the butadiene is polymerized. Typically two-thirds of the styrene charged will be found as a block of polystyrene at the tail end of the polymer chain ... 

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 alkyllithium-initiated copolymerizations of styrene with dienes, especially isoprene and butadiene, have been... 

Commercial random SBR polymers (solution SBR) prepared by alkyllithium-initiated polymerization typically have 32% cis-, A-, 41% trans-, A-, and 27% vinyl-microstructure compared to 8% cw-1,4-, 74% trans-, A-, and 18% vinyl-microstructure for emulsion SBR with the same comonomer composition [3, 221]. Solution SBRs typically have branched architectures to eliminate cold flow [17, 49]. Compared to emulsion SBR, solution random SBRs require less accelerator and give higher compounded Mooney, lower heat buildup, increased resilience, and better retread abrasion index [3]. Terpolymers of styrene, isoprene, and butadiene (SIBR) have been prepared using a chain of single-stirred reactors whereby the steady-state concentration of each monomer and Lewis base modifier at any degree of conversion could be controlled along the reactor chain [3, 222-224]. 

The alkyllithium-initiated anionic copolymerization of diene and styrene monomers continues to be of interest because one can tailor-make copolymers with a wide range of compositional heterogeneity. Recently, kinetic studies have provided rate constant data to further clarify the factors responsible for the predominant incorporation of the less reactive diene monomer in styrene/diene copol3naerizations carried out in hydrocarbon media.They confirm that the magnitude of the rate constants for butadiene-styrene copolymerizations fall in the order results of several... 

Tapered Block Copolymers. The alkyllithium-initiated copolymerizations of styrene with dienes, especially isoprene and butadiene, have been extensively investigated and illustrate the important aspects of anionic copolymerization. As shown in Table 15, monomer reactivity ratios for dienes copolymerizing with styrene in hydrocarbon solution range from approximately 8 to 17, while the corresponding monomer reactivity ratios for styrene vary from 0.04 to 0.25. Thus, butadiene and isoprene are preferentially incorporated into the copolymer initially. This type of copolymer composition is described as either a tapered block copolymer or a graded block copolymer. The monomer sequence distribution can be described by the structures below ... 

In general, random SBR with a low amount of block st5Tene and low amoimts of 1,2-butadiene enchainment (<20%) can be prepared in the presence of small amounts of added potassium or sodium metal alkoxides. Using 0.2 equiv of sodium 2,3-dimethyl-2-pentoxide, the monomer reactivity ratios for alkyllithium-initiated SBR were found to be = 1.1 and rs = 0.1 (182). The resulting copolymer had only... 

Copolymers of 1,3-butadiene and styrene (SBR) are elastomers of great technical importance that are used for automobile tires [465-474]. In addition to a free-radical process, they can be made by anionic initiation with alkyllithium compounds. In polar solvents the reaction rate of styrene anions with 1,3-butadiene is greater than with styrene, whereas in polar solvents this is just the other way around. The copolymerization parameter rj for styrene-butadiene is 0.03 in hexane and 8 in THF r2 is calculated as 12.5 in hexane and 0.2 in THF [465]. Therefore, a strong dependence of the styrene content of the polymers on the degree of conversion is observed in discontinuous polymerizations. 

On of the most unique aspects of living anionic polymerizations is the ability to synthesize block copolymers by sequential monomer addition. Thus, the product of the alkyllithium-initiated polymerization of styrene (Scheme I) can be reacted with a diene such as butadiene or isoprene to produce a living diblock copolymer as shown in eq, 2 for butadiene. It is important to note that the alkyllithium-initiated poly-... 

In general, random SBR with a low amount of block styrene and low amounts of 1,2-butadiene enchainment (<20%) can be prepared in the presence of small amounts of added potassium or sodium metal alkoxides. " For example, at 50 °C in the presence of as little as 0.067 equivalents of potassium t-butoxide in cyclohexane, the amormt of bound styrene was relatively independent of conversion, in contrast to the heterogeneity observed in the absence of randomizer, that is, tapered block copolymer formation. " The polybutadiene microstmcrnre obtained tmder these conditions corresponds to about 15% 1,2-microstmctrue. Using 0.2 equivalents of hydrocarbon-soluble sodium 2,3-dimethyl-2-pentoxide, the monomer reactivity ratios for alkyllithium-initiated SBR were fotmd to be of re = 1.1 and rs = 0.1. The resulting copolymer had only 5% block styrene and 18% 1,2-vinyl microstmctrue. It was found that there is a very narrow compositional window ([RONa]/[RLi]) at... 

Ceresa drew attention to the fact that out of 1400 copolymers only 5% of the products had been isolated with a reasonable degree of purity and only 20 species had been properly characterized. Exploitation of anionic polymerization, however, soon improved the situation markedly. The use of alkyllithium initiators, which were initially evaluated by Stavely and co-workers at Firestone for the synthesis of high-cis polyisoprene, in the preparation of block copolymers from styrene and butadiene, and styrene and isoprene, is particularly noteworthy. 

Lithium and alkyllithiums in aliphatic hydrocarbon solvents are also used to initiate anionic polymerization of 1,3-butadiene and isoprene.120,183-187 As 1,3-butadiene has conjugated double bonds, homopolymerization of this compound can lead to several polymer structures. 1,4 Addition can produce cis-1,4- or tram-1,4-polybutadiene (19, 20). 1,2 Addition results in a polymer backbone with vinyl groups attached to chiral carbon atoms (21). All three spatial arrangements (isotactic, syndiotactic, atactic) discussed for polypropylene (see Section 13.2.4) are possible when polymerization to 1,2-polybutadiene takes place. Besides producing these structures, isoprene can react via 3,4 addition (22) to yield polymers with the three possible tacticites ... 

The work by Morton and Ells (60) showed that this difference in reactivity was due to differences in the rate with which the different monomers reacted with the different alkyllithiums (styryl or butenyl). Styryllithium ends reacted rapidly with butadiene, but a butenyl-lithium end reacted quite slowly with styrene. Butadiene was polymerized to near exclusion of styrene during the initial part of the reaction. Special solvation of the catalyst by the polymerizing butadiene was not the cause of this copolymerization. 

Hay and co-workers reported that the Mn increased linearly with conversion at a molar ratio of 1 2. However, at high ratios of n-BuLi to TMEDA, the initiator became only 50% efficient. This finding is rather surprising as the addition of TMEDA to alkyllithium compounds enhanced the rate of the polymerization and without TMEDA, at Hay s polymerization temperature, no polymerization of the 1,3-butadiene took place. The explanation advanced by these authors was that the allylic lithium complex of polybutadiene is complexed with two TMEDA molecules and that complex 11 is the propagating species. 

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. 

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