Isoprene polymerization, lithium

The use of alkaU metals for anionic polymerization of diene monomers is primarily of historical interest. A patent disclosure issued in 1911 (16) detailed the use of metallic sodium to polymerize isoprene and other dienes. Independentiy and simultaneously, the use of sodium metal to polymerize butadiene, isoprene, and 2,3-dimethyl-l,3-butadiene was described (17). Interest in alkaU metal-initiated polymerization of 1,3-dienes culminated in the discovery (18) at Firestone Tire and Rubber Co. that polymerization of neat isoprene with lithium dispersion produced high i7j -l,4-polyisoprene, similar in stmcture and properties to Hevea natural mbber (see ELASTOLffiRS,SYNTHETic-POLYisoPRENE Rubber, natural). 

The refined grade s fastest growing use is as a commercial extraction solvent and reaction medium. Other uses are as a solvent for radical-free copolymerization of maleic anhydride and an alkyl vinyl ether, and as a solvent for the polymerization of butadiene and isoprene usiag lithium alkyls as catalyst. Other laboratory appHcations include use as a solvent for Grignard reagents, and also for phase-transfer catalysts. 

Solvent polarity is also important in directing the reaction bath and the composition and orientation of the products. For example, the polymerization of butadiene with lithium in tetrahydrofuran (a polar solvent) gives a high 1,2 addition polymer. Polymerization of either butadiene or isoprene using lithium compounds in nonpolar solvent such as n-pentane produces a high cis-1,4 addition product. However, a higher cis-l,4-poly-isoprene isomer was obtained than when butadiene was used. This occurs because butadiene exists mainly in a transoid conformation at room temperature (a higher cisoid conformation is anticipated for isoprene) ... 

Finally it should be stressed that the complexation affects the microstructure of poly dienes. As was shown by Langer I56) small amounts of diamines added to hydrocarbon solutions of polymerizing lithium polydienes modify their structure from mainly 1,4 to a high percentage of vinyl unsaturation, e.g., for an equivalent amount of TMEDA at 0 °C 157) the fraction of the vinyl amounts to about 80%. Even more effective is 1,2-dipiperidinoethane, DIPIP. It produces close to 100% of vinyl units when added in equimolar amount to lithium in a polymerization of butadiene carried out at 5 °C 158 159), but it is slightly less effective in the polymerization of isoprene 160>. 

When a mixture of styrene and 1,3-butadiene (or isoprene) undergoes lithium-initiated anionic polymerization in hydrocarbon solution, the diene polymerizes first. It is unexpected, since styrene when polymerized alone, is more reactive than, for example, 1,3-butadiene. The explanation is based on the differences of the rates of the four possible propagation reactions the rate of the reaction of the styryl chain end with butadiene (crossover rate) is much faster than the those of the other three reactions484,485 (styryl with styrene, butadienyl with butadiene or styrene). This means that the styryl chain end reacts preferentially with butadiene. 

Copolymerizations initiated by lithium metal should give the same product as produced from lithium alkyls. Usually the radical ends produced by electron transfer initiation have so short a lifetime they can have no influence on the copolymerization. This is true for instance in the copolymerization of isoprene and styrene (50). The product is identical if initiated by lithium metal or by butyllithium. With the styrene-methylmethacrylate system, however, differences are observed (79,80,82). Whereas the butyllithium initiated copolymer contains no styrene at low conversions, the one initiated by lithium metal has a high styrene content if the reaction is carried out in bulk and a moderate one even in tetrahydrofuran. These facts led O Driscoll and Tobolsky (80) to suggest that initiation with lithium occurs by electron exchange and that in this case the radical ends are sufficiently long-lived to produce simultaneous radical and anionic reactions at opposite ends of the chain. Only in certain rather exceptional circumstances would the free radical reaction be of importance. Some of the conditions required have been discussed by Tobolsky and Hartley (111). The anionic reaction should be slow. This is normally true for lithium based catalysts in hydrocarbon solvents. No evidence of appreciable radical participation is observed for initiation by sodium and potassium. The monomers should show a fast radical reaction. If styrene is replaced by isoprene, no isoprene is found in the copolymer for isoprene polymerizes slowly by free radical initiation. Most important of all, initiation should be slow to produce a low steady concentration of radical-anions. An initiator which produces an almost instantaneous and complete electron transfer to monomer produces a high radical concentration which will ensure their rapid mutual termination. 

A more detailed description of the mechanism of isoprene polymerization by lithium compounds has been given (99, 104). The poly-isoprenyllithium first complexes with isoprene in the cis-form. The complex subsequently rearranges to form a transition state in the form of a six-membered ring. H CH... 

Korotkov and Rakova (53) found that butadiene was more active in copolymerization with isoprene with lithium catalyst, although in homopolymerization isoprene is three times faster. Korotkov and Chesnokova (33) studied the copolymerization of butadiene and styrene with n-butyl lithium in benzene. Butadiene polymerized before much of the styrene was consumed. They claimed the formation of block polymers consisting initially of polybutadiene and the polystyrene chain attached. 

It has been found recently that lithium diisopropylamide is able to initiate isoprene polymerization at 80° C to an oligomer having the alkylamino group at the end of the polymer chain. The reaction conditions are more severe than those in previous study. 

This hypothesis was disproved by Worsfold413,415) who investigated the effect of butadiene on the rate of addition of styrene to lithium polystyrene. Only qualitative results were reported the rapidity of the butadiene addition prevented quantitative investigation. However, a closely similar system, isoprene - styrene, was more amenable to a quantitative study415) because the addition of isoprene to lithium polystyrene is somewhat slower. The general pattern of co-polymerization is the same as in the butadiene - styrene system. The effect of small amounts of isoprene on the rate of styrene addition was investigated and the retarding effect demanded by the Korotkov mechanism was not observed. ... 

The microstructures of the polybutadienes, butadiene-styrene copolymers, and polyisoprenes were determined by infrared spectroscopic methods (1,3). The spectra of alkali metal-catalyzed polybutadienes and polyisoprenes show that other reactions occur during polymerization in addition to those involving cis- and trans 1,4, 1,2, and 3,4 additions. For sodium and potassium polybutadienes and polyisoprenes, the absorbances of the bands arising from these additional structures could be taken into account satisfactorily by the methods described. No foreign structures are found in lithium-catalyzed polyisoprenes and the additional band foimd near 14.2 microns in polybutadiene spectra does not appear to affect the cis-1,4 band at 14.7 microns. (Cesium and rubidium, as well as additives such as dimethoxy-tetraglycol, affect the polymerization of butadiene so markedly that it was not possible to obtain satisfactory analyses of such polymers. The effect of these catalysts in isoprene polymerizations does not appear to be so marked and satisfactory analyses were obtained by the method described. 

Alkali Metals The direct use alkali metals and alkaline-earth metals as initiators for anionic polymerization of diene monomers as first reported in 1910 is primarily of historical interest because these are uncontrolled, heterogeneous processes [4]. One of the most significant developments in anionic vinyl polymerization was the discovery reported in 1956 by Stavely and coworkers at Firestone Tire and Rubber Company that polymerization of neat isoprene with lithium dispersion produced high di-l,4-polyisoprene, similar in structure and properties to Hevea natural rubber [47]. This discovery led to development of commercial anionic solution polymerization processes using alkyllithium initiators. 

O Driscoll, Yonezawa, and Higashimura proposed a mechanism for steric control. In isoprene polymerization the terminal charges are complexed with the metal cations. These cations are close to the active centers through the occupied zr-orbitals of the chain ends and the unoccupied p-orbitals of the lithium ions, hi the transition state the monomers are conqilexed with the cations in die same way. The lithium cations are assumed to be in hybridized tetrahedral sp configurations with four vacant orbitals. The chain ends are presumed to be allylic and the diene monomers are bidentate. During the propagation steps both the monomers and chain ends complex with the same counterions ... 

It is known that lithium organyls are associated in apolar media (see Section 18.3). For example, in the polymerization of isoprene with lithium alkyls, RCH2Li, a lithium isoprenyl is first formed ... 

The values of kp can be obtained by plotting kohs versus [PS Mt+]. The order of reactivity [rate constants in brackets are in units of L/(mol-s)] of alkali metal counterions is Li [0.9] < Na [3.4-6.51 < K [20-34] < Cs [5-24] (27). The trend of increasing reactivity with increasing ionic radius, as also observed in hydrocarbon solution, has been taken as evidence for contact ion pairs as the reactive propagating species. Similar behavior has been observed for isoprene polymerization in diethyl ether (e = 4.34) the propagation rate constant assigned to the lithium contact ion pair is 3.2 L/(mol s) (70). 

Details of production processes for the polymerization of isoprene by lithium and alkyllithium compounds do not appear to be available. However, the reaction proceeds under mild conditions (i.e., at room temperature or slightly above and at atmospheric pressure) and a typical process is probably not unlike that described above for co-ordination catalysts. 

Alkenes. —Reviews on Ziegler-Natta catalysis and the stereoregular and sequence-regular polymerization of butadiene have been published and the stereoselective oligomerizations of isoprene by lithium and palladium catalysts have been compared. Semi-empirical MO calculations suggest that Ziegler-Natta polymerization proceeds via a bis-alkene complex and a metallacyclo-pentane intermediate. ... 

From the time that isoprene was isolated from the pyrolysis products of natural mbber (1), scientific researchers have been attempting to reverse the process. In 1879, Bouchardat prepared a synthetic mbbery product by treating isoprene with hydrochloric acid (2). It was not until 1954—1955 that methods were found to prepare a high i i -polyisoprene which dupHcates the stmcture of natural mbber. In one method (3,4) a Ziegler-type catalyst of tri alkyl aluminum and titanium tetrachloride was used to polymerize isoprene in an air-free, moisture-free hydrocarbon solvent to an all i7j -l,4-polyisoprene. A polyisoprene with 90% 1,4-units was synthesized with lithium catalysts as early as 1949 (5). 

Al—Ti Catalyst for cis-l,4-PoIyisoprene. Of the many catalysts that polymerize isoprene, four have attained commercial importance. One is a coordination catalyst based on an aluminum alkyl and a vanadium salt which produces /n j -l,4-polyisoprene. A second is a lithium alkyl which produces 90% i7j -l,4-polyisoprene. Very high (99%) i7j -l,4-polyisoprene is produced with coordination catalysts consisting of a combination of titanium tetrachloride, TiCl, plus a trialkyl aluminum, R Al, or a combination of TiCl with an alane (aluminum hydride derivative) (86—88). 

Alkali Metal Catalysts. The polymerization of isoprene with sodium metal was reported in 1911 (49,50). In hydrocarbon solvent or bulk, the polymerization of isoprene with alkaU metals occurs heterogeneously, whereas in highly polar solvents the polymerization is homogeneous (51—53). Of the alkah metals, only lithium in bulk or hydrocarbon solvent gives over 90% cis-1,4 microstmcture. Sodium or potassium metals in / -heptane give no cis-1,4 microstmcture, and 48—58 mol % /ram-1,4, 35—42% 3,4, and 7—10% 1,2 microstmcture (46). Alkali metals in benzene or tetrahydrofuran with crown ethers form solutions that readily polymerize isoprene however, the 1,4 content of the polyisoprene is low (54). For example, the polyisoprene formed with sodium metal and dicyclohexyl-18-crown-6 (crown ether) in benzene at 10°C contains 32% 1,4-, 44% 3,4-, and 24% 1,2-isoprene units (54). 

The earliest SIS block copolymers used in PSAs were nominally 15 wt% styrene, with an overall molecular weight on the order of 200,000 Da. The preparation by living anionic polymerization starts with the formation of polystyryl lithium, followed by isoprene addition to form the diblock anion, which is then coupled with a difunctional agent, such as 1,2-dibromoethane to form the triblock (Fig. 5a, path i). Some diblock material is inherently present in the final polymer due to inefficient coupling. The diblock is compatible with the triblock and acts... 

If desired, the potassium salt can be converted into the allyllithium derivative by metal exchange with lithium bromide46. Butyllithium/potassium 2,2,6,6-tetramethylpiperidide depro-tonates isoprene without polymerization to give 2-methylene-3-butenyllithium47. 

Closer examination of the data pertaining to polymerization of lithium polyisoprene in aliphatic hydrocarbons suggests that kpcl and kptt are negligible in comparison with k, and k, = k IS, i.e. the addition of isoprene whether to cis or trans active... 

The initiation of polymerization of styrene and isoprene in benzene by t-butyl lithium reveals some complexities129) (e.g. zero order kinetics in monomer) not observed in the reaction proceeding in cyclohexane. Further studies of that system are needed. 

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