Butyllithium initiated system

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. 

COLEMAN and Fox (18) have pointed out that the non Bernoullian sequence distribution observed in some of these systems can be formed without the hypothesis of penultimate effects. All that is required is that two or more types of active species be present which do not rapidly interconvert. Each can add monomer at its own rate and with its own characteristic regulating effect. No penultimate effect is necessary but the sequence distribution will be non-Bernoullian. This type of mechanism is particularly attractive in the explanation of stereoblock polymer formation in the lithium alkyl systems in toluene with small amounts of ether present. The presence of at least two species of active centres has been inferred from an examination of polymer fractions obtained from butyllithium initiated polymerizations (19) in toluene. The change in molecular weight distribution with time suggests the presence of two... 

The initiation events involving dienes and styrene in hydrocarbon solvents have been thoroughly and accurately studied by the application of UV and visible spectroscopy. The archetype of such studies is the now classic 1960 study of Worsfold and Bywater 94) on the n-butyllithium-styrene system in benzene. The reaction was found to follow the relationship ... 

These results indicated that the conventional spectroscopic method of determining the completion of the initiation reaction is fraught with potential uncertainties, insofar as the /-butyllithium-styrene system is concerned. Hsieh s method 106) of analyzing for residual initiator (via GC measurements for isobutane), is unencumbered with such uncertainties. 

But, 1,2- or vinyl BR can be polymerised in the atactic, the syndiotactic or the isotactic form. Hence, five different configurations can be obtained by polymerisation reactions with butadiene (CH2=CH-CH=CH2) as monomer. The product obtained depends on the catalyst system used but is usually a mixture of 1,4 cis-, 1,4 trans- and atactic 1,2-BR. The commercial processes using Co-, Ni- or Ti-based catalyst systems, for instance, produce BR with a 1,4 cis-BR content higher than 90 %wt. But butyllithium initiated homopolymerisation of butadiene results in a product with 1,4 trans-BR contents up to 60 %wt. 

The stereoregularity of polystyrenes prepared by anionic polymerization is predominantly syndiotactic (racemic diad fraction P = 0.53-0.74) and the stereoregularity is surprisingly independent of the nature of the cation, the solvent, and the temperature, in contrast to the sensitivity of diene stereochemistry to these variables [3, 156]. The homogeneous alkyllithium-initiated polymerization of styrene in hydrocarbon media produces polystyrene with an almost random (i.e., atactic) microstructure for example, was 0.53 for the butyllithium/toluene system [3, 191, 192]. A report on the effect of added alkali metal alkoxides showed that polystyrene stereochemistry can be varied from 64% syndiotactic triads with lithium f-butoxide to 58% isotactic triads with potassium f-butoxide [193]. 

Microreactors have also been used for ionic polymerization or polycondensation processes. Nagaki et al. [136] have synthesized polystyrene-poly(alkyl methacrylate) block copolymers by butyllithium initiated anionic polymerization in an integrated flow microreactor system. A high level of control of molecular weight was achieved at temperatures between -28 and +24 °C due to fast mixing, fast heat transfer, and residence time control. Santos and Metzger... 

A good example of this kinetic behavior was found in the study of the n-butyllithium-styrene system in benzene, in which a kinetic order dependency on n-butyllithium concentration was observed, consistent with the predominantly hexameric degree of association of n-butyllithium (Worsfold and By water, 1960). However, this expected correspondence between the degree of association of the alkyllithium compound and the fractional kinetic order dependence of the initiation reaction on alkyllithium concentration was not always observed (Young et al., 1984). One source of this discrepancy is the assumption that only the unassociated alkyllithium molecule can initiate polymerization. With certain reactive initiators, such as 5 c-butyllithium in hexane solution, the initial rate of initiation exhibits approximately a first-order dependence on alkyllithium concentration, suggesting that the aggregate can react directly with monomer to initiate polymerization (Bywater and Worsfold, 1967a). A further... 

The large majority of stereoselective acrylate polymerizations afford either isotactic or syndiotactic-biased material and since heterotactic polymers have only been prepared on rare occasions, they are not covered in depth in this chapter. The principal initiating system for the production of heterotactic poly(alkyl methacrylates) is a stoichiometric 1 1 mixture of tcrt-butyllithium and bis(2,6-di-tert-butylphenoxy)methylaluminum. The heterotactic content is particularly pronounced at low temperatures and increases in the order methyl < ethyl < propyl < allyl methacrylates. For example, at -95 °C, poly(allyl methacrylate) with 95.8% mr triad content is obtained after reduction to poly(methacrylic acid) and subsequent methylation, highly heterotactic PMMA results. ... 

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 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. 

A number of other polar monomers have been polymerized with butyllithium, nominally in hydrocarbon or aromatic solvents. In almost all cases the monomer concentration was so high that the effective dielectric constant was much greater than in a pure hydrocarbon. All show rather complex behaviour. The degree of polymerization of the polymer formed is always much higher than the initial monomer-catalyst ratio so that a simple scheme involving only initiation and propagation reactions is not applicable. Only precipitable polymer was isolated, so it is not sure if the low initiator efficiencies are due to low polymer formation or to side reactions of butyllithium with the monomer. In addition most systems studied stop before complete conversion of the monomer. Evidently the small fraction of active polymer chains formed... 

The presence of residual initiator in the polymerization leading to the polystyrene whose distribution is shown in Fig. 8 b was verified as follows 108). At the completion of the polymerization, both THF and monomer were added to the system. The intensity of the absorption band of poly(slyryl)lithium (Xinax 334 nm) was found to increase to an extent which demonstrated that ca. 75% of the added t-butyllithium remained at the completion of the first polymerization. 

A kinetic study has been carried out on the system butyllithium plus styrene in benzene. Although published reports by Tobolsky (85) and Welch (88) differ in their interpretations, it appears that these differences may be due to the fact that Welch examined a wider range of initiator concentrations. Welch showed that no termination occurs in this system and that the molecular weight was a linear function of the ratio monomer/ butyllithium. The rate results showed... 

The initiation reaction is particularly susceptible to the presence of trace amounts of impurity or added polar substances. Small amounts of tetrahydro-furan accelerate the rate, which becomes too fast to measure by conventional methods at quite low tetrahydrofuran concentrations. This behavior is presumably connected with the dissociation of the hexamer of butyllithium and its replacement by solvated free butyllithium. The addition of salt produces different and perhaps specific behavior in different systems. So far only the effect of lithium ter -butoxide has been investigated. In benzene the initiation rate... 

A considerable amount of work has been done to optimize the preparation of the di-.v-butyllithium adduct of 1,3-diisopropenylbenzene (ra-DIPB). Under the the right conditions, an effective initiator can be prepared with minimal m-DIPB dimerization [13]. The principle drawback of this system is that a polar additive is required, which has implications for the microstructure, as outlined above. The addition of 2mol of butyllithium to l,3-bis(l-phenylethenyl)ben-zene (DDPE) can be carried out in pure hydrocarbon solvents. However, efficient diinitiation requires the addition of a polar modifier or alkoxide salt [14], Another drawback to this approach is the high cost of the diolefin. 

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