Styrenes reactions with organolithiums

More recently, a number of different copolymer structures have been prepared from butadiene and styrene, using modified organolithiums as polymerization initiators ( 4). Organolithium initiated polymerizations have gained prominence because stereo-control is combined with excellent polymerization rates, and the absence of a chain termination reaction facilitates control of molecular weights and molecular weight distributions ( 5). 

Among the variations in chemical structure of these triblock copolymers developed in our laboratories were the use of poly(a-methyl styrene)(43, 44) as end blocks, and poly(alkylene sulfides) (42, 45 ) 311 d polydimethylsiloxanes (43, 46) as center blocks. The reactions of cyclic sulfides with organolithium is illustrated in Figure 10. Both the propylene sulfide and the methyl thietane can be used for the center block with styrene or a-methylstyrene end blocks, but the chemistry shown in Figure 10... 

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,... 

One of the most surprising and useful aspects of the organolithium chemistry of 1,1-diphenylethylene is that reactions with simple and polymeric organolithiums form the corresponding 1,1-diphenylalkyllithiums which are effective initiators for the anionic polymerization of styrene and diene monomers [64]. As described in Sect. 2.2, the pKa (DMSO) value of diphenylmethane (32.2) is much lower than the estimated pKaS (DMSO) of toluene (43) and propene (44) (see Table 2) [43]. These pKa differences correspond to an energy difference of > 64.5 kj/mol thus, the diphenylmethyl carbanion is 64.5 kj/mol more stable than either the benzyl carbanion or the allyl carbanion. Since these latter car-... 

Fractional kinetic orders of homogenous reactions in solution may point to association of a particular reagent. The kinetics of the initiation step of styrene polymerization in the presence of n-BuLi (equation 33) is in accordance with the assumption that this organolithium compound in a nonbonding solvent forms aggregates of six molecules on the average" . 

The above process, Eq. (25), is in conflict with the currently available theoretical results (Table 5) regarding the dissociation enthalpies of aggregated organolithiums. A similar conclusion was reached by Brown in 1966 70>. This assessment is fortified by the fact that the measured 941 energy of activation for the reaction of styrene with n-butyllithium, 18 kcal/mole, is a value far lower than that required if the calculated dissociation enthalpy of the n-butyllithium aggregates is included in the overall energetics of the initiation event. Thus, it would seem that any mechanism which involves only unassociated organolithiums as reactive entities is invalid. 

Nonpolar Media. Because organolithium initiators are soluble in hydrocarbons, the kinetics of these polymerizations have also been studied in these nonsolvating media. A large number of such studies have been carried out (3, 41) mainly on styrene and the dienes. Again the propagation rate is first order with respect to monomer, in accordance with Reaction 13. However, the rate dependence on growing chain concentration has been found to show marked variation from one system to another with the orders varying from one half to much lower values (3, 41). These systems pose... 

As discussed elsewhere in this review, Lewis bases such as tetrahydrofuran are known to promote disaggregation of polymeric organolithium speciesThus, in the presence of excess tetrahydrofuran, both poly(styryl)lithium and poly(isopre-nyl)lithium would be expected to be unassociated (or at least much less associated). Therefore, in the presence of sufficient tetrahydrofuran, the carbonation reaction would take place with unassociated organolithium chain ends and ketone formation (Eq. (73)) would only be an intermolecular reaction (rather than an essentially intramolecular reaction as in the case with the aggregated species) competing with carbonation. In complete accord with these predictions, it was found that the carbonation of poly(styryl)lithium, poly(isoprenyl)lithium, and poly(styrene-h-isoprenyl)lithium in a 75/25 mixture (by volume) of benzene and tetrahydrofuran occurs quantitatively to produce the corresponding carboxylic add chain ends. The observation by Mansson that THF has no apparent influence was complicated by the use of methyl-cyclohexane, which is a Theta solvent for poly(styrene) (60-70 °C) furthermore. 

Figure 3. Kinetic studies of organolithium compounds prepared by the reaction of /i-BuLi with ADPE, styrene, and MDDPE in benzene or benzene/THF mixture ([THF]/[/f-BuLi] = 20/1).

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