Isoprene dilithium initiator

A useful, hydrocarbon-soluble, dilithium initiator has been prepared by the dimerization of 1,1-diphenylethylene with lithium in cyclohexane in the presence of anisole (15 vol%) as shown in equations 14 and 15 (43). Although the initiator was soluble in this mixture, it precipitated from solution when added to the polymerization solvent (cyclohexane or benzene). Therefore, the dilithium initiator was chain extended with approximately 30 units of isoprene to generate the corresponding soluble oligomer. This initiator was used to prepare well-defined polystyrene-6ZocA-polyisoprene-6/oc -polystyrene and poly(a -methylstyrene)-6Zoc -polyisoprene-6ZocA-poly(o -methylstyrene) triblock copolymers with >90% 1,4-microstructure by sequential monomer addition. 

The first publication on the use of 1,1-diphenylalkyllithiums as initiators for diene or styrene polymerization is the pioneering study of Morton and Fetters [49] on the preparation of poly(a-methylstyrene-btock-isoprene-f lock-a-methylstyrene) using the dilithium dimer of 1,1-diphenylethylene, 1,4-di-lithium-1,1,4,4-tetraphenylbutane (6) (see Scheme 4). The dilithium initiator... 

Criterion 1. The dilithium initiator, 6, was soluble (0.01-0.08 mol/1) in a mixture of cyclohexane/anisole (85/15, v/v), but like most dilithium initiators [88, 89] precipitated from solution when added to the polymerization solvent, cyclohexane. Therefore, the dilithium initiator was chain extended with approximately 30 units of isoprene to generate an isoprenyldilithium oligomer which was soluble when added to cyclohexane. Since the final anisole concentration was less than 1vol. %, a high (>85%) 1,4-polyiso-prene center block was obtained (65-70% ds-1,4, 20-25% trans-1,4 and the remainder was 3,4-enchainment). 

Criterion 2. The number average molecular weights of polyisoprenes and poly(a-methylstyrene-l7lock-isoprene-b/ocfc-a-methylstyrene) block copolymers prepared with the initiator, 6, were all within 4% of the number average molecular weights calculated based on the stoichiometry of the reaction (see Eq. 9 note this equation should be modified to Eq. 12 if the moles of dilithium initiator is used in the denominator of Eq. (9)] ... 

McGrath and coworkers [217] demonstrated that the use of the dilithium initiator, 90, provides a very versatile method with respect to the ability to vary the chemical composition of the end blocks after first polymerizing a diene such as butadiene or isoprene. The a,to-dilithiumpolydiene center block can be used to initiate the polymerization of polar monomers to form both end blocks simultaneously. This type of triblock copolymer with polar end blocks cannot be prepared by other block copolymer synthesis procedures. Thus, they prepared poly(tert-butyl methacrylate)-btocfc-polyisoprene-Wock-poly(fcrf-butyl methacrylate) triblock copolymers. However, the resulting triblock copolymers did not exhibit well-defined structures. The molecular weight distributions tended to be broad (M /M =1.10-1.25) or bimodal. 

Foss and co-workers (88) reported ABA copolymers obtained from a new dilithium reagent this organolithium initiator was formed by the addition of sec-butyllithium to m-diisopropenylbenzene in the presence of a small proportion of triethylamine, followed by reaction with isoprene to improve the hydrocarbon solubility. Unfortunately, the starting ma-... 

Anionic polymerization of isoprene was carried out using practically the same method as reported by Rubio and by Morton et al. The bifunctional initiator, 2,4-hexadienyl dilithium was prepared through the reaction of trans,trans-2,4-hexadiene with Li metal in triethylamine. 

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