Diphenylethylene 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 addition of two equivalents of butyl-lithium to one of l,4-bis-(l-phenyl-ethenyl) benzene (and to three other double diphenylethylenes ) in benzene solution produces a dispersion of dilithium initiator." Following solubilization by the addition of a little butadiene, the initiator was capable of polymerizing a second, and much larger quantity of butadiene to polymer of dispersity of 1.2. 

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... 

It is important to consider the possible reasons for the association effects which lead to bimodal molecular weight distributions for polymers formed using 90 as initiator in the absence of added Lewis base or lithium alkoxide. Leitz and Hocker [199] proposed that double diphenylethylene-based dilithium initiators form dimeric dianion aggregates (93) and that is why they are soluble in hydrocarbon solutions compared to other dilithium species. This type of dimeric structure is consistent also with the dimeric association... 

By analogy with the structure of the mcfa-substituted double diphenylethy-lene, 73, which forms a useful dilithium initiator upon addition of 2 moles of sec-butyllithium, the trifunctional diphenylethylene, 94, has been investigated as a precursor for a hydrocarbon-soluble, trilithium initiator, 95, as shown in Eq.(49) [239] ... 

Thus, multifunctional 1,1-diphenylethylenes, such as 73 and 94, are precursors for useful, hydrocarbon-soluble, multifunctional organolithium initiators such as 90 and 95, respectively. Their hydrocarbon solubility appears to be a consequence of a specific type of intermolecular association (e.g., 93) which is favored over the more usual type of 3-dimensional association which leads to insolubility for most dilithium initiators [89, 220]. However, perhaps because of their unique association, these initiators require the addition of either a Lewis base, such as tetrahydrofuran, or a lithium alkoxide salt to initiate rapidly (relative to propagation) and quantitatively. 

Similarly, Tung and co-workers (92a) reported dilithium anionic initiators based upon double 1,1-diphenylethylene compounds, such as bis-[4-(l-phenylethenyl)phenyl] either (21a). 

The use of bis (1,1-diphenylethylenes) and tris( 1,1-diphenylethylenes) as precursors for hydrocarbon-soluble dilithium and trilithium initiators, respectively... 

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