Effect of Lithium Compounds on the SN2 Reaction of Anionic Living Polyisoprene with 4-Bromobutoxy-tert-butyldimethylsilane in Heptane

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The additive effect of lithium tetrahydrofurfuryloxide (LiTHF) and alkyl lithium on the nucleophilic substitution reaction of anionic living polyisoprene with 4-bromobutoxy-tert-butyldimethylsilane (BBS) in heptane was investigated.When BBS was allowed to react with polyisoprenyllithium in heptane in the absence of any additives, considerable amount of dimeric polymers as well as aimed end-functionalized polymer were formed via lithium-halogen exchange followed by the homo coupling of the polymers. Degree of the dimer formation was dramatically suppressed when the reaction was carried out in the presence of LiTHF. Quantitative introduction of silyl-protected hydroxy group was achieved when the reaction was carried out in heptane at 0°C in the presence of LiTHF (5 eq to active chain end). Similar additive effect was observed for n-butyllithium whereas no such an effect was observed for sec-butyllithium.The cross-association of these lithium compounds with the active chain end of anionic living polyisoprene might have prevented the homo coupling of the polymers.
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