Coupling reaction of polyisoprenyllithium with 1,2-dibromoethane

The coupling reaction of polyisoprenyllithium with 1,2-dibromoethane (DBE) gives a multimodal molecular weight distribution (MWD). Species with molecular weights three and four times (P-3, P-4) higher than the base polymer (P) are present beside the main expected one with double molecular weight (P-2) The relative abundancies of P, P-2, P-3, and P-4 depend on the experimental conditions. Gas-phase chromatographic analysis carried out during the coupling reaction show the presence of ethylene whose formation is related to a lithium-bromine exchange reaction competing with direct alkylation. The lithium-bromine exchange reaction is more effective at T < 80 degrees C and results in the formation of allyl bromide-terminated polyisoprene, while direct alkylation is effective at T > 100 degrees C and yields alkyl bromide chain ends. The allylic and alkylic bromides react differently with the remaining polyisoprenyllithium: the former adds only to polyisoprenyllithium yielding P-2, while the later also yields P-3 and P-4 through radical reactions. (C) 1997 John Wiley & Sons, Inc.