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 w ith 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 ana lysis carried out during the coupling reaction show the presence of et hylene whose formation is related to a lithium-bromine exchange reacti on competing with direct alkylation. The lithium-bromine exchange reac tion is more effective at T < 80 degrees C and results in the formatio n of allyl bromide-terminated polyisoprene, while direct alkylation is effective at T > 100 degrees C and yields alkyl bromide chain ends. T he allylic and alkylic bromides react differently with the remaining p olyisoprenyllithium: the former adds only to polyisoprenyllithium yiel ding P-2, while the later also yields P-3 and P-4 through radical reac tions. (C) 1997 John Wiley & Sons, Inc.