Mixed aluminum-magnesium-rare earth allyl catalysts for controlled isoprene polymerization: Modulation of stereocontrol

摘要

The performances of readily available Ln(al-lyl)(2)Cl(MgCl2)(2) center dot (THF)(4) precursors (Ln = Nd, 1; Y, 2; La, 3), in combination with alkyl aluminum activators [MAO, AlMe3, AlEt3, Al(iBU)(3)], have been studied in isoprene polymerization. The catalyst combination 1/MAO (1:30) shows a high activity (average TOFs up to ca. 5 x 10(4) mol (Ip) center dot mol (Nd)(-1) center dot h(-1) at 20 degrees C) and produces polyisoprene in a controlled fashion with up to 98.5% cis content, number-average molecular weights in reasonable agreement with calculated values, and relatively narrow polydispersities index (M) over bar (W)/(M) over bar (n)= 1.20-1.70). The yttrium precursor 2 affords systems with much lower activity and degree of control, but enables the formation of either 1,4-cis-enriched (75%) or 1,4-trans-enriched (91%) polyisoprenes, simply replacing the MAO activator by AlEt3 or AI(iBU)3, respectively.