Living Copolymerization of Ethylene with NorborneneMediated by Heteroligated (Salicylaldiminato) (β-enaminoketonato)Titanium Catalysts

摘要

Three heteroligated (salicylaldiminato)(β-enaminoketonato)titanium com-plexes [3-Bu~t-2-OC_6H_3CH=N(C_6F_5)][(p-XC_4)N=C(Bu~t)CHC(CF_3)O]TiCl_2(3a:X=F, 3b: X = CI, 3c: X = Br) were synthesized and investigated as the catalysts for ethyl-ene polymerization and ethyleneorbornene copolymerization. In the presence ofmodified methylaluminoxane as a cocatalyst, these unsymmetric catalysts exhibitedhigh activities toward ethylene polymerization, similar to their parallel parent cata-lysts. Furthermore, they also displayed favorable ability to efficiently incorporate nor-bornene into the polymer chains and produce high molecular weight copolymers underthe mild conditions, though the copolymerization of ethylene with norbornene leads torelatively lower activities. The sterically open structure of the β-enaminoketonatoligand is responsible for the high norbornene incorporation. The norbornene concen-tration in the polymerization medium had a profound influence on the molecularweight distribution of the resulting copolymer. When the norbornene concentration inthe feed is higher than 0.4 mol/L, the heteroligated catalysts mediated the livingcopolymerization of ethylene with norbornene to form narrow molecular weight distri-bution copolymers (M_w/M_n <1.20),which suggested that chain termination or trans-fer reaction could be efficiently suppressed via the addition of norbornene into thereaction medium. Polymer yields, catalytic activity, molecular weight, and norborneneincorporation can be controlled within a wide range by the variation of the reactionparameters such as comonomer content in the feed, reaction time, and temperature.