Synthesis And Characterization Of Non-Metallocene Rare-Earth-Metal Complexes And Their Catalytic Activity For Olefin Polymerization

Rare-earth-metal complexes were booming for their superior catalytic behavior in polymerizations of ethylene, styrene, or conjugated dienes in the past decade. However, only limited catalyst systems have been used for polymerization of-olefins. Most of them exhibited low formation rates for polymers with low molecular weight. Compared with cyclopentadienyl complexes, the non-cyclopentadienyl complexes, which could be easily prepared and modified in ligand structures, have been widely applied to olefm polymerization. In this thesis, three kinds of rare-earth-metal catalyst systems with non-cyclopentadienyl ligands were developed. Also, their catalytic behaviors were detailed for (co)polymerization of α-olefins or isoprene. The results are summarized as followings:(1) N-heterocyclic carbenes (NHC) are an ideal ligand for coordinating to rare-earth metals, due to their strong σ-donating ability. Trialkyl rare-earth-metal complexes bearing three kinds of NHC ligands were designed and synthesized. Upon activation with two equivalents of [Ph3C][B(C6Fs)4], the scandium complexes with sterically hindered NHC ligands exhibited high activities for a-olefin polymerization, demonstrating that the NHC ligand is capable of stabilizing the presumed cationic active species. The catalyst systems with trialkyl scandium complexes also conducted copolymerization of1-hexene (H) with1,5-hexadiene (HD) to give random atactic H/HD copolymers with a wide range of HD contents(26.6-98.6mol%), a previously unreported copolymer. The cyclization selectivity of the copolymerization is very high. An increase of the HD content in copolymers results in an obvious change of Tg of the copolymers. The Tg value is proportional to the HD content in the copolymer. According to the in situ NMR monitoring, the active species was proposed to be the dicationic complex.(2) A simple Ziegler-Natta catalyst system based on ScCl3was developed. The system could conduct highly isospecific polymerization of1-hexene (mmmm>99%), and the resultant polymers possess high molecular weight(～106). This system proved to be showed an excellent stereoselectivity(mmmm>97%) even at80℃, the highest record for this polymerization at elevated temperatures. The activity showed a strong dependence on the central metal type. Replacing ScCl3(THF)3with YC13(THF)3.5, LaCl3(THF)2or LuCl3(THF)3resulted in nearly complete loss in activity for1-hexene polymerization. The activity and stereoselectivity were strongly affected by the types of aluminum alkyls and organic borates. The change of ligands resulted in different catalytic activities, while it didn’t influence the stereoselectivity. When the chain lengths of monomers were longer than C6, the system will conducted highly isospecific polyolefins of more than99%.(3) NCN-pincer2,6-bis(4’-isopropyl-2’-oxazolinyl)phenyl-ligated rare-earth-metal dichlorides were designed and synthesized, which have C2-symmetric geometry. Upon activation with organic borates and trialkylaluminium, the complexes exhibited highly catalytic activities and more than98%cas-1,4selectivity for isoprene polymerization (up to99.5%). When the monomer to initiator ratio was increased from500to4000, the molecular weights of the resultant polymers increased linearly from11.7x104to49.9x104. Meanwhile, the molecular weight distribution and cis-1,4-selectivity were almost constant. These indicate that the system is probably a single active site to induce controllable polymerization. The activity showed a strong dependence on the central metal type. The order of activities is in agreement with the size of the metal ionic radii. The activity and stereoselectivity was strongly affected by the types of aluminum alkyls, while the types of organic borates influenced the activity only. This system showed a tolerance of change in the polymerization temperature. When polymerization was performed at elevated temperatures, the catalytic activity increased strikingly. The reaction temperature had little effect on the regioselectivity, and high cis-1,4selectivity almost remained even at80℃(cis-1,4>96%). According to the NMR, IR and MS spectra, the alkyl bridged Ln-Al bimetallic cations were proven to be the actual active species.