The recent rapid development of organometallic single‐site catalysts (SSCs) has revolutionized polyolefin research and permitted the production of new specialty polymers and commodity polyolefins with improved properties. These well‐defined catalyst systems generally consist of a transition metal atom, such as titanium, zirconium, hafnium, iron, nickel, or palladium, complexed with an organic ligand set and an initiating group. In most cases, activation with methylaluminoxane or compounds bearing a weakly coordinating anions generates cationic‐active species responsible for olefin coordination and polymer chain growth. Representative examples of SSCs include the bis(cyclopentadienyl) or bis(phenoxyimine)‐type group 4 catalysts as well as the diimine complexes of nickel and palladium. By variation of the organic ligand and thus the steric and electronic environment of the metal center, these catalysts can be tailored to control the olefin polymerization reaction in an unprecedented fashion. Almost any vinyl monomer, irrespective of molecular weight or steric hindrance, can be polymerized by choosing the proper catalyst. Virtually all feasible poly(α‐olefin) microstructures ranging from atactic to isotactic, hemiisotactic, syndiotactic, and stereoblock polymers can be produced by rational modification of the catalyst structure. Functional monomers are readily copolymerized with the less oxophilic late transition metal catalysts. Entirely new materials, not accessible with traditional Ziegler–Natta catalysts, have emerged, including high melting syndiotactic polystyrene and cycloaliphatic polymers. This article provides a broad overview of SSCs and their application in olefin polymerization. Evolution and classification of SSCs, polymerization of ethylene, propylene, as well as functional olefins, macromolecular architecture bearing polyolefin blocks are covered. Finally, heterogenization of SSCs for use in industrial processes is described.
Monocationic complexes of yttrium with various bis-alkyl and bisa l l y l l i g a n d s Y ( C H 2 S i M e 2 P h ) 2 ( T H Fhave been prepared by protonolysis of the corresponding homoleptic tris-alkyl or -allyl complexes using the anilinium borate salt [PhNMe 2 H][B-(C 6 F 5 ) 4 ]. The resulting ion-pair complexes have been isolated and characterized by different techniques such as elemental analysis, 1 H, 13 C, and 89 Y NMR, and EXAFS for the allyl cationic complexMore specifically, a 1 H-coupled 89 Y INEPT sequence has been developed in order to quantify the metal/alkyl ligand stoichiometry of both synthesized neutral trisalkyl and cationic bis-alkyl yttrium complexes. The activity of the cationic complexes toward ethylene and isoprene homopolymerization has been assessed. In presence of TiBA, polyethylene was produced with activities ranging from 6 to 26 kg PE mol Y −1 h −1 bar −1 . The molar mass of the yielded polymers shows a bimodal distribution. Under similar conditions, polyisoprene was produced up to full conversion of the monomer. The microstructure of the yielded polyisoprene displayed mainly cis-1,4-units (ca. 60−70%) and 3,4-units (ca. 20−30%). Only a few percent of trans-1,4 units was revealed.
A new class of nickel catalysts for stereospecific polymerization of butadiene Ni(COD) 2 /[X][B(C 6 F 5 ) 4 ] (X ¼ CPh 3 or HNMe 2 Ph) was investigated and compared to the conventional ternary system Ni(O 2 CC 7 H 15 ) 2 /BF 3 $OEt 2 /AlEt 3 . Both families of catalysts showed high activity and stereospecificity but catalysts based on the relatively non-coordinating anion B(C 6 F 5 ) 4À displayed lower molar masses. The combinations of Ni(O 2 CR) 2 with AlEt 3 /[CPh 3 ][B(C 6 F 5 ) 4 ] and of Ni(COD) 2 with BF 3 $OEt 2 , [CPh 3 ] [B(C 6 F 5 ) 4 ]/BF 3 $OEt 2 and AlEt 3 /BF 3 $OEt 2 were also implemented. The polymerization tests using the resulting catalysts demonstrated that in the case of the industrial catalyst Ni(O 2 CR) 2 /BF 3 $OEt 2 /AlEt 3 , the formation of the active species is based on the reduction of Ni(O 2 CR) 2 in the presence of AlEt 3 followed by the re-oxidation of nickel(0) to nickel(II) by butadiene leading to the bis(allyl) complex Ni(C 12 H 18 ). A cationic active species is finally obtained by the activation of Ni(C 12 H 18 ) with a fluorinated compound formed by the reaction of BF 3 $OEt 2 with AlEt 3 .
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