Nonmetallocene catalysts

Nonmetallocene single-site catalysts are basically catalytically active metal complexes that do not have cyclopentadineyl-type ligands. Many different metals with a wide variety of spectator ligands have been shown to have ethylene polymerization activity. The basic role of the spectator ligands in all these cases is to impart stability to the catalytic intermediates, without compromising activity. 

To stabilize the catalytic intermediates, the ligands must be of optimum bulk. They must allow free access to the alkenes for coordination, but prevent the coordinatively unsaturated intermediates from coming too close to each other, and decomposing into metal. In the choice of ligand for these complexes, the ease of synthesis is also an important consideration. 

Thus 6.16-6.19 and 6.21 are all Schi f base complexes. Schiff bases are relatively easy to synthesize by simple condensation reactions between the appropriate aldehyde or ketone and ArNH. Note that in some of these ligands, bulky substituents such as isopropyl groups are present. 

By fine-tuning reaction conditions, MAO-activated 6.16 can be made to give PE ranging in structure from almost completely linear to moderately branched. Complex 6.17, where the metal is iron, is also highly active for ethylene polymerization but produces a strictly linear, high-molecular-weight polymer. 

Complex 6.17 where the metal is cobalt is notably less active than its iron analogue. As cobalt-based catalysts are used for industrial PB manufacture, this complex has also been evaluated as a single-site catalyst for 1,3-butadiene polymerization. 

Only very few rare-earth-based nonmetaUocene compounds were used as styrene polymerization catalysts (Fig. 7.7). 

Another notable samarium(II)-based catalyst system is Sm(OAr)2(THF)3 (53). While it is inactive as a catalyst for styrene polymerization at 0.1 MPa, 

Neutral and anionic samarium and neodymium species 54 and 55 with allyl ligands were reported to give syndiotactic-rich polystyrene [29]. Although the presence of lithium aUyl in equilibrium suggests an anionic polymerization mechanism, the formation of a syndiotactic-rich product indicates the participation of rare-earth metals in the polymerization. 

Recently, hydride complexes supported by guadinate ligands 56 and 57 were found to be active in styrene polymerization, but only for the smaller lanthanides [30,31]. Both the lutetium and the ytterbium complexes show low conversion rates (the Lu compound converts 100 equiv within 3 days) the products are highly syndiotactic, and the polymer produced by the ytterbium compound was reported to have a high melting temperature of 289-293 C (Af = 90,000 g/mol, MJM = 2.6). 

The introduction of group 3 metal complexes as catalyst precursors for syndiospecific styrene polymerization has resulted in the following new features  

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The resting state of the catalyst is believed to be an amine adduct of the catalytic active Ln-amide. For lanthanocene catalysts such an amido amine species of the type Cp 2Ln(NFiR)(NH2R) has been spectroscopically and crystallographically characterized [103]. Amines, coordinating solvents and other external bases may adversely affect the reactivity of the rare-earth metal center, in particular if the metal center is readily accessible. Sterically open anya-lanthanocenes and constrained-geometry catalysts (CGC) [27,104,108] and more recently also sterically readily accessible nonmetallocene catalysts [101,115,116] have displayed product inhibition (leading to apparent first-order kinetics) or substrate inhibition (resulting in self-acceleration). 

Recent Advances in Nonmetallocene Catalysts for Stereoselective Propylene... 

Nonmetallocene Catalysts Based on Early Transition Metals. 158... 

Nonmetallocene Catalysts Based on Late Transition Metals. 160... 

RECENT ADVANCES IN NONMETALLOCENE CATALYSTS FOR STEREOSELECTIVE PROPYLENE POLYMERIZATION... 

Cyclopolymerization of Nonconjugated Dienes with Nonmetallocene Catalysts 497... 

With reference to Fig. 48, it was forecast that the application of single-site metallocene technology will define the first in a series of new S-curves [68] for the polyolefin industry. The second in the series will be driven by the emerging nonmetallocene catalysts. In this S-curve, product design capability will expand to include polar monomer incorporation and control of intermolecular distributions [68]. After this second technology wave the direction of development will shift to control of intramolecular architecture. 

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