Iron-Based and Cobalt-Based Olefin Polymerisation Catalysts

Abstract

This chapter describes the brief history of iron or cobalt catalysts for olefin polymerisation and oligomerisation. Since the discovery in 1998 that 2,6-bis(imino)pyridine iron and cobalt halides, on activation with MAO, can convert ethylene to highly linear polyethylene, numerous reports have been concerned with ligand modification, mechanisms for precatalyst activation, identifying the active species and understanding the mode of propagation/chain transfer. In addition, heterogenisation of this class of catalysts, their incorporation into macrocycles/ polymers and their use in reactor blending/tandem catalysis has seen some important developments; alternative ligand architectures that can support active iron and cobalt catalysts are also discussed as are different types of olefinic monomer.

An Unsymmetrical Iron(II) Bis(imino)pyridyl Catalyst for Ethylene Polymerization: Effect of a Bulky Ortho Substituent on the Thermostability and Molecular Weight of Polyethylene

“…Among the o - sec -phenethyl substitution analogues 1 − 3 , complex 2 , containing methyl substituted for H on another ortho position, exhibits slightly better catalytic activity and produces much higher molecular weight polymer than the single-ortho-substituent analogues 1 and 3 . As shown in Figure , the polymers produced by the catalysts exhibit broad or bimodal molecular weight distributions that are in accordance with the previous observation from other iron bis(imino)pyridyl catalysts . The broad or bimodal molecular weight distributions can be attributed to the coexistence of two chain-transfer processes in the polymerizations: β-H transfer to monomer or metal and chain transfer to alkylaluminum. , The results obtained herein indicate that the bulky ortho disubstitution on the aniline can efficiently suppress the chain transfer reactions, especially for β-H transfer.…”

“…Important progress in the area of late-transition-metal catalysts using iron(II) bis(imino)pyridyl for ethylene polymerization has been achieved in the past few decades . In contrast to the nickel and palladium α-diimine catalysts, which typically produced highly or moderately branched polyethylene, tridentate iron(II) bis(imino)pyridyl catalysts, in spite of their higher activity, produced highly linear polyethylene with low molecular weight and broad molecular weight distribution.…”

“…Generally, bulkiness on imine carbon can increase catalytic activity for ethylene polymerization and the molecular weight of the polyethylenes (Fe/ketimine > Fe/aldimine), , while the steric hindrance of the 2,6-substitution on the aniline moiety can decrease catalytic activity (Me > Et > i -Pr) but give an obvious increase in the molecular weight of the products (Me < Et < i -Pr). − , Iron(II) catalysts lacking ortho alkyl substituents only produced oligomers or trace polymer. ,, One drawback of these catalysts is their poor thermal stability. The activity and molecular weight dropped rapidly as the reaction temperature increased . A bulky macrocyclic iron(II) bis(imino)pyridyl complex can restrain the catalytically active iron center from deactivation and produce higher molecular weight polymers …”