Olefin Oligomerizations and Polymerizations Catalyzed by Iron and Cobalt Complexes Bearing Bis(imino)pyridine Ligands

Abstract

IntroductionThe capacity of 2,6 -bis(arylimino)pyridine -iron(II) and -cobalt(II) halide complexes, on activation with methylalumoxane ( MAO ), to act as exceptionally active catalysts (comparable with Group 4 metallocenes) for the conversion of ethylene to high density polyethylene ( HDPE ) was fi rst disclosed independently by Brookhart, Bennett and ourselves in 1998 ( Figure 5.1 ) [1 -7] . The signifi cance of this discovery was further highlighted by the complete absence at the time of examples of iron and cobalt complexes in olefi n polymerization applications. In comparison with early transition metal -based catalysts such as metallocenes, these late transition metal systems offer some distinct advantages which range from their relative ease of handling and manipulation, their reduced oxophilicity, to the cost -effectiveness and low toxicity of the metal centers employed. Moreover, by simple modifi cation of the 2,6 -bis(arylimino)pyridine ligand frame, exceedingly effi cient iron and cobalt oligomerization catalysts can also be generated, facilitating the formation of highly linear α -olefi ns with near ideal Schulz -Flory distributions. Since these initial fi ndings the past decade has witnessed a dramatic growth in reports associated with this class of late transition metal catalysts, and clear structure -activity relationships have emerged.This chapter is concerned with highlighting some of the more notable advances that have come to light as a result of identifying key factors that infl uence catalyst performance, particularly those related to precatalyst structure. The importance of the initiator and the role played in chain transfer is probed. Current mechanistic understanding is examined from both a spectroscopic and a computational viewpoint while efforts to prepare well -defi ned iron or cobalt alkyl catalysts are discussed. Efforts to heterogenize these homogeneous catalysts are briefl y reviewed, as is their use in multi -component catalysis.The reader is referred to a series of reviews for more general overviews of postmetallocene α -olefi n polymerization catalysts [8 -12] , while recent reviews relating to the versatility of 2,6 -bis(imino)pyridines and to iron -and cobalt -based catalysis itself have also been documented [13 -15] .

Bi- and tri-dentate imino-based iron and cobalt pre-catalysts for ethylene oligo-/polymerization

“…4,5,43,48 Thus, numerous variations of the original tri-dentate 2,6-bis(imino)pyridyl ligand have 65 been reported, with many of them maintaining the same [N, N, N]-metal core in the catalyst systems. 6,7,18,19,49,50 The Alt group investigated a family of bis(arylimino)pyridine iron(II) complexes bearing ω-alkenyl substituents (Scheme 10) for ethylene oligomerization and 70 polymerization. 51 Complexes bearing substituents both at the 2-and 6-positions of the iminophenyl rings only produced polyethylene, whereas if only one ortho imino group was present, the complexes produced, depending on bulk of the substituent, either oligomer/polymer mixtures or only 75 oligomeric mixtures; the reaction conditions could also affect the products formed.…”

“…The reader is directed to a raft of review articles that focus on early and some more recent developments. [6][7][8][9][10][11][12][13][14][15][16][17][18][19] The main driving force for such studies is to access new highly branched polyethylene products, whilst getting away from the 20 more oxophilic nature of early transition metal systems. In the past half-dozen or so years, stable complex pre-catalysts that exhibit high thermal stability have been reported for both cobalt 20 and iron complexes.…”

Recent advances in Ni-mediated ethylene chain growth: Nimine-donor ligand effects on catalytic activity, thermal stability and oligo-/polymer structure

“…Numerous review articles have highlighted the progress in late transition metal pre-catalyst development and its influence on the resulting polyolefinic products [3][4][5][15][16][17][18]. In addition a range of feature articles and book chapters have also focused on iron-and cobalt-based systems [19][20][21][22][23].…”

Paradox of Late Transition-Metal Catalysts in Ethylene Polymerization

“…There were numerous review articles highlighting the progress in late transition metal pre-catalyst development and its structure influence on the resulting polyolefin products. [4][5][6][7][8][9][10][11][12][13][14] Despite unique properties, the late transition metal catalysts suffer from poor thermostability that has limited their potential applications in industry (polymerization usually at 70-110 °C), which gives the impression of the researcher that it is difficult to be employed in the industrialized process system. Thus, the research of the late transition metal catalyst shows a downward trend in recent years.…”