Nuclearity and cooperativity effects in binuclear catalysts and cocatalysts for olefin polymerization

Abstract

A series of bimetallic organo-group 4 ''constrained geometry'' catalysts and binuclear bisborane and bisborate cocatalysts have been synthesized to probe catalyst center-catalyst center cooperativity effects on olefin enchainment in homogenous olefin polymerization and copolymerization processes. Significant nuclearity effects are found versus mononuclear controls, and the effect can be correlated with metal-metal approach distances and ion pairing effects. Novel polymer structures can be obtained by using such binuclear catalyst͞cocatalyst systems.catalysis ͉ polymer ͉ polyolefin E nzymes achieve superior reactivity and selectivity, in part, by their efficacy in creating high local reagent concentrations and special, conformationally advantageous active site-substrate proximities and interactions (1-4). In this regard, the possibility of unique and more efficient abiotic catalytic transformations based on cooperative effects between adjacent active centers in multinuclear transition metal complexes is currently of great interest. Within the context of the rapidly advancing and technologically significant field of homogeneous single-site olefin polymerization catalysis (5-10), the conjecture that cooperative effects involving two or more metal centers in close proximity might achieve more efficient chain propagation and͞or novel polymer architectures motivated the research that is the subject of this contribution. Single-Site Olefin Polymerization ProcessesOne of the most exciting developments in the areas of catalysis, organometallic chemistry, and polymer science in recent years has been the intense development of new polymerization technologies based on well defined single-site metallocene and coordination complex olefin polymerization catalysts (5-10). The active catalytic species is typically generated by combining a transition-metal organometallic precursor with an activator or cocatalyst. The resulting electrophilic͞coordinatively unsaturated, strongly ionpaired species then undergoes rapid olefin enchainment. In optimum cases (e.g., group 4 metal and cyclopentadienyl-type ligand), catalysts with truly exceptional productivities and selectivities for high-molecular-weight polyolefins are produced. Constrained Geometry Catalysts: Testbeds for Multinuclear Cooperativity EffectsGroup 4 ''constrained geometry'' catalysts (CGCs) (A; Fig. 1) are well known single-site polymerization agents (16, 17) that produce unusual branched, hence far more processable, polyethylenes with high productivity and selectivity. One of the key features of these catalysts is the coordinatively open nature of the active site, which allows rapid enchainment of sterically encumbered olefin comonomers into the polyethylene backbone. Under certain conditions, these catalysts yield vinyl-terminated, chain-transferred macromonomers (a macromolecule that acts as a monomer) that diffuse away and then undergo competitive reinsertion into a growing polymer chain to produce macromolecules with long chain branching (Fig. 19, which is published ...

Ethylene Polymerization Using (Imino)vanadium(V) Dichloride Complexes Containing (Anilido)methyl-pyridine, -quinoline Ligands–Halogenated Al Alkyls Catalyst Systems

“…Both the activity, and the norbornene incorporation, in the ethylene/norbornene copolymerization were also affected by the Al cocatalyst employed [7,8]. We thus speculated that a reason for the observed difference would be due to a formation of the different catalytically-active species, catalyst/cocatalyst nuclearity effect (assumed in Scheme 1) [5,[11][12][13]. The activity decreased upon addition of CCl 3 CO 2 Et (for example, [14]), which can be commonly used as effective additives to improve the catalyst stability [8], clearly suggesting that the active species were, thus, different from those prepared from vanadium(III), (IV) complexes [1][2][3][4][5]14].…”

Group 4 metal complexes for homogeneous olefin polymerisation: a short tutorial review

“…Inspired by the role proximate metal centres play in increasing localised reagent concentration within enzymes, multimetallic species have been introduced to enhance several chemical transformations such as the Diels-Alder [167] and Stecker [168] reactions [169,170]. In olefin polymerisation, a wide range of bimetallic catalysts have been employed to try and find any cooperative effect that would arise from the complimentary interaction of the polymeric chain between the adjacent metal centres.…”

“…Fig. 12 Covalently tethered Group 4 bimetallic constrained geometry catalysts (M = Ti or Zr) [170] employed, only a single site is seen, synonymous with the predominance of a single homogeneous catalytic species [187]. Other heterobimetallic species, using both early and late transition metals, have been synthesised in order to increase the efficiency of ''tandem catalysis'', where the polymer properties produced by different metal centres are incorporated into the same polymer product [188][189][190][191].…”

Dimerization of ethynylaniline to a quinoline derivative using a ruthenium/gold heterobimetallic catalyst

“…14 In these examples, the different metal centers in heterobimetallic catalysts provide advantages including the formation of products different from those obtained using mononuclear complexes. There has been much recent interest in the use of gold complexes as catalysts for synthetic transformations.…”