Controlling the molecular weights of polyethylene waxes using the highly active precatalysts of 2-(1-aryliminoethyl)-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridylcobalt chlorides: synthesis, characterization, and catalytic behavior

Abstract

A series of 2-(1-aryliminoethyl)-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridylcobalt chlorides were synthesized and characterized using FT-IR and elemental analysis, and the molecular structures of complexes , and were confirmed to present a pseudo-square-pyramidal or trigonal-bipyramidal geometry around the cobalt center using single-crystal X-ray diffraction. Upon activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all cobalt precatalysts gave high activities up to the level of 10(7) gPE mol(-1) (Co) h(-1) toward ethylene polymerization, being one of most active cobalt-based precatalysts. In comparison with cobalt analogues, the title precatalysts generally possessed longer lifetime along with good thermo-stability; moreover, the resultant polyethylenes were highly linear and unimodal in most cases.

Thermally stable and highly active cobalt precatalysts for vinyl-polyethylenes with narrow polydispersities: integrating fused-ring and imino-carbon protection into ligand design

“…Notably, this good thermal-stability for Co4/MAO coupled with good catalytic performance is superior to that reported for both [2-(1-(arylimino)ethyl)-9-arylimino-5,6,7,8-tetra hydrocyclohepta [b]pyridyl]cobalt dichloride (D) 12 and [α,α'-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridyl]cobalt chlorides (E) 13 . The GPC curves obtained for polyethylenes produced using Co4/MAO at different Al/Co ratios (Figure 4) indicate only gradual changes of molecular weight and similar molecular weight distributions.…”

“…6i In general, the combination of high activity and good thermal stability of these cobalt systems exceeds that reported for previously documented cobalt catalysts, 4,12 including [2-(1-(arylimino)ethyl)-9-arylimino-5,6,7,8-tetrahydrocyclohepta [b] , entry 5, Table 3). The origin of the enhanced performance characteristics is uncertain but it may be that the imino C-phenyl in these systems is preventing deprotonation in a way that could potentially occur for imino C-methyl counterpart D and hence stabilizing the catalyst at higher temperature.…”

“…On prolonging the reaction time (entries 7, 1113, Table 2) the activities smoothly decreased, indicating there was no induction time in the catalytic reaction; similar observations were noted for their imino C-methyl counterparts (D, Scheme 1). 12 Furthermore, the molecular weights of the resultant polyethylenes gradually increased with the longer reaction times as borne out by changes in the GPC curves ( Figure 3). Table 2).…”

“…Significantly however, the sevenmembered ring system (D, Scheme 1) exhibits the highest activity among these cobalt systems, the longest catalyst lifetime and displays reasonable thermal stability. 12 Subsequent work has seen the development of alternative ligand sets incorporating two fused rings namely α,α'-dibenzylidene-2,3:5,6-bis(pentamethylene)pyridine 13 (E, Scheme 1), their cobalt derivatives display high activity in ethylene polymerization. 13 Herein, we are concerned with the synthesis of a new family of cobalt(II) chloride precatalysts bearing the unsymmetrical tridentate ligands of 2-(1-aryliminobenzylidene)-9-arylimino-5,6,7,8-tetrahydrocyclo hepta [b]pyridines (F, Scheme 1), in which a single fused seven-membered ring and an imino C-phenyl group are incorporated into the ligand design.…”

“…The coordination geometry of Co3 can be best described as distorted square-pyramidal, which is similar to related N,N,NCoCl 2 structures reported elsewhere. 10,12 …”