Abstract
The bis(arylimino)pyridines, 2‐[CMeN{2,6‐{(4‐FC6H4)2CH}2–4‐NO2}]‐6‐(CMeNAr)C5H3N (Ar = 2,6‐Me2C6H3 L1, 2,6‐Et2C6H3 L2, 2,6‐i‐Pr2C6H3 L3, 2,4,6‐Me3C6H2 L4, 2,6‐Et2–4‐MeC6H2 L5), each containing one N′‐2,6‐bis{di(4‐fluorophenyl)methyl}‐4‐nitrophenyl group, have been synthesized by two successive condensation reactions from 2,6‐diacetylpyridine. Their subsequent treatment with anhydrous cobalt (II) chloride gave the corresponding N,N,N′‐CoCl2 chelates, Co1 – Co5, in excellent yield. All five complexes have been characterized by 1H/19F NMR and IR spectroscopy as well as by elemental analysis. In addition, the molecular structures of Co1 and Co3 have been determined and help to emphasize the differences in steric properties imposed by the inequivalent N‐aryl groups; distorted square pyramidal geometries are adopted by each complex. Upon activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), precatalyts Co1 – Co5 collectively exhibited very high activities for ethylene polymerization with 2,6‐dimethyl‐substituted Co1 the most active (up to 1.1 × 107 g (PE) mol−1 (Co) h−1); the MAO systems were generally more productive. Linear polyethylenes of exceptionally high molecular weight (Mw up to 1.3 × 106 g mol−1) were obtained in all cases with the range in dispersities exhibited using MAO as co‐catalyst noticeably narrower than with MMAO [Mw/Mn: 3.55–4.77 (Co1 – Co5/MAO) vs. 2.85–12.85 (Co1 – Co5/MMAO)]. Significantly, the molecular weights of the polymers generated using this class of cobalt catalyst are higher than any literature values reported to date using related N,N,N‐bis (arylimino)pyridine‐cobalt catalysts.
Highlights
The discovery of bis(imino)pyridine-cobalt complexes, more than twenty ago, that can efficiently mediate the formation of linear polyethylenes represents a major landmark in late transition metal ethylene polymerization catalysis (A, Scheme 1) [1,2,3]
With a view to further balancing high activity with high molecular weight, we report our findings concerning the application of bis(arylimino)pyridine-cobalt catalysts appended with one fixed 2,6-bis{di(4-fluorophenyl)methyl}-4-nitrophenyl unit (E, Scheme 1)
We reasoned that the combination of the para-nitro and ortho-difluorobenzyhydryl groups would have a beneficial effect on the molecular weight while the presence of the fluoro substituents would influence the catalytic activity and the thermal stability
Summary
The discovery of bis(imino)pyridine-cobalt (and iron) complexes, more than twenty ago, that can efficiently mediate the formation of linear polyethylenes represents a major landmark in late transition metal ethylene polymerization catalysis (A, Scheme 1) [1,2,3]. An in-depth study of these precatalysts for ethylene polymerization is undertaken with two types of aluminum co-catalyst, and the effects of temperature, Al:Co molar ratio, run time and pressure on catalytic activity and polymer molecular weight explored.
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