Post-functionalization of narrowly dispersed PE waxes generated using tuned N,N,N′-cobalt ethylene polymerization catalysts substituted with ortho-cycloalkyl groups

Multifluoro-Modification Enhancing Catalytic Activity and Thermal Stability of Bis(imino)pyridylcobalt Chlorides for Linear Polyethylene

The multifluoro aniline, 2,6-bis[bis(4-fluorophenyl)methyl]-4-trifluoromethoxy-phenylamine, is meticulously synthesized and utilized to form a series of bis(imino)pyridine derivatives. These derivatives react with cobalt chloride to give six bis(imino)pyridylcobalt (II) complexes (alkyl: 2,6-dimethyl Co1; 2,6-diethyl Co2; 2,6-diisopropyl Co3; 2,4,6-trimethyl Co4; 2,6-diethyl-4-methyl Co5; 2,6-bis[bis(4-fluorophenyl)methyl]-4-trifluoromethoxy Co6). High activity and good thermal stability in ethylene polymerization are achieved by these cobalt complexes in the presence of MAO or MMAO. Notably, superior polymerization results are observed in n-hexane compared to toluene with peak activity reaching 1.55×107 g (PE) mol-1 (Co) h-1 at 70 °C in n-hexane. Moreover, the resulting polyethylenes are highly linear, featuring vinyl terminal group, and exhibit a wide range of molecular weights (Mw: 22.5-285 kg mol-1) along with high melting points (Tm >127.3 °C).

Probing the Catalytic Degradation of Unsaturated Polyolefin Materials via Fe-Based Lewis Acids-Initiated Carbonyl-Olefin Metathesis

Degradation and recyclability of polymeric materials, including extensively used polyolefins, are becoming increasingly necessary. Chemically stable saturated polyolefin backbones make their degradation frustratingly challenging. The current effective strategy is to create cleavable defects, e.g., C═C double bonds along the backbone, and subsequently depolymerize them via cross-metathesis reaction with olefins. High-value chemicals or reusable polymeric segments are obtained. This two-step protocol provides operable means for alleviating plastics problems. There are several approaches to introduce unsaturation into a polymer backbone, like dehydrogenation or copolymerization of olefins and conjugated dienes. However, for the second step, to conduct a cross-metathesis reaction, only noble metal catalysts can be used most of the time. Regardless of their limited availability, the fact that these organometallics are unfavorably sensitive to impurities would raise barriers in industrial practices. Herein we employed earth-abundant and inexpensive iron-based Lewis acids to initiate carbonyl-olefin metathesis reactions between ketone/aldehyde reagents and unsaturated polyolefins. After explorations in poly(diene)s and industrial thermoplastic elastomers, we extended this protocol to degrade low-density polyethylene (LDPE). Low-molecular weight PE wax-like products were obtained as useful chemicals. This catalytic degradation system is expected to enable the development of more efficient metathesis strategies to promote degradation of polyolefins and pave sustainable ways for reuse of polymeric materials.

Achieving Linear α-Macro-olefins in Ethylene Polymerization through Precisely Tuned Bis(imino)pyridylcobalt Precatalysts with Steric and Electronic Parameters

Synthesis of functional polyethylene from ethylene alone is tricky and heavily dependent on both the type and structure of the precatalyst and the choice of cocatalyst used in the polymerization. In the present study, a series of cobalt precatalysts was prepared and investigated for ethylene polymerization under various conditions. By incorporation of strong electron-withdrawing groups (F and NO2) and a steric component (benzhydryl) into the parent bis(imino)pyridine ligand, the catalytic performance of these precatalysts was optimized. On activation with MAO or MMAO, these precatalysts with relatively open structure achieved unprecedented ethylene polymerization rates at 60 °C (up to 27.6 × 106 g mol-1 h-1) and remained effective at temperatures up to 100 °C. Chain growth reactions were moderate, resulting in polyethylene with molecular weights up to 61.0 kg/mol and broad bimodal dispersity index. High crystallinity and melt temperature indicated a strictly linear microstructure, as further confirmed by high-temperature 1H/13C NMR measurements. Of significant note that chain termination predominantly occurred through β-elimination (up to 84.5%), yielding vinyl-terminated long-chain olefins. These functional α-macro-olefins are valuable as precursors for postfunctionalization, expanding the potential applications of polyethylene across various sectors.

Bis(imino)-6,7-dihydro-5H-quinoline-cobalt complexes as highly active catalysts for the formation of vinyl-terminated PE waxes; steps towards inhibiting deactivation pathways through targeted ligand design

A set of five related bis(imino)-6,7-dihydro-5H-quinoline-cobalt(ii) complexes, [2-(ArN = CPh)-8-(NAr)-C9H8N]CoCl2 (Ar = 2,6-Me2C6H3Co1, 2,6-Et2C6H3Co2, 2,6-i-Pr2C6H3Co3, 2,4,6-Me3C6H2Co4, 2,6-Et2-4-MeC6H2Co5), have been synthesized in reasonable yield by the template reaction of cobalt(ii) chloride hexahydrate, 2-benzoyl-6,7-dihydro-5H-quinolin-8-one and the corresponding aniline. The molecular structures of Co1 and Co4 highlight both the differences in the two imino-carbon environments (phenyl-capped chain vs. cyclic) and also the steric properties exerted by the bulky N imine-aryl groups. On pre-treatment with either modified methylaluminoxane (MMAO) or methylaluminoxane (MAO), all complexes proved productive catalysts for the polymerization of ethylene. In particular, Co1/MAO was the most active reaching a very high level of 1.62 × 107 g PE per mol (Co) per h over a 30 minute run time. Owing to the presence of the imino-phenyl substituent, Co1-Co5 were able to exhibit good thermal stability by displaying appreciable catalytic activity at temperatures between 50 and 80 °C, generating polyethylenes with narrow dispersities (M w/M n range: 1.66-3.28). In particular, the least sterically bulky precatalysts, Co1 and Co4 formed polyethylene waxes (M w range: 1.94-5.69 kg per mol) with high levels of vinyl unsaturation as confirmed by high temperature 1H/13C NMR spectroscopy and by IR spectroscopy.