Improvements in catalysis for the copolymerization of ethylene with polar comonomers

Van Dyke, Brian

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https://hdl.handle.net/2142/117838 Copy

Description

Title

Improvements in catalysis for the copolymerization of ethylene with polar comonomers

Author(s)

Van Dyke, Brian

Issue Date

2022-12-09

Director of Research (if dissertation) or Advisor (if thesis)

Guironnet, Damien S

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Catalysis
• Copolymerization
• Late-transition metals

Abstract

Polyolefins are the most abundant polymer by volume, making up over half of the 300 million tons of plastics produced in 2015. This is not without a reason: polyolefins such as polypropylene and polyethylene (PE) have numerous applications and advantages in processability, chemical resistance, strength, and low toxicity. Much work has been done on developing organometallic catalysts capable of synthesizing polyolefins, however there are still some remaining challenges in the field relating to the incorporation of polar comonomers These challenges can be addressed through in improvements in late-transition metal catalysis, and three will be approached in this work: the copolymerization of polar olefins with ethylene, the non-alternating copolymerization of ethylene and carbon monoxide (CO), and the stereocontrolled copolymerization of propylene with vinyl silyl ethers (VSEs). The copolymerization of ethylene with polar olefins greatly improves the adhesion properties of nonpolar commodity polyolefins, producing cutting-edge materials with novel physical properties and potential applications. While much progress has been made in this field of polyolefin catalysis, the structure-function relationships that govern catalyst properties such as activity, polar monomer incorporation and molecular weight control are still non-trivial. To shed light on the relationships, we set out to examine nickel- and palladium-catalyzed polyolefin synthesis with new N-acylamidine based ligands. These ligands are easily accessible through a modular three-step synthesis that allow for the facile tuning of ligand steric and electronic properties. By modulating these ligand features and examining their effect on catalytic activity, insight of structure-function relations can be obtained. A variety of N-acylamidine ligands were synthesized containing Acyl, thiobenzoyl, and phosphoryl chelating moieties. While the palladium-methyl complexes of these ligand were able to be synthesized and isolated, none showed significant activity towards ethylene homopolymerization, preventing any investigation into copolymerizations with polar olefin monomers. Attempts to synthesize the nickel-methyl analogues were unsuccessful. The inability of these ligands to both polymerize ethylene and support a nickel-methyl species was attributed to poor coordination strength, as observed by decomposition under polymerization/metalation conditions. Attempts to improve coordination strength through synthesizing tetradentate bis-N-acyl amidines were unsuccessful, as well as metalation attempts with group (IV) metals. The chemical stability of its hydrocarbon backbone of PEs makes it mostly unsensitive to hydrolysis or photodegradation, leading to rampant plastic pollution. Increasing the rate of photodegradation of these materials is possible through the incorporation of carbonyl monomers from the copolymerization of ethylene and CO. Late transition metal catalysts can copolymerize ethylene and CO to form photodegradable polyketones. The significantly stronger coordination of carbon monoxide over ethylene results in the formation of a purely alternated polyketone that has dissimilar physical properties to commodity polyolefins. A few catalysts have been developed to synthesize HDPE analogs with <5% CO – Pd-Phosphinosulfonates, Ni-Phosphinophenolates, and Pd-diphosphazane monoxide catalysts. However, these catalysts exhibit low polymerization activity. We report the utilization of a highly-active biaryl-substituted Pd-phosphinosulfonate catalyst for the non-alternating copolymerization of CO and ethylene. Polyethylene with CO content varying between 0.1-50% CO were synthesized with up to 170x higher activity and 15x larger Mn than the commercial cationic Pd-bisphosphine complexes. Fine analysis of the polymer microstructure at low CO content suggests that the copolymerization does not follow the classical random Mayo-Lewis model. At low CO content, greater than statistical amount of alternated ketone segments are present in the polymer. We attribute this to a penultimate monomer effect, which may be either kinetic (insertion rate) or thermodynamic (monomer coordination) in origin. Attempts to elucidate the origins of this effect using copolymerization parameters were proved to be mathematically impossible and attempts to determine them were compounded by an extreme sensitivity to CO by the Pd-Phosphinosulfonate complex. PEXb, a crosslinked ethylene/vinyl silyl ether (VSE) copolymer, is used in construction applications as a piping material and wire insulator. However, the isotactic polypropylene (iPP) analogue of PEXb has not been reported and could potentially be an impactful material by merit of the higher strength of iPP compared to HDPE. The ethylene/VSE copolymer cross-linked to form PEXb can be synthesized using diimine-supported cationic nickel-methyl complexes, which usually lack stereocontrol in polypropylene polymerization due to facile “chain-walking” (consecutive β-hydride elimination/re-insertion). In 2005, Coates and coworkers reported an axially-shielded diimine nickel(II) bromide complex capable of suppressing chain-walking and polymerizing propylene in a stereocontrolled manner. However, this complex requires a alkyl aluminum initiator, which is incompatible with polar comonomers. We set out to synthesize a cationic nickel methyl complex supported by this axially shielded diimine to enable the stereocontrolled copolymerization of propylene with VSEs. The nickel (II) dimethyl complex of this ligand was able to be isolated and characterized by nuclear magnetic resonance spectroscopy. Future work will be directed toward the synthesis of the final cationic Nickel methyl complex and copolymerization studies.

Graduation Semester

2022-12

Type of Resource

Thesis

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