Parisa Mehrkhodavandi

Over the years, numerous neutral indium catalysts have been studied for polymerization reactions. However, recent research conducted by Mehrkhodavandi and co-workers has shed light on the potential of cationic indium species in catalytic applications. In a comprehensive study, the impact of carefully designed hemilabile ligands on the stability, reactivity, and catalytic behavior of indium complexes is explored. Specifically, a family of cationic alkyl indium complexes supported by hemi-salen type ligands with hemilabile heterocyclic pendant donor arms are investigated. The stability and stable lifespan of these complexes directly correlate with the affinity of the pendant donor group to the indium center. The reactivity of the complexes exhibits the opposite trend in the cationic ring-opening polymerization of epoxides. Notably, the most stable complex, bearing a pyridyl donor arm, displays the remarkable ability to polymerize racemic lactide without an external initiator. The same air-stable cationic alkyl indium complex with the pyridyl pendant arm is employed to selectively produce high-molecular-weight cyclic poly(lactide) (c-PLA). This complex enables the reproducible synthesis of c-PLA with low dispersity (Đ ∼ 1.30) and molecular weights reaching up to 416,000 g mol-¹. Importantly, the complex remains active even after prolonged exposure to high-humidity air. Furthermore, it could form high-molecular-weight c-PLLA, c-PDLA, and their stereocomplex without inducing epimerization. The polymerization occurs through a cooperative Lewis-pair-based coordination–insertion mechanism, involving the coordination of monomers to the cationic indium center and the ring-opening of lactide facilitated by the pyridine donor group. Finally, these complexes are used to delve into the mechanism of the selective synthesis of spiro-orthoesters, a unique pre-sequenced monomer capable of producing perfectly alternating poly(ether-alt-ester). Structure function investigations and computational studies were conducted to elucidate the reaction mechanism. These findings reveal that this reaction follows Michaelis-Menten type saturation kinetics, which can be attributed to the low Lewis acidity of the indium catalysts, providing a basis for their exceptional selectivity.

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The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.

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Many efforts have been made to develop sustainable polymers that are functional as well as ultimately degradable in response to worldwide plastic pollution. Accordingly, these have prompted the development of new catalytic systems that can actively and selectively polymerize a variety of monomers. One excellent candidate is indium-based catalysts because of its low toxicity, functional group tolerance, and exceptional stability toward air and moisture. Herein, a series of neutral and cationic indium complexes were synthesized and their reactivities were explored for polymer synthesis.Monometallic cationic indium alkyl complex supported by a chiral amino-imino phenolate ligand catalyzed the highly selective coupling of epoxides and lactones to form spiroorthoesters. The quantitative conversion of both substrates allowed the synthesis of a series of functionalized spiroorthoesters by expanding the substrate scope to epoxides with various functional groups and lactones with different ring sizes. The double ring-opening polymerization of spiroorthoesters with analogous cationic indium alkyl complex resulted in the formation of the perfectly alternating copolymer of epoxide and lactone, poly(ether-alt-ester), which could be further modified via post-polymerization cross-linking and degraded in a basic medium.The reactivity of monometallic neutral and cationic indium alkyl complexes supported by a chiral diamino phenolate or a chiral amino-imino phenolate ligand was investigated for homopolymerization of methyl methacrylate and cyclic esters. The neutral complexes were active in the homopolymerizations of those monomers, whereas the cationic complexes only catalyzed one of those polymerizations depending on the counterions. In addition, block copolymers of methyl methacrylate with racemic lactide or epsilon-caprolactone were obtained via sequential copolymerizations with the neutral complex.Finally, bimetallic neutral and cationic indium complexes tethered by a binaphthol linker were investigated for homopolymerization of racemic lactide and copolymerization of CO₂ and cyclohexene oxide. A comparison of reactivity revealed that bimetallic complexes outperformed monometallic complexes, and the superior reactivity of the bimetallic system could be explained by cooperation between two indium centers, attributed to optimal In−In spacing and flexible distance by the binaphthol linker. These results are the first examples of tethered indium complexes acting cooperatively and show the potential of tethered indium complexes as promising catalysts for polymerization application.

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In this thesis, cellulose nanocrystals (CNC) fibers were chemically modified to produce anionic (pCNC) and cationic (nCNC) polyelectrolyte hydrogels to fabricate ionic diodes. In the beginning, the rheological behaviour of cationic and anionic CNC was studied and compared to that of pristine CNC. It was demonstrated that in the sonicated state, anionic and cationic CNC form hydrogen bonding, which notably contributes to interparticle forces and gel strengths. These structures between individual rods defeat the purpose of flocculation and ultimately leading to a more stable suspension. Moreover, enhanced rheological properties were observed in the case of nCNC in comparison with the pCNC and this may be due to the extensive formation of hydrogen bonding. In addition, the surface-modified cellulose nanocrystals were used to fabricate ionic diodes. Rectification behaviour from two oppositely charged hydrogels doped with cellulose nanocrystals with positive and negative surface charges was observed. It was found that the current−voltage characteristics of the CNC−hydrogel diode are influenced by several parameters including gel thickness, hydrogel concentration, applied voltage, and scanning frequency. Pronounced rectification ratio and high current densities in forward bias occurred as a result of the high surface area followed by a high charge density.Analyzing the experimental data, we demonstrated that unidirectional current response originated from an anisotropic distribution of counterions at the interface between the two gels doped with oppositely charged CNCs. Moreover, the physical mechanism is described quantitatively by an electrochemical model. We investigated and validated the proposed electrochemical mechanism by the Yamamoto-Doi model using experimental data. We demonstrated that the diode works via a physical mechanism that involves the electrochemical generation of hydroxyl ions and protons at the electrodes to create current. Exponential currents (J) in the forward bias were observed while J = A√(-V) in the backward bias, which is in agreement with predictions of the electrochemical model proposed by Yamamoto-Doi ¹. The results of this thesis can be directly utilized to fabricate biodegradable diodes of good, stable rectification performance. Also, this work provides insight on how to control ionic movement in ionic devices.

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The development of synthetic polymers that include more sustainable building blocks is an active area of research. Controlled synthesis of copolymers including biobased segments has shown promising results, as the great diversity of functionalities from renewable resources can be used to tune the properties of macromolecules and often allow for post-functionalization.Herein, a series of discrete cationic indium complexes were synthesized and characterized. The role of counteranions was explored and (±)-[(ONNO)In(THF)2][SbF6] proved a highly active catalyst for the polymerization of epoxides and their co-polymerization with other cyclic ethers such as THF, oxetane and oxepane. This catalyst was also active in the one-pot copolymerization of epichlorohydrin with rac-lactide with good control. Investigation of the role of counteranions and solvent donors on the kinetics of polymerization of epoxides revealed a subtle effect of solvents on initiation rates.The activity of this cationic system was improved by changing the counterion from [SbF6] to a less coordinating tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (BArF4). In this way, copolymerization of different epoxides and rac-lactide was achieved either through a mixture of monomers or via sequential addition to high molecular weight block copolymers. Mechanistic studies and control experiments indicate that the epoxide is polymerized by a cationic mechanism to yield a neutral alkoxide indium species that subsequently polymerizes the lactide by a coordination-insertion mechanism with no significant interference of the two mechanisms under polymerization conditions. The thermal and tensile properties of different block copolymers were studied, revealing mostly amorphous materials. It was possible to control the ductility and stiffness of the copolymers by tuning the nature and chain length of the blocks.Finally, a series of neutral indium complexes with different ligand frameworks containing hemi-labile donors were synthesized and characterized. Alkoxide complexes with iminophenolate ligands bearing morpholine, thiomorpholine and methylpiperazine side donors were active in the polymerization of rac-lactide with very good control over molecular weights. The role of hemi-labile side donors was explored in the synthesis of block copolymers by sequential and simultaneous addition of rac-lactide and ε-caprolactone.

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The classic Dewar-Chatt-Duncanson (DCD) model describes the bonding in transition metal-olefin complexes. In this framework, electron density can be bidirectionally relocated through binary frontier orbital interactions: σ donation and π back donation. Modern spectroscopic and theoretical methods have allowed us to probe the boundary of the framework of this bonding pattern, indicative of the existence of unique DCD model metal complex derivatives. The introduction of this thesis is developed in Chapters 1 & 2, illustrating both the historical context for the development of the DCD model, and its utility in investigating catalyst transfer polycondensation with Ni(0) catalysts in polymer science. Chapter 3 & 4 describe our initial studies using advanced synchrotron-based X-ray absorption spectroscopic methods to comprehensively re-examine this classic electronic structure and provide a framework for the interpretation of synchrotron spectroscopy of nickel complexes. These studies reveal the importance of ancillary ligands to enable strong metal-olefin bonding via ligand-induced backbonding.Insights from these systematic studies afforded the opportunity to examine its relevance in catalytically-relevant systems. Chapter 5 reveals how these insights could be leveraged to stabilize previously elusive analogs of the previously proposed Ni(0) π intermediate in catalyst transfer polycondensation of polythiophenes. The dynamic behaviour of these π -intermediates along the delocalized polymer backbone, i.e. so-called ring-walking along the polymer chain , is explored both via experimental and computational methods. An additional example of DCD-like bonding with unique properties is explored in Chapter 6, where a unique agostic interaction is identified and explained in some low-valent linear Ni(I) complexes, whose electronic structure provides a previously unreported mode of agostic bonding. The application of advanced physical methods to classic problems in organometallic chemistry has afforded new insights into such systems, and revealed new motifs for metal-ligand bonding. These findings provide new opportunities to exploit these bonding motifs in the design of novel organometallic species for catalysis and materials development.

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Series of monodispersed linear and star-shaped polyhydroxybutyrate (PHB)s were synthesized using controlled indium and zinc based complexes through immortal ring opening polymerization of β-butyrolactone (BBL) in the presence of benzyl alcohol, tris(hydroxymethyl)benzene, and dipentaerythritol chain transfer agents. The topologies of the prepared PHBs of various molecular weights were investigated using solution and melt rheological characterizations. The powerlaw relationship between the radius of gyration and hydrodynamic radii of the linear and star PHBs with the molecular weight confirmed that the molecules are self-similar. Reduced values of compactness factor relative to that of linear counterparts and exponential scalling of the zero-shear viscosity of the stars with span molecular weight confirmed the presence of branching on the PHB backbone. A series of racemic and enantiopure zinc complexes were synthesized and fully characterized for the polymerization of BBL to form high molecular weight syndiotactic PHBs (Pr up to 75%). Complex (±)-[(NNHOtBu)ZnOBn]₂ (9) showed unprecedented reactivity and control towards the polymerization of up to 20000 equivalents of BBL in the presence of 5000 equivalents of benzyl alcohol. Isothermal time sweep tests at temperatures above the melting point of the syndio-rich PHBs showed thermally stable behavior of these polymers at temperatures below 140 oC. The zero-shear viscosity of the syndio-riched PHBs was higher than their atactic counterparts and showed a power-law relationship with the molecular weight confirming the linear microstructure and the absence of cyclic or branched species in the melt. The extensional rheometry revealed high melt strength in a range of strain rates as a result of flow induced crystallization.Easy to make, indium-salan complexes were reported for the polymerization of as-received lactide. The solution state characterization of these polymers showed narrow molecular weight distributions with molecular weights closely matching the theoretical molecular weight as an indication of a robust catalytic system. These complexes are capable of polymerizing impure lactide isomers in the melt state under ambient atmospheric conditions to form high molecular weight symmetric star shaped multi-block PLAs with high melting points, up to 197 °C. This catalytic system can also be used for the formation of star-shaped PHB-PLA copolymers in inert atmosphere.

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Synthetic plastics were first introduced 180 years ago, but the materials we have produced are likely to persist on our earth for thousands of years. Global shifts in thinking have urged researchers to focus their attention on bio-derived and biodegradable polymers. One such polymer is poly(lactic acid) (PLA). Despite its environmental benefits, PLA has several material weaknesses which hinder it’s use as a replacement for commodity plastics. Highly active and selective indium catalysts for the ring-opening polymerization of lactide isomers have recently been developed by the Mehrkhodavandi group. By utilizing these catalysts, modification of tacticity and end-group functionality of PLAs are possible, permitting exploration into the effect of these modifications on chain interactions in PLA. The thermal and rheological behaviours of PLAs with different microstructures were compared. The molecular weight between entanglements was greatest for the syndiotactically enriched PLAs, giving rise to the lowest zero-shear viscosity. In addition, hetero- and isotactically enriched PLA had higher flow activation energies than syndiotactic variants, implying the inclusion of transient aggregate regions within these polymers due to enhanced L- and D-interactions. A series of aryl-capped PLAs were synthesized by living ring-opening polymerization with a chain transfer agent using a previously reported dinuclear indium catalyst, [(NNO)InCl]₂(μ-Cl)(μ-OEt) (A). Thermal, rheological and mechanical techniques were employed to understand the extent and strength of association caused by arylated chain ends. It is shown that the end-group has a greater effect on the properties of low molecular weight PLAs due to the larger number density of aryl end groups; significant interactions can be induced under oscillatory shear conditions in the low frequency flow regime (terminal zone).The lignocellulosic biorefinery industry has been expanding in recent years and now provides researchers access to a range of bio-based composite materials through blending and copolymerization. Lignin-graft-PLA copolymers were synthesized via different routes and the PLA products were analyzed. Polymers were found to have cyclic structures at low lignin loading and star-like structures at higher lignin loading. Rheological studies were undertaken to derive useful structure-property relationships and optimize material properties.

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The development of highly active and stereoselective catalysts for lactide polymerization is an area of continuing interest in asymmetric catalysis. Aluminum complexes supported by (ONNO), tetradentate, bis(iminophenolate) or salen ligands are the most isoselective catalysts for lactide polymerization reported. However, these are sluggish initiators requiring elevated temperatures and multiple days to achieve high monomer conversion. Recently, indium based catalysts have attracted considerable attention as functional group tolerant catalysts for lactide polymerization. In this thesis a family of mononuclear and dinuclear chiral indium alkoxide complexes bearing salen ligands was prepared. Solution state and solid state characterization of these complexes were carried out. These were highly active catalysts for the ring-opening polymerization of lactide, to generate the biodegradable polymer, poly(lactic acid)(PLA). Polymerization behavior and the stereoselectivity of these systems showed a well-controlled and isoselective family of catalysts. An investigation into the effects of ligand modifications revealed a profound dependence of the stereoselectivity on the ortho-aryl substituents. A detailed study was carried out to gain insights into the mechanism of polymerization. This provided evidence for a mechanism consistent with a mononuclear propagating species.Modification of the ligand backbone to a binap functionality was carried out to synthesize the first reported indium salbinap complexes. The ligand shows the ability to coordinate in both a κ² and a κ⁴ coordination mode to a metal centre. However, these complexes were sluggish initiators with modest stereoselectivity for the ring-opening polymerization of lactide.A dinuclear indium catalyst was used to generate triblock copolymers of PLA and poly(hyroxybutyrate)(PHB) via simple sequential monomer addition. After confirming the formation of these A-B-C type PLA-PHB-PLA triblocks, a series of these copolymers with varying monomer composition were prepared and their thermo-mechanical properties were studies.

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We are interested in the biodegradable polymer poly(lactic acid) (PLA) formed from the ring-opening polymerization of lactide. Our promising results on the polymerization of racemic lactide to form isotactically enriched PLA by a dinuclear indium catalyst bearing a chiral diaminophenolate ligand prompted us to investigate several ligand modifications in order to establish detailed structure-activity relationships within these complexes. Modifications to the terminal amine substituents, the central amine donors and the phenolate substituents of our tridentate ligands were undertaken. The factors affecting the stereoselectivity and activity of these indium catalysts were investigated in detail. Finally, pentadentate dinucleating ligands were used to synthesize dinuclear indium complexes with the goal of producing more stereoselective and/or active catalysts. Modifications to our tridentate ligand system led to complications in their coordination to indium, possibly due to flexibility of the ligands leading to aggregation. It was found that bulkier substituents on the terminal amine position of these ligands led to a lowering of the isoselectivity of the resulting indium complexes due to dissociation of the dimers during the polymerization of lactide. Changing the central amine donors from secondary to tertiary amines led to a profound decrease in polymerization rate. The contributions of intramolecular hydrogen bonding in these dimers on their resulting polymerization activity was explored. However, the nature of the amine, not hydrogen bonding, was found to be the determining factor in their activity towards lactide polymerization. Increasing the steric bulk of the phenolate substituents was found to influence the structure of indium dichloride complexes made with these ligands in solution and the solid state. However, these modifications were found to have only minor impact on the lactide polymerization activity and stereoselectivity of the related dinculear indium ethoxide complexes. A family of pentadentate proligands was utilized for the formation of dinuclear indium ethoxide complexes for the polymerization of racemic lactide. However, only one dinuclear indium ethoxide complex could be isolated cleanly. It was found to have low activity in the polymerization of racemic lactide, requiring weeks to reach full conversion. However, the complex was highly stereoselective producing over 90% heterotactic PLA.

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A family of indium complexes were synthesized and their catalytic activity towards thering opening polymerization of lactide to form poly(lactic acid), a biodegradable polymer,were assessed. Racemic and enantiopure mono- and bis-alkoxy-bridged complexes bearingbulky chiral diaminoaryloxy ligands were synthesized and characterized. The reaction of thebis-alkoxy-bridged complexes with water produced mono-hydroxy-alkoxy-bridged dinuclearindium complexes. Investigation of both the mono- and bis-alkoxy-bridged complexesconfirmed dinuclear structures in solution and in the solid state. These dinuclear complexeswere highly active catalysts for the ring-opening polymerization of lactide to form poly(lacticacid) at room temperature. A detailed mechanistic investigation showed that the selectivitiesobtained for the ROP of racemic LA with the mono- and bis-alkoxy-bridged complexes aredifferent and, along with kinetics investigations, suggest a dinuclear propagating species forthese complexes.Additionally, neutral and cationic alkyl indium complexes bearing a chiraldiaminophenoxy ligand were synthesized and characterized. Investigation of the cationiccomplexes in solution by NMR spectroscopy showed the counter anions influenced thedifferent chemical environments at the metal center in solution. The preliminarypolymerization of methyl methacrylate with neutral dialkyl and cationic alkyl indiumcomplexes produced poly(methyl methacrylate). This is the first demonstration of cationicindium complexes for catalytic reactivity not only in solution but also in neat monomer.Finally, a family of cyclic and acyclic Fischer-type carbenes were generated vianucleophilic attack at the carbon atom of a coordinated isocyande on a piano-stool iron(II)complex. All complexes were characterized by IR and NMR spectroscopy and, wherepossible, by single-crystal X-ray diffraction. In particular, rare donor-functionalized acyclic(phosphino)(amino)- and (silyl)(amino)carbenes were generated by a two-step templatesynthesis on the iron(II) complex. The methodology involves the initial formation of ylidenecomplexes followed by reduction of the resulting imine to yield the desired carbenecomplexes. The reversible conversion of an acyclic (sily)(amino)carbene complex to itsylidene precursor via slow deprotonation with hydride was demonstrated.

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