Parisa Mehrkhodavandi

 
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Professor

Research Classification

Inorganic and Organometallic Compounds
Polymers
Chemical Synthesis and Catalysis

Research Interests

Inorganic and Organometallic Chemistry
Polymer Chemistry and Characterization
catalysis
Green Chemistry
Bioproducts
Bio-based polymers

Relevant Degree Programs

 

Research Methodology

Biodegradable polymers
Biopolymers
Catalysis
NMR spectroscopy
Gel permeation chromatography
Chemical Synthesis

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Synthesis and rheological characterization of polyhydroxybutyrate with different topologies and microstructures (2018)

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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Synthesis and rheology of poly(lactide)s and their lignin composites (2017)

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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Chiral indium catalysts for the ring-opening polymerization of cyclic esters (2015)

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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Effects of ligand tuning on dinuclear indium catalysts for the polymerization of lactide (2015)

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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Catalysis and ligand design : living ring opening polymerization of lactide with chiral dinuclear indium catalysts and template synthesis of functionalized carbenes on iron complexes (2013)

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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Master's Student Supervision (2010 - 2018)
Synthesis and reactivity of dinucleating di(diamino)phenolate ligands for enforcing cooperativity (2018)

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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Synthesis and polymerization of spiro-orthoesters catalyzed by a cationic indium complex (2017)

While numerous neutral indium complexes have been reported to be functional group tolerant Lewis acid catalysts for various transformations, there are only a handful of reported cationic indium complexes; these are mostly inactive in catalytic transformations. Herein, Idescribe the synthesis and characterization of a rare cationic indium alkyl complex bearing a chiral tridentate ligand, [(NNO)In(CH₂SiMe₃)(THF)][BArF] (30). Complex 30 is active towards reaction of 1,2-epoxy-7-octene and ε-caprolactone to form the corresponding spiro-orthoester (SOE1), where quantitative conversions of both substrates are achieved under 24 hours at 2.5% catalyst loading at 60 ˚C in benzene. Synthesis of spiro-orthoesters bearing various ether rings are also achieved with different lactone substrates. While preliminary polymerization studies of SOE1 by the cationic indium species 30 suggest the poor activity of this system towards polymerization of SOE1 at low temperature, polymerization of SOE1 complex 30 at 110 ˚C resulted in over 90% conversion of the monomer, and the resulting polymer was tentatively assigned as low molecular weight poly(ether ester). This thesis provides another example of the unusual reactivity of cationic indium complexes, where further investigation of various spiro-orthoesters synthesis and polymerization, as well as exploration of other cationic indium complexes, will be continued in the Mehrkhodavandi group.

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Living and immortal polymerization of cyclic esters with a dinuclear indium catalyst (2012)

The dinuclear indium catalyst [(NNO)InCl]₂(μ-OEt)(μ-Cl) (30), previously reported to be highlyactive and living in the ring-opening polymerization (ROP) of lactide (LA), was synthesized viaa previously published procedure and is studied to be active for the ROP of β-butyrolactone(BBL), ε-caprolactone (CL) and allyl-β-butyrolactone (fBL). A series of diblock copolymersPLLA-b-PDLLA were synthesized for the first time and their thermal properties were studied. Aseries of triblock copolymers PLLA-b-PDLLA-b-PLLA/PDLA were synthesized for the firsttime. Studies on the polymerization of BBL by 30 reveal that it is highly solvent dependent andthe rate is first order in BBL and catalyst concentration. The activation parameters were obtainedfrom an Eyring plot. The polymerization is controlled, providing PHB with molecular weights inagreement to theoretical values up to 300 kDa in narrow distribution. Triblock copolymersPLLA-b-PHB-b-PDLA were synthesized via the sequential addition technique. Polymerizationof CL by 30 showed an unusual slower rate than LA. Copolymers of allyl-β-butyrolactone withLA and BBL were made, supporting the feasibility of incorporating functionality into pendentgroups of polyesters with this catalyst.The catalyst 30 is capable of immortal polymerization of LA and BBL with high loading ofmonomers and alcohols ([BBL]:[30]:[ROH] = 10000:1:100), providing polymers with molecularweights in well agreement with calculations inverse to the total amount of 30 and alcohols. Thetacticity of polymers was not affected by the addition of ethanol. Poly(ethylene glycol)monomethyl ethers with molecular weights of 350 Da and 5000 Da were used as chain transferagent to synthesize PEG and polyester block copolymers via iROP for the first time.Nanoparticles of PEG₁₁₄-b-PLLA₂₀₀ and PEG₁₁₄-b-PLLA₁₀₅ were made through nanoprecipitationand thin film rehydration/dialysis methods.

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