Parisa Mehrkhodavandi

 
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Research Classification

Research Interests

Inorganic and Organometallic Compounds
Polymers
Chemical Synthesis and Catalysis
Bio-based polymers
Bioproducts
catalysis
Green Chemistry
Inorganic and Organometallic Chemistry
Polymer Chemistry and Characterization

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Affiliations to Research Centres, Institutes & Clusters

 
 

Research Methodology

Biodegradable polymers
Biopolymers
Catalysis
NMR spectroscopy
Gel permeation chromatography
Chemical Synthesis

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2021)
Neutral and cationic indium complexes for the synthesis of oxygenated copolymers (2021)

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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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 - 2020)
Synthesis and reactivity of dinucleating di(diamino)phenolate ligands for enforcing cooperativity (2018)

A new single-frame dinucleating di(diamino)phenolate ligand L has been synthesized and fully characterized, and its coordination chemistry with zinc precursors featuring ethyl, alkoxide, acetate and amide leaving groups has been investigated. Reaction of ligand L with diethyl zinc and Zn[N(SiMe₃)₂]₂ led to the formation of a trinulcear zinc complex 1 (L)Zn₃(CH₂CH₃)₄ and a dinuclear zinc amide complex 4 (L)Zn₂[N(SiMe₃)]₂, respectively. Deprotonation of ligand L followed by salt metathesis with Zn(OAc)₂ gave rise to a trinuclear zinc complex 3 [(L)Zn₃](μ-OAc)₄. The alkyl zinc complex 1 reacted with benzyl alcohol to afford a dinuclear alkoxy zinc complex 2. Complexes 1-3 were fully characterized by ¹H-NMR, ¹³C{¹H}-NMR, COSY, NOESY, HSQC and HMBC NMR spectroscopy and elemental analysis. The solid-state structures of Zn complexes 1-4 were characterized by single-crystal X-ray crystallography. The catalytic activities of complex 1 and 2 towards ring opening polymerization of racemic lactide (rac-lactide) have been studied. Complex 2 showed better control over molecular weight and dispersity than complex 1, and generated heterotactically inclined poly(lactic acid). However, complex 2 promoted extensive transesterification and depolymerization reaction. Complex 3 was found to be active to ROP of rac-lactide at high temperatures and it was active to CO₂/epoxide copolymerization.

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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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Dinucleating ligand plateforms supporting zinc and indium catalysts for stereoselective lactide polymerization (2016)

With the recent advances in cyclic ester polymerization to access biodegradable and bio-based plastic, an increasing number of metal-based initiators have been reported to mediate ring opening polymerization with high yield and stereoselectivity. However, the origin of the stereoselectivity and the factors affecting it are yet little understood. The role of the metal nuclearity for the polymerization of lactide has especially been subject to various speculations. In our group, we developed unique asymmetrically bridged indium initiators, explored their ring opening polymerization mechanism and found evidence for a type of tandem catalysis. In order to have a better understanding of the role of the two indium metal centres during polymerization, we aimed to synthesize dinucleating analogues. The syntheses of the first alkoxide bridged indium complex supported by a chiral dinucleating ligand platform, along with its zinc analogue, are reported. Both complexes were synthesized in a one pot reaction starting from a chiral dinucleating bis(diamino)phenolate ligand platform, NaOEt, and respective metal salts. The dinucleating indium species bearing an achiral ligand backbone previously synthesized in our group was also further investigated. Both indium complexes catalyze the ring opening polymerization of racemic lactide to afford highly heterotactic PLA (Pr > 0.85). The indium complex bearing the achiral ligand backbone affords atactic PLA (Pr = 0.46) from meso-LA. The role of the dinucleating ligand structure in catalyst synthesis and polymerization activity is discussed.

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Amino- and imino-supported zinc complexes and their influence on ring opening polymerization and immortal ring opening polymerization of lactide (2015)

The alkoxy zinc complexes (±)-[(NNHOt-Bu)Zn(OCH₂Ph)]₂ 4, (±)-[(NNOt-Bu)Zn(OCH₂Ph)]₂ 5, and 6 (±)-[(NNOCm)Zn(OCH₂Ph)]₂ were prepared from the reactions of the corresponding alkyl zinc complexes (±)-(NNHOt-Bu)Zn(CH₂CH₃) 1, (±)-(NNOt-Bu)Zn(CH₂CH₃) 2, and (±)-(NNOCm)Zn(CH₂CH₃) 3, respectively, with benzyl alcohol (PhCH₂OH). All zinc compounds 1-6 were characterized by ¹H and ¹³C{¹H} NMR spectroscopy. The molecular structures of 2-6 were characterized by single-crystal X-ray crystallography. The alkyl zinc complexes are mononuclear with a tridentate coordination mode and a distorted tetrahedral geometry around the zinc metal centers. The alkoxy zinc catalysts show a dimeric feature containing two zinc metal centers bridging through the benzyl alkoxy oxygen atoms with different coordination modes for all three catalysts. The solution denticities of 4-6 were further investigated through 1D and 2D NMR spectroscopy experiments. Exchange studies with pyridine and complexes 4-6 were also conducted. The catalytic activities of alkyl and alkoxy zinc complexes 1-6 toward ring opening polymerization (ROP) of racemic lactide (rac-LA) have been studied by ¹H NMR spectroscopy, Gel permeation chromatography, and MALDI-TOF mass spectrometry. Experimental results indicate that the alkyl zinc compounds 1-3 show relatively less activity toward ROP of cyclic esters than the alkoxy zinc compounds 4-6. Catalysts 5 and 6 demonstrate a better control over experimental molecular weights and dispersity values of PLA, and more stereoselective than 4. Catalyst 4 undergoes the depolymerization reactions. Different transesterification side reactions operate at low equivalency polymerization studies to a different extent for all alkoxy zinc complexes. Immortal ROP of alkyl zinc compounds 1-3 was investigated in the presence of a chain transfer agent. The obtained results reveal the controlled nature of iROP.

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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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