Browsing by Author "Wang, Xiaosong"

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Bulk Supramolecular Polymerization via “Weak” Hydrogen Bonds and Aromatic Interactions: Recognizing the Potential of Metal Coordination Geometry
(University of Waterloo, 2019-01-25) Lanigan, Nicholas; Wang, Xiaosong
Bulk supramolecular polymers offer an attractive strategy for the development of functional materials. The use of small molecule monomers is of particular interest, because they maximize the potential of a fully reversible polymer backbone. However, the use of small molecules to generate bulk supramolecular polymers has been hindered by the difficulties associated with monomer design. In particular, the tendency of small molecules to crystallize and the lack of understanding of how intermolecular interactions direct the packing of molecules in bulk, make the rational design of novel monomers challenging. To explore the potential of metal coordination geometry, the organometallic molecule CpFe(CO)PPh3CO(CH2)5CH3 (FpC6) was selected as a candidate for use as a monomer to prepare a bulk supramolecular polymer. To elucidate the packing motif and potential intermolecular interactions, crystal engineering of FpC6 was performed. FpC6 forms 5 different crystal structures depending on recrystallization conditions. Crystal engineering revealed that FpC6 interacts through a variety of C—H---O interactions and aromatic embraces, which are influenced by the recrystallization conditions. Depending on recrystallization conditions, FpC6 forms three unique one-dimensional chain structures in the crystalline material. When the crystalline solid-state structure was disrupted by melting, FpC6 displayed typical polymer behaviour, such as the ability to form free-standing films, and fibres could be drawn from the melt. Upon cooling, FpC6 did not immediately recrystallize and instead formed an amorphous solid which could be molded into a variety of shapes. Powder X-ray diffraction of the amorphous solid revealed a distinctive double peak pattern. The double peak pattern matched the inter-monomer distances for a ladder chain structure (identified via crystal engineering) formed by cooperative C—H---O hydrogen bond and aromatic embraces. Differential scanning calorimetry indicated that no recrystallization occurred and that FpC6 underwent a glass transition. These results demonstrated that the C—H---O hydrogen bonds and aromatic embraces are strong enough to stabilize the chain and prevent crystallization. However, FpC6 eventually crystallized over time, indicating that the chains were metastable. The mechanical behaviour of FpC6 was found to be very rich and complex, as it was sensitive to temperature, ageing and thermal/measurement history. Rheological experiments indicated that aged FpC6 demonstrates viscoelastic behaviour, potentially due to the presence of chain entanglements. In order to improve the reproducibility of the mechanical behaviour, the influence of thermal history on FpC6 was investigated to establish a reproducible initial state. Differential scanning calorimetry experiments revealed that the glass transition temperature of FpC6 decreased with increasing upper heating temperature and annealing time, suggesting that the network was not at equilibrium and changed over time. Based on temperature-dependent PXRD experiments of aged FpC6, the decrease in Tg was attributed to the de-polymerization of the supramolecular chains. Annealing FpC6 at temperatures above the Tm indicated that the chains disassembled faster at higher temperatures. Hysteresis experiments showed that no significant polymerization occurred during subsequent cooling or reheating cycles. FpCx (x = 5, 8 and 10) analogues with different alkyl chain lengths were prepared to investigate the effect of alkyl chain length. Preliminary investigation revealed that the FpCx analogues also formed bulk supramolecular polymers with the same chain structure as FpC6. Additionally, as the alkyl chain length increased, the glass transition temperature decreased FpC6 and its analogues are the first reported bulk supramolecular polymers formed via aromatic embraces and weak hydrogen bonding and represent an opportunity for novel monomer design.
Effective Synthesis of Iron-Carbonyl Small Rings and Macrocycles via Migratory Insertion Reactions
(University of Waterloo, 2019-09-23) Leung, Aaron; Wang, Xiaosong
Migratory insertion reactions (MIRs) of PFpR [PFp = (PPh2(CH2)3Cp)Fe(CO)2, R = (CH2)5CH3 or (CH2)4CH=CH2], which involve migratory insertion polymerization (MIP) and intramolecular migratory insertion cyclization (MIC), produce P(PFpR) macrocycles and cyclized PFpR rings, respectively. MIC occurred at the early stage of MIR, while MIP exclusively underwent ring-closing with no linear analogues generated. The effect of solvent, temperature and the concentration of PFpR on MIR was investigated, based on which the competition of MIP and MIC was regulated for the effective synthesis of P(PFpR) or cyclized PFpR rings. Cyclized PFpR was synthesized as the only product in THF with a low concentration of PFpR (1 wt. %), while ring-closing MIP was predominated under the condition at 60 °C with high monomer concentration of PFpR in THF (> 70 wt. %) resulting in P(PFpR) macrocycles with Mn up to 17,500 g/mol. This effective synthesis of ring molecules is attributed to the piano-stool coordination geometry and the low rotation barrier of Cp-Fe bond, and will facilitate further exploration of ring molecules as functional materials and supramolecular building blocks.
Nano-confined water within the membrane of self-assembled vesicles
(University of Waterloo, 2021-10-01) Marquez, Audren; Wang, Xiaosong
The hydrophobic metal carbonyl complexes, known as FpR (Fp = (Cp)Fe(PPh3)(CO)(CO−), Cp = cyclopentadiene, R = C3Bithiophene, C6Pyrene, C6Azobenzene or C6), self-assembles into stable colloids in water. The colloids are known as metal carbonyl vesicles (MCsome), whose membrane contains interstitial water within tetrahedral order in hydrogen bonds. This well-structured water is responsible for the structural integrity of MCsome. This role of interstitial water is influenced by a variety of factors, including aging the MCsome, the non-polar R-group within the membrane and the presence of a water miscible organic solvent. In this thesis, we first examined how the colloid preparation (addition of water to THF solution of FpR) is sensitive to the quality of water used, the order of mixing water and THF solution of FpR and the rate of water addition. Following the colloidal preparation, we investigated the effect of temperature on MCsome. A higher temperature induces the swelling of MCsome, while cooling to a lower temperature shrinks the MCsome. This behaviour is found to be related to the interstitial water as its polarity and cohesive energy is affected the temperature. With higher temperatures, the polarity increases and the cohesive energy decreases, while cooling to a lower temperature decreases polarity and increases cohesive energy. This behaviour is explained by the temperature-dependent change in the interstitial water structure and dynamics. The change in the interstitial water structure and dynamics influences the energy dissipation as indicated by the temperature-dependent fluorescence intensity of FpC6Pyrene where increasing temperature decreases the energy dissipation of the interstitial water. Using the knowledge obtained from the investigation, it was found that the cooling temperature of the colloids is important in the reconstruction of the interstitial water. Colder temperatures are found to restructure the interstitial water more quickly unlike cooling to room temperature. Furthermore, the changes in the temperature-dependent interstitial water are reversible when the temperature is below 40 °C since the restructure of the interstitial water is quick. Therefore, FpR MCsome is an ideal assembly model for the study of interstitial water confined with self-assembled nano-spaces, which is highly desirable knowledge for the design of novel colloids and understanding biological systems.
Nonamphiphilic Assembly of Metal Carbonyl Complexes: Chemical Structure, Kinetics, and Hydrophobic Effect
(University of Waterloo, 2018-08-30) Liu, Dapeng; Wang, Xiaosong
Self-assembly is an effective approach for the synthesis of various nanostructures. Assembly of amphiphilic molecules has been well developed, which are mostly thermodynamic controlled. Although the importance of kinetics and hydrophobic effect in self-assembly of biological molecules, such as proteins, has been well recognized, it remains to be challenging to understand these factors at molecular level. We have investigated the assembling behaviour of a few hydrophobic metal carbonyl polymers P(FpP) (FpP: CpFe(CO)2(CH2)mPPh2, m = 3 or 6) and small molecules FpC6X (Fp head = (PPh3)(Cp)Fe(CO)(CO-); C6X = hydrocarbon tail) in water. As a result, we were able to kinetically control the assemblies resulting in stable colloids in water with narrow size distribution and also gained fundamental knowledge on the hydrophobic effect at molecular level. The background of self-assembly is introduced in Chapter 1. It concludes that the assembly of non-amphiphilic molecules is less studied, but crucial for the understanding of biological supramolecular systems. Chapter 2 discusses the kinetic behaviour of hydrophobic homopolymer P(FpP) in poor solvents. We find that the kinetic pathways for the precipitation of P(FpP) is altered depending on the solution conditions. Kinetically trapped nanospheres and nanowarms with narrow PDI were produced. In Chapter 3, the assembly of P(FpP) in water is discussed. It is found that the rigidity of the backbone is a parameter determining the assembling morphology. With increasing the flexibility, the macromolecules assemble into lamellae, vesicles and irregular aggregates. As discussed in chapter 4, the vesicles formed in water do not have traditional bilayer membrane structure, which can swell upon the addition of THF. The swollen vesicles are colloidal stable and their PDI remains narrow. In addition to P(FpP), Fp acyl derivatives with pyrene or azobenzene (FpC6Azobenzene and FpC6Pyrene) were synthesis for aqueous assembly. The assembly behaviour of FpC6Azobenzene, as influenced by the balance of aromatic interaction and hydrophobic effect of pyrene, is discussed in chapter 5. In DMSO/water or methanol/water systems, aromatic interactions are predominant and drive the assemble into lamellae, while in a THF/water system. hydrophobic effect drives the assembly into vesicles. The role of hydrophobic effect in solution behaviour of the assembled vesicles is discussed in Chapter 6. we find that the hydrophobic hydration of Fe in the MCsomes (aqueous vesicles of metal carbonyl complexes) can be detected by cyclic voltammetry and fluorescence quenching experiments. Moreover, the hydration can be adjusted by the chemical structures of tails as well as solution conditions, such as dilution and pH, which induces the hierarchical self-assembly of MCsomes. The research is summarized in Chapter 7.
Surface modification of cellulose nanocrystal for advanced applications
(University of Waterloo, 2017-09-18) Zhang, Zhen; Wang, Xiaosong; Sèbe, Gilles
Cellulose nanocrystals (CNC) are bio-based nanoparticles, which display an array of interesting properties related to their renewability, biocompatibility, high tensile strength and elastic modulus, high aspect ratio, low density, low coefficient of thermal expansion, and liquid crystal behavior. As a result, CNC can be exploited in a wide variety of applications, which include the fields of composites, catalysts, emulsions or colloidosomes. However, the engineering of innovative nanomaterials from CNC generally requires a fine control of their surface properties by chemical modification, to tailor their dispersive, interfacial and self-assembling properties, or to introduce novel functionalities. In this context, the surface functionalization of cellulose nanocrystals by esterification and Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP) reactions was envisaged, with the objective to develop novel advanced materials. The first chapter describes the state-of-the-art of the field in which the cellulose nanocrystals are presented, with their properties, methods of functionalization and potential applications. A brief overview of the SI-ATRP reaction is also given. In the second chapter, a convenient method is proposed to directly characterize the polymer grafted by SI-ATRP on the CNC surface, without cleaving the polymer from the nanoparticle. The SI-ATRP grafting of polystyrene (PS) at the CNC surface was performed from brominated CNC initiators, in the presence of a sacrificial initiator. With this work, we show that Dynamic light scattering (DLS) is a convenient tool to monitor the polymerization process, while differential scanning calorimetry (DSC) can be used to verify if the polymerization rates of the grafted and free polymers coincide. Finally, we demonstrate that thermogravimetric analysis (TGA) can be used to directly estimate the molecular weight of the PS grafted at the surface of the CNC without cleaving the polymer from the nanoparticle. In the third chapter, both SI-ATRP and Surface-Initiated Activator Re-Generated by Electron Transfer ATRP (SI-ARGET ATRP) were conducted to graft PS and P4VP on the surface of CNC. The SI-ATRP and SI-ARGET ATRP approaches were systematically compared to evaluate the potential benefit of each method. The surface initiating efficiencies (SIE) of the CNC-Br macroinitiators with regards to styrene or P4VP were evaluated by TGA and EA, respectively. The combination of analytical methods such as FT-IR, DLS, DSC, TGA, and EA demontrated that the SI-ARGET ATRP reaction favored the grafting of longer polymer chains with lower grafting densities compared with the classical SI-ATRP method. The impact of catalyst concentration and propagation rate on the differences noted was particularly discussed. In the fourth chapter, pH-responsive P4VP-g-CNC nanohybrids were prepared by SI-ATRP and subsequently used to stabilize gold nanoparticles (Au NPs) and produce recyclable catalysts. The presence of P4VP brushes at the CNC surface led to the growing of Au NPs of lower averaged diameter compared with the diameter of the Au NPs deposited on pristine CNC. The catalytic performance of pristine Au NPs, Au NPs stabilized by CNC (Au@CNC) and Au NPs stabilized by P4VP-g-CNC (Au@P4VP-g-CNC) were then compared, through the measurement of the turnover frequency (TOF) obtained after catalytic reduction of 4-nitrophenol (4NP), used as a model reaction. Compared with pristine Au NPs, the catalytic activity of Au@CNC and Au@P4VP-g-CNC were about 10 and 24 times better, respectively. Moreover, the Au@P4VP-g-CNC material could be conveniently recovered by flocculation at basic pH, and the recycled catalyst remained highly active. In the fifth chapter, Ultraviolet (UV)-responsive poly(cinnamoyloxy ethyl methacrylate) (PCEM) was grafted on CNC using SI-ATRP. The resultant PCEM-grafted CNC (PCEM-g-CNC) exhibits high UV absorption properties and undergoes crosslinking under UV irradiation. When the PCEM-g-CNC nanohybrids were incorporated in poly(vinyl chloride), transparent composite films with UV-blocking properties were obtained. The comparison of the optical and mechanical properties of the films before and after UV-irradiation allowed it to be demonstrated that the PCEM-g-CNC nanoparticles also acted as thermal and UV-stabilizers for PVC. Meanwhile, the tensile mechanical properties of the PVC film were significantly improved, and further increased after UV-irradiation. In the sixth chapter, a facile method to prepare colloidosomes at room temperature is proposed from w/o inverse Pickering emulsions containing silica precursors and stabilized by cinnamate modified CNC (Cin-CNC). Cin-CNC Pickering surfactants were prepared by acylation with an excess of cinnamoyl chloride. The Cin-CNC surface displayed partial wettability with both toluene and water, which allowed stabilization w/o inverse Pickering emulsions. The Cin-CNC particles around the droplets were subsequently locked by cross-linking TEOS or TBOS silica precursors at the water/toluene interface, leading to an intricate network of polysiloxane within the Cin-CNC shell. In optimized conditions, the Cin-CNC/silica colloidosomes obtained displayed a robust shell and slow releasing capacity with regards to encapsulated molecules such as rhodamine B or fluorescent deoxyribonucleic acid (DNA). In the last chapter, we summarized the general conclusions of the thesis and proposed some recommendations for the future work. The cellulose nanocrystals were modified by esterification and surface initiated Atom Transfer Radical Polymerization. The characterization, mechanism and advanced applications of the functionalized CNC were envisaged. Based on the research and results in this project, we also proposed some recommendations for the future work on the modification of CNC and the other advanced applications.
Synthesis and Self-Assembly of Metal Carbonyl Building Blocks for Aqueous Colloids with Aggregation-Induced Functions
(University of Waterloo, 2017-04-06) Murshid, Nimer; Wang, Xiaosong
Metal carbonyl complexes (MCCs) are potentially useful for a range of biomedical applications, including cell imaging and bioassay. However, the poor solubility and stability of MCCs in water remain obstacles to their use in these applications. This thesis addresses this challenge via an investigation of the self-assembly behaviour of hydrophobic Fp (Fp: CpFe(CO)2) or Mp (Mp: CpMo(CO)3) acyl derivatives in water. Hydration of these hydrophobic molecules results in aqueous colloids with aggregation induced functions. The Fp and Mp acyl derivatives, used in this research, synthesized by the migration insertion reaction (MIR) of Fp and Mp alkyl compounds in the presence of phosphine ligands, e.g. triphenylphosphine (PPh3), possessed a highly polarized acyl CO group. This group was readily hydrated via water-carbonyl interactions (WCIs) during the aggregation of the molecules in water. This aggregation, driven by hydrophobic forces, resulted in highly integrated metal carbonyl vesicles (MCsomes) with liposome-like bilayer membranes. The polarized CO groups associated on the surface of the colloid created a strong local electric field, which induced an aggregation-enhanced IR absorption (AEIRA). When the colloids were exposed to a focused continuous-wave near-IR (NIR) laser beam, a strong gradient (trapping) force was generated, allowing laser-trapping of the MCsome. This strong force resulted from the sharp contrast in the refractive index (RI) between the building blocks (RI = ca. 1.8) and water (RI = 1.33). Blue-light–emitting MCsomes were created via the synthesis and self-assembly of a bithiophene tethered Fp acyl derivative. The bithiophene groups, associated within the membrane, generated an aggregation-induced emission (AIE). The separation of the bithiophene group from the metal carbonyl group, by an alkyl spacer, prevented AIE being quenched by the iron elements. The AEIRA, AIE and laser manipulation render the MCsomes potentially useful for vibrational and photoluminescent sensing applications. In addition, the redox activity of the iron on the surface of the colloids in water was explored. Cyclic voltammetry (CV) results showed two oxidation peaks separated by a redox coupling (ΔE½). The value of ΔE½ is inversely related to the separation distance between the adjacent Fp units located at the surfaces of the colloids, and was used to probe the degree of hydration of the hydrophobic domain. Taking advantage of this redox behaviour, the contribution of hydrophobic hydration to the formation and stabilization of the colloids, assembled from Fp acyl derivatives including hydrophobic and amphiphilic molecules, was systematically investigated. The results indicate that the hydrophobic interaction and hydration are significantly important for colloidal stability. In conclusion, a novel group of vesicles, MCsome, with multiple functions has been created via the aqueous self-assembly of Fp or Mp acyl derivatives. Hydrophobic hydration and interaction are crucial forces for the formation and stabilization of the colloids.
Synthesis and Self-Assembly of Metal Carbonyl Organometallic Macromolecules Prepared via Migration Insertion Polymerization
(University of Waterloo, 2016-05-18) Cao, Kai; Wang, Xiaosong
Migration insertion polymerization (MIP) has been developed for the synthesis of air-stable metal carbonyl polymers. CpFe(CO)₂(CH₂)₃PPh₂ (FpP) was synthesized as a monomer. The solution polymerization of FpP at a monomer concentration of 20 wt% generated PFpP oligomers with a DPn of 11, cyclic molecules, and a fraction of THF-insoluble material. On the other hand, bulk polymerization of FpP resulted in soluble macromolecules and no THF-insoluble material was produced. PFpP with a relatively high molecular weight (Mn = 257,00 g/mol was obtained when FpP was polymerized in bulk at 105 °C in the presence of 5 wt% DMSO. The PFpPs are thermally stable up to 180 °C and have a Tg of 99 °C as indicated by TGA and DSC analysis, respectively. End group analysis suggests that PFpPs possess Fp and phosphine end groups. Taking advantage of the reactivity of the Fp end groups, three PFpP amphiphiles, Ph₂PCn-PFpP (n = 6, 10, 18), were synthesized via migration insertion reactions (MIR) of the Fp end groups with alkyldiphenylphosphines. Moreover, [η⁵-Ph₂P(CH₂)₃C₅H₄]Fe(CO)₂(CH₂)₅CH₃ (FpPCp) with an alkylphosphine group tethered on the Cp ring was synthesized. Bulk MIP of FpPCp resulted in PFpPCp oligomers with a Mn of 4200 g/mol and a PDI of 1.73. Preliminary studies indicated that PFpP was able to self-assemble into uniform and stable vesicles in water. Hydration of the carbonyl groups was deemed to be responsible for the stability of the colloids. Using a nano-precipitation technique, the efficient encapsulation of hydrophilic molecules, e.g., PEG, within PFpP vesicles was achieved.
Synthesis and Solution Behaviour of CpFe(CO)(PPh3)[CO(CH2)3N(CH3)2] (FpC3NMe2)
(University of Waterloo, 2017-09-18) Geng, Diya; Wang, Xiaosong
Metal carbonyl (MC) colloids in water have a wide range of biomedical applications. The Fp-acyl (Fp: CpFe(CO)2) derivatives are one kind of MC molecules that can be synthesized through migration insertion reaction (MIR). The molecules assemble into colloids stabilized by water carbonyl interaction (WCI) due to the presence of highly polarized carbonyl (CO) groups. In this work, a new MC compound, CpFe(CO)(PPh3)[CO(CH2)3N(CH3)2] (FpC3NMe2) has been synthesized, in which a dimethylamino (-NMe2) group is attached. FpC3NMe2 is non-surface active and hydrophobic, but can be hydrated in neutral water. FpC3NMe2 assembles in THF/water, but the colloids are unstable and aggregate into crystal-like precipitates. This instability is attributed to the weakened hydrophobic interaction resulting from the hydratable -NMe2 groups. In strong acidic solutions, FpC3NMe2 is found to show amphiphilic characteristics due to the high water solubility of the protonated -NMe2 groups.
Synthesis of Cyclodextrin-Complexed Metal Carbonyl Polymers
(University of Waterloo, 2016-08-10) Zhou, Na; Wang, Xiaosong
The synthesis of metal carbonyl (MC) polymers has been explored for decades, but well-defined and well-characterized MC polymers are rarely reported mainly due to the chemical instability of MC complexes. To address this challenge, we explored the host-guest chemistry of cyclodextrins (CDs) with MC complexes and used it stabilize MC polymers. The MC polymers were prepared via the post-polymerization modification of poly(vinylbenzyl chloride) (PVBC) with cyclopentadienyl dicarbonyl iron (Fp) anions. The resulting Fp-PVBC subsequently interacted with β-CDs, generating β-CD-complexed Fp-PVBC (β-CD/Fp-PVBC). Compared with Fp-PVBC, the stability of β-CD/Fp-PVBC was significantly improved. The chemical structure of β-CD/Fp-PVBC was characterized using a range of techniques including Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR) spectroscopy, thermogravimetric analysis (TGA) and cyclic voltammetry (CV).