Browsing by Author "Xiang, Peng"

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Efficient, robust surface functionalization and stabilization of gold nanorods with quaternary ammonium-containing ionomers as multidentate macromolecular ligands
(2016-04-25) Dong, Zhongmin; Xiang, Peng; Huang, Lingqi; Ye, Zhibin
Surface functionalization of gold nanorods (GNRs) is critical to their applications in various fields. While there are several existing strategies, we report in this article a new general strategy for the surface functionalization of GNRs with quaternary ammonium-containing ionomers as a novel class of multidentate macromolecular surface ligands. A range of tetralkylammoniumcontaining hyperbranched polyethylene- and linear poly(n-butyl acrylate)-based ionomers has been specifically designed and employed in the strategy. Acting as multidentate macromolecular analogues of cetyltrimethylammonium bromide (CTAB), the ionomers have been demonstrated to bind onto the GNR surface by displacing the surface-bound CTAB species via ligand exchange to render CTAB-free ionomer-modified GNRs. By properly designing the enabling ionomers, we have shown that the modified GNRs can be endowed with some desired properties, such as excellent dispersibility in various organic solvents, robust stability under multiple rounds (up to 12 investigated) of high-speed centrifugation in organic solvents, amphiphilicity with dispersibility in both aqueous and organic media, fluorescence, and capability in carrying hydrophobic guest species. This strategy thus provides potential new ways for the construction of novel multifunctional GNR nanocomposites.
Synthesis and characterization of novel ethylene copolymers by palladium-diimine catalysts
(2015-09-24) Xiang, Peng
Late transition metal catalysts, especially Pd-diimine catalysts, have been extensive studied for olefin polymerization. The unique characteristics of Pd–diimine catalysts, including chain walking mechanism, highly electrophilic cationic metal center with reduced oxophilicity, the capability of initiating and catalyzing olefin "living" polymerization, and the sterically tunable–diimine ligands, allow the synthesis of a range of polyolefins and olefin copolymers with special pedant functionalities and unique microstructures. The main objective of this thesis research is to synthesize different new types of polymers and polymer-grafted nanoparticles with Pd–diimine catalysts by utilizing above unique characteristics. A broad class of low-polydispersity ethylene–norbornene (E–NB) copolymers having various controllable comonomer composition distributions, including gradient, alternating, diblock, triblock, and block-gradient, was synthesized through “living”/quasi-living E–NB copolymerization facilitated with a single Pd–diimine catalyst. This synthesis benefits from two remarkable features of the Pd–diimine catalyst, its high capability in NB incorporation and high versatility in rendering E–NB “living” copolymerization at various NB feed concentrations ([NB]0) while under an ethylene pressure of 1 atm and at 15 C. A class of hyperbranched polyethylene ionomers containing positively charged tetralkylammonium ions and different counter anions were first synthesized by direct iv one-pot copolymerization of ethylene with tetralkylammonium-containing acrylate-type ionic liquid comonomers. The use of a Pd–diimine catalyst, which shows excellent stability towards the highly polar ionic group, is key to the direct synthesis. The resulting ionomers properties including structural, thermal, and melt rheological properties have also been demonstrated. In a further study, HBPE ionomers encapsulating self-supported Pd(0) nanoparticles (NPs) as efficient and recyclable supported Pd catalysts were synthesized with a Pd–diimine catalyst. The Pd(0) NPs were immobilized on the ionomer matrix through ionic interaction directly during the copolymerization of ethylene with polymerizable ionic liquid comonomer. The resulting ionomer supported Pd(0) nanocatalysts have been utilized to catalyze carbon-carbon cross coupling reactions (Suzuki and Heck reactions) and semi-hydrogenation of alkynes. Moreover, the successful tuning of structural parameters of PE brushes in surface-initiated ethylene “living” polymerization from two types of silica nanoparticles were studied. The brush parameters that are controlled herein include brush length, density, and topology. The PE-grafted silicas with varying brush density and length are also used as nanofillers to construct polymer nanocomposites with an elastomeric ethylene-olefin copolymer (EOC) as the matrix polymer. The effects of brush length and density on the nanofiller dispersion, rheological properties, and tensile properties of the composites are examined (The preparation and characterization of the nanocomposites were carried out by K. Petrie and M. Kontopoulou at Queen’s University).