Hard and soft nanocomposites enabled by rationally designed nonlinear copolymers derived from high functionality polyols
Iocozzia, James A.
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Controlled living polymerization techniques, such as atom-transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer polymerization (RAFT), have enabled the synthesis of well-defined monodisperse polymer structures possessing many different compositions including telechelic (chain end) and pendant (side chain) functionality amenable to post polymerization modification by various approaches. One such approach is the immensely useful class of reactions known as “click” chemistry that enables many different materials to be easily connected to each other in high yield and under mild conditions. The most popular click chemistry approach is copper-catalyzed azide-alkyne click chemistry (CuAAC). In parallel, the rational design and synthesis of high functionality polyols such as cyclodextrins and hyperbranched polyglycerols (HPG) has offered the opportunity to produce many new, complex and highly nonlinear polymer architectures. Only in the last fifteen years have the techniques for producing HPG via anionic ring-opening multibranching polymerization (ROMBP) improved to the point where samples of sufficiently controlled molecular weight and low polydispersity have been realized. Consequently, these and related high functionality polyols can be used as polymer macroinitiators in conjunction with ATRP to produce densely-packed polymer and copolymer structures spanning multiple dimensions and length scales with useful shapes and retained functionalities with many immediate applications in nanoscale materials science and technology. HPG, and related polyol-based star-like copolymers, can be used as nanoreactors for the synthesis of varied nanocrystalline (hard) and wholly polymeric (soft) nanomaterials. This is because the globular structure, dense surface arms, and retained functionality naturally lend themselves to the production of nanoparticle structures of various types. More importantly, HPG carries several advantages over established polyols including the ability to control the degree of functionality and polymer size. In addition, HPG possesses inherent biocompatibility and functionality owing to the numerous ether groups present within its structure. In this dissertation, we investigated the synthesis of well-defined β-cyclodextrin and HPG-based star-like polymers and copolymers produced by ATRP in conjunction with CuAAC click chemistry and other surface chemistries for applications in three key areas: (1) Promesogen-capped β-cyclodextrin-based star-like polymer templates for improved inorganic dispersion in liquid crystal solution; (2) HPG-based dense star-like polymer templates for the production of functional inorganic (hard) nanomaterials with retained nanoscale properties and solubility; (3) Azido-HPG hyperbranched polymers for producing biocompatible, wholly polymeric (soft) nanoparticles. These areas of fundamental research are related through their use of high functionality polyols, ATRP and simple, robust linker chemistries to produce novel and useful nanomaterials for applications in displays, optics, ferroelectrics, and biomaterials among others by varying the types of polymers grown, the surface moieties attached and the methods of crosslinking. The specific details of each of these research areas are summarized below. First, a β-cyclodextrin-based promesogen-capped star-like polymer template (21-arm star-like poly(acrylic acid)-CNBP) was produced by a combination of ATRP and click chemistry. The CNBP capping moiety (4-isocyano-4’-(prop-2-yn-1-yloxy)biphenyl) is structurally similar to cyanobiphenyl-type liquid crystals, such as 5CB, commonly encountered in LCD displays. The inner poly(acrylic acid) (PAA) core of the polymer is used to template superparamagnetic iron oxide nanoparticles (Fe3O4) within the core. This leads to iron oxide nanoparticles capped with the CNBP promesogen molecules. The advantages of this approach to producing liquid crystal-capped nanoparticles are twofold. First, the resulting CNBP-capping molecules are covalently tethered to the inner core. This improved the stability of the overall nanocomposite as the ligands cannot readily detach under elevated temperatures or changes in pH. Second, the inner PAA core can be used to template many other inorganic nanomaterials in the future making this approach highly generalizable. This star-like polymer template successfully enabled the formation of superparamagnetic iron oxide nanoparticles and showed good dispersion in liquid crystal solvents at useful loading percentages for property improvement. The aim of this polymer-based templating strategy is to incorporate such templated nanomaterials into LC displays to reduce the switching voltage and increase the switching speed, thus producing low energy, high performance display technologies. Second, an HPG-b-polystyrene star-like polymer template (HPG-b-PS) was produced using ring-opening multibranching polymerization (ROMBP) and ATRP. Various batches were crafted possessing different numbers of arms with different lengths. The templating abilities of the various batches were investigated under different reaction conditions. HPG-based templates can successfully template Au nanoparticles through coordination between the inner ether moieties of HPG and the inorganic precursor molecules while the outer PS-arms stabilize and isolate the nanoparticles during formation and thereafter. By adjusting these parameters it was discovered that a critical arm length is required to stabilize templated Au nanoparticles of a given size in solution. It was also found that an optimal solvent composition existed wherein distinct and well-dispersed nanoparticles can be produced. Nanomaterials templated herein demonstrated excellent long term solubility and stability in organic solvents while also retaining their desirable nanoscale properties. The HPG-b-PS template has been successfully applied to directing the growth of Ag and Fe3O4 nanoparticles as well with similar stability measures. The aim of the solution-based HPG-b-PS templating strategy is to extend its use to templating other well-defined industrially-relevant metal oxides that currently can only be produced in energy intensive hydrothermal techniques yielding irregular shapes. Third, azide-functionalized HPG is synthesized (HPG-4-N3-MBE) for use in the production of wholly polymeric soft nanoparticles. This is achieved by UV-induced activation of azides to highly reactive nitrene intermediates that can combine with each other to form stable azo crosslinks. By performing the UV-induced crosslinking in a dilute regime, it is possible to promote intramolecular crosslinking to yield small unimolecular and multimolecular particles that remain well dispersed in solution. Particle formation occurs fairly quickly over less than 40 minutes. The aim of this work is to produce biocompatible soft nanoparticles that can serve as nanocarriers for drugs and other functional molecules for targeted drug delivery and water remediation. From only a few simple, robust reactions including ATRP, click chemistry and nitrene homocoupling, and high functionality polyols such as β-CD and HPG, it is possible to craft a diverse array of both hard and soft nanocomposite structures easily and at low cost. By understanding how polymer architecture and chemical functionality inform on the resulting nanostructures, several different interesting materials have been realized by applying different post polymerization modification strategies to high functionality polyols to yield polymer-tethered inorganic nanoparticles, promesogen-capped nanoparticles and wholly polymeric soft nanoparticles. These materials have potential applications in optoelectronics, surface/film modification, antifouling and biotechnology among many others.