Synthesis and self-assembly behavior of multi-arm star amphiphilic polyelectrolyte systems and their applications in the delivery of biomolecules [thesis]

Elaine Weiguo He
Amphiphilic polyelectrolytes have attracted increasing attention due to their potential applications in the field of pharmaceutical science and biotechnology. The biocompatible polymers can self-assemble into nano-scale micelles, which provide similar structure and function as natural carriers. Biocompatibility, size and morphologies of the vehicles are important considerations in the design of suitable delivery systems, where they can be accomplished by manipulating block compositions,
more » ... mpositions, structure and lengths. Novel multi-arm star shape PEO was grafted with weak polybase or polyacid to produce stimuli-responsive amphiphilic polyelectrolytes. The three-dimensional branched structure offers greater proportion of end functional groups compared to linear structure of identical molecular weights, which induces greater solubility and more attractive aggregation behavior. The four-arm poly(ethylene oxide)-b-poly(2-(diethylamino)ethyl methacrylate) (PEOb-PDEAEMA) and poly(ethylene oxide)-b-poly(methacrylic acid) (PEO-b-PMAA) block copolymers with tetrahedronal structure were successfully synthesized by the atom transfer radical polymerization technique to yield well-defined amphiphilic block copolymers of narrow polydispersity. The polymerization degree of four-arm PEO-b-PDEAEMA block copolymer was determined from the relative intensities of NMR spectra at 3.66 ppm (-CH 2 CH 2 O of the PEO block) and 4.02 ppm (i ATTENTION: The Singapore Abstract OCH 2 CH 2 N-of the PDEAEMA block), obtained a chemical structure of 4-arm PEO 56 -b-PDEAEMA 74 . The polymerization degree of the four-arm PEO-b-PtBMA copolymers was calculated from the peak intensity ratio of 3.58 ppm (-OCH 2 CH 2 O-) of PEO block and 1.40 ppm (-C(CH 3 ) 3 ) of tBMA block, to yield 4-arm PEO 56 -b-PtBMA 88 block copolymer. The self-assembly behaviors of four-arm PEO-b-DEAEMA in aqueous solution were studied as a function of pH. At low pH, DEAEMA (2-(diethylamino)ethyl methacrylate) segments were protonated, and the polymeric chains exist as fully extended star shape unimers. By increasing the pH of environment, the DEAEMA groups were deprotonated and became hydrophobic resulting in the formation of spherical core-shell micelle comprising of a hydrophobic DEAEMA core surrounded by a folded hydrophilic four-arm PEO corona. The hydrodynamic size of the micelle and conformational transitions was studied in detail at various pH. In addition, the ionic strength in solution also controls the self-assembly behavior of amphiphilic polymeric systems since it mediates the electrostatic interactions between the macroions, counterions, and solvent molecules. The effect of salt on the aggregation behavior of four-arm polyelectrolyte was investigated, where salt concentration alters the electrical potential surrounding the polyions and suppresses electrostatic repulsions of charged polymeric segments. ii ATTENTION: The Singapore Abstract The star shape four-arm PEO-b-PDEAEMA block copolymer was evaluated as a potential vector for gene delivery since it could condense therapeutic DNA forming a core-shell structure that can cross a number of biological barriers. At physiological pH, the star shape four-arm PEO-b-PDEAEMA block copolymer possessed positively charged amine groups that interacted with negatively charged plasmid DNA to form polymer/DNA complexes. The mechanism and physicochemical properties of the complex formation were investigated at various molar ratios of amine and DNA segments (N/P). The capacity of the star block polymer to condense DNA was demonstrated through gel electrophoresis and ethidium bromide exclusion assay. The hydrodynamic radius of polyplexes were investigated by dynamic and static light scattering, where approximately 15 polymeric chains were required to condense a plasmid DNA. The addition of monovalent salt into the polymer/DNA mixture significantly alters the size of the complexes, which would have an impact on cell transfection. The conformation transition of four-arm PEO-b-PMAA block copolymer over the course of neutralization was investigated. The multi-arm block copolymer existed as an extended unimer at high pH due to the negatively charged carboxylate and hydrophilic PEO segments. The block copolymers self-assembled into core-shell micelles and large spherical aggregates that flocculated at very low degree of neutralization (α). Such behavior was controlled by the fine balance of electrostatic, iii ATTENTION: The Singapore Abstract iv hydrophobic and hydrogen bonding interaction forces. The thermodynamic parameters obtained from the isothermal titration calorimetric technique at different salt concentrations indicated that the energy to extract a proton from a charged polyion was reduced with the addition of salt that favors the neutralization process. The four-arm PEO-b-PMAA block copolymer serves as a reservoir for drug loading through the combination of electrostatic attraction, hydrogen bonding and hydrophobic interactions. With the star shape architecture, the polymer possesses higher densities of terminal functional groups and three-dimensional tetrahedronal structure that induces different association properties and interactions with drug compared to linear structured polymer of identical molecular weights. The negatively charged carboxylate on polymer chains interact with cationic drug through electrostatic interaction to form polymer/drug complexes that were stabilized by biocompatible hydrophilic PEO segments. The hydrodynamic radius (R h ) of the polymer/drug complexes varied from 32 nm to 55 nm for different amounts of drug in the present of polymer solution, which is a suitable size for drug delivery. Drug selective membrane was prepared and the high efficiency selective electrode system was used to monitor the release kinetics of IPH from multi-arm PEO-b-PMAA star polymer. The release exponent was greater than 0.5 indicating non-Fickian type diffusion mechanism and the release behavior was dominated by the chain relaxation induced by ion exchange with the effect of pH. ATTENTION: The Singapore
doi:10.32657/10356/19256 fatcat:uapgi47yk5e5hnaovd62y3e4ry