Rational design of self-assembled nanostructures based on polymers synthesized via aqueous reversible addition-fragmentation chain transfer polymerization

Stacey Kirkland York

Abstract

Recent advances in reversible addition-fragmentation chain transfer (RAFT) polymerization have allowed the rational, bottom-up design of biorelevant assemblies. Utilizing foresight, polymers can be tailored to self-assemble into nano-, micro-, and macroscopic structures. Given the size scale on which rationally-designed polymers can be tailored, they hold significant promise in the biomedical field. For example, nanoscale materials can be designed to carry small-molecule and gene therapeutics while macroscopic structures can be tailored for cell growth scaffolds. The design process begins by selecting monomers, chain transfer agents, and reaction conditions which will yield the desired polymer architecture and composition. The work herein builds on these concepts and may be divided into three sections. In the first section, the synthesis of narrowly dispersed, temperature-responsive BAB block copolymers capable of forming physical gels under physiological conditions is described. A difunctional trithiocarbonate was utilized in the aqueous reversible addition fragmentation chain transfer (RAFT) polymerization of the BAB block copolymer, allowing a two-step synthetic approach to obtain a triblock copolymer with symmetrical outer blocks. The outer B blocks of the triblock copolymers consist of poly( N -isopropylacrylamide) (P(NIPAM)) and the inner A block consists of either poly(acrylamide) (P(AM)) or poly(N,N -dimethylacrylamide) (P(DMA)). The copolymers form reversible physical gels above the phase transition temperature of P(NIPAM) at concentrations as low as 7.5 wt% copolymer. Mechanical properties similar to that of collagen, a naturally occurring polypeptide used as a three dimensional in vitro cell growth scaffold, have been achieved. The mechanical properties of the gels as a function of solvent, polymer concentration, and inner block length are discussed. Structural information about the gels was obtained through pulsed field gradient NMR experiments, confocal microscopy, and small angle x-ray microscopy. In the second section, the reversible formation of ordered physical gels from stimuli-responsive ABA [A=P(DMA), B= P(NIPAM))] triblock copolymers is investigated utilizing dynamic light scattering, small angle x-ray scattering, and low-shear rheometry. As the temperature is increased above the phase transition temperature of the P(NIPAM) segment, triblock copolymers under a critical molecular weight are capable of packing into body-centered cubic arrays. Rheometric tests indicate that the storage moduli of the gels at 50oC are inversely related to the molecular weight of the polymer. In addition, cyclic heating of polymer solutions demonstrates the fast, reversible nature of the physical gelation. In the third section, the facile synthesis of polymer-stabilized Au nanoparticles (AuNPs) capable of forming neutral, sterically stable complexes with small interfering RNA (siRNA) is reported. The amine-containing cationic block of poly(N -2-hydroxypropyl methacrylamide-block-N -[3-(dimethylamino)propyl] methacrylamide) was utilized to promote the in situ reduction of Au+3 (NaAuCl4 ) in solution to Au0 (Au nanoparticles). Subsequently, this nanostructure was utilized to bind siRNA while the nonimmunogenic, hydrophilic block provided steric stabilization. Significant protection against nucleases was demonstrated by enzymatic tests while gene down-regulation experiments indicated successful delivery of siRNA to cancerous cells.