Novel Polymeric Materials: Double-brush Copolymers and Polyelectrolyte Nanocages
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This dissertation focuses on the development of two types of novel polymeric materials, brush polymers and polymeric nanocages. Brush polymers, which are also called densely grafted polymers or molecular brushes, can be synthesized by "grafting from" using polyinitiator, by "grafting through" using macromonomer, or by "grafting onto" using polyfunctional coupling agent. Based on living polymerization techniques, "grafting through" methodology was selected to synthesize the double-brush copolymers (DBCs). Three types of diblock macromonomers with norbornene (NB) functionality at block junctions were prepared by strategic combinations of organic reaction and polymerization. Due to its high functional group tolerance and high reactivity, Grubbs 3 rd generation catalyst was used to polymerize the macromonomers, yielding DBCs. Living polymerization techniques, such as ring-opening polymerization (ROP), reversible addition–fragmentation chain-transfer (RAFT) polymerization, and ring-opening metathesis polymerization (ROMP), were used to achieve good control of the polymeric nanostructures. By using simultaneous RAFT polymerization of styrene (St) and ROP of lactide (LA), well-defined polystyrene/polylactide (PSt/PLA)-based macromonomer was prepared. The PSt/PLA DBCs were synthesized by ROMP of the macromonomer precursor. The Janus conformation of DBCs was verified by TEM after thermal annealing. By changing side chain structure, amphiphilic PLA/poly(ethylene oxide) (PLA/PEO)-based DBCs were prepared and used as surfactants in miniemulsions. As compared with the macromonomer precursor, these DBCs led to significantly more stable miniemulsions. Using RAFT polymerization followed by ROMP, PSt/PEO-based DBCs were synthesized and further utilized as both surfactants and polyfunctional RAFT agents in miniemulsion polymerization of St. Without the presence of cross-linker, miniemulsion polymerization yielded well-defined polymeric nanolatexes with narrow size distributions. With the presence of divinylbenzene (DVB) as the cross-linker, the miniemulsion systems were destabilized during cross-linking polymerization, resulting in vesicular polymeric nanocapsules as the major product. As hollowed polymeric nanoparticles, polymer nanocages have been used for the encapsulation and delivery of active ingredients. With positively charged surface, polyelectrolyte nanocages may have promising applications in gene delivery and other areas. Polyelectrolyte nanocages with monolayer-thick shells were synthesized by interfacial cross-linking of monolayer of vinyl-functionalized surfactant molecules adsorbed by crystallized miniemulsion droplets. The monolayer-thick shell of these nanocages was verified by AFM analysis.