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Abstract

The research work presented in this thesis focuses on the synthesis, characterization and preparation of biologically responsive polymers and their nanoparticles containing sulfur in the main chain. Polymerization using click chemistry is relatively new, simple and easy way for the synthesis of linear polymers. Since the discovery of click reactions a decade ago, researchers were much interested in coupling small molecules but eventually more reports have been published in the recent past were polymers and hyper branched structures were prepared by clicking multifunctional groups. Stimuli responsive linear polymers synthesized by click chemistry is one of the less discussed and interesting fields for polymer chemists due its feasibility and unseen potentials. Three different libraries of polymers, polysulfides, poly (β-hydroxy thioethers) and poly(β-thioesters), were synthesized by thiol-yne reactions, thiol-epoxide ring opening polymerization and thiol-ene reactions.

Intensive research is carried out in the area of biologically responsive nanocarries. Many different approaches and methodologies were adapted in the past years for the development of polymer based drug delivery systems. Stimuli based on enzyme response, chemical, pH or temperature changes are already been thoroughly exploited. Response to oxidation is much less investigated, even though there are few literatures based on oxidation sensitive materials there is still a wide area to be explored and understood. Hydrophobic polymer chains of some of the synthesized polysulfides and poly(β-hydroxy thioethers) can be transformed into more hydrophilic ones by oxidation of sulfur to sulfoxides or sulfones using mild oxidizing agents like hydrogen peroxide. A hydrophobic drug/dye encapsulated in the polymer can be thereby released upon an oxidative trigger. Herein polysulfides and poly(β-hydroxy thioethers) polymers were used to prepare nanoparticles by nanoprecipitation, single emulsion and double emulsion techniques. The nanoparticle morphology, size, zeta potential and stability depend on the method of particle preparation which was optimized to meet the final applications. Oxidation responsive drug/dye release was studied under pathological and physiological concentration of hydrogen peroxide. Cellular uptake and cell viability was studied using Hela cells and HUVEC.

Biodegradable polymers are always a good choice for medical applications. In another approach a new library of poly(β-thioesters) was synthesized via base catalyzed thiol-ene polymerization. The presences of β-thioesters bonds make these polymers liable to hydrolysis and hence biodegradable. Synthesized polymers were further used to prepare surfactant stabilized nanoparticles by nanoprecipitation. Nanoparticle size and surface charge was controlled by changing parameters like polymer and surfactant concentrations. Encapsulated drug/dye release kinetics was then studied; an accelerated release of the payload was observed in more acidic condition and lesser in neutral pH.

In conclusion, three different libraries of linear polymers were synthesized using thiol-yne/ene and thiol-epoxide ring opening polymerization. Nanoparticles were then prepared using these polymers and oxidative and pH dependent stimuli response was studied. One of the interesting aspects of this kind of polymerization is its simplicity, mild conditions and less work out procedures. Some of these polymeric nanoparticles efficiently respond to the stimuli applied, release kinetics and nanoparticle degradation were also studied. Cellular uptake and cell viability results confirms good uptake of nanoparticles with minimal toxicity.