Synthesis of poly(vinyl pyrrolidone-B-vinyl alcohol) double hydrophilic block copolymers for direct preparation of core-shell magnetic nanoparticles

In this thesis, poly(vinyl pyrrolidone-b-vinyl alcohol) coated core-shell magnetite nanoparticles (MNPs) were prepared via simultaneous crosslinking and encapsulation with Fenton reaction. For this purpose, poly(vinyl pyrrolidone-b-vinyl acetate) (P(NVP-b-VAc)) block copolymers with different chain lengths were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization which is a very convenient way to obtain amphiphilic block copolymers. As vinyl alcohol monomer does not exist, poly(vinyl alcohol) were obtained through the hydrolysis of poly(vinyl acetate) (VAc) block. In order to acquire polymer coated magnetite nanoparticles, two different pathways were employed (Series A and Series B). In Series A, poly(N-vinyl pyrrolidone) (PNVP) was synthesized via RAFT polymerization as a macro-CTA(macro-Chain Transfer Agent), this block was kept constant and then various P(NVP-b-VAc) block copolymers were prepared with different PVAc chain lengths via RAFT chain extention was achieved. In Series B, poly(vinyl acetate) was obtained first as and then block copolymerization was performed with different NVP monomer concentrations. In doing so, we wanted to compere effects of core and corona segment lengths on the size of resulting MNPs. In the second part of the study, the crosslinking of block copolymers and encapsulation of iron salts (II/III) were achieved simultenously by Fenton reaction. As it is well known the Fenton reagent (Fe(II)+H2O2) results in formation of iron (III) and a free – OH radical. During the interaction –OH radical abstracts one hydrogen from poly(vinyl alcohol) and generates radicals on the backbone. Recombination of the radicals provides an abtrapt crosslinking. The key point of the process is that the use of 1.5 equivalent of the iron (II). So that, at the end of Fenton reaction a mixture of 1 mol of iron (II) and 2 moles of iron (III) oxides were generated in the capsules of crosslinked PVA. The resulting product was then interacted with ammonia solution to form magnetite (Fe3O4) entrapped within the core of particles. Such a reaction of PVA with Fenton reagent and stoichiometric control of iron (II)/ iron (III) ratio has not been reported so far. The overall result of the process is direct formation of MNPs. Various characterization methods were employed through every step. In between the polymerization steps, 1H NMR (Nuclear Magnetic Resonance), FT-IR (Fourier Transform Infrared Spectrophotometry) spectra for all the polymers and 13C NMR spectra for block copolymers were taken as well as GPC (Gel Permeation Chromatography) chromatograms for molecular weight and PDI (Polydispersity Index) data. The size and shape of MNPs were determined by DLS (Dynamic Light Scattering) measurements and TEM (Transmission Electron Microscopy) images. Finally, TGA (Thermogravimetric Analysis) were employed in order to investigate polymer/magnetite ratio in the core-shell nanoparticle. From the DLS measurements, size of MNPs were obtained as nanoscale materials. The size of MNPs appears to change with different PVA segment lengths. Whereas, the crosslinking density or the chain length of PNVP segments does not have a significant effect on the size of MNPs as expected. This conclusion is supported by SEM (Scanning Electron Microscopy) and TEM images. In the TEM images, the contrast between polymer and magnetites prove that magnetite nanoparticles are formed inside the crosslinked polymers. In those images polymer coatings appear lighter as the magnetites are appeared to be dark dots. Finally, from TGA thermograms, organic and inorganic material content of polymer coated MNPs were obtained. The unburnt residue indicates the amount of magnetite inside the polymer matrix. However, the experimental data is somewhat bigger than the theoretical quantity of magnetite calculated from 1H NMR chain lengths. This occurance is due to the inability of polymers to fully burn under nitrogen atmosphere. In any case, the thermograms show the inorganic residue indicating the presence of magnetite nanoparticles.