Photochemical strategies for macromolecular syntheses in near infrared region
Different photoinitiating systems were carried out for the design of well-defined macromolecular structures using near-infrared (NIR) light via Atom Transfer Radical Polymerization (ATRP) and Copper-Catalyzed Azide-Alkyne Click Reactions (CuAAC). Lower-oxidation state copper salts are required for both processes, however; this brings some limitations into the reactions. In order to mitigate these limitations, the use of light energy has been utilized by starting with higher-oxidation state copper salts and reducing them via photoinduced electron transfer reactions (PET). In addition, there have been huge affords to conduct these reactions in metal-free conditions. For the first time, the use of NIR irradiation for the controlled radical polymerization system via ATRP process in the absence of inorganic catalyst was demonstrated in this thesis. In order to do that, the emitting ultraviolet (UV) light from up-conversion nanoparticles (UCNPs) after NIR laser irradiation was used to excite UV photoinitiator which is 2-isopropyl thioxanthone (ITX). The initiation mechanism involves PET from the electron donor amine to the excited state of ITX in order to form radical anion of ITX. The generated radical anion of ITX reduces the alkyl initiator to generate initiating radicals which allow the addition of the monomers and the polymerization to be proceeded. Additionally, the use of NIR sensitizers (Sens) for both reactions (ATRP and CuAAC), for the first time, were successfully investigated in this thesis. The initiation mechanism involves a photoinduced electron transfer reaction between the exited state of a heptamethine cyanine absorber and CuII/Ligand complex. The oxidation potential of the Sens resulted in a negative free enthalpy in combination with the CuII/Ligand complex considering successful electron transfer reactions after excitation by near-infrared light emitting diodes (NIR LEDs). After the photochemical reaction, the back electron transfer was presented which allows the system to be a catalytic cycle. The characterization of the polymers obtained were analyzed by spectroscopic and chromatographic methods. Controlled molecular weight characteristics were observed and block copolymer synthesis was achieved in both ATRP and CuACC reactions via NIR exposure. As an alternative, similar photocatalytic system using carbon nanodots (CDs) which were prepared from seaweeds was shown to be able to catalyze free radical and controlled radical polymerization processes using blue light emitting diodes. The use of these materials was included in this work because they can be an alternative as biocompatible and green photocatalyst. They showed no toxicity and good efficiency for the synthesis of tailor-made materials.