Işıkla tetiklenen ters elektron gereksinimli Diels-Alder reaksiyonu ile tek zincir polimer nanoparçacıkların hazırlanması
Fabrication of single-chain polymer nanoparticles by light-induced electron demand Diels-Alder reaction
Yazar: ADALET NUR ALTUNKAYA
Danışman: DOÇ. DR. MUHAMMET ÜBEYDULLAH KAHVECİ
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Kimya Ana Bilim Dalı / Kimya Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Kimya = Chemistry ; Polimer Bilim ve Teknolojisi = Polymer Science and Technology

Özet:
Doğadan alınan ilham kimya alanının gelişmesinde büyük bir rol oynamaktadır. Son yıllarda öne çıkan tek zincir polimer nanoparçacıklar (SCNP) da proteinlerin katlanıp düzenlenmesinden ilham alınarak geliştirilmiştir. Tek zincir polimer nanoparçacıklar, tek bir polimer zincirinin özgün bir şekilde katlanması ve çapraz bağlanması sonucu ortaya çıkan nano ölçekli yapıları ifade etmektedir. Bu katlanma, proteinlerin hidrojen bağları ile katlanmasına benzemektedir, ancak polimerlerin katlanması için radikal kenetlenme, metal ligasyonu, click kimyası, supramoleküler etkileşimler gibi birçok kovalent, dinamik kovalent ve kovalent olmayan çapraz bağlanma yöntemleri kullanılmaktadır. Bu reaksiyonlar içinde, özellikle ılımlı koşullarda yüksek verimlilikle kovalent bağlar oluşturan click kimyası dikkat çekmektedir. Click kimyası başlığı altında siklokatılma, nükleofilik halka açılma, karbon-karbon çoklu bağlara katılma ve aldol olmayan karbonil bileşiklerinin reaksiyonları vardır. Siklokatılma reaksiyonlarının bir örneği olan ters elektron gereksinimli Diels-Alder (IEDDA) reaksiyonu elektronca zengin bir dienofil ve elektronca fakir bir konjuge dien arasında meydana gelen önemli bir [4+2] katılma reaksiyonudur. Genellikle 1,2,4,5-tetrazinler ve olefinler arasında gerçekleşen IEDDA reaksiyonunun biyoortogonal olması, hızlı kinetiğe ve yüksek verimliliğe sahip olması gibi avantajları bulunmaktadır. Işıkla tetiklenen ters elektron gereksinimli Diels-Alder (foto-IEDDA) reaksiyonlarında ise geleneksel IEDDA reaksiyonları ışıkla kontrol edilebilmektedir. Dihidtrotetrazin (dHTz) molekülleri IEDDA reaksiyonu için inaktifken bir fotooksidan varlığında ışık ile uyarıldığında yükseltgenerek tetrazin (Tz) molekülüne dönüşmekte ve IEDDA reaksiyonu için reaktif hale gelmektedir.
Bu tez kapsamında, polimer üzerinde bulunan 6-(6-(piridin-2-il)-1,4-dihidro 1,2,4,5-tetrazin-3-il) piridin-3-amin (PPA-dHTz) ve norbornen (Nb) grupları arasında in situ
olarak gerçekleşen foto-IEDDA reaksiyonu kullanılarak molekül içi çapraz bağlı nanoparçacıkların hazırlanması için yeni bir strateji geliştirilmiştir. İlk olarak, 4-okso-4-((6-(6-(piridin-2-il)-1,2,4,5 tetrazin-3-il)piridin-3 il)amino)bütanoik asit (PPA-dHTz-COOH) sentezlenmiştir, ardından poli(metil metakrilat-ko-hidroksietil metakrilat) P(MMA-ko-HEMA), tersinir katılma ayrılma zincir-transfer polimerizasyonu (RAFT) kullanılarak sentezlenmiştir. Daha sonra, PPA-dHTz-COOH ve bisiklo[2,2,1]hept-5-en-2-karboksilik asit (Nb-COOH), esterifikasyon reaksiyonu yoluyla polimerin yan zincirlerine bağlanmıştır. Son olarak, SCNP'leri hazırlamak için öncü polimer, P(MMA-ko-HEMA)-g-PPA-dHTz/Nb, çözeltisi metilen mavisi (8μM) varlığında etanol içerisinde 0,5 mg/mL konsantrasyonda  hazırlanmıştır. Polimer, kırmızı lazer ile  (λ=680nm, 0,3W/cm2) uyarılmış ve dihidrotetrazinlerin tetrazine oksidasyonu UV-Vis spektrofotometresi ile takip edilmiştir.
Bu tez kapsamında, sentezlenen bileşikler, nükleer manyetik rezonans (NMR) spektroskopisi, jel geçirgenlik kromatografisi (GPC) ve kızılötesi spektroskopisi (FT-IR) ile karakterize edilmiştir. Hazırlanan SCNP'lerin termal davranışlarını incelemek için  diferansiyel taramalı kalorimetre (DSC), boyut analizleri için dinamik ışık saçılım (DLS) ve morfolojik yapılarını incelemek için taramalı elektron mikroskobu (SEM) kullanılmıştır. Elde edilen sonuçlara göre SCNP'ler başarıyla hazırlanmıştır.

Abstract:
Nanotechnology is a multidisciplinary field that manipulates matter at the nanoscale, which is about 1 to 100 nanometers. This technology leverages the unique physical, chemical, and biological properties that materials exhibit at such a small scale. The importance of nanotechnology lies in its wide-ranging applications across various fields including medicine, electronics, energy, and materials science. In medicine, for example, nanotechnology is used for targeted drug delivery, which allows drugs to be delivered directly to diseased cells, minimizing side effects and improving treatment efficacy. In electronics, it enables the production of smaller, faster, and more efficient devices. Small size of Nanomaterials gives them a much larger surface area to volume ratio, which can lead to novel physical and chemical properties. Nanomaterials can be classified based on their dimensions, which range from zero-dimensional (0D) to three-dimensional (3D). 
Focusing on nanoparticles, these are 0D nanomaterials with all three dimensions in the nanoscale. They have a wide range of applications, including in drug delivery, imaging, and as catalysts due to their high surface area and tunable physical and chemical properties. Nanoparticles can generally be classified into three fundamental categories: carbon-based, inorganic, and organic. Carbon-based nanoparticles encompass nanomaterials entirely composed of carbon elements in their structure. Inorganic nanoparticles, on the other hand, are particles that do not contain carbon, typically based on metals and metal oxides. Organic nanoparticles are commonly referred to as polymeric nanoparticles. Polymeric nanoparticles are based on polymers which are composed of monomers. Polymers are incredibly versatile materials that constitute a fundamental component across a multitude of industrial sectors and consumer products. These materials find extensive applications in fields including but not limited to textile manufacturing, cosmetic formulation, construction engineering, food processing, and healthcare.
Polymeric nanoparticles (PNPs) are particles crafted from polymeric materials, encompassing nano-sized colloidal organic compounds. Among the common morphological structures of PNPs are nanospheres and nanocapsules. Nanospheres are composed of a dense polymeric matrix, while nanocapsules consist of nano-scaled shells made from polymers. PNPs are employed in biomedical fields due to their numerous advantages compared to other nanoparticles, including high stability, capability for sustained drug release profiles, biocompatibility, and non-toxicity. The most common application of PNPs is in drug delivery systems.
In recent years, single-chain polymer nanoparticles (SCNPs), which have gained prominence, are designed by taking inspiration from the folding and organization of proteins. SCNPs refer to nano-scale structures that emerge from the unique folding of a single polymer chain through cross-linking. A fundamental characteristic of SCNP is their large surface to volume ratio due to their small size. This ratio affects how well these tiny particles work in various uses, like in chemical reactions, detecting substances, delivering drugs, and storing energy. SCNPs have a larger surface area when compared to their size. This larger surface area means they can interact more at the molecular level, making them behave differently than larger particles. SCNPs have found applications in many areas such as drug delivery systems, bioimaging, sensors and catalysts. When preparing SCNPs, it is necessary to work under highly diluted conditions during the folding process to prevent intermolecular cross-linking. Unlike cross-linked polymers, SCNPs are well-dispersed in the solvent. 
The folding of SCNPs is smiliar to the hydrogen bonding in protein folding, yet for polymer folding, a variety of covalent, dynamic covalent, and non-covalent cross-linking methods are utilized, such as radical coupling, metal ligation, click chemistry, and supramolecular interactions. Among these reactions, click chemistry, known for forming covalent bonds efficiently under mild conditions, stands out. Click chemistry includes cycloadditions, nucleophilic ring opening, addition to carbon-carbon multiple bonds, and reactions of non-aldol carbon compounds. A notable example of cycloaddition reactions is the inverse electron-demand Diels-Alder (IEDDA) reaction which is a [4+2] addition reaction occurring between an electron-rich dienophile and an electron-poor conjugate diene. The bioorthogonal nature of the IEDDA reaction, typically between 1,2,4,5-tetrazines and olefins, along with its fast kinetics and high efficiency, are key advantages. In photo-induced inverse electron-demand Diels-Alder (photo-IEDDA) reactions, traditional IEDDA reactions can be controlled with light. Dihydrotetrazine (dHTz) molecules are inactive for the IEDDA but can be converted to tetrazine (Tz) molecules upon irradiation in the presene of a photosensitizer leading to a IEDDA reaction.
In this thesis, a strategy for the preparation of intramolecularly cross-linked nanoparticles via an in situ the light-induced inverse electron demand Diels-Alder photo-IEDDA reaction between the 6-(6-(pyridin-2-yl)-1,4-dihydro-1,2,4,5-tetrazin-3-yl) pyridin-3-amine (PPA-dHTz) and norbornene (Nb) moieties on the polymer has been developed. Firstly, 4-oxo-4-((6-(6-(pyridin-2-yl)-1,2,4,5-tetrazin-3-yl)pyridin-3yl)amino) butanoic acid (PPA-dHTz-COOH) was synthesized and poly(methyl methacrylate-co-hydroxyethyl methacrylate) P(MMA-co-HEMA) were synthesized using reversible addition-fragmentation chain transfer polymerization (RAFT). Subsequently, PPA-dHTz-COOH and bicyclo[2.2.1]hept-5-ene-2-carboxylic acid (Nb-COOH) were grafted onto the polymer via an esterification reaction. Finally, the precursor polymer, P(MMA-co-HEMA)-g-PPA-dHTz/Nb, was prepared in solution at a concentration of 0.5 mg/mL in ethanol in the presence of methylene blue (8 μM). The polymer was excited with a red laser (λ=680 nm, 0.3W/cm2), and the oxidation of dihydrotetrazines to tetrazines was monitored using UV-Vis spectroscopy.
The synthesized substances were characterized using nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), and Fourier-transform infrared spectroscopy (FT-IR). Differential scanning calorimetry (DSC) was employed to investigate the thermal behavior of the prepared SCNPs, dynamic light scattering (DLS) for size analysis, and scanning electron microscopy (SEM) for examining their morphological structures. According to the obtained results, single-chain polymer nanoparticles (SCNPs) were successfully prepared.