Biyo bazlı 1,3-benzoksazinlerin sentezi ve karakterizasyonu
Synthesis and characterization of bio-based 1,3-benzoxazines
Yazar: MERVE ERDEĞER
Danışman: DOÇ. DR. FÜSUN ŞEYMA KIŞKAN
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
Dizin: Biyopolimerler = Biopolymers ; Organik kimya = Organic chemistry ; Yeşil kimya = Green chemistry

Özet:
Benzoksazinler, oksazin halkasının benzen halkasıyla birleşmesiyle oluşan heterosiklik bileşiklerdir. Benzoksazinlerin bazı izomerleri, tedavi edici olarak ilaç endüstrisinde önemli bir yere sahiptir. Ayrıca polibenzoksazinler, polimer endüstrisinde yaygın olarak kullanılan epoksi, poliester ve fenolik reçineler gibi reçinelere önemli bir alternatif oluşturmaktadır. Benzoksazin reçineleri termal kararlılık, düşük su emme, yanmazlık ve sıfıra yakın hacim değişimi, kimyasal direnç ve düşük duman üretimi gibi üstün özelliklerinden dolayı otomotiv, havacılık ve elektronik endüstrilerinde kompozit ve kaplama malzemesi olarak, ayrıca yapıştırıcı ve enkapsülan olarak kullanılmaktadır. Benzoksazin halkasına farklı gruplar eklenerek kullanım alanlarına göre özellikleri değiştirilebilmektedir. Benzoksazin sentezinin kolaylığı, sentezde kullanılan başlangıç bileşiklerinin ucuz ve kolaylıkla ulaşılabilir olması nedeniyle literatürde çok çeşitli türlerde benzoksazinler bulunmaktadır. 
Polimer sektöründe genellikle petrol kaynaklı ham maddeler kullanılmaktadır. Yenilenemeyen fosil yakıtlardan elde edilen kimyasalların yüksek karbon emisyonu ve çeşitli şekillerde doğal yaşama zararları bulunmaktadır. Son yıllarda karşımıza çıkan atık sorunları, çevresel etkiler ile insan ve çevre sağlığına zararlı kimyasalların kullanımının hükümetler tarafından kısıtlandırılması sebebiyle polimer sektöründe sürdürülebilirlik ilkeleri benimsenmiştir. Bu gibi nedenlerden dolayı endüstride yaygın olarak kullanılan petrol bazlı ürünlerin biyo bazlı, doğada çözünebilen veya geri dönüştürülebilen alternatiflerinin geliştirilmesi popülerlik kazanmıştır. 
Benzoksazin sentezinde sıklıkla kullanılan kimyasallar insan sağlığına zararlıdır ve endokrin sistemini bozucu olarak kabul edilmektedir. Bu tür başlangıç maddeleri yerine yenilenebilir ham madde kaynaklı kimyasalların kullanımı üzerine çalışmalar yoğunlaşmaktadır. Bu çalışmalarda çoğunlukla bitki bazlı fenoller ile birlikte yenilenebilir aminlerin sentezi sonucunda biyo bazlı benzoksazinler elde edilmektedir. 
Bu tez çalışmasında, biyo bazlı fenoller kullanılarak benzoksazin sentezleri yapılması hedeflenmiştir. Tezin ilk aşamasında tepkime koşullarını belirlemek ve tepkime çözücüsünü  seçmek amacıyla petrol bazlı olan fenol ve Jeff amin ile 1,3-benzoksazin sentezi yapılmıştır ve analizlerde referans olarak kullanılmıştır. Kloroform çözücüsü yapılan denemeler sonucunda en verimli sonucu vermiştir. Çalışmanın devamında, Jeff amin ile biyo bazlı fenol olarak vanilin, timol ve karvakrol kullanılarak Mannich kondenzasyon tepkimesi sonucunda biyo bazlı 1,3-benzoksazin bileşikleri başarıyla sentezlenmiştir. Bileşiklerin yapıları 1H-NMR, 13C-NMR, FT-IR gibi spektroskopik yöntemlerle kanıtlanmıştır. Ayrıca bileşiklerin DSC, TGA cihazlarıyla termal gravimetrik analizleri de yapılmıştır. Bu yöntemler sayesinde bileşiklerin, camsı geçiş ve polimerleşme sıcaklıkları, kül verimleri, aktivasyon enerjisi gibi özellikleri karşılaştırılmıştır.

Abstract:
Benzoxazines are formed by the combination of an oxazine ring with a benzene ring, categorizing them under the class of heterocyclic compounds. These compounds hold significant value in various industries due to their unique properties and versatile applications. Several isomers of benzoxazines are particularly noteworthy in the pharmaceutical industry, where they exhibit a range of therapeutic effects and are utilized in the formulation of numerous drugs.
Beyond their pharmaceutical applications, polybenzoxazines have emerged as a considerable alternative to traditional resins like epoxy, polyester, and phenolic resins, which are widely used in the polymer industry. Polybenzoxazine resins are highly regarded for their superior properties, including exceptional thermal stability, low water absorption, non-flammability, near-zero volume change upon curing, chemical resistance, and low smoke production during combustion. These attributes make them highly desirable for use in composite materials, coatings, adhesives, and encapsulants, particularly within the automotive, aerospace, and electronics industries.
The ability to modify the properties of benzoxazines by introducing various functional groups to the oxazine ring further enhances their adaptability to different applications. This customization allows for the tailoring of benzoxazine-based materials to meet specific performance requirements, thereby broadening their scope of use. The ease of synthesis and the availability of inexpensive and accessible starting materials have led to the development of a wide array of benzoxazine derivatives documented in the literature.
The use of petroleum-based raw materials is common in the polymer industries. Chemicals derived from non-renewable fossil fuels have high carbon emissions and damage the environment in several ways. In recent years, sustainability principles have been adopted in the polymer industry due to waste problems, environmental impacts, and the restriction by governments of the use of chemicals harmful to human health and nature. These environmental and regulatory pressures have encouraged the development of bio-based, biodegradable, and recyclable alternatives to petroleum-based products widely used in the industry.
The chemicals frequently used in the synthesis of benzoxazines are toxic to human health and are considered to disrupt the endocrine system. To address these concerns, studies are intensifying on the use of renewable raw materials as starting materials instead of damaging fossil fuel-based chemicals used in the benzoxazine industry. In these studies, renewable amines and plant-based phenolic compounds are predominantly used in the synthesis of bio-based benzoxazines. This shift towards green chemistry not only reduces the environmental footprint of benzoxazine production but also aligns with global sustainability goals.
Researchers are actively exploring the potential of various natural sources to develop bio-based benzoxazines. A prominent method in the literature involves converting petroleum-based phenol into bio alternatives, given that bio-based phenols are more abundant in nature than other starting compounds. Consequently, numerous syntheses of benzoxazines from bio-based phenols have been conducted, primarily focusing on plant-derived sources. Among the most commonly used bio-based phenols are vanillin, eugenol, coumarin, catechol, chavicol, arbutin, thymol, and carvacrol. The structure of the plant-based phenol employed in the synthesis significantly influences the properties of the resulting benzoxazines, enabling the creation of materials with diverse characteristics. Despite this variability, a common feature of these bio-based benzoxazines is their excellent thermal properties, making them highly suitable for a range of advanced applications.
Moreover, lignocellulosic phenols like lignin and guaiacol, along with long carbon chain phenols such as cardanol and urushiol, are also being utilized in the synthesis of bio-based benzoxazines. In addition to these well-known phenols, researchers have recently begun to explore the potential of less commonly used bio-based phenols, such as sesamol, diphenolic acid, terpenediol, and daidzein, to develop innovative bio-based benzoxazine monomers. This emerging research is unveiling new possibilities for creating advanced materials with unique properties, further expanding the horizons of sustainable polymer science.
Alongside bio-based phenols, bio-based amines are also widely utilized in the sustainable synthesis of benzoxazine monomers. Prominent examples of these amines include furfurylamine, stearylamine, chitosan, and amine derivatives of rosin. Given that amines derived from natural sources are less common than phenols, industry frequently employs furfurylamine, known for its high cross-linking capacity, and stearylamine, which features long alkyl chains. The type of amine used can impart desirable properties to the resulting benzoxazine, enabling its application in a wide range of fields. These applications include hydrophobic and anti-corrosion coatings, as well as the production of composite matrices requiring thermal stability. The versatility provided by these bio-based amines enhances the overall functionality and sustainability of benzoxazine materials.
Additionally, although not very common, there are a few publications where bio-based benzaldehyde is used instead of formaldehyde in benzoxazine synthesis. The aim of using benzaldehyde is to produce a completely bio-based benzoxazine. However, its widespread use is limited due to the reduced reaction efficiency caused by the steric hindrance of the benzene ring.
In this thesis, it is aimed to synthesize benzoxazines using bio-based phenols. In the first stage of the thesis, 1,3-benzoxazine was synthesized with petroleum-based phenol and Jeff amine to determine the reaction conditions and select the reaction solvent and was used as a reference in the analyses. Jeffamine was selected as the amine derivative in the study to offer flexibility and durability to the structure, owing to the presence of its long alkyl chains. Afterwards, bio-based 1,3-benzoxazine compounds were successfully synthesized as a result of the Mannich condensation reaction using Jeff amine and vanillin, thymol and carvacrol as bio-based phenols. 
The structures of the compounds have been proven by spectroscopic methods such as 1H-NMR and 13C-NMR. In addition, thermal gravimetric analyzes of the compounds were performed with DSC and TGA devices. With these methods, the properties of the compounds, such as glass transition and curing temperatures, char yields and activation energy, were compared. 
According to the 1H-NMR and 13C-NMR results, the chemical structures of all synthesized compounds have been confirmed, demonstrating compatibility with the NMR spectra found in the literature. 
Benzoxazine monomers undergo thermal polymerization at high temperatures through cationic ring-opening polymerization. This process typically occurs between 180–250°C, depending on the specific structure of the benzoxazine. The polymerization is exothermic and can be monitored using differential scanning calorimetry (DSC). DSC thermograms are used to measure the peak polymerization (curing) temperatures of the monomers, providing valuable insights into their thermal behavior.
According to the DSC results, the polymerization temperature (Tmax) for the phenol-based benzoxazine compound was measured at 235.24℃. In comparison, the Tmax for vanillin-based benzoxazine was 211.72℃, for thymol-based benzoxazine was 262.84℃, and for carvacrol-based benzoxazine was 255.59℃. The presence of the aldehyde group in vanillin contributes to the polymerization process, resulting in a lower Tmax. Furthermore, the results indicate that benzoxazines synthesized with thymol and carvacrol exhibit higher polymerization temperatures than those synthesized with the reference phenol. This suggests that bio-based benzoxazines possess superior thermal properties.
From the peak temperatures obtained by scanning at specific rates, the activation energy (Ea) of the monomer can be calculated using the Kissinger and Ozawa equations, which are derivatives of the Arrhenius equation. When these calculations were performed, the activation energies of the ring-opening polymerization reactions were found to be consistent across both methods, aligning well with values reported in the literature.
Thermal stability of the synthesized 1,3-benzoxazine monomers after thermal polymerization was measured by thermal gravimetric analysis (TGA). For the phenol-based polymer, the T5% value is 341℃, the T10% value is 371℃, and the Tmax value is 405℃. In the case of vanillin-based polybenzoxazine, the T5% value is 254℃, the T10% value is 275℃, and the Tmax value is 541℃. For thymol-based polybenzoxazine, the T5% value is 290℃, the T10% value is 312℃, and the Tmax value is 375℃. Lastly, for carvacrol-based polybenzoxazine, the T5% value is 231℃, the T10% value is 248℃, and the Tmax value is 306.4℃.
Upon examining the char yields of the obtained polymers, it was observed that the phenol-based polymer exhibited a 45.5% char yield, while the thymol-based polymer showed a 23% char yield. This difference is attributed to the substituent occupying the para position in thymol, which results in a lower degree of cross-linking.
In summary, novel polybenzoxazines with exceptional thermal stability were successfully synthesized using bio-based phenols vanillin, thymol, and carvacrol alongside Jeffamine as the amine component.