Publication: Tiyol-epoksi tepkimelerinin hesapsal incelenmesi
Abstract
Kürlenen epoksi reçineler, olağanüstü mekanik mukavemet, yüksek kimyasal direnç, daha az büzülme, mükemmel yapışma ve azaltılmış stres gibi bazı değerli özellikler göstermesi nedeniyle endüstriyel olarak talep edilmektedirler. Öte yandan, kırılgan malzemelerin elde edilmesi bu tür reçinelerin ana dezavantajıdır. Epoksi reçinelerle karşılaştırıldığında, tiyol-en polimerizasyonu, oksijen inhibisyonuna direnç ve homojen bir polimer ağının üretimi gibi bazı belirgin özellikler sağlar. Epoksi reçineler gibi, tiyol-en ürünlerinin de büzülme özelliği belirgindir. Bu nedenle, tiyol-en/ tiyol-epoksi sistemi için ikili kürleme prosedürü geliştirmek, her iki polimerizasyon tekniğinin sinerjik etkisinin yolunu açmıştır. Bu kapsamı altında tiyol-epoksi reaksiyonları, esasen iki ana mekanizmada incelenmiştir: radikal aracılı ve anyonik tiyol-epoksi reaksiyonları. 1. Kuantum kimyasal araçlar kullanılarak tiyol-en/ tiyol-epoksi sistemlerinin çakışma problemini analiz etmek için radikal aracılı tiyol-epoksit reaksiyonları için analog bir tiyol-en mekanizması önerildi. Tiyol-en reaksiyonlarının aksine, tiyollerin epoksitlere katılma reaksiyonun nispeten yavaş olduğu saptanmıştır (hız sabiti değerleri <10-4 M-1s-1). Ancak ikili kürleme sistemlerinin çakışmasına zemin hazırlayan zincir transferinin oldukça hızlı gerçekleştiği saptanmıştır (hız sabiti değerleri >10-2 M-1s-1). Tiil radikallerinin yüksek stabilitesi, epoksi halkası esnekliği ve katılma reaksiyonlarından oluşan alkoksit radikalinin kararsızlığı, reaksiyon enerjisi ve kinetiği için temel unsurlar olarak vurgulanmıştır. Bu çalışmadaki bulgulara dayanarak kürleme aşaması çakışmasını önlemek için sıcaklığın kontrolü ve belirli tiyollerin kullanılmasına dikkat çekilmiştir. 2. Literatür taramasına dayanarak, tiyol-epoksi reaksiyonunun baz katalizli mekanizma ile ilerlediği bildirilmektedir. Bu kapsamında, iyonik reaksiyonların ortamın polaritesinden önemli ölçüde etkilendiğini göz önünde bulundurarak, anyonik reaksiyonları gerçekçi bir şekilde modellemek için ortam polaritesini dikkate alan SMD çözücü hesaplamaları da ek olarak gerçekleştirildi.
Due to its beneficial qualities, which include decreased stress, great adhesion, low shrinkage, strong chemical resistance, and remarkable mechanical strength, curing epoxy resins are in high demand in the industry. However, the primary disadvantage of this type of resin is that it might yield brittle products. Thiol-ene polymerization offers notable advantages over epoxy resins, such as creating a uniform polymer network and resisting oxygen inhibition. However, thiol-ene compounds exhibit less shrinkage stress than epoxy resins. Thus, a synergistic impact of both polymerization processes has been made possible by the development of a sequential dual curing procedure for the thiol-ene/ thiol-epoxy system. Thiol-epoxy reactions are essentially investigated in two primary mechanisms under the purview of this PhD thesis: radical mediated and anionic thiol-epoxy reactions. 1. An analog thiol-ene mechanism was proposed for radical-mediated thiol-epoxide reactions to analyze the overlap problem of the thiol-ene/ thiol-epoxy systems using computational approaches. The addition reaction to epoxides proceeds more slowly than the thiol-ene reactions (the rate constants <10-4 M-1s-1). Hence, the chain transfer, which makes it possible for two curing systems to overlap, happens quickly (the rate constants >10-2 M-1s-1). The primary factors influencing the reaction's energetics and kinetics were highlighted as the high stability of thiyl radicals, epoxy ring strain, and instability of the alkoxide radical produced by the addition reaction. Based on this study's findings, temperature control and the use of certain thiols are strongly advised to prevent curing step overlap. 2. Based on the literature review, thiol-epoxy reactions are reported to proceed via a base-catalyzed mechanism. Within the scope of this Ph. D. thesis, we also perform implicit solvent calculations that consider the ambient polarity to model the anionic reactions realistically, considering that ionic reactions are affected significantly by the polarity of the medium.
Due to its beneficial qualities, which include decreased stress, great adhesion, low shrinkage, strong chemical resistance, and remarkable mechanical strength, curing epoxy resins are in high demand in the industry. However, the primary disadvantage of this type of resin is that it might yield brittle products. Thiol-ene polymerization offers notable advantages over epoxy resins, such as creating a uniform polymer network and resisting oxygen inhibition. However, thiol-ene compounds exhibit less shrinkage stress than epoxy resins. Thus, a synergistic impact of both polymerization processes has been made possible by the development of a sequential dual curing procedure for the thiol-ene/ thiol-epoxy system. Thiol-epoxy reactions are essentially investigated in two primary mechanisms under the purview of this PhD thesis: radical mediated and anionic thiol-epoxy reactions. 1. An analog thiol-ene mechanism was proposed for radical-mediated thiol-epoxide reactions to analyze the overlap problem of the thiol-ene/ thiol-epoxy systems using computational approaches. The addition reaction to epoxides proceeds more slowly than the thiol-ene reactions (the rate constants <10-4 M-1s-1). Hence, the chain transfer, which makes it possible for two curing systems to overlap, happens quickly (the rate constants >10-2 M-1s-1). The primary factors influencing the reaction's energetics and kinetics were highlighted as the high stability of thiyl radicals, epoxy ring strain, and instability of the alkoxide radical produced by the addition reaction. Based on this study's findings, temperature control and the use of certain thiols are strongly advised to prevent curing step overlap. 2. Based on the literature review, thiol-epoxy reactions are reported to proceed via a base-catalyzed mechanism. Within the scope of this Ph. D. thesis, we also perform implicit solvent calculations that consider the ambient polarity to model the anionic reactions realistically, considering that ionic reactions are affected significantly by the polarity of the medium.