External Control in Atom Transfer Radical Polymerization
thesisposted on 13.01.2022, 19:50 by Sajjad Dadashi SilabSajjad Dadashi Silab
This dissertation describes the advances in external control in atom transfer radical polymerization (ATRP). Chapter 1 provides an overview of the fundamentals and advances of reversible deactivation radical polymerization (RDRP) techniques. In particular, the development of ATRP and the advances in use of external means for initiating the ATRP catalytic systems are highlighted.
In Chapter 2, I presented an in-depth mechanistic analysis of various ATRP systems regarding temporal control with external stimuli. Temporal control was studied in ATRP using zerovalent metals, light, and chemical redox agents. The effect of polymerization components in providing temporal control was studied. The effect of the activity of the Cu catalysts was highlighted to demonstrate the importance of developing highly active catalytic systems for achieving excellent temporal and on-demand control over polymerization. Results of this research offered a deep mechanistic understanding of the contribution of ATRP components in polymerizations induced by external stimuli. Furthermore, we developed a redox switchable ATRP system that enabled on demand control over the activity of the Cu catalyst and hence the polymerization. By applying reducing or oxidizing agents such as ascorbic acid and ferrocenium salts or air, the Cu catalyst was switched on or off, respectively. As a result, the polymerization was switched multiple times between on and off states.
As discussed in Chapter 2, we investigated photoinduced ATRP which was triggered using UV light. To explore new possibilities for carrying out photoinduced ATRP and address challenges associated with the use of UV light (i.e., high energy and low depth of penetration), developing photocatalytic systems based on visible or near infrared (NIR) light is necessary. Chapter 3 presented our research in advancing photoinduced ATRP systems by taking advantage of the visible or NIR lights. We explored dual photoredox catalytic systems comprised of suitable photocatalysts used for generation of the Cu activators under visible light irradiation (green or red). In the first section of this chapter, I showed the development of a heterogeneous photocatalyst comprised of conjugated microporous polymers of phenothiazine as a highly versatile photocatalyst for activating copper-catalyzed ATRP. Using dimethoxybenzene as a crosslinker under Friedel-Crafts reaction offered the photocatalyst of being heterogeneous in nature as well as extending the conjugation throughout the network via aromatic linkages. The heterogeneous photocatalyst enabled performing copper-catalyzed ATRP under green or red-light irradiation, and offered reusability for several reactions with high photocatalytic efficiency.
In subsequent chapters, the development of new catalytic systems for external control in ATRP other than Cu complexes is presented. Although Cu-based catalysts provide excellent control over ATRP, developing new catalytic systems would open new possibilities for the enhanced ATRP performance. For example, iron-based complexes are of great importance because of the abundance of iron on earth’s crust and its involvement in biological events. These features offer a great opportunity for developing environmentally benign iron-based catalytic systems for ATRP.
In Chapter 4, I first reviewed the fundamentals and possibilities of using iron catalysts in ATRP. Next, I presented our research on development of an iron-based photoinduced ATRP system, which provided well-controlled polymerization of methacrylate monomers using ppm levels of the iron catalyst under blue light irradiation. We showed that irradiation of the catalyst with FeBr4− anion under blue light promoted a ligand-to-metal charge transfer that resulted in a homolytic cleavage of the Fe-Br bond. Therefore, the activator FeII was generated photochemically to start the polymerization. This system was studied in detail regarding the photoreduction mechanism, the scope of applicable monomers such as fluorinated and non-fluorinated methacrylates in synthesis of homo and block copolymers, and temporal control. Subsequently, I studied the effect of halogen and reaction medium in polymerization control in iron-catalyzed ATRP in the presence of halide anions as ligands. Because of the stronger Fe-Cl bond in the Cl-based initiating systems, inefficient deactivation led to poorly controlled polymers with high dispersity. However, Br-based initiating systems provided well-controlled ATRP because of the fast and efficient Br exchange (dispersity <1.2). Moreover, we discovered that the high stability of the anionic deactivator FeBr4− in polar solvents led to diminished rates of deactivation and therefore polymers with large dispersity values were obtained (>1.6).
Chapter 5 discussed use of organic photoredox catalysts in ATRP. Organo-catalyzed ATRP (O-ATRP) systems provide the possibility of eliminating the use of metal-based catalysts. The photocatalysts in the excited state activate the dormant chain ends via an electron transfer. The excited state photocatalyst is much more reducing than a ground state. In this work we extended the use of phenothiazine as a visible light photocatalyst in O-ATRP of methacrylate monomers. We showed that extended conjugation imparted by introducing a phenyl ring to the catalyst’s core resulted in a red shift in the absorption of the photocatalyst and thus enabled ATRP under visible light irradiation. Well-controlled polymerization of various methacrylate monomers was observed with excellent temporal control modulated by light on/off periods.
In Chapter 6, I presented a new concept in catalyzing ATRP by using iodine-based initiating systems. The alkyl iodide initiator was generated in situ by a halogen exchange reaction using iodide salts and a bench-top stable alkyl bromide, ethyl α-bromophenylacetate. The alkyl iodide was activated by visible light irradiation in the presence of iodide salts as catalysts. We studied iodine-mediated photoATRP in aqueous media which enabled fast and well-controlled ATRP of a water-soluble methacrylate monomer under blue, green, or yellow light irradiation. We found that this system offered excellent control over the kinetics of polymerization that occurred only under light irradiation. The complexation of with the iodide salts with the chain ends provided labile bonds for photochemical generation of radicals. Moreover, we demonstrated that this system was oxygen tolerant as polymerizations were well-controlled in the presence of residual oxygen without the need for deoxygenation processes. The results of this chapter offered new possibilities in designing ATRP catalytic systems that can be carried out under mild conditions and controlled by photochemical processes.
Finally in Chapter 7, I summarized the results of research studies presented in this dissertation on development of ATRP catalytic systems controlled by external stimuli. I also provided an outlook for possible implications of these findings as well as new directions for advancing the field of RDRP in general and specifically for ATRP systems controlled by external stimuli.
- Doctor of Philosophy (PhD)