Mn2(CO)10 based visible-light photo initiating systems for distinct macromolecular structures

Light is a particularly fascinating stimulus because it can be precisely modulated in terms of wavelength, polarization direction and intensity, allowing spatial and temporal of the chemical reactions. Photochemical reactions involve the absorption of light to create an excited species that may undergo a number of different reactions such as dissociation, isomerization, abstraction, electron or energy transfer, and bond formation. These reactions have been studied quite extensively in various fields including organic chemistry, molecular biology and electronics etc. Photoinduced chemical reactions can advantageously be utilized in the field of polymer chemistry. Among them, photoinitiated polymerization which is a process that transforms monomers into polymers under light irradiation, has many advantages over other polymerization methods. It is fast, uses little energy, and readily occurs at room temperature. It has been estimated that energy costs can be reduced 30% by switching from thermal polymerization to photoinitiated polymerization. Therefore, it has been the basis of numerous conventional applications in surface coatings, printing inks, adhesives, microelectronics, printing plates and three dimensional imaging and micro-fabrication processes. Additionally, there is a huge number of photoinitiators for such photo-induced systems. Among them, dimanganese decacarbonyl (Mn2(CO)10) in conjunction with organic halides appears as an ideal photoinitiating system for the preparation of polymers with various topologies. Additional attractive features of the transition metal carbonyl compound include efficient light absorption in the visible region and solubility in a wide variety of reactive monomers. Many different applications of Mn2(CO)10 chemistry including initiation of free radical polymerization, promotion of cationic polymerization, mechanistic transformation, graft copolymerization, iodine degenerative transfer polymerization, preparation of telechelics and hyperbranched polymers have been reported and reviewed. Taking account of the unique advantages of Mn2(CO)10 photochemistry, in this thesis, we focused on the development of new Mn2(CO)10 based photochemical approaches for the synthesis of macromolecular structures with various architectures. In the first part of the thesis, polyolefin graft copolymers were prepared by combining ring-opening metathesis polymerization (ROMP), hydrobromination, and visible light-induced free radical polymerization. First, cis-cyclooctene (COE) was polymerized via ROMP in the presence of a chain transfer agent and quantitatively hydrobrominated to give bromo functional polyethylene (PE-Br). Subsequent irradiation of PE-Br in the visible range using dimanganese decacarbonyl (Mn2(CO)10) initiated free radical polymerization of tert-butyl acrylate (tBA) resulting in the formation of polyethylene-graft-poly(tert-butylacrylate) (PE-g-PtBA). The effect of Mn2(CO)10 concentration and irradiation time on the grafting density and efficiency was evaluated. Then, the tBA moieties of PE-g-PtBA were hydrolyzed into acrylic acid functionalities by acidolysis to obtain hydrophilic polyethylene-graft-poly(acrylic acid) (PE-g-PAA). In the second part, a new photoredox catalyst system for Atom Transfer Radical Polymerization (ATRP) is developed on the basis of visible light photocatalysis using Mn2(CO)10 that initiates and controls the polymerization at ambient temperature. The polymerization was performed by Mn2(CO)10/alkyl halide system with visible- or sunlight in the presence of parts per million (ppm) copper catalysts. The photogenerated •Mn(CO)5 radicals are not only able to abstract halogen atoms from alkyl halides to generate carbon centered radicals but also reduce the copper(II) bromide (CuIIBr2) to copper(I) bromide (CuIBr) directly, which was used as activator in the ATRP of vinyl monomers such as methyl methacrylate, methyl acrylate and styrene. The method was also used to synthesize graft copolymers from commercially available poly(vinyl chloride) without additional modification. Finally, a new photoinitiating system for living cationic polymerization of vinyl ethers is reported. In the current approach, visible-light irradiation of Mn2(CO)10 in the presence of an alkyl bromide results in the formation of carbon-centered radicals. The photochemically generated radicals were then oxidized by diphenyliodonium ions to the corresponding cations. These cations can add vinyl ether monomers, which are then rapidly deactivated by the bromide anions to give α-halide functional end groups. Poly(vinyl ether) chains are then grown through successive photoinduced radical oxidation/addition/deactivation (PROAD) in a controlled manner. The living nature of the system is evaluated through kinetics studies and block copolymer formation.