New approaches for photoinduced step-growth polymerizations

Abstract

Photoinduced reactions are receiving a tremendous interest recently due to the global need for renewable and more efficient procedures over conventional methods. Several photoinduced approaches were developed and applied over conventional techniques to increase the ecological and economical benefits of the industrial productions since it provides more efficient reaction conditions by involving lesser chemicals. All of the polymerization mechanisms including radical, anionic and cationic polymerizations have benefited over the advancements in the photochemistry. Scientists across the globe have developed novel photoinitiators or sensitizing systems to create successful polymerization methods to be adapted over conventional techniques. Thus, enhanced the growing speed of industries including dentistry, additive manufacturing, coatings and electronics. However, most of the reactions that utilized photochemical methods were chain or ring opening polymerizations. Step-growth polymerizations on the other hand, were investigated scarcely compared to the previously mentioned alternatives. Step-growth polymerizations are mechanistically differ from the chain polymerizations. Photochemistry can be used to generate reactive species either in radical or cationic form to initiate the corresponding polymerizations using suitable monomers. However, for the step-growth polymerization, two different bifunctional monomers have to be used or generated in order to obtain polymers which limits the options for scientists. In the scope of the thesis, novel perspectives towards step-growth polymerizations involving substitution reactions have been presented and developed. SN1 type substitution reactions can occur in the presence of a good leaving group, heat, polar solvents and may be strong acid catalysts. Conditions tend to be very susceptible to humid and other nucleophilic impurities. Also applied heat catalyzes elimination reactions that is generating impurities. All the mentioned conditions brings up a challenge to obtain a pure product and safe conditions. However, this reactions can be conducted under ambient conditions by photochemical means using the harmony between dimanganese decacarbonyl chemistry and iodonium salts. Dimanganesedecacarbonyl chemistry has been investigated thoroughly by Bamford et al. They have revealed its affinity towards halogens that is directly bonded to organic structures. With a quantum efficiency that is close to unity, dimanganese decacarbonyl absorbs light at visible region and disassociates from Mn–Mn bond to create manganese pentacarbonyl radical which can homolytically abstract halogens. Organoradicals generated by halogen abstraction can be oxidized to corresponding carbocations to initiate cationic polymerization or in this case, used as electrophiles to react with the nucleophiles present in the reaction media. Diols were used as nucleophiles initially to obtain polyethers derivatives. Later on, this approach was further extended to the aromatics to obtain Friedel-Crafts type electrophilic aromatic substitution reactions yielding poly(phenylene methylene) (PPM) derivatives. Iodonium salts are known for their ability to initiate cationic polymerizations under irradiation with the suitable light. However, their oxidizing characteristic can be experienced even without light but under the presence of oxidizable organo radicals. Diphenyliodoniumhexafluorophosphate (DPI) was selected as the iodonium salt for this synthesis method since it contains a small chromophore group that is transparent in visible light region and bearing non nucleophilic counter anion. As the nucleophilic counteranions are known to terminate the living cations to halt the ongoing cationic reactions, counter anions like PF6–, BF4– or AsF6– are preferred since they are unable to make a covalent bond with the living cations. Dibromoxylene and dimethoxybenzene were initially used as the electrophile and nucleophile respectively for the reaction. Fluorescent polymers have been obtained from the reactions. Even though no direct conjugation is present through the polymer chain, arising from the hyperconjugation, varying fluorescence properties occurred due to electron density of the benzene rings. By taking advantage of the benzylic bromines at the polymer chain ends, radical polymerizations have been done and block copolymers consisting of PPM and poly(methyl methacrylate) (PMMA) were synthesized and fibers of the blocks were prepared using electrospinning technique. Later on dibromomethane has been utilized in the synthesis of the PPM derivatives. Generated bromomethylenium cation after the abstraction of the bromine and oxidation to the corresponding cation, resulting electrophile is susceptible to be attacked by the dimethoxybenzene nucleophile. Subsequently generated dimethoxybromomethyl benzene is a more reactive species compared to dibromomethane which increases the rate of polymerization through the process. After the kinetic investigation with different concentrations, a shift to the novel chain-growth condensation (CCP) mechanism has been observed. Resulting in easier reactions to be conducted and higher molecular weights. The approach in this thesis is the first application of photochemistry in the CCP mechanism.