Combining step-growth and chain-growth polymerizations in one pot: Light-induced fabrication of conductive nanoporous PEDOT-PCL scaffold

Abstract

The process of chemically combining monomers to form larger molecules is called polymerization. These monomers can be one type or more than one type, they can be categorized in two different ways according to the monomer types the polymer contains. Polymers containing a single type of monomer are called homopolymers, and polymers containing more than one type of monomer are called copolymers. Since Carothers' work on polyamides and polyesters, polymers have gained great importance due to the new needs which are brought by developing technology. Today, industrial polymers consist of thousands of repeat units. The ways of how these polymers are obtained, which are used in every field in our modern world, is as important as their properties. Two methods generally used to obtain polymers; chain-growth polymerization and step-growth polymerization. In chain-growth polymerization, the chain is started with the help of an initiator, then the chain grows over time by adding monomers to the resulting chain, the end of the chain is active in this process, and the polymerization process is completed by terminating the active end. In chain-growth polymerization, the expansion of macromolecules occurs by adding monomers one by one, resulting in a steady decrease in monomer concentration. The average chain length remains almost constant throughout the reaction. Unlike chain-growth, step-growth polymerization can be achieved by utilizing the reactions between monomers containing two active groups, for this reason, a decrease in monomer concentration is observed in the initial stage of the reaction. Due to concerns about the cost and environmental impact of thermochemical methods which used to obtain polymers, photochemical methods have recently gained importance thanks to the advantages they provide in these matters and have begun to used in different areas such as coatings, curable dental filling materials, microelectronics and inks. While the traditional thermochemical method passes the energy barrier required for the reaction to start, with the increase in temperature, photoinitiators are used in photochemical reactions. These initiators, which become excited with the help of UV-visible light, form free radicals or ions required to obtain the polymer. Since most monomers cannot produce reactive species when exposed to ultraviolet light, photoinitiators are introduced into the reaction media. In order to active species to be formed effectively, the wavelength range of the light source used, must match with the absorption range of the photoinitiator. Photoinitiated free radical polymerization is one of the methods used in industry, as it is a suitable method for acrylates, unsaturated polyesters and polyurethanes. The availability of photoinitiators that work in the near ultraviolet region or the visible region makes photoinitiated free radical polymerization a good alternative. In photoinitiated free radical polymerization, which consists of 4 steps: photoinitiation, propagation, chain transfer and termination, in the first step, initiator decomposes under light and forms a radical species, and when this radical interacts with the monomer, the initiation step occurs. Monomers are added to the radicals formed in the propagation step, and the polymer chain begins to form which continues throughout the step, growing chains may terminate by removing hydrogen from the reaction environment, or new radicals may form and continue the chain elongation. In the termination step, termination may occur in two ways, termination by combination or disproportionation. In combination termination, termination occurs by the joining of two chains, while in disproportionation termination, two polymer molecules are formed by hydrogen transfer. Conductive polymers, or conjugated polymers, have been applied in tissue engineering, electrochromic devices, sensors, batteries and organic solar cells, due to their optical, electrical and electrochemical properties since their discovery in 1977. Conjugated polymers, which basically contain alternating sigma and pi bonds, exhibit optical and electrochemical properties that attract the attention of scientists thanks to the delocalized pi bonds they obtain. Conjugated polymers can consist of aromatic heterocycle structures such as thiophene, furan and pyrrole. Hideki Shirakawa, who synthesized polyacetylene using a version of Natta's method, obtained a silvery thin film using 1000 times more catalyst than normal. With the analysis of the film, its semiconductor feature was noticed and the conductivity of the film was increased with exposing the film to halogen vapor through a process called doping. Since the discovery of conjugated polymers, scientists tried to obtain new polymers based on existing conductive polymers. In 1988, researchers at Bayer company succeeded in synthesising 3,4-ethylenedioxythiophene, a thiophene derivative. Poly(3,4-ethylene dioxythiophene) stands out thanks to its properties such as good electrical conductivity, thermal stability and biocompatibility. Due to these advantages, it has been used in different fields over the past 40 years, for example, drug delivery systems, organic field effect transistors, light emitting diodes, organic solar cells, wearable electronics. When compared with polythiophene, PEDOT offers better thermal stability in doped state and electrical conductivity due to extended electron delocalization. On the other hand, like many conductive polymers, the solubility problem also have been seen for PEDOT. Different methods have been tried to overcome this, one of the most common one is the PEDOT:PSS system. Although not completely soluble in water, this system provides a stable solution in dispersed form. Poly(ε-caprolactone), a linear polyester, has been used with conductive polymers for different purposes. Its biocompatibility, hydrophobicity and low melting point have enabled it to be used in tissue engineering and drug delivery systems. Scaffold and sensor applications with conductive polymers have been introduced to the literature. In this thesis study, it is aimed to synthesize PEDOT-PCL scaffold with phenacylbromide as a single-component photoinitiator. Chain-growth polymerization of ε-caprolactone was initiated by the HBr released into the media during the step-growth polymerization of EDOT. The obtained polymers were characterized by analyzes such as proton nuclear magnetic resonance (1H NMR), Fourier transfrom infrared (FT-IR), Ultraviolet and visible light (UV-vis) and Fluorescence spectroscopies. Their electrical conductivity properties were investigated. Moreover, the surface and morphological properties of resulting products were analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).