Optimization And Mechanism Of Microwave-assisted Synthesis Of Conjugated Polymers

Microwave technology has been widely used in organic chemistry, polymerchemistry, heterocyclic chemistry, material sciences, biochemical processes, medicinalchemistry, nanotechnology, combinatorial chemistry and green chemistry, as a way toprovide microwave energy. The most important characteristic is that it can accelerate thereaction rate, and reduce the reaction time. Microwave-enhanced chemistry is based onthe “microwave dielectric heating” effects. This phenomenon is dependent on theability of a specific material to absorb microwave energy and convert it into heat at agiven frequency and temperature is determined by the so-called loss factor (tan). Theelectric component of a microwave field causes heating by two main mechanisms:dipolar polarization and ionic conduction. Not only is direct microwave heating able toreduce chemical reaction times from hours to minutes, but it is also known to increaseyields, reduce side reactions, inhibit thermal decomposition, reduce environmentalpollution, improve reproducibility and so on. The initial slow uptake of the technologyhas been attributed to its lack of available control systems, and the controllability andreproducibility are poor. Microwave synthesis has developed rapidly with the evolutionof microwave theory and using dedicated instruments.Conjugated polymers have attracted tremendous attentions in the past decadesbecause of their potential applications in organic light emitting diodes, photovoltaicdevices and field effect transistors. The quality of conjugated polymers, which involvesthe molecular weights and the structural defects, strongly influence their electronic and optical properties. Generally, metal-catalyzed coupling reactions are utilized to prepareconjugated polymers, which take long reaction time under high temperature. However,the synthesis of these conjugated polymers with high quality is often a challenge forchemists. Microwave irradiation technology is a relatively new research field for thesynthesis of conjugated polymers. There are some reports related to the development ofmicrowave-assisted synthesis of conjugated aromatic polymers in a short time, however,the optimum conditions of different reactions may have large differences. Hence, thesystematical investigation of microwave-assisted synthesis of conjugated polymers isvery important.Based on the advantages and great potential of microwave in organic synthesis,Herein, by utilizing microwave as heating resource and PDHFs as model, we focused onthe following aspects.Firstly, we systematically study the effect of reaction conditions on themicrowave-assisted Suzuki polymerization to optimize performance. This involvescontrolled instrument parameters (the mode of microwave irradiation, microwave power,reaction temperature and reaction time) and the internal parameters of the reactionsystem (solvents, catalyst species and catalyst concentrations, monomer ratio and O2).The optimized condition is using Coupling SPS mode, THF as solvent,1mol%PdCl2(dppf) as catalyst at150W,130°C and14min. We can demonstrate that themolecular weight of microwave-assisted polymerization was improved, and the reactiontime of microwave-assisted polymerization was reduced by two orders of magnitudecompared with conventional polymerization. The choice of appropriate reaction modeand control reaction conditions precisely is key considerations to obtain high qualitypolymers in the microwave-assisted Suzuki polymerization. Cross-linked polymers maybe formed under harsh reaction conditions, which involve high microwave-assistedpower and reaction temperature, as well as long reaction time. A tentative mechanism,which is related to the microwave-assisted polymerization, was proposed.Secondly, we investigated microwave-assisted Yamamoto polymerization.Compared with the conventional heating, microwave-assisted polymerization can reducereaction time, and save the activation process of catalyst before polymerization, and improve the efficiency of synthesis. According to the set of different reaction conditions,microwave reaction mode may change its way of heating, and there is no insoluble gelsemerged, no matter how to change the reaction conditions in Yamamoto polymerization.And then, we investigated the insoluble gels, which formed sometimes with changedreaction conditions in Suzuki polymerization. The chemical structures of insoluble gelswere confirmed by XPS and elemental analysis, it consists of three elements, they are C,H and O. The thermal properties further proved the existence of cross-linking structure.The decomposition temperature of the insoluble gels is as high as730°C, and the glasstransition temperature was not observed from the insoluble gels. We observedfluorenone, hydroxyl group and C-O-C structures are introduced in the insoluble gelsfrom the IR spectra. We supposed that H2O might take part in the Suzuki couplingreaction under microwave severe condition, which could make the cross-linking reactionoccur and form gels.Thirdly, we investigated FeCl3-mediated oxidative polymerization for thenon-electron-rich system by microwave synthesis technology, and we optimized themicrowave power, reaction temperature, reaction time, solvent species, solvent volumesand monomer concentrations. The optimized condition is using4mL CHCl3as solvent,0.05M monomer at20W,50°C and25min. In contrast to the Suzuki polymerizationand Yamamoto polymerization, the monomer structure of the oxidative polymerization issimple, and the number-average molecular weight of obtained polymers up to63K. TheXPS profiling of purified PDHF products show no signs of element Fe, which indicatedFe content below the detecting limits of XPS. The employment of MW and selection ofappropriate solvent volume were critical for the success of the polymerization thatdemonstrates excellent Mnand narrow PDI. And the FeCl3exhibiting catalytic activityfor oxidation polymerization must exist in the solid state.