Microwave-Assisted Polymer Synthesis: State-of-the-Art and Future Perspectives

Microwave irradiation is a well-known method for heating and drying materials and is utilized in many private households and industrial applications for this purpose. It offers a number of advantages over conventional heating, such as noncontact heating (circumventing the decomposition of molecules close to the walls of the reaction vessel), instantaneous and rapid heating (resulting in a uniform heating of the reaction liquor), and highly specific heating (with thematerial selectivity emerging from thewavelength of microwave irradiation that intrinsically excites dipolar oscillation and induces ionic conduction). Microwave ovens operate with electromagnetic nonionizing radiation with frequencies between 300 GHz and 300 MHz. The corresponding wavelengths span a range from 1 mm to 1 m, exhibiting the medial position of microwaves between infrared and radio waves. Most commercial microwave systems, however, utilize an irradiation with a frequency of 2 450MHz (wavelength l1⁄4 0.122 m) in order to avoid interferenceswith telecommunication devices. The corresponding electric fields oscillate 4.9 10 times per second and consequently subject dipolar species and ionic particles (as well as holes and electrons in semiconductors or metals) to perpetual reorientation cycles. This strong agitation leads to a fast noncontact heating that is (approximately) uniform throughout the radiation chamber. A large number of reactions, both organic and inorganic, undergo an immense increase in reaction speed under microwave irradiation compared with conventional heating. Apart from this main advantage, significant improvements in yield and selectivity have been observed as a consequence of the fast and direct heating of the reactants themselves. Furthermore, high-pressure synthesis is easily Summary: Monomodal microwaves have overcome the safety uncertainties associated with the precedent domestic microwave ovens. After fast acceptance in inorganic and organic syntheses, polymer chemists have also recently discovered this new kind of microwave reactor. An almost exponential increase of the number of publications in this field reflects the steadily growing interest in the use of microwave irradiation for polymerizations. This review introduces the microwave systems and their applications in polymer syntheses, covering step-growth and ring-opening, as well as radical polymerization processes, in order to summarize the hitherto realized polymerizations. Special attention is paid to the differences between microwave-assisted and conventional heating aswell as the ‘‘microwave effects’’.