Dielectric heating polymers

Abstract Current microwave-assisted protocols for reaction on solid-phase and soluble supports are critically reviewed. The compatibility of commercially available polymer supports with the relatively harsh conditions of microwave heating and the possibilities for reaction monitoring are discussed. Instrmnentation available for microwave-assisted solid-phase chemistry is presented. This review also summarizes the recent applications of controlled microwave heating to sohd-phase and SPOT-chemistry, as well as to synthesis on soluble polymers, fluorous phases and functional ionic liquid supports. The presented examples indicate that the combination of microwave dielectric heating with solid- or soluble-polymer supported chemistry techniques provides significant enhancements both at the level of reaction rate and ease of purification compared to conventional procedures. 

Microwave curing of polymeric materials requires the presence of dipolar materials for effective modification to occur through dielectric heating [45]. This is not an essence in the case of EB modification of polymers which requires the presence of only labile reactive site, e.g., hydrogen in the polymeric stmcture. 

Although feasible, as shown by Erwin and Suh (4), using dielectric heating as an energy source is rather limited in polymer processing practice as a primary melting mechanism. 

Abstract Recent developments in the microwave-assisted synthesis of heterocycles are surveyed with the focus on diversity-oriented multi-component and multi-step one-pot procedures. Both solution- and solid-phase as well as polymer-supported methodologies for the preparation of libraries of heterocycles are reviewed. Advantages of microwave dielectric heating are highlighted by comparison with conventional thermal conditions. 

An alternative solution to the workup issue relied on the attachment of CH-acidic compounds 64 to a soluble polymer support (PEG-4000). The approach improved the yields of the dihydropyrimidinones 66 by the use of a 2-fold excess of other components—urea and aldehyde in the microwave-assisted solvent-free cyclocondensation [118]. Another single-step approach towards 4,5-disubstituted pyrimidines was based on cyclocondensation of a variety of aromatic, heterocyclic and aliphatic ketones, formamide and HMDS as the ammonium source [119]. The high temperature (215 °C) required to effect the formation of pyrimidines was secured by microwave dielectric heating in sealed vessels (Scheme 45). 

One of the main drawbacks of the Wittig reaction is the formation of unwanted triphenylphosphine oxide. A new route, which makes use of polymer-supported triphenylphosphine and microwave dielectric heating has been developed (Scheme 13), which yields the required alkene without the triphenylphosphine oxide. An alternative strategy for separation of the product alkene from unwanted phosphine oxide by-product is to carry out the Wittig reaction in a fluorous solvent using a perfluorinated ylide such as (45). One drawback of this... 

The noble metals were the favorite metals for demonstrating the usefulness of the microwave operation in conducting the polyol reaction. Among the noble metals platinum was synthesized most frequently. Polymer-stabilized platinum colloids with nearly uniform spherical shape were prepared by Yu and coworkers by microwave dielectric heating [172]. The average diameters of the as-prepared platinum colloids were 2-4 nm with a narrow size distribution in regard to the preparation conditions. 

Polymer Laboratories Ltd. (U.K.) is developing applications of related complexed material to capacitors and dielectric heating. 

Welding at High Frequency Dielectric heating is accomplished with electrodes that are connected to a generator of voltage at high frequency. This method is very efficient but is restricted to polymers that exhibit a high electrical loss factor like PVC and other polar polymers. 

Dielectric loss n. Energy dissipated as heat within a polar material subjected to a rapidly alternating, strong electric field. Ku CC, Liepins R (1987) Electrical properties of polymers. Hanser Publishers, New York. See dielectric heating. 

A low dissipation factor is important for plastic insulators in high-frequency applications such as radar equipment and microwave parts smaller values mean better dielectric materials with less dielectric heating. A high dissipation factor is important for polymers that are to be heated in a radio frequency or microwave oven for welding, drying, etc. 

Dielectric heating occurs when a dielectric material is placed in an electric field that alternates at high frequency. A dielectric material is an electric insulator and it has a low conductivity (high resistivity). Materials with a resistivity higher than 10 ohm-cm are generally considered to be dielectric most polymers fall into this cate-... 

Table 5.1 Relative Response of Various Polymers to Dielectric Heating...

From an applications point of view, it may be that the filler yields an unexpected advantage reduction of dielectric heating, and concomitant reduction in power loss. As the tan 8 peak is reduced, less energy would be transferred from an AC source to the polymer in the range of near -10 to 0°C. 

---