Monomode reactors, microwave-assisted

Caouthar et al. (2005) synthesized some new aromatic polyamides based on diphenylamino isosotbide. Diphenylamino isosoibide reacts with several diacyl chlorides in the presence of a small amount of N-metltylpyrrohdone (NMP) in a monomodal microwave reactor. Microwave-assisted polymerizations in the presence of NMP led to faster polymerizations and higher molecular weight products as compared to starrdand polymerization methods. The polymers were obtairred with irrhererrt viscosities between 0.22 and 0.73 dL/g, corresporrding to molectrlar weight trp to 140,000 g/mol. 

One of the first published microwave-assisted synthesis of benzothiazoles is the condensation of a dinucleophile such as 2-aminothiophenol, with an ortho-ester (neat) in the presence of KSF clay in a mono-mode microwave reactor operating at 60 W under a nitrogene atmosphere [ 12] (Scheme 12). Traditional heating (oil bath, toluene as solvent and KSF clay) gave the expected products in similar yields compared to the microwave experiments but more than 12 h were required for completion. Solvent-free microwave-assisted syntheses of benzothiazoles was also described by attack of the dinucleophiles cited above on benzaldehydes and benzaldoximines [13] (Scheme 12). This methodology was performed in a dedicated monomode microwave reactor... 

As already mentioned above, a different strategy to achieve high throughput in microwave-assisted reactions can be realized by performing automated sequential microwave synthesis in monomode microwave reactors. Since it is currently not feasible to have more than one reaction vessel in a monomode microwave cavity, a robotic system has been integrated into a platform that moves individual reaction... 

Monomode reactors Prolabo, Synfhe-wave S402 and S1000 (actually not on the market). CEM, STAR system 2 and 6 and Discover. Personal Chemistry, Smith Synthesizer and Smith Creator. Temperature measurement is one of the main problems in microwave-assisted reactions. See Ref. [2] for temperature-measurement systems. 

Another example of microwave-assisted PSR chemistry involves the rapid conversion of amides to thioamides by use of a polystyrene-supported Lawesson-type thio-nating reagent. By use of microwave irradiation at 200 °C in sealed vessels (monomode reactor), a range of secondary and tertiary amides was converted within... 

Pursuing their efforts on the Niementowski reaction and its possibilities, Besson and coworkers have recently extended the family of fused quinazolinones which can be obtained via the microwave-assisted Niementowski reaction from the starting amidines (43). The authors described rapid and convenient access to pentacy-clic 6,7-dihydro-5a,7a,13,14-tetraazapentaphene-5,8-diones (44) structurally related to well studied terrestrial alkaloids (e.g. rutaecarpine and luotonine A) [50e]. The strong thermal effect, because of graphite-microwaves interaction, was particularly efficient in these reactions, in which the quinazolinone and the piperazine rings are fused. Only 10% by weight of graphite was used, in pressurized monomode reactors (Scheme 9.13). 

The key point to successful synthesis under microwave irradiation is the use of equipment specially designed for chemical laboratories. Monomodal microwave equipment has overcome the uncertainties associated with the use of domestic microwave ovens. These reactors offer much more precise tanperature and pressure control, and the software provides simplified process monitoring, which results in accurate, reproducible reaction conditions. The energy transfer in a microwave-assisted reaction is rapid, the decomposition of the substances can be avoided, and high yields can be obtained in short reaction times by proper programming of the temperature. When the desired temperatme is reached, the power is automatically reduced and maintained by the software during the reaction period. 

Free radical copolymerization of methyl methacrylate and styrene as well as butyl methacrylate with styrene or isoprene in toluene under microwave irradiation (monomode microwave reactor) has also been carried out (Fellows, 2005). However, no changes in reactivity ratios were observed although more detailed studies were required for the copolymerization of butyl methacrylate and isoprene. The microwave-assisted polymerization procedure accelerated the polymerizations by a factor of 1.7, may be due to an increase in radical flux. It was proposed that the increased radical flux under microwave irradiation is due to rapid orientation of the radicals that are formed from decomposition of the azoisobutyronitrile. This orientation would reduce the number of direct terminations by recombination of the two radical fragments under microwave irradiation and thus, cause a higher radical flux. 

Stange et al. (2006) investigated the free radical copolymerization of methyl methaciy late and styrene with different initiators in different solvents nsing both microwave (monomode microwave reactor) and thermal heating. Agarwal et al. (2005) studied the copolymerization of 2,3,4,5,6-pentafluorostyrene and phei lmaleimide. They revealed that a higher initial polymerization rate and a lower final monomer conversion for the microwave-assisted procedure were there as compared to thermal heating. 

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