Microwave irradiation polymeric reactions

Cycloaddition reactions often require the use of harsh conditions such as high temperatures and long reaction times. These conditions are not compatible with sensitive reagents or products such as natural products. The applicability of Diels-Alder cycloadditions is, moreover, limited by the reversibility of the reaction when a long reaction time is required. The short reaction times associated with microwave activation avoid the decomposition of reagents and products and this prevents polymerization of the diene or dienophile. All these problems have been conveniently solved by the rapid heating induced by microwave irradiation, a situation not accessible in most classical methods. With the aid of microwave irradiation, cydoaddition reactions have been performed with great success [9, 10]. 

Since very recently the number of papers on microwave-assisted polymerization reactions has been growing almost exponentially [3], the purpose of this chapter is to provide useful details about the application of microwave irradiation to polymer chemistry during the last few years. A survey of past achievements in polymer synthesis and polymer composites can be found in review papers [1-8] whereas fundamentals of electromagnetic heating and processing of polymers, resins and related composites have been summarized by Parodi [9]. 

Another example where PEG played the role of polymeric support, solvent, and PTC was presented by the group of Lamaty [72]. In this study, a Schiff base-proteded glycine was reacted with various electrophiles (RX) under microwave irradiation. No additional solvent was necessary to perform these reactions and the best results were obtained using cesium carbonate as an inorganic base (Scheme 7.64). After alkylation, the corresponding aminoesters were released from the polymer support by transesterification employing methanol in the presence of triethylamine. 

A similar conclusion has been drawn during an examination of the Diels-Alder reaction of 6-demethoxy-/J-dihydrothebaine with methylvinylketone using microwave irradiation [110]. When performed under conventional heating conditions, extensive polymerization of the dienophile was observed whereas reaction is much more cleaner under microwave activation (Eq. 61). 

Diels-Alder cydoadditions of vinylpyrazoles under dassical conditions require highly reactive dienophiles and extreme conditions, i. e. high pressures and temperatures (8-10 atm and 120-140 °C) for long reaction times (several days), and usually afford moderate yields only [62]. The main obstacle to these reactions is extensive polymerization of the reagents. l-Phenyl-4-vinylpyrazole (69) reacted with dimethyl acetylenedicarboxylate (26b) within 6 min under the action of microwave irradiation to afford the adducts 70 and 71 in 72% overall yield (Scheme 9.19) [63], The cydoad-... 

Z- Configuration is typical of the majority of a-aryl(hetaryl)-/V-alkylaldo-nitrones. The isolation of -isomers in the condensation of aromatic aldehydes with iV-j3-]ihenyletli Tliydroxylamine has been described (155). The synthesis of a, N -diary lnitrones gives best results if acidic catalysis is employed (156), or when clay is used as a catalyst (157). Significant reduction of reaction time and increase in the yields of nitrones can be achieved if microwave irradiation is used (158, 159). On the basis of polymeric arylaldehydes, the synthesis of polymeric a,-diarylnitrones has been described (160). 

Also, Kerep and Ritter reported a radical chain transfer agent as a dual initiator, FRP-1 [45]. The first step builds on the fact that hydroxyl groups are much better nucleophiles in enzymatic ROP than thiols. Due to the chemoselectivity of the enzyme, PCLs with predominantly thiol endgroups were obtained, which were subsequently used as macroinitiator for styrene. The authors report that the reaction yield can be further increased by microwave irradiation. Although thiols provide less control over the radical polymerization than RAFT agents, the subsequent radical polymerization successfully leads to the synthesis of PCL-Z -PS. 

Similar to the solvent-free approaches discussed in Section 3.2, a combinatorial approach also has been employed to scale-up the synthesis of desired compounds100. The microwave-assisted reactions are performed on solvent-swollen polymeric beads and are classified herein as being carried out in the presence of a solvent. Examination of these supports after 20 min of microwave irradiation (700 W) revealed that neither the appearance nor swelling behaviour of the beads had altered65. 

The best solvent from an ecological point of view is without doubt no solvent. There are many great reactions that can already be carried out in the absence of a solvent, for example numerous industrially important gas-phase reactions and many polymerizations. Diels-Alder and other pericyclic reactions are also often carried out without solvents. Reports on solvent-free reactions have, however, become increasingly frequent and specialized over the past few years. Areas of growth include reactions between solids [5], between gases and solids [6], and on supported inorganic materials [7], which in many cases are accelerated or even made possible through microwave irradiation [8]. 

Lindsley and co-workers developed a general procedure towards the collection of diverse heterocyclic scaffolds from common 1,2-diketone intermediates 96. Substituted quinoxalines 97, fused pyrazolo [ 4,5-g ] quinoxalines 98 and imidazolo[3,4-g]quinoxalines 99 as well as pyrido[2,3-fo]pyrazines 100 and Ihicno[3,4-fo Ipyrazincs 101 have been prepared in excellent yields [132] (Scheme 54), employing optimized reaction conditions (microwave heating of equimolar mixtures of 1,2-diketone 96 and diamine components at 160 °C for 5 min in 9 1 MeOH - AcOH). The use of microwave irradiation resulted in reduced reaction times (5 min vs. 2-12 hours), improved yields as well as the suppressed formation of polymeric species a characteristic of traditional... 

Microwave irradiation was applied to the atom transfer radical polymerization (ATRP) of azo-containing acrylates. The polymerization was greatly promoted and the reaction time was shortened from several days to about 1 h. The functional... 

Samanta et al. [23] conveniently synthesized the 1,5-disubstituted imidazoles (xxv) on a polymeric support using base-promoted 1,3-dipolar cycloaddition reaction of p-toluenesulfonylmethyl isocyanide (TOSMIC) with immobilized imines under microwave irradiation. 

Since the first edition of this book appeared in 2003, the field of solvent-free organic synthesis has undergone an explosion in research and I have received considerable response from readers. With the aid of this information I have completely updated this book. In this second edition, over 200 examples of new sol-vent-free organic reactions, published in the journals during 2003 to 2007, have been added. The most obvious changes in this book are the two new chapters one on polymerization (Chapter 14) and the other on supramolecular complexa-tion (Chapter 15). Additionally, all the sections on solvent-free reactions under microwave irradiation have been deleted, since Professor A. Loupy covers this area in his book on Microwaves in Organic Synthesis (Wiley-VCH, 2006). 

Improvements to the regime for Doebner-Miller ring closures include the use of a two-phase orgaific/ aqueous acid system" to minimize alkene polymerization and the nse of indinm(ni) chloride on silica with microwave irradiation." It is sigifificant that the accepted and proved regiochemistry for these cyclisations is reversed when the reaction is carried out in trifluoroacetic acid, imine formation being the first step, at least for unsaturated 2-keto esters. "... 

Microwave irradiation has been successfully applied in polymer chemistry (Ref [10] and Chapter 14 of this book) - for the synthesis and processing of polymers, e.g. for modification of the surface and cross-linking, and also in the degradation of polymers. Microwave plasmas also have been used in the polymerization and surface modification of materials. The enhanced reaction rates have been attributed to thermal effects - although for some reactions it seems the advantages arise from the selective excitation of one of the educts involved. Shifts in selectivity have also been observed. 

De la Hoz et al. describe several of the advantages of microwave irradiation by this method in Diels-Alder and 1,3-dipolar reactions of ketene acetals [41]. A specific improvement is the absence of polymerization of the ketene acetals. The same... 

Chain polymerization reactions under microwave irradiation conditions have been investigated for both free-radical and controlled living polymerization and ringopening polymerization. 

Free-radical polymerization reactions have recently been studied for different monomers, for example mono and disubstituted vinyl monomers and dienes. The bulk polymerization of vinyl monomers (e.g. vinyl acetate, styrene, methyl methacrylate, and acrylonitrile) has been investigated by Amorim et al. [10]. The reactions were conducted in the presence of catalytic amounts of AIBN (or benzoyl peroxide). It was found that the rate of polymerization depends on the structure of the monomers and the power and time of microwave irradiation. In a typical experiment 10.0 mL of each monomer and 50 mg AIBN was irradiated in a domestic microwave oven for 1 to 20 min to afford the polymers polystyrene, poly(vinyl acetate), and poly(methyl methacrylate) with weight-average molecular weights 48 400, 150 200, 176700 g mol, respectively (Scheme 14.1). The experiments were performed without temperature control. 

Emulsion polymerization of methyl methacrylate under the action of pulsed microwave irradiation was studied by Zhu et al. [11], The reactions were conducted in a self-designed single-mode microwave reaction apparatus with a frequency of 1250 MHz and a pulse width of 1.5 or 3.5 ps. The output peak pulse power, duty cycles, and mean output power were continuously adjustable within the ranges 20-350 kW, 0.1-0.2%, and 2-350 W, respectively. Temperature during microwave experiments was maintained by immersing the reaction flask in a thermostatted jacket with a thermostatic medium with little microwave absorption (for example tetrachloroethylene). In a typical experiment, 8.0 mL methyl methacrylate, 20 mL deionized water, and 0.2 g sodium dodecylsulfonate were transferred to a 100-mL reaction flask which was placed in the microwave cavity. When the temperature reached a preset temperature, 10 mL of an aqueous solution of the initiator (potassium persulfate) was added and the flask was exposed to microwave irradiation. 

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