Microencapsulated amines

Scheme 2.44 Three-component domino Heniy/Michael reaction catalysed by a combination of a chiral preformed nickel catalyst and a microencapsulated amine catalyst.

Scheme 7.55 Three-component domino Henry-Michael reaction catalysed by microencapsulated amine catalysis and chiral nickel catatysis.

In 2006, Poe et al. reported a cascade reaction employing two incompatible catalysts, one of which was microencapsulated [19]. In this case, an organic amine was encapsulated and used in conjunction with a nickel-based Lewis acid catalyst (Scheme 5.4). 

Coreactants, in microencapsulation, 16 444 Coreactive curing agents, 10 388-389, 392-411, 418. See also Curing agents amine functional, 10 392 401 carboxylic functional polyester and anhydride, 10 401—406 cyanate ester, 10 411 cyclic amidine, 10 410 isocyanate, 10 410 melamine-, urea-, and phenol-formaldehyde resins,... 

In this case, unilamellar vesicles with a large capture volume were prepared by the reverse phase evaporation technique and alginate was used to microencapsulate the liposome s. The alginate spheres were double coated, first with poly-L-lysine and then with polyvinyl amine (Wheatley and Langer in press). 

A significant amount of development is currently occurring relative to latent catalysts because of interest in their long shelf life, high reactivity, and single-component adhesive formulations. Present technologies involve absorption of acidic or basic catalysts in molecular sieves, formation of Lewis acid salts or other amine salts, microencapsulation of amines, and other novel segregation methods. 

Amines have also been microencapsulated within small cellulosic or polyelectrolyte capsules. This is a method for keeping the amine separate from the epoxy resin during storage. When the user decides to initiate cure, the capsules are broken, usually in the application process, and the amine is free to react with the epoxy resin. A successful example of this type of product is an epoxy adhesive that can be preapplied to machine screw threads. When the screw is ultimately threaded into place, the shearing action causes the capsule to break. Bond strengths are generally low for this type of adhesive, but this may not be important in certain applications. 

Aqueous solutions can also by microencapsulated in high concentration [6]. To prepare the reverse phase W/0 (Water-in-Oil) emulsions care must be taken to select monomers that will remain in the dispersed water droplets during the emulsion stage. If the monomers diffuse from suspended droplets into the continuous phase polymerization will happen throughout the emulsion and not at the interface as intended. No microcapsules will be formed. This problem has been addressed utilizing carboxy-functional polymers to associate with amine functional reactive monomers dissolved into the water droplets [7]. Shellwalls are formed at the W/0 interface by addition of the oil-soluble monomers to the continuous oil phase. Without the carboxy-functional protective polymers amine monomers would have partitioned out of dispersed water droplets and into the oil phase. Microcapsules would not have been produced. 

The reaction involves the amine-catalyzed conversion of an aldehyde into a nitroalkene by reaction with nitromethane followed by a transition-metal-catalyzed Michael addition of p-dicarbonyl compounds in the same reaction vessel. Typically, amine catalysts and nickel complexes are incompatible due to their tendency to chelate and to render each other inactive. However, microencapsulation of poly(ethyleneimine) (PEI) forms catalyst 140, which can successfully be used in tandem with the nickel-based catalyst 141 (Figure 3.6). 

Microencapsulation by interfacial polycondensation is a usefiil method to microencapsulate a liquid core material. Especially, polyurea and polyurethane microcapsules have been extensively investigated in various industries [209]. For example, aliphatic hexamethylene diisocyanate (HMDI) and aliphatic ethylene diamine (EDA) have been used to prepare polyurea microcapsules containing insecticide called diazinon [210]. A urea linkage is formed immediately by the reaction between an amine and an isocyanate group (see Figure 4.31), and a polyurea is synthesized by the reaction between an amine with two or more amine groups and an isocyanate with two or more isocyanate groups. 

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