Hawker adduct

The SFRP or NMP has been studied mainly using the stable free radical TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) or its adducts with, e.g., styrene derivatives. It is based on the formation of a labile bond between the growing radical chain end or monomeric radical and the nitroxy radical. Monomer is inserted into this bond when it opens thermally. The free radical necessary to start the reaction can be created by adding a conventional radical initiator in combination with, e.g., TEMPO or by starting the reaction with a preformed adduct of the monomer with the nitroxy radical using so-called unimolecular initiators (Hawker adducts). 

FIGURE 3.19 General concept of NMRP highlighted using TEMPO as stable radical and some examples for advanced stable nitroxide radicals and Hawker adducts. 

Hawker et al. prepared the 1-phenylethyl adduct of BPPN, i.e., 2,2,5-trime-thyl-3-(l-phenylethoxy)-4-phenyl-3-azahexane, (TMPAH, Fig. 13) and found that it was useful for the controlled homopolymerizations of St, nBA, acrylonitrile, and N,N-dimethylacrylamide [71]. For example, the homopolymerization of DMA resulted in polymers with Mn=4000-55,000 with Mw/Mn=l.15-1.21. TMPAH was also used to prepare random copolymers containing St or nBA and the above monomers, in addition to copolymers with MMA, acrylic acid, 2-hy-droxyethyl acrylate (HEA),and glycidyl acrylate. As with DEPN, it was necessary to add the free nitroxide to mediate the polymerization rate, but the resulting... 

The thermal lability of the R—C—O—N bond system controls the reversibility of the chain termination and limits also the use of NMRP. SFRP of styrene at about 130°C is studied intensively. In this case, high control and high molar mass products could be achieved. It was found that the thermal autopolymerization of the styrene monomer plays an important role in the mechanism of the reaction. Therefore, first experiments using different monomers in the presence of TEMPO and a radical initiator failed with regard to the control. However, new nitroxide adducts with a different R—O—N bond stabiUty have been developed, for example, by Hawker [14], which work also for styrene derivatives as well as for acrylates. End group functionalization in NMRP can be achieved by using a functional radical initiator in combination with a stable radical or functionalized nitroxide adducts. 

The stable radical-mediated polymerization represents a promising approach to defined homopolymers and block copolymers. Recently, Hawker and Hedrick [71] demonstrated the potential use of the living radical polymerization for the preparation of fimctional polymers. Two strategies were followed. The first method involves the synthesis of functionalized monoadducts of styrene and TEMPO and their use in styrene polymerization (Scheme 23), The hydroxy functionalized initiators were prepared by hydrolyzing the ester functionality of the adduct (Scheme 24). Protected amino groups were also introduced by further modification of the latter. Hydroxy groups were treated with iV(terr-butoxycarbonyl)-4-aminophenol, as illustrated in Scheme 25. 

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