Acyclic phosphonylated nitroxides

More recently, the CRP method has been improved signi cantly with the discovery of very ef cient acyclic nitroxides such as the stable acyclic phosphonylated nitroxide radical SGI (Fig. 11.15), which can control the polymerization of much broader range of monomers than cyclic nitroxides like TEMPO. The radical SGI has a larger equilibrium constant than most other nitroxides, including TEMPO, and is therefore suitable for polymerization temperatures as low as 90°C (SGI K = 1.9x10" mol dm at 125°C TEMPO K = 2.1x10"" moldm- at 125°C). 

Acyclic / -phosphonylated nitroxides and the corresponding derivatives were developed by Farcetetfl/. [32],andlater onbyDufilseta/. [33] that are efficient at lower temperatures, not only for styrene polymerization, but also for that of alkylacrylates, acrylamides, dienes, etc. A typical example of these nitroxides is the so-called SGI (N-tert-butyl-N-fl-diethylphos-phono-2,2-dimethylpropyl)nitroxide) of the following structure ... 

Ab initio emulsion polymerisations of styrene were also conducted at 90°C, using the stable acyclic phosphonylated nitroxide radical SGI (NIO) as a mediator together with a water-soluble redox initiator (Lansalot et al, 2000). A long induction period was observed, assigned to the formation of water-soluble alkoxyamines before nucleation. In this system, molar mass of the polymer increased with conversion following the theoretical line, but the MMD was rather broad (PDI between 2.0 and 2.5). Rather small particles were obtained (average diameter was 120 nm) with a broad particle size distribution. It was also found that a few per cent of coagulum formed usually. 

Benoit D, Grimaldi S, Robin S et al. (2000) Kinetics and mechanism of controlled free-radical polymerization of styrene and n-butyl acrylate in the presence of an acyclic beta-phosphonylated nitroxide. J Am Chem Soc 122 5929-5939... 

Grimaldi, S., Finet, J.-P., Le Moigne, F. et al. (2000) Acyclic jS-phosphonylated nitroxides A new series of counterradicals for living Vcontrolled free radical polymerization. Macromolecules, 33,1141-1147. 

SGI-mediated polymerization in miniemulsion SGI (Figure 15) is a versatile nitroxide suitable for the polymerization of styrene, acrylates, and even methacrylates (when copolymerized with 10mol.% styrene " or acrylonitrile to impart livingness to the system). SGI is an acyclic P-phosphonylated nitroxide with a more favorable equilibrium constant than TEMPO. For highly reactive monomers such as BA, additional SGI is required to maintain a controlled polymerization. 

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