Controlled radical polymerization azobisisobutyronitrile

A RAFT polymerization system consists of initiator, monomer, chain transfer agent, solvent, and temperature. RAFT polymerization can be performed by simply adding a chosen quantity of an appropriate RAFT agent (thiocarbonylthio compounds) to a conventional free radical polymerization. Usually the same monomers, initiators, solvents and temperatures can be used. Because of the low concentration of the RAFT agent in the system, the concentration of the initiator is usually lower than in conventional radical polymerization. Radical initiators such as Azobisisobutyronitrile(AIBN) and 4,4 -Azobis(4-cyanovaleric acid)(ACVA) are widely used as the initiator in RAFT. RAFT polymerization is known for its compatibility with a wide range of monomers compared to other controlled radical polymerizations. These monomers include (meth)acrylates, (meth)... 

Rasmussen and co-workers. Chapter 10, have shown that many free-radical polymerizations can be conducted in two-phase systems using potassium persulfate and either crown ethers or quaternary ammonium salts as initiators. When transferred to the organic phase persulfate performs far more efficiently as an initiator than conventional materials such as azobisisobutyronitrile or benzoyl peroxide. In vinyl polymerizations using PTC-persulfate initiation one can exercise precise control over reaction rates, even at low temperatures. Mechanistic aspects of these complicated systems have been worked out for this highly useful and economical method of initiation of free-radical polymerizations. 

Carbon suboxide s susceptibility to radical polymerization was also examined by an attempted copolymerization of equimolar quantities of carbon suboxide and styrene initiated by azobisisobutyronitrile at 60°C in toluene solution. The infrared spectrum of the polymeric product was identical with that of a styrene homopolymer control, and no carbonyl absorption was detected. 

Obviously, the use of nitroxides as agents to control polymerizations remains interesting, however, their limitations makes one wonder whether there are other stable radicals which may be superior. Unfortunately, there are a limited number of stable radicals available, and those that have been studied have not been particularly successful. Earlier studies with galvi-noxyl radicals have been followed recently with the use of triazolinyl radicals and verdazyl radicals 1. However, in the only reported use of verdazyl radicals to mediate styrene polymerizations no control was observed with 2, an adduct of the 1,3,5-triphenylverdazyl radical and the 2-(2-cyano-2-propyl) radical derived from 2,2 -azobisisobutyronitrile (AIBN), at reaction temperatures between 80 °C and 120 In the case of the triazolinyl radicals, a spirotriazolinyl radical controlled the polymerization of styrene reasonably weU, but was only moderately effective for methyl methacrylate ... 

Some of the most important critical points in RAFT polymerizations are the relative concentrations of the free radical initiator, the CTA, and the monomer, since these will establish the delicate balance between the dormant and active species. Acrylate and methacrylate derivatives can be successfully polymerized using 2-cyano-2-butyl dithio benzoate (CBDB) as a CTA. However, the amount of free radical initiator (a, a-azobisisobutyronitrile (AIBN) is used in general) compared to CTA determines the rate of control over the polymerization. Therefore, eight different acrylates or methacrylates were polymerized with different ratios of CTA to AIBN [54]. The structures of the monomers and the design of the experiment are shown in Fig. 6. 

Most c/s-azoalkanes do this on heating, or on irradiation with shorter-wavelength radiation, and photolysis of azoalkanes provides a convenient source of certain radicals the photochemical process can be more readily controlled than the thermal reaction. Radicals produced in this way may be employed as initiators for the polymerization of alkenes. and a widely used compound for this purpose is azobisisobutyronitrile (ABIN, 5.20). 

Shaver and coworkers [319] investigated the mechanism of bis(imino)pyridine ligand framework for transition metal systems-mediated polymerization of vinyl acetate. Initiation using azobisisobu-tyronitrile at 120°C results in excellent control over poly(vinyl acetate) molecular weights and polymer dispersities. The reaction yields vanadium-terminated polymer chains which can be readily converted to both proton-terminated poly(vinyl acetate) or poly(vinyl alcohol). Irreversible halogen transfer from the parent complex to a radical derived from azobisisobutyronitrile generates the active species. 

---