Diphenylphosphinoyl radicals

Phosphinoyl radicals are much more reactive towards olefinic compounds than carbon-centered radicals. For example, the rate constants for the addition of diphenylphosphinoyl radicals (see Scheme 10.2) to vinyl monomers are of the order 10 to 10 m s , i.e. one or two orders of magnitude larger than those... 

Polymerization kinetics. Radical polymerization of vinyl monomers initiated by the diphenylphosphinoyl radical has been investigated by time-resolved CW and FT EPR. The diphenylphosphinoyl radicals (DPO) were generated by photo-cleavage of 2,4,6-trimethylbenzoyldiphenylphosphine (TMDPO) by laser pulses of 335 nm and 10 mJ in benzene solutions. The reaction scheme of the initiation and the first polymerization step are shown in Figure 13. Well-resolved hyperfine structures of the starting radicals (first addition step) were observed for vinyl acetate, ethyl vinyl ether, styrene, methyl methacrylate and... 

Diphenylphosphinoyl radicals 16 were generated by hydrogen-abstraction from diphenylphosphine oxide 15 by rm-butoxyl radicals. Thus, heating a mixture of di-r rr-butyl peroxalate (5 x IQ- M), the aminoxyl 1 (1.2 x 10-2 M) and diphenylphosphine oxide (1 x 10-2 M) in acrylonitrile at 60 C for 70 mins, gave a mixture of the addition product 17 and the phosphinate ester 18 in the ratio of 1 0.28 (Scheme 5). Clearly, the addition of diphenylphosphinoyl radicals to acrylonitrile is competitive with aminoxyl trapping, and as expected, increasing the aminoxyl concentration (to 5 x 10 2 M) resulted in an increase in the ratioof 17 18(to 1.0 0.78). 

Analogous results were obtained when diphenylphosphinoyl radicals were generated by photolysis of the acylphosphine oxide 19. In addition to the products 17 and 18, the ester 20 was also formed as expected22 (Scheme 6). 

That diphenylphosphinoyl radicals exhibit nucleophilic behaviour was demonstrated by employing vinyl acetate instead of acrylonitrile. Thus, treatment of diphenylphosphine oxide (1 X 10 2 M) with di-/ rr-butyl peroxalate (5 x lO- M) and the aminoxyl 1 (5 x 10-2 M) in vinyl acetate at 60 C for 70 mins, gave a mixture of the addition products 21 and 22 as well as the phosphinate ester 18 in a ratio of 1.02 0.33 1.0 respectively. The much lower ratio of addition product 22 to phosphorus radical trapping product 18, compared with the corresponding ratio of 17 18 in the case of acrylonitrile, shows that diphenylphosphinoyl radicals are nucleophilic in nature. 

The relative reactivity of diphenylphosphinoyl radicals towards acrylonitrile and vinyl acetate was also determined in a competition experiment using a mixture of the two monomers. Thus,... 

Under similar reaction conditions, dimethoxyphosphoryl radicals were found to be much less nucleophilic than diphenylphosphinoyl radicals. Thus, a 2 1 molar mixture of vinyl acetate and acrylonitrile gave approximately equal amounts of the two addition products 23 and 24 indicating that acrylonitrile is only twice as reactive as vinyl acetate toward addition of dimethoxyphosphoi7l radicals. Suiprisingly, the ratio of /err-butoxyl radical addition O ONR2 O ONR2... 

Abstract The radical trapping technique employing the stable aminoxyl (nitroxide) 1 as a radical scavenger, has been used to study (a) the initiation stage in the mechanism of formation of alternating copolymers (b) the reaction of diphenylphosphinoyl and dimethyl phosphoryl radicals with monomers (c) the unusual polymerisation characteristics of maleates and fumarates. 

---