Radical stability transition metal salts

A number of authors have studied the reductive termination step. The classical work by Hermann and Nelson (121) showed that the reduction of the titanium in alkyltitanates occurred readily with the more alkylated species. The same authors (122) showed that the stability of the alkyl metal decreased markedly when the number of alkyls per metal atom increased. This is parallel to increased anionicity of the alkyl. Vanheerden (123) pointed out that the disproportionation of alkyl-titanium was bimolecular. Cotton (124) showed that the coupling occurs when phenyl Grignard reagents are mixed with various transition metal salts. This is through disproportionation or combination of the alkyl radicals. 

The nature of a transition metal is not essential for this redox reaction. However, one of the reaction products, namely, anion radical SOJ, can be complexed by a transition metal in a higher oxidation state. This leads to some stabilization of SOJ and increases its effective concentration. In other words, further reactions with organic substrates are facilitated (Fristad Peterson 1984). Cuprous and ferrous salts are preferable. 

It was noted that the efficiency is increased in the presence of amines (Scott, 1993b), so the effectiveness in conjunction with aromatic amine free-radical (CB-D) stabilizers or the amine end groups of nylon would be expected. It is also noted that cuprous salts are often used as the iodide, and the effective alkyl-scavenging ability of iodo compounds has been demonstrated (Henman, 1979). Copper salts are also effective in the melt stabilization of polyesters, but in this case the effectiveness is improved by using a hindered phenol antioxidant. This performance is in contrast to the strong prodegradant effect of copper and other transition metals on the polyolefins as discussed later. 

The discovery of stable, quasi-metallie, radical-cation salts has stimulated many studies. A few studies concerning the metallic nature will be briefly discussed here. Most of the results were obtained with FA2X because of its simple, well-documented structure and phase transition and its relatively high stability. 

---