Triazolinyl radicals

A few studies have appeared on systems based on persistent nitrogen-centered radicals. Yamada et al.2"1 examined the synthesis of block polymers of S and MMA initiated by derivatives of the triphenylverdazyl radical 115. Klapper and coworkers243 have reported on the use of triazolinyl radicals (e.g. 116 and 117). The triazolinyl radicals have been used to control S, methacrylate and acrylate polymerization and for the synthesis of block copolymers based on these monomers [S,243 245 tBA,243 MMA,243 245 BMA,245 DMAEMA,24 5 TMSEMA,247 (DMAEMA-Wbc/fc-MMA),246 (DMAEMA-Woc -S)246 and (TMSEMA-6/ocfc-S)247]. Reaction conditions in these experiments were similar to those used for NMP. The triazolinyl radicals show no tendency to give disproportionation with methacrylate propagating radicals. Dispcrsitics reported arc typically in the range 1.4-1.8.2"43 246... 

The triazolinyl radical 116 is thermally unstable with a half-life of -20 min at 95 °C. The compound 117 is stable under similar conditions. The decomposition mechanism involves loss of a phenyl radical and formation of a stable aromatic triazene (Scheme 9.26).24 This provides a mechanism for self regulation of the stable radical concentration during polymerization and a supplemental source of initiating radicals. 

S) " ]. Reaction conditions in these experiments were similar to those used for NMP. The triazolinyl radicals show no tendency to give disproportionation with methacrylate propagating radicals. Di.spcrsitics reported arc typically in the range... 

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 ... 

Another class of counter radicals, introduced by Mullen et al. and resulting in a controlled polymerization of acrylates and methacrylates, are triazolinyl radicals. A molecular mass of ca. 60,000 was achieved at a conversion around 35% [271]. Besides, the addition of small amounts of camphorsulfonic acid (CSA) [272] and FMPTS [273,274] was examined. Thereby, a reduction in the concentration of free nitroxide during the polymerization to a level around 5x 10 M resulted in an improvement of polymerization rates and consequently a higher molecular mass, but at a cost in the narrowness of molecular mass distribution. 

Thermal homolytic cleavage of a weak bond in a covalent species which reversibly provides a growing radical and a less reactive radical (a persistent or stable free radical) (Scheme 1). There are several examples of persistent radicals but it seems that the most successful are nitroxides [13,14,16], triazolinyl radicals... 

Klapper, M., Brand T., Steenbock, M., and Mullen, K. (2000). Triazolinyl radicals towards a new mechanism in controlled radical polymerization. ACS Symp. Ser. 768 152-166. 

Various stable radicals such as nitroxide, triazolinyl, trityl, and dithiocarbamate have been used as the mediating or persistent radical (deactivator) for SFRP. Nitroxides are generally more efficient than the others. Cyclic nitroxide radicals such as 2,2,6,6-tetramethyl-l-piper-idinoxyl (TEMPO) have been extensively studied. SFRP with nitroxides is called nitroxide-mediated polymerization (NMP). Polymerization is carried out by two methods that parallel those used in ATRP [Bertin et al., 1998 Georges, 1993 Flawker, 1997 Flawker et al., 2001], One method involves the thermal decomposition of an alkoxyamine such as... 

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