Cumyl-TEMPO

More recently, the unusual time-dependence of the radical concentrations (9) was also observed directly.22 Like many other /V-alkoxyamines (trialkyl-hydroxylamines), 2-phenyl-2-(2, 2, 6, 6 tetrameth-ylpiperidine-l -oxyl)propane (cumyl-TEMPO) (Scheme 10) cleaves thermally reversibly into a persistent nitroxide radical (TEMPO) and a transient carbon-... 

Figure 2. Time dependence of the TEMPO concentration during thermolysis of cumyl-TEMPO in fe/t-butylbenzene for different initial cumyl-TEMPO concentrations at 83 °C. The solid lines confirm the f1/3-dependence (eq 18).

The reversible thermal homolysis (kj) of cumyl-TEMPO 8 (R R NOR) affords TEMPO 2 (R R NO ) and the cumyl radical 9 (R ) in equal amounts. The main reaction, which the persistent nitroxide 2 undergoes, is the eoupling (kc) with the transient cumyl radical 9. Radical 9, however, ean either eouple with 2 or undergo self-termination predominantly by dimerization but also disproportionation. Note that hydrogen transfer (also ealled disproportionation) between 2 and 9 also occurs, affording the hydroxylamine 10 and a-methylstyrene 11. This leads to consumption of both 2 and 9. The impact of this side reaction on NMP is discussed in Section 3.5.3. 

Numerically calculated time dependences of concentrations of 2 (TEMPO) and 9 (cumyl) for thermolysis of 8 (cumyl-TEMPO) in double logarithmic presentation. Temperature 83°C, [8]o=10 M, d = 2.3 x... 

The concentration of the transient radicals 9 reaches a maximum value of about 10 M after 10 ms and then starts to decrease. At this point, only 0.035% of cumyl-TEMPO 8 has decomposed. 

Scheme 10.6 Alkoxyamine formation by trapping of the cumyl radical by TEMPO.

TEMPO, which is commercially available, traps carbon-centred radicals with rate constants an order of magnitude lower than the diffusion-controlled limit in most organic solvents at <120°C (e.g. kc = 3.1 x 108 dm3 mol-1 s 1 with benzyl radical at 50°Cin tert-butylbenzene) [6], and somewhat more slowly if the radical is sterically congested (e.g. kc = 5.7x 107 dm3 mol-1 s 1 with cumyl radical under the same conditions, Scheme 10.6) [6]. Non-Arrhenius behaviour or non-temperature dependence has been observed for several radical coupling reactions [6, 7]. 

Reactions that convert the alkoxyamines to hy-droxylamines and alkenes can strongly limit the monomer conversion. These are either usual radical disproportionations between the nitroxide and the propagating radicals or concerted alkoxyamine decays. Both pathways lead to an exponential decrease of the concentration of the dormant chains with rate constant kdec = f)kd, where fn is the fraction of the side reaction concurring with radical coupling of alkoxyamine decay.57 kdec can be measured from the decay of the dormant alkoxyamine chains under nonscavenging conditions, and its relation with kd provides fn. From data of Fukuda et al. one can deduce fn = 0.4% for a TEMPO-polystyryl compound and fn = 1.1% for a di-tert-butylnitroxide-poly-terf-butylacrylate macroinitiator both at 120 °C.53,55 Similar small values of fn hold for TEMPO-cumyl (Scheme 10),22 TEMPO-1-phenylethyl,112 and a better mimetic compound for TEMPO-polystyryl.113 In these cases, fn probably represents the usual radical disproportionation. A much larger fn 25% holds for TEMPO—... 

On the other hand, strategy B tries to reduce termination reactions, for example, dicumyl formation from two cumyl radicals (see Eq. (2.5)). This has been achieved by the addition of TEMPO (2,2,6,6,-tetramethyl-l-piperidinyloxy), a radical component that reacts reversibly with the cumyl radical, thereby stabilizing it. TEMPO allows the cumyl radical to proceed to the oxidation reaction step instead of reaction with another cumyl radical to dicumyl. 

Approximately 19% of 8 has decomposed at the end of the intermediate period. The decomposition products are TEMPO in an amount corresponding to the decomposed 8, and the dead products of the cumyl radical. Without the back-reaction between 2 and 9, the decomposition of 8 would obey classical first order kinetics described by eqn (4.6), and 8 would behave as a classical radical initiator, such as azo-isobutyronitrile (AIBN), i.e., its half life time (ti/2 = ln2/kd) would be only 301 s at 83 °C. 

Figure 4.2 Time dependence of the TEMPO concentration during thermolysis of cumyl-TEMP08.(a)intert-butylbenzene,[8]o = 1.16x 10 % A=5.10x lO M =2.52x 10 M (b) in tert-butylbenzene, [8]q = 2x 10 M, [galvinoxyl] = 5x 10 M.

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