Polystyryl alkoxyamines

The arm-first synthesis of star microgels by initiating polymerization or copolymerization of a divinyl monomer such as diviny lbenzene or a bis-maleimide with a polystyryl alkoxyamine was pioneered by Solomon and coworkers.692 693 The general approach had previously been used in anionic polymerization. The method has now been exploited in conjunction with NMP,692 6 ATRP69 700 and RAFT.449 701 702 The product contains dormant functionality in the core. This can be used as a core for subsequent polymerization of a monoene monomer to yield a mikto-arm star (NMP ATRP704). 

Fig. 10.5 Arrhenius plots for k,j of the polystyryl alkoxyamine adducts (P—I) where T is N088Me ( ) and N088Bn (o) [7].

Table 1 displays rate data for alkoxyamine-termi-nated polymers and low molecular model compounds and shows some important trends. At about the same temperature, the dissociation rate constants Ad of alkoxyamines (Schemes 12 and 30) with the same leaving radical (polystyryl, 1-phenylethyl) increase in the order 3 (TEMPO) < 6 < 8 (DEPN) < 1 (DBNO) by a factor of about 30. Acrylate radicals dissociate markedly slower than styryl radicals from 1 (DBNO), but there is no appreciable difference for 8 (DEPN). The dependence of Ad on the nitroxide structure has been addressed by Moad et al.104 They found the order five membered ring < six membered ring < open chain nitroxides and pointed out additional steric (compare 3 and 6) and polar effects. 

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—... 

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