Thermolysis of free-radical initiators: tert-butylazocumene and its 1,3- and 1,4-bisazo and 1,3,5-trisazo analogues.
Four tert-butylazocumenes (4-7) were prepared from the corresponding cyanobenzenes, and their nitrogen evolution kinetics and products were analyzed. In combination with TEMPO, the simplest compound, tert-butylazocumene (4), shows promise as a "one-radical" initiator of styrene polymerization. The ABNO-trapped cumyl radical 29 is a particularly stable trialkylhydroxylamine, whose thermolysis half-life is 2.1 h at 150 degrees C. Taking advantage of this stability, we trapped the cumyl radical centers from 7 to afford tris adduct 32a. While the behavior of the meta bisazoalkane 6 can be mostly predicted from that of 4, the para isomer 5 exhibits both unusual products and kinetics, attributed to the formation of quinodimethane 33 via azo-containing radical 34. In fact, flash vacuum pyrolysis of 5 allowed observation of the (1)H and (13)C NMR spectra of 33, whose persistence even at ambient temperature showed that this quinodimethane is far more stable than the parent 36. Finally, evidence is presented that 7 is an initiator of star polymerization.
Engel
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Department of Chemistry, Rice University, P.O. Box 1892, Houston, Texas 77251, USA.
Pan
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Ying
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Alemany
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