Low-Temperature “Living” Radical Polymerization of Styrene in the Presence of Nitroxides with Spiro Structures

Bulk radical polymerizations of styrene (St) in the presence of 7-aza-15-oxodispiro[5.1.5.3]hexadecane-7-yloxyl ( 4 ), 7-aza-15-hydroxydispiro[5.1.5.3]hexadecane-7-yloxyl ( 5 ), and 1-aza-2,2-dimethyl-4-hydroxyspiro[5.6]dodecane-1-yloxyl ( 6 ) were carried out at 70−110 °C. The ln( M 0 / M t ) vs time plots were linear, and the M n 's of the formed poly(St)'s increased linearly with conversion. Furthermore, the polydispersities ( M w / M n ) were always below 1.3, showing that the polymerization proceeded in a “living” fashion. To increase the polymerization rates, small amounts of camphorsulfonic acid (CSA) or 1,1‘-azobis(cyclohexane-1-carbonitrile) (ACN) were added to the polymerization systems. Addition of 0.50 equiv (vs nitroxide) of CSA increased the polymerization rate by 1.8−2.8 times, and addition of 0.125−1.0 equiv (vs nitroxide) of ACN increased the polymerization rate by 2.3−11.5 times, keeping the “living” nature. The (quasi)equilibrium constants ( K ) of the C−O bond homolysis and its reformation were determined at 80 °C using the corresponding alkoxyamines from 5, 6, and TEMPO; the K constants for 5 and 6 are much larger than that for TEMPO. The bond dissociation energies of the C−O bonds of alkoxyamines calculated by semiempirical and ab initio molecular orbital methods supported the above results.