Tributylborane/p-quinone system: reversible and irreversible inhibition in the styrene polymerization

Polymerization of styrene with tributylborane in the presence of various p -quinones (2,3-dimethyl-1,4-benzoquinone, 1,4-naphthoquinone, menadione, duroquinone, 2,5-di-tert-butyl-1,4-benzoquinone) has been investigated. It was found that in spite of dual nature of p -quinone and the presence of tributylborane in the initial mixture, the semi-quinone macroradical is capable to “chain regeneration”. Neither solvent introduction nor use of a more reactive triethylborane affects this process. The insensitivity of this reaction to external conditions may be caused by the cell effect. For some p -quinones in the presence of tributylborane, the realization of reversible-deactivation radical polymerization is observed. Mechanism of the chain termination reactions was established by the MALDI-TOF technique. It consists of reversible inhibition causing the formation of active macromolecules as well as irreversible inhibition causing the formation of “dead” macromolecules. The ratio of these directions depends on the relative reactivity of p -quinone ( k z / k p ). The higher is k z / k p , the lower is the probability of irreversible inhibition. Polymers obtained in the presence of the tributylborane/ p -quinone system can re-initiate polymerization. Post-polymers with distinct molecular weight characteristics ( Đ = 1.12 – 1.19) were obtained during this synthesis. Only polystyrene macro-radicals are formed during polymerization re-initiation. This fact has been proven by ESR spectroscopy. The macroinitiator polymers isolated at the initial conversions have the same reactivity regardless of the p -quinone nature. The linear dependence of M n with conversion, the polydispersity lessening, and the constant concentration of macro-radicals indicate realization of reversible-deactivation radical polymerization. Graphical abstract