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 (D = 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. [GRAPHICS] .