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Inhibition of vinyl polymerization by lignins Straforelli, Jean-Baptiste

Abstract

The mechanism of inhibition and retardation, caused by lignins, on the polymerization of methyl methacrylate (MMA) in vacuum and in presence of air was investigated. Polymerizations of MMA were carried out with the use of azobisisobutyronitrile (AIBN) as the initiator at 50°C. The effect of lignin origin was studied by means of appropriately substituted phenols, used as lignin model compounds (LMC). When polymerizations were conducted in vacuum, neither an inhibition period (IP) nor a retardation was noted with low LMC concentration (10⁻³ -10⁻² M) and 5 mg/ml of AIBN. However, under these conditions, large IP occurred in presence of air. At higher LMC concentrations (0.1-2 M), inhibition and retardation became evident also in vacuum. The length of the IP varied according to the 0.6 power of phenol concentration in vacuum, and to the 0.4 power of phenol concentration in presence of air. If was found that inhibition and retardation resulted from transfer reactions of poly(MMA) radicals with LMC. In order to study the effects of nuclear substituents on these reactions, chain transfer constants for a series of LMC were determined. They were found to increase in the following order: 4-n-propylphenol (3.2 x 10⁻⁴ ), 4-n-propylguaiacol (8.3 x 10⁻⁴), and 4-n-propylsingol (54.6 x 10⁻⁴). The transfer reactions were shown to be governed chiefly by the resonance stabilization of the phenoxy radicals formed. In presence of air, however, the influence of nuclear substituents is determined by their polar character. Indeed, the substituent effects on transfer reaction rates were best analyzed on the basis of the following Hammett equation: log[(IP[sub substituted phenol])/(IP[sub phenol])] = ρσ where (σ) represents the Hammett polar substituent constant, and (ρ) is a reaction constant. A value of -0.83 was obtained for the constant p, indicating that electronegative radicals, such as peroxy-poly(MMA) radicals, are actually reacting with LMC. These results were qualitatively confirmed by experiments with coniferous and pored wood kraft lignins.

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