Abstract
The radical-scavenging activities of two thiols, eight (thio)barbituric acid derivatives and six chain-breaking phenolic antioxidants were investigated using the induction period method for polymerization of methyl methacrylate (MMA) initiated by thermal decomposition of 2,2’-azobisisobutyronitrile (AIBN) and monitored by differential scanning calorimetry (DSC). The induction period (IP) for the thiols 2-mercaptoethanol (ME) and 2-mercapto-1-methylimidazole (MMI) was about half that for phenolic antioxidants. Except for the potent inhibitor 5,5-dimethyl-2-thiobarbituric acid (3), the IP for thiobarbituric acid derivatives was about one tenth of that for phenolic antioxidants. The IP for 1,3,5-trimethyl-2-thiobarbituric acid (1) and 5-allyl-1, 3-dimethyl-2-thiobarbituric acid (7) was less than that of the control, possibly due to inhibition by a small amount of atmospheric oxygen in the DSC container. The ratio of the chain inhibition to that of chain propagation (CI/CP) for the thiols and thiobarbituric acid compounds except for 1, 3 and 7 was about 10 times greater or greater than that for phenolic compounds. A kinetic chain length (KCL) about 10% greater than that of the control was observed for 1, suggesting that 1 had chain transfer reactivity in the polymerization of MMA. The average molecular weight of polymers formed from thiobarbituric acid derivatives is discussed.
Highlights
Quantitative inhibition of auto-oxidation by various synthetic and natural phenols and organic sulfur compounds has been investigated by many researchers [1,2,3,4,5,6,7,8,9]
(2-methoxy-4-propenylphenol) with thiol, thiophenol, mercaptothiazoline or mercaptomethylimidazole using the induction period method for polymerization of methyl methacrylate (MMA) initiated by thermal decomposition of AIBN under nearly anaerobic conditions
In the light of currently available data, we investigated the radical-scavenging activities of two thiols (2-mercaptoethanol, 2-mercapto-1-methylimidazole), eightbarbituric acid derivatives and six chain-breaking phenolic antioxidants (p-cresol, eugenol, hydroquinone, phenol, thymol, α-tocopherol) using the induction period method for polymerization of MMA initiated by thermal decomposition of AIBN
Summary
Quantitative inhibition of auto-oxidation by various synthetic and natural phenols and organic sulfur compounds has been investigated by many researchers [1,2,3,4,5,6,7,8,9]. 1,3-dimethylthiourea was reported to be detectable at higher oxygen tensions, this compound had lower radical-scavenging activity [2]. Both chlorobenzene-styrene [1] and linoleic acid-aqueous systems [2] have been investigated under fully oxygenated conditions, and some of the compounds described above were not evaluable due to their interaction with oxygen as a biradical. We investigated the radical-scavenging activity of the reaction products of isoeugenol (2-methoxy-4-propenylphenol) with thiol, thiophenol, mercaptothiazoline or mercaptomethylimidazole using the induction period method for polymerization of MMA initiated by thermal decomposition of AIBN under nearly anaerobic conditions. Even at relatively small concentrations, these products enhanced the initial rate of polymerization (IRP) value by about 10%
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