Prevulcanization of isoprene rubber (IR) latex was investigated by latex-state 13 C-NMR spectroscopy to understand its mechanism in latex stage. The prevulcanization was carried out at 50 to 90°C with sulfur (S) and zincdibutyldithiocarbamate (ZDBC) as vulcanizing agents. The prevulcanized IR latex was subjected to latex-state 13 C-NMR spectroscopy. The signal at 44 ppm was assigned to secondary carbons adjacent to carbons linking to S atoms, whereas the signal at 58 ppm was assigned to tertiary and quaternary carbons linking to S atoms. The time evolution of the small signals at 44 and 58 ppm was investigated at various temperatures, and kinetic constants were determined at each temperature. The activation energy of the prevulcanization of IR latex was estimated from the kinetic constants to be about 80 kJ/mol.

DFT computational studies with B3LYP/6-31G(d) level theory has been carried out for peroxy radical cations which are formed by the reaction of molecular oxygen with aliphatic and aromatic olefin radical cations. The calculations have led us to find rearrangement reactions of the peroxy radical cations to the isomeric carbonyl oxide radical cations. The results show that the peroxy radical cations in which their positive charges at the olefinic carbons cannot be stabilized due to the structural requirements have a great tendency to undergo the rearrangement. Thus, 2-methylpropene radical cation does not afford a peroxy radical cation formed by the addition of molecular oxygen at 2-position of the olefinic carbon, and DFT calculations to locate the structure were found to give a rearranged ethyl methyl carbonyl oxide radical cation, which is significantly more stable than the peroxy radical cation. It was also found that carbonyl oxide radical cations, depending on the structures, are as stable as, or even more stable than, the isomeric peroxy radical cations which are formed by the addition of molecular oxygen to the aromatic olefin radical cations derived from trans-stilbene and b-methylstyrene. The results also indicate that the C-O bonds are shorter than the O-O bonds in the alkyl carbonyl oxide radical cations examined, which suggests that oxygen lone pair delocalization to the carbonyl carbon play an important role for the determinaton of the structures and their stability.Although reactions of olefin radical cations with molecular oxygen have been studied in greater detail, [1][2][3][4][5][6][7][8][9][10][11] no systematic studies on regiochemistry of the oxygen addition toward the olefin radical cations have been reported. During the course of our computational studies on the regiochemistry of the addition of molecular oxygen to various types of olefin radical cations, we have found that peroxy radical cations (Scheme 1) in which the formal positive charge generated at the olefinic carbon cannot be stabilized through its delocalization with substituents attached tend to rearrange to corresponding carbonyl oxide radical cations (Scheme 2). These findings may open a new route of generating carbonyl oxide radical cations and extend the horizon of isomerism of the carbonyl oxygen-containing radical cations. Although extensive experimental, [12][13][14][15][16][17] and theoretical studies [18][19][20][21] have been carried out on chemistry of carbonyl oxides, little attention has so far been directed to its one-electron removed reactive species studies of which may certainly lead to topics of great mechanistic interest. [22] We would like to report in this communication the first finding of structures and stability of carbonyl oxide radical cations which may be derived form peroxy radical cations formed by the addtion of molecular oxygen to the olefin radical cations.Initial calculations have been performed for the reaction of molecular oxygen with ethene radical cation 1 +. . To our best knowledge, experimental...