The mechanism of biscitraconimides (BCI) as antireversion agents for sulfur vulcanized diene rubber was investigated choosing squalene as a model substrate. Curing of squalene with sulfur in the presence of accelerator, activator and BCI at 170°C gave BCI bridged adducts. The products were identified by spectroscopic methods as being results of a Diels-Alder reaction of BCI (dienophile) with conjugated polyenes that are formed as a result of reversion. The rate at which the sulfur network degrades is comparable to the rate in which BCI adduct is formed. As a result of this, a nearly constant crosslink density is obtained after optimum cure time (t90). Based on this concept, the advantage of using 1,3-bis-(citraconimidomethyl) benzene (BCI-MX), a representative of the BCI-class, in NR based compounds is examined.

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The relation between the electron spin polarization of the donor triplet state of the photosynthetic reaction center from Rhodopseudomonas viridis and the redox state of the primary acceptor The hypothesis [( 1986) Photobiochem. Photobiophys. 11,95 -1001 that the temperature dependence of the electron spin polarization (ESP) pattern of the Am = k 1 EPR spectrum of the triplet state PR of the Rhodopseudomonas viridis reaction center is caused by magnetic interaction between the reduced menaquinoneiron complex Q-Fe*+ and the electron spin on I-(reduced bacteriopheophytin b), which is part of the radical pair P+I-(P' is the oxidized primary electron donor P960) has been investigated. It was found that the AEAEAE ESP pattern of the EPR spectrum detected at T> 20 K changes into the usual AEEAAE pattern, when Q-Fe*+ is photochemically converted into Q*-Fez+. This demonstrates that the presence of Q-in Q-Fe*+ is a necessary condition to obtain the AEAEAE ESP pattern.

The mechanism of anti-reversion activity of biscitraconimides (BCI) during rubber vulcanization has been investigated. Biscitraconimides form crosslinks by reaction with chain modifications which are formed during the reversion process. These additional crosslinks compensate for the sulfidic crosslinks that are lost during the reversion. To allow structural assignments of the adducts formed between rubber and citraconimides, squalene has been used as a simple model system in combination with the currently used additives and sulphur. The products were identified as Diels—Alder adducts of conjugated polyenes, formed during reversion, with citraconimide. The nuclear magnetic resonance (NMR) spectroscopic assignments of the products are discussed. Furthermore the quantification of squalene-BCI adducts via NMR and mass spectrometry (MS) using 13C labelled 1,3-bis (citraconimidomethyl) benzene (BCI-MX) is described. It was found that under the applied conditions (2 h, 170 °C), approximately 80% of the citraconimide groups are converted into squalene-BCI adducts. Diels—Alder adducts are the main reaction products of the reaction. Products from an ene reaction are formed as minor component (less than 20%).

A method is developed to detect electron paramagnetic resonance (EPR) transient signals with frequency components in the range from dc to 1 MHz and a low initial signal-to-noise ratio (e.g., 0.1 after one sweep). This method uses 200-kHz magnetic field modulation on a conventional EPR spectrometer, using a modified lock-in detector: the tuned amplifier is replaced by a broadband preamplifier with a high-pass filter and the low-pass filter of the lock-in detector is replaced by a broadband amplifier. The transients are generated incoherently from the magnetic field modulation, and subsequently averaged. The function of the (removed) low-pass filter is taken over by the averaging. An instrumental response time of less than 1 μs is obtained. This was determined by measuring the rise time of the triplet state of the photosynthetic reaction center of Rhodopseudomonas sphaeroides R-26 at 165 K. This method, direct detection, and high-frequency (1–2 MHz) modulation are discussed. The S/N ratio of the method described in this paper is similar to that of the other two methods. However, when large linewidths (>0.5 mT) are met, it gives a superior S/N ratio as compared to high-frequency (1–2 MHz) modulation methods, due to the higher maximum modulation amplitude. When the transients of interest contain frequency components in the low-frequency noise regime (dc–50 kHz), the described method is to be preferred over direct detection because of its better S/N ratio.