Dielectric Properties of Free Radical InitiatorsInvestigation of Thermal Decomposition Products

Studies into the phenomena that explain the dielectric properties of azo initiators (AMBN and V70) at high temperatures are reported in this paper. Previous studies have successfully related the variation in dielectric properties of these species below 110 °C to either phase changes or the thermal production of free radicals. In the latter case the marked increase in their response was attributed to the free radicals having more prominent dipoles than their initiator precursors. This study reports the results of experiments designed to explain their observed dielectric characteristics within the temperature range of 110–150 °C. At these elevated temperatures, the decomposition half-life of the initiators studied should be of the order of few seconds. However, in both this and the previous reports, the dielectric response is found to remain at a significant level for several hours. The two prime explanations for the unexpected duration of the increased dielectric properties are (i) the presence of microwave induced protected radicals or (ii) the dielectric properties of the initiator decomposition products. The observations made in this study were subsequently used to define that the latter of these is the key to the observed phenomenon

Mechanistic Investigation into the Accelerated Synthesis of Methacrylate Oligomers via the Application of Catalytic Chain Transfer Polymerization and Selective Microwave Heating

The synthesis of methyl methacrylate (MMA) oligomers by catalytic chain transfer polymerization (CCTP) is demonstrated to be significantly accelerated by the use of microwave heating. The CCTP reactions, which use a cobalt-based catalyst to very efficiently control the molecular weight of the final polymer, were conducted in both a conventional oil bath and a CEM Discover microwave reactor with a target set point of 80 °C. The required reaction time was shown to be reduced from 300 to 3 min, while also retaining control over the polymerization. Additionally, for the first time the bulk temperature of these catalyzed polymerizations was monitored in both heating methods by the use of internal optical fiber sensors. It was demonstrated that, to monitor the temperature of the reaction correctly, it is essential to use an optical fiber sensor rather than the external IR sensor supplied with the reactor. The acceleration in the synthesis during microwave heating was attributed to selective heating of the radical and oligomeric species within the reaction, which lead to both rapid heating of the reaction bulk to reaction temperature and average reaction temperatures that were higher than the chosen set point. However, comparative reactions carried out under conventional heating (CH) conditions at the true reaction temperature of the microwave experiments (MWH) showed that MWH was able to produce significantly greater yields than the CH experiments after only 3 min, indicating the existence of a real selective heating effect during the reaction. Three methods have been investigated to optimize the acceleration achieved in the MWH experiments while retaining control and yield levels within the MWH experiments. These were varying the; solvent concentration, initiator concentration and chain transfer agent concentration. It was demonstrated that by understanding the influence of the microwave heating that it was possible to retain control over the molecular structure of the product polymer at the accelerated rate