Reaction calorimetry in supercritical fluids: a study of the dispersion polymerization of methyl methacrylate in supercritical carbon dioxide

This thesis is devoted to the study of the dispersion polymerization of methyl methacrylate in supercritical carbon dioxide (scCO2), using a poly(dimethylsiloxane) macromonomer (PDMS macromonomer) as stabilizer. Supercritical fluids (SCF) and SCF mixtures are characterized by a temperature and a pressure above their critical point(s), which is the last point on the vaporization line of a pure component. This means that these fluids operate from moderate to high pressure. They can be used in various processes ranging from extractions, nanoparticle formation for controlled drug release, chemical reactions and polymer processing. Nowadays, the best candidate for SCF processing is carbon dioxide. The fundamental motivation of using scCO2 as a solvent is based on its potential to replace harmful chemical organic volatile compounds (VOCs) in order to develop more sustainable and environmentally friendly chemical processes. At this point, the crucial role of CO2 in the development of the so-called "green chemistry" comes on the stage. CO2 is a natural abundant compound with low toxicity exhibiting no inflammability. This last property is very advantageous considering the cost investments spent by the chemical industry to control the safety of the chemical processes using highly flammable compounds like VOC solvents. As expected, environmental arguments are not sufficient to motivate the development of new chemical process routes. Therefore, additional arguments to use SCFs have to be found, and they do exist. As supercritical fluids are compressible fluids they can exhibit liquid-like and gas-like properties, which can be tuned easily by varying the operating conditions, like pressure and temperature. This fundamental behavior of SCF is their main asset and demonstrates their superiority to develop more flexible processes. The polymer industry is one of the industries that uses the largest volumes of organic solvents and sometimes halogenated ones, well known to destroy the ozone layer. The use of scCO2 gives to chemists and engineers the opportunity to develop more sustainable polymer processes, considering the numerous chemical and physical advantages of carbon dioxide. The processing of scCO2 for polymer production is no more than fifteen years-old. This means that a certain quantity of knowledge has been acquired but still a lot of unknowns hinder their promotion at industrial level. This work is inserted in this context and finds there its main motivations. This thesis is composed of two different but intrinsically connected approaches of the dispersion polymerization of the methyl methacrylate (MMA) in scCO2. A part of this thesis is devoted to the development of techniques allowing the on-line monitoring of polymerizations in scCO2 at "larger" scale, conducted from an engineering approach. The intrinsically connected part is devoted to the understanding of the fundamental phenomena that govern the dispersion polymerization of MMA in scCO2, its kinetic features and the product characteristics of the polymer produced, being conducted from a chemical approach of the subject. Up to now, most of the studies dealing with polymerization reactions in scCO2 are realized in small autoclaves between 2 and 60 ml allowing pertinent fundamental analysis but with poor similarities with an industrial reactor. A keystone of this work is the development of a supercritical reaction calorimeter composed of a 1.3 liters high pressure reactor allowing the kinetic study of the dispersion polymerization, which in turn gives a direct insight into the parameters that control the dispersion polymerization stability, efficiency and mechanism. Based on an adapted heat balance, the calorimeter can give the profile of monomer conversion (thermal conversion). Furthermore, the volume of the reactor allows inserting on-line sensors inside the reactor. This possibility led to the development of an ultrasonic sensor to monitor the polymerization process. The combination between the calorimetric information and the sensor signal shows the potential of these sensors to monitor polymerization reactions in scCO2. By measuring the speed of sound evolution during the course of the polymerization, it is possible to calculate the composition of the medium and thus evaluate the monomer conversion. The analysis of the effects of temperature, stirring speed and impeller types demonstrate that the dispersion polymerization of MMA can be effective under a wide range of operating conditions using the PDMS macromonomer as stabilizer. The experiments point out that the stability of the dispersion polymerization and in turn the rate of polymerization, the polymerization loci1 and the polymer quality depend strongly on the stabilizer concentration but more fundamentally on its degree of solubility in the carbon dioxide. Phase behavior measurements demonstrate that the 5'000 g/mol PDMS macromonomer exhibits a relative good solubility in carbon dioxide and that this solubility can be greatly improved by the presence of the monomer in the mixture. In fact, the monomer acts as a cosolvent for the PDMS macromonomer improving the solvency of the scCO2 (polarity, density). This study demonstrates that the concentration of the monomer in the reacting medium is a key parameter to control the stability of the dispersion throughout the polymerization process. A dispersion polymerization is characterized by polymer-rich particles dispersed in a continuous phase, i.e. the CO2-rich continuous phase. The results give evidence of the fact that the polymerization can take place in both phases depending on the concentration of stabilizer and its solubility in the reacting medium. In this case, bimodal molecular weight distributions and intermediate rate of polymerization are observed. When only one reaction locus is active, monomodal molecular weight distributions are obtained. Furthermore, the locus of the polymerization influences directly the rate at which the polymer is produced and the degree of polymerization. In order to complete the study, a model has been developed demonstrating that diffusion limitations are operative in the dispersion polymerization of MMA in scCO2. A gel effect is present in the polymer-rich phase that leads to the increase of the molecular weight of the polymer produced, as observed also experimentally. Moreover, the presence of this gel effect occurring inside the polymer-rich particles explains the auto-acceleration of the polymerization rate as the conversion increases. The model demonstrates that in the case of an effective dispersion polymerization of the methyl methacrylate in scCO2 the main reaction loci are the polymer-rich particles. ______________________________ 1 A dispersion polymerization is composed of polymer-rich particles dispersed in the CO2-rich continuous phase. The polymerization locus is referred to the phase into which the polymerization can take place.

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