Investigation of the effects of various parameters on the individual polymerizations of methyl methacrylate and styrene in supercritical CO2 medium

In this work, due to its being non-toxic and alternative to many organic solvents, and having mild critical properties, supercritical CO2 (scCO2) is used as the polymerization medium for homogenous dispersion polymerizations of methyl methacylate and styrene in the presence of surfactants synthesized in this work or supplied commercially.Initially, cloud point experiments were carried out with methyl methacylate and styrene monomers in a 105 ml stainless steel view cell to determine the minimum reaction pressures to be used. Next, p(HDFDA-co-SiMA) was synthesized to be used as the surfactant for the polymerization in the scCO2 medium. Additionally, the fluorine containing commercial surfactants (Clariant Nuva DT, Clariant Nuva HPC and Krytox 157FSL) were provided. The cloud points of each of p(HDFDA-co-SiMA) and commercial surfactants were determined. To reflect the polymerization solution the cloud points of each monomer+surfactant binary system were also determined.In the polymerization reactions, the effects of pressure, reaction time, amount of surfactant and initiator on the polymerization yield, molecular weight distribution, structure and morphology of the products were investigated by using various analytical techniques. Dispersion polymerizations of methyl methacrylate and styrene were carried out in scCO2 using various surfactants in the presence of the initiator 2,2'-azobisisobutyronitrile (AIBN) in a 40 ml stainless steel reactor at 65°C and pressure range of 15-40 MPa. Screening tests yielded the optimum pressure of 35 MPa. Polymerization times of 4-48 h were used, and different optimum polymerization times (12-36 h) were obtained with different surfactants. The initiator concentrations were changed in the range of 1-20% (w/w) to investigate their effect on the product. However, 1% (w/w) concentration was used for the rest of the experiments. 0-20% (w/w) surfactant concentrations were used to investigate the surfactant concentration effect. However, 5% (w/w) concentration was used generally for the rest of the experiments. Molecular weights and particle size distributions of the products varied between 1000-140 000 g/mol and 1-2.5 ? m, respectively in the screening tests.The positive effects of surfactants on the yields, particle size distributions and morphologies of both polymethyl methacrylate (PMMA) and polystyrene (PS) at the optimum reaction conditions decreased in the following order: [p(HDFDA-co-SiMA)] > Clariant Nuva surfactants > Krytox 157FSL. However, the yields of PMMA were much higher then PS for all surfactants. The yields of both PMMA and PS, which were dry, white, free-flowing and micron-size, varied between 85-95% at the optimum conditions.Finally, polymerization kinetics of methyl methacrylate and styrene were investigated at 15-360 minutes reaction times with yields of less than 10% by determining the reaction order, reaction rate constant and the rate equation, and the molecular weights and the morphologies of the products obtained at these conditions were comparatively studied. It has been observed that the ratio of rate constants of PMMA and PS formations (~ 4) is compatible with the ratio of optimum polymerization times (~ 3).