ScCO2 as polymerization medium for high value polymers: two recent cases

Abstract

In recent years the use of supercritical carbon dioxide (scCO2) in polymer processing and polymerization reactions has merged as a “green” alternative to replace both organic solvents, environmentally hazardous (VOCs), and water, which use involves the production of large amount of waste streams. ScCO2 is non-toxic, inexpensive, and often exhibits high solubility in polymer matrixes. Moreover, it causes polymer plasticization, thus favoring the diffusion of low MW species (monomers, initiators or additives) in the polymer phase. In this work, the use of scCO2 as polymerization medium for two different special polymers is discussed. Fluorinated copolymers mainly find application in environmentally aggressive systems due to their high chemical and thermal resistance. In the case of vinylidene fluoride precipitation reaction in scCO2, bimodal MWDs are typically obtained. Different mechanisms have been proposed to explain such heterogeneity, such as chain-transfer to polymer and transport limitations of the radicals between supercritical and polymer phase. Here we show that carrying out the reaction in a stable dispersion, thus increasing the area between the two phases, monomodal MWDs are obtained, thus confirming the second proposed mechanism. Dendronized polymers are macromolecules which consist of a polymer backbone to which dendrons (tree like structures) of increasing size (generation) are attached at regular distances. The highly branched dendritic layer causes the denpol to change from a random coil polymer to a cylindrical nanoobject with set of features which make them unique macromolecules for a variety of applications. G3 macromonomers had already been synthesized and polymerized under conventional conditions (bulk and solution). Recently we carried out batch reactions in scCO2, using AIBN as initiator. Molar mass determinations with GPC show average molecular weights on the order of several millions, much higher then those obtained under conventional homogeneous conditions. The denpols obtained were checked by SFM for their structural integrity and mostly linear chains were observed. A series of trials with different conditions has been carried out in order to elucidate the reaction mechanism and to determine the role of scCO2