Polymer Synthesis in Supercritical Carbon Dioxide

The primary consideration is that polymer synthesis in SCFs are generally carried out at high pressure consequently, it is important to analyze the influence of this parameter on the polymerization process. The increase of pressure can affect a polymerization process by  

Apart from pressure effects, which are not unique to SCFs, some other peculiar aspects of polymerization in SCFs are related to the physico-chemical properties of compounds under supercritical conditions. 

In traditional liquid solvents, the polymerization reaction rates are often limited by the local increase in viscosity during the process, as this lowers the mass transfer rate of the monomer to the reaction site. A lower viscosity and a higher diffusion coefficient in SCFs each contribute to overcome this limitation, however, allowing the polymerization rate to be significant up to high value of monomer conversion. 

The initiation step could also be positively affected by the above-mentioned transport properties, as the efficiency factor f assumes higher values with respect to conventional liquid solvents due to the diminished solvent cage effect One further advantage is constituted by the tunability of the compressibility-dependent properties such as density, dielectric constant, heat capacity, and viscosity, all of which offer additional possibilities to modify the performances of the polymerization process. This aspect could be particularly relevant in the case of copolymerization reactions, where the reactivity ratios of the two monomers, and ultimately the final composition of the copolymer, could be controlled by modifying the pressure of the reaction system. 

Apart from polymerization processes with gaseous monomers above their critical points-for example, the synthesis of low-density poly(ethylene) - several SCFs have been tested as inert reaction media, such as ethane, propane, butane, and C02. Among these, scC02 is by far the most widely investigated, because it links positive fluid effects on the polymers with environmental advantages this makes scC02 the main candidate as an alternative to traditional solvents used in polymer syntheses. 

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