PDF Publication Title:
Text from PDF Page: 004
546 Chemical Reviews, 1999, Vol. 99, No. 2 Kendall et al. and other28-33 work, the plasticization of polymers by CO2 is well-established. The highly plasticized state of the polymer can result in increased polymerization rates by the enhanced diffusion of monomer into the polymer.34-36 The effects of this plasticization can be studied by controlling the reaction pressure and temperature. The plasticization of polymers with SCFs can also be used to lower the melt and solution viscosities37-39 and affect polymer morphology with supercritical drying40 or foaming.41-43 Polymer plasticization by supercritical CO2 can also be used in the removal of residual monomer.1 The high transport properties of supercritical CO2 such as high diffusivity and low viscosity coupled with the propensity of CO2 to plasticize polymers were em- ployed to remove N-vinylcarbazole from poly(N- vinylcarbazole).10 Supercritical CO2 treatment of polymer resulted in lowering monomer levels from 3.66% to 0.1%. In this system, traditional solvents were unsuitable for removal of residual monomer. Supercritical fluid CO2 has indeed been shown to be a promising solvent in which to perform polym- erization reactions. Chain-growth routes such as free- radical polymerization of styrenics, acrylates, and methacrylates, cationic polymerization of isobutylene, vinyl ethers, and styrene, and transition metal- catalyzed polymerization of norbornene and copo- lymerization of epoxides and CO2 have been reported. Step-growth reactions in CO2 have produced poly- carbonates, polyamides, polyesters, polypyrrole, polyphenoxides, and silica gels. In addition, the numerous supercritical CO2 processing techniques for polymers that are being developed become simplified when the synthesis of the polymer is performed in supercritical CO2. These examples illustrate the versatility and importance of supercritical CO2 as a solvent for polymerization reactions. II. Chain-Growth Polymerizations in CO2 The major types of chain-growth polymerization routes include free-radical, cationic, anionic, and metal-catalyzed reactions. Most chain-growth poly- merizations in CO2 have focused on free-radical polymerizations, but there are a number of reports in the areas of cationic and metal-catalyzed reactions. We are unaware of any reports of anionic polymer- izations in CO2: reactive anions would attack the Lewis acid CO2, and thus anionic polymerizations in CO2 are unlikely unless extremely weak nucleophiles are involved, such as siloxanolates. Initial breakthroughs in the 1960s in the use of compressed CO2 as a continuous phase for polymer- izations, especially in cationic and free-radical pre- cipitation polymerizations, were followed by very little activity in the 1970s and 1980s. The 1990s, however, have seen an explosion of research in this area. The next breakthrough in the use of CO2 as a polymerization medium was realized when siloxanes and amorphous fluoropolymers were identified as polymeric materials which had high solubilities in CO2 at easily attainable temperatures (T < 100 °C) and pressures (P < 350 bar).1 This realization opened up new areas of research in CO2, mainly homoge- neous polymerizations as well as dispersion and emulsion polymerizations. A. Free-Radical Polymerizations Free-radical polymerizations can be classified as either homogeneous or heterogeneous reactions. In a homogeneous polymerization all components, in- cluding monomer, initiator, and polymer, are soluble throughout the duration of the reaction; a hetero- geneous polymerization contains at least one in- soluble component at some point during the reaction. Because the terminology to describe heterogeneous polymerization processes has been used inconsis- tently in the literature,44 a brief treatment of this subject is necessary to avoid confusion. The four most widely studied heterogeneous processes (precipita- tion, suspension, dispersion, and emulsion) can be clearly distinguished on the basis of the initial state of the polymerization mixture, the kinetics of polym- erization, the mechanism of particle formation, and the shape and size of the final polymer particles.45 Other heterogeneous processes which are not pres- ently of industrial importance and which will not be discussed here in detail include miniemulsion46,47 polymerization and microemulsion48 polymerization. In a precipitation polymerization, an initially homo- geneous mixture of monomer, initiator, and solvent becomes heterogeneous during the reaction as in- soluble polymer chains aggregate to form a separate polymer phase. In a suspension polymerization, on the other hand, neither the monomer nor the initiator are soluble in the continuous phase. The resulting polymer is also insoluble in the continuous phase, which simply acts as a dispersant and heat-dissipa- tion agent during the polymerization. As a result of the high solubility of many common monomers and organic initiators in compressed CO2, suspension polymerizations in CO2 are not common and will not be presented herein. Dispersion and emulsion poly- merizations constitute the colloid-forming polymer- ization methods that have been recently explored using CO2. A brief discussion of the traditional definitions of these colloid-forming processes follows. A dispersion polymerization begins as a homo- geneous mixture because of the solubility of both the monomer and the initiator in the continuous phase. Once the growing oligomeric radicals reach a critical molecular weight, the chains are no longer soluble in the reaction medium and phase separation occurs. At this point the surface active stabilizing molecule adsorbs to or becomes chemically attached to the polymer colloid and prevents coagulation or ag- glomeration of the particles. In this respect, a disper- sion polymerization is often considered to be a “modified precipitation polymerization”.49 Polymeri- zation persists both in the continuous phase and in the growing polymer particles. Since the initiator and monomer are not segregated or compartmentalized, dispersion polymerizations do not follow Smith- Ewart kinetics. However, enhanced rates of polym- erization are often observed due to the autoacceler- ation (Gel or Trommsdorf) effect within a growing polymer particle. The product from a dispersion polymerization exists as spherical polymer particles,PDF Image | Polymerizations in Supercritical Carbon Dioxide
PDF Search Title:
Polymerizations in Supercritical Carbon DioxideOriginal File Name Searched:
desimonepolymerization.pdfDIY PDF Search: Google It | Yahoo | Bing
NFT (Non Fungible Token): Buy our tech, design, development or system NFT and become part of our tech NFT network... More Info
IT XR Project Redstone NFT Available for Sale: NFT for high tech turbine design with one part 3D printed counter-rotating energy turbine. Be part of the future with this NFT. Can be bought and sold but only one design NFT exists. Royalties go to the developer (Infinity) to keep enhancing design and applications... More Info
Infinity Turbine IT XR Project Redstone Design: NFT for sale... NFT for high tech turbine design with one part 3D printed counter-rotating energy turbine. Includes all rights to this turbine design, including license for Fluid Handling Block I and II for the turbine assembly and housing. The NFT includes the blueprints (cad/cam), revenue streams, and all future development of the IT XR Project Redstone... More Info
Infinity Turbine ROT Radial Outflow Turbine 24 Design and Worldwide Rights: NFT for sale... NFT for the ROT 24 energy turbine. Be part of the future with this NFT. This design can be bought and sold but only one design NFT exists. You may manufacture the unit, or get the revenues from its sale from Infinity Turbine. Royalties go to the developer (Infinity) to keep enhancing design and applications... More Info
Infinity Supercritical CO2 10 Liter Extractor Design and Worldwide Rights: The Infinity Supercritical 10L CO2 extractor is for botanical oil extraction, which is rich in terpenes and can produce shelf ready full spectrum oil. With over 5 years of development, this industry leader mature extractor machine has been sold since 2015 and is part of many profitable businesses. The process can also be used for electrowinning, e-waste recycling, and lithium battery recycling, gold mining electronic wastes, precious metals. CO2 can also be used in a reverse fuel cell with nafion to make a gas-to-liquids fuel, such as methanol, ethanol and butanol or ethylene. Supercritical CO2 has also been used for treating nafion to make it more effective catalyst. This NFT is for the purchase of worldwide rights which includes the design. More Info
NFT (Non Fungible Token): Buy our tech, design, development or system NFT and become part of our tech NFT network... More Info
Infinity Turbine Products: Special for this month, any plans are $10,000 for complete Cad/Cam blueprints. License is for one build. Try before you buy a production license. May pay by Bitcoin or other Crypto. Products Page... More Info
| CONTACT TEL: 608-238-6001 Email: greg@infinityturbine.com | RSS | AMP |