Supercritical free radical polymerization

We have also investigated the kinetics of free radical initiation using azobisisobutyronitrile (AIBN) as the initiator [24]. Using high pressure ultraviolet spectroscopy, it was shown that AIBN decomposes slower in C02 than in a traditional hydrocarbon liquid solvent such as benzene, but with much greater efficiency due to the decreased solvent cage effect in the low viscosity supercritical medium. The conclusion of this work was that C02 is inert to free radicals and therefore represents an excellent solvent for conducting free radical polymerizations. 

Finally, the use of stable free radical polymerization techniques in supercritical C02 represents an exciting new topic of research. Work in this area by Odell and Hamer involves the use of reversibly terminating stable free radicals generated by systems such as benzoyl peroxide or AIBN and 2,2,6,6-tetramethyl-l-piperidinyloxy free radical (TEMPO) [94], In these experiments, styrene was polymerized at a temperature of 125 °C and a pressure of 240-275 bar C02. When the concentration of monomer was low (10% by volume) the low conversion of PS which was produced had a Mn of about 3000 g/mol and a narrow MWD (PDI < 1.3). NMR analysis showed that the precipitated PS chains are primarily TEMPO capped, and the polymer could be isolated and then subsequently extended by the addition of more styrene under an inert argon blanket. The authors also demonstrated that the chains could be extended... 

Scheme 1. Free radical polymerization of fluorocarbon acrylates in supercritical C02.

Z. Guan, Homogeneous free radical polymerization in supercritical carbon dioxide, University of... 

Guan, Z. Combes, J. R. Menceloglu, Y. Z. DeSimone, J. M. Homogeneous Free Radical Polymerizations in Supercritical Carbon Dioxide 2. Thermal Decomposition of 2,2 -Azobis(isobutyronitrile), Macromolecules 1993, 26, 2663-2669. 

As a comparison and reproduction to recent works done by DeSimone and coworkers [2], attempts were made in the free radical polymerization of acrylic acid (AA) in supercritical carbon dioxide (SCCO2) with Azoisobutyronitrile (AIBN) as initiator. 

In this article we describe the phase behavior of a microemulsion system chosen for the free radical polymerization of acrylamide within near-critical and supercritical alkane continuous phases. The effects of pressure, temperature, and composition on the phase behavior all influence the choice of operating parameters for the polymerization. These results not only provide a basis for subsequent polymerization studies, but also provide data on the properties of reverse micelles formed in supercritical fluids from nonionic surfactants. 

Radical polymerization can be carried out under homogenous as well as heterogenous conditions. This difference is classified based on whether the initial mixture and/or final product are homogenous or heterogenous. Some homogenous mixtures become heterogenous as polymerization proceeds as a result of insolubility of the resulting polymer in the reaction media. There are many other specialized processes that are used to synthesize materials via free-radical polymerization. These include interfacial polymerization, gas phase reactions ( popcorn polymerization ), as well as the use of specialized media like supercritical fluids. Current research efforts include the study of such... 

CO2 is completely nonflammable. This property provides a tremendous advantage for some traditionally hazardous chemical processes and reactions. For example, fluoroethylene monomers used for the production of tetrafluoroethylene (TFE) are rendered nonexplosive when mixed with CO2. In addition, highly reactive free-radical polymerization of these monomers can be carried out directly in a supercritical CO2 continuous phase. 

Beuermann S, Buback M, Busch M. Free-radical polymerization in reactive supercritical fluids. In Jessop PG, Leitner W, eds. Chemical Synthesis Using Supercritical Fluids. Weinheim, Germany Wiley-VCH, 1999 326-350. 

Numerous examples of block copolymers formed in supercritical C02 via the bifunctional initiator approach have been reported [54], Perhaps the most common approach is to incorporate eROP with free-radical polymerization-the general scheme for this methodology is shown in Figure 13.3. Howdle et al. [55] was the first to report the synthesis of a block copolymer by the bifunctional initiator approach in supercritical C02 and showed the simultaneous eROP of e-caprolactone with controlled free radical polymerization of methyl methacrylate by atom transfer radical polymerization (ATRP)-at this time simultaneous eROP and ATRP had not been reported in any media. The bifunctional initiator incorporated both a primary hydroxyl group (as an initiation site for eROP of e-caprolactone) and a bromine moiety (for initiation of ATRP). Howdle showed that... 

The use of the dual initiator strategy has opened up a whole new area of block copolymer synthesis using enzymes and over the previous five years many reactions and systems have been extensively reported. In general, the previous reports have shown that the use of supercritical C02 has allowed the two reaction mechanisms of eROP and free radical polymerization to occur simultaneously to yield well-defined block and graft copolymers. 

This report has been the only publication involving enzymatically-catalyzed free radical polymerization in supercritical C02, perhaps due to the difficulty in adequately matching monomer or polymer compatible systems with the solvent. 

Ethylene is compressed to 2,700 bar and a free-radical initiator, e.g., trace amounts of oxygen or a peroxide, is injected into the feed stream to promote the free-radical polymerization. The polyethylene polymer that is formed remains dissolved in the supercritical ethylene phase at the operating temperature, which ranges from 140 to 250°C. The heat of reaction is removed by through-wall heat transfer when the tubular reactor is used and by regulating the rate of addition of initiator when the autoclave reactor is used. 

The results of a set of three graft copolymers produced by free radical polymerization are shown in figure 9.15 (DeSimone et al., 1988b). The points on the curves represent the composition of the fractions obtained by supercritical chlorodifluoromethane fractionation of the parent copolymer. The... 

Two PMMA-g-PDMS copolymers were also prepared with roughly similar composition (20 wt% and 26 wt% PDMS) and with the same molecular weight PDMS grafts (M = 10,000) by free radical polymerization and by anionic polymerization. The copolymers were first extracted of any unincorporated methacryloxy-terminated PDMS using supercritical carbon dioxide then they were fractionated with supercritical chlorodifluoromethane. Each fraction was characterized in the same manner as described for the three polymers depicted in figure 9.15 and the results are shown in figure 9.16 (DeSimone et al., 1990). The differences in chemical composition distribution profiles of the copolymers... 

During the 1960s and 1970s Ehrlich and coworkers compiled a comprehensive picture of the free-radical polymerization of ethylene in supercritical fluid... 

Takahashi, T. 1980. Absolute rate constants for the free radical polymerization of ethylene in the supercritical phase. Ph.D. diss.. State Univ. of New York at Buffalo. 

We have also studied the kinetics of free radical initiation in CO2 using azobis(isobutyronitrile) (AIBN) as an initiator [35]. These experiments were accomplished using high pressure UV spectroscopy, and illustrated that AIBN decomposes more slowly in CO2 than in traditional hydrocarbon solvents, yet the initiator efficiency is much greater in CO2 due to the reduced solvent cage effect in the low viscosity supercritical medium. The main conclusion drawn from this work was that CO2 can therefore be employed effectively as a solvent for free radical polymerizations and remains an inert solvent even in the presence of highly electrophilic hydrocarbon radicals. 

Free-Radical Polymerization in Reactive Supercritical Fluids... 

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