Carbon dioxide free-radical polymerization

Romack TJ (1997) Polymerization of fluoroolefins in liquid and supercritical carbon dioxide (free radical). PhD Thesis, University of North Carolina at Chapel HUl, Chapel Hill,NC... 

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. 

Under suitable conditions, anionic polymerization is faster than free-radical polymerization and so can be conducted at lower temperatures. The main reasons are fast initiation by an ionic reaction and absence of an effective termination mechanism. However, the sensitivity to impurities is much greater and choice and control of reaction conditions are more delicate. Water, oxygen, carbon dioxide, and other substances able to react with carbanion chain carriers must be strictly excluded. 

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... 

Cosgrove et al. in 1985 (828a) showed that it was possible to detect hydroxyl radicals in an aqueous extract of the particulate phase of cigarette MSS under metal-mediated conditions. An unidentified alkyl radical and the carbon dioxide anion radical were also observed nnder their experimental conditions. They concluded that the results of their experiment indicated that the major particnlate-phase free radical was a quinone/hydro-quinone redox system in a polymeric matrix and that the radical signal conld become associated with DNA. Unequivocal proof was not provided for these claims. 

In the last three years, reviews and publications by DeSimone and co-workers [52-59] have shown that free-radical polymerizations can be carried out with ease in supercritical fluids such as carbon dioxide. 

Kinetics of Free-Radical Polymerization in Homogeneous Phase of Supercritical Carbon Dioxide ... 

Scheme 9,4 Free-radical polymerization of FOA in supercritical carbon dioxide.

Recent work by several research groups has shown that supercritical fluids can be superior to other solvents for several chemical processes. For example, DeSimone has demonstrated the ability of supercritical CO2 to replace Freons in the free radical polymerization of fluorinatkl acrylate monomers. 34) Noyori has shown that significant rate enhancements can be achieved in supercritical carbon dioxide relative to other solvents for the homogeneous catalytic hydrogenation of carbon dioxide to either formic acid or its derivatives in the presence of triethylamine or triethylamine/methanol respectively, (equation 1). (55-57) As discussed below, we have recently demonstrated that improved enantioselectivities can be achieved in supercritical carbon dioxide for the catalytic asymmetric hydrogenation of several enamides. 5 8)... 

Another example is the polymerization of acrylic acid in supercritical carbon dioxide (20 wt %) with t-butyl hydroperoxide as initiator [24]. The effect of initiator concentration (2 to 6%), temperature (160 to 350 °C) and pressure (185 to 320 bar) have been reported. The polymerization was conducted at 250 °C and 310 bar. Contrary to conventional free radical polymerizations, in these polymerizations, it is reported that the initiator amount does not influence the molecular weight, even though in the absence of the initiator polymerization would not proceed. No explanation for this observation has been provided. Also, at a given temperature, molecular weight was found to decrease with pressure. For example, at 160 °C weight average molecular weight decreased from about 80,000 to 45,000 upon increase of the pressure from 185 to 250 bar while polydispersity was reduced from about 10 to 7. This behavior is somewhat similar to the... 

A recent approach in polymerization studies in our laboratory is the use of binary fluid mixtures of carbon dioxide with a traditional solvent to increase the precipitation thresholds and thus achieve higher molecular weights [26]. In this context AIBN initiated free-radical polymerization of styrene has been reported in mixtures of carbon dioxide with pentane, toluene and sulfur hexafluoride [26]. 

Figure 5. Weight average molecular weight of polystyrene from free-radical polymerization of styrene with AIBN initiator in binary fluid mixtures of carbon dioxide + co-solvent (pentane or toluene) at 6ffC after 5 hr polymerization time. Left Influence of the cosolvent amount at 56 MPa. Right Influence of pressure in solvents containing 90 % carbon dioxide + 10 % toluene or 10 % pentane.

Figure 7. Weight average molecular weight of polystyrene ( x 10 ) from free-radical polymerization of styrene in carbon dioxide + sulfur hexafluoride mixtures. Left. Influence of SF6 content at 56 MPa and 51 °C, 5 hr polymerization time. Right Influence of pressure in the solvent mixture containing 30 % by mass SFe at 51 °C. Pressure dependence of the density (in g/cm ) of the solvent mixture is shown on the right coordinate. [1 MPa = 10 bar],...

A parallel study has reported the synthesis of crosslinked polymer microspheres in supercritical carbon dioxide [54]. Heterogeneous free-radical polymerization of divinyl benzene and ethyl benzene were carried out at 65 C and 310 bar using AIBN initiator to form the crosslinked polymer. It is shown that in the absence of surfactants as stabilizers, polymerization of the mixture containing 80 % divinyl benzene + 20 % ethyl benzene leads to poly(divinylbenzene) microspheres of about 2.4 micron diameter [Figure 14]. In the presence of a carbon dioxide-soluble diblock copolymer as a stabilizer, polymerization of the mixture with the same monomer ratio proceeds as an emulsion and lead to smaller crosslinked particles (ca. 0.3 micron). Thermal analysis shows that the crosslinked polymer that is formed from these polymerizations is stable up to 400 C. 

Beuermann, S., Buback, M., Isemer, C. and Wahl, A. (1998) Homogeneous phase free-radical polymerization of styrene in supercritical carbon dioxide. Paper presented at theM TO ASI on Supercritical Fluids, Kemer, Antalya, Turkey, July 1998. (b) Sabine, B., Buback, M., Isemer, C., and Wahl, A. (1999) Homogeneous free-radical polymerization of styrene in supercritical COiMacromol. Rapid. Commun. 20, 26-32. 

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