Dispersion Polymerizations in Supercritical Carbon Dioxide J. M. DeSimone,". E. Maury, Y. Z. Menceloglu, J. B. McClain, T. J. Romack, J. R. Combes Conventional heterogeneous dispersion polymerizations of unsaturated monomers are performed in either aqueous or organic dispersing media with the addition of interfacially active agents to stabilize the colloidal dispersion that forms. Successful stabilization of the polymer colloid during polymerization results in the formation of high molar mass polymers with high rates of polymerization. An environmentally responsible alternative to aqueous and organic dispersing media for heterogeneous dispersion polymerizations is described in which supercritical carbon dioxide (CO,) is used in conjunction with molecularly engi- neered free radical initiators and amphipathic molecules that are specifically designed to be interfacially active in CO,. Conventional lipophilic monomers, exemplified by methyl methacrylate, can be quantitatively (>90 percent) polymerized heterogeneously to very high degrees of polymerization (>3000) in supercritical CO, in the presence of an added stabilizer to form kinetically stable dispersions that result in micrometer-sized particleswith a narrow size distribution. The dissemination of volatile organic com- pounds, chlorofluorocarbons, and aqueous- waste streams into our environment has prompted considerable worldwide concern, and environmental issues are now of para- mount interest to the chemical industrv. To survive, the chemical industry must bk able to conform to more environmentallv sound practices in the manufacture and processing of uroducts. Herein. we describe the use of supercritical CO, as a dispersing medium for the manufacture (and ~rocessine) of ", polymeric materials. specifiEally, we have developed a dispersion polymerization pro- cess in which polymeric stabilizers, molec- ularly engineered to be interfacially active in CO,, are used to form a stable polymer colloid consisting of a lipophilic dispersed phase in a CO, continuous phase. This procedure allows for the synthesis of high molar mass acrylic polymers in the form of micrometer-sized particles with a narrow size distribution. This polymerization methodology obvi- ates the use of aqueous and organic dispers- ing media in this classical manufacturing route to many' of the polymers made on a large commercial scale such as polystyrene, poly (alkyl acrylate)~,poly (ethylene-co-vi- nyl acetate), poly (vinyl chloride) (PVC) , styrene-butadiene rubber, acrylonitrile-bu- tadiene-styrene terpolymers, poly(acry1ic acid), and poly (acrylamide) (1). Typically, these polymers are synthesized with either water (for water-insoluble polymers) or an organic solvent (for water-soluble poly- mers) used as the dispersing medium. Such heterogeneous polymerizations usually form Department of Chemistry, University of North Carolina at Chapel Hill, CB 3290, Venable and Kenan Labora- tories, Chapel Hill, NC 27599, USA. *To whom correspondence should be addressed. at least two phases in which the starting monomers or the resulting polymer, or both, are in the form of a fine dispersion in an immiscible liquid phase; particle sizes typically range from 50 nm to several mil- limeters and can be controlled with surfac- tants to within a relatively narrow size distribution. There are basicallv four differ- ent heterogeneous polymerization tech- niques-precipitation, suspension, emul- sion, and dispersion processes-which are distinguished by (i) the initial phase behav- ior of the polymerization mixture, (ii) the polymerization kinetics, and (iii) the mech- anism of particle formation (2). This report focuses on the design of a dispersion polymerization in supercritical CO,. Typically, a free radical dispersion polymerization starts as a one-phase, homo- geneous system such that both the mono- mer and the polymerization initiator are soluble in the polymerization medium but the resulting polymer is not (3, 4). As a result, the polymerization is initiated ho- Table 1. Results of MMA polymerizations with F-AIBN and AlBN as the initiator in CO, at 204 bar and 65°C (22); stabilizer is either LMW or HMW poly(F0A). MWD, molecular weight dis- tribution. - Stabilizer Yield <(!$ Particle (wlv %) (") g/mol) MWD size (pm) 0% 2% LMW 4% LMW 1% HMW 2% HMW 4% HMW 0% 2% LMW 4% LMW 2% HMW 4% HMW F-AIBN 77 2.9 277 2.5 2.0 (k0.2) 303 2.3 0.9 (20.3) 163 2.3 193 2.3 2.4 (k0.2) 281 2.5 1.4 (kO.l) AIBN 149 2.8 308 2.3 1.2 (k0.3) 220 2.6 1.3 (k0.4) 315 2.1 2.7 (kO.1) 321 2.2 2.5 (20.2) mogeneously and the resulting polymer phase separates into primary particles. Once nucleated, these primary particles become stabilized by amphipathic mole- cules present in the system that prevent particle flocculation and aggregation. Poly- mer colloids produced by dispersion poly- merizations are usuallv stabilized bv a "steric" mechanism as compared with an electrostatic mechanism that is common to colloidal stabilization in aqueous environ- ments (3-7). Steric stabilization of a colloi- dal dispersion is usually imparted by amphi- pathic macromolecules that become ad- sorbed onto the surface of the dispersed phase. These amphipathic macromolecules contain an anchoring segment, which at- taches to the particle usually by physical adsorption, and stabilizing moieties that are soluble in the continuous phase. The stabi- lizing moieties project into the continuous phase (which needs to be a good solvent for the stabilizing moieties) and prevent floc- culation by mutual excluded volume repul- A I Lipophilic,acrylic-like anchor 0 I I TO2-philic"steric stabilizing moiety Fig. 1. The chemical structures of (A) an-inter- facially active, polymeric stabilizer [poly(FOA)], de- picting the proposed site of anchorage for the C0,-philic steric stabiliz- ing moieties, and (B) a free radical initiator (F- AIBN) that has the solu- bility characteristics to selectively partition into a supercritical CO, phase over either a hydrophilic dispersed or a lipophilic dispersed phase. 356 SCIENCE VOL. 265 15 JULY 1994