J. of Supercritical Fluids 48 (2009) 183–187 Contents lists available at ScienceDirect The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu Cationic polymerization of styrene in scCO 2 and [bmim][PF 6 ] C. Bueno a , V.F. Cabral a,∗ , L. Cardozo-Filho a , M.L. Dias b , O.A.C. Antunes c a Universidade Estadual de Maringá, Departamento de Engenharia Química, Av. Colombo 5790, 87020-900 Maringá, PR, Brazil b Universidade Federal do Rio de Janeiro, Instituto de Macromolécula Professora Eloísa Mano (IMA), CT Bloco J, Cidade Universitária, 21945-970 Rio de Janeiro, RJ, Brazil c Universidade Federal do Rio de Janeiro, Instituto de Química, CT Bloco A 641, Cidade Universitária, 21941-909 Rio de Janeiro, RJ, Brazil article info Article history: Received 29 May 2008 Received in revised form 3 September 2008 Accepted 24 September 2008 Keywords: Cationic polymerization Polystyrene Aluminum chloride Ionic liquid scCO2 abstract This work presents a study on the cationic polymerization of styrene in different reaction media, super- critical CO 2 (scCO 2 ), [bmim][PF 6 ], dichloromethane, and scCO 2 plus [bmim][PF 6 ] using AlCl 3 as initiator at temperatures from 273 to 333 K. The reactions were analyzed in relation to the monomer conversion rate, polymer structures, average molecular weights and molecular weight distribution. The polymerization reaction product was characterized by 1 H and 13 C nuclear magnetic resonance (NMR) and by gel perme- ation chromatography (GPC). In all cases, the styrene polymerization produced oligomers. Reactions using ionic liquid as a solvent led to higher molecular weight and monomer conversion rate. Monomer con- version rates of about 100% and weight average molecular weight (M w ) of 2400 at 298 K were obtained. Reactions with scCO 2 as a solvent yielded low monomer conversion rates (around 50%) and a M w = 2000 at 298 K. The oligolystyrenes presented rr syndiotatic-rich sequences in the microstructure in all reaction conditions. The use of ionic liquid and scCO 2 results in better yields than the use of the other organic solvents solvents. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Considering the present need of sustainable development, it is a rule that new chemical processes must contribute to the improve- ment of life quality. In this context, the continued promotion of development with reduced damages to the environment is a great challenge, which requires new chemical approaches to improve processes aiming, especially, the reduction of wastes and toxic effluents. The use of ionic liquids and supercritical fluids as solvents in chemical processes consists in some of this “green chemistry” approach, particularly due to the inflammability and possibility of recycling of these solvents [1]. Ionic liquids are molten salts at room or close to room tem- perature and pressure conditions. Most often, they are colorless and present relatively low viscosity, practically no vapor pressure, low flammability, high ionic conductivity, and are non-volatile. They are good solvents of a large number of organic and inorganic compounds and immiscible in several organic solvents [2,3]. Appli- cations of ionic liquids as solvents in polymerization processes as described in the literature involve a combination of the ionic liq- uids with haloaluminates (AlCl 4 - ). For example, these acidic ionic liquids were used as solvents and initiators in the cationic oligomer- ization of alkenes by Stenzel et al. [4] These authors reported that ∗ Corresponding author. Tel.: +55 44 3261 4749; fax: +55 44 3261 4792. E-mail address: vladimir@deq.uem.br (V.F. Cabral). the oligomerization of linear olefins using the equivalent of 0.1 mol excess AlCl 3 in the ionic liquid 1-butyl-3-methylimidazolium tetrachloroaluminate ([bmim][AlCl 4 ]) in the presence of ethyl alu- minum dichloride resulted in the formation of atatic oligomers with average molar mass in the range of 650–1620 g/mol and large molar mass distribution. After the discovery of ionic liquids stable at room conditions such as 1-n-butyl-3-methylimidazolium hexaflu- orophosphate ([bmim][PF 6 ]) and 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF 4 ]), their use in different polymeriza- tion processes such as in free radical polymerization and in poly- merization by transition metal-based catalysts [5] has been studied as well as in enzymatic process [6] and in polycondensations [7]. Recently, it was reported the cationic polymerization of styrene with a system formed by CH 3 CH(C 6 H 5 )Cl in association with TiCl 4 in [bmim][PF 6 ] as a initiator [8]. Significant yields were obtained after 98 h of reaction. However, the average molar mass was around 2.000 g/mol, independently of monomer conversion. Based on the present knowledge, [bmim][PF 6 ] was chosen and used in that work in styrene polymerization because it does not have affinity to water, which makes purification easier, and because the initiator used is sensitive to moisture. Supercritical CO 2 (scCO 2 ) has also been used in several poly- merization studies [9–12]. These polymerizations using scCO 2 as solvent can be classified as: homogeneous polymerization, when the polymer is soluble in scCO 2 , and heterogeneous polymeriza- tion, when the polymer precipitates. According to DeSimone et al. [9], only two polymer classes are significantly soluble in scCO 2 : 0896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2008.09.023