Polymer Synthesis in Ionic Liquids

Ambient-temperature ionic liquids have received much attention in both academia and industry, due to their potential as replacements for volatile organic compounds (VOCs) [1-3]. These studies have utilized the ionic liquids as direct replacements for conventional solvents and as a method to immobilize transition metal catalysts in biphasic processes. 

Many organic chemical transformations have been carried out in ionic liquids hydrogenation [4, 5], oxidation [6], epoxidation [7], and hydroformylation [8] reactions, for example. In addition to these processes, numerous synthetic routes involve a carbon-carbon (C-C) bond-forming step. As a result, many C-C bond-forming procedures have been studied in ambient-temperature ionic liquids. Among those reported are the Friedel-Crafts acylation [9] and alkylation [10] reactions, allylation reactions [11, 12], the Diels-Alder reaction [13], the Heck reaction [14], and the Suzuki [15] and Trost-Tsuji coupling [16] reactions. 

In these reactions the system is tuned, for example by adjustment of the reaction temperature and time and modification of the catalyst structure to maximize the quantity of the desired dimers produced, and to minimize the production of higher molecular weight oligomers and polymers. In other reactions it is the opposite 

Polymers are essential to modem society. They are found in every household as plastics, fibers, coatings, detergents, adhesives etc. So, it is not surprising that the use of ionic liquids as solvents for polymerization reactions is now being extensively explored. This is the subject of this chapter and reviews in the literature have already covered aspects of this field in some detail [1]. 

Studies on the dimerization and hydrogenation of olefins with transition metal catalysts in acidic chloroaluminate (iii) ionic liquids report the formation of higher molecular weight fractions consistent with cationic initiation [3-6]. These studies ascribed the occurrence of the undesired side reaction to both Lewis acid and proton-catalyzed polymerization routes. Studies have shown that when protons, from HCl as the source, are dissolved at ordinary temperatures and pressures in the 

Ionic Liquids in Synthesis, Second Edition. P. Wasserscheid and T. Welton (Eds.) Copyright 2008 WILEY-VCH Verlags GmbH Co. KGaA, Weinheim ISBN 978-3-527-31239-9 

Ionic liquid catalyzed polymerization of butene is not limited to the use of pure alkene feedstocks, which can be relatively expensive. More usefully, the technology can be applied to mixtures of butenes, for example, the low value hydrocarbon feedstocks raffinate I and raffinate II have been used. The raffinate feedstocks are principally C4 hydrocarbon mixtures rich in butenes. When these feedstocks are 

This technology has been utilized by BP Chemicals for the production of lubricating oils with well-defined characteristics (e.g. pour point and viscosity index). It is used in conjunction with a mixture of olefins (i.e. different isomers and different chain length olefins) to produce lubricating oils ofhigher viscosity than is obtainable by conventional catalysis [13]. Unichema Chemie BV have applied these principles to more complex monomers, e.g. unsaturated fatty acids, to create a mixture of products [14]. 

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Microwave-Assisted Polymer Synthesis in Ionic Liquids.57... 

Carmichael, A. J. Haddleton, D. M (2003). Polymer Synthesis in Ionic Liquids. In Ionic Liquids in Synthesis, Wasserscheid, P. Welton, T. (Editors), Ionic Liquids in Synthesis. 319-335, Wdey-VHC Verlag, Weinheim... 

Carmichael AJ, Haddleton DM (2003) Polymer synthesis in ionic liquids. In Wasserscheid P, Welton T (eds) Ionic liquids in synthesis. WUey, Weinheim, pp 319-335... 

Marcilla R, De Geus M, Mecerreyes D et al (2006) Enzymatic polyester synthesis in ionic liquids. Eur Polym J 42 1215-1221... 

Kubisa et al. [64] have been exploring the use of chiral ionic liquids in polymer synthesis. Using ionic liquids with a chiral substituent on the imidazolium ring for the ATRP of methyl acrylate gave a small but definite effect on polymer tacticity, with more isotactic polymer formed than in simple [BMIM][PF6]. They also found that the use of ionic liquids led to fewer side reactions. Ionic liquids have been used as solvents in biphasic ATRP to facilitate the separation of the products from the catalysts [65]. 

The controlled synthesis of polymers, as opposed to their undesired formation, is an area that has not received much academic interest. Most interest to date has been commercial, and focused on a narrow area the use ofchloroaluminate(III) ionic liquids for cationic polymerization reactions. The lack of publications in the area, together with the lack of detailed and useful synthetic information in the patent literature, places hurdles in front of those with limited loiowledge of ionic liquid technology who wish to employ it for polymerization studies. The expanding interest in ionic liquids as solvents for synthesis, most notably for the synthesis of discrete organic molecules, should stimulate interest in their use for polymer science. 

There are also a number of other variables to consider when planning the electrochemical synthesis of conducting polymers in ionic liquids. While most of these variables also exist for the synthesis of the polymers in molecular/solvent systems and have been investigated in detail, it is worth considering that the influence of any of these factors may be different when utilizing ionic liquids as the growth medium because of their distinctly different properties. These are discussed in more detail below. 

When ionic liquids are used, this will have a significant effect on the viscosity and hence the conductivity and rate of ion diffusion within the ionic liquids. Growth of conducting polymers at reduced temperatures (as low as — 28 ° C) [4,24] in molecular solvent systems is generally accepted to result in smoother, more conductive films, but we have found that in ionic liquids the significant increase in the viscosity can be problematic. In addition, the temperature used for the conducting polymer synthesis may be limited by the melting point of the ionic liquid [25]. 

Fig. 7.4 The cations and anions utilized to date for the electrochemical synthesis of conducting polymers in ionic liquids, and their abbreviations.

Danielsson et al. [25] have studied the synthesis of PEDOT in ionic liquids that utilize bulky organic anions, l-butyl-3-methylimidazolium diethylene glycol monomethyl ether sulfate and l-butyl-3-methylimidazolium octyl sulfate, the latter of which is a solid at room temperature and thus requires the addition of either monomer or solvent (in this case water) to form a liquid at room temperature. Polymerization in a water-free ionic liquid was only possible in the octyl sulfate species, but the polymerization of EDOT was successful in aqueous solutions of both the ionic liquids (0.1 M). The ionic liquid anions appear to be mobile within the polymer, exchangeable with chloride ions at a polymer/KCl(aq) interface, but it is interesting that when the PEDOT is in aqueous solutions of the ionic liquid, at higher concentrations (0.01-0.1 M) the imidazolium cation can suppress this anion response. The ion mobility in both the ionic liquid and in the polymer film in contact with the solution is significantly increased by addition of water. 

Endres et al. [82] have demonstrated the suitability of an air- and water-stable ionic liquid for the electropolymerization of benzene. This synthesis is normally restricted to media such as concentrated sulfuric acid, liquid SO2 or liquid HF as the solution must be completely anhydrous. The ionic liquid used, l-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, can be dried to below 3 ppm water, and this ionic liquid is also exceptionally stable, particularly in the anodic regime. Using this ionic liquid, poly(para-phenylene) was successfully deposited onto platinum as a coherent, electroactive film. Electrochemical quartz crystal microbalance techniques were also used to study the deposition and redox behavior of the polymer from this ionic liquid (Section 7.4.1) [83]. 

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