Polymerized Ionic Liquids

Ali Eftekhari, Ali Eftekhari

• Editorial: Royal Society of Chemistry
• Año: 2017
• ISBN:
  9781788012218
• Edición: 1
• Categoría: Technology & Engineering
• Categoría: Materials Science

The applications of ionic liquids can be enormously expanded by arranging the organic ions in the form ofa polymer architecture. Polymerized ionic liquids (PILs), also known as poly(ionic liquid)s or polymeric ionic liquids, provide almost all features of ionic polymers plus a rare versatility in design. The mechanical properties of the solid or solid-like polymers can also be controlled by external stimuli, the basis for designing smart materials.

Known for over four decades, PILs are a member of the ionic polymers family. Although the previous forms of ionic polymers have a partial ionicity, PILs are entirely composed of ions. Therefore, they offer a better flexibility for designing a responsive architecture as smart materials. Despite the terminology, PILs can be synthesized from solid organic ionic salts since the monomer liquidity is not a requirement for the polymerization process. Ionicity can also be induced to a neutral polymer by post-polymerization treatments.

This is indeed an emerging field whose capabilities have been somehow overshadowed by the popularity of ionic liquids. However, recent reports in the literature have shown impressive potentials for the future. Written by leading authors, the present book provides a comprehensive overview of this exciting area, discussing various aspects of PILs and their applications as smart materials. Owing to the novelty of this area of research, the book will appeal to a broad readership including students and researchers from materials science, polymer science, chemistry, and physics.

CHAPTER 1

Polymerization in Ionic Liquids

NIKHIL K. SINGHA^a, KUNLUN HONG^b AND JIMMY W. MAYS^*c

^a Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, India

^b Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

^c Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA

^*E-mail: [email protected]

1.1 Introduction

Most liquids used as solvents are composed of neutral molecules. In contrast, ionic liquids (ILs) are salts in the liquid state at ambient or near ambient temperatures. This room temperature ionic liquid state is often achieved by choosing ion pairs where one is organic and has a delocalized charge, or by choosing bulky asymmetric substituents. This causes the ions to be poorly coordinated, resulting in low melting temperatures. In principle, literally millions of ionic liquids with an exceptionally wide range of properties can be produced. This has led to ILs being considered as designer solvents or task specific solvents, where their extremely low vapor pressures offer potential to minimize pollution associated with volatile organic compound (VOC) solvents through recycling.¹

Some examples of common IL cations and anions and their general characteristics are shown in Figure 1.1. Imidazolium- and pyridinium-based ILs feature delocalized cations, whereas quaternary ammonium cations are asymmetrically substituted. A very wide range of properties, including viscosity, hydrophobicity or hydrophilicity, thermal and chemical stability, melting point, flammability and cost, can be tuned by judicious selection of the cation and anion.

Figure 1.1 Some common IL cations and anions and their properties.

Probably the first room temperature IL, ethylammonium nitrate (m.p. 12 °C), was reported by Walden² in 1914. However, there was little interest in ILs until the mid-1970s when they attracted attention as electrolytes for batteries.^3,4 Nowadays, ILs are being intensely investigated in a wide range of applications, including use as solvents in a wide range of chemical processes.⁵ ILs have been used as solvents for Diels–Alder reactions,⁶ hydrogenations,⁷ alkylations,⁸ Friedel–Crafts reactions,⁹ Heck reactions,¹⁰ Suzuki couplings,^11,12 metathesis reactions,^13,14 and many others.

To the best of our knowledge, the first polymerization in ILs was reported in 1990 by Carlin et al.¹⁵ They reported that TiCl₄ and AlEthylCl₂ in AlCl₃/1-ethyl-3-methylimmidazolium chloride ([EMIM]Cl) could polymerize ethylene in low yields. Subsequent work by the same group, where TiCl₄ was replaced by Cp₂TiCl₂, gave higher yields.¹⁶ This pioneering work, using ILs as a reaction medium for polymerization, has inspired numerous researchers over the past quarter of a century to investigate a wide range of different types of polymerization in ILs. While much of this work was inspired by the “green” aspects of ionic liquids (very low vapor pressure and potential for recycling), it quickly became apparent that chemistry could often proceed differently (faster polymerization rates, higher molecular weights, enhanced yields, etc.) in ILs. Providing a review of the field of polymerization in ionic liquids, with particular attention to developments over the past several years, is the subject of this chapter. The reader is referred to earlier reviews in this field for additional details on work in this area.^17–23

1.2 ILs in Conventional Free Radical Polymerization

Free radical polymerization, because of its compatibility with a wide range of monomers having different types of functional groups, is one of the most widely used polymerization techniques. Free radical solution polymerization is of great commercial importance as dilution of radical polymerizations with a solvent, typically a VOC, is effective in controlling viscosity and the exotherm accompanying polymerization. Hong et al.²⁴ noted large increases in the rate of polymerization and much higher molecular weights for free radical polymerization of methyl methacrylate (MMA) in [BMIM]PF₆ as compared to polymerizations carried out under identical conditions in VOCs. These effects were attributed at least in part to the high viscosity of the polymerization medium. A “diffusion-controlled termination” mechanism was proposed to explain the decreased rate of chain termination in these viscous systems. A decrease in termination rate could explain a simultaneous increase in the rate of polymerization and molecular weight.²⁴ In contrast to variations in the rate of polymerization and molecular weight, the polymers synthesized in RTILs have similar glass transition temperatures and microstructures as compared to those obtained in benzene or in bulk, based upon thermal analysis and ¹³C-NMR experiments.²⁵ Since then, other groups^26–28 have reported similar behavior, high molecular weights and rapid polymerization rates, for MMA and other methacrylates in [BMIM]PF₆. Harrison et al.^29,30 used pulse laser polymerization (PLP) techniques to polymerize MMA in [BMIM]PF₆. They found that both the propagation and termination rates, k_p and k_t, respectively, were strongly affected by the presence of [BMIM]PF₆. They attributed the increase in propagation rate to the high polarity of the ionic liquid solution, which reduces the activation energy of propagation via charge–transfer interactions. The termination rate decrease was attributed to the increased viscosity of the polymerization medium. Both the increase of k_p and the decrease of k_t combine to account for a ten-fold increase in the overall rate of polymerization. Subsequent PLP studies^31,32 have reported k_t to be decreased by an order of magnitude and k_p to be increased by a factor of 4 for free radical polymerization in ILs.

Polenz and co-workers recently studied the polymerization of MMA co-initiated by imine bases and found that the reaction was accelerated greatly by even a trace amount of IL.³³ They demonstrated that the polymerization proceeded via a free radical mechanism, with the addition of IL decreasing the activation energy of polymerization and increasing the rate of polymerization. These effects were attributed to interactions between the IL and the imine base. Cheng et al.³⁴ studied free radical polymerization of acrylonitrile in [BMIM]BF₄ using AIBN as an initiator. This team found that ionic liquids are excellent media for obtaining high molecular weight polymers. They attribute this finding to the low chain transfer constants for ILs and their ability to stabilize growing radical chain ends. Puttick et al.³⁵ used NMR to investigate nanoscale domains formed in dialkylimidazolium ILs. When polymerizing MMA in this type of IL, it was shown that the reactants and intermediates have different affinities for nanodomains that form within the IL. Segregation of different species within these domains accounts for the unusually high polymerization rates and increased molecular weights.

Many early studies on free radical polymerization in ILs used ...