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RSC Advances REVIEW View Article Online View Journal | View Issue Glymes as versatile solvents for chemical reactions and processes: from the laboratory to industry Cite this: RSC Adv.,2014,4,11251 Shaokun Tanga and Hua Zhao*b Glymes, also known as glycol diethers, are saturated non-cyclic polyethers containing no other functional groups. Most glymes are usually less volatile and less toxic than common laboratory organic solvents; in this context, they are more environmentally benign solvents. However, it is also important to point out that some glymes could cause long-term reproductive and developmental damage despite their low acute toxicities. Glymes have both hydrophilic and hydrophobic characteristics that common organic solvents lack. In addition, they are usually thermally and chemically stable, and can even form complexes with ions. Therefore, glymes are found in a broad range of laboratory applications including organic synthesis, electrochemistry, biocatalysis, materials, Chemical Vapor Deposition (CVD), etc. In addition, glymes are used in numerous industrial applications, such as cleaning products, inks, adhesives and coatings, Received 6th October 2013 batteries and electronics, absorption refrigeration and heat pumps, as well as pharmaceutical Accepted 9th December 2013 formulations, etc. However, there is a lack of a comprehensive and critical review on this attractive DOI: 10.1039/c3ra47191h subject. This review aims to accomplish this task by providing an in-depth understanding of glymes' www.rsc.org/advances physicochemical properties, toxicity and major applications. 1. Introduction based). The general structure of PEG-based glymes and their common names are illustrated in Scheme 1. Glymes, i.e. glycol diethers, are saturated polyethers containing Most glymes are completely miscible with both water and no other functional groups. When compared with glycols (such hydrocarbon solvents, and could solvate alkali cations. In as polyethylene glycols, PEGs), glymes do not carry free hydroxyl addition, glymes have many other favorable properties groups and thus are aprotic polar and chemically inert including being liquid at a wide range of temperatures (typi- Published on 18 December 2013. Downloaded 9/28/2021 8:32:39 PM. compounds. There are two major types of glymes: ethylene cally >200 C, except monoglyme), low viscosity, high chemical oxide (EO)-based glymes (also known as PEG-based) and and thermal stability, relatively low vapor pressure and low propylene oxide-based glymes (i.e. polypropylene glycol (PPG)- toxicity. Due to these excellent solvent properties, glymes have been widely used in many laboratory and commercial appli- cations including reaction media (i.e. organometallic reac- tions, polymerization, and reactions involving alkali metals, aKey Laboratory for Green Chemical Technology of Ministry of Education, School of oxidations and reductions), extraction solvents (for metals Chemical Engineering & Technology, Tianjin University, Tianjin 300072, China and organics), gas purication, absorption refrigeration, bDepartment of Chemistry and Forensic Science, Savannah State University, Savannah, formulation of adhesives and coatings, textiles, solvents for GA 31404, USA. E-mail: [email protected]; [email protected] Shaokun Tang was born in 1974, and graduated from Tianjin Hua Zhao studied chemistry (BS) and chemical engineering (MS) University (China) with BS, MS and PhD degrees in Chemical at Tianjin University (China) before he earned his PhD degree in Engineering. She was a research fellow at the National University 2002 from New Jersey Institute of Technology (NJIT) and completed of Singapore from 2007 to 2008. She is currently an Associate a post-doctoral training at Rutgers University. He is currently an Professor at Tianjin University. She was awarded “Excellent Associate Professor of Chemistry at Savannah State University. He Teacher” of Tianjin University. Her major research interests focus became fascinated with new solvents at graduate school while on green chemistry (catalytic preparation of biofuels, supercritical working on organic synthesis and enzymatic resolution of amino uid technology for natural compounds extraction, and ionic acids in aqueous ionic liquids. His current research interests liquids for renewable energy), as well as functional materials & include biocatalysis in ionic liquids and glymes, preparation of nanotechnology (controlled synthesis and functionalization of biofuels using ionic liquids and glymes, synthesis of medicinal nanostructured materials). molecules with anti-cancer and anti-HIV properties, and micro- wave-assisted enzymatic reactions. This journal is © The Royal Society of Chemistry 2014 RSC Adv.,2014,4, 11251–11287 | 11251 View Article Online RSC Advances Review Scheme 1 Structure of EO-based glymes, and representative common names and symbols. electronic industry, pharmaceutical formulation, batteries, polyperuorosulfonic acid resin at high temperature and pres- cleaning solutions, etc. sure. Similarly, propylene oxide-based glymes can be prepared Despite numerous studies on glymes, there is a lack of a by replacing ethylene oxide with propylene oxide in the above comprehensive and critical review on this subject to meet the methods. continuing interest in glymes; in recent years, there has been a In addition, Haymore et al.4 described a two-step synthesis of strong focus on their applications in electrochemistry, catalysts, pentaglyme (Scheme 3): the reaction of 2-methoxyethanol with Chemical Vapor Deposition (CVD), nanomaterials, and the sodium metal to yield an alkoxide followed by a nucleophilic 5 dissolution of CO2. This review aims to provide a comprehen- substitution of dichloride. Adcock and Lagow prepared per- sive set of physicochemical properties of glymes and a system- uoroglyme and peruorodiglyme as potential high stability atic overview of their applications, as well as a critical analysis of uids and solvents through a direct uorination method their structure–property–application relationships. To be (uorine gas). focused on the subject, glycol monoethers and glycol ether esters are not discussed herein as their physical properties, – 3. Toxicity of glymes toxicity and applications have been reviewed elsewhere.1 3 In general, glymes exhibit low to moderate acute toxicity (see 2. Preparations of glymes toxicity data in Tables 1 and 2) when compared with common organic solvents (such as toluene, THF and chloroform). Ethylene oxide (EO)-based glymes can be prepared by several Ethylene glycol dimethyl ether (monoglyme) triggered maternal common methods on large scales using ethylene epoxide deaths of pregnant Sprague-Dawley rats at 1000 mg per kg per Published on 18 December 2013. Downloaded 9/28/2021 8:32:39 PM. (Scheme 2): Route 1, ethylene oxide reacts with an alcohol to day and was fetolethal at doses ranging from 120 to 1000 mg per produce glycol monoether, which is further converted to glyme kg per day; a dose of 60 mg per kg per day caused a 7% weight following the Williamson synthesis (i.e. glycol ether reacts with decrease and severe edema in pups surviving to birth.6 When sodium to yield sodium alkoxide, which then reacts with alkyl rats were exposed to 200 ppm diglyme vapor for an extended halide to produce the glyme); Route 2, methylation of glycol period of time (15 Â 6 h), no toxic effect was observed in terms ether with methyl sulfate; Route 3, Lewis acid-catalyzed cleavage of normal blood and urine tests and normal organs by autopsy; of ethylene oxide by ether, typically resulting in glyme mixtures; however, at a higher vapor concentration (600 ppm) for the Route 4, the reaction of ethylene glycol with alcohol catalyzed same period of time, irregular weight gain was observed and Scheme 2 Common routes for glyme synthesis. 11252 | RSC Adv.,2014,4, 11251–11287 This journal is © The Royal Society of Chemistry 2014 View Article Online Review RSC Advances Scheme 3 Synthesis of pentaglyme. autopsy suggested atrophied thymus and congested adrenals than ethylene glycol. The diethylene glycol mono-alkyl ethers although the blood and urine tests were normal.7 and (alkyl) diglymes were more toxic than diethylene glycol, and However, there is a rising concern that glymes may cause triglyme was more toxic than triethylene glycol. (2) Methyl reproductive and developmental harm to exposed workers and ethers usually seem more toxic than ethyl or butyl ethers, except consumers using paint, carpet cleaners, inkjet cartridges and for ethylene glycol monobutyl ether. Similarly, Johnson et al.10 other products. McGregor et al.8 studied the exposure of male found that butyl diglyme was more toxic than diethylene glycol, rats to 250 or 1000 ppm diglyme, and found that diglyme was a but did not induce signicant developmental toxicity to the reproductive toxicant causing increased sperm abnormalities. hydra. Schuler et al.9 examined een glycol ethers for their adverse A review11 on the genetic toxicology of glycol ethers suggested reproductive toxic effects using an in vivo mouse screening that diglyme lacks genotoxic potential in some mutagenicity bioassay; this group found that all mice exposed to glycol ethers tests, but it was a reproductive toxicant in the mouse sperm test with terminal methyl groups, i.e., ethylene glycol monomethyl and male rat dominant lethal test. Repeated daily oral doses of À ether, monoglyme, diethylene glycol monomethyl ether, diglyme at 684 mg kg 1 in a subchronic study of Sprague- diglyme and triglyme produced