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High Performance Polymers in Ionic Liquids: A Review on Prospects for Green Polymer Chemistry. Part I: Polyamides

Iranian Polymer Journal 19 (12), 2010, 983-1004

Shadpour Mallakpour1,2* and Mohammad Dinari1

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(1) Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, 84156/83111, Iran (2) Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan, 84156/83111, Iran

ABSTRACT

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Received 17 July 2010; accepted 8 November 2010

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Key Words:

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he introduction of cleaner technologies has become a major concern worldwide and therefore, the search for alternatives to the unsafe volatile solvents has become the greatest priority throughout the academy and industry. Due to enforcement of policies in protecting the environment, "green chemistry" has received much attention since 1990's which it is still progressing. Green chemistry is understood to be a superior, innovative chemistry which is cost-effective and has minimum deleterious impact on environment. Highly polar conventional solvents such as N,N'dimethylformamide, N,N'-dimethylacetamide, pyridine, N-methylpyrrolidone and chlorinated solvents which are used in polycondensation polymerizations are volatile and most of them are flammable, toxic, quite hazardous and harmful compared to ionic liquids (ILs). Hence, there is a basic obligation for the advancement of new methodologies of polymerizations by using environmentally benign media which could replace the conventional solvents and enhance adequate solubility for polymerization processes. Ionic liquids have great potential as a non-volatile organic medium for polymerization and polymer processing due to their near-zero vapour pressure, nonflammability, low cost and ease of production. However, implementation of ILs as solvents for polymerization processes has afforded some marked advantages such as increased molecular weight and narrower polydispersity in comparison to organic solvents. Owing to their higher performance and characteristics, the demand for high-performance polymers is growing steadily. Hence, the synthesis and properties of high-performance polyamides in the green media using ILs are reviewed to develop a methodology for "green polymer chemistry".

ionic liquids; high-performance polyamides; green polymer chemistry; environmentally friendly methods.

(*) To whom correspondence to be addressed. E-mail: [email protected]

CONTENTS Introduction ............................................................................................................ 984 Polyamides ............................................................................................................ 984 Solution Polycondensation of Diamine and Diacid Chloride .......................... 984 Polycondensation of Diamine and Diacid via Phosphorylation or with Phosphorus-containing Activating Agents ............................................... 985 Polycondensation of Silylated Diamine and Diacid Chloride ......................... 985 Interfacial Polycondensation of Diamine and Diacid Chloride ....................... 985 Volatile Organic Solvents ................................................................................. 986 Ionic Liquids .......................................................................................................... 986 High Performance Polyamides in Ionic Liquids ..................................................... 989 Conclusion ............................................................................................................. 995 Acknowledgement ................................................................................................. 996 Abbreviations ......................................................................................................... 996 References .............................................................................................................. 997

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High Performance Polymers in Ionic Liquids ...

POLYAMIDES

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High performance polymers (HPPs) exhibit exceptional stability upon exposure to some harsh environments and have properties that surpass those of traditional polymers. These materials are defined in many ways depending upon the application and, to some extent, on the organized systems used for developing or employing the materials. Most of the factors that contribute to high performance and heat resistance properties of these polymers are presented as: resonance stabilization, primary bond strength, molecular symmetry, secondary bonding forces, molecular weight and distribution, rigid intrachain structure, cross-linking, mechanism of bond cleavage and additives or reinforcements (fillers, clays, miscellaneous nanoparticles) [1-5]. They are used in many industries ranging from communications to medicine and to transportation [5-9]. Because of the superior performance characteristics for HPPs, the demand for these types of polymeric materials is growing steadily [10-14]. Although several of these applications do not demand uses at high temperatures, but the fabrication process leading to parts or components requires the polymer to be thermally stable. Polyamides (PAs) as one of the main well known families of HPPs are presented here to exhibit the basic principles underlying polymer preparations in ionic liquids.

hydrogen bonding. The hydrogen bonding increases the chain interaction resulting in higher yield stress, fracture stress, impact strength, tear strength, and abrasion resistance [20-23]. Polyamides are used essentially as fibres, films, and filler-containing engineering plastics for special applications [24,25]. Most of the reinforced PAs are filled with glass fibres and to a lesser extent with particles, e.g., talc, kaolin, and mica. For engineering plastics applications, dimensional stability at high temperatures is often sought. Wholly aromatic PAs can be processed from solution either into films or into fibres. These polymers have very good dimensional stability [21,22] and excellent heat resistance [26,27]. The PAs, (AA-BB)n, can be prepared from diamines and diacids. The AB types of PAs are made either from ω-amino acids or cyclic lactams which are the derivatives of the ω-amino acids. Partially aromatic PAs have higher glass transition and melting temperatures compared to their aliphatic counterparts. They can be prepared by various methods, e.g., mono-add systems which both acid and amine functionalities are on the same molecule and di-add systems such as diacids, diacid chlorides or diesters in combination with either aromatic or aliphatic diamines, as well as, the reactions between diisocyanates and diacids which are commonly used for this propose [16]. Wholly aromatic PAs have high glass transition temperatures (>200°C) and, when crystalline, they show very high melting temperatures (>500°C). High-molecular-weight polymers cannot be prepared by melting or melt processing, because many aromatic diacids are decarboxylated and the aromatic diamines are readily oxidized and have a tendency to sublime at elevated temperatures [28]. Their synthesis is usually carried out in solution and due to the very low solubility, special solvents are required to obtain high-molecular-weight polymers. Polyamides are usually synthesized by the following methods.

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INTRODUCTION

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The polyamides (PA)s are high molecular weight materials containing intermittent amide units and the hydrocarbon segments that can be aliphatic, partially aromatic, or wholly aromatic. The type of hydrocarbon segment employed has effects on the chain flexibility and structural regularity, with the latter's importance in the formation of crystalline phase. Polyamides are often called nylons, the trade name given to them by DuPont. Many types of PAs have been studied, e.g., in one of the first patents on PAs almost 31 types are being described [15]. Since then, partially aromatic and wholly aromatic PAs have been successfully developed [16-19]. They have good mechanical properties due to formation of

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Solution Polycondensation of Diamine and Diacid Chloride In this method, a diamine and a diacid chloride reacting in an amide solvent such as NMP, hexamethylphosphoramide (HMPA), or DMAc [29,30].

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the same research group [35]. In this method, several key factors, such as the concentration of monomers, ratio of TPP to monomer, reaction temperature and time, concentration of LiCl and CaCl2, solvent and quantitative ratio of Py to metal salt were considered to have great influence on the molecular weight of the final polymer. Although this type of reaction is accompanied with gelation, still the polycondensation does not cease to proceed. The role of CaCl2 and LiCl salts is quite complicated. They can form complexes with Py, i.e., LiC1-2Py and CaCl2-nPy. They are more soluble in NMP than the salts themselves; while NMP with a higher content of metal salt can solubilize PAs more. Other phosphorus-containing activating agents like: triphenylphosphine [36,37], hexachlorocyclotriphospha-triazene [38], phenylphosphine dichloride [39], or diphenylchlorophosphate [40] can be used in this method, either. Polycondensation of Silylated Diamine and Diacid Chloride While most of the efforts in the synthesis of high molecular weight PAs were oriented towards the activation of diacid component, there has been increasing interest in the activation of diamine component by reacting it with trimethylsilyl chloride. Indeed, high molecular weight PAs have been synthesized by low temperature polycondensation of an N-silylated aromatic diamine with an aromatic diacid chloride. The reaction is usually carried out at -10°C in NMP. Furthermore, reaction proceeds more rapidly and affords PAs with higher molecular weights relative to those obtained from free diamines [41-43].

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Other polar aprotic solvents such as DMF and dimethylsulphoxide (DMSO) cannot be used because they react rapidly with acid chlorides [28]. Consequently, this kind of polycondensation allows the macromolecular chain to continue to grow until the reaction is completed. Therefore, the key factors that are critical for obtaining high molecular weight polymers are the stoichiometry of the monomers, purity and concentration of monomers, temperature of the reaction medium, reaction time, nature of the solvent(s), addition of salt, solubility of the polymer and speed of stirring [28,31,32]. The temperature should be adjusted to ensure that the condensation reaction is much faster than the side reactions. In general, low temperatures are favoured in this method. The solvent should enhance maximum solubility of the polymer formed at the early stage of polycondensation process. The solvation properties of amide solvents can usually be increased by addition of salts, e.g., LiC1 or CaCl2. It is vital to avoid the polymer to precipitate fast, because it may prevent the macromolecular chains to grow further. In this method, the diacid needs to be converted to its diacid chloride derivative. Although the synthesis itself does not seem to be complicated, it usually involves the treatment with thionyl chloride which is an additional reaction step and produces environmentally hazardous SO2 and HC1 gases. The stability of diacid chloride towards hydrolysis is another problem in terms of purification and storage which should be kept in mind. In addition when other functional groups are present on the same molecule, they need to be protected before treating with SOC12. These problems have stimulated the need to seek alternatives [28,33]. Polycondensation of Diamine and Diacid via Phosphorylation or with Phosphorus-containing Activating Agents In 1974, Higashi et al. [34] described a novel procedure to prepare aromatic PAs. This reaction involved the complexation of an acid with triphenylphosphite (TPP) in N-methylpyrrolidone (NMP) and pyridine (Py). CaCl2 and LiCl were used along with NMP to improve the molecular weight of the polymer. The initial study was followed by an extensive investigation of the reaction conditions by

Interfacial Polycondensation of Diamine and Diacid Chloride In the so-called interfacial polycondensation method, the two fast reacting intermediates are dissolved in a pair of immiscible liquids, one of which is preferably water. The water phase generally contains the diamine and usually an inorganic base to neutralize the acid by-product. The other phase contains the diacid chloride in an organic solvent such as dichloromethane, toluene or hexane. The important factors that influence this type of polycondensation have been studied


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Volatile Organic Solvents In all the above methods for the synthesis of PAs, the use of highly polar conventional solvents such as DMF, NMP, DMAc and Py is necessary for polymerization process. Most of these solvents are flammable, toxic, quite hazardous and harmful and generally have several ecological disadvantages which impose heavy costs beyond prices of industrial synthetic methods. Hence, there is a great need for the advancement of new polymerization methodologies in using environmentally benign media which could replace the usual solvents and enhance solubilizations in polymerization process, either. Volatile organic solvents which are of serious concerns in increasing air pollution and worker's health are common reaction media for commercial production of different chemicals, including polymers. These solvents are high on the list of harmful chemicals for two simple reasons: (i) they are used in huge volumes and (ii) they are usually volatile liquids that are difficult to handle [47]. It is an enormous challenge to diminish the amount of volatile organic compounds (VOCs) used in chemically small scale and industrial processes. It is estimated that about 20 million tons of VOCs as one of the major air polluting agents are released to the atmosphere each year [48-50]. VOCs are the most usually used solvents in solution polymerizations, owing to their compatibility with monomers and simplicity of separation, even with their well-documented health and environmental concerns [51]. The separation of VOCs can be costly and is generally ineffective in preventing them to spread into the environment. When VOCs spread into air or water, many health problems emerge ranging from simple discomfort to cancer [52]. Another concern with VOC use is smog formation at elevated concentrations of ozone at ground level [53]. At the same time, as a response to increasing legislative and social pressure and an increasingly "green-conscious" industrial community, researchers have started to examine more eco-friendly and sustainable chemical processes [54]. Toxicity and

recycling considerations are influencing the choice of solvent for industrial reactions. Therefore, the development of more efficient and environmentally friendly processes is mandatory in the coming years [55]. These processes must be designed on the basis of two main characteristics: energy saving to avoid excessive emission of carbon dioxide (CO2) (at least, as long as industry depends on the combustion of fossil fuels as a main source of energy) and depletion of emissions related to harmful VOCs. For these reasons, the development of new chemical processes based on the application of non-volatile materials is necessary [56-59]. Research on chemical and polymer manufacturing has focused on the investigation of different approaches for diminishing the emission of VOCs including solvent-free processes and the use of water, supercritical CO2 and more recently, ionic liquids (ILs) as the reaction media [60]. Water is non-flammable and environmentally benign, but due to solubility issues it cannot be used in moisturesensitive systems, such as ionic polymerizations [61]. The main drawback of the use of CO2 as a reaction medium is the utilization of high-pressure equipments to reach critical conditions in alleviating one of the advantages of free-radical polymerization [62-64].

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in detail and reviewed by Morgan et al. [44-46].

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IONIC LIQUIDS The novel green recyclable solvents, completely composed of ions as liquid at or close to room temperature, are traditionally referred to "ionic liquids" (ILs) [65]. They also include ionic fluid, molten salt, liquid organic salt, fused salt, or neoteric solvent [66-69]. Therefore, the development of neoteric solvents, i.e., ILs, for chemical synthesis holds great promise for green chemistry applications [70]. Interest in these compounds, is often heralded as the green, high-tech media of the future [71-73]. The more useful properties of ILs are as follows: - They are relatively non-volatile which means they do not produce atmospheric VOCs and can be used in low-pressure environments [74-77], - They exhibit good thermal stability and do not decompose over a wide temperature range thereby


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are hydrophilic and if they are used in open vessels, hydration will certainly take place. The hydrophobicity of an IL increases with increasing length of its alkyl chains. Despite their wide spread usage, ILs containing PF6- and BF4- have been reported to decompose in the presence of water, giving off HF. Wasserscheid et al. [87] pointed out that ILs containing halogen anions generally show poor stability in water, and also give off toxic and corrosive species such as HF or HCl. Therefore, they suggested the use of halogen-free and relatively hydrolysis-stable anions such as octylsulphate compounds. The interaction between water and ILs and their degree of hydroscopic character are strongly dependent on anions. The amount of absorbed water is the highest for BF4- and the lowest for PF6- anions. However, [Tf2N]- shows much greater stability in the presence of water as well as having the advantage of an increased hydrophobic character. Therefore, presence of water may have dramatic effect on the reactivity of ILs, and therefore they are usually utilized after a moderate drying process, as water is present in all ILs [54,56,88,89]. As biodegradability has also been a main concern, new families of ILs are derived from renewable feedstock or from ''low cost'' starting materials. These ''Bio-ILs'' are entirely composed of biomaterials. An example can be given by the development of "deep eutectic mixtures" of liquid systems based on choline chloride for which its qualification as "ILs" is still the subject of controversies. Choline can be used as an alternative cation in combination with suitable anion to generate ILs. The biodegradability properties of these ILs have also been reported [90]. It was shown that the introduction of an ester group into long alkyl chains leads to reduced toxicity and improves the eco-toxicity of ILs [90]. Further incorporation of ether groups into the side chain improves the biodegradability of imidazolium-based ILs, while the introduction of the biodegradable octylsulphate anion has a further beneficial effect. Recent work on pyridinium-based ILs demonstrated how the heteroaromatic cationic core can be modified to produce biodegradable examples [90]. As with the imidazolium examples, the inclusion of an ester group in the cation side chain leads to improved biodegradability [90]. High levels of biodegradability

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making it feasible to reactions which require high temperature [74,78-80], - They show a high degree of potential for enantioselective reactions which have a significant impact on the reactivity and selectivity due to their polar and non-coordinating properties and in addition, chiral ILs are used to control the stereoselectivity [75,81], - They are good solvents for a wide variety of chemical processes and can be considered both polar and non-coordinating solvents [77,79], - They are the most complex and versatile solvents that they have the ability to interact via hydrogen bonding, i.e., π-π, n-π, dispersive, dipolar, electrostatic, and hydrophobic interactions [77-79], - They serve as a good medium to solubilitize gases such as H2, CO, O2 and CO2 and many reactions are now being performed using ILs and supercritical CO2 [82], - They can be immiscible with non-polar organic solvents and/or water [80], - They have high ionic character that enhances the reaction rates to a great extent in many reactions including microwave assisted organic synthesis as well as polymerization reactions [83], - Their solubility depends upon the nature of the cations and counter-anions [84], - They have physicochemical properties that can be controlled by judicious selection of the cation and/or anion [85], - They can be stored without decomposition for a long period of time and have good miscibility with other organic solvents or monomers [86]. Ionic liquids also have some potential negative properties, e.g., high viscosity, potential toxicity, moister sensitivity and difficulty in purification. The viscosity of many ILs is relatively as high as one to three orders of magnitude compared to conventional solvents. For a variety of ILs, viscosity has been reported to be in the range of 10 - 500 mPa/s at room temperature. The viscosity of ILs can affect transport properties such as diffusion and may be an issue in practical catalytic applications for the engineering uses. It also plays a major role in stirring, mixing and pumping operations [47,54]. Many ILs are stable in air and moisture conditions. Conversely, most imidazolium and ammonium salts


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Most commonly used cations:

Most commonly used anions: BF4-, PF4-, PF6-, CF3SO3-, (CF3SO3)N-, CH3C6H5SO3-, CF3CO2-, Br-, ClScheme I. Most commonly used cation structures and possible anion types.

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radical [107], living/controlled free radical [108,109], electrochemical, coordination polymerization of olefins via Ziegler-Natta catalysts [110-112], condensation [113-115], ring-opening metathesis [116-120], block [121-125], the enzymatic synthesis of polyesters (PE)s [126] and the statistical polymerization [127] have been carried out in various ILs with the development of air and water-stable ILs. Ionic liquids have also been recently evaluated as non-volatile plasticizers and as external or internal lubricants in several polymers including poly(methyl methacrylate) [128], poly(vinyl chloride) [129], PAs and poly(lactic acid) [130,131]. As demonstrated in Figure 1, opening in 1998, when 105 papers were published on this topic according to SciFinder, the annual number of publications has increased to 4892 publications in 2009, with many contributions of industrial scientists and engineers. Notably, the number of these

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have also been reported in cases where environmentally benign anions such as saccharinate and acesulphamate are included. Several ammonium ILs based on choline have been introduced which are biodegradable and can be readily prepared [90]. Room temperature ILs (RTILs) which are already in common use, typically involve nitrogen- or phosphorus-containing organic cations such as alkylimidazolium, alkylpyridinium, alkylpyrrolidinium, alkylphosphonium, alkyltriazolium, alkyloxazolidinium or alkylmorpholinium and anions like bis(trifluoromethanesulph-onyl)imide, hexafluorophosphate or tetrafluorophosphate. Although these particular cations and anions and their various combinations have already been studied extensively for their potential applications in numerous chemical and physical processes, every year more and more cation- and anion-forming liquid salts at room temperature are reported [87-92]. The most common ILs cations and anions are summarized in Scheme I. There are several review articles on the synthesis, properties, and applications of ILs [93-96]. They have been investigated as a reaction media for syntheses including Diels-Alder [97], Wittig, the Suzuki crosscoupling [98], Heck alkylation [99], hydrogenation [100,101], oxidation [100], reduction [102], extraction [103], preparation of inorganic materials [104], bioprocessing operation [105], catalysis or as electrolytes in electrochemistry [106]. Ionic liquids are also, more and more frequently used by polymer chemists not only as solvents for polymerization processes but also as plasticizers or additives for polymeric materials. In 1988, Oudard et al. [38] first described the polymerization of pyrrole in N-alkylpyridinium chloroaluminate. Since 1997s, many polymerizations, including free

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Figure 1. Annual number of citations for ILs and those containing the term polymer in the years 1998-2009. (Source: SciFinder Scholar).


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[MEIm](CF3SO2)2N). Thus, the polymerization process and the molecular weights of the polymers could be highly influenced by the nature of ILs. Inherent viscosities of the polymers obtained in 1,3dialkylimidazolium bromides were in the range of 0.18 -0.82 dL/g. In another study by Vygodskii et al. [142], ILs have been used as solvents for preparing polymers via direct polycondensation. Direct step growth polymerization reaction of different monomers (dicarboxylic acids and their dihydrazides, diamines, etc.) effectively occurs in ILs under the influence of TPP as activating agent. Owing to ILs usage there is no need for any extra component in such a process, for example, LiCl and Py. It should be mentioned that these additives are necessary participants in direct polyamidation of the same monomers in ordinary molecular solvents, such as NMP. The study of different polycondensation parameters including IL's nature has shown that the combination of ionic media with activating agent, TPP, allows polymer synthesis to be carried out in relatively mild conditions (reaction temperature of 100-140°C) [142]. The influence of ILs nature upon the polymer molecular weight has been studied, as well. For ILs having the same alkyl substituents there is a strong dependency between alkyl length and PA molecular weight, as upon longer alkyl group, there is an increase in viscosity observed. However, for ILs with asymmetrical cation there was no similar clear dependency. Therefore it was depicted that ILs containing n-propyl or iso-propyl alkyl chains seemed to be the best solvents for PAs synthesis. As far as anions are concerned, the best results were achieved in ILs with Br-. When aliphatic dicarboxylic acids (adipic acid) and aromatic diamines are used as initial compounds, high molecular weights PAs (ηinh up to 0.91 dL/g) are formed. In contrast, aliphatic diamines lead to lower PA viscosity characteristics. The similar results are reached upon the same reactions in ordinary organic solvents. Various PAs (ηinh = 0.11-1.10 dL/g), polyamide imides (PAIs) (ηinh = 0.48-1.41 dL/g), polyamide hydrazides (ηinh = 0.56-0.60 dL/g) and polyhydrazides (ηinh = 0.71-1.32 dL/g) have been obtained in quantitative yields with high molecular weights.

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publications that mention both "ionic liquid" and "polymer" in some forms increased from only 3 articles in 1998 to 406 articles in 2009. Therefore, interest in the intersection of ILs and polymer science is increasing even more rapidly than overall interest in ILs only. This probably reflects the realistic approach of polymer scientists and engineers, who are always looking for better ways to synthesize macromolecules, alter their structures and properties and process them more efficiently. In recent years, a number of outstanding reviews have been reported on the topics of both polymers and ILs [132-139]. The utilization of ILs as solvent for polymerization processes have afforded some marked advantages such as increased molecular weight and narrower polydispersity in comparison to organic solvents. Therefore, with the aim of developing green polymer chemistry, the synthesis and properties of high performance PAs in the green media using ILs are considerably developed.

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High Performance Polyamides in ILs Analyzing the works on the use of ILs in organic synthesis shows their efficiency particularly in alkylation and acylation reactions [49]. This is the base for studying the possibility of carrying out reactions of aromatic diamines and tetra- and dicarboxylic acid derivatives resulting in polyimides (PIs) and aromatic PAs in such media by Vygodskii et al. [140,141]. The first information on the use of ILs as a solvents and activated agent for polycondensation reaction were reported by this group. A series of aromatic PAs from indirect polycondensation reaction of diamines such as 5(6)amino-2-(4'-aminophenyl)benzimidazole (ABIZ), APh and 1,4-phenylenediamine (p-PhDA) with terephthaloyl chloride were synthesized in different imidazolium type ILs at different reaction temperatures (0 to 60°C). The choice of polycondensation temperature was being determined by the viscosity of the ILs and solubility of their starting diamines. It was found that PAs with the highest molecular weight were obtained when ABIZ, containing an imidazole group similar to the IL structure, was used. Furthermore, all PAs precipitated from the reaction solution in the course of polycondensation in hydrophobic ILs such as [B2Im]BF4 and


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conventional solvent that lead to the formation of polymers with lower inherent viscosity (0.21 to 0.57 dL/g) in the presence of DBTDL as a catalyst. In the case of TBAB, higher yields and inherent viscosities were obtained. This process is safe and green since toxic and volatile organic solvent such as NMP is eliminated, an indication of the potential of molten TBAB, a benign readily available IL as an efficient catalyst, and has much promise for further applications. Moreover, this methodology offers significant improvements with regard to yield of products, inherent viscosities, thermal stability, cost efficiency and green aspects avoiding toxic catalysts and solvents. Step growth polymerization reaction of different chiral diacid monomer such as 5-(4-methyl-2-phthalimidylpentanoylamino)isophthalic acid, 5-(3-phenyl2-phthal-imidylpropanoylamino)isophthalic acid and 5-(3-methyl-2-phthalimidylpentanoyl-amino) isophthalic acid with several aromatic and aliphatic diisocyanates were performed under microwave

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Ionic liquids based on 1,3-dialkylimidazolium and ammonium have been used as efficient reaction media in the amidation of several carboxylic acids with isocyanates. Mallakpour et al. [143,144] reported the first application of molten IL (MIL) for the synthesis of PAs from the reaction of dicarboxylic acids with diisocyanates. Polycondensation of terephthalic acid (TPA) with various commercially available diisocyanates such as 4,4'-methylene-bis-(4-phenylisocyanate) (MDI), toluylene-2,4-diisocyanate (TDI), isophorone diisocyanate (IPDI) and hexamethylene diisocyanate (HMDI) was performed in a fairly inexpensive and readily accessible MIL, tetrabutylammonium bromide (TBAB) with or without dibutyltin dilaurate (DBTDL) as a catalyst. The polymerization reaction gave similar results in the presence or absence of DBTDL, indicating that there is no need for a catalyst in this process. Various PAs were obtained with high yields and moderate inherent viscosities ranging from 0.36 to 0.71 dL/g. This method was compared with the polymerization reaction in

Scheme II. Synthesis of optically active and thermally stable PAs [145-147].

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method. In order to study the effect of different existing functional groups in the resulting PAs on their thermal stability, the thermal kinetic investigations by TGA and DSC techniques were carried out by using Coats-Redfern equation [149]. One-pot polyamidation reaction of optically active aromatic diacid containing L-alanine [150], L-methionine [151,152], L-leucine [153,154] or L-valine [155,156] and phthalimide moieties with aromatic diamines through direct phosphorylation reaction under microwave irradiation and traditional heating was performed. Polycondensation reactions were examined using NMP/TPP/CaCl2/Py system as the conventional condensing agent and then the benefits of IL/TPP as both polyamidation catalysis and reaction medium were investigated for preparation of wholly aromatic optically active and high performance PAs. Optimization of the polymerization reactions was carried out by varying the type and quantity of ILs, amount of TPP, reaction time and temperature, and catalyst used. The poly-condensation reactions were carried out in different RTILs bearing different alkyl groups and it was found that [Isopr2Im]Br and [Pr2Im]Br are the best ILs for polymerization reaction. The results indicated that the PAs prepared via IL method encompass higher inherent viscosities and yields compared to those prepared under conventional method. By comparison, in the case of IL catalyzed polyamidation, removal of some chemicals (e.g., NMP, CaCl2 and Py) which are essential in conventional methodologies decreases the cost of polymerization as well as the environmental pollution, considerably. Moreover, polymerization reactions via IL/TPP have been completed in shorter period of time (2.5 h vs. 5 h) and it can be a desired aspect from energy saving and commercial point of views especially in the industrial large scales polymerization processes. The above mentioned polymerization reactions were performed under microwave irradiation. The yield and inherent viscosities of resulting polymers were ranging between 91%-95% and 0.47-0.65 dL/g, respectively. It shows that polymerization reactions proceeded in higher yields, moderate inherent viscosities, good thermal stability and shorter reation times under microwave irradiation conditions,

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irradiation and conventional heating technique through direct polycondensation in molten TBAB or traditional solvent like NMP (Scheme II) [145-147]. The reactions were carried out in the presence of a small amount of DBTDL, Py or triethylamine (TEA) as catalysts and with no conventional catalyst. The best results were obtained with DBTDL, under both conditions. Microwave irradiation as a non-conventional energy sources is a very popular and useful technology in polymerization for optimizing and accelerating of chemical reactions. Mallakpour et al. [146] have studied the effect of microwave power levels and duration of heating to provide the optimum reaction conditions (method I). The results revealed that the optimal results were obtained after 3 min at 100% of power level. The inherent viscosities of the resulting polymers under microwave irradiation were in the range of 0.360.63 dL/g and the yields were 74-95%. To compare the microwave-assisted method with conventional heating, the polycondensations were also carried out under conventional heating in TBAB (method II) [146]. When the same experiment was conducted by conventional heating in the presence of TBAB as a solvent, it took much longer time (12 h of heating at 120°C) for completion of the polymerization reactions. Under these conditions, yields and inherent viscosities of the polymers were ranging from 73 to 91% and 0.32-0.56 dL/g, respectively [146]. Mallakpour et al. [148] prepared high performance PAs by polyamidation reaction of a novel diacid containing naphthalimide and flexible chiral groups and different diisocyanates in the presence of a small amount of RTIL that act as a primary microwave absorber. For the first time, electrochemical behaviour and stability of the resulting PAs on multiwall carbon nanotube-modified glassy carbon electrode was reported by this group [148]. Moreover, these electrochemical studies showed that electrochemical decomposition and stability of S-valine-based diacid monomers and the resulting polymers in acidic solution are more complex than in basic solution as the electrochemical behaviour of polymer is very similar to its monomers. Kinetic study was performed on the thermalgravimetric analysis curve, by use of an integral analysis


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temperature, and over 40% char yield at 800°C in nitrogen atmosphere (Scheme III). All these polymers possessed bulky anthracenic and amino acid functionalities in the side chains, showing excellent solubility in various solvents. The novel optically active aromatic PAs with flame retardancy properties were prepared by heating over an oil bath, using a mixture of [Pr2Im]Br and TPP both as reaction media and activator (Scheme IV) [158]. This procedure was a one-pot reaction and did not need to prepare diacid chloride. These polymers presented high thermal stability, with the decomposition temperature above 400°C [158]. The incorporation of tetrabromophthalimide and L-phenylalanine groups into PA's backbone gave polymers a good solubility in common organic solvents. In addition, the interpretation of kinetic parameters (E, ΔH, ΔS and ΔG) of thermal decomposition stages of

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Mallakpour et al. [155,156] claim that since these polymers are optically active and have amino acids in the polymer architecture, they are likely biodegradable, and therefore they are classified as environmental friendly polymers. Recently, Mallakpour et al. [157] synthesized novel chiral aromatic PAs from the reaction of a new diacid monomer, 5-[3-phenyl-2-(9,10-dihydro-9, 10ethanoanth-racene-11,12-dicarbox-imido) propanoylamino], isophthalic acid and different aromatic diamines using [Pr2Im]Br under microwave irradiation. By controlling the concentration of [Pr2Im]Br, reaction time and power level, polymers with high yield and moderate inherent viscosity were accomplished in a very short period of time. The PAs were found to have inherent viscosities in the range of 0.54-0.85 dL/g, glass-transition temperatures (Tg) above 180°C with 10% weight-loss beyond 340°C

Scheme III. Synthesis of anthracene based PAs [157].

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Scheme IV. Preparation of thermally stable and optically active PAs with flame retardancy properties [158].

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aforementioned PAs was evaluated. More recently, Mallakpour et al. [160-163] prepared novel optically active and thermally stable aromatic PAs by safe and fast polyamidation of 5-[4(2-hthalimidiylpropanoylamino)benzoylamino] isophthalic acid [159] or (2S)-4-[(4-methyl-2-phthalimidylpentanoylamino)benzoylamino] isophthalic acid with aromatic diamines or aromatic and aliphatic diisocyanates in polar organic solvent and ILs under microwave irradiation and conventional heating methods. Several high performance PAs containing pendant L-leucinephthalimide or alaninephthalimide and benzamide groups were prepared with good yields through a simple microwave heating method using IL and TPP as a modern and safe methodology (Scheme V). The resulting PAs have inherent viscosity in the range of 0.43-0.81 dL/g. These PAs exhibited an enhanced solubility as compared to those analogous PAs without benzamide unit. It is evident that the introductions of the benza-

mide substituents resulted in increased chain packing of the macromolecules and thus facilitate the diffusion of solvent molecules into the polymer chains and decreased intermolecular interactions, leading to higher solubility. However, the effect of the benzamide group on the T5 and T10 values could be appreciated when these PAs were compared with PAs without benzamide unit, while the other part of the polymer structure remains intact. All polymers showed optical rotation [α] with verifiable optical activity. Polymers prepared by different methods showed different optical rotation, and this fact was attributed to the dependency of the optical rotation on the overall structure and regularity of the resulting polymer chains. Surprisingly, polymers of the same chemical structure polymerized by the same method with different catalysts resulted in different optical rotation values. The following results can be concluded: (1) The combined advantages of microwave irradiation and IL make the polycondensation reactions with safe


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Scheme V. Synthesis of thermally stable PAs having benzamide group [159-163].

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operation, low pollution, rapid access to products and simple workup and on the other hand no need of using volatile and toxic solvents such as NMP in this procedure, make it an environmental friendly and green method, (2) polymerization reactions take place in few minutes and consequently may minimize polymer degradation, (3) in the case of IL catalyzed polyamidation, removal of some chemicals (e.g., NMP, CaCl2 and Py) which are essential in conventional processes, decreases the cost of polymerization reaction and (4) comparison of these two methods by means of the yields and inherent viscosities of resulting PAs indicates that ILs in combination with TPP can be considered as the superior polyamidation agents. An efficient, convenient and practical approach

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for the direct polycondensation of dicarboxylic acid, 5-(3-acetoxynaphthoyl-amino)isophthalic acid with several aromatic diamines was studied in [Isopr2Im]Br under microwave irradiation and conventional heating [164]. The polymerization reaction was effectively run in IL, and TPP as an activating agent, and the resulting photoactive PAs were obtained with high yields and moderate inherent viscosities in the range of 0.44-0.69 dL/g. TGA showed that polymers are thermally stable, with 10% weight loss beyond 390°C, and higher than 60% char yields at 600°C in nitrogen atmosphere. These macromolecules exhibited maximum UV-vis absorption at 265 and 300 nm in DMF solution. Their photoluminescence in DMF solution demonstrated fluorescence emission maxima around 361 and 427 nm for all of


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H N

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O

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O O + Cl

N

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O CH2

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TBAB or

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N N

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n: 2, 4, 8 N(CH3)2 N(CH3)2

Scheme VI. Synthesis of heterocyclic PAs [165].

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resulting polymers were obtained, only low to moderate inherent viscosities were achieved. This could be explained in terms of cyclization during polymerization reaction. Pillai et al. [166] developed a novel and convenient method for the synthesis of a potentially safe non-viral gene delivery vehicle based on the cationic block copolymer of spermine and aspartic acid (ASSP) and coupled it with polyethylene glycol (PEG). The copolymer ASSP was prepared by direct polycondensation in IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([MBIm]PF6), using TPP as the condensing agent under mild reaction conditions. The highly hydrophobic ASSP was transformed into a water soluble hydrophilic micelle by coupling ASSP with PEG using the same IL and 1,1-carbonyl diimidazole as the condensing agent without harsh conditions. The polycationic ASSP-PEG was then used to condense calf thymus and plasmid deoxyribonuclceic acids (DNAs) in Tris-HCl buffer (pH 7.4) to obtain a series of block ionomer complexes with various charge ratios. It was observed that the DNA was condensed to compact particles by its interaction with the copolymer. Since DNA condensation to nano/micrometer sized particles is essential for gene delivery, this research group claimed that the resulted polymer had a potential use as the copolymer for gene delivery applications.

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the PAs. The incorporation of an acetoxynaphthalene group, through an amide unit, into PAs backbone gave polymers with remarkable solubility in common org anic solvents. A facile and proficient synthesis of heterocyclic PAs through polycondensation of 4-(4-dimethylaminophenyl)-1,2,4-triazolidine-3,5-dione with various commercially available aliphatic diacid chlorides (the length of the diacid chlorides chains were 2, 4 and 8 -CH2- moieties, respectively) in the presence of RTILs and MIL was carried out by Mallakpour et al. (Scheme VI) [165]. The effect of various reaction parameters, including the nature of the ILs, its amount, the reaction temperature, and the reaction time were investigated to optimize the conditions of the preparation of heterocyclic PAs. The polymerization proceeded well in ILs without any catalyst and PAs were obtained with high yields and moderate inherent viscosities. The easy work-up procedures, the absence of a catalyst and recyclability of the non-volatile IL used as reaction medium make the method amenable for scale-up operations. In particular, the effect of the nature of the ILs, reaction temperature, and reaction time on the reaction yields and polymer viscosities had been examined and the following conclusions were reached: (1) the best results were achieved in the presence of [Isopr2Im]Br, at temperature of 80°C for 6 h; (2) ILs act both as effective solvents and catalysts to mediate clean polycondensation reactions to yield the desired PAs. In fact the process of polymerization can be carried out without additional catalyst and conventional solvent and the ILs can be easily separated. Although excellent yields for the

CONCLUSION One of today's biggest challenges for coming years is


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chemistry", the synthesis and properties of high performance PAs in the environmental friendly media using ILs as a green and safe media are in the main stream of polymer studies. It is predicted that this new technology will be a popular method for the synthesis of HPPs in laboratory and industrial scales, soon.

ACKNOWLEDGEMENT

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We wish to express our gratitude to the Research Affairs Division of Isfahan University of Technology (IUT) for financial support. Further financial support from National Elite Foundation (NEF) and Center of Excellency in Sensors and Green Research of Isfahan University of Technology are also gratefully acknowledged.

ABBREVIATIONS

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the minimization of industrial pollutions. Worldwide usage of VOCs as industrial solvents currently exceeds $5billion annually, indicating their tremendous volumes employed. One of the largest sectors of the chemical industry is polymer industry, with over 30 million tons of polymers produced annually. A major component of industrial waste is used solvents, and strategies for eliminating solvent vapours will have huge impact on cleaning up the industrial polymer productions. While aqueous reaction media are widely used, employing VOCs is still widely practiced. Today, there is a great need for the advancement of new methodologies for polymerization using environmentally benign media which could replace the usual solvents and give enhanced solubility to polymerization products. Ionic liquids have great potential as non-volatile organic media for polymerization and polymer processing due to the near-zero vapour pressures, non-flammable, inexpensive, excellent microwave absorbing ability and ease of preparation. In addition, polymerization in RTILs may also improve the chemistry of synthesis and the quality of the resulting polymers, through higher reaction rate and molecular weight, easier recycling and reusing the reaction medium. Furthermore the use of ILs as industrial solvents can result in economical, social, and ecological impacts due to their direct effects on the human health and environment. One of the common observations across several techniques used is that the precipitation of the polymer product from the ILs media terminates the reaction. To take advantage of this phenomenon would lead to narrower polydispersity which is scarcely reached. With the synthetic flexibility that ILs show, it may be possible to manipulate the solubility of polymers in the ILs in a sufficiently controlled manner that they can be used as a means to prepare polymers of desired molecular weights. Because of the superior performance characteristics for high performance PAs, the demand for these types of polymeric materials is growing steadily. Non-volatile nature and stability at high temperature make the ILs excellent candidates as reaction medium as well as catalysts for the preparation of these types of polymers. Therefore, with the aim of developing of "green polymer

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[MAIm]Cl

1-Allyl-3-methylimidazolium chloride ABIZ5 (6)-Amino-2-(4'-aminophenyl) benzimidazole [MBIm]Cl 1-Butyl-3-methylimidazolium chloride [MBIm]PF6 1-Butyl-3-methylimidazolium hexafluorophosphate [MBIm]BF4 1-Butyl-3-methylimidazolium tetrafluoroborate [BPy]BF4 1-Butylpyridinium tetrafluoroborate [BPy]CF3COO 1-Butylpyridinium trifluoroacetate APh 3,3-Bis(4'-aminophenyl) phthalide BDA 3,3',4,4'-Biphenyltetracarboxylic dianhydride BD 1,4-Butanediol ZI 1-(1-Butyl-3-imidazolio)butane-4sulphonate CL ε-Caprolactone DNAs Deoxyribonuclceic acids [All2Im]Br 1,3-Diallylidazolium bromide [Bz2im]Br 1,3-Dibenzylimidazolium bromide DBTDL Dibutyltin dilaurate [B2Im]Br 1,3-Dibutylimidazolium bromide [B2Im]PF6 1,3-Dibutylimidazolium


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TPP TDI TsCl TEM VOCs

Triphenylphosphite Toluylene-2,4-diisocyanate Tosyl chloride Transmition electron microscopy Volatile organic compounds

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