Iranian Polymer Journal 19 (12), 2010, 983-1004 ionic liquids; high-performance polyamides; green polymer chemistry; environmentally friendly methods. ( * ) To whom correspondence to be addressed. E-mail: [email protected]ABSTRACT Key Words: High Performance Polymers in Ionic Liquids: A Review on Prospects for Green Polymer Chemistry. Part I: Polyamides Shadpour Mallakpour 1,2* and Mohammad Dinari 1 (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 Received 17 July 2010; accepted 8 November 2010 T 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 methodo- logies 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". 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 Available online at: http://journal.ippi.ac.ir
(*) To whom correspondence to be addressed.E-mail: [email protected]
A B S T R A C T
Key Words:
High Performance Polymers in Ionic Liquids: A Review on Prospects for Green Polymer
Chemistry. Part I: Polyamides
Shadpour Mallakpour1,2* and Mohammad Dinari1
(1) Organic Polymer Chemistry Research Laboratory, Department of Chemistry,Isfahan University of Technology, Isfahan, 84156/83111, Iran
(2) Nanotechnology and Advanced Materials Institute, Isfahan Universityof Technology, Isfahan, 84156/83111, Iran
Received 17 July 2010; accepted 8 November 2010
The introduction of cleaner technologies has become a major concern worldwideand therefore, the search for alternatives to the unsafe volatile solvents hasbecome the greatest priority throughout the academy and industry. Due to
enforcement of policies in protecting the environment, "green chemistry" has receivedmuch attention since 1990's which it is still progressing. Green chemistry is understoodto 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 volatileand most of them are flammable, toxic, quite hazardous and harmful compared to ionicliquids (ILs). Hence, there is a basic obligation for the advancement of new methodo-logies of polymerizations by using environmentally benign media which could replacethe 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 assolvents for polymerization processes has afforded some marked advantages such asincreased 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 ofhigh-performance polyamides in the green media using ILs are reviewed to develop amethodology for "green polymer chemistry".
Solution Polycondensation of Diamine and Diacid Chloride .......................... 984Polycondensation of Diamine and Diacid via Phosphorylation or with Phosphorus-containing Activating Agents ............................................... 985Polycondensation of Silylated Diamine and Diacid Chloride ......................... 985 Interfacial Polycondensation of Diamine and Diacid Chloride ....................... 985 Volatile Organic Solvents ................................................................................. 986
High performance polymers (HPPs) exhibit exceptional stability upon exposure to some harshenvironments and have properties that surpass thoseof traditional polymers. These materials are definedin many ways depending upon the application and, tosome extent, on the organized systems used for developing or employing the materials. Most of thefactors that contribute to high performance and heatresistance properties of these polymers are presentedas: resonance stabilization, primary bond strength,molecular symmetry, secondary bonding forces,molecular weight and distribution, rigid intrachainstructure, cross-linking, mechanism of bond cleavageand additives or reinforcements (fillers, clays, miscellaneous nanoparticles) [1-5]. They are used inmany industries ranging from communications tomedicine and to transportation [5-9]. Because of thesuperior performance characteristics for HPPs, thedemand for these types of polymeric materials isgrowing steadily [10-14]. Although several of these applications do not demand uses at high temperatures, but the fabrication process leading toparts or components requires the polymer to be thermally stable. Polyamides (PAs) as one of the mainwell known families of HPPs are presented here toexhibit the basic principles underlying polymerpreparations in ionic liquids.
POLYAMIDES
The polyamides (PA)s are high molecular weightmaterials 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 thechain flexibility and structural regularity, with the latter's importance in the formation of crystallinephase. Polyamides are often called nylons, the tradename given to them by DuPont. Many types of PAshave been studied, e.g., in one of the first patents onPAs almost 31 types are being described [15]. Sincethen, partially aromatic and wholly aromatic PAshave been successfully developed [16-19]. They havegood mechanical properties due to formation of
Polyamides are used essentially as fibres, films,and filler-containing engineering plastics for specialapplications [24,25]. Most of the reinforced PAs arefilled with glass fibres and to a lesser extent with particles, e.g., talc, kaolin, and mica. For engineeringplastics applications, dimensional stability at hightemperatures is often sought. Wholly aromatic PAscan be processed from solution either into films orinto fibres. These polymers have very good dimen-sional stability [21,22] and excellent heat resistance[26,27].
The PAs, (AA-BB)n, can be prepared fromdiamines and diacids. The AB types of PAs are madeeither from ω-amino acids or cyclic lactams whichare the derivatives of the ω-amino acids. Partially aromatic PAs have higher glass transition and meltingtemperatures compared to their aliphatic counter-parts. 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-addsystems such as diacids, diacid chlorides or diestersin combination with either aromatic or aliphaticdiamines, as well as, the reactions between diisocyanates and diacids which are commonly usedfor this propose [16].
Wholly aromatic PAs have high glass transitiontemperatures (>200°C) and, when crystalline, theyshow very high melting temperatures (>500°C).High-molecular-weight polymers cannot be preparedby melting or melt processing, because many aromaticdiacids are decarboxylated and the aromatic diaminesare readily oxidized and have a tendency to sublimeat elevated temperatures [28]. Their synthesis is usually carried out in solution and due to the very lowsolubility, special solvents are required to obtainhigh-molecular-weight polymers. Polyamides areusually synthesized by the following methods.
Solution Polycondensation of Diamine and DiacidChloride In this method, a diamine and a diacid chloride reacting in an amide solvent such as NMP, hexam-ethylphosphoramide (HMPA), or DMAc [29,30].
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Other polar aprotic solvents such as DMF anddimethylsulphoxide (DMSO) cannot be used becausethey react rapidly with acid chlorides [28].Consequently, this kind of polycondensation allowsthe macromolecular chain to continue to grow untilthe reaction is completed. Therefore, the key factorsthat are critical for obtaining high molecular weightpolymers are the stoichiometry of the monomers,purity and concentration of monomers, temperature ofthe reaction medium, reaction time, nature of the solvent(s), addition of salt, solubility of the polymerand speed of stirring [28,31,32]. The temperatureshould be adjusted to ensure that the condensationreaction is much faster than the side reactions. In general, low temperatures are favoured in thismethod. The solvent should enhance maximum solubility of the polymer formed at the early stage ofpolycondensation process. The solvation properties ofamide solvents can usually be increased by additionof salts, e.g., LiC1 or CaCl2. It is vital to avoid thepolymer to precipitate fast, because it may prevent themacromolecular chains to grow further.
In this method, the diacid needs to be converted toits diacid chloride derivative. Although the synthesisitself does not seem to be complicated, it usuallyinvolves the treatment with thionyl chloride which isan additional reaction step and produces environmen-tally hazardous SO2 and HC1 gases. The stability ofdiacid chloride towards hydrolysis is another problemin terms of purification and storage which should bekept in mind.
In addition when other functional groups are pres-ent on the same molecule, they need to be protectedbefore treating with SOC12. These problems havestimulated the need to seek alternatives [28,33].
Polycondensation of Diamine and Diacid viaPhosphorylation or with Phosphorus-containingActivating Agents In 1974, Higashi et al. [34] described a novel procedure to prepare aromatic PAs. This reactioninvolved the complexation of an acid with triphenylphosphite (TPP) in N-methylpyrrolidone(NMP) and pyridine (Py). CaCl2 and LiCl were usedalong with NMP to improve the molecular weight ofthe polymer. The initial study was followed by anextensive investigation of the reaction conditions by
the same research group [35]. In this method, severalkey factors, such as the concentration of monomers,ratio of TPP to monomer, reaction temperature andtime, concentration of LiCl and CaCl2, solvent andquantitative ratio of Py to metal salt were consideredto have great influence on the molecular weight of thefinal polymer. Although this type of reaction isaccompanied with gelation, still the polycon-densation 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 inNMP than the salts themselves; while NMP with ahigher content of metal salt can solubilize PAs more.Other phosphorus-containing activating agents like:triphenylphosphine [36,37], hexachlorocyclotri-phospha-triazene [38], phenylphosphine dichloride[39], or diphenylchlorophosphate [40] can be used inthis method, either.
Polycondensation of Silylated Diamine and DiacidChloride While most of the efforts in the synthesis of highmolecular weight PAs were oriented towards the activation of diacid component, there has beenincreasing interest in the activation of diamine com-ponent by reacting it with trimethylsilyl chloride.Indeed, high molecular weight PAs have been synthesized by low temperature polycondensation ofan N-silylated aromatic diamine with an aromaticdiacid chloride. The reaction is usually carried out at-10°C in NMP. Furthermore, reaction proceeds morerapidly and affords PAs with higher molecularweights relative to those obtained from free diamines[41-43].
Interfacial Polycondensation of Diamine andDiacid Chloride In the so-called interfacial polycondensation method,the two fast reacting intermediates are dissolved in apair of immiscible liquids, one of which is preferablywater. The water phase generally contains the diamineand usually an inorganic base to neutralize the acidby-product. The other phase contains the diacid chlo-ride in an organic solvent such as dichloromethane,toluene or hexane. The important factors that influ-ence this type of polycondensation have been studied
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in detail and reviewed by Morgan et al. [44-46].
Volatile Organic SolventsIn all the above methods for the synthesis of PAs, theuse of highly polar conventional solvents such asDMF, NMP, DMAc and Py is necessary for poly-merization process. Most of these solvents are flammable, toxic, quite hazardous and harmful andgenerally have several ecological disadvantageswhich impose heavy costs beyond prices of industrial synthetic methods. Hence, there is a greatneed for the advancement of new polymerizationmethodologies in using environmentally benignmedia which could replace the usual solvents andenhance solubilizations in polymerization process,either.
Volatile organic solvents which are of serious concerns in increasing air pollution and worker'shealth are common reaction media for commercialproduction of different chemicals, including polymers. These solvents are high on the list of harmful chemicals for two simple reasons: (i) theyare used in huge volumes and (ii) they are usuallyvolatile liquids that are difficult to handle [47]. It isan enormous challenge to diminish the amount ofvolatile organic compounds (VOCs) used in chemically small scale and industrial processes. It isestimated that about 20 million tons of VOCs as oneof the major air polluting agents are released to theatmosphere each year [48-50].
VOCs are the most usually used solvents in solution polymerizations, owing to their com-patibility with monomers and simplicity of separation, even with their well-documented healthand environmental concerns [51]. The separation ofVOCs can be costly and is generally ineffective inpreventing them to spread into the environment.When VOCs spread into air or water, many healthproblems emerge ranging from simple discomfort tocancer [52]. Another concern with VOC use is smog formation at elevated concentrations of ozoneat ground level [53].
At the same time, as a response to increasing legislative and social pressure and an increasingly"green-conscious" industrial community, researchershave started to examine more eco-friendly and sustainable chemical processes [54]. Toxicity and
recycling considerations are influencing the choice ofsolvent for industrial reactions. Therefore, the development of more efficient and environm-entally friendly processes is mandatory in the comingyears [55]. These processes must be designed on the basis of two main characteristics: energy savingto 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 harmfulVOCs. For these reasons, the development of newchemical processes based on the application of non-volatile materials is necessary [56-59]. Researchon chemical and polymer manufacturing has focused on the investigation of different approachesfor diminishing the emission of VOCs including solvent-free processes and the use of water, super-critical CO2 and more recently, ionic liquids (ILs) as the reaction media [60]. Water is non-flammable and environmentally benign, but due to solubilityissues it cannot be used in moisturesensitive systems, such as ionic polymerizations [61]. Themain drawback of the use of CO2 as a reaction medium is the utilization of high-pressure equipments to reach critical conditions in alleviatingone of the advantages of free-radical polymerization[62-64].
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 neotericsolvent [66-69]. Therefore, the development of neoteric solvents, i.e., ILs, for chemical synthesisholds great promise for green chemistry applications[70]. Interest in these compounds, is often heralded asthe green, high-tech media of the future [71-73]. Themore useful properties of ILs are as follows:
- They are relatively non-volatile which meansthey do not produce atmospheric VOCs and can beused in low-pressure environments [74-77],
- They exhibit good thermal stability and do notdecompose over a wide temperature range thereby
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making it feasible to reactions which require hightemperature [74,78-80],
- They show a high degree of potential for enantioselective reactions which have a significantimpact on the reactivity and selectivity due to theirpolar 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 ofchemical processes and can be considered both polarand non-coordinating solvents [77,79],
- They are the most complex and versatile solventsthat they have the ability to interact via hydrogenbonding, i.e., π-π, n-π, dispersive, dipolar, electrostat-ic, and hydrophobic interactions [77-79],
- They serve as a good medium to solubilitizegases such as H2, CO, O2 and CO2 and many reactions are now being performed using ILs andsupercritical CO2 [82],
- They can be immiscible with non-polar organicsolvents and/or water [80],
- They have high ionic character that enhances thereaction rates to a great extent in many reactionsincluding microwave assisted organic synthesis aswell as polymerization reactions [83],
- Their solubility depends upon the nature of thecations and counter-anions [84],
- They have physicochemical properties that canbe controlled by judicious selection of the cationand/or anion [85],
- They can be stored without decomposition for along period of time and have good miscibility withother organic solvents or monomers [86].
Ionic liquids also have some potential negativeproperties, e.g., high viscosity, potential toxicity,moister sensitivity and difficulty in purification.
The viscosity of many ILs is relatively as high asone to three orders of magnitude compared to conven-tional solvents. For a variety of ILs, viscosity hasbeen reported to be in the range of 10 - 500 mPa/s atroom temperature. The viscosity of ILs can affecttransport properties such as diffusion and may be anissue 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
are hydrophilic and if they are used in open vessels,hydration will certainly take place. The hydrophobic-ity of an IL increases with increasing length of itsalkyl chains. Despite their wide spread usage, ILscontaining PF6
- and BF4- have been reported to
decompose in the presence of water, giving off HF.Wasserscheid et al. [87] pointed out that ILs con-taining halogen anions generally show poor stabilityin water, and also give off toxic and corrosive speciessuch as HF or HCl. Therefore, they suggested the useof halogen-free and relatively hydrolysis-stableanions such as octylsulphate compounds.
The interaction between water and ILs and their degree of hydroscopic character are stronglydependent on anions. The amount of absorbed wateris the highest for BF4
- and the lowest for PF6- anions.
However, [Tf2N]- shows much greater stability in thepresence of water as well as having the advantage ofan increased hydrophobic character. Therefore, presence of water may have dramatic effect on thereactivity of ILs, and therefore they are usually utilized after a moderate drying process, as water ispresent 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. Anexample can be given by the development of "deepeutectic mixtures" of liquid systems based on cholinechloride for which its qualification as "ILs" is still thesubject of controversies. Choline can be used as analternative cation in combination with suitable anionto generate ILs. The biodegradability properties ofthese ILs have also been reported [90].
It was shown that the introduction of an estergroup into long alkyl chains leads to reduced toxicityand improves the eco-toxicity of ILs [90]. Furtherincorporation of ether groups into the side chainimproves the biodegradability of imidazolium-basedILs, while the introduction of the biodegradable octylsulphate anion has a further beneficial effect.
Recent work on pyridinium-based ILs demons-trated how the heteroaromatic cationic core can be modified to produce biodegradable examples [90]. Aswith the imidazolium examples, the inclusion of anester group in the cation side chain leads to improvedbiodegradability [90]. High levels of biodegradability
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have also been reported in cases where environmen-tally benign anions such as saccharinate and acesul-phamate are included. Several ammonium ILs basedon choline have been introduced which are biodegrad-able and can be readily prepared [90].
Room temperature ILs (RTILs) which are alreadyin common use, typically involve nitrogen- or phosphorus-containing organic cations such asalkylimidazolium, alkylpyridinium, alkylpyrrolidini-um, alkylphosphonium, alkyltriazolium, alkyloxazoli-dinium or alkylmorpholinium and anions like bis(trifluoromethanesulph-onyl)imide, hexafluoro-phosphate or tetrafluorophosphate. Although theseparticular cations and anions and their various combinations have already been studied extensivelyfor their potential applications in numerous chemicaland physical processes, every year more and morecation- and anion-forming liquid salts at room temperature are reported [87-92]. The most commonILs cations and anions are summarized in Scheme I.
There are several review articles on the synthesis,properties, and applications of ILs [93-96]. They havebeen investigated as a reaction media for synthesesincluding Diels-Alder [97], Wittig, the Suzuki cross-coupling [98], Heck alkylation [99], hydrogenation[100,101], oxidation [100], reduction [102], extraction [103], preparation of inorganic materials[104], bioprocessing operation [105], catalysis or aselectrolytes in electrochemistry [106].
Ionic liquids are also, more and more frequentlyused by polymer chemists not only as solvents forpolymerization processes but also as plasticizers oradditives for polymeric materials. In 1988, Oudard etal. [38] first described the polymerization of pyrrolein N-alkylpyridinium chloroaluminate.
Since 1997s, many polymerizations, including free
radical [107], living/controlled free radical [108,109],electrochemical, coordination polymerization ofolefins via Ziegler-Natta catalysts [110-112], conden-sation [113-115], ring-opening metathesis [116-120],block [121-125], the enzymatic synthesis of poly-esters (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 liquidshave also been recently evaluated as non-volatileplasticizers and as external or internal lubricants inseveral polymers including poly(methyl methacry-late) [128], poly(vinyl chloride) [129], PAs andpoly(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 in2009, with many contributions of industrial scientistsand engineers. Notably, the number of these
Figure 1. Annual number of citations for ILs and those con-taining the term polymer in the years 1998-2009. (Source:SciFinder Scholar).
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988 Iranian Polymer Journal / Volume 19 Number 12 (2010)
Scheme I. Most commonly used cation structures and possible anion types.
Most commonly used anions: BF4-, PF4-, PF6-, CF3SO3-, (CF3SO3)N-, CH3C6H5SO3-, CF3CO2-, Br-, Cl-
Most commonly used cations:
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 scienceis increasing even more rapidly than overall interestin ILs only. This probably reflects the realisticapproach of polymer scientists and engineers, whoare always looking for better ways to synthesizemacromolecules, alter their structures and propertiesand process them more efficiently. In recent years, anumber of outstanding reviews have been reported onthe topics of both polymers and ILs [132-139].
The utilization of ILs as solvent for poly-merization processes have afforded some markedadvantages such as increased molecular weight andnarrower polydispersity in comparison to organic solvents. Therefore, with the aim of developing greenpolymer chemistry, the synthesis and properties ofhigh performance PAs in the green media using ILsare considerably developed.
High Performance Polyamides in ILsAnalyzing the works on the use of ILs in organic synthesis shows their efficiency particularly in alkylation and acylation reactions [49]. This is thebase 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 etal. [140,141]. The first information on the use of ILsas a solvents and activated agent for polycondensa-tion reaction were reported by this group.
A series of aromatic PAs from indirect poly-condensation reaction of diamines such as 5(6)-amino-2-(4'-aminophenyl)benzimidazole (ABIZ),APh and 1,4-phenylenediamine (p-PhDA) withterephthaloyl chloride were synthesized in differentimidazolium type ILs at different reaction tempera-tures (0 to 60°C). The choice of polycondensationtemperature was being determined by the viscosity ofthe ILs and solubility of their starting diamines. It wasfound that PAs with the highest molecular weightwere obtained when ABIZ, containing an imidazolegroup similar to the IL structure, was used.Furthermore, all PAs precipitated from the reactionsolution in the course of polycondensation in hydrophobic ILs such as [B2Im]BF4 and
[MEIm](CF3SO2)2N). Thus, the polymerizationprocess and the molecular weights of the polymerscould be highly influenced by the nature of ILs.Inherent viscosities of the polymers obtained in 1,3-dialkylimidazolium bromides were in the range of0.18 -0.82 dL/g.
In another study by Vygodskii et al. [142], ILshave been used as solvents for preparing polymers viadirect polycondensation. Direct step growth poly-merization reaction of different monomers (dicar-boxylic acids and their dihydrazides, diamines, etc.)effectively occurs in ILs under the influence of TPPas activating agent. Owing to ILs usage there is noneed for any extra component in such a process, forexample, LiCl and Py. It should be mentioned thatthese additives are necessary participants in directpolyamidation of the same monomers in ordinarymolecular solvents, such as NMP.
The study of different polycondensation parameters including IL's nature has shown that thecombination of ionic media with activating agent,TPP, allows polymer synthesis to be carried out in relatively mild conditions (reaction temperature of100-140°C) [142]. The influence of ILs nature uponthe polymer molecular weight has been studied, aswell. For ILs having the same alkyl substituents thereis a strong dependency between alkyl length and PAmolecular weight, as upon longer alkyl group, there isan increase in viscosity observed. However, for ILswith asymmetrical cation there was no similar cleardependency. Therefore it was depicted that ILs con-taining n-propyl or iso-propyl alkyl chains seemed tobe the best solvents for PAs synthesis.
As far as anions are concerned, the best resultswere achieved in ILs with Br-. When aliphatic dicarboxylic acids (adipic acid) and aromaticdiamines are used as initial compounds, high molecular weights PAs (ηinh up to 0.91 dL/g) areformed. In contrast, aliphatic diamines lead to lowerPA viscosity characteristics. The similar results arereached upon the same reactions in ordinary organicsolvents. 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) andpolyhydrazides (ηinh = 0.71-1.32 dL/g) have beenobtained in quantitative yields with high molecularweights.
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Ionic liquids based on 1,3-dialkylimidazolium andammonium have been used as efficient reaction mediain the amidation of several carboxylic acids with isocyanates. Mallakpour et al. [143,144] reported thefirst application of molten IL (MIL) for the synthesisof PAs from the reaction of dicarboxylic acids withdiisocyanates. Polycondensation of terephthalic acid(TPA) with various commercially available diiso-cyanates such as 4,4'-methylene-bis-(4-phenyliso-cyanate) (MDI), toluylene-2,4-diisocyanate (TDI),isophorone diisocyanate (IPDI) and hexamethylenediisocyanate (HMDI) was performed in a fairly inex-pensive and readily accessible MIL, tetrabutylammo-nium bromide (TBAB) with or without dibutyltindilaurate (DBTDL) as a catalyst. The polymerizationreaction gave similar results in the presence orabsence of DBTDL, indicating that there is no needfor a catalyst in this process. Various PAs wereobtained with high yields and moderate inherent vis-cosities ranging from 0.36 to 0.71 dL/g. This methodwas compared with the polymerization reaction in
conventional solvent that lead to the formation ofpolymers with lower inherent viscosity (0.21 to 0.57 dL/g) in the presence of DBTDL as a catalyst. Inthe case of TBAB, higher yields and inherent viscosities were obtained. This process is safe andgreen since toxic and volatile organic solvent such asNMP is eliminated, an indication of the potential ofmolten TBAB, a benign readily available IL as anefficient catalyst, and has much promise for furtherapplications. Moreover, this methodology offers significant improvements with regard to yield ofproducts, inherent viscosities, thermal stability, costefficiency and green aspects avoiding toxic catalystsand solvents.
Step growth polymerization reaction of differentchiral diacid monomer such as 5-(4-methyl-2-phthal-imidylpentanoylamino)isophthalic acid, 5-(3-phenyl-2-phthal-imidylpropanoylamino)isophthalic acid and5-(3-methyl-2-phthalimidylpentanoyl-amino) isoph-thalic acid with several aromatic and aliphatic diisocyanates were performed under microwave
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Scheme II. Synthesis of optically active and thermally stable PAs [145-147].
irradiation and conventional heating techniquethrough direct polycondensation in molten TBAB ortraditional solvent like NMP (Scheme II) [145-147].The reactions were carried out in the presence of asmall amount of DBTDL, Py or triethylamine (TEA)as catalysts and with no conventional catalyst. Thebest results were obtained with DBTDL, under bothconditions.
Microwave irradiation as a non-conventional energy sources is a very popular and useful technology in polymerization for optimizing andaccelerating of chemical reactions.
Mallakpour et al. [146] have studied the effect ofmicrowave power levels and duration of heating toprovide the optimum reaction conditions (method I).The results revealed that the optimal results wereobtained after 3 min at 100% of power level. Theinherent viscosities of the resulting polymers undermicrowave irradiation were in the range of 0.36-0.63 dL/g and the yields were 74-95%. To comparethe microwave-assisted method with conventionalheating, the polycondensations were also carried outunder conventional heating in TBAB (method II)[146]. When the same experiment was conducted byconventional heating in the presence of TBAB as asolvent, it took much longer time (12 h of heating at120°C) for completion of the polymerization reactions. Under these conditions, yields and inherentviscosities of the polymers were ranging from 73 to91% and 0.32-0.56 dL/g, respectively [146].
Mallakpour et al. [148] prepared high performancePAs by polyamidation reaction of a novel diacid containing naphthalimide and flexible chiral groupsand different diisocyanates in the presence of a smallamount of RTIL that act as a primary microwaveabsorber. For the first time, electrochemical behaviourand stability of the resulting PAs on multiwall carbonnanotube-modified glassy carbon electrode wasreported by this group [148]. Moreover, these electrochemical studies showed that electrochemicaldecomposition and stability of S-valine-based diacidmonomers and the resulting polymers in acidic solution are more complex than in basic solution asthe electrochemical behaviour of polymer is very similar to its monomers.
Kinetic study was performed on the thermalgravi-metric analysis curve, by use of an integral analysis
method. In order to study the effect of different existing functional groups in the resulting PAs ontheir thermal stability, the thermal kinetic investiga-tions by TGA and DSC techniques were carried out byusing Coats-Redfern equation [149].
One-pot polyamidation reaction of optically activearomatic diacid containing L-alanine [150], L-methio-nine [151,152], L-leucine [153,154] or L-valine[155,156] and phthalimide moieties with aromaticdiamines through direct phosphorylation reactionunder microwave irradiation and traditional heatingwas performed. Polycondensation reactions wereexamined using NMP/TPP/CaCl2/Py system as theconventional condensing agent and then the benefitsof IL/TPP as both polyamidation catalysis and reaction medium were investigated for preparation ofwholly aromatic optically active and high performance PAs. Optimization of the polymerizationreactions was carried out by varying the type andquantity of ILs, amount of TPP, reaction time andtemperature, and catalyst used. The poly-condensa-tion reactions were carried out in different RTILsbearing different alkyl groups and it was found that[Isopr2Im]Br and [Pr2Im]Br are the best ILs for poly-merization reaction. The results indicated that the PAsprepared via IL method encompass higher inherentviscosities and yields compared to those preparedunder conventional method.
By comparison, in the case of IL catalyzedpolyamidation, removal of some chemicals (e.g.,NMP, CaCl2 and Py) which are essential in conventional methodologies decreases the cost ofpolymerization as well as the environmental pollution, considerably. Moreover, polymerizationreactions via IL/TPP have been completed in shorterperiod of time (2.5 h vs. 5 h) and it can be a desiredaspect from energy saving and commercial point ofviews especially in the industrial large scales polymerization processes.
The above mentioned polymerization reactionswere performed under microwave irradiation. Theyield and inherent viscosities of resulting polymerswere ranging between 91%-95% and 0.47-0.65 dL/g,respectively. It shows that polymerization reactionsproceeded in higher yields, moderate inherent viscosities, good thermal stability and shorter reationtimes under microwave irradiation conditions,
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Mallakpour et al. [155,156] claim that since thesepolymers are optically active and have amino acids inthe polymer architecture, they are likely biodegrad-able, and therefore they are classified as environmen-tal friendly polymers.
Recently, Mallakpour et al. [157] synthesizednovel chiral aromatic PAs from the reaction of a new diacid monomer, 5-[3-phenyl-2-(9,10-dihydro-9, 10-ethanoanth-racene-11,12-dicarbox-imido) propanoy-lamino], isophthalic acid and different aromaticdiamines using [Pr2Im]Br under microwave irradiation. By controlling the concentration of[Pr2Im]Br, reaction time and power level, polymerswith high yield and moderate inherent viscosity wereaccomplished in a very short period of time. The PAswere found to have inherent viscosities in the range of0.54-0.85 dL/g, glass-transition temperatures (Tg)above 180°C with 10% weight-loss beyond 340°C
temperature, and over 40% char yield at 800°C innitrogen atmosphere (Scheme III). All these polymerspossessed bulky anthracenic and amino acid function-alities in the side chains, showing excellent solubilityin various solvents.
The novel optically active aromatic PAs withflame retardancy properties were prepared by heatingover an oil bath, using a mixture of [Pr2Im]Br andTPP both as reaction media and activator (Scheme IV)[158]. This procedure was a one-pot reaction and didnot need to prepare diacid chloride. These polymerspresented high thermal stability, with the decomposi-tion temperature above 400°C [158]. The incorpora-tion of tetrabromophthalimide and L-phenylalaninegroups into PA's backbone gave polymers a good solubility in common organic solvents. In addition,the interpretation of kinetic parameters (E, ΔH, ΔSand ΔG) of thermal decomposition stages of
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992 Iranian Polymer Journal / Volume 19 Number 12 (2010)
Scheme III. Synthesis of anthracene based PAs [157].
aforementioned PAs was evaluated. More recently, Mallakpour et al. [160-163]
prepared novel optically active and thermally stablearomatic PAs by safe and fast polyamidation of 5-[4-(2-hthalimidiylpropanoylamino)benzoylamino]isophthalic acid [159] or (2S)-4-[(4-methyl-2-phthal-imidylpentanoylamino)benzoylamino] isophthalicacid with aromatic diamines or aromatic and aliphat-ic diisocyanates in polar organic solvent and ILsunder microwave irradiation and conventional heat-ing methods. Several high performance PAs contain-ing pendant L-leucinephthalimide or alaninephthal-imide and benzamide groups were prepared withgood yields through a simple microwave heatingmethod using IL and TPP as a modern and safemethodology (Scheme V). The resulting PAs haveinherent viscosity in the range of 0.43-0.81 dL/g.These PAs exhibited an enhanced solubility as compared to those analogous PAs without benzamideunit.
It is evident that the introductions of the benza-
mide substituents resulted in increased chain packingof the macromolecules and thus facilitate the diffu-sion of solvent molecules into the polymer chainsand decreased intermolecular interactions, leading tohigher solubility. However, the effect of the benzamide group on the T5 and T10 values could beappreciated when these PAs were compared with PAswithout benzamide unit, while the other part of thepolymer 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 theoptical rotation on the overall structure and regulari-ty of the resulting polymer chains. Surprisingly, poly-mers of the same chemical structure polymerized bythe same method with different catalysts resulted indifferent optical rotation values.
The following results can be concluded: (1) Thecombined advantages of microwave irradiation andIL make the polycondensation reactions with safe
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Iranian Polymer Journal / Volume 19 Number 12 (2010) 993
Scheme IV. Preparation of thermally stable and optically active PAs with flame retardancy properties [158].
operation, low pollution, rapid access to products andsimple workup and on the other hand no need ofusing 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 mayminimize polymer degradation, (3) in the case of ILcatalyzed 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 thesetwo 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
for the direct polycondensation of dicarboxylic acid,5-(3-acetoxynaphthoyl-amino)isophthalic acid withseveral aromatic diamines was studied in[Isopr2Im]Br under microwave irradiation and con-ventional heating [164]. The polymerization reactionwas effectively run in IL, and TPP as an activatingagent, and the resulting photoactive PAs wereobtained with high yields and moderate inherent vis-cosities in the range of 0.44-0.69 dL/g. TGA showedthat polymers are thermally stable, with 10% weightloss beyond 390°C, and higher than 60% char yieldsat 600°C in nitrogen atmosphere. These macromole-cules exhibited maximum UV-vis absorption at 265and 300 nm in DMF solution. Their photolumines-cence in DMF solution demonstrated fluorescenceemission maxima around 361 and 427 nm for all of
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994 Iranian Polymer Journal / Volume 19 Number 12 (2010)
Scheme V. Synthesis of thermally stable PAs having benzamide group [159-163].
the PAs. The incorporation of an acetoxynaphthalenegroup, through an amide unit, into PAs backbonegave polymers with remarkable solubility in commonorganic solvents.
A facile and proficient synthesis of heterocyclicPAs through polycondensation of 4-(4-dimethy-laminophenyl)-1,2,4-triazolidine-3,5-dione with various commercially available aliphatic diacid chlorides (the length of the diacid chlorides chainswere 2, 4 and 8 -CH2- moieties, respectively) in thepresence of RTILs and MIL was carried out byMallakpour et al. (Scheme VI) [165]. The effect ofvarious reaction parameters, including the nature ofthe ILs, its amount, the reaction temperature, and thereaction time were investigated to optimize the conditions of the preparation of heterocyclic PAs. Thepolymerization proceeded well in ILs without anycatalyst and PAs were obtained with high yields andmoderate inherent viscosities. The easy work-up pro-cedures, the absence of a catalyst and recyclability ofthe non-volatile IL used as reaction medium make themethod 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 beenexamined and the following conclusions werereached: (1) the best results were achieved in thepresence of [Isopr2Im]Br, at temperature of 80°C for6 h; (2) ILs act both as effective solvents and catalysts to mediate clean polycondensation reactionsto yield the desired PAs. In fact the process of poly-merization can be carried out without additional catalyst and conventional solvent and the ILs can beeasily separated. Although excellent yields for the
resulting polymers were obtained, only low to moderate inherent viscosities were achieved. Thiscould be explained in terms of cyclization duringpolymerization reaction.
Pillai et al. [166] developed a novel and con-venient method for the synthesis of a potentially safenon-viral gene delivery vehicle based on the cationicblock copolymer of spermine and aspartic acid(ASSP) and coupled it with polyethylene glycol(PEG). The copolymer ASSP was prepared by directpolycondensation in IL, 1-butyl-3-methylimidazoli-um hexafluorophosphate ([MBIm]PF6), using TPP asthe condensing agent under mild reaction conditions.The highly hydrophobic ASSP was transformed intoa water soluble hydrophilic micelle by couplingASSP with PEG using the same IL and 1,1-carbonyldiimidazole as the condensing agent without harshconditions. The polycationic ASSP-PEG was thenused to condense calf thymus and plasmid deoxyri-bonuclceic acids (DNAs) in Tris-HCl buffer (pH 7.4)to obtain a series of block ionomer complexes withvarious charge ratios. It was observed that the DNAwas condensed to compact particles by its interactionwith the copolymer.
Since DNA condensation to nano/micrometersized particles is essential for gene delivery, thisresearch group claimed that the resulted polymer hada potential use as the copolymer for gene deliveryapplications.
CONCLUSION
One of today's biggest challenges for coming years is
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Iranian Polymer Journal / Volume 19 Number 12 (2010) 995
N
NN
O O
H H
N(CH3)2
CH2C C
OO
Cl Cl
N
NN
O O
N(CH3)2
C
O
CH2C
O
nm
TBAB or
n: 2, 4, 8
[Isopr2Im]Br
+ n
Scheme VI. Synthesis of heterocyclic PAs [165].
the minimization of industrial pollutions. Worldwideusage of VOCs as industrial solvents currentlyexceeds $5billion annually, indicating their tremendous volumes employed. One of the largestsectors of the chemical industry is polymer industry,with over 30 million tons of polymers producedannually. A major component of industrial waste isused solvents, and strategies for eliminating solventvapours will have huge impact on cleaning up theindustrial polymer productions. While aqueous reac-tion media are widely used, employing VOCs is stillwidely practiced. Today, there is a great need for theadvancement of new methodologies for polymeriza-tion using environmentally benign media whichcould replace the usual solvents and give enhancedsolubility to polymerization products. Ionic liquidshave great potential as non-volatile organic media forpolymerization and polymer processing due to the near-zero vapour pressures, non-flammable, inexpensive, excellent microwave absorbing abilityand ease of preparation. In addition, polymerizationin 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 solventscan result in economical, social, and ecologicalimpacts due to their direct effects on the humanhealth and environment. One of the common observations across several techniques used is thatthe precipitation of the polymer product from the ILsmedia terminates the reaction. To take advantage ofthis phenomenon would lead to narrower poly-dispersity which is scarcely reached. With the synthetic flexibility that ILs show, it may be possibleto manipulate the solubility of polymers in the ILs ina sufficiently controlled manner that they can be usedas a means to prepare polymers of desired molecularweights. Because of the superior performance characteristics for high performance PAs, the demandfor these types of polymeric materials is growingsteadily.
Non-volatile nature and stability at high temperature make the ILs excellent candidates asreaction medium as well as catalysts for the preparation of these types of polymers. Therefore,with the aim of developing of "green polymer
chemistry", the synthesis and properties of high performance PAs in the environmental friendlymedia using ILs as a green and safe media are in themain stream of polymer studies. It is predicted thatthis new technology will be a popular method for the synthesis of HPPs in laboratory and industrialscales, soon.
ACKNOWLEDGEMENT
We wish to express our gratitude to the ResearchAffairs Division of Isfahan University of Technology(IUT) for financial support. Further financial supportfrom National Elite Foundation (NEF) and Center ofExcellency in Sensors and Green Research of Isfahan University of Technology are also gratefullyacknowledged.
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High Performance Polymers in Ionic Liquids ... Mallakpour S et al.
1004 Iranian Polymer Journal / Volume 19 Number 12 (2010)
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