7 Aromatic Polyethers Hans R. Kricheldorf Institute of Technical and Molecular Chemistry, University of Hamburg, Hamburg, Germany I. INTRODUCTION Aromatic polyethers are a group of high-performance engineering plastics which were developed by several chemical companies over the past forty years. Poly(phenyl ene ether)s mostly prepared by oxydative polycondensation of phenols, were the first class of aromatic polyethers which was technically produced and commercialized [1–3]. Perfectly linear poly(arylene ether)s free of side chains can only be obtained by oxidative coupling of 2,6-disubstituted phenols. Therefore, the poly(2,6-dimethylphenylene-oxide), usually called PPO, is the most widely used poly(arylene ether). Its outstanding property is its compatibility with polystyrene (and a few other polymers) so that it is mainly used as component of blends [4]. The oxidative coupling of phenols has intensively been studied between 1950 and 1980 (as discussed in the 1st edition of this handbook) but only few research activities were observed during the past 10 years discussed in the subchapter ‘Various Aromatic Polyethers’. Therefor e the literature evaluated and discussed below mainly concerns poly(ether sulfone)s and poly(ether ketone)s. Furthermore, semi aromatic polyethers and aromatic polysulfides are included in this chapter which mainly covers the literature of the years 1990 through spring 2000 (complementary to the first edition). The characteristic properties and advantages of aromatic polyethers when compared to aliphatic engineering plastics based on an aliphatic main chain are as follows. Aromatic polyethers are less sensitive to oxydati on at all temperatures, and thus, also less inflammable (poly(vinyl chloride) and poly(tetrafluoroethylene) are, of course, exceptions in the case of aliphatic polymers). The thermostability of aromatic polyethers is higher. For all these reasons the maximum service temperature of aromatic polyethers (200– 260  C) is twice as high as that of aliphatic polymers. Furthermore, aromatic polyethers possess higher glass-transition temperatures (T g s) which may be as high as 230  C for commercial poly(ether sulfone)s. The T g s of commercial poly(e ther-ketone)s are lower (typically around 140–150  C) but all commercial poly(ether-ketone)s are semicrystalline materials having melting temperatures in the range of 280–420  C. Either due to high T g or due to a high T m the heat distortion temperature of aro matic polyethers significantly higher than that of aliphatic polyme rs. A few characteristic disadvantages should also be mentioned. Most poly(ether-sulfone)s reported so far and all commercial examples are amorphous with the advantage of a high transparency and the shortcoming of a high sensitivity to the attack of organic solvents. The crystalline poly(ether ketone)s are rather Copyright 2005 by Marcel Dekker. All Rights Reserved. insensitive to organic solvents, but they are sensitive to a cleavage by UV-irradiation quite analogous to low molar mass benzophenones. Finally, it should be mentioned that a typical application of poly(ether sulf one)s and poly(ether-ketone)s is that as matric material in composites. Glas s fiber or carbon fiber are used as reinforcing components. II. POLY(ETHER SULFONE)S Poly(ether-sulfone)s, PESs, may in principle be prepared via four different strategies: 1. Polycondensation of suitable monomers involving an electrophilic substitution of an aromatic ring. 2. Polycondensation of suitable monomers involving a nucleophilic substitution of a chloro, fluoro or nitroaromat activated by a sulfonyl group in para-position. 3. Chemical modification of suitable precursor polymers. 4. Ring-opening polymerization of cyclic oligo(ether-sulfone)s The discussion of synthetic methods and structures presented below will follow this order. A. Syntheses via Electrophilic Substitution The oldest approach known for the preparation of PESs is a polycondensation process involving the electrophilic substitut ion of a phenyl ether group by an aromatic sulfonyl chloride group [3,5,6]. Such polycondensations may be based, either on monomers containing both functional groups in one molecule (Eq. (1)) or by a combination of a nucleophilic and an electrophilic monomer (Eq. (2)). These polycondensations need to be catalyzed by strong Lewis acids such as FeCl 3 , AlCl 3 ,orBF 3 . Characteristic disadvantages of this approach are the need of an expensive inert reaction medium and side reactions such as substitution (including branching) in ortho position of the nucleophilic monomer. Furthermore, this approach is not versatile and limited to a few monomers. Previous research acti vities in this field were report ed in the 1st edition of the Handbook [3], but more recently new activities were not observed. ð1Þ ð2Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. B. Syntheses Via Nucleophilic Substitution The most widely used approach to the preparation of PESs in both academic research and technical production is a polycondensation process involv ing a nucleophilic substitution of an aromatic chloro- or fluorosulfone by a phenoxide ion (Eq. (3)). Prior to the review of new PESs prepared by nucleophilic substitution publications should be mentioned which were concerned with the evaluation and comparison of the electrophilic reactivity of various mono- and difunctional fluoro-aromats [7–10]. The nucleophilic substitution of aromatic compounds may in general proceed via four different mechanism. Firstly, the S N1 mechanism which is, for instance, characteristic for most diazonium salts. Secondly, the elimination-addition mechanism involving arines as intermediates which is typical for the treatment of haloaromats with strong bases at high temperature. Thirdly, the addition– elimination mechanism which is typical for fluorosulfones as illustrated in equations (3) and (4). Fourthly, the S NAR mechanism which may occur when poorly electrophilic chloroaromats are used as reaction partners will be discussed below in connection with polycondensations of chlorobenzophenones. ð3Þ ð4Þ In the case of the addition–elimination mechanism the addition step with the formation of a short lived Meisenheimer complex (Eq. (3)) is the rate determining step. Hence, the electron density of the carbon directly bound to the fluorine (ipso position) is decisive for the reactivity and thus, for the rate of the reaction. In two publications [7,8] the 13 C NMR chemical shifts of various fluoroaromats were determined, compared and shown to be useful indicators of the reactivity of the ipso-carbon. This conclusion was confirmed by calculation of the electron density via the quantum semiempirical PM 3 method in the MOPAC software. In fact, a linear correlation between the calculated electron density and the 13 CNMRd values was obtained. Furthermore, the 19 F NMR chemical shifts were determined for numerous electrophilic fluoroaromats and again a linear correlation with the calculated electron densities, on the one hand, and with the 13 C NMR chemical shifts, on the other, was found [7,8]. These studies proved that the SO 2 group is the strongest activating divalent group. Only the monovalent nitrogroup has a stronger electron-withdrawing effect. The strong electron-withdrawing effect of the SO 2 group has also the consequence that the C-atom directly attached to it is sensitive to a nucleophilic attack. With KF as reagent the cleavage of the PES backbone (back-reaction of Eqs. (3) and (4) was observed at 280  C [9], but it is not clear if the cleavage will be more favored by other cations such as Cs  . Finally, a publication should be mentioned [10] reporting on a partial desulfonylation during the polycondensation of a special ketone- sulfone type monomer. The standard procedure used by most authors for syntheses of new poly(ethersul- fone)s is based on the reaction of equimolar amounts of a difluoro (or dichloro)- sulfone Copyright 2005 by Marcel Dekker. All Rights Reserved. and a bisphenol with dry K 2 CO 3 (equimolar or slight excess) in polar aprotic solvents such as N-methylpyrrolidone (NMP), dimethylacetamide (DMAc) dimethylsulfoxide (DMSO) or sulfolane. In the paper [11] stoichiometric amounts of CsF were applied instead of K 2 CO 3 . However, CsF has no advantage, but it is significantly more expensive. Following the standard procedure with K 2 CO 3 two research groups used commercial 4,4 0 - dichlorodiphenylsulfone (DCDPS) for the preparation of the PESs (5a) [12] and (5b) [13]. The DCDPS was also taken as electrophilic reagent for the preparation of the functionalized oligo(ether-sulfone)s which were modified at the chloro endgroups (6) [14]. Another class of functional PES (7) was synthesized from commercial 4,4 0 -Difluorodi- phenylsulfone (DFDPS) and 1,1-bis(4-hydroxydiphenyl)ethene [15]. The pendant methy- lene group allows for thermal crosslinking of these PESs. DFDPS in combination with various diphenols and 4-fluoro-4 0 -hydroxydiphenylsulfone served as comonomers for the preparation of copoly(ether-sulfone)s having the structure (8). Their properties were evaluated and correlated with their composition and sequence [16]. ð5Þ ð6Þ ð7Þ ð8Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. Most new PESs reported during the past ten years were prepared from new sulfone type monomers or at least from noncommercial monomers. For instance, the difluoroketone-sulfone (9a) was polycondensed with the dihydrodiphenylketone-sulfone (9b) [17]. The same monomers were later used by another resear ch group together with a variety of new fluoro ketone type monomers [19]. Other authors synthesized the naphthalene containing difluordiphenylsulfones (10a) and (10b) as reaction partners of methyl substituted 4,4 0 -dihydroxybiphenyls [19]. Dichloro- or difluoro- diphenylsulfones having a central biphenyl unit (11a) were polycondensed with various commercial diphenols [20,21]. In one of these papers [20] PES derived from the bisphenol (11b) were studied in detail. ð9Þ ð10Þ ð11Þ Thiophene based poly(arylene-ether-sulfone)s were synthesized from monomer (12) and bisphenol-A [22]. Another class of unconventional monomers is outlined in the formulas (13a) and (13b) [23,24]. These monomers have the advantage that its structure can easily be varied at the imide ring, and reactions at the imide ring allow also a modification of the PES itself. In any case these terphenyl monomers co nsiderably raise the glass transition temperature (Tg). Another kinked structure is that of the indan derivatives (14) [25]. They were polycondensed with bisphenol-A and other common diphenols. Two research groups reported on syntheses of more or less fluorinated PESs. Two different synthetic strategies were elaborated. In the first case a difluoro diphenyl disulfone having a fluorinated aliphatic chain segment (15b) was synthesized by oxidation of the corresponding disulfide (15a) [26]. The disulfone (15b) was then polycondensed with Copyright 2005 by Marcel Dekker. All Rights Reserved. a variety of diphenols by means of sodium carbonate in DMAc. The second strategy is characterized by the preparation of PESs having pendant trifluoromethyl groups by polycondensation of the monomers (16a), (16b) and (16c) [27,28]. ð12Þ ð13Þ ð14Þ ð15Þ ð16Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. C. Chemical Modification The normal route of nucleophilic substitution was also applied to syntheses of PESs having a broad variety of pendant functional groups. In most publications dealing with functional PES the reactive substi tuents were subjected to further modifications. Two research groups were inter ested in sulfonated PES which may find potential application as proton transporting membranes in fuel cells. Two different synthetic approaches were explored. The first one is based on polycondensations of a sulfonated DCDPS (17a) with preformed potassium salts of diphenols in DMAc at 170  C [29]. The second approach consists of the sulfonation of preformed PESs [30,31]. When a PES derived from hydroquinone was sulfonated exclusive monosulfonation of the hydroquinone unit was observed (18a). Increasing reactivity of the sulfonating agent did not influence the degree of substitution but the stability of the PES chain. With 91% sulfuric acid no degradation was observed at room temperature, whereas chlorosulfonic acid and oleum caused severe degradation. Fur thermore, PES derived from methyl hydroquinone, dimethylhydro- quinone and trimethylhydroquinone were sulfonated with concentrated sulfuric acid. Complete monosubstitution was found for mono- and dimethyl hydroquinone (18b) and (19a), whereas the sulfonation of the trimethyl hydroquinone units (19b) remained incomplete [31]. ð17Þ ð18Þ ð19Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. Several studies dealt with syntheses of PESs having pendant amino groups. Quite analogous to the syntheses of sulfonated PESs two strategies were explored, (a) polycondensation of aminated monomers, and (b) modification of preformed PES. The first strategy was realized with synthesis and polycondensation of the dichlorodiamino- sulfone (20a) which was synthesized by hydrogenation of 2,2 0 -dinitro-4,4 0 -dichlorodiphe- nylsulfone [32]. Another research group used the corresponding difluorodiaminosulfone [33]. A further difluoromonomer having a pendant amino group is the phosphine oxide (20b) which was used as comonomer together with DFDPS and various diphenols [34]. The second synthetic strategy was realized in such a way that performed PESs were nitrated at the hyd roquinone unit and the nitrogroup was reduced by means of sodium dithionite (21) [35]. Another approach is based on the synthesis of PES, having pendant imide groups (22) [36–38]. Variation of the amine (for instance via transimidization) allows a broad variation of the pendant functional groups including the introdu ction of an amino group. ð20Þ ð21Þ ð22Þ Five more papers reported on various modifications of PES involving introduction and substitution of chloro or bromoatoms. For instance, bromination of the bisphenol-A unit in a commercial PES yield ed the dibromoproduct (23), which was treated with butyllithium. The lithiated PES was then reacted with methyliodide [39] or with tosylazide [40]. The resulting azide groups were finally reduced to amino groups (24). Another modification of brominated PESs utilized palladium complexes as catalysts for the Copyright 2005 by Marcel Dekker. All Rights Reserved. introduction of alkin-type substituents (25). These substituents have the purpose to enable a thermal cure via cyclization or polymerization of the alkin groups [41]. ð23Þ ð24Þ ð25Þ Two papers reported on the chloromethylation of PESs and the further modification of the chloromethyl groups. In the first paper [42] the combination of octylchloromethyl ether and SnCl 4 was used to introduce the CH 2 Cl groups, the combination of octylbromomethyl ether and SnBr 4 yielded CH 2 Br groups (26a) and combinations of chloromethylether þ SnBr 4 or bromomethyl ether and SnCl 4 producing a statistical array of chloro and bromomethyl substituents. Reactions with potassium tert butoxide yielded pendant tert butyl ether group s (26b) with sodium acetate pendant acetate groups were obtained and after alkaline saponification CH 2 OH groups (26b). Furthermore, pendant tosylate groups (27a) and diethylphosphonates (27b) were prepared. With sodium cyanide pendant nitrile groups were formed (28a) which were saponified to yield CH 2 CO 2 H Copyright 2005 by Marcel Dekker. All Rights Reserved. groups. Finally the oxidation of chloromethyl groups with dimethylsulfoxid/NaHCO 3 or with Cr 2 O 2 7 was studied (yielding aldehyde groups (28b) [42]. In the second paper triflicacid was used as solvent and catalyst in combination with butyl or octyl chloro- methyl ether. This system is of course too expensive for any large scale experiments or technical production of functionalized PES. For homo- or copolyether containing hydroquinone an exclusive monosubstitut ion of the hydroquinone unit was found (29a), and finally the transformation of the chloromethyl groups into triethylammonium groups (29b) was studied [43]. PESs having pendant aldehyde groups were prepared from (co-)polycondensations of the diphenol (30a). The aldehyde groups were almos t quantitatively transformed into azomethine groups (30b) [44]. In another publication [45] unsaturated PESs were prepared from 4,4 0 -dihydroxy-trans-stilbene and DFDPS and treated with H 2 O 2 in the presence of a tungsten catalyst whereby epoxide groups suitable for chemical or thermal crosslinking were obtained (31). Finally, a publication dealing with the influence of energy rich irradiation (x-ray, electrons, AR  and N 2  ) on PES should be mentioned [46]. ð26Þ ð27Þ ð28Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. [...]... backbone includes C–N bonds in addition to ether groups 72 Þ 73 Þ Copyright 2005 by Marcel Dekker All Rights Reserved  74 Þ 75 Þ 76 Þ 77 Þ 78 Þ 79 Þ Most syntheses of PEKs showing new structural elements were based on new or noncommercial ‘fluoromonomers’ A difluorodiketone (80a) with a kinked structure designed to reduce the melting temperatures of the PEKs derived from it was prepared from isophthaloyl... substitution from hydroquinone and an excess of DFBP (109) They were used in turn for syntheses of PES block copolymers (110) [ 177 , 178 ] Blockcopolymers containing dimethyl siloxane segments were realized by heating OH-terminated tOEKs (111a) with amine terminated oligosiloxanes (111b) in a two phasic solvent system [ 179 ] Finally the multistep synthesis of A-B-A-triblock copolymers having a central OEK block... combination with an acidic dehydrating agent None of the polycondensation methods described in this section is new, and origin and early exploration of these methods has been reviewed in the 1st edition of this handbook (Chapter 9) ð54Þ In a publication of 1988 [79 ] (not reviewed before) polycondensations of phenoxybenzoyl chloride (Eq (54)) or polycondensations of diphenylether with isophthaloylchloride,... when silylated diphenols (71 b) and a catalytic amount of CsF were used as reaction partners of (71 a) In the second paper it was reported that DMPU is advantageous over NMP when less reactive electrophiles than fluoroketones or fluorosulfones are used (see Section III.F) 70 Þ 71 Þ C Various Structures Most papers reporting on syntheses of PEKs deal with a systematic variation of their structure with the... characterization of telechelic oligo(ether sulfone)s which served as building blocks of triblock copolymers, multiblock copolymers or networks OH-terminated oligomers (32) were prepared by polycondensations of DCDPS with an excess of bisphenol-A [ 47 49] These oligo(ether sulfone)s were reacted with commercial bisepoxides to yield epoxy networks [ 47] They also proved to be useful for syntheses of multiblock... concentrations, large fractions of cyclic OES were obtained and monodisperse cycles (from the dimer to the hexamer) were isolated by column chromatography [76 ,77 ] Two papers [73 ,75 ] describe detailed studies of ring-opening polymerizations conducted in bulk at high temperatures Unfortunately, cyclic OESs possess high melting temperatures (up to 500  C), and only in cases of nonsymmetrical cycles the... ð114Þ 3 Hyperbranched PEKs For the sake of completeness six publications dealing with synthesis and characterization of hyperbranched PEKs should be mentioned [181–186] However a detailed discussion of hyperbranched polymers will be presented in a separate chapter of this handbook D Unusual Synthetic Methods The discussion of unusual methods reported for syntheses of PEKs is subdivided into two strategies:... dimerized and cyclized in the presence of K2CO3 (51) [68] Thirdly, several cyclic OESs were prepared by condensation of diphenols and dihalosulfones via nucleophilic substitution under high dilution (52) [69 75 ] Either mixtures of cyclic OES were isolated and used for ring-opening polymerizations [69 ,70 ] or monodisperse cycles were isolated and characterized [71 75 ] Fourthly, preformed PES was subjected... technology Secondly, the ROP of strained cyclic OESs offers the chance to prepare PESs with very high molecular weight (Mn > 105 Da) Thirdly, sequential copolymerizations with other cyclic monomers may yield a variety of block copolymers Cyclic OESs were prepared in four different ways Firstly, an electrophilic acylation of 1,4-bisphenoxybenzene was performed under high dilution (50) [ 67] Secondly, 1-chloro-40... (72 a) [114] or PEKs prepared from the substituted hydroquinones (72 b–e) [115] Polyelectrolytes of structure (73 ) were prepared by copolycondensation of a sulfonated hydroquinone and unsubstituted hydroquinone [116] The free sulfonic acid served as binding site for the fixation of basic NLO chromophors, such as (74 a,b) and (75 a,b) in the form of their pyridinium salts Several PEKs showing improved solubilities . based on polycondensations of a sulfonated DCDPS (17a) with preformed potassium salts of diphenols in DMAc at 170  C [29]. The second approach consists of the sulfonation of preformed PESs [30,31] fractions of cyclic OES were obtained and monodisperse cycles (from the dimer to the hexamer) were isolated by column chromatography [76 ,77 ]. Two papers [73 ,75 ] describe detailed studies of ring-opening. and early exploration of these methods has been reviewed in the 1st edition of this handbook (Chapter 9). ð54Þ In a publication of 1988 [79 ] (not reviewed before) polycondensations of phenoxy- benzoyl