Handbook of Polymer Synthesis Second Edition Episode 7 pptx

Syntheses via Electrophilic Substitution The oldest approach known for the preparation of PESs is a polycondensation processinvolving the electrophilic substitution of a phenyl ether gro

‘Various Aromatic Polyethers’ Therefore the literature evaluated and discussed belowmainly concerns poly(ether sulfone)s and poly(ether ketone)s Furthermore, semiaromaticpolyethers and aromatic polysulfides are included in this chapter which mainly covers theliterature 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 Aromaticpolyethers are less sensitive to oxydation at all temperatures, and thus, also lessinflammable (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 polyetherspossess higher glass-transition temperatures (Tgs) which may be as high as 230
C forcommercial poly(ether sulfone)s The Tgs of commercial poly(ether-ketone)s are lower(typically around 140–150
C) but all commercial poly(ether-ketone)s are semicrystallinematerials having melting temperatures in the range of 280–420
C Either due to high Tg

or due to a high Tmthe heat distortion temperature of aromatic polyethers significantlyhigher than that of aliphatic polymers A few characteristic disadvantages should also

be mentioned Most poly(ether-sulfone)s reported so far and all commercial examplesare amorphous with the advantage of a high transparency and the shortcoming of a highsensitivity to the attack of organic solvents The crystalline poly(ether ketone)s are rather

insensitive to organic solvents, but they are sensitive to a cleavage by UV-irradiationquite analogous to low molar mass benzophenones Finally, it should be mentioned that

a typical application of poly(ether sulfone)s and poly(ether-ketone)s is that as matricmaterial in composites Glass fiber or carbon fiber are used as reinforcing components

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)sThe discussion of synthetic methods and structures presented below will follow thisorder

A Syntheses via Electrophilic Substitution

The oldest approach known for the preparation of PESs is a polycondensation processinvolving the electrophilic substitution of a phenyl ether group by an aromatic sulfonylchloride group [3,5,6] Such polycondensations may be based, either on monomerscontaining both functional groups in one molecule (Eq (1)) or by a combination of anucleophilic and an electrophilic monomer (Eq (2)) These polycondensations need to becatalyzed by strong Lewis acids such as FeCl3, AlCl3, or BF3 Characteristic disadvantages

of this approach are the need of an expensive inert reaction medium and side reactionssuch as substitution (including branching) in ortho position of the nucleophilic monomer.Furthermore, this approach is not versatile and limited to a few monomers Previousresearch activities in this field were reported in the 1st edition of the Handbook [3], butmore recently new activities were not observed

ð1Þ

ð2Þ

B Syntheses Via Nucleophilic Substitution

The most widely used approach to the preparation of PESs in both academic research andtechnical production is a polycondensation process involving 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 mentionedwhich were concerned with the evaluation and comparison of the electrophilic reactivity ofvarious mono- and difunctional fluoro-aromats [7–10] The nucleophilic substitution ofaromatic compounds may in general proceed via four different mechanism Firstly, the SN1mechanism which is, for instance, characteristic for most diazonium salts Secondly, theelimination-addition mechanism involving arines as intermediates which is typical for thetreatment 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 SNAR mechanism which may occur when poorly electrophilicchloroaromats are used as reaction partners will be discussed below in connection withpolycondensations of chlorobenzophenones

ð3Þ

ð4Þ

In the case of the addition–elimination mechanism the addition step with theformation 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) isdecisive for the reactivity and thus, for the rate of the reaction In two publications [7,8]the 13C NMR chemical shifts of various fluoroaromats were determined, compared andshown to be useful indicators of the reactivity of the ipso-carbon This conclusion wasconfirmed by calculation of the electron density via the quantum semiempirical PM 3method in the MOPAC software In fact, a linear correlation between the calculatedelectron density and the 13C NMR d values was obtained Furthermore, the 19F NMRchemical shifts were determined for numerous electrophilic fluoroaromats and again

a linear correlation with the calculated electron densities, on the one hand, and with the13

C NMR chemical shifts, on the other, was found [7,8] These studies proved that the SO2group is the strongest activating divalent group Only the monovalent nitrogroup has

a stronger electron-withdrawing effect The strong electron-withdrawing effect of the SO2group has also the consequence that the C-atom directly attached to it is sensitive to anucleophilic 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 morefavored 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 fone)s is based on the reaction of equimolar amounts of a difluoro (or dichloro)- sulfone

poly(ethersul-and a bisphenol with dry K2CO3(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

K2CO3 However, CsF has no advantage, but it is significantly more expensive Followingthe standard procedure with K2CO3 two research groups used commercial 4,40-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 thefunctionalized oligo(ether-sulfone)s which were modified at the chloro endgroups (6) [14].Another class of functional PES (7) was synthesized from commercial 4,40-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 withvarious diphenols and 4-fluoro-40-hydroxydiphenylsulfone served as comonomers forthe preparation of copoly(ether-sulfone)s having the structure (8) Their properties wereevaluated and correlated with their composition and sequence [16]

ð5Þ

ð6Þ

ð7Þ

ð8Þ

Most new PESs reported during the past ten years were prepared from new sulfonetype monomers or at least from noncommercial monomers For instance, thedifluoroketone-sulfone (9a) was polycondensed with the dihydrodiphenylketone-sulfone(9b) [17] The same monomers were later used by another research group together with

a variety of new fluoro ketone type monomers [19] Other authors synthesized thenaphthalene containing difluordiphenylsulfones (10a) and (10b) as reaction partners ofmethyl substituted 4,40-dihydroxybiphenyls [19] Dichloro- or difluoro-diphenylsulfoneshaving a central biphenyl unit (11a) were polycondensed with various commercialdiphenols [20,21] In one of these papers [20] PES derived from the bisphenol (11b) werestudied in detail

of the corresponding disulfide (15a) [26] The disulfone (15b) was then polycondensed with

a variety of diphenols by means of sodium carbonate in DMAc The second strategy ischaracterized by the preparation of PESs having pendant trifluoromethyl groups bypolycondensation of the monomers (16a), (16b) and (16c) [27,28].

C Chemical Modification

The normal route of nucleophilic substitution was also applied to syntheses of PESshaving a broad variety of pendant functional groups In most publications dealing withfunctional PES the reactive substituents were subjected to further modifications Tworesearch groups were interested in sulfonated PES which may find potential application asproton transporting membranes in fuel cells Two different synthetic approaches wereexplored The first one is based on polycondensations of a sulfonated DCDPS (17a) withpreformed potassium salts of diphenols in DMAc at 170
C [29] The second approachconsists of the sulfonation of preformed PESs [30,31] When a PES derived fromhydroquinone was sulfonated exclusive monosulfonation of the hydroquinone unit wasobserved (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 degradationwas observed at room temperature, whereas chlorosulfonic acid and oleum caused severedegradation Furthermore, PES derived from methyl hydroquinone, dimethylhydro-quinone and trimethylhydroquinone were sulfonated with concentrated sulfuricacid 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

Several studies dealt with syntheses of PESs having pendant amino groups Quiteanalogous to the syntheses of sulfonated PESs two strategies were explored, (a)polycondensation of aminated monomers, and (b) modification of preformed PES Thefirst strategy was realized with synthesis and polycondensation of the dichlorodiamino-sulfone (20a) which was synthesized by hydrogenation of 2,20-dinitro-4,40-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 werenitrated at the hydroquinone unit and the nitrogroup was reduced by means of sodiumdithionite (21) [35] Another approach is based on the synthesis of PES, having pendantimide groups (22) [36–38] Variation of the amine (for instance via transimidization) allows

a broad variation of the pendant functional groups including the introduction of an aminogroup

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 octylchloromethylether and SnCl4 was used to introduce the CH2Cl groups, the combination ofoctylbromomethyl ether and SnBr4 yielded CH2Br groups (26a) and combinations ofchloromethylether þ SnBr4or bromomethyl ether and SnCl4producing a statistical array

of chloro and bromomethyl substituents Reactions with potassium tert.-butoxide yieldedpendant tert.-butyl ether groups (26b) with sodium acetate pendant acetate groups wereobtained and after alkaline saponification CH2OH groups (26b) Furthermore, pendanttosylate groups (27a) and diethylphosphonates (27b) were prepared With sodium cyanidependant nitrile groups were formed (28a) which were saponified to yield CH CO H

groups Finally the oxidation of chloromethyl groups with dimethylsulfoxid/NaHCO3

or with Cr2O27 was studied (yielding aldehyde groups (28b) [42] In the second papertriflicacid 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 containinghydroquinone an exclusive monosubstitution of the hydroquinone unit was found (29a),and finally the transformation of the chloromethyl groups into triethylammoniumgroups (29b) was studied [43] PESs having pendant aldehyde groups were preparedfrom (co-)polycondensations of the diphenol (30a) The aldehyde groups were almostquantitatively transformed into azomethine groups (30b) [44] In another publication [45]unsaturated PESs were prepared from 4,40-dihydroxy-trans-stilbene and DFDPS andtreated with H2O2in the presence of a tungsten catalyst whereby epoxide groups suitablefor chemical or thermal crosslinking were obtained (31) Finally, a publication dealingwith the influence of energy rich irradiation (x-ray, electrons, AR and N2 ) on PESshould be mentioned [46]

ð26Þ

ð27Þ

ð28Þ

ð30Þ

ð31Þ

Numerous publications describe syntheses and characterization of telechelic oligo(ethersulfone)s which served as building blocks of triblock copolymers, multiblock copolymers

or networks OH-terminated oligomers (32) were prepared by polycondensations ofDCDPS with an excess of bisphenol-A [47–49] These oligo(ether sulfone)s were reactedwith commercial bisepoxides to yield epoxy networks [47] They also proved to be usefulfor syntheses of multiblock poly(ether-esters) [48,49] The polyester blocks either consisted

of poly(ethylene terephthalate) or of LC-poly(ester-imide)s (33) The LC-blocks played therole of a reinforcing component in the PES matrix and showed interesting mechanicalproperties Telechelic poly(ether-sulfone)s having C–F endgroups were also prepared

by copolycondensation of 4-fluoro-40-trimethylsiloxy-diphenylsulfone with small amounts

of DFDPS [50] The molecular weight was controlled by the feed ratio of DFDPS.When small amounts of 4,40-bis(2,4-difluorobenzoyl)diphenyl ether were used as como-nomers, four armed stars with C–F endgroups were obtained Small amounts of silylated1,3,5-trihydroxybenzene as comonomer yielded three-armed stars having OSiMe3 end-groups [50]

ð33Þ

Several research groups prepared oligo(ether-sulfone)s having primary aminoendgroups [51–55] m-Aminophenol, p-aminophenol or a higher aminophenol (34)served as endcapping agents These oligomeric diamines were polycondensed with variousaromatic dicarboxylic acid dichlorides to yield polyamides [53] or they were poly-condensed with bisanhydrides yielding polyamides such as (35) [54] Poly(ether-sulfone-amide)s were also prepared by the inverse approach [56] In this case two ‘sulfonedicarboxylic acids’ (36) were synthesized and polycondensed with various aromaticdiamines via the triphenylphosphite pyridine method

consisting of PES and polydimethylsiloxane blocks were prepared from OH terminatedoligo(ether sulfone)s and diethylamine terminated siloxane blocks (39,40) [58] The sameapproach was reported to yield the poly(ether sulfone disilane)s (41) starting from abis(diethylamino)disilane [59] These polysilanes deserve interest because of theirphotosensitivity Grafting of polysiloxane blocks onto PES (based on bisphenol-A) wasachieved in such a way that the PES was lithiated with nBuLi and reacted withchlorodimethylvinylsilane [60] The Si-H endgroup of a polysiloxane was then added ontothe pendant vinyl group (42,43) In this connection a work describing the radical grafting

of styrene onto maleimide or nadimide endcapped oligo(ether sulfone)s should bementioned [61] In this way PES reinforced polystyrene foams with open pores wereobtained In two papers oligo(ether sulfone)s and oligo(etherketone)s having twoacetylenic endgroups were described [62,63] These oligomers (45) designed to yieldthermostable networks upon thermal cure were prepared by means of the new endcappingagents (44)

ð37Þ

ð38Þ

ð39Þ

ð40Þ

100
C, but the molecular weights remained low (Mn<3000 Da) for all monomers ofstructure (46a), whereas high molecular weights (Mn38,000 Da) were obtained fromthe polycondensations of monomers (47) Finally, a new polycondensation methodshould be mentioned yielding poly(ketone sulfone)s free of ether groups (48) and (49) [66].

ð47Þ

ð48Þ

ð49Þ

All synthetic strategies discussed above have in common to be step growth tions Over the past ten years a new strategy was elaborated and explored based on thering-opening polymerization of cyclic oligo(ether sulfone)s, OESs This chain growth

polymeriza-polymerization has the following advantages and disadvantages when compared to growth polymerizations The main problem is the synthesis of the cyclic monomers, aboveall, when large quantities are needed The advantages are, firstly, a polymerization processwhich does neither need solvents, nor produce byproducts Therefore, the ROP approach

step-is well suited for the reaction-injection molding (RIM) technology Secondly, the ROP ofstrained cyclic OESs offers the chance to prepare PESs with very high molecular weight(Mn>105Da) Thirdly, sequential copolymerizations with other cyclic monomers mayyield 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-hydroxy diphenylsulfone was dimerized and cyclized in the presence of

K2CO3(51) [68] Thirdly, several cyclic OESs were prepared by condensation of diphenolsand dihalosulfones via nucleophilic substitution under high dilution (52) [69–75] Eithermixtures of cyclic OES were isolated and used for ring-opening polymerizations [69,70] ormonodisperse cycles were isolated and characterized [71–75] Fourthly, preformed PESwas subjected to back-biting degradation catalyzed by CsF in DMF at 155
C At lowconcentrations, large fractions of cyclic OES were obtained and monodisperse cycles (fromthe 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 hightemperatures Unfortunately, cyclic OESs possess high melting temperatures (up to

500
C), and only in cases of nonsymmetrical cycles the reaction temperatures could belowered to 290
C Such high temperatures have two disadvantages Firstly, high fractions

of cycles remain unreacted for thermodynamic reasons, and gel particles are formed due topartial crosslinking Anionic polymerizations in concentrated solutions below 250
C havenot been studied yet

ð50Þ

ð51Þ

Finally, a synthetic approach should be mentioned yielding polyamides containingcyclic OESs as part of the repeating unit [78] The cyclic dicarboxylic acid (53c) wasprepared from (53a) and (53b) by a conventional procedure and polycondensed with4,40-diamino-diphenylmethane using the triphenylphosphite-pyridine method

ð53Þ

III POLYETHERKETONES

Most research activities in the field of aromatic polyethers published over the past tenyears concern poly(etherketone)s (in this review the abbreviation PEK is used for allpoly(etherketone)s, not only for those having one ether and one keto group in therepeating unit) Quite analogous to PESs the synthetic methods reported for PEKs may besubdivided into three groups:

1 polycondensations involving an electrophilic substitution (i.e., acylation of aphenoxy group)

2 Polycondensations involving a nucleophilic substitution of a chloro-, fluoro- ornitro-aromat activated by a keto group in para position)

3 modification of suitable precursor polymers

A Syntheses Via Electrophilic Substitution

Various PEKs were prepared via electrophilic substitution processes such as thatexemplarily outlined in equation (54) [79] The problems of this approach are in principlethe same as in the case of PESs An inert expensive solvent is needed, it is difficult to reachhigh conversions without side reactions and the number of useful monomers is lower than

in the case of syntheses based on nucleophilic substitution reactions The electrophilicpolycondensations may be subdivided into two different methods Firstly, acid chloridesare used as electrophilic monomers in combination with a Lewis acid Secondly, freecarboxylic acid served as monomers in combination with an acidic dehydrating agent.None of the polycondensation methods described in this section is new, and origin andearly 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 benzoyl chloride (Eq (54)) or polycondensations of diphenylether with isophthaloylchlor-ide, terephthaloyl chloride and a phenolphthalein dicarboxylic acid dichloride (55) werestudied AlCl3 served as catalyst and the solvent was varied It was found that thehomogeneous polycondensation in nitrobenzene gave lower molecular weights than theheterogeneous reaction in CH2Cl2or (ClCH2)2, (DCE) The chain growth continues inthe precipitated AlCl3-oligomer or -polymer complexes Similar results were found byanother group [80] which did not know the first paper [79] Other authors [81], comparedtwo reaction media: AlCl3/CH2Cl2and H2F2/BF3 The faster reaction (but similar mol-weights) were found in the H2F2system The influence of HCl or Lewis bases on AlCl3catalyzed polycondensatins of diphenylether and terephthaloylchloride was also studied[81] New structures were obtained by AlCl3 catalyzed polycondensations of the oligo-ethers (56a,b) with isophthaloylchloride or adipoylchloride [82] Two other research groupsstudied AlCl3catalyzed polycondensations of isophthaloyl chloride, terephthaloyl chloride

phenoxy-or naphthalene-1,6-dicarbonyl chlphenoxy-oride with diphenyl ether phenoxy-or with the oligoether (57)

in much detail [83–88] Potential defects in the chemical structure (as revealed by1H and

C NMR spectroscopy) and the morphology of the particles which crystallized directlyfrom the reaction mixture were intensively investigated [84–88].

ð58Þ

B Syntheses Via Nucleophilic Substitution

1 Mechanistic Studies

Numerous PEKs were synthesized by the nucleophilic substitution of aromatic

fluoro-or chlfluoro-oroketones, and in this connection several research groups conducted detailedmechanistic studies [7,8,98–110] Using 13C and 19F NMR spectroscopy combined withmodel reactions and computer calculations of electron densities the activating power ofCO-groups for F in para position was compared to that of other electron-withdrawinggroups [7,8] and a significantly weaker activation effect was found When the reactivities

of the dihalobenzonaphthones (66a–d) were compared in polycondensation process withdiphenols the reactivity decreased in the given order (a ! d) The failure of the dichlorocompound (66d) to yield PEKs was attributed to the low reactivity (i.e electrophilicity) ofthe Cl–C bond in aromatic nucleophilic substitution (SNAR) However, several authorsfound in detailed mechanistic studies [99–103] that the chlorobenzophenones easilyundergo a radical side reaction (67–69) yielding saturated chain ends The extent of thisside reaction depends very much on the solvent and to a lesser extent on the redoxpotential of the phenoxide ions The following order of decreasing usefulness (i.e.decreasing molecular weights of the isolated PEKs) of polar solvents was found:

DPSU > DMAc > NMP > TMU > DMPU(diphenylsulfone, dimethylacetamide, N-methylpyrrolidone, tetramethyl urea, 1,3-dimethyl-perhydropyrimidinone 2)

ð66Þ

ð67Þ

ð68Þ

ð69Þ

In DPSU the radical side reactions are almost completely avoidable [10].Furthermore, addition of a radical scavenger may be helpful to raise the molecularweights [100] Another approach consists of the use of special phase transfer catalysts(70a,b) which promote the polycondensation of chlorobenzophenones and diphenols inthe presence of K2CO3[104] These pyridinium salts were selected because they are stable

up to 300
C even under alkaline conditions Another version of this approach is thecombination of these pyridinium salts with an amount of KF Activation of the phenolicOH-groups and under certain reaction conditions a halogen exchange takes place so thatthe far more reactive fluoroketones are formed as reaction intermediates [105] Howeverthe activation of KF by means of the phase-transfer catalysts (68a–c) may have theadditional effect, that the fluoride ions begins to cleave the PEK backbone at temperatures

as low as 160
C From other studies [10,106,107] it was known that KF alone attacks thePEK chains only at temperatures 300
C Transetherification, catalyzed by phenoxideions was also studied by several authors [105–108] Another important aspect investigated

in two papers [109,110] is the influence of the reaction medium on the molecular weight inpolycondensations exclusively involving fluoroketones and the SNAR mechanism In thefirst paper [109] difluorobenzil (71a) was polycondensed with free diphenols and K2CO3

in four different solvents DMSO and sulfolane gave the best results, whereas cleavage

of the PEK backbone was found in NMP and DMPU However, excellent molecularweights were obtained in NMP when silylated diphenols (71b) and a catalytic amount ofCsF were used as reaction partners of (71a) In the second paper it was reported thatDMPU is advantageous over NMP when less reactive electrophiles than fluoroketones orfluorosulfones are used (see Section III.F)

1 PEKs prepared from 4,40-difluorobenzophenone (DFBP) and various diphenols[111–122]

2 PEKs prepared from new fluoroketone monomers and commercial diphenols[123–148]

ð72Þ

ð73Þ

syntheses and polycondensations of new fluoromonomers derived from naphthalene Forinstance, the 2,6-substituted naphthalene monomers (84a), were described in Refs [132]and [133] Syntheses and polycondensations of the 1,5-substituted naphthalenes (84b,c)were reported in Refs [134–136] The chemistry of the ‘1,8-naphthalene monomer’ (85a)was described in Ref [137] From the tetrasubstituted monomer (85b) a kind of comb-likePEK was prepared [138] PEKs derived from a monomer having pendant naphthyl groupswere prepared from (86) and had high Tgs [139] New ‘fluoromonomers’ derived fromindane (87a,b) were synthesized from 4-methyl-a-methylstyrene [140,141] The PEKsderived from them were as expected amorphous Two research groups were interested inpolyethers having alternating sequences of ketone, ether and sulfone groups [142,143] Fortheir syntheses mainly the monomers (88a or b) were used Another research group [144]reported on syntheses of phosphorous containing PEKs from monomer (89a) In this workand in publications discussed below the fluorinated bisphenol-A (89b) was used as one ofthe comonomers Several authors had interest in PEKs containing heterocycles in thebackbone Thiophene containing PEKs were obtained from monomers (90a or b) [145],[146], and benzofurane based PEKs or PESs were prepared from monomers (91a or b)[147] PEKs having pyridine or isoquinoline rings in their backbones were obtained bypolycondensations of the monomers (92a) [133] or (92b) [132] and (92c) [148].

ð80Þ

ð81Þ

ð82Þ

to the ‘standard procedure’ Amorphous PEKs with high Tg were, as expected, isolatedfrom polycondensations of the monomers (95c,d) [154].

characteriza-of these PEKs is doubtful

2 Telechelic oligomers (tOEKs), block-copolymers and networks

A broad variety of telechelic oligo(ether ketone)s, tOECs, was prepared over the past tenyears and used for chain extension via amide, imide or ether groups and for syntheses ofblock copolymers or thermostable networks [158–179] Most research groups haveconcentrated their interest on tOEKs having amino-endogroups [158–166] One group[158–159] synthesized a monodisperse tOEK (97a) which was polycondensed with thedicarboxylic acid dichloride (97b) The thermal and photochemical cis–trans isomerization

of the azogroups in the resulting polyamides was investigated The influence of randomlyincorporated 2,20-bisnaphthy1 ‘kinking units’ was also studied Most amine-terminatedtOEKs were prepared in a ‘one-pot procedure’ from mixtures so DFBP a diphenol andmeta- or para-aminophenol The tOEK prepared from monomer mixture (98) were chainextended with benzophenone-tetracarboxylic anhydride, and the thermal crosslinking wasstudied and ascribed to the formation of ketomine groups (99) [160] Two other researchgroups [161–164] found that the ‘one-pot procedure’ does not give satisfactory results due

to side reactions of the amino groups and developed alternative strategies Either, aN-protected 3-aminophenol was used (100) [161,162] or a fluoroterminated tOEK wasisolated after the polycondensation (101) treated with the K-salt of 3-aminophenol in aseparate step [163,164] Detailed NMR spectroscopic analyses were reported The tOEKsisolated from reaction (100) were chain extended with pyromellitic anhydride, whereas theamine terminated tOEKs derived from (101) were used as reinforcing component in epoxyresins [162] In two publications [165,166], tOEKs endcapped by maleimido groups weredescribed Two synthetic strategies were applied Firstly, amine-terminated tOEKs wereprepared from monomer mixtures such as (98) or from analogous monomer mixturescontaining (102a) instead of DFBP [165] The amino endgroups of the isolated tOEKswere then reacted with maleic anhydride Secondly, the polycondensation process wasperformed in the presence of the maleimido phenol (102b) [165] Finally, the thermal cureand the physical properties of the resulting networks were studied

ð97Þ

to esterify the OH-terminated tOEKs In this connection tOEKs containing styreneendgroups (105) should be mentioned which are suited for radical crosslinking [172].

ð103Þ

ð105Þ

Several research groups elaborated strategies for syntheses of tOEKs having tworeactive fluoro endgroups The ‘electrophilic substitution strategy’ was realized either withmonomer mixtures containing 4-fluorobenzoyl chloride as endcapping agent (106) or withfluorobenzene (107) [173–175] The resulting tOEKs were chain extended with variousshort or long (oligomeric) diphenols Furthermore, blockcopolymers containing PESsegments (108) were prepared by copolycondensation with DFDPS and 4,40-dihydroxy-diphenylsulfone tOEKs having C–F endgroups were prepared via nucleophilic substitu-tion by copolycondensation of 4-fluoro-40-trimethylsiloxy benzophenone and smallamounts of DFDPS [176] The molecular weights were controlled by the feed ratio ofDFDPS F-terminated tOEKs were also synthesized via nucleophilic substitution fromhydroquinone and an excess of DFBP (109) They were used in turn for syntheses ofPES block copolymers (110) [177,178] Blockcopolymers containing dimethyl siloxanesegments were realized by heating OH-terminated tOEKs (111a) with amine terminatedoligosiloxanes (111b) in a two phasic solvent system [179] Finally the multistep synthesis

of A-B-A-triblock copolymers having a central OEK block should be mentioned.Copolycondensation of the monomer mixture (112) yielded tOEK having two Me3SiOendgroups which were acetylated with a large excess of acetylchloride The resulting tOEKbisacetates (113a) were polycondensed with silylated 3,5-bisacetoxy benzoic acid (113b) toyield hyperbranched polyester A-blocks (114) [180]

ð106Þ

ð107Þ

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 ofhyperbranched polymers will be presented in a separate chapter of this handbook

The discussion of unusual methods reported for syntheses of PEKs is subdivided into twostrategies: (1) polycondensation methods, (2) ring-opening polymerization (ROP) of cyclicoligo(ether ketone)s, cOEKs

1 Polycondensation Methods

The synthetic methods discussed in this section have in common that they deviatesignificantly from both standard methods A new polycondensation process based onnucleophilic substitution steps is schematically outlined in (115) In contrast to the

standard procedure no diphenols are required only difluoro- or the less expensivedichloroketones are needed (116a–c) Sodium or potassium carbonate serve as source ofoxide ions yielding the ether bonds under the catalytic influence of silica which is dopedwith traces of CuCl, CuCl2 or CuO [187,188] High molecular weights require hightemperatures (up to 320
C) in DPS as reaction medium The molecular weights decreasewith increasing amounts of silica Silylated phenolate groups formed on the surface of thecatalyst were claimed as reactive intermediates Another new method involving theformation of biphenyl units by C–C coupling is also based on dichloroketones as startingmaterials (117) [189,190] Nickel(II) chloride complexed by triphenylphosphine andbipyridine catalyzes the coupling step at moderate temperatures (80
C) Low to moderatemolecular weights were obtained A problem is the low solubility of the resulting PEKs atthese low reaction temperatures.

to the first two methods The second approach is based on the acylation of stannylatedbenzene (120) This coupling method requires palladium catalysts and may give Mns above20,000 Da, when the polymers are soluble in the reaction medium [195] as it is true for thetert.butylsubstituted PEK of (120).

a Ni0-complex as catalyst [196] However, most syntheses of cOEKs are based on (poly)condensations involving the standard nucleophilic substitution process discussed abovewith the difference that these ring syntheses were conducted under high dilution (pseudohigh dilution method) In one paper [197] a new version of this synthetic approach wasstudied, using a difluoroketimine as electrophilic monomer (122) The iminogroup is easilyhydrolyzed by acid catalysis (123) As discussed in the section ‘Modifications’ belowfluoroketimines are useful monomers for the preparation of crystalline, insoluble PEKs,via an amorphous, soluble precursor polymer, but no advantages are in sight for synthesis

of cOEKs

as nucleophilic monomer [203] cOEKs synthesized from 1,2-bis(4-fluorobenzoyl)benzene(125) were transformed into the corresponding cyclic phthalazines (126a) [200] Howeverstudies of ROP have not been reported yet.

of the RO polymerizations were also reported [206,208,210] A broad variety of catalystswere tested For instance, a mixture of cOEKs having structure (125), which waspolymerized with Na-, K-, and Cs-phenoxide at 340
C and similar weights were found.Furthermore, the K salts of various phenols were compared or their concentration wasvaried In most polymerizations Mns in the range of 26,000–34,000 Da (based on PScalibrated GPC) were achieved Partial crosslinking and a rather high percentage ofunreacted cycles due to the thermodynamic equilibrium are characteristic features of ROP

at temperatures around or above 340
C Using the K and Cs salts of 4,40-biphenyldiolthe cOEKs of structure (125) were also polymerized in refluxing DMF In this waycrosslinking was avoided and lower Mns (15,000 Da) with high polydispersities werefound [206] When the cyclic dimer (127a) was polymerized in the melt at 260–275
Ccrosslinking was again avoided and Mns in the range of 14,000–50,000 Da were obtainedwith catalyst such as CsF, K2CO3or Cs2CO3[208] Lower Mns resulted from initiationwith K- or Cs-phenoxides