Poly(aryl ether) Synthesis - ACS Symposium Series (ACS Publications)

Chapter 12

Poly(aryl ether) Synthesis 1

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Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0624.ch012

Jeff W. Labadie , James L. Hedrick , and Mitsuru Ueda

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1

Argonaut Technologies, Inc., 887 Industrial Road, San Carlos, CA 94070 Research Division, IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099 Department of Materials Science and Engineering, Yamegata University, 4-3-16 Jonan, Yonezawa, Yamagata 992, Japan 2

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Poly(aryl ethers) are an important class of commercial polymers and are a member of the family of materials referred to as engineering thermoplastics (1). Commercial examples include Amoco's poly(aryl ether sulfone) (Udel), ICI's poly(ether ether ketone) ( P E E K ) , and General Electric's Ultem poly(ether imide). They display an attractive balance of properties such as relatively low cost, good processability, excellent chemical resistance, high thermal stability and good mechanical properties. Since the initial report of their synthesis by nucleophilic aromatic displacement polymerization of activated aryl dihalo compounds with bisphenolates (2), significant effort has been devoted towards these polymer systems (3). It is the purpose of this article to review many of the latest developments in the field of poly(aryl ether) synthesis, including mechanistic results, new activating groups and polymer structures, and alternative synthetic routes. The most commonly used synthetic route to poly(aryl ether)s involves generation of an ether linkage by nucleophilic aromatic substitution (S Ar) as the polymer-forming reaction (Scheme I). Early work focused primarily on sulfones and ketones as activating groups for the displacement of halides (2), and nitro groups (4). The role of heterocycles as activating groups was first reported for a 1,3,4-oxadiazole group (2a), and later for the nitrodisplacement polymerization of bis(nitrophthalimides) to afford poly(ether imides) (4b). The nature of the activating group, leaving group, bisphenolate, and solvent all play an important role in the polymer forming reaction, the details of which will be discussed further in the following section. N

Reaction Conditions and Mechanism The primary mechanism for formation of aryl ether linkages involves nucleophilic aromatic substitution of an activated leaving group by phenolate. Polar aprotic solvents, e.g., dimethylsulfoxide ( D M S O ) , N-methylpyrrolidone ( N M P ) and dimethylacetamide ( D M A C ) are required to effect the reaction. The use of dimethylproylene urea has been reported as an alternative solvent

0097-6156/96/0624-0210$12.00/0 © 1996 American Chemical Society In Step-Growth Polymers for High-Performance Materials; Hedrick, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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Poly (aryl ether) Synthesis

which affords increased polymer molecular weights and yields with less reactive aryl fluorides, and it was found to dissolve poorly soluble rigid heterocyclic-based poly(aryl ethers) (5). Special reaction conditions are required for the crystalline ketone based poly(aryl ethers), where diphenylsulfone is used at reaction temperatures above the melting point of the polymer to avoid premature crystallization (2c). Potassium carbonate is the preferred base for bisphenolate generation as the reaction can be carried out in the reactor, in the presence of the dihalide, using toluene to dehydrate the system (6). The key characteristics of effective activating groups for an S A r mechanism are high electron affinity and the presence of a site of unsaturation which can stabilize the negative charge developed along the reaction coordinate through resonance to a hetero atom. This step involves the formation of a Meisenheimer complex (I) which lowers the activation energy of the displacement (Scheme II) (7). Formation of the Meisenheimer complex is the slow rate-determining step, as reflected in the greater reactivity of aryl fluorides relative to chlorides and bromides. The higher reactivity of the fluoride is attributed to its small size and higher electronegativity relative to chloride and bromide. Recently, aryl triflates were shown to be effective leaving groups, displaying a higher reactivity than fluoride (8). A r y l triflate displacement was complicated by competitive S-O cleavage which afforded sulfonate exchange to the phenolate. This exchange limited the phenols which could be used to those which can form activated sulfonates themselves. Nitro groups are readily displaced, but the resulting nitrite ion is reactive and undergoes side reactions, such as those observed with imides if conditions are not carefully controlled (9). In general, fluoride is the most desirable leaving group owing to its reactivity. A complication observed with fluoride is that its nucleophilic nature in polar aprotic solvents leads to a back reaction where fluoride cleaves the aryl ether bond (7a). The generation of high polymer is assisted by the precipitation of fluoride salts from the polymerization to limiting the effect of the back reaction. The aryl ether bonds are also cleaved by phenoxide, which leads to ether interchange in the polymerization (2bja). Although activated aryl fluorides are the most common substrates for poly(aryl ether) synthesis, there is a great deal of interest in the use of aryl chlorides due to their reduced cost. Hergenrother and co-workers showed that a variety of dichlorobenzophenone, phenylsulfone, and bis(benzoyl)benzene monomers afforded high polymer with a variety of bisphenols (10). Percec (11-13) and Mohanty (14) found that in certain cases the polycondensation of a bis(aryl chloride) with a hydroquinone afforded low molecular weight polymer due to a reductive dehalogenation reaction. The degree to which this occurred was dependent on the oxidation potential of the bisphenolate, the solvent, and the polymerization temperature. The results are explained by a competition between a polar and single electron transfer (SET) pathway (Scheme III), where the latter is based on an S 1 mechanism (15). In the S E T pathway, an electron is transferred from the bisphenolate (electron donor) to the aryl halide (electron acceptor) to form a radical anion-radical pair, II. This pair may collapse to form the Meisenheimer complex, I, and proceed to the aryl ether, or separate and eliminate the halide to give a phenyl radical. The phenyl radical can abstract a hydrogen from the solvent to give the dehalognated chain-end. It is also proposed that a aryl ether can form via the S E T pathway rather than strictly the


N

R N

In Step-Growth Polymers for High-Performance Materials; Hedrick, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

STEP-GROWTH POLYMERS FOR HIGH-PERFORMANCE MATERIALS

M

+

+

"0-Ar-0~M +

X - ( ^ Z - 0 - X

•

ζ = so , C = 0 Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0624.ch012

2

Scheme I.

OAr

Z

Ζ

I Scheme II.

"ΟΑΓΟ'

•OArO'

"ΟΑΓΟ'

II

CO OArO"

#

- CO

X + OArO"

CO--OArO"+X" • - Q -

ι - C O - 0 - H Scheme III.



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Poly(aryl ether) Synthesis

polar pathway (14). Since the S E T relies on the bisphenolate acting as the electron donor, the reductive halogenation side reaction is limited to electronrich bisphenolates, e.g., that derived from hydroquinone. The balance between polar and S E T pathways is also dependent on aryl halide structure, with the polar pathway favored to a greater extent with more electronegative leaving groups and with sulfone compared to ketone activating groups (13). The suppression of the S E T pathway can be affected by addition of a radical scavenger, e.g., tetraphenylhydrazine, to the polymerization (14). Addition of 0.01 mole% of tetraphenylhydrazine to the polymerization of 1,3-bis(p-chlorobenzoyl)benzene and hydroquinone resulted in high polymer with an inherent viscosity equivalent to those polymers prepared with the difluoro monomer. In addition, the reductive dehalogenation side reaction can be avoided by the use of diphenylsulfone rather than N M P or D M A C , due to the absence of labile hydrogen atoms towards hydrogen abstraction (16).

Poly(Aryl Ether Ketones) Since the first reports of poly(aryl ether ketones) prepared via nucleophilic aromatic substitution nearly three decades ago, various synthetic strategies have been pursued to prepare semicrystalline materials under less stringent polymerization conditions (i.e., common organic solvents at mild temperatures). O f particular interest is poly(aryl ether ether ketone), P E E K , which is a highly crystalline polymer with high solvent resistance and a T of 145 °C and a T of 340 °C (1,17-19). The polymerization of P E E K is mediated in diphenylsulfone at temperatures in the proximity of the T m . To circumvent the use of high polymerization temperatures and diphenylsulfone as solvent, the synthesis of soluble precursor polymers of P E E K has been investigated. This strategy was applied to ketimine derivatives of 4,4 -difluorobenzophenone (20-22). The ketimine group activates fluoride displacement, allowing polymerization with hydroquinone and, since the substituent on the imine nitrogen retards crystallization, conventional polyether synthetic conditions ( D M A C or N M P , 160-180 °C) can be used without premature precipitation of the polymer. Likewise, an amorphous and soluble poly(ketal ketone) has been reported by nucleophilic displacement polycondensation of an acetal monomer, 2,2-bis(4-hydroxyphenyl)-l,3dioxoline, with 4,4 -difluorobenzophenone at 150-220 °C in an aprotic dipolar solvent (23). Each of the above amorphous/soluble polymer systems could be quantitatively hydrolyzed to produce the parent ketone structure. Sogah (24) and McGrath (25) have reported tert-butyl and phenylsubstituted poly(aryl ether ketones) respectively, which are prepared by nucleophilic substitution reaction of the corresponding substituted hydroquinones and 4,4 -difluorobenzophenone. The resulting polymers were amorphous and highly soluble in common organic solvents as a result of the bulky substituents which suppressed crystallization. The bulky substituents were cleaved with acid in a reverse Friedel-Crafts alkylation reaction to produce the semi-crystalline P E E K (24). A novel approach to semicrystalline poly(aryl ether ketones) involves the reaction of 4,4 -dichlorobenzophenone with sodium carbonate in the presence of a silica/copper catalyst in diphenylsulfone at 2 8 0 - 3 2 0 °C (26). Under these conditions, the sodium carbonate/catalyst combination behaves as a g

m

,

,

/

,


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STEP-GROWTH POLYMERS FOR HIGH-PERFORMANCE MATERIALS

water-equivalent, inserting an ether linkage between benzophenone units. The proposed mechanism involves formation of a silyl ether by the reaction of 4,4'-dichlorobenzophenone with a silanol on the silica surface as a key intermediate. This process was also applied to the synthesis of amorphous poly(aryl ether ketones) and polysulfones. In addition to new routes to crystalline poly(aryl ether ketones), a number of new monomers and polymerization methods have been reported for the synthesis of amorphous poly(aryl ether ketones). The driving force for much of this research has been the development of processable high T materials with tough, ductile mechanical properties. A n important example of this research is the preparation of high molecular weight poly(aryl ketones) derived from 1,3-bis(4-chlorobenzoyl)benzene reported by Hergenrother and co-workers (27). The resulting poly(aryl ether ketones) were high T~ materials with exceptionally tough mechanical properties. Likewise, 1,3-bis(4-chlorobenzoyl)benzene and related ketone-containing monomers were polymerized with the bulky bisphenol 9,9 -bis(3,5-diphenyl-4-hydroxyphenyl)fluorene, to produce a series of high Tg poly(aryl ether ketones) (28). Other new bis(fluorobenzoyl) monomers whicn have been developed include naphthalene- (29J, biphenyl- (30), and indanbased (31) compounds, affording high T , processable amorphous poly(aryl ether ketones) in polymerizations with common bisphenols. Hay has reported the synthesis of poly(aryl ether ketones) containing o-dibenzoylbenzene moieties by the polymerization of 1,2-bis(4-fluorobenzoyl)benzene with various bisphenolates in D M A C (32). Transformation of the o-dibenzoyl(benzene) moiety in the polymer chain to a heterocycle by cyclization with small molecules was developed as a means of increasing the polymer chain stiffness and solvent resistance. It was demonstrated that reaction of the polymer with hydrazine monohydrate in the presence of a mild acid in chlorobenzene converted the poly(aryl ether ketone) to a poly(aryl ether phthalazine), a new class of heterocyclic-containing polyether (Scheme V) (33). Likewise, reaction of the polymer with benzylamine in a basic medium led to amorphous, thermally stable poly(aryl ether isoquinolines) (Scheme V) (34). Another example of the use of the o-benzoyl cyclization strategy is the intramolecular ring closure of poly(aryl ketones) containing 2,2 -dibenzoylbiphenyl units to form poly(aryl ether phenanthrenes) (35-37). Trans-etherification involving cleavage of aryl ether bonds by phenoxide has been observed to be a significant side reaction in poly(aryl ether) synthesis and leads to ether interchange (7a). This process has been exploited in the synthesis of poly(aryl ether ketones) from polymers with related structures. High molecular weight poly(aryl ether ketone) was found to react with 4,4 -difluorobenzophenone in the presence of potassium carbonate in benzophenone or diphenyl sulfone at 300 °C, to afford low molecular weight poly(aryl ether ketone) with fluorophenyl end groups (38). High temperature polycondensation of 4,4'-dihydroxydiphenyl sulfone with 4,4'-bis(4-fluorobenzoyl)biphenyl using diphenyl sulfone as solvent and sodium carbonate as base, led to formation of the expected alternating polymer sequence. Replacement of sodium carbonate by potassium gave the random sequence polymer (39). The ether linkages, being activated by electron-withdrawing groups on both adjacent aromatic rings, are susceptible to trans-etherification as a result of nucleophilic cleavage by fluoride ion as


g

,

g

,

,


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Polyfaryl ether) Synthesis


an intermediate step. Potassium fluoride has greater solubility in dipolar aprotic solvents which promotes fluoride-catalyzed trans-etherification. Generally, the synthesis of poly(aryl ether ketones) by nucleophilic dis placement requires the use of expensive fluoroarylketone monomers. Alterna tively, potassium fluoride or a mixture of potassium fluoride and the phase transfer catalyst N-neopentyl-4-(dialkylamino)pyridinium chloride have been shown to be very effective in promoting the polymer-forming nucleophilic dis placement reaction in poly(aryl ether ketone) synthesis from dichloroaryl ketone monomers (40). This reaction is based on phase-transfer catalyzed chloride/fluoride exchange. New Activating Groups One of the most interesting aspects of poly(aryl ether) research is the multi tude of polymer structures which have been prepared with a wide variety of functionality and backbone constituents. Structural diversity can be intro duced through either the bisphenol or dihalo monomer. One important means of introducing new functional and heterocyclic groups is based on their use as the activating group in the dihalo monomer. Activating groups can be either ortho or para relative to the leaving group, or fused to the aryl halide in the case of heterocycles. The important features of activating groups are that they display high electron affinity and possess a site of unsaturation which can stabilize negative charge by resonance to a hetero atom. The applicability of a potential activating group can often be assessed by the !

deshielding of the protons ortho to the group of interest in the H - N M R , where chemical shifts of 7.5 and greater indicate probable fluorodisplacement (41). Alternatively, correlation of aryl fluoride reactivity with 19

fluorine chemical shift by F - N M R is a useful predictive tool since relative reactivities of non-related activated aryl fluorides have been shown to map well with the fluorine chemical shift (42). In addition to N M R techniques, aryl fluoride reactivity has been estimated using Huckel molecular orbital cal culations to determine the net charge density at the C - F carbon atom (34). There has been a large number of reports over the past five years on poly(aryl ethers) prepared via nucleophilic aromatic substitution polymerization from new activating groups. The resurgence in this area stems from both the microelectronic and composite industries which require materials with a broad scope of thermal, mechanical and dielectric properties. The activating groups can be categorized into several categories including fused heterocycle, pendent heterocycle, perfluoroalkyl, carboxylic acid deriva tive and phosphine oxide. The heterocycle-based activating groups will be covered together in the following section with the heterocycle containing bisphenols. Perfluoroalkyl-activated ether synthesis was pursued as a means of pre paring highly fluorinated polymers which are readily soluble in common organic solvents (43-45). The electron withdrawing effect of the perfluoroalkyl group was determined to be similar to that of a ketone by com parison of the Hammet σ value of the trifluoromethyl group to a ketone group ( a : C F = 0.54, carbonyl = 0.50) and the deshielding of aromatic protons ortho to perfluoroalkyl groups (

Poly(aryl ether) Synthesis - ACS Symposium Series (ACS Publications)

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