Alternative approach to an AB2 monomer for hyperbranched poly(arylene ether ketone imide)s

Article abstract of DOI:10.1080/00397910701771157

An alternative approach to an AB2 monomer, N-[3,5-bis(4-hydroxybenzoyl) benzene]-4-fluorophthalimde, 4, for hyperbranched poly(arylene ether ketone imide)s has been developed. The key steps were a para-position selective electrophilic aromatic substitutio

Full text of DOI:10.1080/00397910701771157

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Alternative Approach to an AB2 Monomer for
Hyperbranched Poly(Arylene Ether Ketone
Imide)s
Zhenning Yu a , Eric Fossum a , David H. Wang b & Loon‐Seng Tan c
a Department of Chemistry , Wright State University , Dayton, Ohio, USA
b University of Dayton Research Institute , Dayton, Ohio, USA
c Polymer Branch , Materials and Manufacturing Directorate, AFRL/MLBP, Air
Force Research Laboratory, Manufacturing Technology Directorate , Dayton,
Ohio, USA
Published online: 28 Jan 2008.
To cite this article: Zhenning Yu , Eric Fossum , David H. Wang & Loon‐Seng Tan (2008) Alternative Approach
to an AB2 Monomer for Hyperbranched Poly(Arylene Ether Ketone Imide)s, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 38:3, 419-427, DOI:
10.1080/00397910701771157
To link to this article: http://dx.doi.org/10.1080/00397910701771157
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Synthetic Communicationsw, 38: 419–427, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701771157
Alternative Approach to an AB2 Monomer
for Hyperbranched Poly(Arylene Ether
Ketone Imide)s
Zhenning Yu,1 Eric Fossum,1 David H. Wang,2
and Loon-Seng Tan3
1Department of Chemistry, Wright State University, Dayton, Ohio, USA
2University of Dayton Research Institute, Dayton, Ohio, USA
3Polymer Branch, Materials and Manufacturing Directorate, AFRL/
MLBP, Air Force Research Laboratory, Manufacturing Technology
Directorate, Dayton, Ohio, USA
Abstract: An alternative approach to an AB2 monomer, N-[3,5-bis(4-hydroxyben-
zoyl)benzene]-4-fluorophthalimde, 4, for hyperbranched poly(arylene ether ketone
imide)s has been developed. The key steps were a para-position selective electrophilic
aromatic substitution of fluorobenzene with 5-nitroisophthaloyl dichloride and a sub-
sequent clean conversion of the aryl fluorides to phenol groups using potassium
hydroxide as the nucleophile. The overall yield for the synthesis of 4 was 51.6%.
Keywords: branched polymer, monomers, selectivity
INTRODUCTION
The synthesis of branched polymers, in particular hyperbranched systems, has
received considerable attention over the past decade because of their unique
properties, which include 1) a large number of end groups, 2) low values of
intrinsic viscosity, and 3) generally, good solubility characteristics.[1–6] One
such class of hyperbranched polymers is the hyperbranched poly(arylene
Received in the USA July 17, 2007
Address correspondence to Eric Fossum, Department of Chemistry, Wright State
University, 3640 Colonel Glenn Hwy, Dayton, OH 45435, USA. E-mail: eric.
fossum@wright.edu
419

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Z. Yu et al.
ether)s, HB PAEs, which are branched analogs of the corresponding engineering
plastics.[7] A wide variety of HB PAEs have been prepared and studied, including
HB poly(arylene ether ketone)s,[7–9] PAEK; poly(arylene ether sulfone)s,[10–12]
PAES; poly(arylene ether phosphine oxide)s,[13–15] PAEPO; poly(arylene ether
quinoxaline)s,[16,17] PAEQ; poly(arylene ether imide)s,[18–20] PEI; and poly(ar-
ylene ether ketone imide)s,[21,22] PAEKI.
Typically, the synthesis of an HB PAE is achieved via the nucleophilic
aromatic substitution polycondensation reaction of AB2-type monomers in
which A is either a phenol or aryl fluoride and B is the corresponding
reaction partner. One of the limiting factors in the study of HB PAE
systems is the synthetic effort required to prepare the appropriate AB2
monomer. Any practical synthesis of an AB2 monomer should meet a
number of requirements, including 1) high-yielding, facile, and selective
reactions; 2) a minimum number of steps; and 3) use of low-cost and
readily available reagents.
The most widely studied AB2 monomer for the synthesis of HB PAEKIs
is N-[3,5-bis(4-hydroxybenzoyl)benzene]-4-fluorophthalimde, 4. Two of the
reported synthetic routes to 4 are shown in Scheme 1. We now report on an
improved synthesis of 1, which takes advantage of the selectivity shown by
fluorobenzene for reaction at the para-position during electrophilic aromatic
substitution processes. Additionally, the phenol groups are subsequently intro-
duced, via nucleophilic aromatic substitution, using the low-cost reagent
potassium hydroxide. Although the current method reverts to a synthesis
that involves five steps, the yield and selectivity of several steps is
improved significantly, thus providing an efficient, low-cost, and readily
scalable approach to the synthesis of 4. When compared to Method I
Scheme 1.

Alternative Approach to an AB2 Monomer
421
(Scheme 1),[21] the overall yields in the current synthetic route are improved
considerably compared to Method II.[22] The new synthesis represents a con-
siderable cost savings as the imidization reaction, requiring the most
expensive reagent (4-fluorophthalic anhydride), is carried out in the final
step, limiting expensive material losses.
RESULTS AND DISCUSSION
The previously reported synthetic routes to 4 are shown in Scheme 1. Method I
starts by converting 5-nitroisophthalic acid into 5-nitroisophthaloyl dichlor-
ide, followed by a Friedel–Crafts reaction with an excess of anisole to
provide the corresponding protected bisphenol, 2. Deprotection of the
phenol groups with pyridine hydrochloride and reduction of the nitro group
affords the corresponding aniline derivative, 3. The final product, N-[3,5-
bis(4-hydroxybenzoyl)benzene]-4-fluorophthalimde, 4, is provided in an
overall yield of 32.3%, by the imidization reaction of 3 with 4-fluorophthalic
anhydride. A slightly improved synthesis of 4 has been reported that employs
methanesulfonic acid (MSA) and P2O5 as the condensing agent, thus eliminat-
ing the need to first prepare the diacid chloride.[23]
Method II begins with the imidization reaction of 5-aminoisophthalic
acid, 5, with 4-fluorophthalic anhydride to afford N-(3,5-dicarboxylic
acid benzene)-4-fluorophthalimide, 6, in 84% yield. The bis-phenol moiety
of 4 is introduced via the Friedel–Crafts reaction of 6 with an excess
of anisole, using methanesulfonic acid/P2O5 as the condensing agent, to
provide N-[3,5-bis(4-methoxybenzoyl)benzene]-4-fluorophthalimide, 7, in
74% yield. The final step involves demethylation of the phenol groups,
using BBr3 in dichlorobenzene at 808C for 24 h, to afford the desired 4 in
an overall yield of 56.6% yield. Method 2 represents a substantial improve-
ment in the overall yield (56.6% versus 32.3%) for the synthesis of 4 with a
concurrent increase in efficiency by reducing the number of reaction steps
from five to three. However, the reduction in the number of reaction steps
comes with a significant increase in the overall cost of the synthesis. For
example, 4-fluorophthalic anhydride is a relatively expensive reagent, but it
is introduced in the first step of the synthesis. The starting material,
5-aminoisophthalic acid, 5, is considerably more expensive than the corre-
sponding nitro analog. Additionally, demethylation of the phenol groups
using BBr3 often proves to be problematic, especially when a large excess
of the reagent is utilized.
Development of a low-cost, efficient synthesis of 4 is vital if the corre-
sponding HB PAEKIs are to be fully studied and exploited on an industrial
scale. The current synthetic route to 4, Method III, shown in Scheme 2,
involves five steps, as in method 1; however, the yields and selectivity of
the reactions are improved significantly and it takes advantage of low-cost
reagents. The fundamental difference, versus the prior two methods, is the

422
Z. Yu et al.
Scheme 2.
reaction in which the phenolic moieties are introduced into the molecule.
Rather than relying on a demethylation of methoxy protected phenol groups
using pyridine hydrochloride or BBr3, the phenol groups are introduced via
a high-yielding nucleophilic aromatic substitution reaction of activated aryl
fluorides using low-cost potassium hydroxide as the nucleophile.
Thus, Method III begins with the aluminum chloride–promoted Friedel–
Crafts reaction of the diacid chloride of 1 with an excess of fluorobenzene to
provide the bis-phenol precursor, 3,5-bis(4-fluorobenzoyl)-5-nitrobenzene, 8,
in 82% yield. The selectivity of the fluoro group for directing electrophilic
aromatic substitution at the para-position is typically much greater than that
observed for the methoxy group. In fact, for benzoylation, the selectivity is
reported to be 100% for the para-position.[24,25] This may help to explain
the significantly improved yield for the Friedel–Crafts reaction in method 3
relative to that for method 1 (82% versus 48%). At this stage, direct conver-
sion of the aryl fluoride groups in 8 to the corresponding phenol groups was
unsuccessful, in part, because of a competing substitution reaction of the
nitro group. Therefore, the nitro group was first reduced to the amine using
hydrogen gas and Pt on carbon under acidic conditions to form the
ammonium salt in situ. The free amine was unstable to air, thus the
ammonium hydrochloride salt was isolated. Conversion of the aryl fluoride
groups to phenols was achieved cleanly in 97% yield by the use of KOH in
a mixture of DMSO and water, followed by acidification. The yield of 3,
after the first four steps, was 62% versus 45% at the same stage using
method 1, representing a significant improvement. In addition, the melting
point of 3, prepared via method 3, was 258–2598C compared to 249.5–
250.58C for samples of 3 prepared via method 1, which may be indicative
of a slightly higher purity material.

Alternative Approach to an AB2 Monomer
423
Introduction of the 4-fluorophthalimide was achieved by reaction of 3 and
4-fluorophthalic anhydride, in refluxing acetic acid, to afford the desired AB2
monomer 4, in 83% yield. The imidization step required only 2.5 h in
refluxing acetic acid, whereas the reaction in NMP (Method I) required
heating to 2008C for up to 18 h to reach completion. The structure of 4 was
confirmed by comparing its analytical data (1H and 13C NMR spectra) to
the reported literature values. Interestingly, the melting point of 4 prepared
by method 3 was 296–2988C compared to 274–2768C for samples of 4
prepared by either methods 1 or 2. The overall yield for method 3 was
51.6%, compared to 32.3% (Method I) and 56.6% (Method II).
Although the overall yield for Method III is slightly lower than that for
method 2, 51.6 versus 56.6%, it has a number of significant advantages in
terms of the ability for scale-up. First, the most expensive reagent,
4-fluorophthalic anhydride, is introduced in the last step of the synthesis,
thus representing significant cost savings. Second, the Friedel–Crafts
reaction with fluorobenzene provides a more selective method to introduce
phenolic precursor groups, in the para positions, relative to anisole. Third,
starting with the less expensive 5-nitroisophthalic acid rather than 5-aminoi-
sophthalic acid, is more cost effective. Finally, introduction of the phenol
groups via nucleophilic aromatic substitution reaction of aryl fluorides with
potassium hydroxide affords an extremely low-cost and efficient alternative
to demethylation of the methoxy groups using either pyridine hydrochloride
or BBr3.
EXPERIMENTAL
Materials
Fluorobenzene, 5-nitroisophthalic acid, 4-fluorophthalic anhydride, thionyl
chloride, DMSO, and aluminum chloride were purchased from Aldrich
and used as received. Toluene, ethyl acetate, and ethanol were used as
received. Unless otherwise noted, all reactions were performed under a
nitrogen atmosphere, and all transfers were done using syringes or cannula
as necessary.
Instrumentation
1H and 13C NMR spectra were obtained using a Bruker Avance DMX 300-
MHz instrument operating at 300 and 75.5 MHz, respectively. Samples
were dissolved in DMSO-d6. Elemental analyses were obtained from
Midwest Microlabs, Inc., Indianapolis, IN. Melting points were determined
using a Mel-Temp capillary melting-point apparatus and are uncorrected.

424
Z. Yu et al.
Procedures
5-Nitroisophthaloyl Dichloride
5-Nitroisophthalic acid (12.67 g, 60 mmol), 35 mL of thionyl chloride, and
two drops of DMF were placed in a 250-mL round-bottomed flask,
equipped with a stir bar, reflux condenser, and nitrogen inlet. The mixture
was heated to and held at a gentle reflux until all solids were dissolved in
thionyl chloride. Excess thionyl chloride was removed under reduced
pressure to afford 14.43 g (97.0%) of 5-nitroisophthaloyl chloride as a light
yellow solid, which was used without any further purification.
3,5-Bis(40-fluorobenzoyl)nitrobenzene, 8
A solution of 14.88 g (60 mmol) of 5-nitroisophthaloyl dichloride, dissolved
in 45 mL of fluorobenzene, was added slowly via an addition funnel, to a
stirred suspension of 18.0 g (0.135 mol) of aluminum chloride in 15 mL fluor-
obenzene. Solids were formed after the addition of 5-nitroisophthaloyl
dichloride was completed. The mixture was heated to and kept at reflux for
12 h, at which point it was cooled to room temperature and quenched by
pouring into a large excess of cold 10% HCl. Ethyl acetate, 100 mL, was
added to the mixture, and the organic layer was separated. The water layer
was extracted three times with 50 mL of ethyl acetate. The organic layers
were combined and washed with deionized DI water, dilute HCl, then DI
water again. The solvents were removed by vacuum to afford a yellow
solid. Recrystallization of the crude yellow solid from a mixture of 30 mL
of toluene and 150 mL of ethanol provided 18.10 g (82.1%) of yellow
crystals with a melting-point of 122–1248C. 1H NMR (DMSO-d6; d in
ppm): 7.43–7.48 (t, J 8.8 Hz, 4H); 7.94–7.99 (dd, J1 8.8 Hz, J2 5.5 Hz,
4H); 8.33–8.34 (t, J 1.4 Hz, 1H); 8.67–8.68 (d, J 1.4 Hz 2H).13C NMR
(DMSO-d6; d in ppm): 115.8 (d, J 22 Hz); 126.9; 132.3 (d, J 2.8 Hz); 133.0
(d, J 10 Hz); 135.5:138.6; 147.8; 165.2 (d, J 253 Hz); 191.7. Anal. calculated
for C20H11F2NO4: C, 65.40%; H, 3.02%; N, 3.81%. Found: C, 65.01%; H,
2.96%; N, 3.76%.
3,5-Bis(4-fluorobenzoyl)aniline Hydrochloride Salt
A 500-mL high-pressure bottle was charged with 17.90 g (49 mmol) of 3,5-
bis(40-fluorobenzoyl) nitrobenzene, 0.9 g of palladium on activated carbon
(5% palladium content), 5 mL of 37% HCl, and 300 mL of 95% ethanol.
The bottle was placed on the hydrogenation apparatus, and hydrogen gas
was charged and discharged five times, followed by agitation at 75–85 psi
for 24 h. The reaction mixture was filtered through a cake of Celitew 545 to
remove the catalyst, and the solvent was removed via rotary evaporation.
The solid was slurried in 100 mL of ethyl acetate to afford 13.80 g (80.7%

Alternative Approach to an AB2 Monomer
425
yield) of ammonium hydrochloride salt as a white solid with a melt point
1
(decomposing) of 195–1988C. H NMR (DMSO-d6; d in ppm): 7.41–7.47
(t, J 8.8 Hz, 4H); 7.65 (t, J 1.2 Hz, 1H); 7.79 (d, J 1.2 Hz, 2H); 7.90–7.95
(dd, J1 8.8 Hz, J2 5.5 Hz, 4H); 8.78 (br, 3H). 13C NMR (DMSO-d6; d in
ppm): 115.8 (d, J 22 Hz); 124.1; 125.3; 132.7 (d, J 10 Hz); 133.0 (d, J
2.8 Hz); 138.0; 139.4; 164.9 (d, J 252 Hz); 191.7.
3,5-Bis(40-hydroxybenzoyl)aniline, 3
To a 500-mL round-bottomed flask equipped with a stir bar, 6.75 g (102 mmol)
of 85% potassium hydroxide, 25 mL of DI water, and 25 mL of DMSO were
charged. The mixture was preheated to 1208C and held at this temperature, at
which point 6.38 g (17.1 mmol) of 3,5-bis(40-fluorobenzoyl)aniline hydrochlo-
ride salt, dissolved in 35 mL of DMSO, were added dropwise via an addition
funnel. The mixture was held at 1208C for 12 h, cooled to room temperature,
and poured into 200 mL of DI water. The mixture was neutralized by adding
10% HCl, until the pH was approximately 8. The resulting yellow solid was
isolated by filtration. The solid was dissolved in 90 mL of ethanol and
filtered, and the resulting solution was diluted with 360 mL of water. The
resulting mixture was heated until it turned into a clear solution, which was
allowed to cool to rt to afford 5.51 g (97.0%) of yellow crystals with mp
1
258–2598C. H NMR (DMSO-d6; d in ppm): 5.73 (br, 2H); 6.92–6.95 (d, J
8.6 Hz, 4H); 7.02 (t, J 0.9 Hz, 1H); 7.16–7.17 (d, J 0.9 Hz, 2H); 7.72–7.75
(d, J 8.6 Hz, 4H); 10.48 (br, 2H).13C NMR (DMSO-d6; d in ppm): 115.2;
117.2; 117.6; 128.0; 132.4; 138.6; 148.9; 161.9; 194.3. Anal. calculated for
C20H15NO4: C, 72.06%; H, 4.54%; N, 4.20%. Found: C, 71.46%; H, 4.59%;
N, 4.15%.
N-[3,5-Bis(40-hydroxybenzoyl)benzene]-4-fluorophthalimide, 4
To a 250-mL round-bottomed flask with stir bar, 8.40 g (25.2 mmol) 3,5-
bis(40-hydroxybenzoyl) aniline, 4.19 g (25.2 mmol) of 4-fluoro phthalic
anhydride, and 75 mL of acetic acid were added. The mixture was heated to
and held at reflux for 2.5 h and then allowed to cool to room temperature.
The solids were isolated by filtration and washed with acetic acid, and then
small quantities of ethanol and ethyl acetate to afford 8.20 g of a light green
powder (71.8% yield). An additional 1.25 g (10.9% yield) was recovered
from the acetic acid filtrate by precipitating it from water, followed by reslur-
rying in a small amount (15 mL) of acetic acid and filtering. The melting point
of the final product was 296–2988C. 1H NMR (DMSO-d6; d in ppm): 6.93–
6.96 (d, J 8.7 Hz, 4H); 7.71–7.74 (m, 1H); 7.79–7.81 (d, J 8.7 Hz, 4H); 7.89–
7.92 (dd, J1 7.4 Hz, J2 2.2 Hz, 1H); 7.97 (t, J 1.5 Hz, 1H); 8.05–8.07 (m, 1H);
8.08–8.09 (d, J 1.5 Hz, 2H). 105 (br, 2H) 13C NMR (DMSO-d6; d in ppm):
111.3 (d, J 25 Hz); 115.4; 121.6 (d, J 24 Hz); 126.3 (d, J 9.6 Hz); 127.2;
127.8 (d, J.23 Hz) 129.0; 130.5; 132.0; 132.7; 134.6 (d, J 10 Hz); 138.5;

426
Z. Yu et al.
162.4; 165.5 (d, J 2.9 Hz); 165.8; 165.9 (d, J 254 Hz); 192.5. Anal. calculated
for C28H16FNO6: C, 69.85%; H, 3.35%; N, 2.91%. Found: C, 69.88%; H,
3.53%; N, 2.82%.
ACKNOWLEDGMENTS
The authors acknowledge funding support from the Air Force Materials and
Manufacturing Directorate and Air Force Office of Scientific Research
under the Contract FA8650-05-D-5807, Delivery Order 0009.
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