Novel direct polycondensation of 4-hydroxybenzoic acid by means of reaction-induced crystallization of oligomers under nonstoichiometric condition

Article abstract of DOI:10.1021/ma0258307

The direct polycondensation of 4-hydroxybenzoic acid (HBA) in the presence of 4-alkyloxybenzoic anhydride by means of the reaction-induced crystallization of oligomers was described. Thermal properties and crystal structure of poly(p-oxybenzoyl) (POB) were examined to support the formation of high molecular weight POB. Thermal properties of the obtained crystals were measured by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).

Full text of DOI:10.1021/ma0258307

Volume 36, Number 14
J uly 15, 2003
© Copyright 2003 by the American Chemical Society
Communications to the Editor
Sch em e 1. Dir ect P olycon d en sa tion of HBA
Novel Dir ect P olycon d en sa tion of
4-Hyd r oxyben zoic Acid by Mea n s of
Rea ction -In d u ced Cr ysta lliza tion of
Oligom er s u n d er Non stoich iom etr ic
Con d ition
Ku n io Kim u r a ,* Sh in -ich ir o Koh a m a , a n d
Yu h ik o Ya m a sh ita
Faculty of Environmental Science and Technology,
Okayama University, 3-1-1 Tsushima-naka,
Okayama 700-8530, J apan
Received November 13, 2002
Revised Manuscript Received May 26, 2003
In tr od u ction . Direct polycondensation of polyester
and polyamide is a very attractive polymerization
method, and numerous types of condensation reagents
have been developed so far.1-13 The previous direct
polycondensations were carried out in homogeneous
solution, and stoichiometry between two functional
groups in the polymerization system is of great impor-
tance to prepare high molecular weight polymers. We
had been studying the morphology control of aromatic
polyesters by reaction-induced crystallization of oligo-
mers during solution polymerization and prepared suc-
cessfully poly(p-oxybenzoyl) (POB) whiskers from 4-ac-
etoxybenzoic acid (ABA) in liquid paraffin at 320 °C.14-19
In the series of these studies, it has been found that
high molecular weight POB could be prepared from ABA
in liquid paraffin even under nonstoichiometric condi-
tion between acetoxy and carboxyl groups coexisting
4-alkyloxybenzoic acid.20 The polycondensation proceeds
under nonstoichiometric condition through following
mechanisms; when DPn of oligomers exceeds a critical
value, they are crystallized to form the crystals. End-
free oligomers are preferentially crystallized due to the
lower solubility. Though end-capped oligomers are also
crystallized, polycondensation proceeds with eliminating
end-capping groups by trans-esterification just when
they are crystallized. On the basis of the above polym-
erization mechanisms, direct polycondensation of 4-hy-
droxybenzoic acid (HBA) with aromatic acid anhydride
will work well to yield POB.
This paper describes our new finding of direct poly-
condensation of HBA in the presence of 4-alkyloxyben-
zoic anhydride such as 4-ethoxybenzoic anhydride (EOBA
anhydride) and 4-hexyloxybenzoic anhydride (HOBA
anhydride) by means of the reaction-induced crystal-
lization of oligomers.
Exp er im en ta l Section . Typical solution polymeri-
zation procedure was described as follows: Into a
cylindrical vessel equipped with gas inlet and outlet
tubes were placed 0.23 g of HBA (1.67 × 10-3 mol), 0.52
g of EOBA anhydride (1.67 × 10-3 mol), and 20 mL of
liquid paraffin. This mixture was placed into an oil bath
and heated to 320 °C under a slow stream of nitrogen
with stirring. When the mixture became clear solution,
the stirring was stopped. The solution became turbid
after several minutes at 320 °C due to the crystalliza-
tion, and then the polymer crystals were formed. The
solution temperature was kept at 320 °C for 6 h.
Afterward, the crystal suspension was filtrated at 320
°C. The obtained polymer crystals were washed with
n-hexane and acetone and then dried. After the filtrate
was allowed to cool, the compounds dissolved in liquid
* To whom correspondence should be addressed. Telephone and
Fax: 086-251-8902; e-mail: polykim@cc.okayama-u.ac.jp.
10.1021/ma0258307 CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/13/2003

5044 Communications to the Editor
Macromolecules, Vol. 36, No. 14, 2003
Ta ble 1. P olym er iza tion of HBA in th e P r esen ce of 4-Alk yloxyben zoic An h yd r id e in Liqu id P a r a ffin a
content of 4-alkyloxybenzoic acid unitc (mol %)
dissolved
DSCe
concn of 4-alkyloxybenzoic
polymer
morphology of
the crystals
anhydride in feedb (mol %) yield (%)
polymer
compounds
DPnd
Tcf (°C) ∆H (J /g)
øEa
0
0
0
0
18
30
37
39
10
20
30
40
50
60
2.00
1.45
1.30
2.56
75.7
73.3
83.2
80.9
49
68
76
38
slab
slab
fibril, pillar
fibril, pillar
324
323
326
320
34.8
32.2
32.6
38.4
øHa
0-60
0
a
b
Polymerizations were carried out at 320 °C for 6 h. ø is defined as {[4-alkyloxybenzoic anhydride]/([HBA] + [4-alkyloxybenzoic
anhydride])} × 100 (mol %). c Content of 4-alkyloxybenzoic acid unit is defined as {[4-alkyloxybenzoic acid unit]/([4-oxybenzoyl unit] +
d
[4-alkyloxybenzoic acid unit])} × 100 (mol %). DPn was estimated by the molar ratio of 4-hydroxybenzoic acid and 4-alkyloxybenzoic
acid by quantitative analysis of end groups based on the assumption that hydroxyl and other end groups were negligible in this
polymerization. e DSC was measured with a scanning rate of 10 °C min-1 f Crystal-crystal transition temperature.
.
F igu r e 1. Scanning electron micrographs of POB crystals in the system of EOBA anhydride at øEa of (a) 30, (b) 40, (c) 50, and
(d) 60 mol %.
paraffin at 320 °C were precipitated by pouring the
filtrate into n-hexane. The precipitates were collected
by filtration and washed with n-hexane.
When EOBA anhydride was added, the crystals were
formed during polymerization at øEa of 30-60 mol %
with the yield of 18-39%. Any crystals were not
precipitated at øEa of less than 30 mol % and more than
70 mol %. Scanning electron micrographs of the ob-
tained crystals are shown in Figure 1. They exhibit very
clear crystal habits such as slab, fibril, and pillar. As
we had reported previously, there exist two major modes
for phase separation of oligomers during solution po-
lymerization, of which one is the crystallization of
oligomers and another is liquid-liquid phase separation
of oligomers.22-24 If the oligomers are precipitated by
the crystallization, the oligomer crystals are formed in
the solution, and then the polymer crystals having clear
crystal habit are finally formed. In contrast to this, if
the oligomers are phase-separated by the liquid-liquid
phase separation, the microspheres are obtained through
the formation of the microdroplets. In this case, the
crystal habit completely disappeared and the spherical
morphology appeared. In this study, the products ex-
Resu lts a n d Discu ssion . EOBA anhydride21 and
HOBA anhydride21 were used as condensation reagents
as shown in Scheme 1. Polymerizations of HBA were
carried out in liquid paraffin at 320 °C with coexisting
4-alkyloxybenzoic anhydride. These anhydrides possess
alkyloxy group to avoid the sublimation during high-
temperature polymerization. Table 1 summarizes the
results of polymerization. The molar ratio of 4-alkyloxy-
benzoic anhydrides in feed (ø) is described as follows:
ø (mol %) )
[4-alkyloxybenzoic anhydride]
× 100
[HBA] + [4-alkyloxybenzoic anhydride]
øEa and øHa stand for the molar ratio of EOBA anhydride
and HOBA anhydride in feed, respectively.

Macromolecules, Vol. 36, No. 14, 2003
Communications to the Editor 5045
the crystals, and then it is constant after the yield is
leveled off. The number of polymer chains (Np) in the
crystals was calculated according to the following equa-
tion to know the polymerization behavior.
yield × N
Np )
(120.1 × DPn + 166.2)
where yield is the yield of crystals (grams) and N is the
Avogadro’s number.
The plot of Np as a function of time is also shown in
Figure 2. Np decreases with polymerization time despite
the yield increases and then becomes constant after 6
h. This decrement of Np reveals the occurrence of solid-
state polymerization in the crystals. The termination
of this solid-state polymerization is not clear yet, but it
may be due to the defect or disordering of polymer chain
ends in the crystals. Analysis of the composition of
EOBA moiety in the crystals and the dissolved com-
pounds at øEa of 50 mol % elucidates that most of EOBAs
are left in the liquid phase during polymerization. From
these results, direct polycondensation of HBA in the
presence of EOBA anhydride proceeds as follows. HBA
reacts first with EOBA anhydride to form activated
monomer EBBA. EOBA by-produced in situ breaks the
stoichiometry between the 4-alkyloxybenzoyloxy group
in EBBA and the carboxyl group. The ester-acid
exchange reaction between the 4-ethoxybenzoyloxy group
and the carboxyl group forms oligomers in the solution.
When the DPn of oligomers exceeds the critical value,
the oligomers are phase-separated to form the crystal.
The solid-state polymerization takes place in the crys-
tals with eliminating EOBA, and high molecular weight
POB is formed even under the nonstoichiometric condi-
tion. As described above, the crystals were not precipi-
tated in the case of HOBA anhydride. The oligomers
prepared with HOBA possess a longer alkyloxy end
group, and this enhances the miscibility of oligomers.
Therefore, the oligomers are hardly phase-separated.
Thermal properties and crystal structure of POB are
very susceptible to molecular weight, and they are
examined to support the formation of high molecular
weight POB. Thermal properties of the obtained crystals
were measured by DSC and TGA.26 It is well-known
that POB crystal exhibits a reversible first-order solid-
solid transition at around 350 °C differing from the
melting process, and it is regarded as a transition to
pseudohexagonal packing of polymer chains due to a
rotation of 1,4-phenylene ring around the s-bond in the
para position.27-31 The obtained crystals exhibit the
crystal-crystal transition temperature at 320-326 °C
with ∆H of 32-38 J g-1, as shown in Table 1. The
temperature of 5 wt % loss for the crystals prepared at
øEa of 50 mol % for 20 h is 512 °C, which is comparable
to that for POB prepared from ABA. WAXS patterns
(Cu KR) of the obtained POB crystals show very sharp
diffraction peaks being accounted for by the crystal unit
of POB crystals. These results strongly support the
formation of high molecular weight POB.
F igu r e 2. Plots of (a) DPn and yield of polymers and (b)
number of polymer chains (Np) in crystals prepared at øEa of
50 mol % as a function of polymerization time.
hibit the clear crystal habit, and therefore, it can be
thought that they are formed by the crystallization of
oligomers. On the other hand, crystals were not formed
in the presence of HOBA anhydride at any øHa’s. FT-IR
spectra of the crystals and the dissolved compounds in
liquid paraffin prepared at øEa of 50 mol % for 6 h were
measured to confirm the chemical structure. The char-
acteristic peaks of POB such as ester carbonyl group
appear clearly at 1739 and 1261 cm-1, and the hydroxyl
group is not detected in the spectrum of the crystals.
The spectra of the crystals are identical with that of
POB. The result of the elemental analysis result of the
crystal is in good agreement with the calculated value.
These confirm the formation of POB as the crystals.25
The peaks of carboxylic anhydride appearing at 1773
and 1706 cm-1 disappear completely, and the ester
group is clearly observed in the spectrum of the dis-
solved compounds in liquid paraffin. The hydroxy group
in HBA is not detected. This result shows that HBA is
completely converted into 4-(4-ethoxybenzoyloxy)benzoic
acid (EBBA), and oligomer is formed by the ester-acid
exchange reaction with eliminating EOBA. POB is
insoluble in any solvents, and hence DPn cannot be
directly measured by NMR and GPC. In this study, DPn
of the obtained polymer in the crystals was determined
by the molar ratio of HBA and EOBA after hydrolysis
of polymers, assuming that hydroxyl end group did not
exist in the polymer chains, which was consistent with
the results of FT-IR spectra.20 DPns are also shown in
Table 1. DPns of the obtained polymers are from 38 to
76. It increases with increasing øEa up to 50 mol % and
then decreases over 50 mol %. Polymerization at øEa of
50 mol % gives the highest DPn of 76. The influence of
øEa on DPn can be explained as follows. At øEa of 50 mol
%, all HBAs are completely converted into the EBBA,
and the molar ratio of two functional groups, which are
the 4-ethoxybenzoyloxy group in EBBA and the carboxyl
group, is the highest value of 0.5. In other words, øEa of
50 mol % gives the least nonstoichiometric condition
among øEa’s. When øEa is less than 20 mol %, the
stoichiometry is largely out of balance, and oligomers
are not formed enough to be phase-separated because
of the low concentration of activated monomers. There-
fore, the crystals are not precipitated at øEa of less than
20 mol %. On the other hand, the crystals were not
obtained at øEa of more than 70 mol %. In this case,
excess EOBA anhydride reacts with the carboxyl end
group of oligomers to form the anhydride end group, and
the oligomers cannot be phase-separated due to the
increase of miscibility into liquid paraffin. Figure 2 plots
DPn and the yield of the polymer crystals as a function
of polymerization time at øEa of 50 mol %. DPn increases
from 18 for 1 h to 76 for 6 h with increasing the yield of
It is concluded that this study provides a new direct
polycondensation of HBA with EOBA anhydride as a
condensation reagent, and the phase separation of
oligomers is of great importance to prepare high mo-
lecular weight POB.
Refer en ces a n d Notes
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Chem. Ed. 1981, 18, 2879.

5046 Communications to the Editor
Macromolecules, Vol. 36, No. 14, 2003
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(21) EOBA anhydride was synthesized according to following
procedure. Into a flask equipped with a condenser and a
gas inlet tube were placed 15.0 g of EOBA purchased from
Aldrich Co. Ltd. (0.090 mol) and 120 mL of thionyl chloride.
Several drops of N,N-dimethylformamide were added into
this solution under a nitrogen atmosphere. The mixture was
stirred at 25 °C for 12 h. Excessive thionyl chloride was
evaporated. Extraction with hot n-hexane gave crude 4-ethox-
ybenzoyl chloride at a yield of 12.4 g (74.4%). Into a flask
equipped with a dropping funnel, a thermometer, and a gas
inlet tube were placed 11.2 g of EOBA (0.067 mol), 10.2 g of
triethylamine (0.101 mol), and 100 mL of dried tetrahydro-
furan under a slow stream of nitrogen. A solution of 12.4 g
of 4-ethoxybenzoyl chloride (0.067 mol) in 50 mL of dried
tetrahydrofuran was added dropwise through a dropping
funnel to the mixture at 5 °C for 20 min. The mixture was
stirred at 5 °C for 2 h and then at 25 °C for 6 h. HCl-
triethylamine salt was filtrated and tetrahydrofuran was
evaporated. The obtained solids were washed with water.
Washed solids were dissolved in ethyl acetate and dried over
sodium sulfate. Recrystallization from ethyl acetate gave
18.1 g of 4-ethoxybenzoic anhydride. The yield was 85.5%.
Tm (°C): 114. IR (KBr; cm-1): 2980 (aliphatic C-H stretch-
ing), 2856 (aliphatic C-H antisymmetric stretching), 1773
(anhydride CdO antisymmetric stretching), 1706 (anhydride
CdO symmetric stretching), 1604 (aromatic CdC stretch-
ing), 1026 (anhydride C-O stretching). 1H NMR (CDCl3; δ,
ppm): 1.40 (CH3CH2O, t, 6H), 4.03-4.10 (CH3CH2O, m, 4H),
6.90 (phenyl, d, 4H), 8.03 (phenyl, d, 4H). HOBA anhydride
was synthesized as the similar procedure to EOBA anhy-
dride described above. The yield of HOBA anhydride was
83.0%. Tm (°C): 49. IR (KBr; cm-1): 2931 (aliphatic C-H
stretching), 2858 (aliphatic C-H antisymmetric stretching),
1778 (anhydride antisymmetric stretching), 1719 (anhydride
CdO symmetric stretching), 1604 (aromatic CdC stretch-
ing), 1031 (anhydride C-O stretching). 1H NMR (CDCl3; δ,
ppm): 0.90 (CH3CH2(CH2)3CH2O, t, 6H), 1.75-1.85 (CH3CH2
(CH2)3CH2O, m, 4H), 1.26-1.52 (CH3CH2(CH2)3CH2O, m,
12H), 4.03 (CH3CH2(CH2)3CH2O, t, 4H), 6.95 (phenyl, d, 4H),
8.07 (phenyl, d, 4H).
(22) Kimura, K.; Yokoyama, F.; Yamashita, Y. Prepr., IUPAC
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Meeting PPS-14, 1998, 577.
(25) Anal. Calcd for C352H206O101 at øEa of 30 mol %: C, 69.87%;
H, 3.43%. Found: C, 69.73%; H, 3.62%. Anal. Calcd for
C
485H282O139 at øEa of 40 mol %: C, 69.90%; H, 3.41%.
Found: C, 69.96%; H, 3.55%. Anal. Calcd for C541H314O155
at øEa of 50 mol %: C, 69.91%; H, 3.41%. Found: C, 69.81%;
H, 3.53%. Anal. Calcd for C275H162O79 at øEa of 60 mol %:
C, 69.83%; H, 3.45%. Found: C, 69.82%; H, 3.61%.
(26) DSC were performed on Perkin-Elmer DSC-7 with a scan-
ning rate of 10 °C min-1 in a nitrogen atmosphere. TGA
were performed on Perkin-Elmer TGA-7 with a scanning
rate of 10 °C min-1 in a nitrogen atmosphere.
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MA0258307