Self-assembling polycondensation for preparation of poly(p-oxybenzoyl-alt-p-mercaptobenzoyl) whisker

Article abstract of DOI:10.1021/ma030046n

Self-assembling polycondensation was examined to prepare the whisker of poly(p-oxybenzoyl-alt-mercaptobenzoyl). Polymerizations were carried out in liquid paraffin (LPF) and aromatic solvents such as Therm S 800 and 1000 at 300°C for 6 h. Polymerization c

Full text of DOI:10.1021/ma030046n

4268
Macromolecules 2003, 36, 4268-4275
Self-Assembling Polycondensation for Preparation of
Poly(p-oxybenzoyl-alt-p-mercaptobenzoyl) Whisker
Ka zu fu m i Koba sh i,† Ku n io Kim u r a ,*,† Yu h ik o Ya m a sh ita ,† Tetsu ya Uch id a ,‡ a n d
Yosh im itsu Sa k a gu ch i§
Faculty of Environmental Science and Technology, Okayama University, 3-1-1 Tsushima-naka,
Okayama 700-8530 J apan, Faculty of Engineering, Okayama University, 3-1-1 Tsushima-naka,
Okayama 700-8530 J apan, and Toyobo Research Center, Toyobo Co., Ltd., 1-1 Katata 2-Chome,
Ohtsu 520-0292 J apan
Received J anuary 21, 2003; Revised Manuscript Received April 22, 2003
ABSTRACT: Self-assembling polycondensation was examined to prepare the whisker of poly(p-oxybenzoyl-
alt-p-mercaptobenzoyl). Polymerizations were carried out in liquid paraffin (LPF) and aromatic solvents
such as Therm S 800 and 1000 at 300 °C for 6 h. Polymerization concentration was 1.0 wt/vol %. The
whisker was not obtained by the random copolymerization of p-acetoxybenzoic acid and S-acetyl-4-
mercaptobenzoic acid at the molar ratio of 0.5 in feed. However, the whiskers were successfully prepared
by the polymerization of S-(4-acetoxybenzoyl)-4-mercaptobenzoic acid (OS) and 4-(S-acetyl-4-mercapto-
benzoyl)oxybenzoic acid (SO). The whisker prepared from OS (POS) in LPF was 18 µm in average length
and 0.4 µm in average width, and that from SO (PSO) was slightly longer than POS. The whiskers prepared
in aromatic solvents were much longer than those in LPF, for which the length was 36-49 µm. These
whiskers consist of the alternating polymer chains, and the polymer chains align along the long axis of
the whisker. The sequence regularity enhanced the crystallizability of the oligomers, and this led to the
formation of whiskers. The oligomer formation rate was much higher than that of transesterfication
reaction rate, and this large difference in these two rates made the oligomer precipitate with maintaining
the alternating sequence. Further polymerization occurred in the crystals, and the whiskers having the
alternating sequence were finally completed. The difference in the two rates increased with the decrease
of the miscibility between the oligomer and the solvent, and therefore, OS and LPF were more
advantageous to maintain the alternating sequence.
In tr od u ction
merization occurs topochemically in the needlelike
crystals, and the whiskers consisting of high molecular
weight extended polymer chains are eventually formed.4
Aromatic polyester whiskers have been also prepared
by other groups,9-11 and other aromatic polymer whis-
kers besides polyesters such as poly(ester-imide)12 and
poly(azomethine)13 have been synthesized so far. It has
been recognized that the self-assembling polyconden-
sation by means of the reaction-induced crystallization
of oligomers is a valuable method for the morphology
control of intractable polymers.
The preparation of the copolymer whisker is of great
importance to tailor the novel functional polymeric
materials. Recently, we have been studying the mor-
phology of copolymers prepared by the polymerization
of p-acetoxybenzoic acid (ABA) and S-acetyl-4-mercap-
tobenzoic acid (AMBA).14 The crystal habit disappeared
with copolymerization due to the lack of the crystalli-
zability of oligomers. This article describes our new
finding on the self-assembling polymerization to prepare
poly(p-oxybenzoyl-alt-p-mercaptobenzoyl) (POB-alt-PMB)
whiskers and the influence of the sequence regularity
on the morphology as shown in Scheme 1.
Wholly aromatic polymers have been receiving much
attention as candidates for high performance polymeric
materials. They have excellent properties due to their
rigid structures, such as thermal stability, mechanical
properties, chemical resistance, and so on. However,
they exhibit neither solubility or meltability, and the
intractability makes them inaccessible for processing by
conventional techniques. Several approaches have been
attempted to improve the trade off relationship between
their properties and intractability from the aspect of not
only the chemical modification of polymer structures1
but also the processing techniques.2
We have been studying on the morphology control of
rigid polymers during solution polymerization, and
succeeded in the synthesis of whiskers of poly(p-oxy-
benzoyl) (POB), poly(p-mercaptobenzoyl) (PMB), and
other aromatic polyesters by polymerization in liquid
paraffin (LPF).3-8 These whiskers are formed by the
reaction-induced crystallization of oligomers during
solution polymerization. The polymer chains are aligned
along the long axis of the whiskers, and they show single
crystal nature. The formation mechanism of these
whiskers contains the following three steps: (1) When
the degree of polymerization of the oligomers exceeds a
critical value, they are precipitated from the solution
to form lamellae. (2) The lamellae pile up in the form
of needlelike crystals with spiral growth. (3) Postpoly-
Exp er im en ta l Section
Ma ter ia ls. ABA was purchased from TCI Co. Ltd. and
recrystallized from ethyl acetate. AMBA was prepared accord-
ing to the previous papers.14-18 LPF was purchased from
Nacalai Tesque Co. Ltd. Therm S 800 (TS8), a mixture of
isomers of triethylbiphenyl, and Therm S 1000 (TS10), a
mixture of isomers of dibenzylbenzene, were obtained from
Nippon Steel Chemical Co. Ltd. LPF, TS8, and TS10 were
purified by vacuum distillation.
* To whom correspondence should be addressed. Telephone and
Fax: 086-251-8902. E-mail: polykim@cc.okayama-u.ac.jp.
† Faculty of Environmental Science and Technology, Okayama
University.
Mon om er Syn th esis. S-(4-Acetoxyben zoyl)-4-m er ca p -
toben zoic Acid (OS). Into a three-necked flask equipped with
a dropping funnel, a thermometer, and a gas inlet tube were
‡ Faculty of Engineering, Okayama University.
§ Toyobo Co., Ltd.
10.1021/ma030046n CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/23/2003

Macromolecules, Vol. 36, No. 12, 2003
Poly(p-oxybenzoyl-alt-p-mercaptobenzoyl) 4269
Sch em e 1. P olym er iza tion System s
placed 1.79 g of 4-mercaptobenzoic acid (12.5 mmol), 2.49 g of
p-acetoxybenzoyl chloride (12.5 mmol) synthesized from ABA
and thionyl chloride, and 40 mL of dried tetrahydrofuran. The
solution of 1.42 g of triethylamine (15.0 mmol) was added drop-
wise through the dropping funnel under the slow stream of
nitrogen at 5 °C. The reaction temperature was kept at 5 °C
for 2 h and then 25 °C for 12 h. The reaction mixture was fil-
trated to separate the triethylamine-HCl salts, and acidified
by diluted HCl solution. The white precipitates were collected
and washed with water. Recrystallization from ethyl acetate
gave 1.80 g (45%) of white OS crystals. Purity was checked by
HPLC. Product characteristics were as follows. Tm: 236 °C, FT-
IR (KBr) (cm-1): 3300-2500 (OH), 3078 (aromatic C-H), 1749
(ester CdO), 1678 (thioester CdO), 1701 (carboxylic acid Cd
Ch a r a cter iza tion . The morphology of the products was
observed on SEM (Hitachi S-3500N). The length and the width
of the whiskers was determined by the average of over 80
observation values. The density of the crystals was measured
by the flotation method using bromoform and toluene at
25 °C.
N ) 8Y/{W2 × F (3L - 2W/tan(θ/2)}
(1)
where N ) number of whiskers, Y ) yield of whisker, W )
average width of whisker, L ) average length of whisker, F )
density of whisker, and θ ) tip angle of whisker. The number
of whiskers (N) was calculated according to eq 1 assuming that
the cross section of the whiskers was hexagonal like the POB
whisker.3
1
O). H NMR (ppm); 2.4 (3H, s, acetyl), 7.3 (2H, d, aromatic),
The IR spectra were measured on FT-IR spectrometer (FT/
IR-410, J ASCO Co. Ltd). The electron diffraction was per-
formed on TEM (J EOL 2000EXII). WAXS was conducted on a
Rigaku 4012K2 with nickel-filtered Cu KR radiation (35 kV,
20 mA). The chemical structure of oligomers was characterized
by 1H NMR (Varian Unity-500) operating at 500 MHz. The
NMR spectra were measured in the mixed solvents of CF3-
COOH/CDCl3 or CF3COOH/CD2Cl2. The composition of poly-
mers was determined by HPLC (Waters 600E) after hydrolysis
of polymers. Thermal properties were evaluated by DSC
(Perkin-Elmer DSC-7) with a scanning rate of 10 °C ‚ min-1
in nitrogen atmosphere. The nematic transition temperature
was measured on an optical microscope (Yanaco MP-500D)
equipped with a hot stage under crossed polarization.
7.7 (2H, d, aromatic), 8.1 (2H, d, aromatic), 8.2 (2H, d,
aromatic).
4-(S-Acetyl-4-m er ca p toben zoyl)oxyben zoic Acid (SO).
SO was synthesized by a procedure similar to that of OS.
Purity was checked by HPLC. Product characteristics were as
follows. Tm: 219 °C, FT-IR (KBr) (cm-1): 3300-2500 (OH),
3068 (aromatic C-H), 1745 (ester CdO), 1687 (thioester Cd
O), 1703 (carboxylic acid CdO). 1H NMR (ppm); 2.5 (3H, s,
acetyl), 7.4 (2H, d, aromatic), 7.6 (2H, d, aromatic), 8.2 (2H, d,
aromatic), 8.3 (2H, d, aromatic).
P olym er Syn th esis. Solution polymerizations were carried
out in several solvents at 300 °C for 6 h. Polymerization
concentration was 1.0 wt/vol % based on polymer weight and
solvent volume. Polymerization of OS in LPF was described
as a typical procedure. Into a cylindrical flask equipped with
a mechanical stirrer and a gas inlet tube were placed 0.25 g
of OS (0.78 mmol) and 20 mL of LPF. The reaction mixture
was heated under a slow stream of nitrogen up to 300 °C with
stirring. The stirring was stopped after the monomers were
completely dissolved. The temperature was maintained at 300
°C for 6 h. The products were collected by vacuum filtration
at 300 °C and washed with n-hexane and acetone. The filtrate
was poured into n-hexane, and the precipitated oligomers
which were dissolved in LPF at 300 °C were collected by
filtration. Characteristics of the polymer products were as
follows. FT-IR (KBr) (cm-1): 3067, 2922, 1735, 1673, 1587,
1500, 1399, 1261, 1203, 1156, 1054, 1011, 892, 742.
Polymerization of SO and random copolymerization of ABA
and AMBA at molar ratio of 0.5 in feed were carried out by
procedures similar to the polymerization of OS. Characteristics
of the polymer products were as follows. FT-IR (KBr) (cm-1):
for polymer prepared from SO, 3068, 2924, 1736, 1672, 1591,
1502, 1398, 1261, 1203, 1157, 1055, 897, and 754; for copoly-
mer prepared from ABA and AMBA, 3058, 2924, 1732, 1670,
1586, 1498, 1396, 1259, 1160, 1057, 1012, 890, and 736.
Melt polymerization of OS was carried out as follows. Into
a cylindrical flask equipped with a mechanical stirrer and a
gas inlet tube was placed 1.0 g of OS. The flask was heated
under a slow stream of nitrogen up to 250 °C, and the
temperature was maintained for 30 min. Then the mixture
was polymerized at 280 °C for 30 min and 300 °C for 2 h. The
polymer melt was solidified with time. The reduced pressure
was applied, and the polymerization was continued at 300 °C
and 1.0 mmHg for 1 h.
Resu lts a n d Discu ssion
Mor p h ology of P olym er Cr ysta ls. Polymerization
of ABA and AMBA in LPF at a molar ratio of 0.5 in
feed gave a random copolymer having spherical mor-
phology, and the whisker was not formed as previously
reported.14 This morphological feature was attributed
to not only the lack of crystallizability of random co-
oligomer but also propensity for liquid-liquid phase
separation of co-oligomers due to lower freezing point
of co-oligomers. OS and SO were synthesized as mono-
mers to introduce the alternating sequence regularity
into polymer chains. The solution became turbid at the
early stage of polymerization due to the precipitation
of oligomers, and then the polymer precipitates were
obtained after 6 h. The results of polymerizations of
these monomers are summarized in Table 1 in compari-
son with the result of random copolymerization. In
contrast to the random copolymerization, both the poly-
merization of OS and SO in LPF yielded the whiskers
which grew radially from center point as shown in
Figure 1. The whiskers prepared from OS in LPF
(POSLPF) are 18 µm in average length and 0.4 µm in
average width. The whiskers prepared from SO in LPF
(PSOLPF) are slightly longer and wider than POSLPF, for
which the average length is 23 µm and the average
width is 0.6 µm. These morphological features reveal
that these whiskers are formed by the crystallization

4270 Kobashi et al.
Macromolecules, Vol. 36, No. 12, 2003
Ta ble 1. Resu lts of P olym er iza tion s
composition of
a
b
polymer
code
Ts
tt
yield
(wt %)
morphology
of products
p-oxybenzoyl unit
solvent
monomer
(°C)
(min)
in products (mol %)
LPF
LPF
LPF
TS10
TS10
TS8
ABA + AMBA
220
265
245
215
205
215
200
10
12
36
18
45
33
20
26.9
54.0
40.1
61.0
33.9
31.7
21.4
spherical
needlelike
needlelike
needlelike
needlelike
needlelike
needlelike, not clear
57
49
50
42
48
45
46
POSLPF
PSOLPF
POSTS10
PSOTS10
POSTS8
PSOTS8
OS
SO
OS
SO
OS
SO
TS8
a
b
Temperature at which monomer is completely dissolved. Time when the solution becomes turbid.
F igu r e 1. Scanning electron micrographs of (a) POSLPF and (b) PSOLPF
.
F igu r e 2. FT-IR spectra of (a) OS and POSLPF
.
of oligomers as well as the POB whisker. The composi-
tions of the p-oxybenzoyl unit and the p-mercaptoben-
zoyl unit in POSLPF and PSOLPF were in good agreement
with those of the monomers. These whiskers are totally
insoluble into organic solvents, and thereby the chemical
structure and the sequence regularity cannot be directly
evaluated by NMR. Accordingly, the chemical structure
of the obtained whiskers was analyzed by FT-IR. The
spectra of the POSLPF prepared for 6 h and OS are
shown in Figure 2 as representatives. The peaks for Cd
O of ester group and thioester group appeared newly
at 1735 and 1673 cm-1, respectively. The peaks for Cd
O of acetyl group at 1749 cm-1 and carboxylic acid at
1701 cm-1 and that for OH of carboxylic acid at 3300-
2500 cm-1 of OS disappeared significantly. This result
confirms that the whiskers are composed of high mo-
lecular weight polymers. Selected area electron diffrac-
tion was performed to estimate the sequence regularity.
The diffraction patterns of POSLPF and PSOLPF do not
show the true fiber pattern with cylindrical symmetry,
and they consist of the sharp spots of lower to higher
order diffraction as clearly observed in Figure 3. These
are due to the single-crystal nature of these whiskers.
F igu r e 3. TEM and selected area electron diffraction pattern
of (a) POSLPF and (b) PSOLPF
.
The spots on the meridian of PSOLPF are slightly diffuse,
and this may be accounted for by the smaller crystalline
area or the axial shifted structure found in aromatic
polyazoles.19 The meridians of these patterns correspond
to the long axes of the crystals, and the polymer chains
align along the long axes of the whiskers. The observed
fiber identity periods of POSLPF and PSOLPF are 12.73
and 12.96 Å, respectively, which correspond to the
calculated fiber identity period of POB-alt-PMB. It can
be concluded that these polymers maintain the alter-
nating sequence, and the sequence regularity leads to
the formation of the whiskers.
As described, the length and width of the POSLPF are
slightly shorter than those of the PSOLPF whiskers. To
understand a difference in the size, the number of the

Macromolecules, Vol. 36, No. 12, 2003
Poly(p-oxybenzoyl-alt-p-mercaptobenzoyl) 4271
F igu r e 4. Scanning electron micrographs of (a) POSTS8 and (b) PSOTS8
.
Ta ble 2. Size P a r a m eter s a n d Nu m ber of Wh isk er s
Ta ble 3. Secon d -Or d er Ra te Con sta n ts for F or m a tion of
Cr ysta llized Oligom er a n d Tr a n s-Ester ifica tion a
size parameter
formation
polymer
code
length
width
tip angle
(deg)
no. of
rate constant of
trans-esterification
(µm)
(µm)
whiskers (×1010
)
crystallized oligomer rate constant k2t
monomer solvent
k
2f (L‚mol-1‚min-1
)
(L‚mol-1‚min-1
)
k2f/k2t
POSLPF
PSOLPF
POSTS10
PSOTS10
POSTS8
18.1
21.8
48.6
37.6
35.6
0.38
0.59
0.41
0.58
0.41
4.3
3.9
2.0
3.1
2.8
6.7
2.0
2.2
0.9
1.6
OS
LPF
TS8
LPF
1.68
0.16
0.60
1.90 × 10-2
3.60 × 10-2
7.10 × 10-2
88.6
4.4
8.3
SO
a
Polymerizations were carried out at 300 °C. Initial concentra-
tion was 3.90 × 10-2 mol‚L-1
.
whiskers generated in polymerization solution (N) was
calculated according to eq 1 with the shape parameters,
the yields and the densities of the whiskers. The shape
parameters and the calculated Ns are summarized in
Table 2. POSLPF and PSOLPF have the same density of
1.51 g‚cm-3. N of POSLPF is 6.7 × 1010, which is 3.4 times
larger than that of PSOLPF. Many more nuclei used in
the whisker formation are generated in the polymeri-
zation of OS compared with that of SO, and this results
ultimately in forming the shorter whiskers. It is well-
known that critical radius of nucleus (r*) and nucleation
rate (J ) depend on the degree of supersaturation as
follows.20,21
ferences in Ts and tt indicate the higher miscibility of
the SO oligomers than that of the OS oligomers. Se-
condary, the rate constants for the formation of crystal-
lized oligomers (k2f) in LPF are estimated as shown in
Table 3. The k2f of OS is 2.8 times larger than the k2f of
SO. The kinetics will be discussed in detail in the
following section. It can be explained on the basis of
these results that the higher degree of supersaturation
of the OS oligomer than that of the SO oligomer results
in the formation of a larger number of nuclei, and hence,
the length of the POSLPF become reasonably shorter.
The miscibility of the oligomer and the solvent gov-
erns N leading to the length control of the whiskers,
and the higher miscibility which results in the lower
degree of supersaturation of the oligomers makes the
whiskers longer due to the smaller N. Therefore, the
solvent effect on the size of the whisker was examined
with two kinds of aromatic solvents which were TS10
and TS8. The polymerization results and the morphol-
ogy of the crystals are shown in Table 1 and Figure 4.
The polymerizations of OS and SO in these aromatic
solvents yielded the whiskers with the lower yields than
that in LPF. In the polymerization of SO in TS8, pro-
ducts having unclear morphology were also formed
together with the whiskers. The sizes of the obtained
whiskers are also summarized in Table 2. The whiskers
prepared from OS in TS10 (POSTS10) are 49 µm in aver-
age length and 0.4 µm in average width, and those in
TS8 (POSTS8) are 36 µm in average length and 0.4 µm
in average width. Concerning SO, the whiskers pre-
pared in TS10 (PSOTS10) are 38 µm in average length
and 0.6 µm in average width. The whiskers prepared
in aromatic solvents are much longer than those in LPF.
The observed densities of POSTS10, POSTS8, and PSOTS10
were 1.52, 1.52, and 1.51 g‚cm-3, respectively. The Ns
in aromatic solvents tabulated in Table 2 are from one-
fourth to one-third of those in LPF, and this is due to
the higher miscibility anticipated from Ts and tt. The
longer whiskers can be prepared in aromatic solvents
and the length is tunable by the miscibility between the
oligomer and the solvent.
∆µ ) kT ln(1 + σ) σ ) (C - Ce)/Ce
r* ) 2vγ/∆µ
J ) ν+ q exp(-∆G*/kT) )
ν+ q exp(-16πγ3v2/3∆µ2kT )
where µ ) chemical potential, C ) concentration of
solute, Ce ) equilibrium concentration, r* ) critical
radius of nucleus, v ) volume of molecule, γ ) density
of surface energy, J ) nucleation rate, ν+ ) rate of
crystallization of one molecule into critical nucleus, and
q ) density of free molecule.
When the degree of supersaturation (σ) increases, ∆µ,
which is a driving force for nucleation, becomes larger.
This large ∆µ leads to small r* and large J , indicating
that many more nuclei having smaller radius are
formed. There seems to exist several parameters to
influence the degree of supersaturation such as misci-
bility between oligomer and solvent, formation rate of
oligomer, concentration, temperature and so on. Poly-
merizations were carried out at the same concentration
and temperature, and thereby the former two param-
eters are more dominant in this study. First, the mis-
cibility between the oligomer and the solvent is dis-
cussed. The temperature at which the monomer is com-
pletely dissolved during heating (Ts) and the time when
the solution becomes turbid (tt) are adaptable as criteria
to estimate the miscibility between the oligomer and the
solvent, and the lower Ts and the later tt imply the
higher miscibility. The Ts of SO is 20 °C lower and the
tt of SO is 24 min later than those of OS. These dif-
WAXS patterns of these whiskers show that the
diffraction peaks are quite sharp, and the diffuse halo
attributed to amorphous parts is not detected at all as

4272 Kobashi et al.
Macromolecules, Vol. 36, No. 12, 2003
F igu r e 6. Polymerization time dependency of length, width
and yield of (a) POSLPF and (b) PSOLPF
.
nucleation on the spiral step leading to the increase of
the length is easier than that on the lateral side surface
because of the advantage of surface energy. However,
when the degree of supersaturation of the oligomers is
quite high, secondary nucleation occurs on not only the
spiral step but also the lateral side surface leading to
the increase of the width. To evaluate the degree of
supersaturation of the oligomers during polymerization
more precisely, the k2fs were converted into the rate per
one nucleus (k′2f). The k2f of OS is larger than that of
SO as described above, but the k′2f of SO is 3.0 × 10-11
L‚mol-1‚min-1 which is 20% larger than that of OS of
2.5 × 10-11 L‚mol-1‚min-1. This slightly higher degree
of supersaturation of the oligomers leads to much
growth on the lateral side surface of PSOLPF whiskers.
F igu r e 5. WAXS intensity profiles of various whiskers.
Ta ble 4. Ava r a ge Dia m eter of Cr ysta llin e Ar ea (D)
The oligomers dissolved in solution were analyzed by
1H NMR, and the spectra are shown in Figure 7. In the
polymerization of OS in LPF, the average molar ratio
of 1,4-phenylene group and the acetyl end group of the
dissolved OS oligomers is ca. 2, and it is constant during
polymerization as shown in Figure 8. This fact suggests
that OS is mainly left in the solution and dimers are
precipitated in the form of the needlelike crystals. The
peak of acetyl proton of S-acetyl-4-mercaptobenzoyl unit
is newly found at 2.55 ppm which is not included in OS,
and its peak intensity increases with time. The appear-
ance of this peak means the occurrence of transesteri-
fication reaction in the solution via the mixed anhydride
formation mechanism.22-26 The molar ratio of S-acetyl
end group in acetyl end groups of the oligomers in-
creases with time as also shown in Figure 8. The trans-
esterification reaction brings about the randomization
of the regular sequences of the oligomers and ultimately
the polymers. The oligomers are continuously precipi-
tated with time in this polymerization system, and this
results in the gradual decrease of the concentration of
the oligomers in the solution. Thus, the transesterifi-
cation reaction was followed within 10 min until the
precipitation began. The formation of the dimers was
monitored by the recovery of OS dissolved in the solu-
tion. It has been reported that both the polycondensa-
tion of ABA27-30 and the transesterification of aromatic
copolyesters31-34 obeyed second-order kinetics. Figure
9 shows that both the transesterification reaction and
the formation of oligomers obey second-order kinetics
and the rate constants are determined from the slopes
as shown in Table 3. The OS dimers are formed 88.6
times faster than the transesterification reaction in
LPF. It can be said that the OS dimers are mainly
precipitated to form the whiskers while keeping the
regular sequences. On the other hand, the average mo-
lar ratio of 1,4-phenylene group and acetyl end group
of Wh isk er s
polymer code
D (Å)a
POSLPF
PSOLPF
POSTS10
PSOTS10
POSTS8
346
247
223
266
214
a
D was estimated by using Scherrer’s equation from the
reflective width at 2θ of 19.5°.
shown in Figure 5. They possess the extremely high
crystallinity. There is some similarity between these
WAXS profiles and the peak positions are almost the
same. The average diameters of the crystalline area (D)
of the whisker were estimated by Scherrer’s equation
with the sharpest peak around 19.5° as shown in Table
4. The Ds of these whisker are on the order of 300 Å for
POSLPF and 200 Å for the other whiskers. POSLPF
possesses better packing regularity than other whiskers
because of higher sequence regularity.
Gr ow th Mech a n ism of Wh isk er . To understand
the growth mechanism of POB-alt-PMB whisker, the
yields and the sizes of the whiskers were followed during
the polymerization of OS and SO in LPF. The length
and the width of both POSLPF and PSOLPF increase with
the yields as shown in Figure 6, and the length increases
preferentially rather than the width. The large incre-
ments of the length and the width were observed at the
early stage of the polymerization within 60 min in both
cases. This is due to the higher degree of supersatura-
tion of the oligomers in the initial stage of the poly-
merization. The increment of the width of PSOLPF in
the initial stage is much larger than that of POSLPF. If
these whiskers are formed by the spiral growth as well
as the POB whiskers, the increase of whisker length is
caused by which oligomer lamellae pile up along the
long axis of the needlelike crystals. The secondary

Macromolecules, Vol. 36, No. 12, 2003
Poly(p-oxybenzoyl-alt-p-mercaptobenzoyl) 4273
F igu r e 7. 1H NMR spectra of (a) OS, (b) dissolved oligomers prepared from OS for 72 min in LPF, (c) SO, and (d) dissolved
oligomers prepared from SO for 96 min in LPF.
of the SO oligomers dissolved in LPF is ca. 2.5, and the
molecular weight of the precipitated SO oligomers are
larger than that of OS oligomers, which is attributed
to the higher miscibility as described before. The trans-
esterification and the formation rate of the SO oligomers
are also evaluated as shown in Table 3. The transes-
terification reaction of SO occurs more rapidly than OS,
and the SO oligomers are precipitated 8.3 times faster
than the transesterification reaction. The SO oligomers
are also precipitated to form the whiskers while keeping
the regular sequences. Though the oligomers are pre-
cipitated more rapidly than transesterification reaction
in both polymerizations, the difference in the two rate
constants of OS is much larger than that of SO. Hence,
OS is of greater advantage for the formation of POB-
alt-PMB whiskers.
Concerning the polymerization of OS in TS8, the
average molar ratio of 1,4-phenylene group and acetyl
end group of the dissolved oligomers is ca. 3.0. This
indicates that the oligomers having higher molecular
weight are precipitated in TS8 than those in LPF due
to the higher miscibility. The reactions of OS in TS8
also obey the second-order kinetics and their rate
constants are estimated as shown in Table 3. The k2f of
OS in TS8 is 0.16 L‚mol-1‚min-1, which is 10.5 times
smaller than that in LPF. The transesterification reac-
tion for which the k2t is 3.6 × 10-2 L‚mol-1‚min-1 occurs
1.9 times faster than that in LPF. The precipitated
oligomers are formed 4.4 times faster than the trans-
esterification reaction in TS8, but the difference in the
two rate constants of OS in TS8 is much smaller than
that in LPF. As shown in Table 1, there are some

4274 Kobashi et al.
Macromolecules, Vol. 36, No. 12, 2003
F igu r e 8. Polymerization time dependency of (a) molar ratio of 1,4-phenylene group and end group of dissolved oligomers prepared
from OS and SO, and content of end groups of dissolved oligomers prepared from (b) OS and (c) SO in LPF.
F igu r e 9. Kinetics of second-order reaction for (a) formation
of crystallized oligomers and (b) transesterification reaction
in the polymerization of OS (b) and SO (O) in LPF. [Ca]:
Concentration of crystallized oligomer. [Ca′]: Concentration of
S-acetyl or O-acetyl group. [Cao] ) [Cao′] ) 3.90 × 10-2 mol‚L-1
.
deviations between the composition of polymer crystals
prepared in aromatic solvents and monomers. The
whiskers are rich in the p-mercaptobenzoyl unit. This
indicates that the oligomers having randomized se-
quence are precipitated to form whiskers. It can be
supposed that the polymerization of OS and SO in
aromatic solvents is advantageous to prepare the longer
whiskers but disadvantageous to maintain the alternat-
ing sequence compared with that in LPF.
The change in the molecular weight of POSLPF was
qualitatively investigated during polymerization by FT-
IR. The spectrum between 1800 and 1615 cm-1 are
made up by the superposition of four CdO peaks, which
F igu r e 10. (a) FT-IR spectrum of POSLPF prepared for 15 min
and (b) polymerization time dependency of absorbance ratio
of CdO of thioester group and CdO of carboxylic acid group
of POSLPF
.
are those of acetyl at 1749 cm-1, thioester at 1673 cm-1
,
,
ester at 1735 cm-1, and carboxylic acid at 1701 cm-1
Th er m a l P r op er ties of Wh isk er s. Thermal proper-
ties of the resulting whiskers are summarized in Table
5 compared with those of the random copolymer pre-
pared from ABA and AMBA at a molar ratio of 0.5 in
feed. It is known that the POB and PMB crystals show
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 molecule by a rotation of 1,4-phenylene ring
around σ-bond in the para position.35-39 The transition
temperature (Tt) and its enthalpy (∆H) are susceptible
to both crystallinity and packing of polymer chains. The
crystals having higher crystallinity and closer packing
and they are resolved by the combined Lorentzian and
Gaussian function as illustrated in Figure 10. The
absorbance ratio of CdO of thioester group and that of
ester group is constant during polymerization. The
others become smaller with time. The change in the
absorbance ratio of CdO of ester group and that of
carboxylic acid is monitored to clarify the change in
molecular weight. This absorbance ratio increases lin-
early with time. This indicates that the solid-state
polymerization occurs very effectively in the crystals as
well as the POB whiskers,3 and the molecular weight
increases with maintaining the alternating sequences.

Macromolecules, Vol. 36, No. 12, 2003
Poly(p-oxybenzoyl-alt-p-mercaptobenzoyl) 4275
Refer en ces a n d Notes
Ta ble 5. Th er m a l P r op er ties of Cop olym er s
DSCb
(1) For example: Cassidy, P. E. Thermally Stable Polymers,
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(2) For example: Seymour, R. B.; Kirshenbaum, G. S. High
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c
polymerization
methoda
polymer
code
Tt
∆H
Tn
monomer
(°C)
(J /g)
(°C)
ABA + AMBA
OS
SP
SP
MP
SP
d
377
418
369
408
POSLPF
PSOLPF
277.3
6.25
3.65
SO
281.6
(3) Yamashita, Y.; Kimura, K. Polymeric Materials Encyclopedia;
CRC Press: Boca Raton, FL, 1996; 8707.
a
SP: solution polymerization in LPF. MP: melt polymerization.
b
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a
heating rate of 10 °C/min in nitrogen. c Tn: Nematic
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d
show higher Tt and larger ∆H.7,40 Concerning random
copolymer, the transition was not detected. POSLPF and
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The whiskers of POB-alt-PMB are successfully pre-
pared by the polymerization of OS and SO. They possess
extremely high crystallinity. They consist of the alter-
nating polymer chains and the polymer chains aligned
along the long axis of the whisker. The chemical
structure of the monomer and the miscibility between
the oligomer and the solvent are very important pa-
rameters used to control the length and the regular
sequence. Concerning the chemical structure of the
monomer, SO gives an advantage in making the longer
whiskers rather than OS due to the higher miscibility,
which brings about the formation of a smaller number
of nuclei. However, OS gives an advantage in maintain-
ing the alternating sequence because the difference in
the rate constant between the oligomer formation and
the transesterification reaction is much larger than that
of SO. Concerning the solvent, aromatic solvents yield
the longer whiskers than LPF, but they give a disad-
vantage in maintaining the alternating sequence due
to the higher miscibility. The formation mechanism of
the whiskers can be proposed as follows; When the
molecular weight of the oligomers exceeds the critical
value, they are precipitated prior to the randomization
by transesterification reaction and form the needlelike
crystals. Further polymerization occurs in the crystals
and the whiskers of POB-alt-PMB are finally completed.
These whiskers exhibit better thermal properties com-
pared with the random copolymers.
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