Hydrolytic Polycondensation of Bisphenol‐A‐Bischloroformate Catalyzed by Phase‐Transfer Catalysts

Abstract

The hydrolytic interfacial polycondensation of bisphenol‐A‐bischloroformate was performed with four different phase‐transfer (PT) catalysts: N ‐butylpyridinium bromide, triethylbenzylammonium (TEBA) chloride, tetrabutylammonium hydrogen sulfate, and tetraphenylphosphonium bromide. These polycondensations were conducted at 5 or 35 °C initial reaction temperature. The resulting polycarbonates were characterized by viscosity and SEC measurements and by MALDI‐TOF mass spectrometry. The four PT catalysts gave quite different results with respect to molecular weight and formation of cyclic polycarbonates. The highest molecular weights (number average, $\overline M _{\rm n} \approx 215\;{\rm kDa}$ and weight average, $\overline M _{\rm w} \approx 600\;{\rm kDa}$ ) were obtained with TEBA‐Cl. Lower temperatures and high feed ratios of TEBA‐Cl proved to be favorable for both high molecular weights and high fractions of cycles. Cyclic polycarbonates were detectable in the mass spectra up to 14 kDa (technical limit of the measurements). Low molecular weights in combination with unreacted chloroformate groups proved that the other PT‐catalysts were less efficient under the given reaction conditions.
MALDI‐TOF mass spectrum of the polycarbonate No. 3b .
magnified image MALDI‐TOF mass spectrum of the polycarbonate No. 3b .

Full text

Hydrolytic Polycondensation of
Bisphenol-A-Bischloroformate Catalyzed by
Phase-Transfer Catalysts
a
Hans R. Kricheldorf,*
1
Sigrid Bo
¨hme,
1
Gert Schwarz,
1
Claus-Ludolf Schultz
2
1
Institut fu
¨r Technische und Makromolekulare Chemie, Bundesstr. 45, D-20146 Hamburg, Germany
Fax: 040 42838 6008; E-mail: kricheld@chemie.uni-hamburg.de
2
BAYER AG, ZF-ZAU, Geba
¨ude R-79, D-47812 Krefeld, Germany
Received: April 11, 2003; Revised: June 24, 2003; Accepted: June 27, 2003; DOI: 10.1002/macp.200350028
Keywords: bisphenol-A; cyclic polycarbonates; interfacial polycondensation; phase-transfer catalysts
Introduction
The interfacial hydrolytic polycondensation of bisphenol-
A-bischloroformate (BABC) has recently found increasing
interest, because it allows syntheses of cyclic oligocarbo-
nates and cyclic polycarbonates in high yields.
[1– 10]
The
most widely used catalyst for such syntheses of cyclic oligo/
polycarbonates is triethylamine.
[1– 10]
Quite recently, we
have demonstrated
[11]
that other tert. amines such as pyri-
dine or 4-(dimethylamino)pyridine may also be useful when
the reaction conditions are accordingly optimized. Little is
known about the catalytic efficiency of phase-transfer cata-
lysts (PT) such as tetraalkylammonium or tetraarylphos-
phonium salts. Only in one paper,
[4]
Et
4
N

OH and Bu
4
N

Br were studied as potential catalysts, but no reaction
was observed. Those experiments were conducted accord-
ing to the pseudo-high dilution method at temperatures
in the range of 45 –50 8C. On the other hand, it is well
documented
[12– 17]
that PT catalysts are useful for syntheses
of polycarbonates from diphenols and monomeric, dimeric,
or trimeric phosgene. Therefore, it was the purpose of the
present work to provide more information about the influ-
ence of PT catalysts on the hydrolytic polycondensation of
BABC with focus on the formation of cyclic oligo- and
polycarbonates (Scheme 1).
Full Paper: The hydrolytic interfacial polycondensation
of bisphenol-A-bischloroformate was performed with four
different phase-transfer (PT) catalysts: N-butylpyridinium
bromide, triethylbenzylammonium (TEBA) chloride, tetra-
butylammonium hydrogen sulfate, and tetraphenylphospho-
nium bromide. These polycondensations were conducted at
5or358C initial reaction temperature. The resulting poly-
carbonates were characterized by viscosity and SEC mea-
surements and by MALDI-TOF mass spectrometry. The four
PT catalysts gave quite different results with respect to molec-
ular weight and formation of cyclic polycarbonates. The
highest molecular weights (number average, Mn215 kDa
and weight average, Mw600 kDa) were obtained with
TEBA-Cl. Lower temperatures and high feed ratios of TEBA-
Cl proved to be favorable for both high molecular weights
and high fractions of cycles. Cyclic polycarbonates were
detectable in the mass spectra up to 14 kDa (technical limit of
the measurements). Low molecular weights in combination
with unreacted chloroformate groups proved that the other
PT-catalysts were less efficient under the given reaction
conditions.
MALDI-TOF mass spectrum of the polycarbonate No. 3b.
1636 Macromol. Chem. Phys. 2003,204, 1636– 1642
Macromol. Chem. Phys. 2003,204, No. 13 DOI: 10.1002/macp.200350028 ß2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
a
Polymers of Carbonic Acid: Part 34.

Experimental Part
Materials: Bisphenol-A bischloroformate (BABC) was kindly
provided by Bayer AG (Uerdingen, Germany) and was used as
received. TEBA-Cl tetrabutylammonium hydrogen sulfate
(TBA-HS) and tetraphenylphosphonium bromide were pur-
chased from Aldrich Co. (Milwaukee, WI, USA) and used as
received. Pyridinium butyl bromide (BP-Br) was synthesized
by refluxing equimolar amounts of pyridine and butyl bromide
in dry toluene. It was isolated as a white solid which was dried
over phosphorus pentoxide in vacuo. Dichloromethane was
distilled over phosphorus pentoxide.
Butylpyridinium Bromide
Dry pyridine (1 mol) and butyl bromide (1.1 mol) were dis-
solved in dry toluene (1 L) and refluxed for 10 h with stirring.
Afterwards the reaction mixture was allowed to cool down with
stirring. The crystallized product was filtered off, washed with
toluene and dry diethyl ether, and dried in vacuo over P
4
O
10
.
Yield 81%, m.p. 96– 99 8C. The elemental analyses agreed
with the calculated values.
Polycondensations
A) With TEBA-Cl at þ58C (No. 3b, Table 1)
A solution of BABC (20 mmol) in dry dichloromethane
(200 mL) and a solution of NaOH (84 mmol) in water (200 mL)
were cooled to 4 –5 8C. TEBA-Cl (4 mmol) was then added to
the aqueous solution and both solutions were mixed with an
‘‘Ultraturrax’’ high-speed stirrer. Under external cooling with
an ice/NaCl mixture, the internal temperature had reached
15 8C after 10 min. After 10 min, the high-speed stirrer was
replaced by a normal stirrer (several hundred rpm’s) and the
stirring was continued for 50 min without cooling.
The organic phase was diluted with CH
2
Cl
2
(1 L), the water
phase was separated, and the organic phase was washed with
0.1 Nhydrochloric acid (twice) and water (three times). The
organic phase was dried with Na
2
SO
4
, concentrated to ca.
400 mL, and precipitated into methanol. Yield: 59%, Z
inh
¼
4.82 dL/g.
B) With Ph
4
PBr at 35 8C (No. 8, Table 1)
A solution of BABC (20 mmol) in dry dichloromethane
(200 mL) and a solution of NaOH (84 mmol) and Ph
4
PBr
(4 mmol) in water (200 mL) were heated to 35 8C and mixed
with an ‘‘Ultraturrax’’ high-speed stirrer. After 10 min, the
high-speed stirrer was replaced by a normal stirrer (running at
several hundred rpm’s) and the stirring was continued for
50 more min. with an external water bath thermostated at
35 8C. The reaction mixture was worked up as described above.
Yield: 67%, Z
inh
¼0.40 dL/g.
All other polycondensations were conducted analogously.
Measurements
The inherent viscosities were measured in CH
2
Cl
2
with an
Ubbelohde viscometer thermostated at 20 8C.
The MALDI-TOF mass spectra (m.s.) were recorded on a
Bruker Biflex III mass spectrometer equipped with a nitrogen
laser (l¼337 nm). All spectra were recorded in the reflec-
tion mode with an acceleration voltage of 20 kV. The irradia-
tion targetswere prepared from a CHCl
3
solutionusing dithranol
as matrix and K-trifluoroacetate as dopant. In selected cases,
the polymer/dithranol ratio was varied.
The 400 MHz
1
H NMR spectra were recorded on a Bruker
Avance 400 FT NMR spectrometer in 5 mm o.d. sample tubes
using CDCl
3
/TMS as solvent and shift reference.
The SEC measurements were performed on a Hewlett-
Packard HP 1050 apparatus in CH
2
Cl
2
at 30 8C. Five
Lichrogel
1
columns having pore sizes of 4, 40, 400 (2x), and
4000 A
˚were used. The elution curves were evaluated with a
triple detector ‘‘Viskotec-TDA 301’’ combined with the soft-
ware ‘‘Viskotec-Tri SEC’’.
Results and Discussion
Preparative Aspects
Four different PT catalysts were used and compared in
the present work, namely: butylpyridinium bromide, tri-
ethylbenzylammonium chloride (TEBA-Cl), tetrabutyl-
ammonium hydrogen sulfate and tetraphenylphosphonium
bromide. The hydrophilic character of these PT catalysts
decreases in the given order. With all four PT-catalysts ex-
periments were conducted at two different initial reaction
temperatures, namely 5 and 35 8C. Due to the exothermal
Scheme 1.
Hydrolytic Polycondensation of Bisphenol-A-Bischloroformate ... 1637
Macromol. Chem. Phys. 2003,204, No. 13 www.mcp-journal.de ß2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

character of the polycondensations, these initial reaction
temperatures increased by 10 28C in the course of
the experiments. The lower temperature was selected
because it was found in a previous study using triethyl-
amine as catalyst
[10]
that this relatively low temperature was
advantageous to obtain high molar mass and cyclic poly-
carbonates by interfacial hydrolysis of BABC. The higher
temperature was limited to 35 8C to avoid extensive eva-
poration of dichloromethane.
The reaction mixtures were worked up in such a way that
the interfaces of the phase-separation and washing proce-
dures were discarded to avoid contamination of the poly-
carbonates with sodium ions. Contamination with sodium
ions is unfavorable for MALDI-TOF m.s. because Na

-
doped linear polycarbonates of structure La (Scheme 1) are
difficult to distinguish from K

-doped cyclic polycarbo-
nates (Cin Scheme 1). This analytical problem is easily
eliminated by efficient K

-doping of the Na

-free poly-
carbonates prior to the MALDI-TOF measurements. The
work-up procedure used in this (and previous) studies has,
in turn, the shortcoming of low yields (although the yields of
crude Na-contaminated polycarbonates were above 90%).
A comparison of all four PT-catalysts with regard to the
inherent viscosities of the isolated polycarbonates gave
several unexpected results (Table 1). Firstly, when com-
pared at the low initial reaction temperature, only one
PT-catalyst, namely TEBA-Cl, gave high molar mass poly-
carbonates (Nos. 3a and 3b, Table 1). The reproducibility of
this good result and the reproducibility of the poor result
obtained with TBA-HS were checked (Nos. 5a and 5b).
This outcome is particularly interesting and conspicuous
because high molar mass polycarbonates were obtained
with both PT-catalysts in the interfacial polycondensation
of bisphenol-A with diphosgene.
[18]
Secondly, when the influence of the temperature was
studied, two catalysts (BP and TEBA) gave lower molecular
weights than the experiments performed at 5 8C, whereas
the other two catalysts yielded higher molar masses at the
higher temperature. An exhaustive explanation of these
seemingly contradictory temperature effects cannot be given
at this time, but a preliminary interpretation will be presen-
ted on the basis of the MALDI-TOF m.s. discussed below.
The good results obtained with TEBA-Cl as catalyst
prompted us to study the influence of the catalyst/BABC
ratio, because it was found for polycondensations of
bisphenol-A with diphosgene
[18]
that results depend very
much on the TEBA/bisphenol-A ratio. These polyconden-
sations were performed at 35 8C and the results were sum-
marized in Table 2. A clear trend was detectable. The
molecular weights dramatically increased with higher
Table 1. Hydrolytic polycondensations of BABC (Catalyst/BABC ¼4/20 mmol, NaOH/BABC ¼84/20 mmol, total reaction time 1 h)
catalyzed with various PT-catalysts at an initial temperature of 5/35 8C.
Exp. No. Catalyst Temp. Yield Z
inha)
Mei
b)
Reaction products
d)
8C % dL/g Da
1 Butylpyridinium bromide 5 56 0.26 <1 000 C, (La), Ld, Le
2 Butylpyridinium bromide 35 56 0.11 <1 000 C, (La), Ld, Le
3a TEBA-Cl 5 65 5.23 >15 000 C
3b TEBA-Cl 5 63 4.85
c)
>15 000 C
4 TEBA-Cl 35 55 2.00 >15 000 C
5a Bu
4
NHSO
4
5 51 0.36 2 700 C, Ld, Le
5b Bu
4
NHSO
4
5 59 0.28 <1 000 C, Ld, Le
6Bu
4
NHSO
4
35 65 1.80 >14 000 C
7Ph
4
PCl 5 50 0.26 2 400 C, Ld, Le
8Ph
4
PCl 35 67 0.40 6 500 C, Le
a)
Measured at 20 8C with c¼2 g/L in CH
2
Cl
2
.
b)
Masses where the peaks of cyclic and linear polycarbonates display equal intensities.
c)
SEC measurement with triple detection gave: Mn215 000 Da, Mw620 000 Da.
d)
Based on MALDI-TOF m.s.
Table 2. TEBA-Cl catalyzed hydrolytic polycondensations of
BABC with an initial temperature of 35 8C (NaOH/BABC ¼84/
20 mmol, reaction time 1 h). Variation of the catalyst/BABC ratio.
Exp.
No.
TEBA-Cl
a)
Yield Z
inhb)
Mei
c)
Reaction
products
d)
BABC % dL/g Da
1 1/20 53 0.36 3 000 C, Ld, Le
2 2/20 67 0.87 9 000 C, Ld, Le
3 4/20 55 2.00 >15 000 C
4 8/20 56 4.31 >15 000 C
5 12/20 58 4.80 >15 000 C
a)
In mmol.
b)
Measured at 20 8C with c¼2 g/L in CH
2
Cl
2
.
c)
Masses where the peaks of cyclic and linear polycarbonates
display equal intensities.
d)
Based on MALDI-TOF m.s.
1638 H. R. Kricheldorf, S. Bo
¨hme, G. Schwarz, C.-L. Schultz
Macromol. Chem. Phys. 2003,204, No. 13 www.mcp-journal.de ß2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

TEBA/bisphenol-A ratios and finally reached a value
(No. 5, Table 2) comparable to the best values obtained at
þ58C with a smaller amount of TEBA-Cl (Nos. 3a and 3b,
Table 1). In this connection it should be mentioned that
sample No. 3b, Table 1, was characterized by a SEC mea-
surement evaluated with a triple detection (Figure 1). A
number average molecular weight (Mn) of 215 10
3
Da and
a weight average molecular weight (Mw) of 620 000 Da
were found, proving that the high viscosity values indeed
represent high molar masses. The results summarized in
Table 2 also suggest that high molecular weights (and high
fractions of cycles) might be obtained with other PT-
catalysts, provided that the reaction conditions (e.g. feed
ratio and temperature) are optimized for every individual
PT-catalyst.
Mechanistic Aspects
The MALDI-TOF m.s. of all polycarbonates prepared
in this work displayed the peaks of cyclic oligo- and
polycarbonates. Yet, as exemplarily illustrated in
Figure 2–4, the content of cycles varied over a broad
Figure 1. SEC elution curve of the polycarbonate No. 3b, Table 1.
Figure 2. MALDI-TOF mass spectrum of the polycarbonate No. 1, Table 1.
Hydrolytic Polycondensation of Bisphenol-A-Bischloroformate ... 1639
Macromol. Chem. Phys. 2003,204, No. 13 www.mcp-journal.de ß2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

range. In agreement with previous studies on polyester
[19]
and polycarbonates,
[10,11,18,20]
the overriding trend was
higher fractions of cycles with increasing molecular weight.
The MALDI-TOF m.s. of the high molecular weight
samples Nos. 3a,3b, and 4, Table 1, or Nos. 3–5, Table 2,
exclusively exhibited the mass peaks of cyclic oligo- and
polycarbonates (Figure 3 and 4). In order to illustrate such
trends by a simple number, the Mei values were listed in
Table 1 and 2. These Mei values are defined as the molar
mass where the peaks of cycles and linear species (regard-
less of their end-groups) show equal intensities. This defini-
tion is based on the fact that the cycles are the predominant
class of oligomers and polymers in the low molar mass
range, whereas the linear chains dominate the high molar
mass range.
From polycarbonates having inherent viscosities
2.0 dL/g MALDI-TOF mass spectra were obtained
displaying mass peaks up to 14 000 Da. In the case of
samples 4and 6of Table 1 or sample No. 3of Table 2, only
peaks of cycles were detected in this mass range. When
the molecular weights were higher (Nos. 3a þ3b, Table 1,
and Nos. 4þ5, Table 2), the observable mass range of
the MALDI-TOF m.s. was limited to 9 000 Da. The high
content of high molar mass chains which did not ‘‘fly’’
reduced the signal-to-noise ratio significantly, as exem-
plarily illustrated in Figure 3. To find out, if cycles of
higher masses can be detected after fractionation, part
of sample 3b, Table 1, was subjected to fractionation by
SEC with preparative columns. The low molar mass part
was subdivided into twelve fractions, as illustrated in
Figure 1. Unfortunately, only the fractions 10– 12 gave
satisfactory MALDI-TOF m.s. The best m.s. obtained from
fraction 11 is displayed in Figure 4 and proves the exclusive
formation of cyclic polycarbonates up to masses around
14 000 Da. The high molar mass wing of this fraction covers
the mass range of fraction 10 and part of the mass range
of fraction 9 indicating that the cycles also dominate in
those fractions.
The analyses of mass peaks of linear chains provided the
following interesting information. Chains terminated by
two OH-end-groups (La in Scheme 1) resulting from the
hydrolysis of chloroformate groups were almost absent in
all samples of this work. The prevailing type of linear chains
had two methylcarbonate end-groups (Le in Scheme 2) and
another class of linear chains possessed one methylcarbo-
nate chain end (Ld in Scheme 2). The existence of methyl-
carbonate end-groups in low molar mass polycarbonates,
such as No. 1, Table 1 (Figure 1), was evidenced by
1
HNMR
spectra which displayed a singlet signal at 3.90 ppm typical
for aromatic methylesters or methylcarbonate groups. The
formation of methylcarbonate end-groups can be explained
by the precipitation of polycarbonates having chlorofor-
mate end-groups (Lb and Lc in Scheme 2) into methanol.
When the crude polycarbonates were precipitated into
moist diethyl ether and stored overnight prior to filtration,
the MALDI-TOF m.s. revealed strong peaks of La chains,
whereas Ld and Lcchains were absent. These results demon-
strate that it is highly useful and informative to precipitate
the crude polycarbonates into methanol, because the forma-
tion of La chains does not tell, if the hydrolysis occurred
Figure 3. MALDI-TOF mass spectrum of the polycarbonate
No. 3b, Table 1.
Figure 4. MALDI-TOF mass spectrum of fraction 1b of the
polycarbonate No. 3b, Table 1.
1640 H. R. Kricheldorf, S. Bo
¨hme, G. Schwarz, C.-L. Schultz
Macromol. Chem. Phys. 2003,204, No. 13 www.mcp-journal.de ß2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

during the polycondensation or in the course of the work-up
procedure.
Altogether the MALDI-TOF m.s. clearly prove that
most PT-catalysts were rather inefficient in hydrolyzing
the chloroformate groups under the conditions used for
the experiments of Table 1. Efficient chain growth and
cyclization require 50% of the chloroformate groups to
be hydrolyzed (Scheme 3). Apparently, the transport of
OH ions into the organic phase (which is in principle
known from PT catalysis in organic chemistry)
[21]
is
inefficient, and is thus the rate determining step. The
results documented in Table 2 and the results obtained
from the direct phosgenation of bisphenol-A using
PT-catalysts indicate that larger amounts of PT catalysts
(i.e. >20 mol-% relative to BABC or bisphenol-A) are
required for a satisfactory hydrolysis and polycondensa-
tion process. In the case of tert. amine type catalysts, such
as triethylamine or 4-(N,N-dimethylamino)pyridine, the
mechanism is different. This amine can react with
chloroformates yielding acylammonium ions which are
hydrophilic (Scheme 4). Therefore, such tertiary amines
efficiently catalyze the hydrolysis of chloroformate
groups, and relatively low amine/BABC ratios are
beneficial for the entire polycondensation process.
[18,20]
Nonetheless, the results obtained in this work prove for
the first time, that PT-catalysts may be useful for the
preparation of high molar mass polycarbonates and
high fractions of cycles via interfacial hydrolytic poly-
condensation of BABC.
Scheme 2.
Scheme 3.
Scheme 4.
Hydrolytic Polycondensation of Bisphenol-A-Bischloroformate ... 1641
Macromol. Chem. Phys. 2003,204, No. 13 www.mcp-journal.de ß2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Conclusion
Although the use of PT-catalysts in interfacial polycon-
densations of diphenols with phosgene or diphosgene has
been described by several research groups, a detailed study
dealing with the role of PT-catalysts in the hydrolytic poly-
condensation of BABC was missing. A few preliminary
results obtained with the pseudo-high dilution method at
45–50 8C were totally negative.
[3]
The results of this work
clearly demonstrate that high molar mass polycarbonates
can be obtained from PT-catalyzed hydrolytic polyconden-
sations of BABC when the structure of the catalyst and the
reaction conditions are optimized. In agreement with our new
theory of kinetically controlled polycondensations,
[19,22]
the fraction of cycles increases when the reaction conditions
favor high molecular weights.
Acknowledgement: We wish to thank Dr. R. Wehrmann (Bayer
AG, Uerdingen) for a generous gift of chemicals.
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