Synthesis of Hydroxyl-TerminatedPolycarbonates of

Controlled Number-Average Molecular Weight zy

zyxwv
J. S. RIFFLE, E. SHCHORI,* A. K. BANTHIA,? R. G. FREELIN,Z T. C.
WARD, and J. E. MCGRATH,**Department of Chemistry and Polymer
Materials and Interfaces Laboratory, Virginia Polytechnic Institute and
State University, Blacksburg, Virginia 24061

Synopsis
Hydroxyl-terminatedpolycarbonates are important starting materials for the synthesis of multiblock
copolymers. Earlier papers from our laboratory and elsewhere have demonstrated their utility in

zy
zyxw
siloxane, aryl ether, and ester systems. One synthesis problem that must be addressed is the control
of the number-average molecular weight and hence block size of the polycarbonate oligomeric pre-
cursor. The facile phosgene-hydroxyl reaction is often difficult to monitor precisely. The present
article describes a novel, simple, and convenient technique for the synthesis of hydroxyl-terminated
polycarbonates of well-controlled number-average molecular weight. The approach involves an
in situ blocking of some of the phenolic groups either prior to or during phosgenation. The protecting
group are easily removed after the polymerizationis complete. In a practical laboratoq experiment
the technique does not require any additional step beyond that necessary for the preparation of
nonfunctional polycarbonates of controlled molecular weight. The method is demonstrated in this
article with the polycarbonate of bisphenol-A via the use of trimethylchlorosilane, trifluoroacetic
anhydride, and trifluoroacetic acid as blocking groups. Ultraviolet, lgF-NMR, and 'H-NMR

zy
measurements as well as vapor-pressure osmometry were used to characterize the oligomers.

INTRODUCTION
Hydroxyl-terminated polycarbonates are interesting building blocks for
making block ~opolymers.~"J0-13 One synthesis problem which must be ad-
dressed, however, is the control of number-average molecular weight (M,) and,
hence, block size of the oligomeric polycarbonate precursor. Methods previously
utilized to obtain this control have included (1)the use of a crystalline bisphe-
nol-bischloroformateintermediate, and (2) direct phosgenation monitored by
a spectrophotometric technique.9 In the past it has been difficult to achieve
desired molecular weights usingthe facile direct phosgenation route. This article
describes a novel, convenient technique for the attainment of reactive polycar-
bonates of well-controlled (M,) using this simpler direct phosgenation method
of synthesis.
The approach involves blocking of a desired number of the phenolic end groups
with various protecting groups prior to phosgenation. These protecting groups

* Present address: Membrane Filtration Technology, Ltd., Kiryat Weizmann P.O.B. 2057, Re-
hovot 76200 Israel.

zyx
t Present address: Department of Materials Science, Indian Institute of Technology, Kharagpur
721302 India.
Present address: Film Division, 3M Center, St. Paul, MN 55144.
* * To whom all correspondence should be addressed.

Journal of Polymer Science: Polymer Chemistry Edition, Vol. 20,2289-2301 (1982)

0 1982 John Wiley & Sons, Inc. CCC 0360-63761821082289-13102.30
 zyxw
zyxwv
2290 RIFFLE ET AL.

zyxw
are designed to produce bonds that are easily removed after the phosgenation
is complete. The technique is demonstrated herein with polycarbonates derived
from bisphenol-A and using trimethylchlorosilane (TMCS, l),trifluoroacetic

zyxwvu
anhydride (TFAA, 2), and trifluoroacetic acid (TFA, 3), as the blocking re-
agents:
0
II 0
CF3--C II
CH,--Si-Cl 0‘ CF3-C-OH
I CF3-C
/
CH:, II
0
1 2 3

EXPERIMENTAL
The suggested synthetic method differs slightly depending on the blocking
reagent used. However, the procedures using trifluoroacetic anhydride and
trifluoroacetic acid are identical with the exception of the calculation for the

zyxw
required amounts of the blocking agents themselves. Hence, a typical procedure
using trimethylchlorosilane is provided separately whereas the fluorinated re-
agents are combined.

zyxwvu
Preparation of Polycarbonates Utilizing Trimethylchlorosilane for
Control of ( M , )
High-purity bisphenol-A (usually Union Carbide high-purity UCAR grade)
was used as received. Fisher practical-gradeTMCS was distilled over P205 prior
to use.
The apparatus consisted of a three-necked, round-bottomed flask equipped

zyxwvut
with a mechanical stirrer, combined argon and phosgene inlet (the gases must
of course be bubbled through the reaction mixture, not into the air space above
the solution), and condenser connected to a caustic trap. The argon-purged
reaction vessel is charged with the bisphenol-A together with an approximately
equal amount of dry tetrahydrofuran. The required amount of TMCS [deter-
mined by ( X , ) N 2 / X , where ( X , ) is the degree of polymerization (number
of repeat units per chain) and X is the molar ratio of blocking reagent to bis-
phenol-A] is added and the mixture is stirred at room temperature for 15-30 min.
Pyridine (ca. 2 mol per mole bisphenol-A) and enough methylene chloride to
make up a ca. 10%solution are added and the phosgene gas is introduced. After
formation of the polycarbonate, water is slowly (care!) added to decompose excess
phosgene. The solution is fmt washed with dilute hydrochloric acid. Hydrolysis
of the trimethylsilyl ether end groups is achieved by clarifyingthe organic phase
with 10-15% of methanol, adding 1cm3concentrated aqueous HC1/100 mL so-
lution, and subsequently stirring the homogeneous mixture for one hour. After
this, the solution is again washed twice with water. Finally, the polymer is iso-
lated by coagulation in isopropanol followed by filtration and vacuum drying
to a constant weight at ca. 80°C.
 HYDROXYL-TERMINATED POLYCARBONATES

zyxwvut
2291

zyxwvutsrqp TABLE I
Synthesis of Bisphenol-A Polycarbonates of Controlled Molecular Weights

zy
Blocking la (M")
reagent x Solvent Theoretical Experimental
TMCS 10.73 CHzClJpyridine 5,450 5,300
TMCS 19.70 CHzClJpyridine 10,010 9,100
TMCS 19.70 THF/CHZClJpyridine 10,010 11,900
TMCS 40.87 THF/CHzClJpyridine 20,760 20,600
TMCS 9.84 THF/CHzClJpyridine 5,000 5,500

TFAA 11.34 THF/CHzClDEAb/pyridine 2,880 3,000

TFAA 19.68 THF/CH2ClflEAb/pyridine 5,000 5,800
TFAA 59.0 THF/CHzC1flEAb/pyridine 15,000 16,000
TFAA 12.36 THF/pyridine 3,140 2,268

zy
TFAA 10.30 THF/pyridine 2,617 2,679

zyxwvut
zyxwv
TFA 5.0 THF/CH&lflEAb/pyridine 2,540 2,700
TFA 19.33 CHzCldpyridine 9,820 9,100
TFA 5.92 THF/pyridine 3,000 2,658
TFA 11.06 THFKHzClJpyridine 5,600 6,000
TFA 21.93 THFKHzClJpyridine 11,140 10,900
a 1/X= molar ratio of bisphenol-A to blocking,reagent.
b TEA is Triethylamine. These experiments were done using THF/TEA in a prestep to poly-
merization analogous to that used with TMCS.

The procedure described below utilizing trifluoroacetic anhydride or trifluo-

roacetic acid shows THF as the solvent. It should be noted that dichloromethane
can be substituted as the solvent and that several of the reactions listed in Table
I were done in dichloromethane.

Preparation of Polycarbonates Utilizing TrifluoroaceticAnhydride or

TrifluoroaceticAcid for Control of ( Mn)
Trifluoroacetic anhydride obtained from Eastman was freshly distilled prior
to use. Trifluoroacetic acid obtained from Aldrich Chemicals (purity 99%)and
high-purity bisphenol-A (Union Carbide high-purity UCAR grade) were used
as received.
The required apparatus consisted of a three-necked, round-bottomed flask
equipped with a mechanical stirrer, combined argon and phosgene inlet, and
Dean Stark trap topped with a condenser connected to a caustic trap. A silicone
oil bath is used for heating. The reaction vessel is charged with a ca. 10% solution

zyxw
of bisphenol-A dissolved in tetrahydrofuran and the pyridine (4 moles per mole
bisphenol-A). Methylene chloride is added in the amount of approximately 3wo
(v/v) of the volume of tetrahydrofuran used and then is subsequently distilled
off to dehydrate the system. The required amount of trifluoroaceticanhydride
(determined by (X, ) = 1/X, where X is the molar ratio of blocking reagent to
bisphenol-A) or trifluoroacetic acid (determined by ( X , ) = 2 / X ) is added. The
justification for using these expressions is presented below. Phosgene (three
times the stoichiometric amount) is introduced over a 45-min period, then the
mixture is allowed to stir under argon for an additional hour. After polymer-
ization has been completed, water is slowly added to decompose residual phos-
 zyxw
zyxw
2292 RIFFLE E T AL.

zyxwv
gene, the solution is washed several times with dilute (ca. 3%)hydrochloric acid,
then with water several more times. Finally, the polymer is coagulated in
methanol, isolated by filtration, and dried to a constant weight under vacuum
at approximately 80°C.

zyx
Molecular weights (M,) of the oligomers after hydrolysis of the protecting
groups were assessed via a UV spectrophotometric technique: using either a
Hitachi model 100-60or a Perkin-Elmer 552 instrument.
High-resolution proton NMR of the TMCS-derivatized polymers and 19F-
NMR of trifluoroacetyl-terminated compounds were performed using a Varian
EM-390 spectrometer at 84.7 MHz and 3OOC. 1,2-Difluorotetrachloroethane
used for the 19Flock signal and reference was obtained from Peninsular Chemical
Research, Lancaster, PA. Fisher certified-grade (99 mol % pure) methylene
chloride was used as the proton NMR lock and reference signal. Samples used
for lgF-NMRspectra were @en directly from the reaction mixture. Therefore,
solvent and base concentrations used for 19Fanalysis were those utilized in the
reaction procedure. Molecular weight distributions were assessed by gel per-
meation chromatography (Waters 6000-A pump, Waters R-400 D.R.I. detector,
Waters styragel columns, dioxane solvent).

RESULTS AND DISCUSSION

The total reaction for formation of the reactive polycarbonates of controlled
molecular weights can be envisioned as occurring in three steps [formulated in
eqs. (I),(II),and (III), respectively, using TMCS as the blocking reagent]: (1)
capping of the desired number of phenolic end groui)s, (2) formation of the

zyxwvu
polymer, and (3) hydrolysis of the protecting groups (shown on following page).
A basic assumption for calculating the theoretical degree of polymerization is
of course that only one hydroxyl group on any bisphenol-A molecule is protected.
The success of the method can be explained using step polymerization theory1*
where phosgene and bisphenol-A are considered to be in perfect stoichiometry
and 4,

zyxwvut
written as
CH,

for example, is considered to be the monofunctionalmolecular weight regulator.

For this case, r , the parameter describing stoichiometric imbalance, can be

+
F = NA/(NB” ~ N B ’ ) (1)
The terms to be used are defined as follows: NB’ is the number of moles of tri-
methylsilyl-capped bisphenol-A, 4 (note that this equals the number of moles
of end-capping reagent); NB the number of phenolic end groups before reaction
of the end-blocking reagent; NB” the number of phenolic end groups on un-
capped bisphenol-A after reaction of the end-blocking reagent; and N A the
number of moles of chlorine on the theoretical amount of phosgene needed.
Since
+ +
(X,)= (1 r)f(l r - 2rp) (2)
HOa--I-@OH
-
zy
zyxwvuts
HYDROXYL-TERMINATED POLYCARBONATES

+ H,C-Si-ClI
I
THF z 2293

HOa-#-@Cd-Cf13

I zyxw -CH,Cl,

rr

as p
zyx
zyxwvuts
zyxwv
- 1, i.e., “complete”phosgenation, the expression reduces to
( X , ) = (1 + r M 1 - r ) (3)
It is necessaryto point out that in order to react nearly all of the phenolic groups
present gaseous phosgene must be introduced to the reaction vessel in amounts
far exceeding theory (i.e., about three times the calculated amount). Most of
 zyxw
zyx
zyxw
zyxwvutsr
zyx
zyxwvuts
2294 RIFFLE ET AL.

zyxwv
the excess is no doubt due to physical loss plus interaction with residual moisture
in the system. Moreover, the “classical” equation shown above can be reduced

zyxw
to a useful simplified expression. It should be recognized that in eq. (4) which
defines r
NA = NB” = NB - ~ N B ’
Substituting these equalities into the expression for r [eq. (411, we obtain
t = (NB- NB’)/NB
The degree of polymerization (Xn) is defined as the number of repeat units per
(4)

(5)

chain (i.e., not counting the bisphenol-A terminal unit). The expression for the
limiting value of (X,) is then given by

Furthermore, if the final bisphenol-A terminal unit in the polymer is also in-
cluded, then the expression becomes simply
xn=----
- NB - (2) (moles bisphenol-A)
(7)
NB’ (moles monofunctional capping reagent)

difunctional capping reagents, i.e.,

- 212 1
xn=---
x -x
zyx
where x is the ratio of moles monofunctional capping reagent to moles bisphenol
A. This final form of the calculation can be easily adapted to accommodate

When TMCS is used as the blocking reagent, three separate steps probably
occur consecutively as formulated in eqs. (1)-(111). This is postulated even
though the total reaction is performed in one pot and no intermediates need be
isolated. Number-average molecular weights of the final hydroxy-functional

Fig. 1. Proton NMR spectrum of trimethylsilyl-ether-cappedbisphenol-Apolycarbonate.

 HYDROXYL-TERMINATED POLYCARBONATES 2295 z

m zyxwv
zyxwv
zyx 293 270 233 (m.)

zy
Fig. 2. UV spectrum of hydroxyl-terminatedbisphenol-Apolycarbonate.

zyx
oligomers prepared using TMCS are in close agreement with the theoretical
values in all cases (see Table I). The quantitative efficiency of the hydrolysis
procedure used is demonstrated by a comparison of end-group analyses per-
formed with proton NMR and ultraviolet ~pectroscopy.~ A ratio of proton NMR
peak integrals (integral of silicon methyl peak to that of aromatic peak) of a
trimethylsilyl ether-terminated oligomer which was coagulated in isopropanol
prior to hydrolysis of the end groups yielded an (M,) of 2430 g/mol (see Fig. 1).
It is expected that the workup procedure used for preparation of the trimeth-
ylsilyl ether-terminated polymer sample would have extracted any excess TMCS
or its by-products. Therefore, all silicon methyl groups in the NMR spectrum
should have been in the form of polycarbonate end groups. Following subse-
quent acid-catalyzed hydrolysis of the end groups, the silicon methyl peak was
observed to disappear from the NMR spectrum and UV analysis of the hydroxyl
groups yielded a quite similar (M,) figure of about 2,300 g/mol (see Fig. 2).
Both trifluoroacetyl capping reagents provide a much simpler experimental
procedure than that using trimethylsilylchloride. Neither a capping step prior
to polymerization nor a “posthydrolysis step” is necessary twing these reagents.
19F-NMRanalysis of the reaction mixture before and after phosgene addition
indicates that, under the reaction conditions, trifluoroacetyl esterificationtakes
place during polymerization (i.e., only in the presence of phosgene and not in
a “precapping step”). Therefore, a step analogous to that depicted in eq. (I)is
not required. Trifluoroacetyl “caps” are easily removed in the normal workup
procedure for the polymer, which includes the addition of water followed by
coagulation in methanol. Hence almost no additional procedures beyond those
necessary for ordinary polycarbonate synthesis are necessary in order to produce
accurate molecular weight control.
2296 zyxw
zyxwvutsr
zyxwvuts
1
zyxwvut
zyxwv u -$-$-U
RIFFLE ET AL.

-7.9 P.P.M

*
ESTER zyxwvu
-7.00 P.P.tl

to coc12.

In order to show that esterification was indeed taking place during polymer-
ization, lgF-NMRspectra were run on the reaction mixtures before, during, and
after phosgene addition. In agreement with Dorn et al.l0 (spectra in the article
cited were run in deuterochloroform), the trifluoroacetyl ester of bisphenol-A
in dichloromethane appears at -7.42 ppm from 1,2-difluorotetrachloroethane.
zy
Fig. 3. lSF-NMR spectra of the polycarbonate capping reaction with trifluoroacetic acid prior

It resonates at slightly higher field in tetrahydrofuran (-7.64 to -7.67 ppm) (see

Figs. 3 and 4). The direction and approximate magnitude of the solvent shift
in tetrahydrofuran is also in agreement with literature values.ll l9F-NMR
spectra of reaction mixtures using each of the two fluorine-containing blocking
reagents were run immediately after addition of the reagents. In both cases,
only one fluorine peak appeared which resonates at -7.93 to -7.94 ppm (pre-
sumably due to some type of pyridine complex with the blocking reagents). A
spectrum of the sample taken from the trifluoroacetic anhydride reaction was
checked again after a two-day period and no changes were observed. Hence it
was evident that the desired ester was not forming at this stage in the reaction.
A second l9F-NMR spectrum of the reaction mixture using trifluoroacetic acid
was run after the stoichiometric amount of phosgene had been introduced (see
Fig. 3). Both ester and a new fluorine-containingcomplex or reagent (resonating
at -8.13 ppm) were present, whereas the original peak at -7.93 to -7.94 ppm
had disappeared. Finally, after three times the stoichiometric amount of
phosgene had been bubbled through the reactions, the spectra showed only one
 HYDROXYL-TERMINATED POLYCARBONATES 2297

zyxwvut
zyxwvutsrq
zyxwvut
zyx
Fig. 4. 19F-NMR spectra of the polycarbonate capping reaction with trifluoroacetic anhydride
prior to COC12.
peak,the ester, to be present in both cases (using either trifluoroaceticanhydride
or trifluoroacetic acid as blocking agents) (see Figs. 3 and 4).
Hydrolysis of these protecting groups in the workup procedure was demon-
strated by the disappearance of the lgF-NMRpeak coupled with the growth of
the shoulder due to the phenolic end groups at 287 nm in the UV s p e ~ t r u m . ~
Although the mechanism of trifluoroacetylation is not understood, two plau-
sible routes [schematically given in eqs. (IV) and (V)] using trifluoroacetic acid
as the blocking reagent can be envisioned. Note that these equations are sepa-

zyxw
<Mn> =
10,900

<Mn> =
6000

Fig. 5. GPC of polycarbonates prepared with trifluoroacetic acid as molecular weight regu-
lator.
2298 zyxw
zyxw
zyxwvutsr RIFFLE ET AL.

zyxw
rated on the basis of the first step (i.e., whether it is the acid or the bisphenol
which first reacts with phosgene) (shown on following page): Similar routes to
the ester utilizing trifluoroacetic anhydride are shown in eqs. (VI) and (VII).
For the case of the reaction of an acid halide with a phenol, previous investi-
gators have rejected12and acceptedl3 the mechanism proceeding through the
acid chloride intermediate [eq. (IV) route 21, mainly on the basis of whether or
not carboxylic anhydrides appeared in the infrared spectra of their products.
The assumption made was that if acid chloride intermediates had been present,
then carboxylic anhydrides should exist in the product. This reasoning, however,
cannot be applied to the work described here since it has been shown that even
if trifluoroacetic anhydride were being formed, it would not survive the reac-
tion.

0
II
CF,-C--OH + Cl--C--CI
0
II
0
- CF3-C-eC-Cl
0
ll
0
0
zyx
Molecular weight distribution curves of polymers synthesized by these
methods were assessed using gel permeation chromatography. It should be noted

II

0
U
HO*/? 2.-\

0
II
cF3-C-Cl
I zyx (IV)

0 0
II H O W O - ! ! - C I

0 0

3Fc-!!H
-O
0* (V)
HYDROXYL-TERMINATED POLYCARBONATES

O-Y
T

6
8
-x zyx
zyxwvutsrq
zyx
zyx
$ zyxw
0--u

O-Y
8
x

zyxw
dI
6
u
I
O=V

0"
9
z9

----t
I
I
I

"=Y
I
0

6
2299 zy

8
X

0I

"=Y
2300 zyxwvutsr
zyxw
zyxw
zy OH+
RIFFLE ET AL.

0
I
Cl-C-CI-
0

of approximately Gaussian shape result (see Fig. 5). zyxw

that these distributions were of particular interest in the cases of polymers
prepared with the fluorinated reagents since polymerization and hydroxyl
capping had occurred somewhat simultaneously. In all cases, unimodal curves

Table I shows a comparison of theoretical and experimental number-average

molecular weights obtained using the three different blocking reagents and
various solvent combinations. Calculated values are in reasonable agreement
with experimentally obtained values in all cases. No advantage was observed
in using either tetrahydrofuran or dichloromethanewith any of the three blocking
groups. It was established, however, that a step prior to polymerization was not
useful with either fluorine-containing reagent. For this reason, coupled with
the fact that trifluoroacetyl groups are hydrolyzed in the normal polycarbonate
workup procedure, one may conclude that the method using the fluorinated re-
agents is easier and less time consuming than the procedure using TMCS. In
the future, it would be of interest to evaluate the use of the methods described
here for the production of polycarbonates of differing structures.
The authors would like to thank the National Science Foundation, Polymers Program, for partial

zyx
support of this research under grant DMR-8022545.

References
1. T.C. Ward, A. J. Wnuk, E. Shchori, R. Viswanathan, and J. E. McGrath, in Multiphase
Polymers, S. L. Cooper and G. M. Estes,Eds., Am. Chem. SOC.Adv. Chem. Ser., No. 176,American
Chemical Society, Washington, DC, 1979,p. 293.
2. A. Noshay and J. E. McGrath, Block Copolymers, Overview and Critical Survey, Academic,
New York, 1977,pp. 335-354,416-423.
3. R. P. Kambour, J. E. Corn, S. Miller, and G. E. Niznik, J. Appl. Polym. Sci., 20, 3275
(1976).
4. J. E.McGrath, T. C. Ward, E. Shchori, and A. J. Wnuk, Polym. Eng. Sci., 17,647 (1977).
967-998 (1981).
HYDROXYL-TERMINATED POLYCARBONATES zyx
zy
zyx 2301

5. J. S. Riffle, R. G. Freelin, A. K. Banthia, and J. E. McGrath, J. Macromol. Sci. Chem., A15,

6. J. E.McGrath, J. S. Riffle, D. W. Dwight, T. F. Davidson, D. C. Webster, and A. K. Banthia,

Macromolecules, manuscript in preparation.

(1980).

zyxwv
7. S. Tang, E.A. Meinecke, J. S. Riffle,and J. E. McGrath, Rubber Chem. Technol.,53,1160-1170

8. J. S.Riffle, Ph.D. Thesis, Virginia Polytechnic Institute and State University, 1981.
9. E.Shchori and J. E. McGrath, J. Appl. Polym. Sci.Appl. Polym. Symp., 34,103 (1978).
10. P. Sleevi, T. E. Glass, and H. C. Dorn, Anal. Chem., 51,1931 (1979).

zyxwvu
11. M. S.Sleevi, Thesis, Virginia Polytechnic Institute and State University, 1979.
12. E. P. Goldberg, S. F. Strause, and H. E. Munro, Polym. Prepr. Am. Chem. SOC. Diu. Polym.
Chem., 5,233 (1964).
13. D. C. Prevorsek, B. T. Dibona, and Y. Kesten, J. Polym. Sci. Polym. Chem. Ed., 18.75
(1980).
14. G. Odian, Principles of Polymerization, 2nd ed., Wiley, New York, 1981.

Received February 1,1982

Accepted April 2,1982