Concerning the mechanism of the ring opening of propylene oxide in the copolymerization of propylene oxide and carbon dioxide to give poly(propylene carbonate)

Article abstract of DOI:10.1021/ja0394232

The reactions between (TPP)AlX, where TPP = tetraphenylporphyrin and X = Cl, O(CH₂)₉CH₃, and O₂C(CH ₂)₆CH₃, and propylene oxide, PO, have been studied in CDCl₃ and hav

Full text of DOI:10.1021/ja0394232

Published on Web 08/14/2004
Concerning the Mechanism of the Ring Opening of Propylene
Oxide in the Copolymerization of Propylene Oxide and
Carbon Dioxide To Give Poly(propylene carbonate)
Malcolm H. Chisholm* and Zhiping Zhou
Contribution from the Department of Chemistry, The Ohio State UniVersity,
100 West 18th AVenue, Columbus, Ohio 43210
Received November 4, 2003; E-mail: chisholm@chemistry.ohio-state.edu
Abstract: The reactions between (TPP)AlX, where TPP ) tetraphenylporphyrin and X ) Cl, O(CH2)9CH3,
and O2C(CH2)6CH3, and propylene oxide, PO, have been studied in CDCl3 and have been shown to give
(TPP)AlOCHMeCH2X and (TPP)AlOCH2CHMeX compounds. The relative rates of ring opening of PO follow
the order Cl > OR > O2CR, but in the presence of added 4-(dimethylamino)pyridine, DMAP (1 equiv), the
order is changed to O2CR > OR. From studies of kinetics, the ring opening of PO is shown to be first order
in [Al]. Carbon dioxide inserts reversibly into the Al-OR bond to give the compound (TPP)AlO2COR, and
this reaction is promoted by the addition of DMAP. The coordination of DMAP to (TPP)AlX is favored in
the order O2C(CH2)6CH3 > O2CO(CH2)9CH3 . O(CH2)9CH3. The microstructure of the poly(propylene
carbonate), PPC, formed in the reactions between (TPP)AlCl/DMAP and (R,R-salen)CrCl and rac-PO/S-
PO/R-PO and CO2, has been investigated by 13C {1H} NMR spectroscopy. The ring opening of PO is
shown to proceed via competitive attack on the methine and methylene carbon atoms, and furthermore
attack at the methine carbon occurs with both retention and inversion of stereochemistry. On the basis of
these results, the reaction pathway leading to ring opening of PO can be traced to an interchange associative
mechanism, wherein coordination of PO to the electrophilic aluminum atom occurs within the vicinity of the
Al-X bond (X ) Cl, OR, O2CR, or O2COR). The role of DMAP is two-fold: (i) to labilize the trans Al-X
bond toward heterolytic behavior, and (ii) to promote the insertion of CO2 into the Al-OR bond.
Introduction
The discovery of the heterogeneous-catalyzed copolymeri-
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by extensive studies of heterogeneous systems, although none
was sufficiently efficient to bring PPC to the market place.19-52
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10.1021/ja0394232 CCC: $27.50 © 2004 American Chemical Society

Ring Opening of Propylene Oxide
A R T I C L E S
Scheme 1. The Monometallic Mechanism Proposed for the
CHO/CO2 Copolymerization Reaction by Darensbourg
[Reproduced with Permission from Ref 54. Copyright 2002
American Chemical Society]
More recently, homogeneous systems have been developed,53-58
and, as a result of the creative and insightful independent work
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J. AM. CHEM. SOC. VOL. 126, NO. 35, 2004 11031

A R T I C L E S
Chisholm and Zhou
Figure 1. 1H NMR (500 MHz, CDCl3) spectrum of PO ring-opened product by (TPP)AlCl, showing two regioisomers.
Table 1. Catalyst Systems Showing Differing Capabilities in PO
In an attempt to clarify this matter, we have examined the
stereochemistry and kinetics of the initial ring-opening event
of PO by various aluminum porphyrin and salen initiators. We
also examined the influence of the promoter DMAP on this
event and on the insertion of carbon dioxide into the Al-OR
bond. We describe herein these findings.
Homopolymerization and PO/CO2 Copolymerization Reactions, as
Well as CO2 Incorporation
CO incorporation
2
temp
(°C)
PO
in PO/CO
2
copolymerizationd
c
catalystsa
cocatalystsb
homopolymerization
(TPP)AlCl
(TPP)AlCl
(TPP)AlCl
(TPP)CrCl
(TPP)CrCl
(salen)CrCl
(salen)CrCl
zinc glutarate
Union Carbide
25
25
25
25
25
25
25
60
25
yes
yes
yes
yes
yes
noe
no
0.13
0.43
0.48
0.07
0.40
0.49
0.49
0.50
no
EtPh3PBr
DMAP
Results and Discussion
Studies of the Initial Ring Opening of Propylene Oxide.
The reactions between (TPP)AlX initiators, where TPP )
tetraphenylporphyrin and X ) Cl, O(CH2)9CH3, and O2C(CH2)6-
CH3, and PO give the ring-opened products (TPP)AlOCHMe-
CH2X as the major products at short reaction times (Experi-
mental Section in Supporting Information). Because of the large
magnetic anisotropy of the porphyrin ring,104,106-108 the proton
resonances of signals of CH, CH2, and Me groups within ca. 5
Å of the Al center appear notably shifted upfield relative to
Me4Si. Consequently, it is relatively easy to monitor these
DMAP
DMAP
yes
yes
a The polymerization reactions were carried out in neat PO (2 mL),
1:1500 molar ratio used for (TPP) and (salen) metal catalyst systems, and
0.1 g of catalysts used for zinc glutarate and Union Carbide systems.105
b One equivalent of cocatalyst was added in the reactions. c The reactions
were carried out in the absence of CO2. d The reactions were carried out at
49 atm of CO2 pressure. The molar fractions of CO2 in the resulting
polymers were used to evaluate the CO2 incorporation, based on the 1H
NMR spectra of the products. e No polymer formed under the condition
described in footnote a. At elevated temperatures and high catalyst
concentrations, poly(propylene oxide) can be obtained.
1
reactions in solution by H NMR spectroscopy. Furthermore,
one can unequivocally distinguish between the regioisomers of
ring opening, (TPP)AlOCH2CHMeX versus (TPP)-
AlOCHMeCH2X.
isotactic junctions.91,94-100 In contrast, the copolymerization of
PO/CO2 yields regioirregular PPC with a ratio of HT:HH+TT
junctions that is catalyst dependent.55,58,91,101 Moreover, the
addition of Lewis bases, such as Br-, 1-methylimidazole, or
4-(dimethylamino)pyridine (DMAP), greatly enhance PPC
production at the expense of PPO.53,54,57,102-104 With porphyrin
and salen complexes of Al(III) and Cr(III), this begs the question
of how the alkyl carbonate and PO react within the same face
of the metal center.
A careful examination of the spectrum in this region reveals
that both regioisomers are formed but that the major regioisomer
(TPP)AlOCHMeCH2X predominates by ca. 9:1, when X ) Cl.
1
The H NMR spectrum revealing the presence of the (TPP)-
AlOCHMeCH2Cl and (TPP)AlOCH2CHMeCl isomers is shown
in Figure 1. The minor regioisomer was prepared independently
from the reaction between (TPP)AlCH3 and the chloro alcohol
S-ClCHMeCH2OH (Experimental Section in Supporting Infor-
mation). The ring opening of PO by (TPP)AlCl is extremely
rapid, but the very low solubility of (TPP)AlCl in CDCl3 and
C6D6 precludes the study of the kinetics of this reaction by NMR
methods.
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CH3 and O2C(CH2)6CH3 rendered these derivatives soluble in
CDCl3 and to some extent in C6D6. The choice of the
n-octanoate ligand is intended to model for the ring opening of
PO by an alkyl carbonate ligand, vide infra.
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1
followed by H NMR spectroscopy under pseudo first-order
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11032 J. AM. CHEM. SOC. VOL. 126, NO. 35, 2004

Ring Opening of Propylene Oxide
A R T I C L E S
Figure 2. 1H NMR (500 MHz, CDCl3) spectra of the reaction between (TPP)AlO(CH2)9CH3 and PO, showing the disappearance of initiator and appearance
of product with time.
The influence of added 4-(dimethylamino)pyridine (DMAP)
was examined in the following manner. In a NMR experiment,
1 equiv of DMAP was added to a CDCl3 solution of (TPP)-
AlX, where X ) O(CH2)9CH3 or O2C(CH2)6CH3. In each case,
the signals associated with the aromatic protons of DMAP were
shifted upfield and were significantly broadened relative to
signals of DMAP in neat CDCl3 (Figure S3 in Supporting
Information). At 25 °C, DMAP is reversibly coordinating to
the aluminum porphyrin center, and there is a significant
concentration of the 1:1 adduct. Upon cooling to -50 °C, this
equilibrium becomes frozen out on the NMR time scale for X
) O2C(CH2)6CH3 and O2CO(CH2)9CH3 (see later). The 2,6-
protons of the aromatic ring in DMAP appear at δ 0.56 ppm
upon complexation to the Al center, which contrasts with δ 8.2
ppm for the free molecule in CDCl3. The 3,5-aromatic protons
shift similarly from δ 6.5 ppm to 4.0 ppm on complexation,
and the NMe resonances shift from δ 3.0 ppm to 2.0 ppm. In
the case of X ) O(CH2)9CH3, the equilibrium could not be
frozen out on the NMR time scale because the equilibrium
constant for binding of DMAP was much smaller. At ambient
temperature, the 1H signals of DMAP represent a time average
between [free] and [complexed], which allows for the estimation
of Keq for binding. At 25 °C in CDCl3, we estimate Keq to be
5.8 × 10-3 mM-1 for X ) O(CH2)9CH3, 1.7 mM-1 for X )
O2C(CH2)6CH3, and 0.51 mM-1 for X ) O2CO(CH2)9CH3,
which reveals that DMAP binds in the order O2CR > O2COR
. OR. This order follows roughly the inverse order of the
basicity of the oxygen donor ligands.
Figure 3. Plots of -ln(It/I0) versus reaction time for the ring-opening
reaction of the first PO molecule by initiators, in the absence and in the
presence of 1 equiv of DMAP. I0 is the initial initiator concentration, and
It is the initiator concentration at time t. (TPP)AlOR ) (TPP)AlO(CH2)9-
CH3, (TPP)AlOOCR ) (TPP)AlO2C(CH2)6CH3. In all of the reactions, [Al]
) 15 mM, [PO] ) 720 mM.
reactions were followed at 25 °C in CDCl3 by the disappearance
of the Al-X proton signals (Figure 2). For the reaction involving
X ) O(CH2)9CH3, the product of ring opening was almost
exclusively the (TPP)AlOCHMeCH2O(CH2)9CH3 isomer with
the other regioisomer being barely detectable. However, for the
n-octanoate initiator, both regioisomers were formed in an
approximate ratio of 4:1, where the major isomer once again
represents the formation of the secondary alkoxide by ring
opening of PO at the methylene carbon. However, with time
this ratio changes as the primary alkoxide, the product of ring
opening of PO at the methine carbon, reacts faster with PO than
does the n-octanoate. The secondary alkoxide also reacts with
PO but at a significantly slower rate than the primary. Thus,
we can state that in the initiation step the reactivity order for
(TPP)AlX compounds is X ) Cl > O(CH2)9CH3 > O2C(CH2)6-
CH3 (Figure 3). We also determined, by varying the concentra-
tion of (TPP)AlO(CH2)9CH3 from 5 to 15 mM in four different
experiments, that the rate of ring opening of PO was first order
in (TPP)AlO(CH2)9CH3 (Figure S1 in Supporting Information).
The straight-line plots shown in Figure 3 also confirm this.
The kinetics of the ring opening of PO by the (TPP)Al-X
compounds in the presence of 1 equiv of DMAP was also
determined by NMR studies at 25 °C by monitoring the
disappearance of the R-CH2 protons of the alkoxide or car-
boxylate group signal. As shown in Figure 3, the influence of
added DMAP was very dramatic in enhancing the rate. Now,
the ring-opening event is much faster by the carboxylate than
the alkoxide, although even the alkoxide is accelerated by an
order of magnitude. We also noticed that the kinetic profiles
of these two initiators in the presence of DMAP were poly-
nomial rather than linear (Figure S4 in Supporting Informa-
tion). This is because the secondary alkoxide products are less
strongly coordinated by DMAP than the initiators, and the
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Figure 4. 1H NMR (500 MHz, CDCl3) spectrum of R-PO ring-opened products by (R,R-salen)AlCl, showing three isomers. (1) (R,R-salen)AlOCH2C(S)-
HMeCl; (2) (R,R-salen)AlOC(R)HMeCH2Cl; (3) (R,R-salen)AlOCH2C(R)HMeCl. (a and a′ represent CH2 signals, and b represents CH signals derived from
PO units, respectively.)
Scheme 2. The Stereochemistry in the PO Ring-Opening Event
Involving Attack at Methine Carbons by (R,R-Salen)AlCl
effective [DMAP]:[initiator] ratio increases with time. Thus, the
apparent rate constants increase with time in these two reaction
systems.
Salen aluminum chloride is not capable of polymerizing PO
under the conditions comparable to those of the porphyrin
analogue, nor will it act in copolymerizing PO and CO2.
However, it will ring open PO to generate a chloroalkoxide.
Because of this, we have studied the ring opening of S-PO and
R-PO by the compound containing Jacobsen’s chiral ligand
bound to aluminum, (R,R)-N,N′-bis(3,5-di-tert-butyl-salicylidene)-
1,2-cyclohexenediaminoaluminum chloride, (R,R-salen)AlCl.109-111
The ring opening of R-PO yields a 2.3:1 mixture of (R,R-salen)-
AlOC(R)HMeCH2Cl and (R,R-salen)AlOCH2CHMeCl. The
latter compound exists as a mixture of R- and S-OCH2CHMeCl
isomers in the ratio of 2:1, respectively. The related reaction
between S-PO and (R,R-salen)AlCl gave (R,R-salen)AlOC(S)-
HMeCH2Cl, and apparently only (R,R-salen)AlOCH2C(R)-
1
HMeCl in the ratio of 5:1. The H NMR spectrum revealing
the products derived from the ring opening of R-PO is shown
in Figure 4, and other related spectra are given in Figure S5 in
the Supporting Information.
The compound (R,R-salen)AlOC(S)HMeCH2Cl was crystal-
lographically characterized (Figure S6), and the aluminum was
shown to be in a square pyramidal coordination environment
with the â-chloroalkoxy ligand of S stereo configuration in the
apical site. The molecular structure is unexceptional,110,111 and
details will be reported fully elsewhere. The formation of (R,R-
salen)AlOCH2C(S)HMeCl was confirmed by its independent
synthesis from the reaction between (R,R-salen)AlMe and the
chiral alcohol HOCH2C(S)HMeCl as described in the Experi-
mental Section in the Supporting Information. This compound
ferentially but not exclusively attacked in the ring-opening event.
Moreover, the use of the chiral salen aluminum template shows
that there is a preference in the ring opening of R-PO for the
formation of the OCH2C(R)HMeCl alkoxide when attack occurs
at the methine carbon. The ring opening of S-PO by attack at
the methine carbon also preferentially forms the OCH2C(R)-
HMeCl ligand. The preferential formation of the â-chloroalkoxy
ligand with the R-stereocenter is clearly a result of the R,R-
was also crystallographically characterized and is shown in
Figure S6 in the Supporting Information.
These studies of (R,R-salen)AlCl clearly parallel those of the
porphyrin analogue in revealing the ring opening of PO occurs
salen template, but its formation in the reaction with R-PO
occurs with retention of stereochemistry at the methine carbon,
by competitive pathways where the methylene carbon is pre-
whereas for S-PO it is formed by inversion (Scheme 2).
The Carbon Dioxide Insertion Reaction. Carbon dioxide
reacts reversibly with (TPP)AlOR compounds to give (TPP)-
(109) Brandes, B. D.; Jacobsen, E. N. Synlett 2001, 1013-1015.
(110) Jacobsen, E. N. Acc. Chem. Res. 2000, 33, 421-431.
(111) Hansen, K. B.; Leighton, J. L.; Jacobsen, E. N. J. Am. Chem. Soc. 1996,
118, 10924-10925.
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Ring Opening of Propylene Oxide
A R T I C L E S
Figure 5. The percentages of alkyl carbonate and alkoxide versus the
[DMAP]:[(TPP)AlO(CH2)9CH3] ratio for the 13CO2 insertion with (TPP)-
AlO(CH2)9CH3.
AlO2COR compounds.104,112 This equilibrium is both temper-
ature and CO2 pressure dependent, that is, dependent of [CO2]
in solution, and when the CO2 atmosphere is removed only the
alkoxide is recovered. In our studies, we have examined the
equilibrium shown in 1 in CDCl3 at 25 °C under 4 atm of
13CO2 by 1H and 13C NMR spectroscopy. The concentration of
aluminum complex was 11 mM.
Figure 6. 13C{1H} (150 MHz, CDCl3) NMR spectra of the carbonate
carbon region of regioregular oligoether carbonate compounds (n ≈ 10),
having a HH carbonate junction. Carbonate compounds in (a) have random
stereosequences, while those in (b) have all isotactic sequences.
(TPP)Al-O(CH2)9CH3 + 13CO2 yk1z
k-1
(TPP)Al-O213CO(CH2)9CH3 (1)
inversion, and that the observed carbonate signals corresponded
to syndiotactic junctions for both HH and TT when S-PO was
used. We have now prepared a polyether carbonate with a HH
carbonate junction of both random stereosequence and one
where the HH carbonate is isotactic (Figure 6). From this, we
conclude that the syndiotactic HH carbonate carbon-13 signal
is upfield in PPC, and this is consistent with the previously
suggested assignment for the PPC derived from S-PO/CO2 and
a zinc glutarate catalyst.
We have now examined the carbonate 13C signals of PPC
prepared by (TPP)AlCl/DMAP and (R,R-salen)CrCl catalyst
systems. As shown in Figure 7, the regio- and stereo-sequences
differ quite significantly with the catalyst system employed. The
PPC derived from (TPP)AlCl/DMAP has fewer TT and HH
junctions, and Coates recently reported that (salen) cobalt(III)
acetate produces approximately 80% HT regiosequences, while
(BDI) zinc acetate produces almost random TT:HT:HH ≈ 1:2:
1.55,58
At 25 °C, under these conditions, the equilibrium favors the
alkoxide. Upon the introduction of DMAP, however, the
equilibrium shown in 1 is shifted markedly to the right, as shown
in Figure 5. At a high concentration of DMAP (5 equiv), the
alkoxide was completely converted to carbonate. When CO2
pressure was released under N2 atmosphere, ∼95% of the Al
complexes existed as carbonate in solution. However, if the
solvent and CO2 pressure were removed and the products were
dried under vacuum, the alkoxide was recovered. We note in
this system that CO2 insertion/deinsertion is kinetically rapid
with respect to the ring opening of PO.
Polymer Microstructure of Polypropylene Carbonate,
PPC. We have previously examined the polymer microstructure
of PPC formed in the zinc glutarate-catalyzed copolymerization
of PO and CO2 by use of proton decoupled 13C NMR (13C-
{1H}) spectroscopy.91 Specifically, we used rac-PO, S-PO, and
a 50:50 mixture of rac + S-PO, and we were thereby able to
make assignments for the stereosequences i and s for the TT,
HT, and HH carbonate junctions as defined below.
We can reasonably assume that HT carbonate junctions
normally retain the stereochemistry at the methine carbon as a
consequence of the preferential ring opening at the methylene
carbon. However, when ring opening at the methine carbon is
relatively competitive, HT junctions may also be formed with
either double inversion (backside attack) or with randomization
of stereochemistry as shown in Scheme 3.
The maximum information concerning the stereochemistry
of the ring-opening event can be gleaned from an examination
of the HH carbonate 13C signals of PPC derived from enantio-
merically pure PO. As shown in Figure 8, the (TPP)AlCl/DMAP
system yields PPC derived from S-PO with both i and s HH
(and TT) junctions, although the syndiotactic junctions are
preferred by approximately a 2:1 ratio. Rather interestingly, the
PPC derived from S-PO and (R,R-salen)CrCl has both i and s
HH (and TT) carbonate junctions, but the ratio is now in the
inverse order, i > s (Figure 8).
Zinc glutarate was thus shown to be stereoselective as a
catalyst in forming HH and TT junctions. In an earlier
publication, we assumed that the ring-opening event at the
methine carbon occurred by backside attack, that is, with
(112) Takeda, N.; Inoue, S. Bull. Chem. Soc. Jpn. 1978, 51, 3564-3567.
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A R T I C L E S
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Scheme 3. The Chain Growing Pathway Showing HT Carbonate
Junction (*) Formation Involving Double Attack at Methine Carbons
in the PO/CO2 Copolymerization (No Mechanistic Information Is
Implied by This Scheme, Which Merely Outlines the
Regiochemical Consequence of Attack on the Methine Carbon)
PO and CO2 yields PPO in preference to PPC (Table 1).
Furthermore, we note that CO2 insertion into the Al-OR bond
is promoted by the addition of DMAP. Both of these factors
assist in producing a regular alternating copolymer and serve
to minimize the number of ether-rich linkages as denoted by
HT′ and HH′ signals in the carbonate 13C{1H} spectra (Figure
7). The CO2 incorporation into the polymer formed in the PO/
CO2 copolymerization with (TPP)AlO(CH2)9CH3 can be in-
1
creased from 0.20 to 0.50 (molar fraction determined by H
NMR) by the addition of 1 equiv of DMAP, yielding a perfectly
alternating copolymer as shown by mass spectrometry (Figure
9).
We propose that the role of DMAP is to labilize the ligand
trans to it, either the alkoxide or the alkyl carbonate. The fact
that HH and TT junctions can be formed with retention of
stereochemistry clearly implies that an incipient carbonium ion
is captured by a neighboring nucleophile. The PO and the alkyl
carbonate must be cis to one another. We pictorially represent
this DMAP-promoted PO interchange associative process by
the sequence shown in Scheme 5. The PO is activated by
coordination to the aluminum center as the Al-O2COR bond
is weakened by the trans-effect of the DMAP ligand. Which
oxygen of the carboxylate group is involved in attacking the
PO carbon (Schemes 3 and 4) is unknown.
Figure 7. 13C{1H} (150 MHz, CDCl3) NMR spectra of the carbonate
carbon region of PPC made from (A1) zinc glutarate and rac-PO, (B1)
(TPP)AlCl/DMAP and rac-PO, and (C1) (R,R-salen)CrCl and rac-PO. (HH′
and HT′ are ether-rich carbonate signals.)
We can now envisage the (TPP)AlX-catalyzed reactions on
the basis of the equilibria and cycles shown in Scheme 6. In
the absence of DMAP, the ring opening of PO by alkoxide
dominates even in the presence of CO2, such that very little
carbonate is incorporated into what is essentially poly(propylene
oxide) (Table 1). The addition of DMAP serves to (1) increase
the equilibrium concentration of the aluminum alkyl carbonate
at the expense of the aluminum alkoxide, and (2) vastly increase
the rate of ring-opening PO by the alkyl carbonate ligand
(relative to the alkoxide), such that alternating copolymerization
of PO and CO2 to yield PPC now dominates. As shown in
Figures 7 and 8, there are few ether linkages in the PPC, and
this can be contrasted with the carbonate incorporation into PPO
in the absence of DMAP as shown in Table 1 and the spectra
(Figure S7) presented in the Supporting Information.
When R-PO is employed with the chiral salen CrCl catalyst,
the ratio of i:s junctions again changes and is now in favor of
s. From this, we can conclude that the salenCr catalyst prefers
to open the PO molecule to generate the primary alkoxide of S
stereochemistry by attack at the methine carbon (Scheme 4).
A Proposed Mechanism for the (TPP)AlCl/DMAP Copo-
lymerization of PO and CO2. Our studies have shown that
the ring opening of PO by (TPP)AlX compounds occurs by
competitive attack on the methylene and methine carbons. When
X ) O(CH2)9CH3, the reaction is first order in [Al], giving
predominately the secondary alkoxide by attack at the methylene
carbon. When X ) O2C(CH2)6CH3, the regiochemistry of ring
opening is less selective, and the ring opening is notably slower
than that for X ) O(CH2)9CH3. However, upon addition of
DMAP, we note that the rate of ring opening of PO is
accelerated greatly, and now the carboxylate group is much more
active in ring-opening PO relative to the alkoxide. This is
particularly important as (TPP)AlCl alone in the presence of
Concluding Remarks
The observation that the ring opening of PO by (TPP)AlO-
(CH2)9CH3 is first order in aluminum complex supports the
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11036 J. AM. CHEM. SOC. VOL. 126, NO. 35, 2004

Ring Opening of Propylene Oxide
A R T I C L E S
Figure 8. 13C{1H} (150 MHz, CDCl3) NMR spectra of the carbonate carbon region of PPCs made from (A2) zinc glutarate and S-PO, (B2) (TPP)AlCl/
DMAP and S-PO, (C2) (R,R-salen)CrCl and S-PO, and (C3) (R,R-salen)CrCl and R-PO. (HH′ and HT′ are ether-rich carbonate signals.)
Scheme 4. The Preferential Stereochemistry Involved in the Formation of HH Carbonate Junctions during the Formation of PPC by the
Catalyst Derived from (R,R-Salen)CrCl
earlier study on the kinetics of PPO formation employing (TPP)-
AlCl as initiator. These workers determined that the rate of PPO
formation was first order in [Al] and first order in [PO].113-116
Others have shown that (TPP)AlCl immobilized on a support
will also ring open PO to give PPO.117 The mechanism is thus
unequivocally mononuclear and different from that described
by Jacobsen for (R,R-salen)CrN3.110,111,118
the Al-alkoxide, and (2) it labilizes the carboxylate ligand, and
by inference the alkyl carbonate ligand, toward ring opening
of PO. In the latter reaction, it is evident the DMAP binds more
strongly to (TPP)AlO2CX species (X ) R or OR) than it does
to the parent alkoxide complex. Thus, whereas in the absence
of added DMAP the (TPP)Al system is only effective in the
homopolymerization of PO, when DMAP is added, alternating
copolymerization of PO and CO2 is kinetically favored.
The stereochemical studies of the ring opening of PO in these
systems reveal that attack occurs at both the methine and the
methylene carbons, and furthermore when attack occurs at the
methine carbon the ring-opening event can occur with retention
or inversion of stereochemistry.119,120 Collectively, these findings
We have shown that added DMAP serves to accelerate two
principal reactions. (1) It promotes the insertion of CO2 into
(113) Yoo, Y.; McGrath, J. E. Pollimo 1994, 18, 892-894.
(114) Yoo, Y.; McGrath, J. E. Pollimo 1994, 18, 1063-1070.
(115) Nuytens, F.; Lopitaux, G.; Faven, C.; Coqueret, X. Macromol. Symp. 2000,
153, 17-25.
(116) Lopitaux, G.; Ricart, G.; Faven, C.; Coqueret, X. Macromol. Chem. Phys.
2002, 203, 2560-2569.
(117) Uno, H.; Takata, K.; Mizutani, Y. React. Polym. 1991, 15, 121-129.
(118) Konsler, R. G.; Karl, J.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120,
10780-10781.
(119) March, J. AdVanced Organic Chemistry: Reactions, Mechanisms, and
Structure, 4th ed.; Wiley: New York, 1992.
(120) Parker, R. E.; Isaacs, N. S. Chem. ReV. 1959, 59, 737-799.
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J. AM. CHEM. SOC. VOL. 126, NO. 35, 2004 11037

A R T I C L E S
Chisholm and Zhou
Scheme 5. Proposed Interchange Associative Pathway for PO
Ring Opening by (TPP)AlX/DMAP
serve to support a reaction pathway wherein PO and the growing
chain are on the same side of the porphyrin ring. The
coordination of DMAP in the trans position serves to labilize
the Al-OR or AlO2CX (X ) R or OR) bonds toward
dissociation and uptake of PO in an interchange substitution
process. Upon coordination to the electrophilic aluminum center,
the PO is activated toward carbonium ion-like behavior, and
the attack by the nucleophilic growing chain leads to its
enchainment in the polymerization reaction in a relatively
nonregio- and nonstereoselective manner.
We propose that the use of added ligands, such as 1-meth-
ylimidazole and Br- in related reactions, parallels the effects
noted here for DMAP, but to a different degree. Many parallels
appear to exist with the findings of Darensbourg in the
copolymerization of CHO/CO2 and PO/CO2 at the salen
chromium(III) center,54,56 but contrast with the bimetallic
mechanism of Jacobsen for ring opening of cyclopentene
oxide110,111,118 and that of Coates for the bimetallic zinc-
catalyzed copolymerization of CHO and CO2.72 Clearly, more
needs to be known about the details of these reaction profiles.
An understanding of the factors controlling the reactions shown
Figure 9. MALDI-TOF mass spectrum of PPC prepared with the (TPP)-
AlO(CH2)9CH3/DMAP catalyst system. An ) H-[(C3H6O)n-alt-(CO2)n-1]-
O(CH2)9CH3‚Na+; Bn ) H-[(C3H6O)n-alt-(CO2)n]-O(CH2)9CH3‚Na+; Cn
) H-[(C3H6O)n-alt-(CO2)n-2]-O(CH2)9CH3‚Na+. Peaks (m/z): A38 (4014),
A39 (4116), A40 (4218), B38 (4058), B39 (4160), B40 (4262), C39 (4072),
C40 (4174), C41 (4276).
in Scheme 6 should allow for the development of highly active
systems for the catalytic production of PPC and related
polycarbonates.
Further work is in progress.
Scheme 6. Proposed Mechanism for PO/CO2 Copolymerization by (TPP)AlX Systems, Where Nu Represents the Added Lewis Base
Promoter
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Ring Opening of Propylene Oxide
A R T I C L E S
Acknowledgment. We thank the Department of Energy,
Office of Basic Sciences, Chemistry Division for financial
support of this work, Drs. J. C. Gallucci and N. J. Patmore at
The Ohio State University for X-ray experiments, and Professor
C. M. Hadad at The Ohio State University and Dr. E. M.
Dexheimer at BASF for valuable discussions. This paper is
dedicated to Ian P. Rothwell in memorium.
AlX. 1H NMR (500 MHz, CDCl3) spectra of (TPP)AlX/DMAP
mixtures at 25 and -50 °C. Kinetic plots of -ln(It/I0) versus
reaction time for ring opening of PO by (TPP)AlX/DMAP. 1
H
NMR spectra revealing the products in the reactions between
(R,R-salen)AlCl and S (and R)-POs. ORTEP drawings of the
(R,R-salen)AlOC(S)HMeCH2Cl and (R,R-salen)AlOCH2C(S)-
1
HMeCl molecules. H and 13C NMR spectra of the polymer
prepared in the PO/CO2 polymerization reaction catalyzed by
(TPP)AlCl alone. This material is available free of charge via
the Internet at http://pubs.acs.org.
Supporting Information Available: Experimental Section.
Graphs pertaining to the kinetics for ring opening of PO by
varying concentrations of (TPP)AlO(CH2)9CH3. Kinetic plots
of I0/It versus reaction time for ring opening of PO by (TPP)-
JA0394232
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