Aliphatic polycarbonates produced from the coupling of carbon dioxide and oxetanes and their depolymerization via cyclic carbonate formation

Article abstract of DOI:10.1021/ma2002323

The (salen)CrCl/onium salt catalyzed coupling reactions of several oxetane derivatives and carbon dioxide are reported. The oxetanes investigated contain substituents in the 3-position covering a range of steric requirements. The oxetanes examined include, 3,3-dimethyloxetane, 3-methoxymethyl-3-methyloxetane, and 3-benzyloxymethyl-3-methyloxetane. The rates of reaction of these oxetanes with CO₂ were found to be significantly slower than the corresponding process with the parent oxetane monomer. Furthermore, in these instances the formation of copolymer was found to proceed via the preformed cycloaddition product, i.e., the six-membered cyclic carbonate, to a greater extent and increasing with the steric bulk of the substituents on oxetane. For these sterically more hindered oxetanes, the CO₂ coupling reaction carried out in toluene at 110 °C reached an equilibrium product distribution of copolymer to cyclic carbonate which increased in cyclic carbonate product with increasing steric requirements of the oxetane monomer. For example, the catalyzed coupling of the parent oxetane and CO₂ provides a copolymer to cyclic carbonate ratio of greater than 95%, whereas the corresponding product distribution for 3-benzyloxymethyl-3-methyloxetane was observed to be 60%. The catalytic rate of depolymerization of a purified sample of the copolymer afforded from 3-benzyloxymethyl-3-methyloxetane and CO₂ to the corresponding cyclic carbonate, 5-benzyloxymethyl-5-methyl-1,3-dioxan-2-one, was found to be greatly retarded when carried out in an atmosphere of CO ₂.

Full text of DOI:10.1021/ma2002323

ARTICLE
pubs.acs.org/Macromolecules
Aliphatic Polycarbonates Produced from the Coupling of Carbon
Dioxide and Oxetanes and Their Depolymerization via Cyclic
Carbonate Formation
Donald J. Darensbourg,* Adriana I. Moncada, and Sheng-Hsuan Wei
Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
S Supporting Information
b
ABSTRACT: The (salen)CrCl/onium salt catalyzed coupling reac-
tions of several oxetane derivatives and carbon dioxide are reported.
The oxetanes investigated contain substituents in the 3-position cover-
ing a range of steric requirements. The oxetanes examined include, 3,3-
dimethyloxetane, 3-methoxymethyl-3-methyloxetane, and 3-benzylox-
ymethyl-3-methyloxetane. The rates of reaction of these oxetanes with
CO2 were found to be significantly slower than the corresponding
process with the parent oxetane monomer. Furthermore, in these
instances the formation of copolymer was found to proceed via the
preformed cycloaddition product, i.e., the six-membered cyclic carbo-
nate, to a greater extent and increasing with the steric bulk of the substituents on oxetane. For these sterically more hindered
oxetanes, the CO2 coupling reaction carried out in toluene at 110 °C reached an equilibrium product distribution of copolymer to
cyclic carbonate which increased in cyclic carbonate product with increasing steric requirements of the oxetane monomer. For
example, the catalyzed coupling of the parent oxetane and CO2 provides a copolymer to cyclic carbonate ratio of greater than 95%,
whereas the corresponding product distribution for 3-benzyloxymethyl-3-methyloxetane was observed to be 60%. The catalytic rate
of depolymerization of a purified sample of the copolymer afforded from 3-benzyloxymethyl-3-methyloxetane and CO2 to the
corresponding cyclic carbonate, 5-benzyloxymethyl-5-methyl-1,3-dioxan-2-one, was found to be greatly retarded when carried out
in an atmosphere of CO2.
’ INTRODUCTION
corresponding polycarbonate, ring-opening polymerization
(ROP) of six-membered cyclic carbonates readily occurs to
provide its polycarbonate counterpart with complete retention
of carbon dioxide.4 In previous studies, we have addressed this
issue by examining the copolymerization of oxetane with carbon
dioxide to determine the origin of the polycarbonate, i.e.,
whether it is afforded by alternating enchainment of the como-
nomers CO2/oxetane or the ROP of initially formed trimethy-
lene carbonate (Scheme 1).5
For catalyst systems comprised of (salen)CrCl complexes in
the presence of onium salt initiators, the reaction can be tuned to
proceed selectively via the route involving the ROP of preformed
trimethylene carbonate. This was achieved by the judicious
choice of onium salt and reaction conditions of temperature
and pressure. In this manner, polycarbonates are afforded which
contain only trace quantities of ether linkages in the copolymer
backbone. As indicated in eq 2 in most instances the copolym-
erization of oxetane and CO2 provides small quantities of
trimethylene carbonate (TMC) as a coproduct. This observation
suggests that the free energy change associated with the ROP of
Four-membered cyclic ethers, oxetanes, have been under-
utilized because of their commercial inaccessibility and laborious
synthesis. Nevertheless, these small molecules are currently
receiving much attention in medicinal chemistry and drug
delivery studies.1 Our interest in these cyclic ethers is their ability
to react with carbon dioxide to provide polycarbonates.2 These
studies were inspired by the numerous successful catalytic
systems investigated for the coupling of oxiranes and CO2 to
yield an array of polycarbonate materials.3 Equations 1 and 2
illustrate these processes, which are generally carried out at
modest pressures of carbon dioxide, i.e., < 3.0 MPa.
Received: February 1, 2011
Although the five-membered cyclic carbonates formed from
Revised:
March 8, 2011
epoxides and CO2 are thermodynamically more stable than the
Published: March 24, 2011
r
2011 American Chemical Society
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|

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Scheme 1
McAlees with some modifications.8 A solution of 3-methyl-3-oxetane-
methanol (50 g, 0.489 mol) in THF (∼100 mL) was added via syringe to
a suspension of sodium hydride (23.5 g, 60% in mineral oil) in THF
(1 L) that was previously cooled to 0 °C using an ice/water bath. After
the reaction mixture was allowed to warm to room temperature, the
solution was stirred for 24 h. Dimethyl sulfate (86.4 g, 0.685 mol) was
added dropwise (exothermic reaction) via syringe to the reaction
solution that was previusly cooled to 0 °C using an ice/water bath.
After warming the solution to room temperature, the mixture was stirred
for 24 h at ambient temperature. A solution of sodium hydroxide (30 g in
50 mL of water) was then added, and most of the THF was removed by
distillation. The residue was extracted with diethyl ether, and the ether
extract was dried over Na2SO4 and vacuum distilled to give 3-methox-
ymethyl-3-methyloxetane (MMO) (36 g, 63.3%). 1H NMR (300 MHz,
CDCl3): δ 4.45 (d, 2H, OCH2), 4.30 (d, 2H, OCH2), 3.40 (s, 2H, CH2),
3.35 (s, 3H, OCH3), 1.26 (s, 3H, CH3).
Synthesis of 3-Benzyloxymethyl-3-methyloxetane. A ben-
zene solution (50 mL) of 3-methyl-3-oxetanemethanol (20 g, 0.20 mol)
and benzyl bromide (33.5 g, 0.20 mol) was stirred with 50% sodium
hydroxide aqueous solution (80 mL) and tetra-n-butylammonium
bromide (9.5 g 0.03 mol). After stirring for 2 days at room temperature,
the organic layer was collected and the organic solvent was removed.
The desired product was afforded as a colorless liquid in 70% yield (27.1
g) after distillation under reduced pressure. 1H NMR (300 MHz,
CDCl3): δ 7.37ꢀ7.24 (m, 5H, ꢀAr), 4.58 (s, 2H, ꢀCH2ꢀAr), 4.54
(d, 2H, ꢀOCHH2), 4.37 (d, 2H, ꢀOCHH2), 3.53 (s, 2H, CH2), 1.34 (s,
3H, CH3).
TMC to poly(TMC), although negative, is not very large in
magnitude. Hence, it should in principle be able to depolymerize
these polycarbonates into their monomer units, six-membered
cyclic carbonate, under suitable reaction conditions (eq 3).6
Herein we wish to explore other four-membered cyclic ether
monomers for the synthesis of copolymers from carbon diox-
ide, thereby generating other polycarbonates with different
properties.
Synthesis of 2-Methoxymethyl-2-methyl Malonic Acid
Diethyl Ester. This compound was prepared according to the
procedure reported by Doherty.9 A tetrahydrofuran solution (40 mL)
of diethyl methylmalonate (5 g, 0.0287 mol) was cooled to ꢀ78 °C and
treated with a 1.64 M solution of n-butyllithium in hexanes (17.5 mL,
0.0287 mol). The resulting mixture was stirred rapidly and after
warming to room temperature, was transferred dropwise via cannula
to a tetrahydrofuran solution (30 mL) of chloromethyl methyl ether
(2.9 g, 0.0287 mol). After the reaction solution was stirred overnight,
the solvent was removed under vacuum and the residue extracted into
diethyl ether (2 ꢁ 30 mL), washed with water (2 ꢁ 30 mL), dried over
MgSO4, filtered, and the solvent was removed to afford the desired
ester as a pale yellow/colorless oil in 80% yield (5.02 g). 1H NMR (300
MHz, CDCl3): δ 4.13 (quart, 4H, J = 7.0 Hz, CH2CH3), 3.66 (s, 2H,
CH2), 3.28 (s, 3H, OCH3), 1.42 (s, 3H, CH3), 1.19 (t, 6H, J = 7.1 Hz,
CH2CH3).
’ EXPERIMENTAL SECTION
Reagents and Methods. Unless otherwise specified, all syntheses
and manipulations were carried out on a double-manifold Schlenk
vacuum line under an atmosphere of argon or in an argon filled glovebox.
Toluene and tetrahydrofuran were freshly distilled from sodium/ben-
zophenone. Ethanol and methanol were freshly distilled from Mg/I2.
Diethyl ether, dichloromethane, and hexanes were purified by an
MBraun Manual Solvent Purification System packed with Alcoa F200
activated alumina desiccant. 1,1,2,2-tetrachloroethane (TCE) (TCI)
was freshly distilled over CaH2. 3-Methyl-3-oxetanemethanol (Alfa
Aesar) was used as received. Triethylamine was freshly distilled over
CaH2 before use. Ethyl chloroformate (Aldrich), diethyl methylmalo-
nate (Alfa Aesar), n-butyllithium (Aldrich), lithium aluminum hydride
(Alfa Aesar), chloromethyl methyl ether (Aldrich), sodium hydride
(60% in mineral oil) (Alfa Aesar), dimethyl sulfate (Alfa Aesar),
potassium hydroxide (EMD), ethylenediamine (Aldrich), 1,2-phenyle-
nediamine (ACROS), chromium(II) chloride (Alfa Aesar), sodium
hydroxide (EMD), sodium sulfate (EMD), and magnesium sulfate
(EMD) were used as received. Tetra-n-butylammonium azide was
stored in the freezer of the glovebox upon arrival. Bone-dry carbon
dioxide supplied in a high-pressure cylinder and equipped with a liquid
dip tube was purchased from Scott Specialty Gases. The corresponding
salen ligands and chromium complexes were synthesized as described in
the literature.7
Synthesis of 2-Benzyloxymethyl-2-methyl Malonic Acid
Diethyl Ester. In a similar manner to that above, a tetrahydrofuran
solution (40 mL) of diethyl methylmalonate (5 g, 0.0287 mol) was
cooled to ꢀ78 °C and treated with a 2.5 M solution of n-butyllithium in
hexanes (11.5 mL, 0.0287 mol). The mixture was stirred and after warming
to room temperature, was transferred dropwise via cannula to a tetra-
hydrofuran solution (35 mL) of chloromethoxymethylbenzene (4.50 g,
0.0287 mol). After being stirred overnight, the solvent was removed
under vacuum and the residue extracted into diethyl ether (2 ꢁ 30 mL),
washed with water (2 ꢁ 30 mL), dried over MgSO4, and filtered. The
solvent was removed to afford the product as a colorless oil in 96% yield
(8.14 g). 1H NMR (300 MHz, CDCl3): δ 7.37ꢀ7.24 (m, 5H, ꢀAr), 4.54
(s, 2H, ꢀCH2ꢀAr), 4.18 (q, 4H, ꢀCH2CH3), 3.81 (s, 2H, CH2), 1.53
(s, 3H, CH3), 1.23 (t, 6H, ꢀCH2CH3).
1H NMR spectra were acquired on Unityþ 300 MHz and VXR 300
MHz superconducting NMR spectrometers. Molecular weight determi-
nations (Mn and Mw) were carried out with Viscotek Modular GPC
apparatus equipped with ViscoGEL I-series columns (H þ L), and
Model 270 dual detector comprised of RI and Light Scattering detectors.
High-pressure reaction measurements were performed using an ASI
ReactIR 1000 reaction analysis system with stainless steel Parr autoclave
modified with a permanently mounted ATR crystal (SiComp) at the
bottom of the reactor (purchased from Mettler Toledo).
Synthesis of 2-Methoxymethyl-2-methyl-1,3-propane-
diol. This compound was prepared according to the procedure reported
by Doherty.9 A solution of 2-methoxymethyl-2-methyl malonic acid
diethyl ester (5 g, 0.0229 mol) in tetrahydrofuran (20 mL) was added
dropwise via cannula to a stirred suspension of LiAlH4 (4.36 g, 0.115
mol) in tetrahydrofuran (80 mL), at 0 °C. The reaction mixture was
allowed to warm to room temperature and stirred for an additional 4 h.
Synthesis of 3-Methoxymethyl-3-methyloxetane. This de-
rivative of oxetane was prepared according to the procedure reported by
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After cooling to 0 °C, the resulting suspension was diluted with diethyl
ether (100 mL) and quenched by addition of water (10 mL), followed by
KOH (2.8 g in 10 mL of water), and finally water (10 mL), and stirred for
1 h. After hydrolysis was complete, the resulting mixture was filtered and
the solids were washed with diethyl ether (2 ꢁ 25 mL). The organic
fractions were combined, and dried over MgSO4, and the solvent was
removed to afford 2-methoxymethyl-2-methyl-1,3-propanediol as a
colorless oil in 90% yield. H NMR (300 MHz, CDCl3): δ 3.65 (d,
2H, J = 10.7 Hz, OCH2), 3.54 (d, 2H, J = 10.7 Hz, OCH2), 3.37 (s, 2H,
CH2), 3.33 (s, 3H, OCH3), 0.79 (s, 3H, CH3).
X-ray Structural Studies. Single crystals of (salen)Cr(III)-
Cl oxetane (complex 6) were obtained by layering hexanes into a
3
saturated dichloromethane solution of the corresponding (salen)Cr
(III)Cl complex (N,N0-bis(3-methoxy-5-tert-butylsalicylidene)-1,2-phe-
nylenediimine chromium(III) chloride) containing 20 equiv of MMO.
Single crystals of 5-methoxymethyl-5-methyl-1,3-dioxan-2-one were
isolated after several months.
1
For both structures, a Bausch and Lomb 10ꢁ microscope was used to
identify suitable crystals. Each crystal was coated in paratone, affixed to a
nylon loop, and placed under streaming nitrogen (110 K) in a Bruker-D8
Adv GADDS X-ray diffractometer. Space group determinations were
made on the basis of systematic absences and intensity statistics. Both
crystal structures were solved by direct methods and were refined by full-
matrix least-squares on F2. All hydrogen atoms were placed in idealized
positions and refined with fixed isotropic displacements parameters
equal to 1.2 (1.5 for methyl protons), times the equivalent isotropic
displacements parameters of the atoms to which they were attached. All
non-hydrogen atoms were refined with anisotropic displacement
parameters.
The following are the programs that were used: data collection and
cell refinements; FRAMBO Version 4.1.05 (GADDS),11 data reduc-
tions; SAINTPLUS Version 6.63,12 absorption correction; SADABS,13
structural solutions; SHELXS-97,14 structural refinement; SHELXL-
97;15 molecular graphics and preparation of material for publication;
SHELXTL, version 6.14,16 and X-Seed, version 1.5.17
General Procedure for Copolymerization Reactions of
3-methoxymethyl-3-methyloxetane and CO2. In a typical
experiment, the appropiate amount of catalyst, cocatalyst (n-Bu4NN3),
and 4 g of MMO were delivered via the injection port into a 300 mL
stainless steel Parr autoclave reactor that was previously dried in vacuo
overnight at 80 °C. The autoclave was then pressurized with 3.5 MPa of
CO2 and the temperature was increased to 110 °C. The monomer:
catalyst:cocatalyst ratio was maintained at 275:1:2, and the reaction was
run for the corresponding reaction time. After the reaction was stopped,
the autoclave was put into ice, cooled down to 10 °C, and vented in a
fume hood. The percent conversion to products was determined based
on the amount of oxetane monomer left in the reaction solution as
ascertained by 1H NMR in CDCl3:MMO: δ 4.45 (d, 2H, OCH2), 4.30
(d, 2H, OCH2), 3.40 (s, 2H, CH2), 3.35 (s, 3H, OCH3), 1.26 (s, 3H,
CH3). Furthermore, the quantities of 5-methoxymethyl-5-methyl-1,3-
dioxan-2-one, polycarbonate, and ether linkages in the copolymer were
determined by integrating the peak area of the corresponding reso-
nances in CDCl3: Polycarbonate: δ 4.07 (s, 4H, OCH2), 3.31 (s, 3H,
OCH3), 3.26 (s, 2H, CH2), 1.0 (s, 3H, CH3), cyclic carbonate: δ 4.27 (d,
2H, OCH2), 4.02 (d, 2H, OCH2), 3.31 (s, 3H, OCH3), 3.28 (s, 2H,
CH2), 1.03 (s, 3H, CH3), and ether linkages: δ 0.9 (s, 3H, CH3), with
other resonances being obscured by the intense polymer signals.
Synthesis of 2-benzyloxymethyl-2-methyl-1,3-propane-
diol. A tetrahydrofuran solution (30 mL) of 2-benzyloxymethyl-2-
methylmalonic acid diethyl ester (8.14 g, 0.0277 mol) was added
dropwise via cannula to a stirred suspension of LiAlH4 (5.25 g, 0.138
mol) in tetrahydrofuran (100 mL) at 0 °C. The reaction mixture
was allowed to warm to room temperature and stirred overnight. After
cooling to 0 °C, the resulting suspension was diluted with diethyl ether
(100 mL) and quenched by addition of water (10 mL), followed by
KOH (3 g in 1 0 mL of water), and finally water (10 mL), and stirred for
a further 1 h. After hydrolysis was complete, the resulting mixture was
filtered and the solids were washed with diethyl ether (2 ꢁ 25 mL). The
organic fractions were combined, and dried over MgSO4. The solvent
was removed to afford 2-benzyloxymethyl-2-methyl-1,3-propanediol as
1
a white solid in 87% yield (5.06 g). H NMR (300 MHz, CDCl3): δ
7.40ꢀ7.28 (m, 5H, ꢀAr), 4.52 (s, 2H, ꢀCH2Ar), 3.70 (d, 2H,
ꢀOCHH2), 3.61 (d, 2H, ꢀOCHH2), 3.47 (s, 2H, CH2), 2.39 (m,
2H, OH), 0.83 (s, 3H, CH3).
Synthesis of 5-Methoxymethyl-5-methyl-1,3-dioxan-2-
one. This compound was synthesized according to the procedure
reported by Endo for the synthesis of trimethylene carbonate with a
slight modification.10 Triethylamine (21.4 g, 0.211 mol) was added
dropwise via syringe to a solution of 2-methoxymethyl-2-methyl-1,3-
propanediol (13.5 g, 0.100 mol) and ethyl chloroformate (21.7 g, 0.201
mol) in 700 mL of THF at 0 °C over a period of 30 min. The reaction
mixture was stirred overnight at room temperature. The precipitated
triethylamine hydrochloride salt was isolated by filtration, and the filtrate
was concentrated under vacuum. The oily residue was vacuum distilled
to afford 5-methoxymethyl-5-methyl-1,3-dioxan-2-one as colorless oil.
After a period of several months colorless crystals grew and were
1
successfully analyzed by X-ray crystallography. H NMR (300 MHz,
CDCl3): δ 4.27 (d, 2H, OCH2), 4.02 (d, 2H, OCH2), 3.31 (s, 3H,
OCH3), 3.28 (s, 2H, CH2), 1.03 (s, 3H, CH3).
Synthesis of 5-Benzyloxymethyl-5-methyl-1,3-dioxan-2-
one. Triethylamine (5.11 g, 0.05 mol) was added dropwise via syringe
to a solution of 2-benzyloxymethyl-2-methyl-1,3-propanediol (5.06 g,
0.0241 mol) and ethyl chloroformate (5.25 g, 0.0483 mol) in 50 mL of
tetrahydrofuran at 0 °C over a period of 30 min. The reaction mixture
was stirred overnight at ambient temperature. The precipitated triethy-
lamine hydrochloride salt was removed by filtration, and the filtrate was
concentrated under vacuum. Colorless oil of 5-benzyloxymethyl-5-methyl-
1,3-dioxan-2-one (3.85 g, 70%) was afforded after chromatography
purification (EA:hexane = 1:1, Rf ∼ 0.45). 1H NMR (300 MHz,
CDCl3): δ 7.42ꢀ7.26 (m, 5H, ꢀAr), 4.53 (s, 2H, CH2ꢀAr), 4.36 (d,
2H, ꢀOCHH2), 4.07 (d, 2H, ꢀOCHH2), 3.41 (s, 2H, CH2), 1.11 (s,
3H, CH3). 13C NMR (300 MHz, CDCl3): δ 148.17, 137.38, 128.34,
127.86, 127.49, 73.86, 73.48, 71.01, 32.92, 17.32. Anal. Calcd for
C13H16O4: C, 66.09; H, 6.83. Found C, 66.14; H, 7.04.
Substrate Binding and Ring-Opening Step Examined by
Infrared Spectroscopy. 3-methoxymethyl-3-methyloxetane binding
and ring-opening step studies were examined by solution infrared
spectroscopy. The catalytic system used in these studies was a (salen)
CrIIICl (50 mg) complex (N,N0-bis(3,5-di-tert-butylsalicylidene)-1,2-et-
hylenediimine chromium(III) chloride) in the presence of n-Bu4NN3 as
cocatalyst and using TCE as the solvent (4 mL).
Copolymerization Reaction Monitored by in situ IR Spec-
troscopy. In a typical experiment, the appropiate amount of complex
5a, cocatalyst, (n-Bu4NN3), and oxetane monomer (8 g) were dissolved
in 6 mL of toluene and delivered via the injection port into a 300 mL
stainless steel Parr autoclave reactor that was previously dried in vacuo
overnight at 80 °C. The monomer:catalyst:cocatalyst ratio was main-
tained at 150:1:2. The autoclave is modified with a 30 bounce SiComp
window to allow for the use of an ASI ReactIR 1000 system equipped
with a MCT detector. In this manner a 128-scan background spectrum
was collected after the reaction mixture was heated to 110 °C. The
autoclave was pressurized with 3.5 MPa of CO2, and the infrared spectro-
meter was set to collect one spectrum every 3 min the corresponding
reaction time. Profiles of the absorbance at 1750 cmꢀ1 (polymer) and at
1770 cmꢀ1 (cyclic carbonate) with time were recorded after baseline
correction. After the reaction was stopped, the autoclave was cooled
down to room temperature and vented in a fume hood. The reaction
solution was analyzed by 1H NMR spectroscopy in the same manner as
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above, to determine the percent conversion to products, and the
percentages of polycarbonate, cyclic carbonate, and ether linkages.
Depolymerization of Poly(3-benzyloxymethyl-3-methy-
loxetane carbonate). Poly(3-benzyloxymethyl-3-methyloxetane
carbonate) (60 mg, 0.25 mmol), (N,N0-bis-(3,5-di-tert-butylsalicyli-
dene)-1,2-ethylenediimine chromium(III) chloride (3 mg, 0.005
mmol), and n-Bu4NN3 (2.8 mg, 0.01 mmol) were dissolved in 2.4 mL
of d-toluene in the glovebox and transferred into a J. Young NMR tube
(1.2 mL). The tube was evacuated at liquid nitrogen temperature and
refilled with argon or CO2. The tube was placed in a 110 °C oil bath and
the reaction was monitored by 1H NMR spectroscopy.
Polymerization of Poly(3-benzyloxymethyl-3-methyloxe-
tane carbonate) from 5-Benzyloxymethyl-5-methyl-1,3-di-
oxan-2-one. 5-Benzyloxymethyl-5-methyl-1,3-dioxan-2-one (60 mg,
0.25 mmol), (N,N0-bis-(3,5-di-tert-butylsalicylidene)-1,2-ethylenedii-
mine chromium(III) chloride (3 mg, 0.005 mmol), and n-Bu4NN3
(2.8 mg, 0.01 mmol) were dissolved in 2.4 mL of d-toluene in the
glovebox and transferred into a J. Young NMR tube (1.2 mL). The tube
was placed in a 110 °C oil bath and the reaction was monitored by 1H
NMR spectroscopy.
Figure 1. Thermal ellipsoid plot of 5-methoxymethyl-5-methyl-1,3-
dioxan-2-one. Ellipsoids are at the 50% level. H atoms are omitted for
clarity.
Table 1. Selected Bond Distances and Angles for 5-Methox-
ymethyl-5-methyl-1,3-dioxan-2-onea
O(1)ꢀC(1)
1.357(12)
1.187(12)
1.460(12)
1.418(13)
118.3(10)
119.7(9)
105.6(8)
111.4(8)
O(3)ꢀC(1)
O(1)ꢀC(4)
O(4)ꢀC(6)
Statistical Deconvolution of FTIR Spectra. FTIR spectra were
deconvoluted using Peakfit, version 4.12 (Peakfit for Windows, v. 4.12;
SYSTAT Software Inc., San Jose, CA, 2003). Statistical treatment was a
residuals method utilizing a combination GaussianꢀLorentzian summa-
tion of amplitudes with a linear baseline and Savitsky-Golay smoothing.
O(3)ꢀC(1)ꢀO(1)
O(2)ꢀC(1)ꢀO(1)
C(2)ꢀC(3)ꢀC(4)
C(5)ꢀO(4)ꢀC(6)
’ RESULTS AND DISCUSSION
a Units of bond angles and bond distances are deg and Å, respectively.
The oxetane, 3-methyl-3-oxetane methanol (1), is commer-
cially available and can be readily converted to the corresponding
methoxy or O-benzyl derivatives. Derivatization of 1 prior to
copolymerization with CO2 is required in order to avoid rapid
chain transfer reactions between the growing polymer chain and
the alcoholic monomer, thereby prohibiting polymer formation.
For example, depronation of 1 with NaH in mineral oil followed
by the treatment with dimethyl sulfate generated 3-methoxy-
methyl-3-methyl oxetane (MMO) (2) in 63% yield (eq 4).8 In
subsequent postsynthetic modification of these copolymers via
deprotection of the ꢀOH function it is possible to prepare
hydrophilic polycarbonates (eq 5).18,19
As illustrated in Figure 2 the oxetane monomer and products
from the coupling reactions of monomer 2 with carbon dioxide
1
are easily accounted for using H NMR spectroscopy. In this
instance the relative quantities of copolymer along with any
accompanying ether linkages and cyclic carbonate can be
assessed by integrating the areas under the peaks of the corre-
sponding resonances at 1.00, 0.90, and 1.03 ppm, respectively. In
situ infrared monitoring of the reaction’s progress can be
achieved by observing the growth of the carbonyl vibrations of
the copolymer and cyclic carbonate in a mixture of toluene and
oxetane monomer at 1750 and 1770 cmꢀ1, respectively.
We present here copolymerization studies of oxetane deriva-
tives and CO2 employing the (salen)CrCl/onium salt catalyst
systems utilized in our previous investigations (Figure 3). In-
itially complex 5a in the presence of n-Bu4NN3 was used to
examine the selectivity and catalytic activity for copolymer
formation from the coupling of monomer 2 and carbon dioxide.
The copolymerization reactions were performed under identical
reaction conditions, i.e., 110 °C, 3.5 MPa CO2 pressure, and the
monomer:catalyst:cocatalyst ratio was maintained at 275:1:2.
The results of this study are summarized in Table 2. The product
1
mixtures were analyzed by H NMR spectroscopy, with quan-
tities of copolymer, cyclic carbonate and ether linkages in the
copolymer determined by integrating the proton resonances at
1.00, 1.03, and 0.90 ppm, respectively. As is readily evident from
Table 2, the yield of copolymer greatly exceeds the cyclic
carbonate in all instances. It should also be pointed out that
the reaction occurs on a much slower time scale than the
oxetane/CO2 process and results in a much greater amount of
cyclic carbonate product at high conversion.2,5 100% CO2
content corresponds to a completely alternating copolymer with
no ether linkages; i.e., it represents the maximum allowable CO2
content in the copolymer. Some researchers prefer to define a
completely alternating copolymer with a CO2 content of 50%.
The corresponding six-membered cyclic carbonates can be
prepared from the respective 1,3-propanediol with ethylchlor-
oformate in the presence of stoichiometric quantities of
triethylamine.10 For example, 5-methoxymethyl-5-methyl-1,3-
dioxan-1-one (4), was prepared from 2-methoxymethyl-2-meth-
yl-1,3-propanediol and its structure was confirmed by X-ray
crystallography. Figure 1 contains a thermal ellipsoid drawing
of 4 with a list of selected bond distances and bond angles
provided in Table 1.
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Figure 3. Structures of the (salen)CrIII chloride complexes utilized as
catalysts for the copolymerization of oxetanes and CO2.
effects of altering the diimine backbone of the Cr(III) salen
complex while maintaining the di-tert-butyl groups in the 3,5-
positions of the phenolate moiety (entries 3 and 4, Table 3). As
is seen in Table 3, the catalytic behavior of the chromium salen
complexes is greatly affected by changing the diimine backbone
from cyclohexylene to ethylene, with the chromium salen
complex with the ethylene backbone displaying higher catalytic
activity. This is most likely due to the flexibility imparted to the
chromium salen complex by the ethylene backbone compared
to the cyclohexylene backbone. Hence, binding of a bulkier
oxetane to the chromium center would be more feasible. It is
important to point out that the percentage of CO2 incorpora-
tion in the copolymers was lower than the typical CO2 fixation
observed in polycarbonates obtained from oxetane and CO2
catalyzed by the (salen)CrCl catalytic system. In general, the
observed Mn values were found to be significantly lower than
the theoretical value. For example, an observed Mn value for the
copolymer afforded in entry 3 of Table 3 of 11 400 is con-
siderably lower than the anticipated value of 34 000. This is
thought to be due to a chain transfer mechanism arising from
the presence of trace quantities of water in the system.21ꢀ23 The
polydispersity indices were general around 1.30.
Substrate Binding and Ring-Opening Studies. Since ox-
etane monomers containing substituents in the 3-position are
sterically more encumbering than oxetane, a comparative study
of the binding of monomer 2 with that of the parent oxetane to
the Cr(III) center in (salen)CrCl was conducted employing
infrared spectroscopy.2c For this investigation complex 5a in the
presence of 2 equiv of n-Bu4NN3 was utilized, where the vN
3
Figure 2. 1H NMR in CDCl3 of (A) MMO, (B) polycarbonate
obtained from MMO and CO2, and (C) cyclic carbonate.
stretching vibrations provide accessible probes for both binding
and ring-opening steps. The results of this study are depicted in
Scheme 2 and Figure 4.
In subsequent studies, the effects of altering the nature of the
substituents on the phenolate rings and the diimine backbone
of the salen ligand in the (salen)CrCl complex on the reactivity
and selectivity of the coupling reaction of 2 and CO2 were
examined. The results of this investigation are summarized in
Table 3. As noted in Table 3, upon retaining the phenylene
backbone of the diimine while changing the substituents in the
3,5-positions of the phenolate groups (entries 1 and 2 in
Table 3) reveals the Cr(III) salen derivative containing the
bulky di-tert-butyl groups to be more active. This is consistent
with previous observations reported for the copolymerization
of oxetane and CO2 catalyzed by the (salen)CrCl catalyst
system.2,5 On the other hand, for the copolymerization of
cyclohexene oxide and CO2 catalyzed by chromium salen
complexes, higher catalytic activity was obtained in complexes
containing methoxy and tert-butyl groups in the 3 and 5
positions of the phenolate rings.20 We have also studied the
As indicated in Scheme 2, upon addition of 2 equiv of n-
Bu4NN3 to (salen)CrCl in weakly coordinating tetrachlor-
oethane solvent, the anionic six-coordinate bis(azide) species
(salen)Cr(N3)2ꢀ readily forms at ambient temperature. This is
apparent in the vN stretching region with the appearance of an
3
infrared band at 2047 cmꢀ1 with a shoulder at 2057 cmꢀ1. It
ꢀ
should be noted here that the n-Bu4Nþ salt of (salen)Cr(N3)2
anion has been fully characterized by X-ray crystallography.2c,24
Addition of 100-fold excess of 3-methoxymethyl-3-methyloxe-
tane to the bis(azide) complex displaces some of the azide anion
as seen by an increase in the vN absorption of the free azide ion
concentration at 2009 cmꢀ1, wi3th a concomitant decrease in the
concentration of (salen)Cr(N3)2ꢀ (yellow line, Figure 4). More-
over, a new νN3 stretching band appears at 2061 cmꢀ1 which is
assigned to (salen)Cr(N3) oxetane. Upon stirring of this reaction
3
mixture for 24 h at ambient temperature, no significant changes
in the infrared spectrum resulted, indicative of the ring-opening
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Table 2. Copolymerization of 3-Methoxymethyl-3-methyloxetane (MMO) and CO2 Catalyzed by Complex 5a in the Presence of
n-Bu4NN3 at Various Reaction Timesa
time (days)
% polycarbonate
% cyclic carbonate
% CO2 content
% conversion
1
2
3
77.6
85.4
85.7
22.4
14.5
14.2
75.4
73.6
87.6
23.3
52.7
76.7
a Copolymerization conditions: Catalyst loading = 0.012 mol %, 4 g of MMO, 2 equiv of n-Bu4NN3, M/I = 275, 3.5 MPa of CO2, at 110 °C. Percent
conversion to products, product distributions, and % of CO2 content were determined by 1H NMR spectroscopy.
Table 3. Copolymerization of 3-Methoxymethyl-3-methyloxetane (MMO) and CO2 Catalyzed by (salen)CrIIICl Complexes.a
entry
complex
R1
R2
R3
R4
% poly carbonateb
% cyclic carbonateb
% CO2 contentb
% conversionb
1
2
3
4
5b
5c
5a
5d
ꢀC4H4ꢀ
ꢀC4H4ꢀ
H
tert- butyl
OCH3
tert- butyl
tert- butyl
tert- butyl
tert- butyl
87.3
85.1
85.7
84.1
12.7
14.9
14.2
15.9
80.5
64.3
87.6
81.6
42.4
38.3
76.7
37.2
H
tert- butyl
tert- butyl
(1R,2R)-C4H8ꢀ
a Copolymerization conditions: Catalyst loading = 0.012 mol %, 4 g of MMO, 2 equiv of n-Bu4NN3, M/I = 275, 3.5 MPa of CO2, at 110 °C for 3 days.
b Percent conversion to products, product distributions, and % of CO2 content were determined by 1H NMR spectroscopy.
Scheme 2. Ring-Opening Step of MMO Catalyzed by
ꢀ
(salen)Cr(N3)2
process of MMO requiring higher temperatures. Indeed, heating
the reaction mixture at 110 °C led to oxetane ring-opening by
azide as indicated by the organic azide band at 2100 cmꢀ1. After
the reaction solution was heated for 24 h at 110 °C, the main
infrared stretching band observed was that of organic azide. It
should be noted that an analogous experiment employing oxetane
as monomer, led to oxetane ring-opening by azide at 110 °C after
only 3 h.2c Hence, these results indicate that the ring-opening of
monomer 2 by azide is much slower than that of the parent
oxetane. This is consistent with 3-methoxymethyl-3-methyloxe-
tane being more sterically hindered than oxetane. Additionally, the
presence of electron donating substituents on the 3-position of
trimethylene oxide electronically retards the ring-opening step.
Figure 4. Spectra of TCE solutions of chromium salen chloride
complex with 2 equiv of n-Bu4NN3 (blue line), after addition of 100
equiv of MMO at ambient temperature and stirred for 3 h (pink line),
and after stirring the reaction solution at 110 °C for 24 h (green line).
derivatives do not differ significantly, i.e., 2.0492 (11) and 2.056
(8) Å, respectively.
Copolymerization of 2 and Carbon Dioxide Monitored by
in situ IR Spectroscopy. Figure 6 shows the reaction profiles of
both copolymer and cyclic carbonate formation for the copo-
lymerization reaction of MMO and CO2 carried out at 110 °C
and 3.5 MPa in the presence of complex 5a along with 2 equiv of
n-Bu4NN3. It is clearly observed under these reaction conditions
that the formation of cyclic carbonate is enhanced over the
formation of copolymer during the early stages of the coupling
reaction, followed by a slow decrease in cyclic carbonate con-
centration over time. Concomitantly, the formation of polycar-
bonate is initially inhibited, followed by rapid copolymer production
over the remaining course of the reaction. These results are
consistent with formation of copolymer at least in part via ring-
opening of the preformed cyclic carbonate and the presence of an
equilibrium between the cyclic carbonate and the copolymer
products which only slightly favors the copolymer product at
equilibrium. Importantly, the percentage of copolymer in the
product mixture is about 80% at 110 °C based on the assumption
X-ray crystallography was utilized in conjunction with the vN
3
infrared spectral data (vide supra) to verify that MMO binding to
the chromium center occurs without ring-opening at ambient
temperature, similar to what was observed for the oxetane
monomer.2b X-ray quality single crystals of the adduct
(complex 6) formed between complex 5c and monomer 2 were
isolated from a hexanes layered saturated dichloromethane
solution of 5c and a 20-fold excess of 2. A thermal ellipsoid
representation of complex 6 is depicted in Figure 5, with selected
bond distances and bond angles listed in Table 4. This clearly
shows that the sterically encumbering 3-methoxymethyl-3-
methyloxetane monomer is capable of strongly binding to the
(salen)Cr(III) center prior to ring-opening. Noteworthy is the
observation that the CrꢀO(oxetane) bond distances in the
parent oxetane and the 3-methoxymethyl-3-methyloxetane
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Table 4. Selected Bond Distances and Angles for Complex 6a
Cr(1)ꢀCl(1)
2.297(3)
2.056(8)
1.920(6)
94.5(8)
94.4(8)
78.9(9)
89.6(10)
Cr(1)ꢀO(1)
Cr(1)ꢀO(5)
O(1)ꢀC(1)ꢀC(2)
O(1)ꢀC(3)ꢀC(2)
C(3)ꢀC(2)ꢀC(1)
C(3)ꢀO(1)ꢀC(1)
a Units for bond distances and bond angles are Å and deg, respectively.
Figure 7. (A) Three-dimensional stack plot of IR spectra collected every 3
min during the copolymerization reaction of 3-benzyloxymethyl-3-methy-
loxetane and CO2. (B) Reaction profiles obtained after deconvolution of
selected IR spectra, indicating copolymer and cyclic carbonate formation
with time. Reaction carried out at 110 °C in toluene, at 3.5 MPa of CO2
pressure, in the presence of complex 5a and 2 equiv of n-Bu4NN3.
Figure 5. Thermal ellipsoid plot of complex 6. Ellipsoids are at the
50% level.H atoms are omitted for clarity.
on 1H NMR experiments as presented in Table 2. It is also to be
noted here that the product apportionment is much more in
favor of the copolymer (>95%) for the reaction involving CO2
and the parent oxetane monomer.2
On the other hand, the coupling reaction of CO2 with the
sterically more bulky oxetane, 3-benzyloxymethyl-3-methyloxe-
tane (3), afforded an equilibrium distribution of copolymer to
cyclic carbonate of about 60% at 110 °C. The reaction profiles for
the production of copolymer and cyclic carbonate from the
coupling of monomer 3 and CO2 are illustrated in Figure 7.
These observations strongly suggest that the steric requirements
of the substituents on the 3-position of oxetane govern the
relative stabilities of the cyclic carbonate and polycarbonate
afforded from CO2 and oxetanes. Consistent with these qualita-
tive observations is the data reported in Figure 8 for the reaction
between 3,3-dimethyloxetane and carbon dioxide carried out
under similar catalytic conditions as those in Figure 6. In this
instance the equilibrium product distribution was observed to be
88% in favor of copolymer. Hence, the equilibrium distribution
of copolymer to cyclic carbonate produced from the coupling of
oxetanes and CO2 was found to decrease in the order: oxetane
>3,3-dimethyloxetane >3-methoxymethyl-3-methyloxetane >3-
benzyloxymethyl-3-methyloxetane. A similar observation was
reported by Endo and co-workers when examining the ring-
opening polymerization of a series of six-membered cyclic
carbonates.6 In the study by Endo and co-workers the order of
percent conversion of cyclic carbonate to polycarbonate was:
Figure 6. (A) Three-dimensional stack plot of IR spectra collected
every 3 min during the copolymerization reaction of MMO and CO2.
(B) Reaction profiles obtained after deconvolution of selected IR
spectra, indicating copolymer and cyclic carbonate formation with time.
that the extinction coefficients for the νc=o vibrations in the two
products are similar. This observation in turn is consistent with
more definitive quantification of the product distribution based
We have initiated an investigation of the equilibrium process in
reaction 6 as catalyzed in toluene by complex 5a in the presence
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Figure 9. Depolymerization of poly(5-benzyloxymethyl-5-methyl-1,3-
dioxan-2-one) to the corresponding cyclic carbonate in toluene at
110 °C as catalyzed by complex 5a in the presence of 2 equiv of
n-Bu4NN3.
Scheme 3
Figure 8. (A) Three-dimensional stack plot of IR spectra collected
every 3 min during the copolymerization reaction of 3,3-dimethylox-
etane and CO2. (B) Reaction profiles obtained after deconvolution of
selected IR spectra, indicating copolymer and cyclic carbonate formation
with time. Reaction carried out at 110 °C in toluene, at 3.5 MPa of CO2
pressure, in the presence of complex 5a and 2 equiv of n-Bu4NN3.
of two equivalents of n-Bu4NN3. It is noteworthy that computa-
tional studies have shown that conformational isomerization
between the two
the reverse process. This fits with our observation of the oxetane/
CO2 coupling process, where the reaction appears to be proceed-
ing by initial formation of cyclic carbonate prior to ring-opening
polymerization to copolymer since these reactions were per-
formed at elevated pressures of CO2.
’ CONCLUSIONS
Herein, we have extended our studies of the coupling
reaction of CO2 with oxetanes, focusing on oxetanes that are
doubly substituted at the 3-position with substituents of varying
steric requirements. Presented in these investigations were
3-methyloxetanes further substituted at the 3-position with
methyl, ꢀCH2OMe, and ꢀCH2OCH2Ph. Formation of the
copolymer was found to proceed via preformed six-membered
cyclic carbonate to a greater extent as the steric bulk of the
substituents in the 3-position increased. That is, the degree of
copolymer formation from oxetane and CO2 by way of ring-
opening polymerization of the first formed cyclic carbonate
increased in the order listed below at 110 °C. Furthermore, the
rate of the CO2/oxetane coupling process was found to
decrease in this order.
conformers of closely related six-membered cyclic carbonates occurs
with a low activation barrier and with little thermodynamic differ-
ence between the equatorial vs axial conformers.25 It is of interest to
note here that the crystals isolated of 5-methoxymethyl-5-methyl-
1,3-dioxan-2-one (4) were of the axial conformer.
Figure 9 depicts the rates of depolymerization of poly(5-methyl-5-
benzyloxymethyl-1,3-dioxan-2-one) and the concomitant formation
of the corresponding cyclic carbonate for reactions carried out in
toluene at 110 °C in both argon and CO2 atmospheres.26 As is
evident in Figure 9 the rate of depolymerization to cyclic
monomer is greatly retarded by carbon dioxide. This is consistent
with views based on cycloaddition studies involving epoxides and
CO2, where the process proceeding via the carbonate species has
a significantly higher energy of activation compared to that
occurring by way of an alkoxy species (Scheme 3).27 On the
other hand, the forward reaction in eq 6 to provide copolymer
from the cyclic carbonate proceeds at 110 °C at a faster rate than
The thermal stability of the six-membered cyclic carbonate vs
copolymer increased as well with an increase in the steric
requirements of the substituents in the 3-position of the oxetane
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monomer. For example, only a trace of trimethylene carbonate is
seen upon copolymerizing oxetane and CO2, whereas, the
coupling of 3-benzyloxymethyl-3-methyloxetane and CO2 af-
forded an equilibrium product distribution of copolymer to cyclic
carbonate of 60% at 110 °C. A noteworthy observation was that
the rate of the isolated purified polycarbonate proceeding to the
equilibrium distribution of polycarbonate and cyclic carbonate
was greatly retarded by the presence of carbon dioxide. An
ongoing effort in our laboratory is to develop an effective
postsynthetic hydrogenation process for converting the benzy-
loxymethyl copolymer to the hydrophilic polycarbonate contain-
ing pendant hydroxyl groups.
(10) Ariga, T.; Takata, T.; Endo, T. Macromolecules 1997,
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’ ASSOCIATED CONTENT
(19) Westwood, G.; Horton, T. N.; Wilker, J. J. Macromolecules
2007, 40, 3960–3964.
(20) Darensbourg, D. J.; Mackiewicz, R. M.; Billodeaux, D. R.
Organometallics 2005, 24, 144–148.
(21) Nakano, K.; Kamada, T.; Nozaki, K. Angew. Chem., Int. Ed.
2006, 45, 7274–7277.
S
Supporting Information. Tables and cif files providing
b
crystallographic data for compound 4 and complex 6. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(22) Sugimoto, H.; Ohtsuka, H.; Inoue, S. J. Polym. Sci., Part A:
Polym. Chem. 2005, 43, 4172–4186.
’ AUTHOR INFORMATION
(23) Darensbourg, D. J.; Fitch, S. B. Inorg. Chem. 2007,
47, 5474–5476.
Corresponding Author
*E-mail: djdarens@mail.chem.tamu.edu. Fax (979)845-0158.
(24) A PPNþ salt of (salen)Cr(N3)2ꢀ has also been characterized by
X-ray crystallography. See: Darensbourg, D. J.; Mackiewicz, R. M. J. Am.
Chem. Soc. 2005, 127, 14026–14038.
’ ACKNOWLEDGMENT
(25) Kuramshina, A. E.; Bochkor, S. A.; Kuznetsov, V. V. Russ. J. Gen.
Chem. 2009, 79, 787–790.
We gratefully acknowledge the financial support from the
National Science Foundation (CHE 05-43133) and the Robert
A. Welch Foundation (A-0923).
(26) The polycarbonate used in this study was made from monomer
3 and CO2 catalyzed by complex 5a in the presence of 2 equiv of n-
Bu4NN3 at 3.5 MPa of CO2 at 110 °C. The purified copolymer isolated
had 12% ether linkages or an 88% CO2 content and exhibited a Mn value
of 4326 and PDI = 1.32 with a Tg of ꢀ2.16 °C.
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