Mechanistic Study of Palladium-Catalyzed Hydroesterificative Copolymerization of Vinyl Benzyl Alcohol and CO

Article abstract of DOI:10.1021/acs.organomet.9b00091

The copolymerization of vinyl benzyl alcohol (VBA) and carbon monoxide (CO) to give a new polyester poly(VBA-CO) has been achieved via palladium-catalyzed hydroesterification. Reaction conditions involve moderate temperatures, moderate to low CO pressures, and low catalyst loadings to give a low molar mass (M_n 3-4 kg/mol) polymer as a 2:1 mixture of linear to branched repeat units. The polymer molar mass increase is consistent with a step-growth polymerization mechanism, and ester yields of >97% are achieved within 24 h. However, increases in M_n cease beyond 16 h. Control experiments indicate that the degree of polymerization is limited due to a combination of side reactions such as alcoholic end-group oxidation, hydroxycarbonylation, and alcohol acetylation, which lead to the degradation of monomeric and polymeric end groups. When a less promiscuous substrate is used such as 10-undecenol, higher molar masses (M_n 16 kg/mol) are achieved. This method has the potential to be a mild route to new polyester architectures with appropriate mitigation of side reactions.

Full text of DOI:10.1021/acs.organomet.9b00091

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Mechanistic Study of Palladium-Catalyzed Hydroesterificative
Copolymerization of Vinyl Benzyl Alcohol and CO
Gereon M. Yee,† Tong Wang,† Marc A. Hillmyer, and Ian A. Tonks*
Department of Chemistry, University of MinnesotaTwin Cities, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United
States
S
* Supporting Information
ABSTRACT: The copolymerization of vinyl benzyl alcohol (VBA) and carbon monoxide (CO) to give a new polyester
poly(VBA-CO) has been achieved via palladium-catalyzed hydroesterification. Reaction conditions involve moderate
temperatures, moderate to low CO pressures, and low catalyst loadings to give a low molar mass (Mn ∼ 3−4 kg/mol)
polymer as a ∼2:1 mixture of linear to branched repeat units. The polymer molar mass increase is consistent with a step-growth
polymerization mechanism, and ester yields of >97% are achieved within 24 h. However, increases in Mn cease beyond 16 h.
Control experiments indicate that the degree of polymerization is limited due to a combination of side reactions such as
alcoholic end-group oxidation, hydroxycarbonylation, and alcohol acetylation, which lead to the degradation of monomeric and
polymeric end groups. When a less promiscuous substrate is used such as 10-undecenol, higher molar masses (Mn ∼ 16 kg/mol)
are achieved. This method has the potential to be a mild route to new polyester architectures with appropriate mitigation of side
reactions.
INTRODUCTION
■
Polyesters constitute ubiquitous classes of polymers in use
today, finding application in a wide range of products,
including water bottles, films for packaging, and clothing.1
Figure 1. Palladium-catalyzed hydroesterification.
Additionally, the ability of the constituent ester linkages to
cleave hydrolytically has led to their rise as biodegradable and
chemically recyclable materials.2−7 The synthesis of polyester-
based materials is typically achieved via condensation polymer-
ization of dicarboxylic acids with diols,1 a step-growth method
that liberates water as a byproduct.8 Despite the development
of other more controlled synthetic methods such as the ring-
opening transesterification polymerization of cyclic esters9,10 or
the copolymerization of cyclic anhydrides and epoxides,11−13
condensation polymerization remains the predominant meth-
od for the preparation of most polyesters.14 For some
monomers such as long-chain aliphatic diols/diacids, other
methods of polymerization are difficult or ineffective, and
condensation polymerization remains the most viable method
of synthesis.15,16
Alternatively, ester linkages can be formed via palladium-
catalyzed hydroesterification, whereby an olefin, alcohol, and
carbon monoxide (CO) are coupled (Figure 1) to give either
the linear or branched esters.17 A notable industrial application
of this reaction is in the first step of the Lucite α-process,
which couples ethylene, methanol, and CO to make methyl
propionate.18 Hydroesterification of propyne and methanol has
also been used in the direct synthesis of methyl methacrylate,19
and the related hydroxycarbonylation reaction has been used
to produce acrylic acid from acetylene.20 A key challenge of
this chemistry is the control of olefin insertion regioselectivity.
However, recent advances in ligand design have led to catalyst
systems capable of delivering high substrate conversions with
remarkable regioselectivities for a wide range of substrates,21,22
including particularly difficult-to-activate internal and tetrasub-
stituted olefins.23 From the standpoint of polymer synthesis,
hydroesterificative polymerization has the potential to provide
access to polyesters derived from hydroxy olefins, many of
which can be obtained directly from renewable sources.24−26
Additionally, the regioselective control of these systems has the
potential to provide direct access to a wide range of branched
polymer architectures without having to preinstall branch
points via multistep monomer syntheses.27,28
Received: February 11, 2019
© XXXX American Chemical Society
A
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To date, there are only two examples of hydroesterificative
polymerization. The earliest report from 2009 utilized
Co2(CO)8 or Co4(CO)12 precatalysts, activated by pyridine
at high temperatures and CO pressures to give poly-
(dodecyloate) from 10-undecenol with moderate molar masses
(max Mn ∼ 23 000 g/mol) and degrees of polymerization
(∼140).29 Monomer conversions were generally high (>99%),
but long reaction times (∼65 h) were necessary to achieve high
degrees of polymerization. More recently, the same polymer-
ization was demonstrated using palladium-catalyzed hydro-
esterification, which the authors showed to be capable of giving
poly(dodecyloate) under much milder reaction conditions (30
bar CO, 120 °C, 0.2% Pd loading).30 Although the latter report
goes into mechanistic considerations such as possible side
reactions and catalyst dependence, a more detailed under-
standing of the factors influencing molar mass growth and side
product formation is warranted given the promise of this route
to polyesters.
Figure 2. Hydroesterification of styrene with benzyl alcohol.
Herein, we report a palladium-catalyzed hydroesterificative
polymerization of vinyl benzyl alcohol (VBA) to give a new
polymer (poly(VBA-CO)), which can be achieved at mild
reaction temperatures (75−100 °C) and CO pressures (120
psig). Our choice of VBA as a substrate for this chemistry
stemmed from the fact that both the benzylic alcohol and the
styrenic alkene are particularly reactive for the respective
nucleophilic and insertion steps. However, benzylic alcohols
and styrene also undergo facile oxidation and reduction
reactions. As such, we hypothesized that VBA should be a
suitable substrate to explore productive catalytic polyester
formation, as well as an excellent model for observing any
nonproductive reactions that might compete with or hinder
polymer formation. Furthermore, we reasoned that intra-
molecular hydroesterification would be discouraged due to the
anticipated high ring strain of the product paracyclophane
lactones. VBA can also be viewed as a model for other aromatic
olefinic alcohols such as eugenol or other species that may be
derived from the decomposition of lignocellulosic biomass, and
thus the resulting poly(VBA-CO) could provide some insight
as to how these bio-based materials might behave. Importantly,
we highlight some key pitfalls with the Pd-catalyzed system,
which speak generally of the drawbacks of hydroesterificative
polymerizations but which may be circumvented with the
judicious choice of monomer and/or catalyst system.
from either hydrolysis of the product esters or from the
hydroxycarbonylation of styrene (both from adventitious
water).32 The molar amounts of side products in this reaction
are small; they can, however, potentially play an important role
in inhibiting the formation of high molar mass species in the
step-growth polymerization of VBA (vide infra).
With these results, the hydroesterificative polymerization of
VBA was attempted using analogous conditions (Table 1).
After 24 h, analysis of the crude reaction mixture by 1H NMR
spectroscopy showed high ester yields (98%) (Table 1, entry 1;
see SI for full NMR characterization).33 Consumption of CO
gas over the course of the reaction also tracked well with NMR
yields of benzyl ester, suggesting that CO consumption is
primarily associated with ester formation and not involved in
significant side product formation. Analysis of the crude
reaction mixture by size exclusion chromatography (SEC)
showed the presence of a macromolecular species with an
estimated number average molar mass (Mn) of ∼3 kg/mol
(based on comparison to polystyrene standards), correspond-
ing to an estimated degree of polymerization (Xn) of ∼17.
Thus, although palladium-catalyzed hydroesterification of VBA
does indeed lead to the formation of polyester, under these
conditions, only relatively low molar mass material was
obtained.
With these initial results in hand, we sought to understand
the influence of catalyst loading, reaction temperature, and CO
pressure (see the SI for gas uptake curves and SEC data).
These parameters, along with the corresponding benzyl ester
yields, linear/branched ratios, molar masses, dispersities, and
degrees of polymerization, are shown in Table 1. An initial
screen of catalyst loadings showed that Pd loadings as low as
0.5% were suitable to achieve high monomer conversions and
estimated Mn values of ∼3 kg/mol. Increasing the Pd loading
to 1% resulted in virtually no change in Mn. Increasing Pd
loading to 4% led to a significant decrease in overall molar
mass. This is likely due to the greater propensity for alcohol
oxidation by the Pd(OAc)2 precatalyst, thus rendering lower
catalyst loadings advantageous despite the sacrifice in reaction
rate. Running the reaction in the absence of BHT (entry 4)
resulted in a minor decrease in Mn. Increasing the relative
loadings of TsOH·H2O and PPh3 (entries 5 and 6) also led to
lower overall molar mass values. Extending the reaction time as
long as ∼8 days (entry 7) did not lead to any increase in
product molar mass, evidence that the polymerization reaction
RESULTS AND DISCUSSION
■
To test the viability of using VBA as a hydroesterification
substrate, we first explored the reaction between styrene and
benzyl alcohol (Figure 2, see the Supporting Information (SI)
for characterization data). Using 1 M concentration of both
substrates in toluene and a 1/1/4 mol % loading of Pd(OAc)2,
TsOH·H2O, and PPh3, respectively, high conversion (94%) of
styrene and benzyl alcohol to the corresponding branched and
linear (branched/linear ratio = 38:62) benzyl esters was
1
achieved after 16 h, as shown by H NMR spectroscopy and
gas chromatography flame ionization detection (GC-FID; see
Figures S3−S5 for 1H NMR and GC characterization). A small
amount (<1%) of benzaldehyde was formed, which likely
results from benzyl alcohol oxidation (a commonly proposed
entry point into the catalytic cycle;31 Scheme S1) to give the
active [Pd-H]+ species. Other side products were also
observed, including benzyl acetate resulting from the
esterification of acetic acid (derived from Pd(OAc)2), as well
as hydrocinnamic acid and 2-phenylpropionic acid resulting
B
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Table 1. Ester Yield, Molar Mass (Mn), Dispersity (Đ), and Degree of Polymerization (Xn) for the Copolymerization of Vinyl
Benzyl Alcohol and CO under Varying Conditions
a
b
c
d
e
entry
Pd/H+/PPh3 (%) T (°C) pressure (psig) ester yield (%)
CO uptake (%)
linear/branched ratio Mn (kg/mol)
Mw/Mn (Đ) Xn
1
2
3
1/1/4
0.5/0.5/2
4/4/16
1/1/4
1/2/8
1/4/16
1/1/4
1/1/4
1/1/4
1/1/4
1/1/4
75
75
75
75
75
75
75
50
100
75
75
120
120
120
120
120
120
120
120
120
10
98 (91)
98 (90)
98 (89)
99 (90)
97 (89)
92 (85)
>99 (88)
53 (51)
80 (73)
68 (69)
97 (90)
94
88
88
85
92
90
65:35
63:37
70:30
65:35
67:33
68:32
63:37
44:66
74:26
70:30
71:29
3.0
3.1
1.6
2.7
2.3
1.9
3.3
∼0.8
1.8
1.8
1.8
1.8
1.9
1.8
2.0
2.0
1.8
1.7
1.5
1.8
17
17
8
15
12
10
18
3
f
4
5
6
7
g
8
9
10
52
80
88
99
9
3
9
h
∼0.8
1.7
h
11
30
a
b
Reaction conditions: 1 M VBA (in toluene), 1% butylated hydroxytoluene (BHT), 24 h reaction time. Based on standardization of aliphatic 1H
1
NMR signals against dimethylterephthalate as an internal standard. Values in parentheses are based on the benzyl ester H NMR integration.
c
d
e
Given as a percent mmol of VBA. Determined from calibration against polystyrene standards. Defined as [Mn − (296.14)]/(mass of the repeat
f
g
h
unit); mass of expected end groups = 296.14 g/mol. Run with no BHT. Run for 192 h in a Fisher-Porter tube. 16 h runs.
has stopped by 24 h. Increasing or lowering the reaction
temperature to 100 or 50 °C (entries 8 and 9, respectively) led
to notable reductions in both ester yield and molar mass.
Interestingly, reducing the CO pressure as low as 30 psig
(entry 11) did not seem to significantly impact the overall
conversion to ester but gave significantly lower Mn values than
for the 120 psig reaction. Further reduction of the CO pressure
to 10 psig slowed down the reaction, resulting in lower
conversion and low Mn values.
Purified polymer could be isolated by the removal of
palladium using thiol-functionalized silica gel, followed by
precipitation from cold methanol to give a precipitate (Figure
S7), which stiffened upon the removal of solvent in vacuo at 50
°C. Thermogravimetric analysis (TGA, Figure S14, left) of
polymer isolated from entry 1 of Table 1 shows that the
material is stable toward mass loss, with only 1% mass loss at
∼230 °C and 5% mass loss at ∼350 °C. Differential scanning
calorimetry (DSC) shows a single endothermic event
consistent with a glass transition (Tg) at ∼9 °C (Figure S14,
Figure 3. Plot of ester yield vs time for catalytic reactions of VBA with
CO (conditions: 1 M VBA in toluene, 1% Pd(OAc)2, 1% TsOH·H2O,
4% PPh3, 1% BHT as a radical inhibitor, 10% dimethylterephthalate
as an internal standard, PCO = 120 psig). The error bars are
propagated from the error in 1H NMR integration. The red trace is a
single-exponential fit to the data. Data from the 48, 96, and 192 h time
points are from separate reactions.
right).
To better understand the time dependence of the reaction,
as well as the changes in molar mass as a function of monomer
conversion, we evaluated the reaction using the conditions
listed in entry 1 of Table 1. Small aliquots (50 μL) of reaction
solution were taken at various time points and analyzed by 1H
NMR spectroscopy (Figures S15−S18). Ester yields of >97%
were achieved within 16 h of reaction and yields at longer
reaction timeseven up to 192 hare within error of this
(Figure 3).
Analysis of the crude reaction mixtures by SEC (Figure 4)
showed that there is an increase in Mn with increasing reaction
time over the range of ∼6−16 h (molar masses for earlier time
points were too low to be reasonably estimated by SEC).
Plotting the degree of polymerization versus time (Figure 4,
inset) shows this increase to be linear, consistent with an
externally catalyzed step-growth polymerization mechanism.34
However, beyond 16 h, there is a clear drop-off in Mn growth,
and extended reaction times, even up to 192 h (8 days), did
not result in an increase in molar mass (Table 1, entry 7).
Thus, we began to suspect some other mechanism for the
observed molar mass limitations besides less than optimal
conversions. Low catalyst longevity was an obvious suspect,
which we hypothesized could be happening at higher
conversions when the reaction significantly slows down,
allowing high degrees of monomer conversion but preventing
an increase in conversion at longer reaction times. A second
hypothesis was that end-group degradation was leading to a
stoichiometric imbalance of propagating functional groups and
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Figure 5. Plot of degree of polymerization (Xn) versus fractional
conversion (p). The black data points were estimated using diffusion-
ordered spectroscopy (DOSY) (see Experimental Section and the SI
for details) and the blue data points were measured by SEC and
calibrated against polystyrene standards. The red line represents the
fit of the data (shaded region represents 95% confidence interval of
the fit) to eq 1.
Figure 4. SEC traces showing the shift to shorter elution times and
thus higher molecular weight (MW) as the polymerization reaction
progresses from 6 to 24 h. The inset shows a plot of Xn versus reaction
time, highlighting the linear growth in molar mass over time as well as
the drop-off in molar mass growth beyond 16 h.
thus limiting the maximum degree of polymerization
achievable (vide infra).
Catalyst longevity over the course of the reaction was
evaluated by running a reaction to completion (16 h, see
Figure S19) followed by the addition of a second aliquot of
monomer solution. After an additional 42 h of reaction, the
second batch of monomer had reached 82% ester yield,
demonstrating that the catalyst is still active even after the
second monomer addition. In a separate experiment, the
addition of a second bolus of catalyst to a reaction solution that
had been run to completion (16 h), followed by reaction for
another 26 h, did not result in changes in molar mass (Figures
S20 and S21). Thus, catalyst deactivation does not seem to be
the origin of molar mass limitations.
Figure 6. Proposed formation of benzaldehyde and ethyl benzene side
products.
Another factor to consider in step-growth polymerizations is
the preservation of 1:1 stoichiometry between reacting
functional groups, as expressed by the Carothers equation
(eq 1)
1 + r
Xn =
1 + r − 2rp
Figure 7. Reaction of VBA in the absence of CO.
(1)
where p represents the fractional conversion and r is the
stoichiometric ratio of reactive functional groups. In the limit
of p → 1, Xn depends solely on r, and even small imbalances in
functional group stoichiometry can have a dramatic limitation
on molar mass.34 The plot of Xn vs p for the present system
along with the corresponding fit to eq 1 is shown in Figure 5.
From the fit, a value of r ∼ 0.88 was estimated, consistent
with a significant stoichiometric imbalance. Based on this
analysis and given the fact that only relatively low degrees of
polymerization are achieved despite the observed high yields of
ester, we hypothesize that molar masses are limited by
functional group degradation over the course of these
reactions. Notably, the presence of vinylic and benzyl alcohol
end groups for the purified polymer could not be
time-of-flight (MALDI-ToF) mass spectrometry was similarly
not definitive (Figure S23).
There are several plausible pathways for the loss of
propagating functional groups in this reaction. First, as
mentioned previously, alcohol oxidation is a common pathway
of activation in Pd-catalyzed hydroesterification reac-
tions.30,35,36 Indeed, small peaks at ∼10 ppm, consistent with
the formation of aldehydes,37 are observable in the H NMR
1
spectrum of crude reaction mixtures. Multiple peaks are
observed, including 4-vinylbenzaldehyde, likely indicating that
both monomeric and polymeric alcohol moieties are oxidized.
Furthermore, Pd catalysts have also been known to couple
alcohol oxidation with CC and CX (X = O, N)
reductions via transfer hydrogenation38,39 (Figure 6).
1
unambiguously identified by H or 13C NMR spectroscopy.
Further analysis by matrix-assisted laser desorption ionization
D
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When reactions were carried out under an atmosphere of N2
in the absence of CO (Figure 7), the yield of 4-vinyl-
benzaldehyde is significantly enhanced compared to normal
catalytic conditions, and 4-ethylbenzaldehyde40 and 4-ethyl
benzyl alcohol41 were also observed in both the H NMR
1
spectrum (Figure S29) and GC−MS chromatogram (Figure
S30) for the crude reaction mixture. This evidence is
consistent with transfer hydrogenation operating in con-
junction with catalyst activation through benzyl alcohol
oxidation.
In addition to alcohol oxidation and transfer hydrogenation,
other plausible side reactions are possible, but spectroscopic
evidence for their involvement during catalysis is limited. In
some instances, we have also observed the formation of
hydrovinylation products (Figure S15), which occurs most
likely via a Pd-catalyzed insertion mechanism42,43 or cationic
addition44 (Figure 8). Other possible side products include the
Figure 9. Degradation of polymer by methanolysis.
1
Figure 10. H NMR spectrum of crude degradation product from
reaction shown in Figure 8, which is made up of methylated and
acetylated linear and branched monomeric and dimeric repeat units,
of poly(VBA-CO).
Figure 8. Proposed route to hydrovinylation products.
The rich side chemistry of VBA prompted us to explore
hydroesterificative polymerization of 10-undecenol, which we
expected to be less easily oxidized. Reactions were carried out
according to the conditions shown in Table 2 and are provided
along with their results. Under typical conditions for the
polymerization of VBA, as described in entry 1 of Table 1, we
observed that the reaction is much slower than with VBA.
However, we found that increasing the concentration to 1.67
M gave ester yields of 77% (entry 1, Table 2), and increasing
the loading of acid and PPh3, as well as increasing the
temperature to 100 °C, gave high ester yields (∼94%) within
16 h (entry 2, Table 2). Notably, this polymer could be easily
precipitated from MeOH to give a white solid (Figure S32,
left). The Mn value of the isolated polymer was determined by
end-group analysis (Figure 11), whereby the 1H NMR
integration of the methylene protons α to the ester oxygen
(denoted by the dark blue dots in Figure 11) was compared to
the integration of the methylene protons α to the alcohol
oxygen (light blue dot in Figure 11). The ratio of these 2
values is a quantitative measure of the number of ester groups
per alcohol end group, indicating approximately 79 esters per
alcohol, which corresponds to a polyester molar mass of Mn ∼
14 kg/mol.45 In addition to the alcohol end group, an olefinic
corresponding hydroxycarbonylation products 3-phenyl- and
2-phenylpropionic acids, which we have observed in the
hydroesterification of styrene and benzyl alcohol by gas
chromatography (vide supra, Figure S5). Thus, a portion of
the unwanted side reactions that occur during VBA polymer-
ization is directly tied to TsOH(H2O): either through the
oxidation of a substrate to generate the active [Pd-H]+ species
or through hydroxycarbonylation.
Since the analysis of end groups or defects using NMR
spectroscopy or mass spectral analysis was ambiguous, we
attempted to degrade samples of the isolated polymer into the
respective methyl esters of the corresponding repeat units. This
was done via digestion of polymer in the presence of MeOH at
170 °C for 24 h, using Zn(OAc)2 (20 mol % loading) as a
catalyst (Figure 9). Analysis of the reaction mixture by SEC
after digestion showed no molar masses of >1 kg/mol, and
1
analysis by H NMR spectroscopy (Figure 10) revealed new
resonances consistent with benzyl alcohol moieties, as well as
methyl and acetate ester groups. Further analysis by electro-
spray ionization mass spectrometry showed masses for the
methylated and acetylated linear and branched monomeric and
dimeric repeat units, thus supporting the degradation of the
polyester backbone by methanolysis (Figure S31). However,
efforts to isolate and identify polymer end groups or defected
repeat units were unsuccessful.
1
end group is also observed at ∼5.4 ppm in the H NMR
spectrum. The integration for the olefin is about half that of the
alcoholic methylene groups, indicating that there must be
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propensity for the loss of end-group fidelity, but that the
small amount of end groups that are lost may still limit Mn.
Table 2. Ester Yield, Molar Mass, Dispersity, and Degree of
Polymerization Data for the Copolymerizations of 10-
Undecenol and CO under Various Conditions
a
CONCLUSIONS
■
We were able to demonstrate the use of hydroesterificative
polymerization toward synthesizing the new polyester poly-
(VBA-CO), resulting in high conversions of monomer to ester
but giving polymer product with a low molar mass. Although
VBA ultimately proved to be a poor substrate for productive
hydroesterificative polymerization, the reactive nature of the
benzyl alcohol and styrene functionalities allowed us to identify
key side chemistries. By characterizing the changes in molar
mass growth as a function of monomer conversion, we were
able to definitively support a step-growth polymerization
mechanism and identify the loss in 1:1 stoichiometry between
the alcohol and vinylic functional groups as a likely cause for
inhibition in molar mass growth. We hypothesize that although
such side reaction-induced end-group deactivation is likely to
be general for hydroesterificative polymerization as a whole,
the influence of such factors can likely be mitigated by the
judicious choice of a catalyst system and substrate. We
demonstrate the latter point in the successful polymerization of
10-undecenol to give poly(dodecyloate), in similar molar mass
and polymer characteristics, as reported previously, but with
the use of a simpler catalyst system.30 Ultimately, the lessons
learned here will assist in the further design and optimization
of practical hydroesterificative polymerization reactions.
T
P
ester yield
(%)
Mn
Mn/M
cw
bc
,
d
entry (°C)
(psig)
(kg/mol)
(Đ)
Xn
e
1
2
3
4
75
100
100
100
120
120
120
30
77
95
98
89
0.7(1.2)
14(10)
16(12)
2.0(2.8)
1.8
1.8
1.8
1.8
1.5
69
79
8
f
a
Conditions: 1.67 M 10-undecen-1-ol in toluene (3 mL, 5 mmol), 1%
Pd(OAc)2 (0.05 mmol), 4% TsOH·H2O (0.2 mmol), 8% PPh3 (0.4
mmol), 16 h reaction time. Molar mass determined by H NMR
end-group analysis; value in parenthesis determined by comparison to
polystyrene standards. Determined by SEC in tetrahydrofuran
(THF) vs polystyrene standards. Defined as [Mn − (396.61)]/
(mass of the repeat unit). Conditions: 1.67 M 10-undecen-1-ol in
b
1
c
d
e
toluene (3 mL, 5 mmol), 1% Pd(OAc)2 (0.05 mmol), 1% TsOH·H2O
(0.05 mmol), 4% PPh3 (0.2 mmol). 72 h reaction time.
f
EXPERIMENTAL SECTION
■
Materials and Methods. Solvents and reagents were purchased
from Sigma-Aldrich, STREM, and Oakwood Chemicals and used
without further purification unless otherwise noted. Deuterated
chloroform (CDCl3) was purchased from Cambridge Isotope
Laboratories and used without further purification. A Biotage
Endeavor 8-port parallel pressure reactor was used for optimization
experiments (Figure S2). Gas chromatography mass spectrometry
(GC−MS) was performed using an HP 6890 GC system (HP-5
column, 30 m length, 0.25 mm ID, 0.5 μm film thickness; temperature
program: 50 °C for 1.5 min, 20 °C/min to 290 °C, hold 6.5 min)
equipped with an HP 5973 mass selective detector. Gas
chromatography flame ionization detection (GC-FID) was performed
using an Agilent 7890 series gas chromatograph system (HP-5
column, 30 m length, 0.32 mm ID, 0.25 μm film thickness;
temperature program: 50 °C for 1.5 min, 20 °C/min to 290 °C,
hold 6.5 min) equipped with a Polyarc reactor for quantitative carbon
detection. Size exclusion chromatography (SEC) was performed in
tetrahydrofuran (THF) using an Agilent 1260 Infinity LC system
equipped with three Waters Styragel columns (HR6, HR4, HR1, 5
μm particles of poly(styrene-divinylbenzene)) connected in series and
fitted with a Wyatt OPTILAB T-rEX refractive index detector.
Determination of molecular weights and polydispersities was made by
calibration against polystyrene standards. Differential scanning
calorimetry (DSC) analysis was performed on a TA Instruments
Discovery DSC using hermetically sealed aluminum T-zero pans.
Scans were conducted under a nitrogen atmosphere at a heating/
cooling rate of 10 °C/min unless otherwise noted. Thermogravimetric
analyses (TGA) were performed on a TA Instruments Q500 under a
nitrogen atmosphere at a heating rate of 10 °C/min. Mass
spectrometric measurements using matrix-assisted laser desorption/
time-of-flight (MALDI/ToF) mass spectrometry were performed on
an AB-Sciex 5800 MALDI/TOF mass spectrometer in positive
reflector mode using dithranol as the matrix. THF solutions of
polymer (10 mg/mL), dithranol (20 mg/mL), and sodium
trifluoroacetate (5 mg/mL) were mixed in a 2:10:1 ratio, and a 0.5
μL aliquot of the final solution was applied to the sample plate for
analysis. Masses were externally calibrated against a standard mixture
Figure 11. 1H NMR spectrum of precipitated polymer from the
reaction shown in entry 2, Table 2.
another type of end group present, likely alkyl from the
reduction of the olefin. Estimation of Mn by SEC using
polystyrene standards was found to be ∼10 kg/mol, reasonably
1
similar to the molar mass determined by H NMR analysis.
The dispersity of the polymer was found to be ∼1.8, similar to
what was observed for reactions with VBA and consistent with
a step-growth polymerization mechanism. Extending the
reaction time to 72 h (entry 3, Table 2) resulted in a slight
increase in molar mass, likely due to a slight increase in
conversion (see Figure S32, right, for SEC comparison).
1
Notably, in both the 16 and 72 h reactions, the H NMR
spectra obtained for the precipitated polymer samples show a
small peak centered at around ∼9.78 ppm, consistent with
some degree of aldehyde end groups. Furthermore, carrying
out the polymerization reaction in the absence of CO
(analogous to Figure 7) showed analogous side products to
the reaction with VBA but in much lower yields (Figures S33
and S34). Thus, we propose that the increased values of Mn
observed for 10-undecenol are consistent with a lower
F
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Article
of small peptides (leucine encephalin, bradykinin 2-9, angiotensin I,
Glu-fibrinogen, and ACTH 18-39). H NMR spectra were recorded
(∼88.7 mL) was charged with Pd(OAc)2 (0.0112 g, 0.0500 mmol),
TsOH·H2O (0.0380 g, 0.200 mmol), and PPh3 (0.0906 g, 0.400
mmol). A toluene solution of 10-undecenol (3 mL at ∼1.67 mL, 5.00
mmol) was subsequently added followed by sealing of the reaction
vessel and pressurization/venting of the vessel with CO (4×, ∼120
psig), after which the vessel was pressurized to the desired pressure.
The reaction was then heated to the desired temperature and allowed
to react for 16 h, after which the vessel was vented to air and the
solution transferred to a 20 mL scintillation vial. To the solution was
added dimethylterephthalate (0.0971 g, 0.500 mmol) as an internal
standard. Analysis by 1H NMR spectroscopy and SEC was performed
on a 50 μL aliquot, which had been concentrated in vacuo and
redissolved in either 1 mL of CDCl3 (NMR) or 2 mL of THF (SEC).
Conversions for each reaction were approximated as the yield of
polymer repeat unit, as determined by the integration of the
corresponding aliphatic ester −PhCH2− protons. For SEC analysis,
dissolved Pd was removed using thiol-functionalized silica gel
(SiliaMetS Thiol from SiliCycle) and either stirring the sample
overnight or sonicating for 1 h at 40 °C, followed by filtration.
Molecular weight determination by end-group analysis was done on
samples of polymer that had been precipitated from MeOH and
isolated by filtration to remove any unreacted monomer.
1
on Varian Inova 500 MHz, Bruker Avance III 500 MHz, Bruker
Avance III HD 500 MHz, or Bruker Avance III HD 400 MHz
spectrometers. 13C, correlation spectra (COSY), heteronuclear single-
quantum correlation spectra (HSQC), and heteronuclear multiple-
bond correlation spectra (HMBC) were recorded on a Bruker Avance
III HD 500 MHz spectrometer. Chemical shifts are reported with
respect to tetramethylsilane (TMS).
Safety Considerations. Carbon monoxide (CO) is a highly toxic
and odorless gas, and extreme caution must be exercised when
working with high pressures of CO, both from an exposure standpoint
and from explosion risk. All reactions should be performed in a
properly working fume hood, with active CO monitoring at all times.
It is recommended that work involving CO be carried out during
normal working hours such that in the event of exposure, proper
emergency response can be made. Pressure vessels should be checked
for cracks prior to pressurization and properly secured with clamps,
weights, etc. Once pressurized, vessels should be placed behind an
appropriate blast shield when left unattended.
Synthesis of Vinyl Benzyl Alcohol (VBA). Synthesis was
modified from a previously reported procedure.46 A 2 L round-
bottom flask was charged with NaOH (0.904 g, 22.6 mmol), 0.95 L of
water, and a large stir bar. The flask was fitted with a reflux condenser
and heated to reflux, at which time Bu4NBr (73.1 g, 226 mmol) was
added with an additional 50 mL of water, as well as 4-vinyl benzyl
chloride (32 mL, 226 mmol). The reaction was stirred vigorously, and
upon reaching reflux, the suspension went from a deep yellow-orange
color to a much lighter yellow. At this point, the reaction was heated
for an additional 40 min with vigorous stirring, after which the flask
was cooled in an ice bath. To the aqueous mixture was added 50 mL
of 2 M HCl solution, and the aqueous phase was extracted with
EtOAc (5 × 100 mL). The combined organic phases were washed
with 2 M HCl (50 mL), water (50 mL), and brine (3 × 50 mL) and
dried over sodium sulfate. After filtration to remove sodium sulfate,
the organic phase was concentrated by rotary evaporation to give a
crude yellow oil. Nonpolar impurities were removed by passing
through a plug of silica gel (∼200 g on a 600 mL frit) first with
hexanes (1.5 L), followed by washing with EtOAc (1 L), and finally
concentrated by rotary evaporation to give a light yellow oil. This was
subsequently distilled in fractions by Kugelrohr distillation, followed
by recrystallization from hexanes, to give a colorless, crystalline solid,
which melts slightly above room temperature (11.2 g, 36.9% yield).
General Procedure for the Catalytic Hydroesterificative
Polymerization of Vinyl Benzyl Alcohol (VBA). The correspond-
ing reaction vessel (3 oz. Fisher-Porter tube or an appropriate test
tube of ∼15 mL volume for reactions performed in the parallel
pressure reactor) was charged with Pd(OAc)2 (0.0112 g, 0.0500
mmol), TsOH·H2O (0.0095 g, 0.0500 mmol), PPh3 (0.0453 g, 0.200
mmol), and BHT (0.0110 g, 0.0500 mmol). A toluene solution of
VBA (∼1 M, 5.00 mmol VBA, 4.4 mL of toluene) was subsequently
added followed by sealing of the reaction vessel and pressurization/
venting of the vessel with CO (4×, ∼120 psig), after which the vessel
was pressurized to the desired pressure. The vessel was then heated to
the desired temperature and allowed to react for 24 h, after which the
vessel was cooled to ∼30−40 °C, vented to air, and the solution
transferred to a 20 mL scintillation vial. To the solution was added
dimethylterephthalate (0.0971 g, 0.500 mmol) as an internal standard.
General Procedure for the Determination of Polymer
Conversions and Molecular Weights as a Function of Reaction
Time. A 3 oz. (∼87 mL) Fisher-Porter tube was charged with
Pd(OAc)2 (0.0112 g, 0.0500 mmol), TsOH·H2O (0.0095 g, 0.0500
mmol), and PPh3 (0.0453 g, 0.200 mmol). A stock solution of VBA
(1.0 M), dimethylterephthalate (0.10 M), and BHT (0.01 M) was
added (5 mL) and the tube was sealed. The vessel was pressurized/
vented with CO (4×, ∼120 psig) before finally being pressurized to
120 psig and heated at 75 °C in an oil bath. At specified time points,
the reaction was briefly stopped by venting to air and removing from
heat, and a 50 μL aliquot of reaction solution was removed for
analysis. The vessel was quickly resealed, repurged with CO, as
described above, and heated again for the allotted time. Aliquots taken
for analysis were concentrated in vacuo to remove toluene and
1
redissolved in 1 mL of CDCl3 for analysis by H NMR and DOSY
spectroscopies. After spectroscopic analysis, these samples were
subsequently sonicated at 40 °C with thiol-functionalized silica gel,
filtered, and concentrated in vacuo before being redissolved in 2 mL
of THF for analysis by SEC.
General Procedure for the Determination of Polymer
Molecular Weights by Diffusion-Ordered Spectroscopy
(DOSY). Polymerization samples for DOSY analysis were prepared
by taking 50 μL aliquots of reaction solution, concentrating in vacuo,
and redissolving the crude residue in 1 mL of CDCl3 containing 5%
TMS as a small molecule reference. Based on the amount of starting
substrate, the total concentration of monomer in each sample is at
most 50 mM. For generation of the polystyrene calibration curve, 1−2
mg of polystyrene was dissolved in 1 mL of CDCl3. Concentrations
were kept low to avoid changes in solution viscosity between samples.
Spectra were collected using a Bruker Avance III AV 500 MHz
spectrometer equipped with a broad-band probe with a z-axis gradient
coil. Data collection was performed using the standard Bruker pulse
sequence ledbpgp2s, which utilized a longitudinal eddy current delay
of 5 ms and a gradient recovery delay of 0.2 ms, as well as smoothed
square bipolar gradient pulses for eddy current correction and two
spoiled gradients. A diffusion time (Δ) of 100 ms was used for all
samples, whereas the gradient pulse duration (δ) was varied from 1.4
to 1.8 ms. For each experiment, the gradient strength was
incremented linearly in 32 steps from 2 to 98% of the maximum
gradient strength (54 G/cm). Diffusion constants were determined
from single-exponential fits to plots of peak area (I) versus (γgδ)2(Δ
− δ/3), which comes from the Stejskal−Tanner equation (eq 2)
1
Analysis by H NMR spectroscopy and SEC was performed on a 50
μL aliquot, which had been concentrated in vacuo and redissolved in
either 1 mL of CDCl3 (NMR) or 2 mL of THF (SEC). Conversions
for each reaction were approximated as the yield of polymer repeat
unit, as determined by the integration of the corresponding benzyl
ester −PhCH2− protons or alkyl protons derived from hydro-
esterification of the styrenic olefins. For SEC analysis, dissolved Pd
was removed using thiol-functionalized silica gel (SiliaMetS Thiol
from SiliCycle) and either stirring the sample overnight or sonicating
for 1 h at 40 °C, followed by filtration.
(γgδ)2(Δ−δ/3)D
I
I0
= e
(2)
where γ is the gyromagnetic ratio of the nucleus, g is the gradient
strength, δ is the gradient pulse duration, Δ is the diffusion time, and
D is the diffusion coefficient. Thus, from eq 2, single-exponential fits
General Procedure for the Catalytic Hydroesterificative
Polymerization of 10-Undecenol. A 3 oz. Fisher-Porter tube
G
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give the diffusion constant D as the slope of the exponential function.
Whenever possible, the decay of multiple peaks was fitted and their
diffusion constants averaged. Molecular weights were determined by
comparison to a plot of log(Drel) vs log(MW) (Figure S22) generated
for a set of polystyrene standards, where Drel is the diffusion constant
of polystyrene normalized using the diffusion constant of TMS as a
small molecule reference according to eq 347
glucarodilactone, and 2,5-bis(hydroxymethyl)furan subunits. Polym.
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log(D ) = log(DTMS,avg) − log(DTMS) + log(D
)
rel
poly
(3)
Using relative diffusion constants in this way helps account for
systematic errors between samples such as changes in viscosity or
issues with convection.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00091.
(11) Sanford, M. J.; Pena Carrodeguas, L.; Van Zee, N. J.; Kleij, A.
̃
W.; Coates, G. W. Alternating Copolymerization of Propylene Oxide
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Renewable Aliphatic Polyesters with High Glass Transition Temper-
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G. W. Development of Highly Active and Regioselective Catalysts for
the Copolymerization of Epoxides with Cyclic Anhydrides: An
Unanticipated Effect of Electronic Variation. J. Am. Chem. Soc. 2016,
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(13) Fieser, M. E.; Sanford, M. J.; Mitchell, L. A.; Dunbar, C. R.;
Mandal, M.; Van Zee, N. J.; Urness, D. M.; Cramer, C. J.; Coates, G.
W.; Tolman, W. B. Mechanistic Insights into the Alternating
Copolymerization of Epoxides and Cyclic Anhydrides Using a
(Salph)AlCl and Iminium Salt Catalytic System. J. Am. Chem. Soc.
2017, 139, 15222−15231.
(14) Zhang, X.; Fevre, M.; Jones, G. O.; Waymouth, R. M. Catalysis
as an Enabling Science for Sustainable Polymers. Chem. Rev. 2018,
118, 839−885.
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Long-Chain C26 Monomers and Their Polymers. Macromol. Chem.
Phys. 2012, 213, 2220−2227.
Experimental setup, as well as all characterization data
1
(including representative 1D H and 13C NMR spectra,
2D NMR spectra, GC-FID, GC-MS, and ESI-MS data,
TGA and DSC data, SEC traces, gas-uptake curves)
(PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: itonks@umn.edu.
ORCID
Marc A. Hillmyer: 0000-0001-8255-3853
Ian A. Tonks: 0000-0001-8451-8875
■
Author Contributions
†G.M.Y. and T.W. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(16) Stempfle, F.; Ortmann, P.; Mecking, S. Long-Chain Aliphatic
Polymers to Bridge the Gap between Semicrystalline Polyolefins and
Traditional Polycondensates. Chem. Rev. 2016, 116, 4597−4641.
(17) Wu, X. F.; Fang, X.; Wu, L.; Jackstell, R.; Neumann, H.; Beller,
M. Transition-metal-catalyzed carbonylation reactions of olefins and
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interaction of methyl propanoate with silica. Phys. Chem. Chem.
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Financial support for this work was provided by the Minnesota
Corn Growers’ Association. M.A.H. acknowledges the Center
for Sustainable Polymers at the University of Minnesota, a
National Science Foundation supported center for Chemical
Innovation (CHE-1413862). Equipment for the Chemistry
Department NMR facility were supported through a grant
from the National Institutes of Health (S10OD011952) with
matching funds from the University of Minnesota. I.A.T. is a
2017 Alfred P. Sloan Fellow.
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