A Synthetic Polyester from Plant Oil Feedstock by Functionalizing Polymerization

Article abstract of DOI:10.1002/anie.201810914

Catalytic functionalization/polymerization of castor oil-derived undecenol yields an aliphatic polyester in a single step under mild conditions. The key to selective formation of linear high melting polyester is highly active carbonylation catalysts that at the same time do not undergo strong isomerization.

Full text of DOI:10.1002/anie.201810914

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Accepted Article
Title: Synthetic Polyester from Plant Oil Feedstock by Functionalizing
Polymerization
Authors: Ye Liu and Stefan Mecking
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10.1002/anie.201810914
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Synthetic Polyester from Plant Oil Feedstock by Functionalizing
Polymerization
Ye Liu, and Stefan Mecking*
Abstract: Catalytic functionalization/polymerization of castor oil-
derived undecenol yields an aliphatic polyester in a single step under
mild conditions. The key to selective formation of linear high melting
polyester is highly active carbonylation catalysts that at the same
time do not undergo strong isomerization.
branched polymer microstructure.[11]
Scheme 2. Suggested formation of the active species from a diphosphine-
coordinated Pd(II) catalyst precursor, accounting for the adverse effect of
excess aldehyde. P∧P is e. g. dtbpx (bis(di-tert-butyl)phosphino-o-xylene).
Thermoplastic polymers are currently prepared almost
exclusively from fossil feedstocks. In view of their limited range,
alternative renewable sources are desirable in the long term.[1-3]
Polyesters are one of the most important classes of polymers,
and indeed the more recently developed and commercialized
biomass-based polymers are thermoplastic polyesters.[3-6] The
preparation of the corresponding monomers can employ a
fermentation step, with carbohydrate, most often glucose, as a
feedstock.[7] By comparison to fermentation, chemical synthetic
routes in which the original molecular structure of the plant
biomass employed is substantially retained are attractive as they
can be efficient in terms of feedstock utilization and reaction
space-time-yields, and provide novel properties. Particularly,
fatty acids from plant oils can yield longer methylene sequences
Alkoxycarbonylation of olefins in principle is a well studied
reaction. It is used industrially for the production of methyl
propionate from ethylene, CO and methanol.[12,13] Compared to
ethylene, substituted olefins are less reactive, but a number of
advanced catalyst systems have been reported recently.[14-16]
These can even isomerize an internal double to selectively form
the product ester group at a remote position, e.g. methyl oleate
can be carbonylated to the terminal linear 1,19-diester.[17-19]
Notwithstanding this background, the target reaction poses
several challenges. Catalysts that are highly active and/or
terminal selective in olefin alkoxycarbonylation are also often
very active for isomerization.[20,21] For example, the industrially
used
extremely
active
catalyst
for
ethylene
as
a a further
repeat unit motif.[8,9] Targeting polymers,
methoxycarbonylation with methyl oleate as a substrate affords
the aforementioned isomerizing alkoxycarbonylation. With
undecenol as a substrate for polymerization, double bond
isomerization can be detrimental when it converts this substrate
to undecanal. Formation of this saturated aldehyde not only
consumes monomer, but also via acetal formation with two
further molecules of substrate can disturb the stoichiometric
balance vs. the intial AB-monomer composition.
functionalization in addition to the carboxy group originating from
the fatty acid feedstock is required in order to obtain difunctional
repeat units.
Scheme 1. Carbonylation polymerization of undecenol derived from castor oil.
To enhance the utilization of renewable feedstocks,
straightforward synthetic schemes are desirable. We now report
on an efficient functionalization and polymerization in a single
step via alkoxycarbonylation of undec-10-ene-1-ol (Scheme 1).
The latter is readily available from castor oil by thermal cracking
and hydrogenation.[10] However, a functionalizing polymerization
to date requires high pressure (200 atm) and temperature (160
oC), using [Co2(CO)8] as a catalyst, and unselectively forms a
[*]
Dr. Ye Liu, Prof. Dr. Stefan Mecking
Chair of Chemical Materials Science, Department of
Chemistry,University of Konstanz
Figure 1. 1H NMR (left) and 31P{1H} NMR (right) spectra of CD2Cl2 solutions of
[(dtpbx)Pd(OTf)2] containing an excess (40 equiv.) of methanol (bottom),
undecanol (center), and undecenol (top), respectively (room temperature).
Universitätsstraße 10, 78457 Konstanz (Germany)
E-mail: stefan.mecking@uni-konstanz.
Dr. Ye Liu, present address:
To probe these anticipated issues, the well-established
isomerization active carbonylation catalyst based on the bulky
electron rich diphosphine dtpbx was studied in NMR tube
experiments. Dissolution of the single component precursor
State Key Laboratory of Fine Chemicals
Dalian University of Technology
Dalian 116024 (China)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201810914
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[(dtpbx)Pd(OTf)2] in a CD2Cl2/methanol mixture resulted in clean
accounted for a reversal reaction of the formation for the active
species (Scheme 2).
complete
conversion
to
the
hydride
complex
[(dtpbx)PdH(solvent)]+, in agreement with previous studies[20]
(Scheme 2 and Figure 1, bottom. Note that Pd-H formation is
already observed with 1 eq MeOH, and is complete with only 10
equivalents). As expected the resulting hydride solution is very
active for olefin isomerization, as probed by addition of 1-decene
(cf. Supporting Information, Table S2, entries 13-18). Higher
alcohols are known to be less reactive, but also with undecanol
instead of methanol the hydride is formed cleanly (Figure 1,
center). This suggests that the hydroxyl function of the
undecenol monomer is also capable of activating the catalyst
precursor to the active hydride species effectively. However,
with undecenol as the alcohol component, no hydride species
Scheme 3. Diphosphines studied in this work.
1
In view of these partly contradictive requirements, a range of
diphosphines (Scheme 3) anticipated to vary in their propensity
for isomerization and terminal carbonylation were studied for the
targeted carbonylation polymerization of undecenol in pressure
reactor experiments (Table 1). Indeed, with dtpbx and related
structures (bpx and dtbpp), only small amounts of low molecular
weight polyesters are formed and significant aldehyde formation
occurs (entries 1-3). By contrast with pytbpx14 – known to be
extremely reactive in olefin methoxycarbonylation – polyester is
formed as the major product (entries 4-6). This is surpassed by
xantphos, which converts the undecenol completely to polyester
that also has a substantially higher molecular weight of Mn 1.7 ×
were observed (Figure 1, top). H NMR resonances at 4.46 and
9.77 indicate the formation of aldehyde and acetal products
(Figures S1 and S3). The reducing agent in Pd-hydride
formation is considered to be the alcohol present, even though
the expected (aldehyde) stoichiometric organic products have
not been confirmed.[22] However, addition of excess nonanal to
separately prepared Pd-H species results in their consumption
(Figure S2), and the same is expected for undecanal formed by
isomerization of the monomer. This NMR scale observation is in
accordance with low efficiency towards carbonylation with dtbpx-
based Pd catalysts in pressure reactor experiments (vide infra).
For the case studied here, the adverse effect of aldehyde can be
Table 1. Carbonylation polymerization of undec-10-en-1-ol.[a]
[d]
Entry
diphosphi
ne ligand
Temp
(oC)
CO
(bar)
Time
(h)
Con. to (%)[b]
Polyester
yield (g)
Linear
Mn
Mw/Mn
Tm
Sel. (%)[c]
(kg/mol)[d]
(oC)[e]
polyest
er
aldehyde
and acetal
isomerized
olefin
1
dtbpx
90
30
30
30
30
30
30
30
30
30
30
20
10
30
30
10
30
24
24
24
24
72
24
24
24
48
48
24
24
24
24
24
24
29
43
39
90
95
89
99
>99
>99
33
>99
97
47
88
82
68
24
5
47
53
28
10
5
---
---
---
---
---
2
bpx
90
<0.1
---
98
90
89
86
86
71
71
70
---
1.7
---
1.2
---
---
3
dtbpp
90
33
0
---
4
pytbpx
pytbpx
pytbpx
xantphos
xantphos
xantphos
xantphos
xantphos
xantphos
doapp
90
1.6
1.7
1.3
1.7
1.9
1.9
<0.1
1.9
1.6
<0.1
1.5
1.0
0.9
3.3
2.7
4.1
10.3
13.6
17.4
---
1.5
1.9
1.4
1.8
1.8
1.8
---
64, 70
65, 68
61, 67
55, 60
55, 63
54, 60
---
5
90
0
6
120
90
<1
0
11
2
7
8
120
120
50
0
0
9
0
0
10
11
12
13
14
15
16
0
15
1
120
120
90
0
71
74
88
98
98
89
12.9
5.8
1.8
3.3
3.1
1.22
1.7
1.5
1.2
1.4
1.3
1.9
56, 63
56, 63
51
0
3
0
53
12
15
22
adtbpb
adtbpb
tbcypb
90
<1
3
73, 75
68, 71
50, 55
90
90
10
aReaction conditions: PdOAc2/diphosphine/MSA/olefin = 1/2/10/500, molar ratio,1.9 g undec-10-en-1-olin 4.0 mL toluene. bDetermined by 1H NMR.
cDetermined from integration of 13C NMR carbonyl resonances. dDetermined by GPC in THF, calibrated with polystyrene standards. eDetermined by DSC.
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10.1002/anie.201810914
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104 g mol-1 (entry 9). Note that this value is on the same order as
In view of these findings, the isomerization behavior of
selected catalysts was studied further by NMR spectroscopy,
under conditions similar to pressure reactor polymerizations (but
in the absence of CO). In all cases, isomerization of the terminal
double bond of the undecenol substrate occurred even at room
temperature or slightly above, and at 90 °C no terminal olefin
remained. However, only with dtbpx and also adtbpb (though
less rapid) full isomerization across the entire C11 chain to form
the aldehyde occurred, while this was not the case for xantphos
and pytbpx (Figure S4). The identified microstructures of
polyesters formed with different diphosphine ligands are
confirmed by selectivities observed in alkoxycarbonylation of
model olefins (1- and 2-octene) with undecanol (Figure S16).
In summary, carbonylation polymerization of undecenol can
yield polyesters at moderate conditions, namely 10 to 20 atm of
CO pressure with virtually full conversion of the substrate to the
desired polyester. In general terms, any polycondensation
requires sufficient reaction rates even at low concentrations of
the reacting groups, as present in the final stages of the reaction,
to achieve reasonably high molecular weights. It is notable that
the catalytic carbonylations studied here can fulfill this criterion.
The key to achieving these high conversions is catalysts very
active for carbonylation, which at the same time do not promote
extensive isomerization. The latter results in loss of monomer
and catalyst deactivation by the aldehyde formed. Despite these
constraints, even high linearities and consequently higher
melting points can be achieved with appropriately chosen
catalysts. The case of adtbpb shows that in achieving this a
certain propensity for isomerization can be tolerated, providing
that carbonylation is competitive with isomerization. This can
also serve as a guideline for further developing this chemistry.
molecular weights of commercial polyester materials.
A
molecular weight distribution of Mw/Mn close to 2 indicates a well
behaved nature of the catalytic polycondensation reaction.
Differences in the melting points (Table 1) of the polyesters
obtained with different diphosphines can be related to their
linearities. Branched units that disturb crystallization, resulting
from carbonylation of a secondary alkyl, are mainly present in
the form of methyl branches (Figures 2, S6-S9, S23 and S25).
Notably, with the mixed-substituted benzyl backbone adtbpb,
formation of virtually entirely linear polyester was possible.
Hereby, a peak melting point of Tm 75 °C could be achieved
(Table 1, entries 14 and 15, Figure S27). This matches with a
linear polyester sample prepared by traditional polycondensation
of 12-hydroxydodecanoic acid for comparison.[23]
Note that
carbonylation polymerization) and
a
high isomerizing ability (problematic in this
high linear selectivity
a
(desirable) are not contradictive (also cf. entry 2 with bpx), but in
alkoxcarbonylation of olefins in general often go hand in hand,
as selectivity depends on the preference for CO insertion into
the terminal alkyl vs. branched alkyls.
End group analysis of the polyesters by TOSCY and DOSY
NMR spectroscopy and MALDI-TOF spectrometry (Figures S10-
S14) reveals terminal –OH and internal olefin end groups.
Molecular weights from NMR spectroscopy agree reasonably
well with molecular weights determined by GPC, considering
that GPC vs. polystyrene standards usually overestimates
molecular weights for hydrocarbon backbone polymers. A
considerable deviation is found for the higher molecular weight
polyesters formed with xantphos. This is due to cycle formation,
as evident from MALDI-TOF (Figure S15). It is well established
that in polycondensations a considerable portion of cycles are
formed when high conversions are achieved, and the
concentration of reacting groups is very low which favors
intramolecular cyclization.[24,25]
Experimental Section
Carbonylation polymerization was carried out in a 20 mL stainless steel
reactor equipped with a glass inlet and a magnetic stirring bar, heated in
a metal block. The reactor was purged repeatedly with nitrogen. Undec-
10-en-1-ol, Pd(OAc)2, additional diphosphine ligand, methanesulfonic
acid (MSA) and toluene were weighed in a Schlenk tube equipped with a
magnetic stirr bar in a glovebox. The mixture was stirred vigorously and
cannula-transferred into the reactor in a nitrogen stream. A static
pressure of 20-30 bar CO was applied, and the reactor was heated to the
desired temperatures under stirring. After the designated reaction time,
the reactor was cooled to room temperature and the CO pressure was
released slowly. An aliquot was taken out and CDCl3 was added for NMR
spectroscopy analysis to determine conversion and the product
distribution. The reactor and glass inlet were washed with
dichloromethane, and the combined phases were removed in vacuum.
To completely remove any unreacted monomer which could disturb
further analysis, the remaining crude polymer was repeatedly redissolved
in dichloromethane, precipitated with methanol, isolated by filtration, and
washed several times with methanol. The collected polymer was dried in
vacuum at 50 °C prior to determination of the yield by weighing and NMR,
GPC and DSC analysis.
Figure 2. 13C NMR carbonyl resonances of polyesters obtained with different
diphosphine ligands: adtbpb (bottom; Table 1, entry 14), pytbpx (center; Table
1, entry 6); and xantphos (top; Table 1, entry 9).
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Keywords: sustainable polymers
resources• polyesters • carbonylation polymerization • atom
economy
•
catalysis renewable
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