Selective Polymerization Catalysis from Monomer Mixtures: Using a Commercial Cr-Salen Catalyst To Access ABA Block Polyesters

Article abstract of DOI:10.1002/anie.201801400

ABA triblock polyesters are synthesized using a commercially available chromium salen catalyst, in one pot, from monomer mixtures comprising epoxide, anhydride and lactone. The catalysis is highly selective and applies a single catalyst in two distinct pathways. It occurs first by epoxide/anhydride ring-opening copolymerization and subsequently by lactone ring-opening polymerization. It is used to produce various new ABA polyester polyols; these polyols can undergo post-functionalization and chain-extension reactions. The ability to use a commercial catalyst and switchable catalysis with monomer mixtures is expected to facilitate future explorations of new classes of block polymers.

Full text of DOI:10.1002/anie.201801400

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Accepted Article
Title: Selective Polymerization Catalysis From Monomer Mixtures:
Using A Commercial Cr-Salen Catalyst to Access ABA Block
Polyesters
Authors: Charlotte Katherine Williams and Tim Stoesser
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201801400
Angew. Chem. 10.1002/ange.201801400
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Selective Polymerization Catalysis From Monomer Mixtures:
Using A Commercial Cr-Salen Catalyst to Access ABA Block
Polyesters
Tim Stößer and Charlotte K. Williams*
[
5]
Abstract: ABA triblock polyesters are synthesized using
a
structures. Experimental and theoretical studies suggested that
the selectivity resulted from kinetic and thermodynamic control by
commercially available chromium salen catalyst, in one pot, from
monomer mixtures comprising epoxide, anhydride and lactone. The
catalysis is highly selective and applies a single catalyst in two distinct
[
6]
the metal-chain end group. This process is different from
terpolymerization because two different polymerization cycles are
accessed and the dominant catalytic cycle is switched by the
chemistry of the catalyst-polymer chain end group. It is also
preferable to tandem or multi-functional catalysis because a single
catalyst is active in both catalytic cycles. Very recently, Rieger
and co-workers demonstrated switch catalysis, using a di-zinc β-
diiminate catalyst, to prepare block/random copolymers from β-
butyrolactone, cyclohexene oxide and carbon dioxide. It is
important to understand the generality of this new switch catalysis,
particularly its applicability beyond zinc complexes. Its uptake
should be facilitated and accelerated by determining whether it
also applies to commercially available polymerization catalysts.
The process requires a single catalyst active for both lactone
ROP and epoxide/anhydride ROCOP (Scheme 1). We targeted a
pathways.
It occurs first by epoxide/anhydride ring-opening
copolymerization and subsequently by lactone ring-opening
polymerization. It is used to produce various new ABA polyester
polyols; these polyols can undergo post-functionalization and chain
extension reactions. The ability to use a commercial catalyst and
switchable catalysis with monomer mixtures is expected to facilitate
future explorations of new classes of block polymers.
[7]
[
8]
In polymerization catalysis, the ability to selectively control
(
block) sequence using monomer mixtures remains a significant
[
1]
challenge.
Nature overcomes this problem with exquisite
selectivity and catalyzes thousands of reactions to form different
biopolymers, including sugars/peptides/DNA, where precisely
defined sequence determines function. Synthetic mimics of
biosynthesis are most successful when applying sequential
monomer coupling reactions, analogous to artificial peptide
commercial Cr-salen catalyst [SalcyCrCl] which is applied with
equimolar addition of co-catalyst (PPNCl).
[7b, 7d, 7f, 9]
The catalyst
system has precedent for other alternating copolymerizations
[7b, 7d, 7f, 7h, 9]
(
ROCOP),
carbonate
copolymerization of dihydrocoumarin and propylene oxide.
and related systems are active in cyclic
[2]
synthesis.
Nonetheless such processes are not especially
ROP,
-butyrolactone ROP and for the
suitable for synthetic polymer production – they are generally too
labor intensive and time-consuming to evaluate new block
copolymers.
[9d, 10]
Firstly, the Cr-catalyst system was tested separately for ROCOP
and ROP, under conditions relevant to subsequent switch
catalysis (Table S1). Each polymerization occurred with high
conversion and selectivity, but the resulting polyesters showed
bimodal molar mass distributions; indeed similar bimodality was
This work describes a method to prepare block polyesters
which are relevant as degradable and, in some cases, bio-
renewable materials useful as elastomers, fibers, healthcare
[3]
materials, drug delivery vectors and in electronics. Generally,
block polyesters are synthesized by sequential polymerization
reactions and/or with macro-initiators – such methods can be
limited by both the conversion efficiency, intermediary purification
steps and by the nature of the repeat unit chemistry. An attractive
alternative would be to develop catalytic processes which
selectively enchain particular blocks, but such switchable catalysis
remains under-developed. In 1985, Inoue and co-workers
reported an Al-porphyrin catalyst able to prepare block polyesters
from mixtures of epoxide, anhydride and lactone. Nonetheless
understanding the reaction was hindered as the sole lactone
investigated was -butyrolactone which can ring-open at two sites
complicating enchainment mechanisms. In 2014, we reported
switch catalysis using a single homogeneous di-zinc catalyst
which selectively polymerized mixtures of lactone (CL), epoxide
observed previously for CO
2
/CHO ROCOP using Cr-salen
[7h]
catalysts.
To control the molar mass distribution,
polymerizations were conducted under immortal conditions, i.e.
with the addition of various amounts of 1,2-cyclohexanediol (CHD,
5-20 equiv.) (Fig. S1). The excess alcohol controls molar mass via
rapid and reversible exchange equilibria; the use of a diol ensures
formation of ABA triblock copolymers. Under optimized conditions
(
>10 equiv. CHD), both DL ROP and NBA/CHO ROCOP formed
polyesters showing monomodal molar mass distributions with
predominantly dihydroxyl chain end groups (Fig. S2, S3). It
should be noted that a small fraction (~5 mol%) of chloro-initiated
chains should be present; these chains do not affect the GPC
traces but were detected by MALDI-ToF (Fig. S3).
Having demonstrated that Cr-salen catalysts were active for
the separate polymerizations, catalysis using mixtures of
DL/CHO/NBA, again with 10 equiv. CHD, was investigated. The
reaction was successful and ABA type polyesters were formed
with narrow, monomodal molar mass distributions (Fig. S1; A =
[
4]
(
CHO) and carbon dioxide to form single block polymer
[*]
T. Stößer, Prof. Dr. C. K. Williams
PDL, B = PCHNBE). The reaction was monitored with regular
removal of aliquots which were analyzed using
Department of Chemistry, Oxford University
Chemical Research Laboratory, 12 Mansfield Road, Oxford, OX1
1
H NMR
3
TA, UK.
spectroscopy to determine conversion; this was achieved by
integration of monomer signals vs. an internal standard (Figs. 1,
S6, S7). Over the first 1.5 h, only CHO/NBA ROCOP occurred
E-mail: charlotte.williams@chem.ox.ac.uk
Supporting information for this article is provided via the link at the
end of the document. All experimental data, spectra and results are
included in the supporting information
producing
the
alternating
polyester
(PCHNBE).
1
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COMMUNICATION
Figure 1
:1:10:100:125:100, Tol-d
GPC data (M values are obtained using calibration against PS standards). Conversion vs. time data are obtained by analysis of the H NMR integrals, using
mesitylene as an internal standard (Fig. S4, S5, Table S2).
.
Polymerizations using mixtures of NBA, CHO and DL to form block polyesters (top). Reaction conditions: [SalcyCrCl]:PPNCl:CHD:NBA:CHO:DL =
1
8
(2.5 M), 100 °C. Polymerization conversion vs. time plots (bottom, right) and evolution of molar mass vs. conversion as illustrated by
1
n
This was evidenced by
a
rapid reduction in anhydride
epimerized isomer (Fig. S13). Such signals are not assigned to
transesterification since reaction of the polymer with an efficient
transesterification catalyst (DBU) significantly increased both the
number and intensity of intermediary chemical shift signals (Fig.
S15). Furthermore, the switch catalysis was also generalized to
concentration and the concomitant growth of signals assigned to
PCHNBE. The ROCOP catalysis was quite efficient, showing a
-1
TOF ~ 67 h . Importantly, over this time period there was almost
no change in the signals assigned to the lactone (<3 % by NMR,
Table S2) and no evidence for any epoxide homopolymerization.
After 1.5 h, the anhydride was fully consumed and subsequently
DL ROP occurred slowly, reaching completion after 48 h (Fig. 1,
13
1
produce a block polyester from PA/CHO/DL, its C{ H} NMR
spectrum showed only signals for the two blocks with no
intermediate signals, consistent with a lack of transesterification
reactions (Fig. S26-S29). The DOSY NMR spectrum of the
polymer formed from DL/CHO/NBA shows a single diffusion
coefficient for all signals, consistent with block polyester
formation. The analogous blend of constituent polymers shows
two diffusion coefficients (Fig. S16). The reaction of its hydroxyl
end-groups with 2-chloro-4,4,5,5-tetramethyl dioxaphospholane
-1
TOF ~ 3 h ). The resulting polymer contains 66% cis ester
functionalities, and 34% isomerized trans ester linkages, as
determined by analysis of the polyester degradation products
(
Fig. S9).
The spectroscopic and TOF values clearly indicate the
high catalytic selectivity for CHO/NBA/DL, with ROCOP
occurring before ROP, but such selectivity could form either
triblock polyester (PDL-b-PCHNBE-b-PDL) or a mixture of both
polymers (PCHNBE + PDL). In order to characterize the polymer
composition, aliquots were analyzed by GPC (Fig. 1). The molar
masses increase with conversion and in all cases samples show
narrow, monomodal distributions (1.11<Ð<1.32), such data are
strongly indicative of block polyester formation. Signals
31
1
enabled analysis by P{ H} NMR spectroscopy to differentiate
[
9b]
the end-groups.
The homopolymers show different signals
(146.5 = PCHPE and 147.1 ppm = PDL) and the block polymer
shows only one signal, at 147.1 ppm, consistent with its ABA
structure (Fig. S17,S18, Table S3). Finally, the crude block
polymer was purified using hexane which is known to dissolve
the homopolymer (PDL). There was no detectable homopolymer
in the hexane and the composition of the block polymer
remained the same before/after purifications (Fig. S10). Thus, all
the analytical tests confirmed the formation of single block
polymer structure, i.e. PDL-b-PCHNBE-b-PDL.
1
corresponding to both blocks were observed by H NMR
1
3
1
spectroscopy and C{ H} NMR spectroscopy (Fig. S10-S14).
1
3
1
The C{ H} NMR spectrum also showed low intensity signals at
intermediate chemical shifts which are assigned to the minor
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The catalysis is proposed to occur via two different
polymerization cycles that are linked by a common Cr-alkoxide
molar mass block polyesters formed (Fig. S22, S23). DSC
analyses showed amorphous structures with a single glass
intermediate (Scheme 1). It is important to note that a third cycle, transition temperature, which is indicative of block miscibility.
involving epoxide ring-opening polymerization (forming ether
linkages) is not accessed even over prolonged reaction times,
as confirmed by H NMR and MALDI analysis (Fig. S4-S5). Our
hypothesis is that large differences in rate of anhydride vs.
lactone insertion and a high barrier to Cr-carboxylate reaction
The block polyester composition was directly controlled by the
monomer composition in the mixture and glass transition
temperatures could be tuned from -30-111 °C (Table S4 and
Figs. S24, S25). Several factors contribute to (micro)-phase
separation including polymer architecture, block volume
1
with lactone (and Cr-alkoxide with epoxide) control the selectivity. fractions, the Flory-Huggins interaction parameter () and
[
13]
More generally, the above rationale also allows for scenarios
where both polymerization rates are similar and a statistical
copolyester might form (vide infra).
degree of polymerization.
Here, the block miscibility is most
likely a consequence of the low molar masses, which are
expected to fall below the entanglement molar masses.
Next, these hypotheses were specifically tested using
other monomers and conditions. Firstly, experiments were
conducted to confirm the reactivity of the Cr-carboxylate
intermediate. The preliminary kinetic analyses indicate ROCOP
rates which are zero order in anhydride concentration (Fig. S7,8)
Table 1. Polymerizations using mixtures of CHO, DL and various different
anhydrides.
[a]
A (E)
DL Conv.
(%)
[d]
TOFROCOP/
[e]
–
i.e. during ROCOP the catalyst resting state is the Cr-
Anhydride (A)
PA
Conv.
[b]
M
n
(Ð)
[
b]
TOFROP
(%)
carboxylate species. To test the stability of this intermediate
towards reaction with lactone, a polymerization reaction was
conducted using excess anhydride and lactone vs. epoxide
(
DL:NBA:CHO, 200:200:125). ROCOP proceeded until the
6,200
1.20)
9
9 (76)
99
64
(
epoxide was consumed (~ 60% NBA conversion) producing only
alternating polyester, PCHNBE. At this point no further
conversion of any monomer occurred even over prolonged
reaction times (4 days) and despite the presence of excess DL
Figs S26-S35
THPA
(
200 equiv.) (Fig. S19). In order to confirm the catalyst had not
4
,000
decomposed, the polymerization was ‘switched-on’ by the
addition of a further 200 equiv. of epoxide. At which point,
ROCOP was resumed until complete anhydride consumption
which was followed by DL ROP leading to triblock polyester
formation (Fig. S20, S21). It is important to note that the
proposed pathway does not examine the intimate mechanism or
active catalyst structure(s) which have been the subject of
9
9
9 (79)
9 (74)
99
99
48
22
(1.26)
Figs S36-S45
3
,500
NBA
Figs S10-14
(1.32)
[
7b,7d,9b,9c,11,12]
TCA1
extensive investigations.
6
,700
9
9 (77)
99
32
(1.20)
Figs S46-S55
5
,800
9
9
9 (67)
9 (80)
99
99
103
1.4
(1.20)
TCA2
Figs S56-65
CA
6
,700
(1.14)
Scheme 1. The switch catalysis pathways proposed using mixtures of DL,
NBA, CHO.
Figs.S66-S75
[
1
a] Polymerization Conditions: [SalcyCrCl]:[PPNCl]:[CHD]:[CHO]:[DL]:[A] =
To test the catalytic scope, a series of triblock polyesters of
differing compositions were prepared by controlling the relative
amounts of monomers in the mixture. In all cases, high
monomer conversions were achieved and well-defined, low
:1:15:250:200:200, 100 °C, 24-96 h; where A refers to anhydride and E to
1
epoxide. [b] Determined by H NMR spectroscopy, note for E, theoretical max.
conversion = 80% (see relevant figures in the SI). [c] Determined by H NMR
spectroscopy and MALDI-ToF spectrometry (see relevant figures in the SI).
[
1
d] Determined by GPC, in THF at 30 °C, calibrated using polystyrene
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COMMUNICATION
[
15,16]
standards (see relevant figures in the SI). [e] Determined from conversion vs.
time data, at >95% monomer conversion (Tables S5-S9).
linking agents.
All new block polymers also have hydroxyl-
telechelic structures, i.e. they are new polyester polyols.
Polyester polyols are used in polyurethane production and as
proof of chain extension, PDL-b-PCHNBE-b-PDL was reacted
with 4,4’-methylene diphenyl diisocyanate (MDI). The precursor
material showed one glass transition temperature at 26 °C and
contained 39 wt% PDL. After chain extension, the molar mass
increased from ~4 to ~ 70 kg/mol and DSC analysis indicated
phase separation as two glass transition temperatures at 46 and
88 °C were observed (Fig. S80, S81). Such multi-block polymers,
with controllable compositions, may be interesting as rigid
Similar effects were observed for ABA poly(styrene-b-isoprene-
b-styrene): at low M
observed (M = 11,000 g/mol, T
values, two intermediate glass-transition temperatures occur
= 16,000 g/mol, Tg,1= -48 °C, Tg,2 = 43 °C) whilst at higher
, there are two glass transition temperatures, similar to the
homopolymers (M =161,000 g/mol, g,1 = - 65 °C,
n
, a single glass transition temperature was
n
g
= 38 °C), at intermediate M
n
(
M
n
M
n
n
T
[
14]
Tg,2 = 105 °C).
The Cr-salen switch catalyst system was also applied to a
[17]
plastics or thermoplastic elastomers.
range of different monomer mixtures; the influence of the
anhydride structure was investigated (Table 1). Each reaction
and polymer product was fully characterized using a range of
In conclusion, monomer mixtures can be selectively
reacted with a commercially available chromium salen catalyst,
in one pot, to form well-defined ABA triblock polyesters. The
scope of the catalysis is demonstrated using a range of different
mixture compositions and monomers to deliver new polyester
polyols. These polyesters can undergo post-functionalization
reactions to modify the side chain substituents or chain
extension reactions to produce multi-block polyesters. Switch
catalysis, using Cr-salen catalysts, is expected to be applicable
to the preparation of other block and multi-block polymers. The
method should be applied using mixtures of carbon dioxide,
lactones, anhydrides, epoxides to produce new block
polycarbonates, esters and ethers.
1
13
techniques, including NMR analysis ( H, C, COSY, HSQC),
conversion vs. time plots (using both IR and NMR data), GPC,
MALDI and by comparison of composition before and after
isolation. Table 1 provides an overview of composition and
relative rate data; the complete characterization data sets for
each block polymer are provided in the electronic supporting
information (Figs. S26-S75). The majority of monomer mixtures
resulted in selective formation of only ABA triblock polyesters
(
this applies to mixtures of PA, THPA, TCA1 or TCA2 with
CHO/DL). In all these cases, fast and selective ROCOP is
followed by slow ROP (< 5% DL conversion at >99% conversion
of anhydride). Analysis of the relative rates reveals that selective
catalysis occurs when the rate of ROCOP is >20 times that of
ROP, in line with the mechanistic hypothesis (Scheme 1). The
polymerizations all proceed with high monomer conversions and
form block polyesters with mono-modal molar mass distributions. Acknowledgements
ABA type block polyester structures were confirmed through
multiple
experiments,
analogous
to
the
range
of
The authors gratefully acknowledge Climate KIC (Project
characterizations of PDL-b-PCHNBE-b-PDL. Moreover, in every
case the molar mass increased continuously throughout the
reaction. In particular, the polymerization of DL/CHO/PA results
EnCO re), EPSRC (EP/L017393/1, EP/K014668/1) for funding.
2
Keywords: Ring-opening copolymerization • ring-opening
polymerization • switchable catalysis • block copolyesters •
chromium salen catalysts
in formation of
a block polyester containing an aromatic
backbone group. Thus, the aliquots were analyzed by GPC with
both RI and UV detectors: there was a clear evolution in molar
mass using both detection methods (Fig. S33). This finding
confirms covalent bonding between the alternating semi-
aromatic polyester and the PDL blocks. In contrast, the
monomer mixture comprising DL/CHO/camphoric anhydride
[
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This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201801400
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Tim Stößer and Charlotte K. Williams*
Page No. – Page No.
Selective Polymerization Catalysis
From Monomer Mixtures: Using A
Commercial Cr-Salen Catalyst to
Access ABA Block Polyesters
Text for Table of Contents
Block copolyesters are typically made by labour-and time-intensive, stepwise approaches. Here, the use of a commercial catalytic
system enables the synthesis of well-defined block copolyesters in one pot and from monomer mixtures.
This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201801400
COMMUNICATION
This article is protected by copyright. All rights reserved.