Isoselective Lactide Ring Opening Polymerisation using [2]Rotaxane Catalysts

Article abstract of DOI:10.1002/anie.201901592

Polylactide (PLA) is a fully biodegradable and recyclable plastic, produced from a bio-derived monomer: it is a circular economy plastic. Its properties depend upon its stereochemistry and isotactic PLA shows superior thermal-mechanical performances. Here, a new means to control tacticity by exploiting rotaxane conformational dynamism is described. Dynamic achiral [2]rotaxanes can show high isoselectivity (P_i=0.8, 298 K) without requiring any chiral additives and enchain by a chain end control mechanism. The organocatalytic dynamic stereoselectivity is likely applicable to other small-molecule and polymerization catalyses.

Full text of DOI:10.1002/anie.201901592

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Accepted Article
Title: Isoselective Lactide Ring Opening Polymerisation using
[2]Rotaxane Catalysts
Authors: Charlotte Katherine Williams, Jason Y.C. Lim, Nattawut
Yuntawattana, and Paul Beer
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201901592
Angew. Chem. 10.1002/ange.201901592
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Isoselective Lactide Ring Opening Polymerisation using
[2]Rotaxane Catalysts
Jason Y. C. Lim,[a]ǂ Nattawut Yuntawattana,[a] Paul D. Beer*[a] and Charlotte K. Williams*[a]
Abstract: Polylactide (PLA) is a fully biodegradable and recyclable
plastic, produced from a bio-derived monomer: it is a circular
economy plastic. Its properties depend upon its stereochemistry and
isotactic PLA shows superior thermal-mechanical performances.
Here, a new means to control tacticity by exploiting rotaxane
fluxional
enantiomeric
scandium
catalysts
were
heteroselective.[13] Schaper and co-workers showed fluxional
copper catalysts were isoselective.[14] So far, related examples
of dynamic organocatalysts are unknown.
We reasoned that mechanically interlocked molecules
such as rotaxanes could allow for dynamic stereocontrol as
macrocycle and axle components can be stimulated to move
independently. Such macrocycle/axle dynamic behavior
underpins many of their applications, e.g. in molecular
recognition/sensing,[15] molecular machines,[16] switchable
catalysis[17] and artificial ribosome mimics.[18] To test their
potential for rac-LA ROP, rotaxanes 1, 2 and 3 were targeted
(Fig. 1, SI for synthesis). Each comprises a crown ether
macrocycle and an axle, the latter containing both an ammonium
and a thiourea or triazole group. In this form, the macrocycle
resides over the protonated ammonium group secured by strong
intramolecular hydrogen bonding (Fig. S3, S10, S14).[19] After
deprotonation, the amine interacts weakly with the macrocycle
allowing its free movement along the axle (vide infra). This
protonation-dependent dynamic behavior also highlights their
potential to be switched ‘on’ for catalysis.
conformational dynamism is described.
Dynamic achiral
[2]rotaxanes can show high isoselectivity (Pi = 0.8, 298 K) without
requiring any chiral additives and enchain by a chain end control
mechanism. The organocatalytic dynamic stereoselectivity is likely
applicable to other small-molecule and polymerization catalyses.
Polylactide (PLA) is a commercially available bio-derived
thermoplastic; its biocompatibility and biodegradability have
enabled substitution of petro-polymers in medical, packaging
and fibre applications.[1] PLAs thermal-mechanical properties
depend upon its microstructure: atactic PLA is amorphous whilst
isotactic stereoblock PLA is semi-crystalline. Stereoblock PLA
shows an even higher melting temperature (Tm) than isotactic
PLLA.[2] All tacticities of PLA are produced by the ring opening
polymerization (ROP) of lactide. A current challenge is to deliver
stereoblock PLA from racemic lactide (rac-LA) using selective
catalysis.[3]
Organo-catalysts are attractive due to the high
polymerization control exhibited and thioureas,[4] amidines,[5]
phosphazenes[6] or N-heterocyclic carbenes (NHCs) also show
high rates.[7] Very few organocatalysts are isoselective (rac-LA)
and the most successful are chiral.[8] For example, binaphthol-
derived phosphoric acid (kD/kL = 28),[9] chiral synthetic prolines
(Pi = 0.87-0.96)[10] and a chiral thiourea (Pi = 0.82)[11] operate by
enantiomorphoric site control. Fundamentally this mechanism
cannot maintain both high rates and conversions: once the
preferred enantiomer is consumed the remaining enantiomer
reacts very slowly. The alternative chain-end control mechanism
could overcome this limitation but is under-developed. It has
been successfully applied to various metal complexes[12] and
there are intriguing hints it may operate for
a
few
7]
organocatalysts.[6a,
Because CEC relies on in operando
stereochemical interactions, it is much harder to rationally
improve selectivity. We are interested in understanding how
catalyst dynamic processes, e.g. ligand fluxionality, may
enhance stereoselectivity.[12] We previously reported high
isoselectivity yttrium catalysts and correlated ligand fluxionality
with stereocontrol.[12] Okuda and co-workers showed that
Figure 1. Structures of [2]rotaxane catalysts and non-interlocked control
compounds.
Rotaxanes 1-3 were all inactive for rac-LA ROP, even with
added benzyl alcohol, consistent with previous reports that
thioureas alone cannot activate lactide to attack by alcohols.[4a]
Neutral rotaxanes, deprotonated in situ by addition of base, were
active in presence of benzyl alcohol (Table 1, entries 1, 7 and 8).
The rates differ, with 1 reaching 80 % after 4 d, whilst 2 and 3
achieved high conversions within 12 h. All catalysts showed high
polymerization control and produced PLA with predictable molar
masses (Mn), linear evolution of Mn with conversion, and narrow
dispersity (Ð < 1.20) (Fig. 2A, S31-34). End-group analysis,
[a]
Dr J.Y.C. Lim, N. Yuntawattana, Prof. P.D. Beer, Prof. C.K. Williams
Department of Chemistry
University of Oxford
Chemistry Research Laboratory, Mansfield Road, Oxford OX1 3TA.
E-mail: charlotte.williams@chem.ox.ac.uk;
paul.beer@chem.ox.ac.uk
Current affiliation for Dr J.Y.C. Lim
Institute of Materials Research and Engineering
2 Fusionopolis Way, Singapore 138634.
ǂ
Supporting information for this article is given via a link at the end of
the document.
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using ESI mass spectrometry, showed a single set of peaks
corresponding to benzyl ester end-capped chains (Fig. S28-30).
Transesterification side reactions were not observed for 1 and 3,
as evidenced by the 144 peak separations, whilst peak
separations of 72 for 2, indicates transesterification. It is
noteworthy that neutral rotaxane 2 is the first example of a
triazole-containing organocatalyst for lactide ROP.
Three components are required for control: rotaxane, base
and alcohol. Polymerizations conducted without rotaxane were
rapid but uncontrolled yielding atactic PLA (Table 1, entry 2).
Reactions without alcohol were inactive (Table 1, entry 3).
Under the appropriate conditions, 1 produced highly isotactic
PLA (Pi = 0.81; Fig. 2B, S35). Replacing the thiourea group of 1
The NMR spectra of neutral rotaxanes were compared to
understand catalyst structure in more detail. Addition of
equimolar base to 1, results in upfield shifts to resonances H3
and H4, which are immediately adjacent to the ammonium group
(Fig. 3). Such shifts indicate formation of a neutral amine group.
At the same time, thiourea resonances Hα and Hβ shift
downfield, consistent with enhanced thiourea-macrocycle
interactions. No significant changes were observed after the
addition of benzyl alcohol (Fig. S44). Significant broadening of
all other resonances indicates the macrocycle is moving along
the axle slower than the NMR timescale (vide infra). Adding
base to 2, also deprotonates the ammonium group (Fig. S45).
The remaining signals are sharp suggesting the rate of
macrocycle movement along the axle is faster than for 1. The
ROESY NMR spectrum of (neutral) 2 provides unequivocal
evidence of macrocycle movement between amine and triazole
sites, evident from the numerous cross-peaks between the
macrocycle protons and those arising from the aromatic stopper
units on both ends of the axle (Fig. S49).
with the weaker hydrogen bond-donating triazole in
2
significantly reduces stereocontrol (Pi 0.66; Fig. S39).
=
Thiourea accessibility also appears to be important, as 3 shows
lower stereocontrol (Pi = 0.73, Fig. S40). In all cases, analysis of
defect tetrad resonances, in the methine region of the 1H{1H}
NMR spectra, suggests a likely dominant CEC mechanism (see
Figure S35, SI).[20],[21] Despite a small extent of epimerization
occurring during the reaction (13C NMR spectrum of PLA in Fig
S27), the highest stereocontrol, achieved by 1, matches values
for chiral organocatalysts[8], urea/alkoxide systems[22] or
organometallic catalysts[23]. Perhaps more interesting is the
finding that the rotaxane structures, and dynamism (vide infra),
are inherent to stereocontrol. Under identical conditions, the
macrocycle alone yields only atactic PLA, while the protonated
thiourea axle is inactive (SI). The possibility that isoselectivity
results from only thiourea steric congestion can also be ruled out
because the equivalent acyclic catalyst 4 shows lower selectivity
(Pi = 0.69, Fig. S41). Furthermore, increased thiourea steric
hindrance, in 3, reduces stereoselectivity. These observations
point towards an alternative stereocontrol.
Figure 2. (A) Evolution of molar mass with conversion for polymerizations
catalyzed by 1; (B) 1H{1H} NMR spectrum with tetrads labelled. Reaction
conditions: [LA]/[1]/ [KN(SiMe3)2]/ [BnOH] = 50:1:1:1, [LA] = 1.0 M, THF, 298 K
(Pi = 0.81).
Table 1. Polymerisation of rac-lactide with rotaxane catalysts.
Entry[a]
Catalyst
[LA]/ [cat.]/[base]/ [BnOH]
t/ h[c]
Conv.[b]/ %
Mn [c]/ kg mol-
Mncalc./ kg mol-1
Ð
Pi
[d]
1
1
2
3
4
5
6
7
8
9
1
-
50:1:1:1
50:0:1:1
96
1
80
99
0
6.5
8.3
-
5.8
7.1
-
1.08
1.72
-
0.81 ± 0.03
0.56 ± 0.10
-
1
1
1
1
2
3
4
50:1:0:1
48
48
1
50:1:1:0
0
-
-
-
-
100:1:2:2
50:1:1:1 (323 K)
50:1:1:1
99
80
80
90
98
8.8
5.4
5.8
6.2
8.1
7.1
5.8
5.8
6.5
7.1
1.47
1.11
1.16
1.13
1.16
0.54 ± 0.06
0.66 ± 0.02
0.66 ± 0.03
0.73 ± 0.02
0.69 ± 0.01
72
11
3
50:1:1:1
50:1:1:1
5
[a] Polymerization conditions: [LA] = 1.0 M, THF, 298 K, unless stated. [b] Determined by integration 1H NMR spectrum (LA, 4.96-5.04 ppm; PLA, 5.10-5.22
ppm);[c] Determined by SEC analysis, in THF, and applying a correction factor of 0.58 to values;[24] [d] Determined from the 1H{1H} NMR by integration of
normalized methine tetrads and Bernoullian statistics (Fig. S35-S41, SI).[25]
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complexes. Overall, the data indicate that the catalyst is a
neutral rotaxane and DOSY NMR indicates it is monomeric
under the conditions of catalysis (Fig. S48).
To rationalize the differing catalytic performances, a
polymerization pathway is proposed inspired by the two-
component thiourea/amine organocatalysis literature (Scheme
4b]
1B).[4a,
Neutral 1/2 feature thiourea/triazole groups which
activate the lactide monomer and amine groups to activate
benzyl alcohol. This mode of initiator/ monomer activation is only
possible if the macrocycle is free to move along the axle.
Polymerization is initiated, and propagated, by an activated
monomer mechanism. Importantly, for these rotaxanes, the
basicity of the axle secondary amine is considerably enhanced
by interaction with the crown ether macrocycle component
compared to the free neutral axle, as the stability of the crown
+
ether-R2NH2 conjugate acid upon protonation strongly favours
its formation thermodynamically.[19c, 26] Accordingly, the activated
benzyl alcohol attacks the coordinated lactide forming an -
hydroxyl--ester which is subsequently (re)activated and attacks
another lactide. In line with this, the relative rates correlate with
rotaxane amine basicity, and hence capacity for alcohol
activation. Because the rotaxanes also feature other macrocycle
coordination sites, when these sites are strongly coordinating
the amine basicity is reduced.[27] For 2 and 3, macrocycles
occupy the amine site, increasing its basicity and accelerating
rates (Scheme 1B).
Figure 3. Stacked 1H NMR spectra of protonated and neutral rotaxane 1 ([1] =
2 mM, T = 298 K, d8-THF).
Intense ROESY signals correlate with the macrocycle being
preferentially located at the amine site, likely due to its greater
hydrogen bonding acidity compared to the triazole. In contrast,
neutral 1 would be expected to show favorable interactions with
both thiourea and amine sites. Its ROESY NMR reveals stronger
through-space interactions between the macrocycle and the
thiourea (Fig. S47). Accordingly, stronger macrocycle-thiourea
hydrogen-bonding may slow macrocycle translation along the
axle and broaden the NMR spectrum. The 1H NMR spectrum for
neutral 3 also indicates the axle amine group (Fig. S45). ROESY
NMR could not confirm the macrocycle site occupancy due to
the complex, overlapping aromatic resonances (Fig. S50).
Nonetheless, the bulky stopper group may be expected to hinder
thiourea-macrocycle interactions and favour macrocycle
occupation of the amine site.
Only 1 shows high isoselectivity and operates by an
unusual chain end control mechanism. To investigate further,
the rotaxane-lactide binding constants were determined by 1H
NMR titrations. 1 shows a higher binding constant (Ka ≈ 400 M-1)
than
2 (Ka < 1
M-1) (Section S4.2, SI).[28] From the
aforementioned ROESY studies, it also shows significantly
slower macrocycle shuttling. It is tentatively proposed that both
features (strong binding and slow shuttling) allow for preferential
orientation of the growing polymer chain and lactide molecules
for isotacticity.
Control experiments support the neutral rotaxane being the
In conclusion, these are the first rotaxane catalysts for
lactide ROP and they show isoselectivity. The polymerization
selectivity correlates with macrocycle-axle translocation rates
and with sterically-accessible, strong monomer coordination
sites. The conformational dynamism is responsible for both rates
and stereoselectivity and provides a new means by which to
control tacticity. This new type of stereocontrol is expected to be
applicable to transesterifications, transamidations and other ring-
opening polymerizations of epoxides, cyclic carbonates and
lactones.
true
catalyst
(Scheme
1A).
Neither
potassium
hexafluorophosphate, the amine by-product (N(SiMe3)2H) nor
benzyl alcohol initiate polymerization, either individually or in
combinations (Section S3.2). The free axles of 1 and 2 were
also inactive.
A strong base is necessary for rotaxane
deprotonation: replacing KN(SiMe3)2 with DABCO (a weaker
base) results only in limited ROP (7 % after 120 hours). To rule
out formation of rotaxane-potassium complexes, the 1H NMR
spectra of the potassium and sodium salts of 1 were compared.
Both spectra showed identical chemical shifts for all resonances
(Fig. S51). As K+/Na+ coordination would be expected to change
chemical shifts, the active species are unlikely to be Group 1
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Scheme 1. Proposed mechanisms of (A) [2]rotaxane catalyst activation; and (B) lactide ROP with 1, 2 and 3.
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Acknowledgements
Funding from the EPSRC (EP/L017393/1; EP/K014668/1), the
Agency for Science, Technology and Research (A*STAR),
Singapore (JYCL) and the Royal Thai government DPST (NY) is
acknowledged.
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Keywords: lactide • ring-opening polymerisation • isoselective •
rotaxane • organocatalysis
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Entry for the Table of Contents
COMMUNICATION
Jason Y. C. Lim, Nattawut
Yuntawattana, Paul D. Beer* and
Charlotte K. Williams*
Page No. – Page No.
Isoselective Lactide Ring Opening
Polymerisation using [2]Rotaxane
Catalysts
Text for Table of Contents
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