Highly Syndiotactic or Isotactic Polyhydroxyalkanoates by Ligand-Controlled Yttrium-Catalyzed Stereoselective Ring-Opening Polymerization of Functional Racemic β-Lactones

Article abstract of DOI:10.1002/anie.201704283

Reported herein is the first stereoselective controlled ROP of a specific family of racemic functional β-lactones, namely 4-alkoxymethylene-β-propiolactones (BPL^ORs). This process is catalyzed by an yttrium complex stabilized by a nonchiral tetradentate amino alkoxy bisphenolate ligand {ONOO^R′2}^2?, which features both a good activity and a high degree of control over the molar masses of the resulting functional poly(3-hydroxyalkanoate)s. A simple modification of the R′ substituents in ortho and para position on the ligand platform allows for a complete reversal from virtually pure syndioselectivity (P_s up to 0.91 with R′=cumyl) to very high isoselectivity (P_i up to 0.93 with R′=Cl), as supported by DFT insights. This is the first example of a highly isoselective ROP of a racemic chiral β-lactone.

Full text of DOI:10.1002/anie.201704283

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Accepted Article
Title: Highly Syndiotactic to Isotactic Polyhydroxyalkanoates via
Ligand-Controlled Yttrium-Catalyzed Stereoselective Ring-
Opening Polymerization of Functional Racemic beta-lactones
Authors: Romain Ligny, Mikko Hänninen, Sophie M. Guillaume, and
Jean-Francois Carpentier
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201704283
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10.1002/anie.201704283
Angewandte Chemie International Edition
DOI: 10.1002/anie.201((will be filled in by the editorial staff))
Polymer Chemistry
Highly Syndiotactic to Isotactic Polyhydroxyalkanoates via Ligand-
Controlled Yttrium-Catalyzed Stereoselective Ring-Opening
Polymerization of Functional Racemic -lactones**
Romain Ligny,a Mikko M. Hänninen,b Sophie M. Guillaume,a,* and Jean-François Carpentiera,*
Abstract: Reported herein is the first stereoselective controlled ROP
of a specific family of racemic functional -lactones, namely 4-alko-
xy-methylene--propiolactones (BPLORs), using yttrium catalysts
stabilized by nonchiral tetradentate amino-alkoxy-bisphenolate
ligands {ONOOR’2}2, combining both a good activity and a high
degree of control over the molar masses of the resulting functional
poly(3-hydroxyalkanoate)s PHBORs. The simple modification of
ortho,para-R’ substituents on the ligand platform allows a complete
reversal from virtually pure syndioselective (Ps up to 0.91 with R’ =
cumyl) to pure isoselective (Pi up to 0.93 with R’ = Cl) ROP, as
supported by DFT insights. This is the first example of a highly
isoselective ROP of a racemic chiral -lactone.
Extension of stereoselective ROP to other chiral cyclic esters is
much less documented. In particular, -lactones such as the
prototypical -butyrolactone (BL), are more reluctant to ring-open
and few catalysts are able to perform their ROP, even less in a
stereoselective fashion.[7] We have shown that yttrium catalysts
stabilized by nonchiral tetradentate amino-alkoxy-bisphenolate
2
ligands {ONOOR’2
}
offer highly syndiotactic poly(3-hydroxy-
butyrate) (PHB) with a probability of syndio enchainment (Ps) up to
0.96, provided sterically bulky aryl-containing substituents are
installed on the ortho-site of phenolate ligands.[8] This ROP catalysis
has been extended to different racemic -lactones,[9] in particular to
valuable functional ones such as alkyl -malolactonates (i.e., 4-
alkoxycarbonyl--lactones, MLAR; R = allyl, benzyl);[10] in this
Ring-opening polymerization (ROP) of cyclic esters is the method
of choice for preparing polyesters with a bespoke architecture.
Tremendous efforts have been spent in the past two decades for
developing highly effective metal-based or organo-catalysts able to
fine-tune the molar mass, terminal groups and the microstructure of
the polymers.[1] A specific point of major interest in this research
field is the control of tacticity –and, as a result, of physicochemical
properties– when starting from racemic mixtures of chiral cyclic
monomers. Hence, much work has been devoted to the ROP of
racemic lactide (rac-LA) for the synthesis of biosourced degradable
polymer alternatives to regular petrochemical-based plastics.
Heterotactic polylactide (PLA) is quite commonly obtained, most
often as the result of a chain-end stereocontrolled (CEC) mechanism
which minimizes steric interactions between the last inserted
monomer unit in the growing polymer chain and the new incoming
monomer unit, thus resulting in the alternation of monomer units
with opposite (R,R/S,S) configuration. On the other hand, the
preparation of valuable crystalline (sometimes stereocomplexed)
isotactic PLA from the ROP of rac-LA is much more challenging;
only a few catalysts/initiators, essentially based on group 13 (Al,[2]
In[3]), 12 (Zn),[4] 4 (Zr)[5] and 3 (rare-earth)[6] metals, have proved
able to do so with probabilities of isotactic enchainment (Pi) up to
0.97.
latter case, surprisingly, only yttrium catalysts bearing an ortho,para
2
(o,p)-dichloro-substituted {ONOOCl2
}
ligand enabled achieving
highly syndiotactic polymerizations (Ps > 0.95). Regular catalysts
incorporating sterically crowded R’ alkyl/aryl o,p- substituents on
2
the {ONOOR’2
}
platform afforded only modest syndioselectivity.
All these syndioselective reactions operated through a CEC
mechanism, as revealed by NMR studies and also pinpointed by
DFT computations.[11] On the other hand, the best isoselectivity in
the ROP of racemic BL was achieved by a Cr catalyst with a modest
isotacticity bias (Pi ca. 0.65) and no control over molar masses.[12]
We herein report on the first stereoselective ROP of another
class of racemic functional -lactones, namely 4-alkoxymethylene-
-propiolactones (BPLORs), combining both a good catalytic activity
and a high degree of control over the molar masses of the resulting
alkoxy-functional poly(3-hydroxyalkanoate)s PHBORs. It is shown
2
that simple modification of o,p-R’ substituents on the {ONXOR’2
}
platform of yttrium catalysts allows switching completely from
syndioselective to isoselective polymerization of this specific class
of -lactones; quite unexpectedly and uniquely, catalysts bearing an
o,p-dichloro-substituted ligand deliver highly isotactic polyesters (Pi
> 0.90). This is the first example of a highly isoselective ROP of a
racemic chiral -lactone.
[a]
R. Ligny, Dr. S. M. Guillaume, Prof. Dr. J.-F. Carpentier
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS
Université de Rennes 1, F-35042 Rennes (France)
E-mail: sophie.guillaume@univ-rennes1.fr
jean-francois.carpentier@univ-rennes1.fr
[b]
Dr. M. M. Hänninen, Department of Chemistry
University of Jyväskylä, Jyväskylä, P.O. Box 35, FI-40014, Finland
[]
This research was financially supported in part by the Region
Bretagne (Ph.D. grant to R.L.) and by the Academy of Finland (M.
M. H. grant number 274505).
Scheme 1. Stereoselective synthesis of isotactic and syndiotactic PHBORs from
the ROP of racemic BPLORs.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201xxxxxx.
1
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10.1002/anie.201704283
Angewandte Chemie International Edition
between experimental molar mass values (Mn,NMR) and theoretical
ones (Mn,theo), as determined by 1H NMR analyses (from the
isopropoxycarbonyl end-group and main-chain resonances), and
monomer-to-initiator along with BPLOR conversion, respectively,
(ii) the linear growth of experimental molar masses (e.g., Mn,NMR) as
a function of monomer conversion as illustrated with BPLOAll
(Figure S3), as well as (iii) the rather narrow dispersity values (1.07
< ÐM < 1.23) as determined by SEC analysis. The analysis of the
ROP reactions were performed on three racemic BPLOR
monomers (R = allyl, benzyl, methyl) (Scheme 1), which were
readily prepared by carbonylation of the corresponding racemic
epoxides following Coates’ Co-catalyzed protocol (Figure S1 in the
Supporting Information).[13] As for previous studies,[8,10] yttrium-
isopropoxide catalysts/initiators were conveniently generated in situ
from the action of 1 equiv. of iPrOH onto the respective amido
precursors 1a-g that were selected with ligands differing by the
nature of the R’ phenolate substituents.
1
end-groups was conducted by H and 13C{1H} NMR spectroscopy
(Figures S4S9) as well as by ESI mass spectrometry (Figures
S10S17). The latter MS analyses of PHBORs (OR = OAll, OMe)
showed a major set of peaks assignable to polyester chains with
hydroxy and isoproxycarbonyl end-groups, with peaks separated by
142 and 116 amu, respectively.
Catalytic systems with ligands incorporating bulky alkyl o,p-
substituents (tBu, 1e, 1f; cumyl, 1g, 1h) revealed quite active at
20 °C in toluene, allowing typically quantitative conversion of ca.
50100 equiv. of rac-BPLOR within 2 h (Tables 1, S1). On the other
hand, catalysts based on ligands having small Cl (1a, 1b) or Me (1c,
1d) o,p-substituents proved somewhat more sluggish. This
observation is peculiar and might appear counterintuitive at first
sight, since the approach of the monomer is anticipated to be
favored with lower steric hindrance around the metal center; also,
the presence of electron-withdrawing substituents (such as Cl) on
ligands has been reported to often increase ROP reaction rates.[2f]
We also noted that pre-catalyst systems with a NMe2 cap (X) on the
ligand (1b, 1d, 1f, 1h) are systematically more active than their
OMe-capped counterpart (1a, 1c, 1e, 1g), respectively, regardless of
the o,p-substituents (Tables 1, S1). Hence, the dichloro OMe-capped
system (1a), poorly active at room temperature (Table S1), can be
valuably substituted by the dichloro NMe2-capped system (1b)
which offers high BPLOR conversion within reasonable times under
the same operating conditions (Table 1, entry 2). Detailed
monitoring of NMR-scale reactions performed on rac-BPLOAll at
23 °C with the catalyst systems that are not too active, confirmed
these kinetic trends derived from batch experiments, and linear
semi-logarithmic plots established that the reactions are first-order
in monomer (kapp (min1) = 2.43±0.10, 1g; 2.38±0.10 × 102, 1b;
1.07±0.09 × 102, 1d; 0.62±0.03 × 102, 1c; 0.10±0.01 × 102, 1a)
(Table S2, Figure S2). Reactivity of the catalyst systems in THF was
significantly lower,[14] most likely as the result of competitive
coordination of THF vs. monomer onto the metallic center; such a
behavior has been previously observed in the ROP of alike polar
cyclic ester monomers.[10,15]
The stereoselectivity of the ROP reactions was much affected
by the nature of the R’ o,p-substituents on the phenolate ligand. As a
matter of fact, a complete reversal from virtually pure syndiotactic
(Ps up to 0.91) to pure isotactic (Pi up to 0.93) was observed upon
switching from dicumyl- (1g) to dichloro- (1a) substituted ligands in
the catalyst/initiator, respectively (Table 1, entries 1,6).[16] This
striking change in stereoselectivity was best assessed by 13C{1H}
NMR spectroscopy of the resulting PHBORs (Figures S5, S7, S9). In
line with previous observations on related poly(3-hydroxyalkonate)s,
namely PHB,[8] and allyl and benzyl -malolactonates (PMLAR; R =
allyl, benzyl),[10] we observed that the 13C NMR signals of the main
chain carbonyl and methine carbons as well as that of the methylene
side-chains are diagnostic of the tacticity of PHBOR; the case of
PHBOAll is illustrated in Figure 1. Basically, these regions of the
NMR spectra show signals at the diad level, enabling a facile
determination of the Ps/Pi values (i.e., Ps = 1  Pi).[17] To ascertain
the assignments, pure isotactic polymers were independently
prepared by ROP of enantiomerically pure (S)-BPLOAll and (R)-
BPLOMe (Table S1). The 13C NMR spectra of these two isotactic
materials each displayed a single sharp resonance for the three
aforementioned types of carbons. The polymers prepared from rac-
BPLOAll with o,p-dichloro-substituted catalysts/initiators 1a and 1b
showed the same strong signals, along with a minor one either
downfield (C=O and CH) or upfield (CH2), assigned to the syndio
diad. In contrast, the polymers prepared with the dicumyl-
substituted catalysts/initiators 1g and 1h, exhibited a complete
switch in intensity between these two signals for each carbon,
indicative of highly syndiotactic polymers. Similar observations
were made for PHBOMe and PHBOBn (Figures S18S19).
The reactions proceeded with a fair control over the PHBOR
molar masses. This is evidenced by (i) the quite good agreement
s
Table 1. ROP of rac-BPLOR mediated by 1/iPrOH systems in toluene (refer to Table S1 for further experiments).
[d]
[e]
[f]
Temp.
(°C)
Time[b]
(h)
Conv.[c]
(%)
Mn,theo
Mn,NMR
Mn,SEC
[a]
[f]
Entry
BPLOR
Catalyst
[BPLOR]0:[1]0:[iPrOH]0
ĐM
Pr[g]
(g.mol-1)
(g.mol-1)
(g.mol-1)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
OAll
OAll
OAll
OAll
OAll
OAll
OAll
OAll
OMe
OMe
OMe
OBn
OBn
OBn
1a
1b
1c
1d
1g
1g
1g
1h
1b
1g
1h
1b
1g
1h
53:1:1
54:1:1
54:1:1
50:1:1
51:1:1
55:1:1
108:1:1
53:1:1
44:1:1
49:1:1
43:1:1
47:1:1
52:1:1
51:1:1
60
20
20
20
0
20
20
20
20
20
20
20
20
20
72
2
2
2
2
2
2
2
2
2
2
2
2
2
74
94
39
7 550
7 800
3 050
5 300
7 250
7 850
15 290
7 500
4 350
5 750
5 050
9 100
10 000
9 850
7 150
7 850
3 300
5 600
7 950
7 650
15 150
7 450
5 200
5 750
4 950
11 550
11 400
10 050
9 200
10 500
4 300
1.46
1.09
1.12
1.11
1.39
1.19
1.14
1.23
1.06
1.18
1.07
1.08
1.14
1.07
0.07
0.10
0.49
0.51
0.91
0.88
0.84
0.82
0.10
0.81
0.78
0.10
0.88
0.90
74
7 200
100
100
100
100
84
100
100
98
12 150
11 000
18 300
8 600
5 200
8 400
6 850
11 000
16 400
10 500
100
100
a Reactions performed at [BPLOR 0 = 1.0 M. b Reaction time was not necessarily optimized. c Conversion of BPLOR as determined by 1H NMR analysis
]
of the crude reaction mixture. d Molar mass calculated according to Mn,theo = ([BPLOR]0/[1]0 × conv.BPLOR × MBPLOR) + MiPrOH, with MBPLOAll = 142 g.mol1
,
e
f
MiPrOH = 60 g.mol1
.
Molar mass determined by 1H NMR analysis of the isolated polymer, from resonances of the terminal OiPr group. Number-
g
average molar mass and dispersity (Mw/Mn) determined by SEC analysis in THF at 30 °C vs. polystyrene standards (uncorrected values). Pr is the
probability of racemic linkages between BPLOR units as determined by 13C{1H} NMR analysis of the isolated PHBORs.
2
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10.1002/anie.201704283
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isocontrol in the ROP of BPLOR can be ruled out. In this respect, the
present phenomenon is quite different from that previously noted by
Gibson et al. in the ROP of rac-LA with Al-salan type initiators,
since in this latter case, the chloro-substituted systems actually
afforded heterotactic PLA, just as the methyl-substituted systems
did.[2e] As expected from the achiral nature of our initiators, we
checked that both enantiomers of rac-BPLOAll are consumed at the
same rate (Figures S20, S21), and hence that racemic mixtures of
homochiral (S)- and (R)-PHBOAll are produced.
DFT studies have been performed to investigate the experimen-
tally observed stereoselectivity of the catalysts. For the sake of
computational resources, we have concentrated onto smallest
catalysts 1a and 1c with BPLOAll as monomer. It should be noted
that the mechanism of the elementary steps of ROP (ring-opening of
the first monomer) is already well established,[18] hence our focus
was placed on the second step of the reaction determining the
stereochemistry of the polymer (Figure 2) (see the Supporting
Information for details).[19]
The free energies (G) of the first two steps (attack of the
monomer and formation of the four-membered metallacycle) for
both iso- and syndioselective routes are similar. However, the
second transition state leading to syndiotactic PHBOAll (III-SR-TS)
is notably higher in energy (G‡ = 11.6 kcal mol1) compared to the
transition state leading to isotactic polymer (III-SS-TS, G‡ = 6.2
kcal mol1). The origin of this difference was traced back to strong
C–H···Cl interactions between the ring-opened monomer and
Figure 1. Detailed regions of 13C{1H} NMR spectra (125 MHz, CDCl3, 23 °C) of
polymers prepared from the ROP of rac-BPLOAll (except top spectrum, from
enantiopure (S)-BPLOAll) by 1/iPrOH (1:1) systems (Tables 1, S1). ♦ refers to
residual monomer.
aromatic chloride substituents of the {ONXOCl2 2 ligand (Figures 3
}
The tert-butyl substituted systems 1e and 1f induce a slightly
less pronounced syndio-control than their cumyl-substituted
analogues 1g and 1h, respectively (Ps = 0.76 vs. 0.88 and 0.81 vs.
0.82) (Tables 1, S1).[17] Performing the ROP of rac-BPLOAll at lower
temperature (0 °C) with the 1g/iPrOH system produced, as
anticipated, a polymer with a higher Ps value of 0.91 (Table 1, entry
5). Similar trends were observed for the ROP of rac-BPLOMe and
rac-BPLOBn. The formation of highly syndiotactic PHBOR with
“{ONOOR’2}Y” catalysts bearing sterically crowded R‘ (tBu,
CMe2Ph) substituents follows trends previously established in the
ROP of rac-BL,[8,11] rac-MLAR (R = allyl, benzyl),[10] and other -
lactones[9] with related yttrium catalysts. This is consistent with a
CEC mechanism where bulky substituents on the ligand platform
favor discrimination between enantiomers of opposite configuration
in a highly sterically crowded coordination sphere. Accordingly,
systems 1c and 1d that are based on ligands with much less bulky
methyl substituents afforded systematically atactic PHBOR materials
(Ps ≈ Pi ca. 0.50), as clearly illustrated in the 13C NMR spectra
(Figure 1).
and S22, see the Supporting Information). Because the free energy
of III-SS-TS is very low, the actual rate limiting step in the
isoselective route is II-SS-TS with G‡ = 7.1 kcal mol1. Hence, the
activation energy for the formation of isotactic PHBOAll is 4.5 kcal
mol1 lower than for the syndiotactic polymer, which is in excellent
agreement with the experimental result that 1a and 1b produce a
highly isotactic material. It is very important to note that the
strongest C–H···Cl interaction exists between a hydrogen from the
alkoxymethylene group (–CHCH2OCH2CH=CH2) that is replaced in
alkoxycarbonyl--lactones (MLAR) by
a carbonyl group (–
CHC(=O)OCH2–CH=CH2). Not only this result elucidates the
2
completely inverse behavior of {ONXOCl2
}
ligand in the ROP of
rac-MLAR and rac-PBLOR monomers, but it also indicates that the
mechanism of MLAR polymerization, producing highly syndiotactic
PMLA (Ps > 0.95),[10b] is largely different than the above described
route. Furthermore, because the strong interaction between this
specific hydrogen and the aryl chloride substituent appears to be the
main factor affecting the stereocontrol of the catalyst, it explains
why different R-groups in PHBOR (R= allyl, Me, benzyl) do not
have a significant impact onto the stereochemistry of the polymer
(Tables 1, S1).[20] The above calculations were repeated for methyl
The most striking feature is the ability of dichloro-substituted
systems 1a and 1b to give isotactic PHBOR materials. This is a
completely original behavior since the same systems, when applied
to the ROP of rac-LA,[8b] rac-BL[10] and rac-PMLARs,[10] formed
atactic PLA and PHB and highly syndiotactic PMLARs, respectively.
Because Cl has a similar size to methyl, a steric origin of the
substituted catalyst 1c to estimate the differences between the
2
substitution pattern of {ONXOR’2
}
ligand. The ROP of PHBOAll
using 1c as initiator proceeds through a similar route than that with
Figure 2. Energy profile of the proposed ROP mechanism for isotactic (black) and syndiotactic (red) PHBOAll formation with 1a as initiator.
3
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10.1002/anie.201704283
Angewandte Chemie International Edition
1a. However, the energy profile of the reaction is significantly
different (Figure S23). The free energies of almost all intermediates
and transition states on both iso- and syndio-propagation routes are
virtually identical. Interestingly, the difference in G‡ of the rate
determining step (III-SS/SR-TS) is diminishingly small (0.2 kcal
mol1). This result is again in perfect agreement with the
experimental observation that methyl substituted catalysts 1c and 1d
produce practically atactic PHBORs.
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Figure 3. Structure of III-SS-TS showing the shortest CH...Cl distances (blue =
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The thermal properties of the PHBORs were preliminarily
investigated by DSC (Table S3, Figures S24-S29). These analyses
revealed that a distinct glass transition temperature between purely
isotactic (Pi > 0.95) and syndiotactic (Ps > 0.90) polyesters could
only be observed for PHBOMe (Tgiso = 18 °C vs Tgsyndio = ca.
12 °C). For isotactic PHBORs, the alkoxymethylene substituent on
the repeating unit was found to significantly affect this value
(TgPHBOAll = 39 °C, TgPHBOMe = 18 °C, TgPHBOBn  °C). Finally, a
melting transition temperature was only observed for syndiotactic
PHBORs with R = All, Me (TmPHBOAll =  °C –a similar value to
that reported for PMLAAll [10]
– TmPHBOMe =  °C).
,
In conclusion, we have demonstrated for the first time that a
specific class of functional racemic -lactones can be polymerized
either in a syndioselective, or most originally, in an isoselective
fashion by simple tuning of a given catalyst. Chloro substituents on
the tetradentate bisphenolate ligand have a unique influence. DFT
computations hint at the determining role of Cl···H interactions
between the ligand and growing polymer chain in enabling the
isoselective ROP of BPLOR monomers. Most interestingly, the
stereocontrol ability of these o,p-dichloro-substituted yttrium
catalysts appear to be very different between rac-BPLOR and rac-
MLAR monomers, leading respectively to iso- and syndiotactic
PHAs. These unique stereocontrol abilities are prone to be applied
to the preparation of original polymer materials derived from
various cyclic esters.
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Altenbuchner, P. Deglmann, C. Sinkel, R. Loos, B. Rieger,
ChemCatChem, 2015, 7, 3963–3971.
Received: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
Keywords: -lactones • stereoselective catalysis • ring-opening
polymerization • yttrium
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3709-3712; b) J. A. R. Schmidt, E. B. Lobkovsky, G. W. Coates, J.
Am. Chem. Soc. 2005, 127, 11426-11435.
[14] For the ROP of BPLOAll (similar operating conditions), using 1b:
Conv.BPLOAll = traces in THF vs. 94% in toluene (Table 1, entry 2);
4
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10.1002/anie.201704283
Angewandte Chemie International Edition
using 1d: Conv.BPLOAll = 49% in THF (Pr = 0.53) vs. 74% in toluene
(Pr = 0.51) (Table 1, entry 4); using 1g: Conv.BPLOAll = 59% in THF (Pr
= 0.83) vs. 100% in toluene (Pr = 0.88) (Table 1, entry 6).
[15] C. G. Jaffredo, M. Schmid, I. del Rosal, T. Mevel, P. W. Roesky, L.
Maron, S. M. Guillaume, Chem. Eur. J. 2014, 20, 14387–14402.
[16] Systems based on a methoxy-capped ligand proved generally to be
more stereoselective than their NMe2-capped analogues; compare e.g.
1a vs 1b and 1g vs 1h.
[17] The presence of additional signals/shoulders indicates that there is
also information at a larger range (triads) but this could not be further
developed due to the poor resolution of these signals.
[18] J. Fang, M. J. L. Tschan, T. Roisnel, X. Trivelli, R. M. Gauvin, C. M.
Thomas, L. Maron, Polym. Chem. 2013, 4, 360-367.
[19] The large number of carbonyl and ether oxygens able to coordinate to
yttrium center (particularly after the first ring-opening cycle) should
favor the mononuclear complexes during the polymerization process.
The possibility of dinuclear initiation, evoked earlier in the ROP of
rac-MLAR [10b], is most unlikely in the present case, as experiments
performed on a 3-fold range of catalyst concentration indicated the
apparent rate constants to be first order dependent on the yttrium
concentration (see Table S2 and Figure S2 in the Supporting
Information); also, the tacticity was not affected within this range of
concentration (Pm = 0.10).
[20] The activity of 1a/1b in ROP of rac-BPLOR at room temperature in
THF is almost nil and we therefore could not assess whether the
stereoselectivity is affected or not when reactions are performed in
such a coordinating solvent (see [14]).
5
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10.1002/anie.201704283
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Entry for the Table of Contents (Please choose one layout)
Polymer Chemistry
Choose the
microstructure! First
tunable yttrium catalyzed
stereoselective ring-opening
polymerization to highly
syndiotactic or isotactic
polyhydroxyalkanoates from
a racemic mixture of
Romain Ligny, Mikko M. Hänninen,
Sophie M. Guillaume,* Jean-François
Carpentier* __________ Page – Page
functional
alkoxymethylene-lactones.
6
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