Mechanism of the Polymerization of rac-Lactide by Fast Zinc Alkoxide Catalysts

Article abstract of DOI:10.1021/acs.inorgchem.7b02544

The ring-opening transesterification polymerization (ROTEP) of rac-lactide (rac-LA) using L^XZn catalysts (L^X = ligand having phenolate, amine, and pyridine donors with variable para substituents X on the bound phenolate donor; X = NO

Full text of DOI:10.1021/acs.inorgchem.7b02544

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
pubs.acs.org/IC
Mechanism of the Polymerization of rac-Lactide by Fast Zinc
Alkoxide Catalysts
Daniel E. Stasiw,† Anna M. Luke,†,§ Tomer Rosen, Aaron B. League, Mukunda Mandal,
,§
‡
†
†
†
,†
,‡
,†
Benjamin D. Neisen, Christopher J. Cramer,* Moshe Kol,* and William B. Tolman*
†Department of Chemistry, Center for Sustainable Polymers, University of Minnesota, 207 Pleasant Street SE, Minneapolis,
Minnesota 55455, United States
‡
The School of Chemistry, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel
*
S Supporting Information
ABSTRACT: The ring-opening transesterification polymerization (ROTEP) of
X
X
rac-lactide (rac-LA) using L Zn catalysts (L = ligand having phenolate, amine,
and pyridine donors with variable para substituents X on the bound phenolate
donor; X = NO , Br, t-Bu, OMe) was evaluated through kinetics experiments and
2
density functional theory, with the aim of determining how electronic modulation
of the ligand framework influences polymerization rate, selectivity, and control.
After determination that zinc-ethyl precatalysts required 24 h of reaction with
benzyl alcohol to convert to active alkoxide complexes, the subsequently formed
species proved to be active and fairly selective, polymerizing up to 300 equiv of
rac-LA in 6−10 min while yielding isotactic (P = 0.72−0.78) polylactide (PLA)
m
with low dispersities: Đ = 1.06−1.17. In contrast to previous work with aluminum
catalysts for which electronic effects of ligand substituents were significant
X
(
+
Hammett ρ = +1.2−1.4), the L Zn systems exhibited much less of an effect (ρ =
0.3). Density functional calculations revealed details of the initiation and
propagation steps, enabling insights into the high isotacticity and the insensitivity of the rate on the identity of X.
14,15
INTRODUCTION
uated.
Notably, previous studies of AlOR systems have
■
revealed significant rate changes upon variation of the electron-
withdrawing/-donating power of substituents on otherwise
identical supporting ligands. For example, electron-withdrawing
substituents in salen-AlOR catalysts were found to enhance the
rate of ROTEP of CL (for the insertion rate constant k,
New catalytic synthetic methods are needed for the develop-
ment and broader implementation of sustainable polymers that
are both derived from biorenewable feedstocks and readily
1
,2
degraded after use. Understanding the mechanisms by which
existing catalysts operate is a key prerequisite for addressing this
goal. As a privileged class of bioderived monomers, cyclic esters
undergo ring-opening transesterification polymerization
⧧
Hammett ρ ≈ +1.3, ΔΔG = 1.6−2.6 kcal/mol between
4
,5
systems with NO2 and OMe substituents). Analysis by
density functional theory (DFT) pointed to changes in the
bonding in the insertion transition state as being responsible for
the rate differences (greater electron withdrawal shortens the
Al−O(carbonyl) and C(carbonyl)−O(alkoxide) interactions).
On the other hand, other AlOR catalysts exhibit the reverse
trend, where ROTEP is inhibited by electron-withdrawing
(ROTEP) to yield useful sustainable materials, with catalysis
3
by metal alkoxide complexes being a pervasive route. There is
wide agreement that these systems follow a coordination−
insertion mechanism involving initial binding of the monomer
to the catalyst followed by nucleophilic attack and ring-opening
by an alkoxide ligand. However, purposeful catalyst design is
hindered by a general lack of knowledge of many details of the
process, including the nature of the coordination and insertion
steps and the fundamental reasons for changes in rates and
selectivities observed upon varying the supporting ligand and/
or metal ion.
12,13
groups and enhanced by electron-donating substituents.
While these and other results from studies of the well-
−2
−1
controlled but relatively slow AlOR catalysts (k ≈ 10 s )
provide useful insights, it is unclear whether they are
transferable to catalysts that perform ROTEP at much faster
rates. A notable example of the latter is 1 (Figure 1), which was
found to polymerize up to 500 equiv of rac-LA in under 5 min
Significant insights into these issues have been obtained from
kinetics studies of ROTEP of lactide (LA) or ε-caprolactone
−1
−1 16
4
−13
at room temperature (k = 2.2 M s ). However, this
p
(
CL) by aluminum alkoxide (AlOR) complexes.
Such
system only produced atactic PLA with moderate polydisper-
sities (1.3−1.4), and the fact that it exists predominantly as a
catalysts exhibit high molecular weight control and modest
rates that are convenient for monitoring by NMR spectroscopy
and typically feature easily synthesized supporting Schiff-base
ligands that are readily structurally modified so that steric,
overall geometric, and electronic influences may be eval-
Received: October 4, 2017
©
XXXX American Chemical Society
A
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Inorganic Chemistry
Article
Figure 1. Zn complexes used for the polymerization of LA.
dimer renders mechanistic analysis challenging. Recently, a
modified version of the ligand in 1 incorporating a pyridyl
X
group was found to yield exclusively monomeric L Zn (2, X =
t-Bu), which is a fast, controlled, and selective precatalyst for
the polymerization of rac-LA using a benzyl alcohol initiator,
reaching >95% conversion in ∼30 min at ambient temperature
17
with high isotacticity (P = 0.7−0.8).
m
Intrigued by these initial results, we sought to investigate the
effects of varying the electron-donating propensities of
substituents X in 2 on the ROTEP kinetics and stereocontrol.
We report the synthesis and characterization of new variants
Figure 2. Synthesis of proligands and complexes and a representation
of the X-ray crystal structure of 2 (X = Br), showing all non-hydrogen
atoms as 50% thermal ellipsoids. Selected distances (Å) and angles
with X = NO , Br, OMe and a full experimental kinetic analysis
2
of their ROTEP of rac-LA by all four derivatives. Key findings
include discoveries that preconditioning of the complexes with
BnOH results in very high ROTEP rates, that all catalysts yield
PLA with similar isotacticity, and that the polymerization rates
are insensitive to the nature of X, in contrast to what is observed
with AlOR catalysts. We evaluated the bases for these results
through analysis of the reaction pathway by DFT calculations,
which provided insights into the high isotacticity of the system
and a rationalization for the lack of significant rate changes
upon varying X.
(
2
deg): Zn1−O1, 2.0473(11); Zn1−N1, 2.1407(14); Zn1−N2,
.4348(14); Zn1−N3, 2.1999(15); Zn1−C1, 2.0059(17); C1−Zn1−
O1, 114.67(6); C1−Zn1−N1, 113.30(7); C1−Zn1−N2, 104.29(6);
C1−Zn1−N3, 116.42(7); O1−Zn1−N1, 87.43(5); N1−Zn1−N2,
7
3.24(5); N2−Zn1−N3, 76.93(5); N3−Zn1−O1, 88.89(5); O1−
Zn1−N2, 140.84(5); N1−Zn1−N3, 126.62(5).
beginning of the polymerization array (Figure 3; note the
overall sigmoidal form of the curves). It had been previously
suggested that, once the benzyl alcohol initiator was
introduced, rapid exchange between the zinc’s ethyl substituent
RESULTS AND DISCUSSION
Synthesis of Complexes. Using the same method
X
■
and the benzyl alcohol occurred to yield a L ZnOBn complex
(2′) which initiated ROTEP. To explain the apparent induction
period observed under the aforementioned reaction conditions,
however, we hypothesized that the conversion of 2 to 2′ was in
fact slow and that the overall rate observed under these
conditions (>95% conversion in 2.5 h) was in fact slower than
that if formation of 2′ was instantaneous.
reported previously for the synthesis of the proligand and
17
complex 2 (X = t-Bu), the respective proligands and
complexes 2 (X = Br, NO ) were prepared by coupling the
2
appropriate bromomethylphenol with the pyridyl-amine frag-
ment followed by treatment with ZnEt (Figure 2). A different
2
route was found to be necessary to produce the proligand with
X = OMe, involving reductive amination of the indicated
To test the above hypothesis and determine the mixing time
required for full formation of 2′, exchange experiments were
1
13
aldehyde. Each ligand was fully characterized by H, C,
COSY, and HSQC NMR spectroscopy, as well as high-
resolution electrospray ionization mass spectrometry (HR-ESI-
MS; details provided in the Supporting Information).
Complexes 2 were obtained as crystalline solids in good yields
performed, in which a solution of 2 (X = NO , 3.33 mM) in
2
CD Cl was placed in an NMR tube with an equimolar amount
2
2
of benzyl alcohol (Figure 5) and the decay of the ethyl peaks
1
13
(
85−93%), and were characterized by H, C, COSY, and
HSQC NMR spectroscopy, CHN analysis, HR-ESI-MS, and,
for 2 (X = Br), X-ray crystallography. Similar to other reported
17
structures with analogous ligands, the complex is mono-
nuclear with a distorted-square-pyramidal geometry charac-
terized by τ = 0.24 (τ = 0 is square pyramidal and τ = 1 is
5
5
5
1
8
trigonal bipyramidal).
Polymerization Kinetics: H NMR Monitoring. Polymer-
ization of rac-LA was initially monitored by H NMR
1
1
spectroscopy with fixed concentrations of rac-LA (0.9 M <
[
LA] < 1.2 M), precatalyst 2 (3 mM < [2] < 4 mM), and
0
0
benzyl alcohol (3 mM < [BnOH] < 4 mM) in CD Cl , using
0
2
2
1
,4-bis(trimethylsilyl)benzene (8.33 mM) as an internal
standard. In the experiments, benzyl alcohol was added last,
at which point data collection was initiated. Upon inspection of
the H NMR data for the polymerization of rac-LA by 2 (X =
Figure 3. Concentration versus time plot for polymerization of rac-LA
(
300 equiv, 1 M) by 2 (X = NO , 1 equiv, 3.33 mM) and BnOH (1
2
1
1
equiv, 3.33 mM) as monitored by H NMR spectroscopy. Data
NO ) we found evidence of an induction period near the
collection was begun soon after addition of BnOH.
2
B
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Article
associated with 2 (X = NO ) was monitored. Total
2
X
disappearance of the Et peaks from the NMR of the L ZnEt
+
BnOH mixture signifying complete conversion to the ZnOBn
complex occurred after stirring for 24 h (Figure S9 in the
Supporting Information). Although the slow reaction of 2 with
the BnOH was unexpected, there is literature precedent for
19
slow Zn alkyl and alcohol exchanges. While NMR data
support the formulation of 2′ (X = NO ), efforts to isolate it
2
have not been successful. It is likely mononuclear in solution, as
indicated by the results of diffusion ordered spectroscopy
(
DOSY) experiments; hydrodynamic radii r for 2 and 2′ (X =
H
NO ) were found to be 6.22 and 6.98 Å, respectively (Figure
2
S10 in the Supporting Information).
With the knowledge that conversion to a zinc alkoxide was
slow, we “preconditioned” the catalysts by first stirring 2 with
BnOH for 24 h to fully convert it to 2′ before adding rac-LA.
Figure 4. Stacked IR spectra at 30 s intervals for polymerization of LA
1
Subsequent ROTEP was then monitored via H NMR
(black) to PLA (red) to >95% conversion using 2′ (X = NO ;
2
intermediate spectra in gray).
spectroscopy using the same fixed conditions as described
above. Significantly faster polymerizations were observed; with
pure 2′ (X = NO , Br) >95% conversion was reached in just
2
under 400 s (∼6 min), with the same conversion reached with
2
′ (X = t-Bu, OMe) in ∼8 and ∼10 min, respectively. These
enhanced rates in comparison to the previously monitored
polymerizations support the hypothesis that slow conversion
from 2 to 2′ underlies the induction period and that full
conversion to the active zinc alkoxide catalyst is necessary for
rapid ROTEP. However, these new polymerization rates
1
rendered monitoring of the polymerizations by traditional H
NMR spectroscopy methods much more challenging, and so
we turned to an alternative methodology.
Polymerization Kinetics: React-IR Monitoring. In view
of the fast overall conversions observed for the ROTEP
reactions performed by first allowing 2 to react 24 h with
BnOH, we used in situ IR monitoring by React-IR to obtain
accurate and precise kinetic data. Kinetic measurements of the
ring-opening polymerization of LA using Sn(oct)2 were
previously measured with React-IR using single-wavelength
Figure 5. Illustrative plot of experimental data (circles) and
corresponding COPASI fit (red line) for the ROTEP of LA by 2′
(
X = NO ). See Figures S13 and S14 in the Supporting Information
2
for all other such data plots.
2
0−22
three fitting methods revealed results in close agreement
analysis.
In preliminary experiments using this method we
(
within 20%; Table S1 in the Supporting Information); we
obtained variable and inconsistent results depending on the IR
peak selected. This problem was avoided by using spectral
arbitrarily chose to provide in Table 1 those obtained by
−1
COPASI fitting to calibrated data.
deconvolution and global fitting in the 1900−900 cm region
to obtain relative amounts of rac-LA and PLA per spectrum
collected. Known issues with nonlinear absorption response
Table 1. Rate Constants and PLA Characterization Data
20,23
versus analyte concentration
were addressed by use of
kobs (s−1
a
b
complex
)
Mw (kDa)
M (kDa)
n
Đ
Pm
calibration curves for both rac-LA and PLA (∼50 kDa average
2
2
2
2
′ (X = NO2)
′ (X = Br)
′ (X = t-Bu)
′ (X = OMe)
0.014(1)
0.010(1)
63.1
70.4
40.8
41.6
53.8
61.4
35.5
39.2
1.17
1.15
1.15
1.06
0.72
0.76
0.78
0.75
M ), which established correlation of integrated area under the
w
−1
1
(
900−900 cm region of interest to known concentrations
Figure S12 in the Supporting Information).
0.0102(8)
0.006(1)
Polymerizations of rac-LA to PLA were performed in
a
Conditions: [LA] = 1 M: [2′] = 3.33 mM (300:1) in CH Cl at 25
0
0
2
2
triplicate and monitored by React-IR at fixed concentrations
of LA (0.5 M < [LA] < 1.5 M) and 2′ (1 mM < [2′] < 4
b
°
C. theoretical M = 43.3 kDa.
w
0
0
mM). The series of spectra (illustrative data shown in Figure 4)
were separated into two components, rac-LA and PLA, through
spectral deconvolution analysis. Relative concentrations of
Comparison of the kobs values as a function of para
substituent (Table 1) reveals a negligible trend, as illustrated
24
rac-LA and PLA versus time as well as absolute concentration
by only ∼2-fold difference in rate constants. Also, a small ρ
(
using the calibration curves) versus time data were fit with
value of +0.3 is seen in a Hammett plot of the log k values
p
2
5
COPASI software. Excellent agreement with a first-order fit
by all 2′ species showed that the polymerizations follow a
(black line, Figure 6). This finding differs from that seen for
ROTEP of CL by salen-AlOR catalysts (red and blue lines),
pseudo-first-order rate expression: rate law = k [LA] where
which exhibited ρ = +1.2 or +1.5, respectively (where k refers
obs
p
kobs = k [2′] (Figures S13 and S14 in the Supporting
to the catalytic rate constant k in the Michaelis−Menten
p
2
Information; illustrative data are shown in Figure 5). The
expression applicable for these catalysts because they exhibited
saturation kinetics). In addition, the polymerization rates for
the Al catalysts (also at 293 K) varied by approximately 1 order
spectral data were also fit to a pseudo-first-order rate law by the
26
Olis GlobalWorks spectral fitting software. Comparison of the
C
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average molecular weight (M ) and dispersity (Đ) of the
w
resulting polymers (Table 1). Even at the rapid polymerization
rates observed by the 2′ species, polymerization control is
maintained, which is indicated by narrow polymer dispersities
(
Đ = 1.06−1.17). A sample of dry PLA from each kinetic
experiment was dissolved in CDCl and used to determine the
polymer’s isotacticity (P ) via homonuclear-decoupled
3
1
H
m
NMR. Isotactic preference for PLA by all catalysts studied was
also maintained with P values reaching as high as 0.78 (Table
m
1
).
Density Functional Theory Calculations. Quantum
chemical modeling, which has previously offered important
insights into the metal-catalyzed ROTEP of lactide in other
27
systems, was undertaken to (i) offer insight into the observed
isotacticity in the polymer product and (ii) rationalize the
insensitivity of the reaction rate to para substitution of the
phenoxide moiety in the catalyst. In particular, a detailed survey
of stereodistinct isomers for relevant insertion and ring-opening
transition-state structures in the parent system was done using
the M06 family of density functionals (see Calculation Details
in the Supporting Information for details), and the reaction
coordinate shown in Figure 7 was found to have the lowest
barriers for the initiation step of the proposed polymerization
mechanism. The ring-opening transition state (TS4-5) was
predicted to be rate limiting for both enantiomers of rac-LA,
with the S,S enantiomer having a significantly higher barrier
than the R,R enantiomer (16.1 vs 14.0 kcal/mol, respectively)
for the catalyst configuration used. While we note that the
flexibility of the growing polymer chain, together with the
structural similarity of isopropoxide to the portion of the
growing polymer chain coordinated to the metal, suggests that
similar differential energetics may be expected for subsequent
Figure 6. Hammett plots of log k vs σ for 2′ (X = NO , Br, t-Bu,
p
para
2
OMe; black data and linear fit, R = 0.825), with data and fits reported
previously for ROTEP of CL by AlOR catalysts supported by salen
ligands with a three-carbon backbone (red, R = 0.995) or a two-carbon
backbone (blue; R = 0.999) at 298 K. The slopes of the lines
corresponding to the Hammett ρ values are shown.
4
of magnitude (NO > Br > OMe). In a later section, we turn
2
to DFT calculations in order to better understand why a
significantly less pronounced trend in rate as a function of
phenolate substituent was observed for the Zn catalysts in
comparison to the previously studied salen-AlOR systems.
Polymer Characterization. Kinetic experiments were
exposed to air to quench the polymerization reaction within
∼
30 min of completion, and PLA was precipitated by addition
to cold methanol, collected, and dried under reduced pressure
for 48 h. Size exclusion chromatography (SEC) using a
dynamic light scattering detector was used to determine the
Figure 7. M06-2X//M06-L reaction coordinate standard-state free energies for lactide opening (kcal/mol) relative to separated species with line
drawings of relevant stationary points. Results for pathways involving both S,S-lactide (green dotted line) and R,R-lactide (black solid line) are shown
with adoption of a particular configuration at Zn in the precatalyst differentiating these two paths.
D
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propagation steps, we also recognize that some degree of chain-
end control of polymer stereochemistry likely occurs and is not
addressed by our calculations. Working under the assumption
that site control is important, we hypothesize that the difference
in activation free energies between the two LA enantiomers is
one key determinant of the isotacticity observed in the polymer
functionality about Zn buffers the phenol para substitution
effect so that its influence is more limited than it might
otherwise be expected to be.
28
Additional insights into the basis for the ligand substitution
effects may be obtained by comparing the transition-state
structures calculated here to those previously reported for the
4
(noting that changing the configuration at Zn requires a quite
(salen)Al system with the two-carbon backbone (Figure 8). As
high-energy dissociation/recoordination of the growing poly-
mer chain from the metal center). The origin of this 2.1 kcal/
mol energy difference is associated with the boat-like geometry
of the six-membered lactide ring in this ring-opening TS
structure (Figure S22 in the Supporting Information). The boat
is preferred because it permits both methyl groups of the lactide
to be pseudoequatorial while their corresponding geminal
hydrogen atoms are pseudoaxial (the planarity of the ester
linkages that comprise the other four atoms of the ring also
contribute to stabilizing the boat ring form in this TS
X
structure). For the specific L Zn catalyst configuration chosen,
the alternative boats have differing steric interactions. For the
S,S case, a methyl group of the lactide ring is close to and
clashes with the tert-butyl group of the aryloxy ring and the
alkylamino portion of the ligand linker, whereas these
interactions are lessened considerably for the R,R isomer.
Moreover, the methyl group from the alkylamino portion of the
ligand linker points directly toward the unperturbed ester
linkage of the lactide ring for the S,S case, whereas these
interactions are absent in the R,R variant. We hypothesize that
these differences in steric interactions and/or unfavorable
changes in electronic interactions associated with geometry
relaxation to ameliorate the steric clash underlie the TS
energies and are at least partially responsible for the observed
isotacticity in the polymerization.
Figure 8. Optimized transition-state (TS) structure geometries, with
selected atoms identified: (a) previously reported TS structure for
nucleophilic attack on CL by coordinated alkoxide for (salen)Al
catalyst analogous to the Zn catalysts reported here; (b) TS structure
for nucleophilic attack on R,R-lactide with (salan)Zn catalyst (TS2-3);
To assess substitution effects, all of the stationary points in
Figure 7 were substituted at the para position (cf. Figure 3).
Relative energetics for TS4-5, along with CM5 charges on the
Zn atom, are shown in Table 2 for all of the reaction
(c) ring-opening TS structure for the tetrahedral intermediate in the
R,R-lactide/(salan)Zn system (TS4-5). Geometric details are recorded
in Table 3 and Tables S3−S5 in the Supporting Information, with fully
labeled structures shown in Figures S16−S19 in the Supporting
Information.
Table 2. DFT Predicted Free Energies of Activation and Zn
Charges in TS4-5
illustrated for selected bond distances (Table 3; complete data
are provided in Tables S3−S5 in the Supporting Information),
R,R-lactide
S,S-lactide
substituent
ΔG⧧ (kcal/mol)a QZn,CM5 ΔG⧧ (kcal/mol)a QZn,CM5
−
−
−
−
−
−
NO2
Br
13.3
12.9
14.0
14.0
14.0
14.6
0.733
0.728
0.729
0.727
0.726
0.725
15.4
16.9
16.1
16.9
16.9
17.0
0.722
0.718
0.717
0.716
0.715
0.714
Table 3. Selected Bond Lengths (Å) and Bond Length
Variations in Calculated TS Structure for Nucleophilic
Attack on ε-Caprolactone or R,R-LA by Coordinated
Alkoxide as a Function of Para Substitution of the Phenolate
H
t-Bu
OMe
N(Me)2
a
Group(s)
bond
NO2
Br
H
OMe
range
a
Computed as GTS4‑5 − G using M06-2X//M06-L free energies.
4
(
salen)Al
Al−O1
Al−O2
O1−C1
O2−C1
1.893
1.900
1.302
1.744
1.912
1.900
1.295
1.769
1.927
1.905
1.289
1.801
1.929
1.904
1.291
1.784
0.036
0.005
0.013
0.057
coordinates (again, taking the specific configuration at Zn that
favors R,R reactivity). Slightly lower reaction barriers were
predicted for electron-withdrawing substituents in comparison
to those for H or electron-donating substituents, in good
agreement with experimental observations, but the overall
sensitivity to para substitution is predicted to be slight.
Consistent with this observation is the very small variation in
Zn CM5 charges that is predicted over the range of
substituents. Thus, from p-N(Me) to p-NO , there is an
increase in the Zn charge of only 0.008 au: i.e., its Lewis acidity
is modulated very little by para substitution of the phenol. It is
not entirely trivial, of course, to explain why something does not
happen, but we speculate that the extensive nitrogen
LXZn
Zn−O1
Zn−O2
O1−C1
O2−C1
2.245
2.061
1.255
1.874
2.283
2.067
1.253
1.870
2.291
2.065
1.252
1.876
2.285
2.076
1.252
1.879
0.046
0.015
0.003
0.009
2
2
a
Bond labels refer to structures in Figure 8a,b, respectively. Data for Al
bonds are taken from ref 4. Complete computed bond distances for
structures in Figure 8 are given in Tables S3−S5 and Figures S19−S21
in the Supporting Information.
E
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the largest change in TS structure bond lengths for the (salen)
Accession Codes
Al system is found for the O −C bond, which is the bond that
CCDC 1577971 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
2
1
is forming in this mechanistic insertion step (0.057 Å; O2 is the
alkoxide oxygen nucleophilically attacking the carbonyl carbon
of CL, C1). Given such a large change in the position of the TS,
it is not particularly surprising that a significant variation in
⧧
activation free energy (ΔG ) is also observed for polymer-
ization in this system (Figure 6, blue), presumably reflecting the
influence of the varying basicity of the phenolates in the ligand
AUTHOR INFORMATION
Corresponding Authors
*C.J.C.: e-mail, cramer@umn.edu; Twitter, @ChemProf-
Cramer.
■
X
(
of which there are two). In contrast, for the L Zn system,
there is quite little variation in the lengths of the key bonds
being made or broken as a function of para substitution in the
single phenol ring in these catalysts (0.009 Å for O2−C1 in the
insertion step, TS2-3, Table 3). The variation is similarly small
for O5−C1 in the ring-opening step (0.013 Å, TS4-5, Table S5
in the Supporting Information). Interestingly, in both TS
structures, there are significant variations in heteroatom−Zn
*M.K.: e-mail, moshekol@post.tau.ac.il.
*W.B.T.: e-mail, wtolman@umn.edu; Twitter, @WBTolman.
ORCID
Daniel E. Stasiw: 0000-0003-0526-7372
Anna M. Luke: 0000-0001-9486-5635
Mukunda Mandal: 0000-0002-5984-465X
Christopher J. Cramer: 0000-0001-5048-1859
William B. Tolman: 0000-0002-2243-6409
Author Contributions
X
bond distances associated with para substitution of the L
phenolate, but these do not seem to significantly influence the
reacting partners (consistent with the observed lack of much
charge variation at the Zn center described above). We
postulate that these findings derive from the fact that (i) there
X
§
is only a single phenol in the L Zn system in comparison to the
D.E.S. and A.M.L. contributed equally.
two in the (salen)Al system and/or (ii) the Zn−O bonds are
intrinsically about 0.2 Å longer than analogous Al−O bonds,
thereby making Zn less sensitive to substitution effects than Al
simply from the r− dependence of purely electrostatic effects.
Notes
The authors declare no competing financial interest.
1
ACKNOWLEDGMENTS
■
Funding for this project was provided by the Center for
Sustainable Polymers at the University of Minnesota, a
National Science Foundation (NSF) supported Center for
Chemical Innovation (CHE-1413862) and the US-Israel
Binational Science Foundation (2012187). The X-ray diffrac-
tion experiments were performed using a crystal diffractometer
acquired through NSF-MRI Award CHE-1229400. The authors
acknowledge the Minnesota Supercomputing Institute (MSI) at
the University of Minnesota for providing resources that
contributed to the research results reported within this paper.
CONCLUSIONS
■
In this work we prepared and characterized a series of
complexes 2 akin to previously reported efficient ROTEP
1
7
catalysts, but here featuring para substituents with variable
electronic influences. We found that preconditioning with
BnOH to yield putative alkoxide species 2′ led to fast and
reproducible rates of ROTEP of rac-LA (>95% in 6−10 min,
depending on catalyst). Analysis of kinetic data acquired using
in situ React-IR revealed a pseudo-first-order rate expression,
with the reaction yielding highly isotactic polymer. In contrast
to previous work on Al systems which showed that variation of
the electronic influences of para phenolate substituents has a
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ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02544.
Experimental data, concentration versus time plots,
crystallographic information, and calculations (PDF)
(4) Miranda, M. O.; Deporre, Y.; Vazquez-Lima, H.; Johnson, M. A.;
Marell, D. J.; Cramer, C. J.; Tolman, W. B. Understanding the
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