Bis(pyrrolidene) Schiff Base Aluminum Complexes as Isoselective-Biased Initiators for the Controlled Ring-Opening Polymerization of rac-Lactide: Experimental and Theoretical Studies

Article abstract of DOI:10.1021/acs.macromol.5b01381

A series of bis(pyrrolidene) Schiff base aluminum complexes (1-7) were synthesized and characterized by NMR spectroscopy and elemental analysis. All complexes were efficient initiators for the ring-opening polymerizations of l-LA and rac-LA in toluene at

Full text of DOI:10.1021/acs.macromol.5b01381

Article
pubs.acs.org/Macromolecules
Bis(pyrrolidene) Schiff Base Aluminum Complexes as Isoselective-
Biased Initiators for the Controlled Ring-Opening Polymerization of
rac-Lactide: Experimental and Theoretical Studies
Sittichoke Tabthong,† Tanin Nanok,† Pattarawut Sumrit,† Palangpon Kongsaeree,§ Samran Prabpai,§
Pitak Chuawong,‡ and Pimpa Hormnirun*,†
‡
†Laboratory of Catalysts and Advanced Polymer Materials, Department of Chemistry, Faculty of Science, and Department of
Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, and Special Research Unit for Advanced
Magnetic Resonance (AMR), Kasetsart University, Bangkok 10900, Thailand
§Department of Chemistry and Center for Excellence in Protein Structure and Function, Faculty of Science, Mahidol University,
Bangkok 10400, Thailand
S
* Supporting Information
ABSTRACT: A series of bis(pyrrolidene) Schiff base aluminum complexes (1−7) were synthesized and characterized by NMR
spectroscopy and elemental analysis. All complexes were efficient initiators for the ring-opening polymerizations of L-LA and rac-
LA in toluene at 70 °C. Kinetic studies revealed first-order kinetics in monomer and the rates of L-LA and rac-LA polymerizations
decreased in the order of 1,2-benzylene (4) ≫ 1,3-propylene (2) > 2,2-dimethyl-1,3-propylene (3) > 1,4-butylene (5) > rac-1,2-
cyclohexylene (7) > 1,2-ethylene (1) ≫ 1,2-phenylene (6). Microstructure analyses of the resulting polylactides by homonuclear
1
decoupled H NMR spectroscopy disclosed the isotactic-biased stereocontrol of all synthesized complexes, except 5. Isotactic
stereoblock polylactide with a high Pm value of 0.80 was produced by 3. A systematic DFT study on the rac-lactide ring-opening
mechanism initiated by the initiators synthesized in this study revealed the correlation between the structure of backbone linker
and the polymerization activity and stereoselectivity.
Among the large variety of metal-coordination initiators
known to catalyze the ROP of lactide, aluminum alkoxide
complexes modified by ancillary ligands have in particular
attracted interest over the past 20 years. Aluminum complexes
supported by N,N,O,O-tetradentate ligands have been exten-
sively studied as efficient initiators for the ROP of lactide. The
most important breakthrough was the discovery of the chiral
Salen aluminum complexes for the stereoselective polymer-
ization of rac-LA by Spassky and co-workers.11 Some other
chiral Salen aluminum catalysts were reported to exhibit
excellent stereocontrol for the polymerization of rac-LA to give
isotactic stereoblock PLAs via an enantiomorphic-site control
mechanism (SCM).12 Highly stereoselective polymerizations
INTRODUCTION
■
Polylactide (PLA) has become one of the most important
aliphatic polyesters due to its biodegradable and biocompatible
properties and to the possibility of deriving the monomer from
renewable resources.1 PLA and its copolymers have found
interest in many applications ranging from controlled drug
delivery systems, resorbable sutures, artificial tissue matrices,
food packaging to clothing products.2 Even though several
methods have been exploited to prepare PLA, the most
convenient and efficient one is the ring-opening polymerization
(ROP) of lactide (LA), the cyclic diester of lactic acid. Various
metal alkoxide complexes, including the metals aluminum,3
zinc,4 tin,5 yttrium,6 magnesium,7 calcium,8 and the lantha-
nides,9 have been studied in order to achieve better control,
higher activity, and stereoselectivity of the ROP of LA.
Recently, the research on metal alkoxide complexes has been
extensively reviewed.10
Received: June 24, 2015
Revised: September 6, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
via a chain-end control mechanism (CEM) were also achieved
by a number of achiral Salen aluminum complexes.13 The
discovery concerning an efficient stereocontrol using aluminum
complexes with achiral and chiral Salen ligands has stimulated
extensive studies on the development of new catalyst systems.
Gibson and co-workers discovered the closely related achiral
aluminum complexes bearing N,N,O,O-tetradentate amino-
phenoxy (Salan) ligands.14 Stereoselectivity in the polymer-
ization of rac-lactide was remarkably controlled by subtle
change of ancillary ligand substituents, leading to the
production of polylactide with microstructures ranging from
highly isotactic (Pm = 0.79) to highly heterotactic (Pr = 0.96). A
recent study by Kol and co-workers described the switch of
stereochemical control from isoselectivity to hetereoselectivity
employing alkylaluminum complexes bearing chiral 2,2′-
bipyrrolidine-based Salan ligands.15 A highly heterotactic PLA
with a Pr ≥ 0.98 was demonstrated to be produced by an
insertion/autoinhibition/exchange mechanism. Feijen and co-
workers reported the use of chiral Salan aluminum complexes
as efficient catalysts for the ROP of rac-LA and meso-LA.16
Polylactide materials with a Pm value of 0.66 and a Pr value of
0.73 were obtained from rac-LA, whereas PLA with a Pr value of
0.70 was produced from meso-LA. Further investigations on the
ROP of lactide using aluminum complexes bearing symme-
tric13f and asymmetric17 Salan ligands demonstrated that
changes in ligand substitution pattern can lead to different
degrees of complex fluxionality that have dramatic effects on
the rate and stereoselectivity of the polymerization. Recently,
aluminum complexes of N,N,O,O-tetradentate Salalen ligands
were experimentally and theoretically studied for the ROP of
rac-LA by Jones and co-workers.18 PLAs with microstructure
ranging from medium heterotactic to medium isotactic were
obtained. Theoretical study revealed that the ortho substituent
of the ligand is critical for the positioning of the new incoming
LA, leading to different polymer microstructures.18b Very
recently, Kol and Lamberti reported the use of aluminum
complexes containing enantiomerically pure aminomethyl-
pyrrolidine-based Salalen ligands for the stereoselective
polymerization of rac-LA.19 A new gradient isotactic multiblock
microstructure was proposed to operate by a combination of
enantiomorphic-site and chain-end control mechanism.
In the search of new ancillary ligand systems for the ROP of
lactide, the N,N,N,N-tetradentate ligands have found less
popular for the complexation with aluminum precursor. Only
few studies concerning the aluminum complexes of N,N,N,N-
tetradentate ligands were reported.20 Chen and Feijen reported
the synthesis of achiral aluminum alkyl complexes bearing
bis(pyrrolidine) Schiff base ligands.20a These complexes were
shown to produce highly isotactic PLLAs from L-LA without
epimerization of the monomer, isotactic-biased PLA from rac-
LA, and atactic PLA from meso-LA. However, the effect of
catalyst architecture on the stereocontrol found in this initiator
system remains elusive at present. Also, an understanding of the
relationship between the catalyst structure and reactivity has yet
to be elucidated.
In this work, we experimentally investigated the catalytic
performance for rac-lactide and L-LA polymerizations of
aluminum methyl complexes supported by bis(pyrrolidene)
Schiff base ligands with a wide range of backbone linkers. In
addition, density functional theory (DFT) calculations were
conducted to reveal the ROP mechanism and to highlight the
effect of the catalyst structure on the polymerization activity
and the stereoselectivity found in this system.21
RESULTS AND DISCUSSION
■
Synthesis and Analysis of Complexes. The bis-
(pyrrolidene) Schiff base ligands with different backbone
linkers, H2L1−H2L7, were synthesized via a condensation
reaction with 2 equiv of pyrrole-2-carboxaldehyde and 1 equiv
of an α,ω-diamine in ethanol at room temperature according to
the published procedure.22 All ligands were obtained as white
solids in high yields. Treatment of the ligands with
stoichiometric trimethylaluminum (AlMe3) in anhydrous
toluene at 110 °C afforded the corresponding complexes 1−7
as shown in Scheme 1. The complexes were formulated on the
1
basis of H and 13C NMR spectroscopy, elemental analysis,
and, for 3, X-ray crystallography.
Scheme 1. Synthesis of Complexes 1−7
Solution 1H NMR studies revealed the formation of
aluminum complexes 1−7 from the disappearance of the NH
signal of the free ligands and the appearance of protons of the
methyl groups bound to the aluminum center in the high field
23,24
1
1
region of the H NMR spectra.
For example, the H NMR
spectrum of 1 revealed complex second-order multiplets arising
from the diastereotopic methylene protons of the ligand
backbone around δ 4.00−3.66 ppm. The signal ascribed to the
imine and the methyl protons of the aluminum methyl group
appeared at δ 8.33 and −0.97 ppm, respectively, with an
integral ratio of 2:3. This ratio indicated the formation of the
five-coordinate aluminum complex. The spectra of complexes
1−3, 5, and 6 contained only one imine signal (NCH) and a
symmetric pattern of the pyrrolic protons. In the case of 7, the
conformation of the rac-cyclohexyldiimino unit rendered the
two pyrrole rings nonequivalent. Hence, two imine resonances
at δ 8.48 and 8.18 ppm and two sets of pyrrolic protons were
observed. The purity of 1−7 was ensured through correct
elemental analysis.
Single crystals of 3 suitable for X-ray diffraction analysis were
grown from a saturated toluene solution at −20 °C. The
molecular structure of 3 features a monomeric molecule with a
five-coordinate aluminum center in a geometry best described
as distorted square-based pyramidal (Figure 1). The amount of
distortion can be quantified using the geometric criterion τ = (β
− α)/60 proposed in the literature.24,25 The τ value ranges
from 0 (perfectly square pyramidal) to 1 (perfectly trigonal
bipyramidal). In this case, the τ value is 0.18, indicating the
distorted square-based pyramidal geometry around the
aluminum center. The apical position of the pyramid is
occupied by the methyl group, and the basal plane is
constituted of the four nitrogen atoms. Selected bond lengths
and angles are listed in Figure 1 (see Tables S1 and S2 in the
Supporting Information for the crystallographic details).
Ring-Opening Polymerization of Lactide. The series of
complexes 1−7 allowed the systematic investigation of the
B
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
Figure 1. Molecular structure of 3. Selected bond lengths (Å) and angles (deg) are as follows: Al(1)−C(20), 1.967(2); Al(1)−N(11), 2.0535(14);
Al(1)−N(1), 1.9759(14); Al(1)−N(17), 1.952(15); Al(1)−N(7), 2.0334(13); N(11)−C(12), 1.291(2); N(7)−C(6), 1.289(2); C(20)−Al(1)−
N(1), 108.96(17); C(20)−Al(1)−N(17), 115.40(7); C(20)−Al(1)−N(7), 106.28(7); C(20)−Al(1)−N(11), 102.07(7); N(1)−Al(1)−N(17),
92.86(6); N(1)−Al(1)−N(7), 80.44(5); N(2)−Al(1)−N(11), 148.16(6); N(17)−Al(1)−N(7), 137.59(6); N(17)−Al(1)−N(11), 79.92(6);
N(7)−Al(1)−N(11), 84.27(5).
a
Table 1. Polymerization of rac-LA and L-LA Using Complexes 1−7 in the Presence of Benzyl Alcohol
b
c
d
d
e
entry
complex
monomer
time (h)
conv (%)
Mn(theory) (g mol−1
)
Mn(GPC) (g mol−1
)
Mw/Mn
Pm
1
1
2
3
4
5
6
7
1
2
3
4
5
6
7
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
L-LA
48
1
77
98
95
99
82
86
82
73
92
83
99
94
82
87
11 200
14 200
13 800
14 400
11 900
12 500
11 900
10 600
13 400
12 100
14 400
13 700
11 900
8 500
10 400
12 800
13 700
9 900
1.06
1.05
1.03
1.07
1.02
1.11
1.09
1.08
1.04
1.06
1.07
1.05
1.15
1.02
0.61
0.69
0.80
0.74
0.52
0.64
0.73
1
2
3
1
4
0.5
24
200
24
48
1
5
6
11 600
10 900
10 100
10 600
11 200
12 800
12 700
12 500
11 200
7
8
9
L-LA
1
10
11
12
13
14
L-LA
1
1
L-LA
0.5
24
200
24
1
L-LA
1
L-LA
1
L-LA
12 600
1
a
b
[LA]0/[Al] = 100, [Al]/[PhCH2OH] = 1, [LA]0 = 0.42 M, toluene, 70 °C. As determined via integration of the methine resonances (1H NMR) of
c
d
LA and PLA (CDCl3, 400 MHz). Calculated by ([LA]0/[Al]) × 144.13 × conversion + 108.14. Determined by gel permeation chromatography
e
(GPC) calibrated with polystyrene standards in THF and corrected by a factor of 0.58 for PLA. Pm is the probability of meso linkage between
monomer units and was calculated from the homonuclear decoupled 1H NMR spectra of the obtained poly(rac-LA): [mmm] = Pm2 + (1 − Pm)Pm/2;
[mmr] = [rmm] = (1 − Pm)Pm /2; [rmr] = (1 − Pm)2/2; [mrm] = [(1 − Pm)2 + (1 − Pm)Pm]/2.
effects of bis(pyrrolidene) Schiff base ligands on the polymer-
ization activities. Polymerizations of rac-LA and L-LA using 1−7
in the presence of 1 equiv of benzyl alcohol were carried out at
70 °C in toluene. The molar ratio of monomer to initiator was
fixed at 100:1 ([LA]0/[Al] = 100; [LA]0 = 0.83 M; [Al] = 8.33
mM; Mn(theory) = 14 400). All complexes were effective for
the polymerizations of rac-LA and L-LA, as shown in Table 1.
The molecular weights and PDIs (Mw/Mn) were determined by
gel permeation chromatography (GPC) using the Mark−
Houwink correction factor of 0.58.26 For rac-LA polymer-
izations, the corrected Mn values (entries 1−7, Table 1) are
closed to the theoretical values, and narrow PDIs were
observed, indicative of living characteristic. rac-LA polymer-
ization using 1 proceeded to 77% conversion after 48 h (entry
1, Table 1); this polymerization was found to be slower than
the one using 2 or 3, which reached 98% and 95% conversion,
respectively, in 1 h (entries 2 and 3, Table 1). Complex 4 was
much more active than 2 and 3 and showed high monomer
conversion (99%) in 30 min (entry 4, Table 1). Polymer-
izations using 5 and 7 proceeded to 82% conversion (entries 5
and 7, Table 1) in 24 h, whereas 6 polymerized rac-LA to 86%
conversion after 200 h (entry 6, Table 1). In the cases of L-LA,
all polymerizations also proceeded in a controlled fashion,
which led to PLAs with narrow molecular weight distributions
and molecular weights in good agreement with the Mn values
calculated from the monomer conversions (entries 8−14, Table
1). Similar to rac-LA, complex 4 was the most active (entry 11,
Table 1) and the least active catalyst was complex 6 (entry 13,
Table 1).
Aliquots periodically withdrawn from the polymerization
reactions of rac-LA with 1−7 were used to construct plots of
molecular weight versus percent conversion of monomer (with
C
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
[LA]0/[Al] = 50; [Al] = 8.33 mM; [LA]0 = 0.42 M). A linear
correlation between molecular weight and percent conversion,
in conjunction with a narrow PDI, was observed in all cases
(Figure 2 and Figures S1−S6), indicative of a well-controlled
Figure 3. Plot of PLA Mn (versus polystyrene standards) and
molecular weight distribution versus the molar ratio of monomer to
initiator ([LA]0/[Al]) for the polymerization of rac-LA using 3/
PhCH2OH ([Al] = 8.33 mM, toluene, 70 °C).
Figure 2. Plot of PLA Mn (versus polystyrene standards) and
molecular weight distribution as a function of monomer conversion for
a rac-LA polymerization using 7/PhCH2OH ([LA]0/[Al] = 50,
toluene, 70 °C).
logarithmic plots of the rac-LA conversion (ln[LA]0/[LA]t)
versus time, as shown in Figure 4.
In all cases, first-order kinetics in monomers were observed,
as evidenced from the linear relationship between ln([LA]0/
[LA]t) and time. Compared with the closely related bis-
(pyrrolidene) Schiff base aluminum ethyl/2-propanol catalytic
system,20a the rac-lactide polymerization using the aluminum
methyl/benzyl alcohol initiating system showed no induction
period, indicating that the active species form instantaneously
by an in situ alcoholysis reaction utilizing benzyl alcohol. Thus,
the polymerization proceeds according to the rate law in eq 1.
living polymerization and a single site of reaction.9d,12b In
addition, the observation of relatively narrow and constant PDI
values (ca. 1.1) throughout the course of polymerization
suggests that no significant degree of transesterifications
operates in this initiating system.
Further evidence for the well-controlled character of
polymerization is provided by changing the molar ratio of
monomer to initiator ([LA]0/[Al] and the concentration of
aluminum complex was fixed at 8.33 mM. As indicated in Table
2, complex 3 was able to polymerize rac-LA at a high loading of
monomer to initiator ratio ([LA]0/[Al] = 1600:1). The linear
relationship of Mn to [LA]0/[Al] for 3 is depicted in Figure 3.
The number-average molecular weights Mn were close to the
theoretical values, indicative of a well-controlled polymerization
process. The living characteristic was further highlighted by the
low molecular weight distributions observed. In addition, a high
degree of isotacticity was observed up to high monomer-to-
initiator ratios. The Pm values of the resulting polymers
remained unchanged in the range of Pm = 0.78−0.80 (entries
1−5, Table 2).
−d[LA]/dt = kapp[LA]
(1)
where kapp = kp[Al]x and kp is the propagation rate constant. To
determine the order in aluminum (x), the concentration of 3
was varied from 4.16 to 24.96 mM under otherwise identical
conditions ([LA]0 = 0.42 M). The appropriate semilogarithmic
plots are shown in Figure 5. The order in aluminum was
obtained from the slope of the plot of ln kapp versus ln[Al]. As
can be seen in Figure 6, the plot of ln kapp versus ln[Al] is linear
with the noninteger slope of ca. 1.0 (1.17), suggesting the
proportionally intriguing −d[LA]/dt ∝ [Al]. The propagation
rate constant (kp) can be alternatively obtained from the
gradient of the kapp versus [Al] plot, as depicted in Figure 7.
The linear relationship between kapp and [Al] indicates a first-
order kinetics in aluminum concentration, with a kp value of
Kinetic Studies. To determine the reaction kinetics of the
rac-LA polymerizations with 1−7, all reactions were carried out
under the same conditions at 70 °C in toluene ([LA]0/[Al] =
50; [Al] = 8.33 mM; [LA]0 = 0.42 M). The progress of
polymerizations was monitored by 1H NMR spectroscopy. The
apparent rate constants (kapp) were obtained from semi-
(56
2) × 10−6 s−1 mol−1 L at 70 °C. Therefore, the
polymerization of rac-LA initiated by 3 follows the overall rate
equation of the form −d[LA]/dt = kp[LA][3].
a
Table 2. Polymerization of rac-Lactide Using Complex 3 in the Presence of Benzyl Alcohol
b
c
d
d
e
entry
[LA]0/[Al]
time (h)
conv (%)
Mn(theory) (g mol−1
)
Mn(GPC) (g mol−1
)
Mw/Mn
Pm
1
2
3
4
5
100
200
1
6
95
93
96
93
95
13 800
26 900
12 800
26 100
1.03
1.02
1.18
1.11
1.17
0.80
0.78
0.80
0.78
0.78
400
12
48
60
55 500
59 300
800
107 300
219 200
106 400
233 600
1600
a
b
General polymerization conditions: [Al] = 8.33 mM, toluene, 70 °C. As determined via integration of the methine resonances (1H NMR) of LA
c
d
and PLA (CDCl3, 400 MHz). Calculated by ([LA]0/[Al]) × 144.13 × conversion + 108.14. Determined by gel permeation chromatography
e
(GPC) calibrated with polystyrene standards in THF and corrected by a factor of 0.58 for PLA. Pm is the probability of meso linkage between
monomer units and was calculated from the homonuclear decoupled 1H NMR spectra of the obtained poly(rac-LA): [mmm] = Pm2 + (1 − Pm)Pm/2;
[mmr] = [rmm] = (1 − Pm)Pm /2; [rmr] = (1 − Pm)2/2; [mrm] = [(1 − Pm)2 + (1 − Pm)Pm]/2.
D
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
Figure 5. Semilogarithmic plots of the rac-lactide conversion versus
time in toluene at 70 °C with complex 3/PhCH2OH as an initiator
([LA]0 = 0.42 M: I, [Al] = 24.96 mM, [LA]0/[Al] = 17; II, [Al] =
18.72 mM, [LA]0/[Al] = 22; III, [Al] = 12.50 mM, [LA]0/[Al] = 34;
IV, [Al] = 10.41 mM, [LA]0/[Al] = 40; V, [Al] = 8.33 mM, [LA]0/
[Al] = 50; VI, [Al] = 6.24 mM, [LA]0/[Al] = 67; VII, [Al] = 4.16 mM,
[LA]0/[Al] = 101).
Figure 6. Plot of ln kapp versus ln[Al] for the polymerization of rac-
lactide with complex 3/PhCH2OH as an initiator (toluene, 70 °C,
[LA]0 = 0.42 M).
Figure 4. Semilogarithmic plots of rac-lactide conversion versus time
■
●
in toluene at 70 °C with (a) complexes 1 ( ) and 7 ( ), (b)
⧫
⧫
●
▲
▼
complexes 2 ( ), 3 ( ), 4 ( ), and 5 ( ), and (c) complex 6 ( )
([LA]0/[Al] = 50, [Al]/[PhCH2OH] = 1, [LA]0 = 0.42 M, [Al] = 8.33
mM).
The first-order rate constants (kapp) for the polymerizations
of rac-LA and L-LA with 1−7 are listed in Table 3 (see also
Figure S7). For the rac-LA polymerization, changing the
diimine backbone from ethylene to propylene resulted in an
increase in kapp from (36 10) × 10−6 to (787 32) × 10−6
s−1. It was proposed that the C2 backbone could provide more
steric hindrance during the coordination and/or ring-opening
of the lactide, leading to the lower rate.27 The polymerization
initiated by 3 with the gem-methyl groups on the propylene
backbone exhibited a kapp of (368 11) × 10−6 s−1, which was
approximately 2 times slower than the rate of PLA formation
from 2, which was in contrast with the results reported by
Figure 7. Plot of kapp versus [Al] for the polymerization of rac-lactide
with complex 3/PhCH2OH as an initiator (toluene, 70 °C, [LA]0 =
0.42 M).
Gibson,13e Nomura,28 and Chen13i,29 for the Salen aluminum
catalysts and the results obtained by Chen30 for the enolic
E
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
some extent during the polymerization of rac-LA. In the case of
a racemic complexes 4 and 7, the polymerization rate of L-LA
was faster than that of rac-LA, suggesting catalyst inhibition by
the misinsertion behavior during the course of rac-LA
polymerization.31 The slower polymerization rates of rac-LA
compared to L-LA using racemic catalyst were also reported in
other systems.12e,31,32
Table 3. Kinetic Results for the Polymerizations of rac-LA
and L-LA Using Complexes 1−7 in the Presence of Benzyl
a
Alcohol
entry
initiator
kapp (106 s−1) rac-LA
kapp (106 s−1) L-LA
37
1
2
3
4
5
6
7
1
2
3
4
5
6
7
36 10
2
788
3
1309 23
510 27
3422 96
361 12
368 11
Stereochemistry of rac-Lactide Polymerization. The
2843 113
stereochemical microstructures of the PLA samples produced
1
303
3
4
by 1−7 were determined by analyzing the resultant H NMR
0.01
4
4
0.1
signal intensities of the methine region using Bernoullian
statistics.33 All homonuclear decoupled 1H NMR spectra
produced by complexes 1−7 are shown in Figure 8 and
105
211 10
a
General polymerization conditions: toluene as a solvent, 70 °C,
[LA]0/[Al] = 50, [Al]/[PhCH2OH] = 1, [LA]0 = 0.42 M, [Al] = 8.33
mM.
Schiff base aluminum complexes: the complex with a 2,2-
dimethyl-1,3-propylene backbone was more active than that
with a propylene backbone. Changing the backbone to
benzylene (4) gave rise to substantial enhancement of the
polymerization rate (kapp = (2843 113) × 10−6 s−1), which
was ca. 8 times faster than the kapp value of 3. This observation
was in contrast with the closely related Salen aluminum system
in which catalyst containing a benzylene backbone was less
active than that containing a 2,2-dimethyl-1,3-propylene
backbone.13e,i Lengthening the backbone to a butylene linker
(5) led to a kapp ((303 4) × 10−6 s−1) comparable to the one
observed for 3. Comparing 6 with 1 showed a 10-fold rate
decrease when changing the C2 linker from ethylene to
phenylene. However, the replacement of the C2 ethylene linker
with a rac-1,2-cyclohexyl bridge (7) resulted in an increase in
kapp from (36
10) × 10−6 to (105
4) × 10−6 s−1. In
comparison with the series of unsubstituted phenoxy Salen
aluminum complexes under the same conditions ([LA]0/[Al] =
50 and [Al] = 8.3 mM), the kapp value of the most active
complex 4 with the 1,2-benzylene unit was ca. 3 times higher
than that of the most active Salen aluminum complex with 2,2-
dimethyl-1,3-propylene linker (kapp = 1073 × 10−6 s−1).13e Such
a marked influence of the subtle change of backbone linker on
the polymerization rate has also been observed in other
systems.13e,i,27 In the series of complexes, the rates of
polymerization decrease in the order 1,2-benzylene ≫1,3-
propylene > 2,2-dimethyl-1,3-propylene > 1,4-butylene > rac-
1,2-cyclohexylene > 1,2-ethylene ≫ 1,2-phenylene. The results
revealed that the balance of backbone flexibility has a
remarkable influence on the rate of rac-LA polymerization.
In order to obtain the polymerization rate without the
stereocontrol factor involved, the polymerizations of L-LA
polymerizations were carried out. As shown in Table 3, the
same order of polymerization rates as for rac-LA polymer-
izations was observed. If the polymerization proceeds via a
chain-end control mechanism and without stereoselectivity
control, the rate of L-LA and rac-LA polymerizations should be
identical due to the rates of addition of L- and D-monomers to
the growing polymer chain ends are not affected by the
configuration of the last enchained unit.13i It was found that
there were no significant difference in the rates of L-LA
polymerizations initiated by C2 complexes (1 and 6) and C4
complex (5) compared to rac-LA polymerizations. In the cases
of C3 complexes (2 and 3), the polymerizations of L-LA were
much faster than those of rac-LA. The difference in the rates
between L-LA and rac-LA polymerizations suggested that the
more flexible C3 complexes show stereoselectivity control to
1
Figure 8. Homonuclear decoupled H NMR spectra of the methine
region of PLA prepared from rac-lactide at 70 °C in toluene (500
MHz, CDCl3) with (a) 3/PhCH2OH and (b) 7/ PhCH2OH.
Figures S8−S12. According to the data shown in Table 1,
complexes 1−4, 6, and 7 exhibited isotactic selectivity (the Pm
values are in the range of 0.61−0.80). PLA produced by 1 was
moderately isotactic with a Pm value of 0.61 while complex 2
resulted in a slightly higher isoselectivity (Pm = 0.69).
Complexes 3 and 4 showed high isotactic selectivity: Pm
0.80 for 3 and Pm = 0.74 for 4. The Tm value of 153 °C was
obtained for the highly isotactic PLA produced by 3. The
=
F
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
Scheme 2. Ring-Opening Insertion Mechanism for the First (S,S)-LA Monomer by 3 (H Atoms on the Ligand Are Omitted for
Clarity: Pink = Al; Red = O; Blue = N; Gray = C; White = H)
isoselectivity difference between 3 and 4 can be attributed to
the flexibility of the backbone, in which 2,2-dimethyl-1,3-
propylene backbone is more flexible than the 1,2-benzylene
linker.13h Furthermore, when the butylene backbone sub-
stituted complex 5 was examined, almost no stereoselectivity
was observed for the rac-LA polymerization (Pm = 0.52). In the
cases of other C2 linker complexes (6 and 7), the isotactic-
biased polymers were produced (Pm = 0.64 and 0.73,
respectively).
It can be seen that the C3 backbone complexes (2 and 3)
afforded a higher isoselectivity than the C2 ethylene derivative
(1). A similar trend was observed in the unsubstituted phenoxy
Salen aluminum catalysts system studied by Gibson,13e
Nomura,13d and Chen.13i Nomura and co-workers demon-
strated that the more stereoselectivity control in C3 complexes
was attributed to the role of inversion of ligand; i.e., the chiral
geometry of the complex can be inverted by the introduction of
the opposite chiral sense of the polymer terminus during the
propagation reactions if the chiral environment of the complex
is flexible, resulting in the enhance of the efficiency of the
stereodifferentiation of D-LA and L-LA.13d Hence, the higher
degree of stereoselectivity control was accordingly observed in
the more flexible C3 complexes.
groups on the pyrrole rings resulted in a decreased isotactic
enchainment (Pm = 0.60).20a The modification of the backbone
and the phenoxy substituent group of the Salen ligands shows a
dramatic effect on the stereoselectivity of the aluminum Salen
complexes.11−13 The highly isotactic PLA with the Pm value of
0.97 was produced by the aluminum complex with 2,2-
dimethyl-1,3-propylene linkage and the ortho tBuMe2Si
phenoxy substituents.13d The dichloroaluminum−Salen com-
plex with the biphenyl linker, the only derivative in the Salen−
aluminum family, was found to produce heterotactic-biased
PLA (Pr = 0.63).13e In the cases of Salan− and Salalen−
aluminum catalysts, the reverse stereoselectivity from iso-
selectivity to hetereoselectivity can be controlled by subtle
change of ligand substituents.14−19 However, the more
pronounced influence from ligand substituents on the level of
stereocontrol was observed in the Salan−aluminum complexes.
The homonuclear decoupled 1H NMR analysis of the
stereosequence distribution of PLA produced by the most
selective achiral catalyst 3 shows a large mmm tetrad peak,
indicating the formation of an isotactic biased PLA with a Pm
value of 0.80 (Figure 8a). The highly isoselective ROP of rac-
LA by 3 was proposed to proceed via a chain-end control
mechanism (CEM) due to achiral nature of the catalyst. Figure
1
In comparison with the closely related tetradentate Salen−,
Salan−, and Salalen−aluminum catalyst systems, the bis-
(pyrrolidene) Schiff base aluminum complexes produced only
isotactic-biased PLAs, and the level of isoselectivity control was
moderately high with the highest Pm value of 0.80 (from 3).
Chen and Feijen reported that the introduction of the methyl
8b shows the homonuclear decoupled H NMR spectrum of
PLA prepared by the racemic aluminum complex 7. The
isotactic PLA with the Pm value of 0.73 was obtained. It is
worthy to note that the isoselectivity control exerts by 7 via a
site-control mechanism may include the chain-end control after
the misinsertion of the unfavorable enantiomer as previously
G
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
reported by Chisholm13j and recently by Kol19 and
Lamberti.19,32
set as a reference point of the overall reaction. At the transition
state, the activated carbonyl carbon of LA is attacked by the
methoxy species with the O(1)···C(1) separation of 2.08 Å, and
the bond between the O(2) of LA and the aluminum center is
simultaneously formed (see Scheme 2 and Table S4). In this
configuration, the aluminum center adopts a distorted
octahedral geometry in which the O(2) of LA is located in
the cis-equatorial position to the methoxy O(1) donor atom
with an O(2)···Al separation of 2.06 Å and an Al···O(1)
distance of 1.84 Å. With respect to the separated lactide
monomer and initiator, RA1, the formation of the first
intermediate (INT1) has the activation free energy of 4.4
kcal mol−1, and this process is exergonic by 2.4 kcal mol−1 (see
Figure 9). Before the ring-opening step, the INT1 is
reorganized by the rotation of the lactide ring about the
O(2)−C(1) bond to form the second intermediate (INT2).
This results in the close interaction between an acyl oxygen
atom of LA and the aluminum center. The second step of the
coordination insertion mechanism corresponds to the ring-
opening of the inserted lactide to create the new alkoxy
initiating group, also denoted as a growing polymer chain,
bound to the aluminum center. This step proceeds through the
second transition state (TS2) at which the lactide acyl bond
(C(1)−O(3)) is elongated (2.08 Å) with respect to a free LA
(1.35 Å) in concomitance with the formation of a new
aluminum−alkoxide bond (Al−O(3) = 1.89 Å) (see Scheme 2
and Table S5). After the reorganization of the growing polymer
chain, a five-membered metallocyclic product, INT3, is afforded
with a weak interaction between Al and Ocarbonyl (2.11 Å). The
formation of the five-membered chelate was also observed for
aluminum−Salalen initiators for the ROP of rac-LA.18b With
respect to the most stable intermediate (INT1), the calculated
activation free energy for the ring-opening is 9.1 kcal mol−1.
Although the deinsertion of INT1 to RA1 is more likely than
the ring-opening to INT3, the latter process is favored under
thermodynamic control. Overall, the conversion of RA1 into
INT3 is exergonic by 12.0 kcal mol−1.
Computational Investigations. Density functional theory
(DFT) calculations were carried out in order to gain further
insight into the reaction mechanism of this catalytic system.
The underlying influences of the backbone structure on the
energetics and competing reaction pathways reflecting the
catalytic activity and stereoselectivity of the catalysts were
established. All calculations were performed at the M06-2X
level of theory34 with the 6-311G(d,p) basis set.35 This
methodology has been extensively validated for organic and
main-group inorganic systems and has recently been used to
study the polymerization mechanism of ε-caprolactone
catalyzed by Salen−aluminum complexes.21e It was found that
a good agreement in terms of bond length and bond angle
between the X-ray and the optimized structures of 3 is observed
with the standard deviations of the bond lengths and the bond
angles around the aluminum center in the range of 0.06−1.02%
and 0.9−6.18/%, respectively (see also Figure S13 for the
optimized structure of complexes 1−7 with their selected
structure parameters collected in Table S3).
To address the different catalytic activities of complexes 1−7,
the ring-opening insertion of the (S,S)-lactide monomer was
first examined. In order to reduce the computational time, the
benzyloxide initiating group (−OCH2Ph) was replaced by the
methoxide unit (−OCH3). All sequential steps in the ring-
opening insertion of the first lactide monomer proposed for 3
are applied to all initiators in this study, and only the
mechanistic demonstration for 3 is provided for simplification.
The key species proposed in the coordination−insertion
mechanism of the first (S,S)-lactide monomer initiated by 3
is illustrated in Scheme 2, and the calculated free energy profile
is presented in Figure 9 (see Figures S14−S19 for the free
energy profiles of 1, 2, and 4−7).
From a geometric point of view, the key transition states,
TS1 and TS2, involve in the nucleophilic attack and the ring-
opening steps, and hence, their geometries might account for
the rate of polymerization. A close-up on both transition state
geometries using 3 is presented in Figure 10. It can be seen that
the calculated four-membered transition states (TS1 and TS2)
engage in a simultaneous bond cleavage and formation in the
Figure 9. Calculated free energy profile for the ring-opening insertion
of the first (S,S)-LA monomer by 3.
As shown in Scheme 2, the first step of the coordination−
insertion mechanism involves the insertion of the coordinated
LA into the Al−OCH3 bond of the initiator to form the
tetrahedral intermediate (INT1) via a four-membered tran-
sition state, TS1. Unlike the diketiminate metal initiators, where
the lactide monomer strongly coordinates to the positively
charged metal centers,21a,b the direct coordination of this
monomer to the five-coordinate aluminum center is found to
be unstable on the free energy surface. Therefore, the sum of
free energies between isolated complex and free lactide, RA1, is
Figure 10. Optimized geometries of TS1 and TS2 in the ring-opening
insertion of the first (S,S)-LA monomer with 3 (H atoms on the ligand
are omitted for clarity: pink = Al; red = O; blue = N; gray = C; white =
H). θ is the cis-quatorial angle N(1)−Al−N(4).
H
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
equatorial plane of the octahedral geometry. Therefore, the
geometric constraints imposed by the cis-equatorial
θN(1)−Al−N(4) angle of the ligand framework is expected to
have a strong effect on the stability of the transition states. The
magnitude of cis-equatorial θN(1)−Al−N(4) angle is found the be
strongly correlated with the bite angle, N(3)−Al−N(4), of the
N,N chelate of the backbone; i.e., the increase of the bite angle
leads to a significant reduction of the cis-equatorial angle. Since
89.9° (TS1) and 89.0° (TS2). In comparison between C2 and
C3 backbone complexes, the calculated free energy barriers of
the C2 complexes (1, 6, 7) are higher than that of the C3
complexes (2, 3, 4), suggesting the lower polymerization rates.
It is apparent that lengthening the backbone from C2 to C3
linker results in an increase of the bite angle, which allows the
initiator to better accommodate the key transition states
involved in the ROP process. In the case of C2 complexes, the
highest ΔG‡ and ΔG‡ values were observed for 6, which
are in agreement with the lowest experimental polymerization
rate. For the group of complexes containing C3 linker, the lower
ΔG‡TS1 and ΔG‡TS2 values of 2 compared with 3 are also in line
with the higher observed polymerization rate of 2. In this case,
the greater flexibility of the propylene linkage is responsible for
the enhancement of the polymerization rates. However, the
correlation between the bite angles and the observed
polymerization rate cannot be applied in the cases of initiators
4 and 5. A similar contradiction is also observed for 7, in which
the experimental polymerization rate of 7 is higher than that of
1 although the DFT calculations reveal virtually the same values
of bite angles in both TS1 and TS2. These confounding
observations are also reported in the aluminum complexes
featuring Salen-type ligands with different backbones,13e in
which the polymerization rate cannot be explained solely by the
bite angle effect. Therefore, the contributions of unknown
factors are expected during the propagation reactions.
The origin of stereocontrol for rac-lactide polymerization was
investigated by inspecting the insertion of the second lactide
monomer into the first enchained (S,S)- or (R,R)-LA monomer
unit. For simplicity, the (SS,RR) notation was used to represent
the insertion of (R,R)-LA into the first enchained (S,S)-LA
monomer unit. Similar notation patterns are applied for other
assembly modes of insertion. In each initiator system, we
examined the free energy surfaces of the four sets of different
assembly modes leading to isotactic enchainments (SS,SS and
RR,RR sequences) and heterotactic enchainments (SS,RR and
RR,SS sequences).18b,21a,c,e It should be noted that the (RR,RR)
and (RR,SS) assembly modes of insertion are equivalent to the
(SS,SS) and (SS,RR) enantiomeric analogues in the initiator
systems for 1, 2, 3, 5, and 6 due to achiral nature of the
complexes. Therefore, the (RR,RR) and (RR,SS) assembly
modes of insertion in these initiator systems are redefined as i-
(SS,SS) and i-(SS,RR), corresponding to the interconversion of
ligand configuration. Similar to the first monomer insertion
mechanism (see Scheme S1), the second monomer insertion
the cis-equatorial angle is vertically opposite to the O(1)methoxy
−
TS1
TS2
Al−O(2)carbonyl angle in TS1 and to the O(2)carbonyl−Al−
O(3)acyl angle in TS2, its narrowing results in the reduction of
the steric congestion at the metal center and, thus, weakens the
constraints in the transition state complexes. This causes a
reduction of the free energy barriers of the transition states TS1
and TS2 (ΔG‡
and ΔG‡TS2), leading to higher polymer-
TS1
ization activities.
Table 4 shows the calculated bite angles, the cis-equatorial
angles, and the free energy barriers of TS1 and TS2 using
Table 4. Activation Free Energy, Bite Angle, and cis-
Equatorial Angle Values for the Ring-Opening Insertion of
the First Lactide Monomer Using 1−7
TS2
TS2
a
b
c
d
b
c
complex ΔG‡
bite angle
θ
ΔG‡
bite angle
θ
TS1
TS2
1
2
3
4
5
6
7
5.9
2.1
76.3
83.2
85.6
83.5
89.9
75.9
76.9
107.7
101.5
107.6
104.7
98.3
10.0
76.6
83.1
85.4
83.6
89.0
76.0
76.9
105.8
99.8
7.4
9.1
4.4
4.2
105.5
102.7
96.1
8.5
2.3
6.9
10.0
5.2
112.3
15.6
12.1
109.0
106.7
108.5
a
Activation free energy (kcal mol−1) for the insertion transition state
b
(TS1). Angle between donor atom−metal−donor atom, N(3)−Al−
c
d
N(4). The cis-equatorial angle N(1)−Al−N(4). Activation free
energy (kcal mol−1) for the ring-opening transition state (TS2) with
respect to the most stable intermediate (either INT1 or INT2). Bond
angles are in deg.
initiators 1−7. The results can be clearly divided into three
groups according to the number of carbon atoms in the
backbone (C2, C3, and C4 backbones). In view of the molecular
structure, the C2 complexes have bite angles in the range of
75.9°−76.9°, whereas the C3 complexes hold bite angles in the
range of 83.1°−85.6°. The C4 complex 5 has the bite angles of
Figure 11. Free energy profile for the insertion of the second lactide into the first enchained (S,S)-LA complex, INT3, in different assembly modes
mediated by 3 (a) before and (b) after the conformational inversion of the ligand.
I
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
begins with the insertion of LA into the terminal Al−OR bond
of INT3. This step proceeds through the transition state TS1′
to afford the intermediates INT1′ and INT2′. Subsequently,
the ring-opening occurs through TS2′ in which the intracyclic
C−Oacyl and the Ocarbonyl−Al bonds are broken to give the new
alkoxide species which reorganizes to form the intermediate
INT3′.
to take place. These results suggest that the initiator prefers to
continuously polymerize the (S,S)-LA monomer after the first
(S,S)-LA enchainment, leading to the experimentally observed
isotactic PLA. A closer examination of the insertion transition
state structures (TS1′) of these two assembly modes (Scheme
3) reveals that the relatively high free energy of activation in the
slow step of the (SS,RR) insertion corresponds to the repulsive
interactions between the stereogenic center of the inserting
monomer and the α-methyl group of the first enchained unit
while similar steric repulsions are absent in the fast step of the
(SS,SS) insertion.
From an analysis of the free energy profiles (Figure 11),
before and after the conformational inversion of the ligand, it
can be concluded that the combined effects of the last-
enchained monomer unit and ligand conformation are two
important key factors for the mechanistic interpretation of the
isoselective ring-opening polymerization of rac-LA by 3.
Because of the achiral property of the initiator, it should be
expected that the first LA enchainment takes place without
preferential selection of one enantiomer over another.
However, the ligand conformation is induced by the chirality
of the first enchained monomer to produce the chiral
environment around the aluminum center. This results in the
enhancement of the enantiomeric differentiation ability of the
initiator. As can be seen in Figure 11 and Table 5 (initiator 3),
However, the calculations elucidate a totally opposite
scenario when the ligand conformation is in its inverted form
(Scheme 3 and Figure 11). In this case, the enchainment in the
i-(SS,RR) sequence (ΔG‡
= 10.2 kcal mol−1 and ΔG‡
=
TS1′
TS2′
8.8 kcal mol−1) is apparently more favorable than that of the i-
(SS,SS) sequence (ΔG‡ = 13.3 kcal mol−1 and ΔG‡
=
TS2′
TS1′
16.3 kcal mol−1). It should be noted that the inverted form of
the ligand favors the enchainment in the (SS,RR) sequence, and
thus, it would also favor the (RR,RR) enchainment. However,
this conformation of the ligand disfavors the enchainment in
the (SS,SS) assembly mode. A closer inspection of the insertion
transition state (TS1′) of the i-(SS,SS) sequence (Scheme 3a)
reveals that the higher ΔG‡TS1′ of the insertion in the i-(SS,SS)
mode relative to that in the (SS,SS) mode by 5.3 kcal mol−1
results from the repulsive interactions between the methyl
group of the inserting monomer and the ligand framework. In
the case of the i-(SS,RR) insertion (Scheme 3b), it is found that
the α-methyl group of the first enchained unit is able to
position itself away from the stereogenic center of the inserting
monomer. This reorganization of the complex results in the
minimization of the repulsive interactions during the insertion
process. As a result, the free energy of activation in this step
(ΔG‡TS1′) is substantially reduced from that of the (SS,RR)
sequence by 5.3 kcal mol−1.
Table 5. Calculated Activation Free Energies (kcal mol−1)
for the Insertion (TS1′) and the Ring-Opening (TS2′) Steps
of rac-LA in Different Assembly Modes Using 1−7
initiator
assembly mode
ΔG‡
ΔG‡
TS1′
TS2′
1
SS,SS
9.0
14.5
SS,RR
i-SS,RR
i-SS,SS
SS,SS
17.5
12.4
13.9
9.9
11.5
19.8
15.1
11.6
9.9
2
3
4
5
6
7
SS,RR
i-SS,RR
i-SS,SS
SS,SS
16.7
11.7
13.6
8.0
7.3
13.3
13.4
11.0
8.8
In the ring-opening step, the stability of the transition state
(TS2′) is determined by two crucial interaction factors as
shown in Figure S21. The TS2′ of the i-(SS,RR) sequence is the
most stable configuration while that of the i-(SS,SS) is the least
stable one. The repulsive interactions between the α-methyl
group of the first enchained unit and the stereogenic carbon
center of the ring-opening monomer and between the methyl
group of the ring-opening monomer and ligand framework are
absent in the i-(SS,RR) mode while these two repulsive
interactions are present in the i-(SS,SS) sequence. Nevertheless,
in this study, the stability of the INT1 and INT2 is also taken
into account for determining the rate of the ring-opening step.
Even though the i-(SS,RR) insertion is the most kinetically
favorable pathway, this mode of insertion seems less compelling
in the overall reaction. The less stable INT3′ of the i-(SS,SS)
mode of enchainment (Figure 11) by 5.2 kcal mol−1 relative to
that of the (SS,SS) mode indicates that the ligand inversion,
probably through the reorganization of the ring-opened
intermediate to form INT3′, is unlikely to take place during
the propagation in the (SS,SS) sequence. Instead, the
conformational inversion of the ligand is more likely to occur
after the misinsertion of the opposite enantiomeric monomer,
(R,R)-LA, into the growing (S,S)-polymer chain during the
propagation reactions. In this process, the last-inserted (R,R)-
LA would induce the formation of the inverted ligand via
mechanistic crossing from INT2′ of the (SS,RR) insertion
mode to TS2′ of the i-(SS,RR) insertion mode, which has lower
free energy of activation. Therefore, the initiator would
SS,RR
i-SS,RR
i-SS,SS
SS,SS
15.5
10.2
13.3
9.5
16.3
10.5
14.9
16.1
15.0
11.1
15.2
9.9
SS,RR
RR,SS
RR,RR
SS,SS
14.6
11.8
12.3
9.0
SS,RR
i-SS,RR
i-SS,SS
SS,SS
15.5
10.7
12.3
13.0
18.9
15.0
13.5
10.4
17.1
11.0
12.0
18.8
21.1
21.7
23.9
23.6
10.8
11.6
12.4
16.5
SS,RR
i-SS,RR
i-SS,SS
SS,SS
SS,RR
RR,SS
RR,RR
the insertion step in the (SS,SS) mode (ΔG‡
= 8.0 kcal
TS1′
mol−1) is more kinetically favorable than that in the (SS,RR)
mode (ΔG‡TS1′ = 15.5 kcal mol−1). However, the ring-opening
step of the former sequence (ΔG‡
= 13.4 kcal mol−1) is
TS2′
slightly less favored than that of the latter one (ΔG‡TS2′ = 11.0
kcal mol−1). Taking both ΔG‡ and ΔG‡
into account,
TS1′
TS2′
the enchainment in the (SS,SS) route is more statistically likely
J
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
Scheme 3. Insertion Transition States (TS1′) in the Second Lactide Insertion by 3 and Their Corresponding Activation Free
Energies before and after the Conformational Inversion of the Ligand: (a) for the (SS,SS) and (b) for the (SS,RR) Modes of
a
Insertion
a
Double-headed arrows represent the repulsive interactions between two atomic centers; H atoms of the ligand are omitted for clarity: pink = Al; red
= O; blue = N; gray = C; white = H.
preferentially polymerize the (R,R)-LA from this point onward
or until the misinsertion and ligand inversion take place once
again. A similar mechanistic interpretation has also been
proposed by Nomura and co-workers for the stereoselective
ROP of rac-LA initiated by achiral Salen−aluminum complex-
es.13d Because the subsequent insertion of a new monomer is
controlled by the configuration of the last enchained monomer
unit, the experimentally observed stereoselectivity of rac-LA
polymerization mediated by 3 can be rationalized by assuming a
chain end control mechanism.4b,13d Selected bond lengths and
angles of the insertion (TS1′) and ring-opening (TS2′)
transition state structures initiated by 3 are provided in Tables
S7 and S11.
activation free energies of the four assembly modes of insertion
(complexes 4 and 7 in Table 5 and Figures S24 and S27), it was
found that the (SS,SS) mode is the most favorable insertion
pathway in both initiator systems with ΔG‡
and ΔG‡TS2′ of
TS1′
9.5 and 10.5 kcal mol−1, respectively, for 4 and of 10.4 and 10.8
kcal mol−1, respectively, for 7. These computational results are
in line with the experimental data in which the isotactic-biased
PLAs were produced by initiators 4 and 7 (Pm = 0.74 for 4 and
Pm = 0.73 for 7). The next most favorable insertion pathway
was found to be the (RR,RR) mode for 4 and the (RR,SS)
mode for 7. These results suggest that after a single
misinsertion of the opposite enantiomer (the (SS,RR) mode)
the initiator 4 would prefer to polymerize (R,R)-LA from this
point onward or until a new enantiomeric misinsertion occurs
while the catalyst 7 would prefer to retain the (SS,SS) mode of
insertion. It is worth mentioning that these calculation results
did not provide the information on the polymeryl exchange
process12b,13j,32 that occurred during the course of polymer-
ization which is also responsible for stereoselectivity enchain-
ment of rac-LA by the chiral initiators.
By taking both ΔG‡
and ΔG‡
(see Table 5) into
TS1′
TS2′
account, the (SS,SS) assembly mode of enchainment is found
to be the most preferential pathway for all other achiral
aluminum initiators (1, 2, 5, and 6), suggesting the formation of
isotactic-biased PLAs. A decreased level of isoselectivity in these
initiator systems might be attributed to the degree of chiral
environment created around the aluminum center of these
initiators as well as the frequent occurrence of the ligand
inversion during the propagation reactions. The extent of the
latter factor is related to the comparable stabilities of INT3′ in
the (SS,SS) and i-(SS,SS) modes of enchainment (Figures S22,
S23, S25, and S26).
On the other hand, the stereoselectivity of the ring-opening
polymerization of initiators 4 and 7 could be explained by the
enantiomorphic-site control mechanism3a,12a,b,36 because the
inversion of ligand configuration in these initiators does not
occur during the polymerization process. By considering the
CONCLUSION
■
In summary, bis(pyrrolidene) Schiff base aluminum complexes,
1−7, with a series of backbone linkers have been synthesized
and characterized on the basis of NMR spectroscopy, elemental
analysis, and X-ray crystallography for 3. All complexes are
effective initiators for the polymerization of rac-lactide; the
polymerization proceeds efficiently in a living manner. Studies
of the polymerization kinetics in the presence of 1−7
consistently reveal a first-order behavior in monomers. The
K
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
experiments were performed on a Bruker Advance 500 MHz
spectrometer. The 1H and 13C NMR spectra were referenced
internally to the residual proton impurity peaks according to the
literature.37 Elemental analysis data (C, H, N) were obtained from a
LECO elemental analyzer CS-932H. Differential scanning calorimetry
(DSC) analysis was performed under N2 atmosphere with a
NETZSCH DSC 204 F1 Phoenix instrument. The sample was first
heated to 200 °C at a rate of 10 °C min−1 and cooled to 20 °C for two
cycles. Then, the sample was cooled to −100 °C, followed by heating
to 200 °C at a heating rate of 5 °C min−1 to determine the thermal
properties. Gel permeation chromatography (GPC) measurements
were conducted on a Polymer Laboratories PL-GPC-220 instrument
equipped with PLgel 5 μm MIXED-D 300 × 7.5 mm column, and
tetrahydrofuran (THF) was used as the eluent (flow rate: 1 mL min−1
at 40 °C). The number-average molecular weights (Mn) and
polydispersity indice (PDIs) were calibrated against polystyrene
(PS) standards. The Mn values of PLAs were corrected with a
Mark−Houwink factor of 0.58 to account for the difference in
hydrodynamic volume of polystyrene and polylactide.
flexibility of backbone structure is demonstrated to exert a
significant influence on the polymerization rate and the
polymer microstructure. In the series of complexes, the rates
of polymerization decrease in the order 1,2-benzylene (4), 1,3-
propylene (2) > 2,2-dimethyl-1,3-propylene (3) > 1,4-butylene
(5) > rac-1,2-cyclohexylene (7) > 1,2-ethylene (1) ≫ 1,2-
phenylene (6). These results revealed that rate of polymer-
ization generally increases with increasing the flexibility of the
backbone structure, although the steric shielding of the
aluminum center by the long butylene linker may have an
additional role in complex 5. Complexes 1−3 and 6 afford the
isotactic-biased stereoblock polylactide via a chain-end control
mechanism with the highest Pm value of 0.80 observed for 3. In
the case of the chiral aluminum complexes 4 and 7, the
isotactic-biased stereoblock polylactides were proposed to form
by a combination of enantiomorphic-site and chain-end control
mechanism.
DFT studies show that the activation free energies (ΔG‡
General Protocol for the Synthesis of H2L1 to H2L7. The
reaction was performed according to the procedure previously
reported.22 To a stirred solution of pyrrole-2-carboxaldehyde (3.00
g, 31.6 mmol) in ethanol (50 mL) was slowly added the corresponding
diamine (15.8 mmol). The reaction mixture was stirred, and a catalytic
amount of formic acid was added. The reaction mixture was stirred at
room temperature for 4 h and during which time a white precipitate
formed. The white solid was collected by filtration and dried under
vacuum.
TS1
and ΔG‡TS2) for the monomer insertion are inversely related to
the complex bite angles of the key transition states (TS1 and
TS2). Complexes bearing C3 and C4 backbone linkers (2−5)
exhibit lower activation free energies than those containing C2
backbone linkers (1, 6, and 7). The correlations between
activation free energies and bite angles of complexes indicate
that the structure of the linker units has a significant influence
on the polymerization activity. In general, the predicted
catalytic activities of most complexes (1−3 and 6) are in
good agreement with the experimentally observed polymer-
ization rate constants. The inconsistency of the predicted
results of complexes 4, 5, and 7 with the experimental
observations suggests that unknown factors not contemplated
by these calculations may contribute. The studies of the second
monomer insertion in different assembly modes demonstrate
that the steric interactions between the last enchained unit of
the monomer and a stereogenic center of the incoming
monomer together with the conformational interconversion of
ligand are the key factors determining the stereoselectivity of
insertion. We believe that these findings are important for the
development of future catalyst systems through a rational
design approach.
N,N′-Bis((1H-pyrrol-2-yl)methylene)ethane-1,2-diamine (H2L1).
1
Yield: 3.01 g, 89%. H NMR data (399.87 MHz, [D6]DMSO, 300
K): δ 11.42 (br s, 2H, NH), 8.11 (s, 2H, NCH), 6.90−6.87 (m, 2H,
4
3
pyrrole-H), 6.45 (dd, JHH3 = 1.4 Hz, JHH = 3.5 Hz, 2H, Pyrrole-H),
3
6.12 (dd, JHH = 2.6 Hz, JHH = 3.5 Hz, 2H, Pyrrole-H), 3.76 (s, 4H,
NCH2). 13C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 152.6
(N = CH), 130.0 (Pyrrole-C), 121.9 (Pyrrole-CH), 113.4 (Pyrrole-
CH), 108.8 (Pyrrole-CH), 61.4 (NCH2). Elemental analysis for
C12H14N4: C, 67.27; H, 6.59; N, 26.15. Found: C, 67.07; H, 6.68; N,
26.47.
N,N′-bis((1H-pyrrol-2-yl)methylene)propane-1,3-diamine (H2L2).
1
Yield: 3.32 g; 92%. H NMR data (399.87 MHz, [D6]DMSO, 300
K): δ 11.38 (br s, 2H, NH), 8.11 (s, 2H, N = CH), 6.91−6.88 (m, 2H,
4
3
Pyrrole-H), 6.46 (dd, JHH = 1.5 Hz, JHH = 3.5 Hz, 2H, Pyrrole-H),
6.14 (dd, 3JHH = 2.6 Hz, 3JHH = 3.5 Hz, 2H, Pyrrole-H), 3.56 (dt, 4JHH
3
3
= 0.8, JHH = 6.5 Hz, 4H, NCH2), 1.93 (q, JHH = 6.9 Hz, 2H, CH2).
13C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 151.8 (NCH),
130.0 (pyrrole-C), 121.9 (pyrrole-CH), 113.2 (pyrrole-CH), 108.7
(pyrrole-CH), 58.1 (NCH2), 32.5 (CH2). Elemental analysis for
C13H16N4: C, 68.39; H, 7.06; N, 24.54. Found: C, 68.25; H, 7.32; N,
24.77.
EXPERIMENTAL SECTION
■
Materials and Methods. All manipulations with air- and/or
water-sensitive compounds were carried out under a dry nitrogen
atmosphere using standard Schlenk and cannula techniques in oven-
dried glassware or in a glovebox. Toluene was distilled from sodium
benzophenone before use. Hexane and pentane were distilled from
CaH2 prior to use. Benzyl alcohol was refluxed over sodium and then
freshly distilled onto activated 4 Å molecular sieves. All solvents were
degassed prior to use unless stated otherwise. NMR solvents were
dried over 4 Å molecular sieves and degassed prior to use. A 2.0 M
solution of trimethylaluminum in toluene (Aldrich) was used without
purification. 1,2-Ethylenediamine (99%), 1,3-propanediamine (99%),
2,2-dimethyl-1,3-propanediamine (99%), 2-aminobenzylamine (98%),
1,4-butanediamine (99%), trans-1,2-diaminocyclohexane (99%), 1,2-
phenylenediamine (99.5%), pyrrole-2-carboxaldehyde (98%),and
trimethylaluminum (2.0 M solution in toluene) were purchased
from Aldrich and used as received. The monomers rac-lactide
(Aldrich) and L-lactide (Purac) were recrystallized from dry toluene
and then sublimed three times prior to use. All other chemicals are
commercially available and were used as received unless otherwise
stated.
N,N′-Bis((1H-pyrrol-2-yl)methylene)-2,2-dimethylpropane-1,3-di-
amine (H2L3). Yield: 1.17 g, 57%. 1H NMR data (399.87 MHz,
[D6]DMSO, 300 K): δ 11.25 (br s, 2H, NH), 8.08 (s, 2H, NCH),
6.91−6.89 (m, 2H, pyrrole-H), 6.46 (dd, 4JHH = 1.5 Hz, 3JHH = 3.5 Hz,
3
3
2H, pyrrole-H), 6.14 (dd, JHH = 2.6 Hz, JHH = 3.5 Hz, 2H, pyrrole-
H), 3.38 (d, 4JHH = 1.0, 4H, NCH2), 0.97 (s, 6H, C(CH3)2). 13C NMR
data (100.55 MHz, [D6]DMSO, 300 K): δ 151.8 (NCH), 130.1
(pyrrole-C), 121.7 (pyrrole-CH), 112.9 (pyrrole-CH), 108.7 (pyrrole-
CH), 69.4 (NCH2), 36.7 (C(CH3)2), 24.3(C(CH3)2). Elemental
analysis for C15H20N4: C, 70.28; H, 7.86; N, 21.86. Found: C, 70.20;
H, 7.95; N, 22.18.
N,N′-Bis((1H-pyrrol-2-yl)methylene)-2-aminobenzylamine (H2L4).
Yield: 2.49 g, 83%. 1H NMR data (399.87 MHz, [D6]DMSO, 300 K):
δ 11.69 (br S, 1H, NH), 11.41 (br S, 1H, NH), 8.25 (s, 1H, NCH),
8.23 (s, 1H, NCH), 7.33−7.36 (m, 1H Ar-H), 7.25−7.31 (m, 1H
Ar-H), 7.13−7.19 (m, 1H Ar-H), 7.07−7.09 (m, 1H, pyrrole-H),
7.01−7.05 (m, 1H, Ar-H), 6.84−6.88 (m, 1H, pyrrole-H), 6.69−6.74
(m, 1H, pyrrole-H), 6.36−6.40 (m, 1H, pyrrole-H), 6.21−6.25 (m,
1H, pyrrole-H), 6.07−6.12 (m, 1H, pyrrole-H), 4.88 (s, 2H, ArCH2).
13C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 152.9 (NCH),
150.3 (ArC), 150.1 (NCH), 133.5 (ArCCH2), 130.9 (pyrrole-C),
1
The H (399.87 MHz) and 13C (100.55 MHz) nuclear magnetic
resonance (NMR) spectra were recorded on a Varian Unity Inova 400
1
MHz spectrometer at 300 K. The homonuclear decoupled H NMR
L
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
130.2 (pyrrole-C), 128.9 (ArCH), 127.9 (ArCH), 124.9 (ArCH), 123.8
(pyrrole-CH), 122.1 (pyrrole-CH), 117.8 (ArCH), 116.1 (pyrrole-
CH), 113.7 (pyrrole-CH), 10.9.8 (pyrrole-CH), 109.0 (pyrrole-CH),
59.5 (ArCH2). Elemental analysis for C17H16N4: C, 73.89; H, 5.84; N,
20.27. Found: C, 73.92; H, 6.06; N, 20.74.
data (100.55 MHz, [D6]DMSO, 300 K): δ 160.8 (NCH), 135.1
(pyrrole-C), 134.9 (pyrrole-CH), 116.6 (pyrrole-CH), 113.8 (pyrrole-
CH), 66.8 (NCH2), 35.8 (C(CH3)2), 25.9 (C(CH3)2), 22.0 (C-
(CH3)2). Elemental analysis for C16H21N4Al: C, 64.35; H, 7.14; N,
18.91. Found: C, 64.26; H, 7.35; N, 18.72.
N,N′-Bis((1H-pyrrol-2-yl)methylene)butane-1,4-diamine (H2L5).
1
L4AlMe (4). Yield: 0.67 g, 59% (pale yellow solid). H NMR data
1
Yield: 1.89 g, 99%. H NMR data (399.87 MHz, [D6]DMSO, 300
(399.87 MHz, [D6]DMSO, 300 K): δ 8.57 (s, 1H, NCH), 8.49 (s,
1H, NCH), 7.42 (s, 1H pyrrole-H), 7.41 (s, 1H pyrrole-H), 7.32−
7.38 (m, 2H, Ar-H), 7.31−7.33 (m, 1H, Ar-H), 7.23−7.30 (m, 1H Ar-
H), 6.93−6.94 (m, 2H pyrrole-H), 6.95 (d, 4JHH = 1.5 Hz, 2H, pyrrole-
K): δ 11.37 (br s, 2H, NH), 8.09 (s, 2H, NCH), 6.89−6.86 (m, 2H,
4
3
pyrrole-H), 6.45 (dd, JHH = 1.5 Hz, JHH = 3.5 Hz, 2H, pyrrole-H),
6.12 (dd, 3JHH = 2.6 Hz, 3JHH = 3.5 Hz, 2H, pyrrole-H), 3.56−3.50 (m,
4H, NCH2), 1.68−1.63 (m, 4H, CH2). 13C NMR data (100.55 MHz,
[D6]DMSO, 300 K): δ 152.2 (NCH), 130.8 (pyrrole-C), 122.5
(pyrrole-CH), 113.8 (pyrrole-CH), 109.4 (pyrrole-CH), 60.8 (NCH2),
29.2 (CH2). Elemental analysis for C14H18N4: C, 69.39; H, 7.49; N,
23.12. Found: C, 69.29; H, 7.66; N, 23.41.
3
H), 6.80 (d, JHH = 1.5 Hz, 2H, pyrrole-H), 6.35−6.36 (m, 2H,
2
pyrrole-H), 6.30−6.33 (m, 2H, pyrrole-H), 4.94 (d, JHH = 16.3 Hz,
2
1H, N(ArCHH)N), 4.80 (d, JHH = 16.3 Hz, 1H, N(ArCHH)N),
−1.21 (s, 3H, AlCH3). 13C NMR data (100.55 MHz, [D6]DMSO, 300
K): δ 161.4 (CNH), 153.5 (CNH), 144.2 (ArC), 136.4 (pyrrole-C),
135.8 (ArCH), 134.6 (pyrrole-C), 134.5 (ArCH), 130.2 (ArCCH2),
129.0 (pyrrole-CH), 128.5 (pyrrole-CH), 126.2 (ArCH), 119.8
(ArCH), 119.2 (pyrrole-CH), 117.3 (pyrrole-CH), 114.2 (pyrrole-
CH), 113.8 (pyrrole-CH), 56.1 (ArCH2). Elemental analysis for
C18H17AlN4: C, 68.34; H, 5.42; N, 17.71. Found: C, 67.58; H, 5.55; N,
18.02.
N,N′-Bis((1H-pyrrol-2-yl)methylene)benzene-1,2-diamine (H2L6).
1
Yield: 1.11 g, 26%. H NMR data (399.87 MHz, [D6]DMSO, 300
K): δ 11.62 (br s, 2H, NH), 8.25 (s, 2H, NCH), 7.20−7.17 (m, 2H,
ArH), 7.08−7.05 (m, 2H, ArH), 7.04−7.01 (m, 2H, pyrrole-H), 6.69−
6.67 (m, 2H, pyrrole-H), 6.24−6.21 (m, 2H, pyrrole-H). 13C NMR
data (100.55 MHz, [D6]DMSO, 300 K): δ 152.2 (NCH), 145.2
(ArC), 131.3 (pyrrole-C), 125.7 (ArCH), 124.0 (ArCH), 121.5
(pyrrole-CH), 116.2 (pyrrole-CH), 110.0 (pyrrole-CH). Elemental
analysis for C16H14N4: C, 73.26; H, 5.38; N, 21.36. Found: C, 72.85;
H, 5.70; N, 21.40.
L5AlMe (5). Yield: 0.53 g, 81% (white solid). 1H NMR data (399.87
MHz, [D6]DMSO 300 K): δ 8.32 (s, 2H, NCH), 6.92−6.88 (m,
2H, pyrrole-H), 6.66 (dd, 4JHH = 1.0, 3JHH = 3.3, 2H, pyrrole-H), 6.22
3
3
(dd, JHH = 3.4, JHH = 1.8, 2H, pyrrole-H), 3.80−3.67 (m, 2H,
NCH2), 3.57−3.46 (NCH2), 2.05−1.92 (m, 2H, CH2), 1.89−1.77 (m,
2H, CH2), −0.81 (s, 3H, AlCH3). 13C NMR data are not available due
to the limited solubility of complex. Elemental analysis for
C15H19N4Al: C, 63.81; H, 6.78; N, 19.85. Found: C, 64.11; H, 6.72;
N, 9.63.
N,N′-Bis((1H-pyrrol-2-yl)methylene)cyclohexane-1,2-diamine
1
(H2L7). Yield: 0.77 g, 55%. H NMR data (399.87 MHz, [D6]DMSO,
300 K): δ 11.19 (br s, 2H, NH), 7.98 (s, 2H, NCH), 6.81−6.79 (m,
2H, pyrrole-H), 6.34 (dd, 4JHH = 1.4, 3JHH = 3.5, 2H, pyrrole-H), 6.05
3
3
(dd, JHH = 3.5, JHH = 2.6, 2H, pyrrole-H), 3.24−3.18 (m, 2H,
cyclohexyl-CH), 1.84−1.40 (m, 8H, cyclohexyl-CH2). 13C NMR data
(100.55 MHz, [D6]DMSO, 300 K): δ 151.6 (NCH), 130.6
(pyrrole-CH), 122.3 (pyrrole-C), 113.9 (pyrrole-CH), 109.4 (pyr-
role-CH), 74.4 (cyclohexyl-CH), 34.1 (cyclohexyl-CH2), 25.0 (cyclo-
hexyl-CH2). Elemental analysis for C16H20N4: C, 71.61; H, 7.51; N,
20.88. Found: C, 71.47; H, 7.81; N, 20.58.
L6AlMe (6). Yield: 0.33 g, 26% (white solid). 1H NMR data (399.87
4
MHz, [D6]DMSO, 300 K): δ 8.94 (s, 2H, NCH), 7.80 (dd, JHH
=
=
3
3
3.4, JHH = 5.9, 2H, ArH), 7.69 (m, 2H, pyrrole-H), 7.33 (dd, JHH
4
5.9, JHH = 3.3, 2H, ArH), 7.05−7.01 (m, 2H, pyrrole-H), 6.45 (dd,
3JHH = 3.6, JHH = 1.4, 2H, pyrrole-H), −1.13 (s, 3H, AlCH3). 13C
3
NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 151.0 (NCH),
138.3 (pyrrole-C), 137.4 (ArC), 137.2 (pyrrole-CH), 127.4 (ArCH),
120.5 (ArCH), 117.2 (pyrrole-CH), 115.7 (pyrrole-CH). Elemental
analysis for C17H15N4Al: C, 67.54; H, 5.00; N, 18.53%. Found: C,
67.42; H, 5.24; N, 18.32%.
General Protocol for the Synthesis of L1AlMe (1) to L7AlMe
(7). To a stirred suspension of H2L (4.67 mmol) in toluene (30 mL)
was slowly added AlMe3 (2.33 mL of a 2.0 M solution in toluene, 4.67
mmol). The reaction mixture was stirred at 110 °C for 24 h and during
which time a solid precipitated. For L2AlMe (2), L3AlMe (3), L6AlMe
(6), and L7AlMe (7), the solid was formed after subsequent cooling at
−20 °C. The solid was collected by filtration and dried under vacuum.
The synthetic procedures of L2AlMe−L7AlMe are reported in the
Supporting Information.
1
L7AlMe (7). Yield: 0.14 g, 20% (pale yellow solid). H NMR data
(399.87 MHz, [D6]DMSO, 298 K): δ 8.48 (s, 1H, NCH), 8.18 (s,
1H, NCH), 7.43−7.41 (m, 1H, pyrrole-H), 7.22−7.21 (m, 1H,
pyrrole-H), 6.75−6.73 (m, 1H, pyrrole-H), 6.67−6.65 (m, 1H,
pyrrole-H), 6.30−6.27 (m, 1H, pyrrole-H), 6.22−6.20 (m, 1H,
pyrrole-H), 3.11−3.01 (m, 2H, cyclohexyl-CH), 2.70−1.40 (m, 8H,
cyclohexyl-CH2), −0.95 (s, 3H, AlCH3). 13C NMR data (100.55 MHz,
[D6]DMSO, 300 K): δ 150.8 (NCH), 129.9 (pyrrole-C), 121.6
(pyrrole-CH), 113.1 (pyrrole-CH), 108.6 (pyrrole-CH), 73.7 (cyclo-
hexyl-CH), 34.4 (cyclohexyl-CH2), 24.2 (cyclohexyl-CH2). Elemental
analysis for C17H21N4Al: C, 66.22; H, 6.86; N, 18.17%. Found: C,
66.51; H, 7.11; N, 17.90%.
General Polymerization Procedure. In a nitrogen-filled glove-
box, lactide (720 mg, 5.0 mmol) and benzyl alcohol (5.2 μL, 0.05
mmol) were placed in a polymerization ampule. To this ampule was
added a solution of initiator (0.05 mmol) in toluene (6.00 mL)
([monomer]:[Al] = 100:1). The reaction was stirred for the desired
reaction time at 70 °C. At the desired reaction time, the reaction was
quenched with methanol (2−3 drops). The polymer was precipitated
from excess methanol, collected by filtration, and dried in vacuo to a
constant mass. Conversions were determined by integration of the
monomer versus polymer methine resonances in the 1H NMR
spectrum of the crude product (in CDCl3).
L1AlMe (1). Yield: 0.86 g, 83% (white solid). 1H NMR data (399.87
MHz, [D6]DMSO, 300 K): δ 8.33 (s, 2H, NCH), 7.30 (s, 2H,
3
3
pyrrole-H), 6.62 (d, JHH = 3.3, 2H, pyrrole-H), 6.18 (dd, JHH = 3.3,
3JHH = 1.9, 2H, pyrrole-H), 4.00−3.93 (m, 2H, NCH2), 3.74−3.66 (m,
2H, NCH2), −0.97 (s, 3H, AlCH3). 13C NMR data (100.55 MHz,
[D6]DMSO, 300 K): δ 158.6 (NCH), 136.2 (pyrrole-C), 133.7
(pyrrole-CH), 115.6 (pyrrole-CH), 112.8 (pyrrole-CH), 46.9 (N-
(CH2). Elemental analysis for C13H15N4Al: C, 61.41; H, 5.95; N,
22.03. Found: C, 61.38; H, 6.14; N, 22.00.
1
L2AlMe (2). Yield: 0.33 g, 28% (colorless crystals). H NMR data
(399.87 MHz, [D6]DMSO, 300 K): δ 8.28 (s, 2H, NCH), 7.34−
7.32 (m, 2H, pyrrole-H), 6.67 (dd, 4JHH = 0.8, 3JHH = 3.4, 2H, pyrrole-
3
3
H), 6.25 (dd, JHH = 3.4, JHH = 1.9, 2H, pyrrole-H), 3.88−3.76 (m,
4H, NCH2), 2.12−2.01 (m, 1H, CH2), 1.86−1.74 (m, 1H, CH2),
−0.90 (s, 3H, AlCH3). 13C NMR data (100.55 MHz, [D6]DMSO, 300
K): δ 160.3 (NCH), 135.5 (pyrrole-C), 134.8 (pyrrole-CH), 116.7
(pyrrole-CH), 113.8 (pyrrole-CH), 55.6 (NCH2), 30.2 (CH2).
Elemental analysis for C14H17N4Al: C, 62.67; H, 6.39; N, 20.88.
Found: C, 62.05; H, 6.35; N, 20.30.
1
L3AlMe (3). Yield: 0.47 g, 57% (colorless crystals). H NMR data
General Procedure for Kinetic Studies. The polymerizations
were carried out at 70 °C in a glovebox. The molar ratio of monomer
to initiator was fixed at 50:1. At appropriate time intervals, 0.5 mL
aliquots were removed and quenched with methanol. The solvent was
(399.87 MHz, [D6]DMSO, 300 K): δ 8.24 (s, 2H, NCH), 7.39−
7.36 (m, 2H, pyrrole-H), 6.68 (dd, 4JHH = 0.8, 3JHH = 3.4, 2H, pyrrole-
3
3
2
H), 6.26 (dd, JHH = 3.4, JHH2 = 1.9, 2H, pyrrole-H), 3.76 (d, JHH
=
1
12.0, 2H, NCH2), 3.45 (d, JHH = 12.2, 2H, NCH2), 1.05 (s, 3H,
removed in vacuo, and the percent conversion was determined by H
C(CH3)2), 0.84 (s, 3H, C(CH3)2), −0.89 (s, 3H, AlCH3). 13C NMR
NMR in CDCl3.
M
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
Computational Details. All geometry optimizations were carried
out in the gas phase without molecular symmetry constraints using the
hybrid meta exchange-correlation functional with double the amount
of nonlocal exchange34b (M06-2X) level of theory as implemented in
the Gaussian09 program.38 The all-electron basis set 6-311G(d,p) was
applied to all atoms in the systems. Frequency calculations in the
harmonic approximation performed at the same level of theory were
used to verify that the optimized transition states possess only one
imaginary vibrational frequency corresponding to the relevant reaction
coordinate. The free energies were computed at 298 K.
Crystal Structure Determination. X-ray diffraction data were
measured on a Bruker-Nonius kappaCCD diffractometer with graphite
monochromated Mo Kα radiation (λ = 0.710 73 Å) at 150(2) K. The
structure was solved by direct methods with SIR9739 and refined with
full-matrix least-squares calculations on F2 using SHELXL-97.40
Crystallographic data have been deposited at the Cambridge
Crystallographic Data Centre under the reference numbers CCDC
909527. Copies of the data can be obtained, free of charge, on
application to the Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK.
Research and Development Institute (KURDI) is also acknowl-
edged for equipment support. We also gratefully acknowledge
Prof. Philippe A. Bopp for his useful discussions.
REFERENCES
■
(1) (a) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Adv. Mater.
2000, 12, 1841−1846. (b) Chiellini, E.; Solaro, R. Adv. Mater. 1996, 8,
305−313. (c) Amass, W.; Amass, A.; Tighe, B. Polym. Int. 1998, 47,
89−144. (d) Ragauskas, A. J. Science 2006, 311, 484−489.
(2) (a) Albertsson, A.-C.; Varma, I. K. Biomacromolecules 2003, 4,
1466−1486. (b) Gupta, A. P.; Kumar, V. Eur. Polym. J. 2007, 43,
4053−4074. (c) Mainil-Varlet, P.; Rahn, B.; Gogolewski, S.
Biomaterials 1997, 18, 257−266. (d) Langer, R.; Vacanti, J. Science
1993, 260, 920−926. (e) Auras, R.; Harte, B.; Selke, S. Macromol.
Biosci. 2004, 4, 835−864.
(3) (a) Li, G.; Lamberti, M.; Pappalardo, D.; Pellecchia, C.
Macromolecules 2012, 45, 8614−8620. (b) Bakewell, C.; Platel, R.
H.; Cary, S. K.; Hubbard, S. M.; Roaf, J. M.; Levine, A. C.; White, A. J.
P.; Long, N. J.; Haaf, M.; Williams, C. K. Organometallics 2012, 31,
4729−4736. (c) Lamberti, M.; D’Auria, I.; Mazzeo, M.; Milione, S.;
Bertolasi, V.; Pappalardo, D. Organometallics 2012, 31, 5551−5560.
(d) Darensbourg, D. J.; Karroonnirun, O.; Wilson, S. J. Inorg. Chem.
2011, 50, 6775−6787. (e) Normand, M.; Dorcet, V.; Kirillov, E.;
Carpentier, J.-F. Organometallics 2013, 32, 1694−1709. (f) Maudoux,
N.; Roisnel, T.; Dorcet, V.; Carpentier, J.-F.; Sarazin, Y. Chem. - Eur. J.
2014, 20, 6131−6147. (g) Tabthong, S.; Nanok, T.; Kongsaeree, P.;
Prabpai, S.; Hormnirun, P. Dalton Trans. 2014, 43, 1348−1359.
(4) (a) D’Auria, I.; Lamberti, M.; Mazzeo, M.; Milione, S.; Roviello,
G.; Pellecchia, C. Chem. - Eur. J. 2012, 18, 2349−2360.
(b) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.;
Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 3229−
3238. (c) Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam, W.;
Young, V. G., Jr.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc.
2003, 125, 11350−11359. (d) Drouin, F.; Oguadinma, P. O.;
Whitehorne, T. J. J.; Prud’homme, R. E.; Schaper, F. Organometallics
2010, 29, 2139−2147. (e) Wang, H.; Yang, Y.; Ma, H. Macromolecules
2014, 47, 7750−7764.
(5) (a) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; White, A. J. P.;
Williams, D. J. Chem. Commun. 2001, 283−284. (b) Aubrecht, K. B.;
Hillmyer, M. A.; Tolman, W. B. Macromolecules 2002, 35, 644−650.
(6) (a) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 1999, 121,
4072−4073. (b) Cao, T.-P.-A.; Buchard, A.; Le Goff, X. F.; Auffrant,
A.; Williams, C. K. Inorg. Chem. 2012, 51, 2157−2169. (c) Yang, S.;
Du, Z.; Zhang, Y.; Shen, Q. Chem. Commun. 2012, 48, 9780−9782.
(d) Platel, R. H.; White, A. J. P.; Williams, C. K. Inorg. Chem. 2011, 50,
7718−7728. (e) Alaaeddine, A.; Thomas, C. M.; Roisnel, T.;
Carpentier, J.-F. Organometallics 2009, 28, 1469−1475. (f) Buffet, J.-
C.; Kapelski, A.; Okuda, J. Macromolecules 2010, 43, 10201−10203.
(g) Arnold, P. L.; Buffet, J.-C.; Blaudeck, R.; Sujecki, S.; Wilson, C.
Chem. - Eur. J. 2009, 15, 8241−8250. (h) Bakewell, C.; White, A. J. P.;
Long, N. J.; Williams, C. K. Inorg. Chem. 2015, 54, 2204−2212.
(7) (a) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W. J.
Am. Chem. Soc. 1999, 121, 11583−11584. (b) Wu, J.; Chen, Y.-Z.;
Hung, W.-C.; Lin, C.-C. Organometallics 2008, 27, 4970−4978.
(c) Chisholm, M. H.; Eilerts, N. W.; Huffman, J. C.; Iyer, S. S.; Pacold,
M.; Phomphrai, K. J. Am. Chem. Soc. 2000, 122, 11845−11854.
(d) Tang, H.-Y.; Chen, H.-Y.; Huang, J.-H.; Lin, C.-C. Macromolecules
2007, 40, 8855−8860.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b01381.
Synthetic procedure of L2AlMe (2)−L7AlMe (7);
crystallographic data for compound 3 in CIF format;
tables of crystallographic data; plots of PLA Mn and
molecular weight distribution as a function of monomer
conversion for a rac-LA polymerization using 1−6/
PhCH2OH; semilogarithmic plots of L-lactide conversion
versus time using 1−7/PhCH2OH; homonuclear
decoupled 1H NMR spectra of PLA produced by
1,2,4,5,6/PhCH2OH; selected bond lengths and angles
of the optimized geometries of complexes 1−7;
calculated free energy profile for the ring-opening
insertion of the first (S,S)-LA monomer by 1,2,4−7/
PhCH2OH; optimized structures of complexes 1−7;
selected geometrical parameters for TS1 of complexes
1−7; selected geometrical parameters for TS2 of
complexes 1−7; ring-opening insertion mechanism for
the second (S,S)-LA monomer by 3; close-up of the
TS1′and TS2′ structures for the (SS,SS) assembly mode
of insertion by 3; free energy profiles for the second
lactide insertion in different assembly modes mediated by
1,2,4,5,6; selected geometrical parameters for TS1′
structures of complexes 1−7; selected geometrical
parameters for TS2′ structures of complexes 1−7
(PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: fscipph@ku.ac.th (P.H.).
■
Notes
(8) (a) Chisholm, M. H.; Gallucci, J. C.; Phomphrai, K. Inorg. Chem.
2004, 43, 6717−6725. (b) Darensbourg, D. J.; Choi, W.;
Karroonnirun, O.; Bhuvanesh, N. Macromolecules 2008, 41, 3493−
3502. (c) Hill, M. S.; Hitchcock, P. B. Chem. Commun. 2003, 1758−
1759. (d) Cushion, M. G.; Mountford, P. Chem. Commun. 2011, 47,
2276−2278.
(9) (a) Ma, H.; Spaniol, T. P.; Okuda, J. Angew. Chem. 2006, 118,
7982−7985. (b) Sinenkov, M.; Kirillov, E.; Roisnel, T.; Fukin, G.;
Trifonov, A.; Carpentier, J.-F. Organometallics 2011, 30, 5509−5523.
(c) Buchard, A.; Platel, R. H.; Auffrant, A.; Le Goff, X. F.; Le Floch, P.;
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Thailand Research Fund
(TRF) (Grant DIG5180026). S.T. thanks the Graduate School
Kasetsart University of his scholarship for supporting the
publication in an international journal. P.K. and S.P. thank
Mahidol University and the Center for Innovation in Chemistry
(PERCH-CIC) for financial support. Kasetsart University
N
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
Williams, C. K. Organometallics 2010, 29, 2892−2900. (d) Stevels, W.
M.; Maron, L. Chem. - Eur. J. 2008, 14, 5507−5518. (e) Miranda, M.
O.; DePorre, Y.; Vazquez-Lima, H.; Johnson, M. A.; Marell, D. J.;
Cramer, C. J.; Tolman, W. B. Inorg. Chem. 2013, 52, 13692−13701.
(f) Della Monica, F.; Luciano, E.; Roviello, G.; Grassi, A.; Milione, S.;
́
M.; Ankone, M. J. K.; Dijkstra, P. J.; Feijen, J. Macromolecules 1996, 29,
3332−3333.
(10) For a reviews on lactide polymerization, see: (a) O’Keefe, B. J.;
Hillmyer, M. A.; Tolman, W. B. J. Chem. Soc., Dalton Trans. 2001,
2215−2224. (b) Dechy-Cabaret, O.; Martin- Vaca, B.; Bourissou, D.
Chem. Rev. 2004, 104, 6147−6176. (c) Platel, R. H.; Hodgson, L. M.;
Williams, C. K. Polym. Rev. 2008, 48, 11−63. (d) Dove, A. P. Chem.
Commun. 2008, 6446−6470. (e) Thomas, C. M. Chem. Soc. Rev. 2010,
39, 165−173. (f) Wheaton, C. A.; Hayes, P. G.; Ireland, B. J. Dalton
Trans. 2009, 4832−4846. (g) Stanford, M. J.; Dove, A. P. Chem. Soc.
Rev. 2010, 39, 486−494. (h) Wu, J.; Yu, T.-L; Chen, C.-T; Lin, C.-C.
Coord. Chem. Rev. 2006, 250, 602−626. (i) Amgoune, A.; Thomas, C.
M.; Carpentier, J.-F. Pure Appl. Chem. 2007, 79, 2013−2030.
(j) Dijkstra, P. J.; Du, H.; Feijen, J. Polym. Chem. 2011, 2, 520−527.
(k) Guillaume, S. M.; Kirillov, E.; Sarazin, Y.; Carpentier, J.-F. Chem. -
Eur. J. 2015, 21, 7988−8003.
Capacchione, C. Macromolecules 2014, 47, 2830−2841. (g) Borner, J.;
̈
dos Santos Vieira, I.; Pawlis, A.; Doring, A.; Kuckling, D.; Herres-
Pawlis, S. Chem. - Eur. J. 2011, 17, 4507−4512.
̈
(22) Weber, J. H. Inorg. Chem. 1967, 6, 258−262.
(23) Atwood, D. A.; Harvey, M. J. Chem. Rev. 2001, 101, 37−52.
(24) Benn, R.; Rufinska, A. Angew. Chem., Int. Ed. Engl. 1986, 25,
́
861−881.
(25) The τ value ranges from 0 (perfectly square pyrimidal) to 1
(perfectly trigonal bipyradal). (a) Addison, A. W.; Rao, T. N.; Reedijk,
J.; van Rijn, J.; Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984,
1349−1356. (b) Munoz-Hernandez, M.-A.; Keizer, T. S.; Wei, P.;
Parkin, S.; Atwood, D. A. Inorg. Chem. 2001, 40, 6782−6787.
́ ̂ ́
(26) (a) Barakat, I.; Dubois, P.; Jerome, R.; Teyssie, P. J. Polym. Sci.,
Part A: Polym. Chem. 1993, 31, 505−514. (b) Baran, J.; Duda, A.;
Kowalski, A.; Szymanski, R.; Penczek, S. Macromol. Rapid Commun.
1997, 18, 325−333. (c) Kowalski, A.; Duda, A.; Penczek, S.
Macromolecules 1998, 31, 2114−2122.
(11) Spassky, N.; Wisniewski, M.; Pluta, C.; Le Borgne, A. Macromol.
Chem. Phys. 1996, 197, 2627−2637.
(12) (a) Ovitt, T. M.; Coates, G. W. J. Polym. Sci., Part A: Polym.
Chem. 2000, 38, 4686−4692. (b) Ovitt, T. M.; Coates, G. W. J. Am.
Chem. Soc. 2002, 124, 1316−1326. (c) Radano, C. P.; Baker, G. L.;
Smith, M. R. J. Am. Chem. Soc. 2000, 122, 1552−1553. (d) Zhong, Z.;
Dijkstra, P. J.; Feijen, J. Angew. Chem., Int. Ed. 2002, 41, 4510−4513.
(e) Zhong, Z.; Dijkstra, P. J.; Feijen, J. J. Am. Chem. Soc. 2003, 125,
11291−11298.
(27) Platel, R. H.; White, A. J. P.; Williams, C. K. Inorg. Chem. 2008,
47, 6840−6849.
(28) Ishii, R.; Nomura, N.; Kondo, T. Polym. J. 2004, 36, 261−264.
(29) Tang, Z.; Chen, X.; Yang, Y.; Pang, X.; Sun, J.; Zhang, X.; Jing,
X. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 5974−5982.
(30) Pang, X.; Du, H.; Chen, X.; Wang, X.; Jing, X. Chem. - Eur. J.
2008, 14, 3126−3136.
(31) Yu, I.; Acosta-Ramírez, A.; Mehrkhodavandi, P. J. Am. Chem.
Soc. 2012, 134, 12758−12773.
(32) Pilone, A.; De Maio, N.; Press, K.; Venditto, V.; Pappalardo, D.;
Mazzeo, M.; Pellecchia, C.; Kol, M.; Lamberti, M. Dalton Trans. 2015,
44, 2157−2165.
(33) (a) Thakur, K. A. M.; Kean, R. T.; Hall, E. S.; Kolstad, J. J.;
Lindgren, T. A.; Doscotch, M. A.; Siepmann, J. I.; Munson, E. J.
Macromolecules 1997, 30, 2422−2428. (b) Thakur, K. A. M.; Kean, R.
T.; Hall, E. S.; Kolstad, J. J.; Munson, E. J. Macromolecules 1998, 31,
1487−1494. (c) Thakur, K. A. M.; Kean, R. T.; Zell, M. T.; Padden, B.
E.; Munson, E. J. Chem. Commun. 1998, 1913−1914. (d) Chisholm,
M. H.; Iyer, S. S.; McCollum, D. G.; Pagel, M.; Werner-Zwanziger, U.
Macromolecules 1999, 32, 963−973. (e) Zell, M. T.; Padden, B. E.;
Paterick, A. J.; Thakur, K. A. M.; Kean, R. T.; Hillmyer, M. A.;
Munson, E. J. Macromolecules 2002, 35, 7700−7707. (f) Coudane, J.;
Ustariz-Peyret, C.; Schwach, G.; Vert, M. J. Polym. Sci., Part A: Polym.
Chem. 1997, 35, 1651−1658.
(13) (a) Nomura, N.; Ishii, R.; Akakura, M.; Aoi, K. J. Am. Chem. Soc.
2002, 124, 5938−5939. (b) Tang, Z.; Chen, X.; Pang, X.; Yang, Y.;
Zhang, X.; Jing, X. Biomacromolecules 2004, 5, 965−970. (c) Ovitt, T.
M.; Coates, G. W. J. Am. Chem. Soc. 1999, 121, 4072−4073.
(d) Nomura, N.; Ishii, R.; Yamamoto, Y.; Kondo, T. Chem. - Eur. J.
2007, 13, 4433−4451. (e) Hormnirun, P.; Marshall, E. L.; Gibson, V.
C.; Pugh, R. I.; White, A. J. P. Proc. Natl. Acad. Sci. U. S. A. 2006, 103,
15343−15348. (f) Cross, E. D.; Allan, L. E. N.; Decken, A.; Shaver, M.
P. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1137−1146.
(g) Agatemor, C.; Arnold, A. E.; Cross, E. D.; Decken, A.; Shaver, M.
P. J. Organomet. Chem. 2013, 745−746, 335−340. (h) Chen, H.-L.;
Dutta, S.; Huang, P.-Y.; Lin, C.-C. Organometallics 2012, 31, 2016−
2025. (i) Du, H.; Pang, X.; Yu, H.; Zhuang, X.; Chen, X.; Cui, D.;
Wang, X.; Jing, X. Macromolecules 2007, 40, 1904−1913. (j) Chisholm,
M. H.; Gallucci, J. C.; Quisenberry, K. T.; Zhou, Z. Inorg. Chem. 2008,
47, 2613−2624.
(14) Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; White, A. J. P.;
Williams, D. J. J. Am. Chem. Soc. 2004, 126, 2688−2689.
(15) Press, K.; Goldberg, I.; Kol, M. Angew. Chem., Int. Ed. 2015,
DOI: 10.1002/anie.201503111.
(16) Du, H.; Velders, A. H.; Dijkstra, P. J.; Sun, J.; Zhong, Z.; Chen,
X.; Feijen, J. Chem. - Eur. J. 2009, 15, 9836−9845.
(17) Sumrit, P.; Hormnirun, P. Macromol. Chem. Phys. 2013, 214,
1845−1851.
(18) (a) Whitelaw, E. L.; Lorain, G.; Mahon, M. F.; Jones, M. D.
Dalton Trans. 2011, 40, 11469−11473. (b) dos Santos Vieira, I.;
Whitelaw, E. L.; Jones, M. D.; Herres-Pawlis, S. Chem. - Eur. J. 2013,
19, 4712−4716. (c) Hancock, S. L.; Mahon, M. F.; Jones, M. D. Dalton
Trans. 2013, 42, 9279−9285.
(19) Pilone, A.; Press, K.; Goldberg, I.; Kol, M.; Mazzeo, M.;
Lamberti, M. J. Am. Chem. Soc. 2014, 136, 2940−2943.
(20) (a) Du, H.; Velders, A. H.; Dijkstra, P. J.; Zhong, Z.; Chen, X.;
Feijen, J. Macromolecules 2009, 42, 1058−1066. (b) Schwarz, A. D.;
Chu, Z.; Mountford, P. Organometallics 2010, 29, 1246−1260.
(34) (a) Zhao, U.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 1−17.
(b) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241.
(c) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157−167.
(35) Hehre, W. J.; Ditchfie, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257−2261.
(36) Honrado, M.; Otero, A.; Fernan
F.; Garces, A.; Lara-Sanchez, A.; Rodríguez, A. M. Organometallics
2014, 33, 1859−1866.
́ ́
dez-Baeza, J.; Sanchez-Barba, L.
́
́
(37) Gottlieb, H. E.; Kotlyar, B.; Nudelman, A. J. Org. Chem. 1997,
62, 7512−7515.
(38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson,G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
(c) Zpeitia, H. F. S.; Cortes-Llamas, S. A.; Vengoechea-Gom
A.; Rufino-Felipe, E.; Crespo-Velasco, N. T.; Munoz-Hernan
Main Group Chem. 2011, 10, 127−140.
́
ez, F.
dez, M.-A.
́
́
̃
(21) For selected references of DFT studies, see: (a) Marshall, E. L.;
Gibson, V. C.; Rzepa, H. S. J. Am. Chem. Soc. 2005, 127, 6048−6051.
(b) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; Rzepa, H. S.; White, A.
J. P.; Williams, D. J. J. Am. Chem. Soc. 2006, 128, 9834−9843.
(c) Fang, J.; Yu, I.; Mehrkhodavandi, P.; Maron, L. Organometallics
2013, 32, 6950−6956. (d) Barros, N.; Mountford, P.; Guillaume, S.
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
O
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian, Inc.:
Wallingford, CT, 2009.
(39) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115−119.
(40) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, 64, 112−122.
P
DOI: 10.1021/acs.macromol.5b01381
Macromolecules XXXX, XXX, XXX−XXX