Living ring-opening polymerization of lactones by n-heterocyclic Olefin/Al(C6F5)3 lewis pairs: Structures of intermediates, kinetics, and mechanism

Article abstract of DOI:10.1021/acs.macromol.6b02398

The strong Lewis acid Al(C6F5)3, in combination with a strong Lewis base N-heterocyclic olefin (NHO), cooperatively promotes the living ring-opening (co)- polymerization of lactones, represented by δ-valerolactone (δ-VL) and ?-caprolactone (?-CL) in this

Full text of DOI:10.1021/acs.macromol.6b02398

Article
pubs.acs.org/Macromolecules
Living Ring-Opening Polymerization of Lactones by N‑Heterocyclic
Olefin/Al(C F ) Lewis Pairs: Structures of Intermediates, Kinetics, and
6
5 3
Mechanism
Qianyi Wang, Wuchao Zhao, Jianghua He, Yuetao Zhang,* and Eugene Y.-X. Chen‡
†
†
†
,†
†State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China
‡Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States
*
S Supporting Information
ABSTRACT: The strong Lewis acid Al(C F ) , in combina-
6
5 3
tion with a strong Lewis base N-heterocyclic olefin (NHO),
cooperatively promotes the living ring-opening (co)-
polymerization of lactones, represented by δ-valerolactone
(
δ-VL) and ε-caprolactone (ε-CL) in this study. Medium to
high molecular weight linear (co)polyesters (M up to 855 kg/
w
mol) are achieved, and most of them exhibit narrow molecular
weight distributions (Đ as low as 1.02). Detailed investigations
into the structures of key reaction intermediates, kinetics, and polymer structures have led to a polymerization mechanism, in that
initiation involves nucleophilic attack of the Al(C F ) -activated monomer by NHO to form a structurally characterized
6
5 3
zwitterionic, tetrahedral intermediate, followed by its ring-opening to generate active zwitterionic species. In the propagation
cycle, this ring-opened zwitterionic species and its homologues attack the incoming monomer activated by Al(C F ) to generate
6
5 3
the tetrahedral intermediate, followed by the rate-determining ring-opening step to regenerate the zwitterionic species that re-
enters into the next chain propagation cycles. Owning to the living features and lack of transesterification side reactions possessed
uniquely by this Lewis pair polymerization system, well-defined di- and triblock copolymers with narrow molecular weight
distributions (Đ = 1.06−1.15) have been successfully synthesized using this method, regardless of the comonomer addition
order.
INTRODUCTION
is rapid, polymers with high molecular weights are typically
difficult to obtain; for example, the ZROP of ε-CL generates
■
As a powerful polymerization technique that combines the
ability to produce condensation-type polymers with fast chain-
growth polymerization kinetics, the ring-opening polymer-
ization (ROP) of lactones has been widely utilized to produce
the technologically important and environmentally friendly
biodegradable and/or biocompatible aliphatic polyesters, such
as poly(δ-valerolactone) (PVL) and poly(ε-caprolactone)
PCL). Various types of catalysts have been developed for
the synthesis of such polyesters, including different metal
cyclic PCL with number-average molecular weights M ranging
n
21
from 41 to 114 kg/mol.
There have been intense investigations into the cooperative
(
or synergistic) catalytic effects of Lewis acid (LA) and Lewis
1
base (LB) pairs, since the seminal works of Stephan and
22−32
Erker.
These Lewis pairs (LPs) have achieved noteworthy
success in many areas of chemistry, such as activation of small
(
3
3−50
51−60
molecules,
catalytic hydrogenation,
and new reac-
In addition, Lewis pair
has emerged and began to generate
6
1−74
tivity/reaction development.
polymerization (LPP)
complexes exhibiting impressive catalytic performance in the
ROP of cyclic esters and, more recently, a series of simple,
75,76
2
,3
some exciting results on polymerization catalysis. For instance,
we employed aluminum LA-based, especially strongly acidic,
sterically encumbered alane Al(C F ) , LPs with several classes
often commercially available but highly effective organocatalysts
4
−12
as well.
Among them, N-heterocyclic carbenes (NHCs) are
the most successful and widely studied organocatalysts for the
6
5 3
1
3−17
of LBs including NHCs and phosphines for the rapid addition
polymerization of conjugated polar vinyl monomers or Michael
acceptors such as linear methyl methacrylate, cyclic and
naturally renewable α-methylene-γ-butyrolactone, and γ-meth-
ROP of lactones.
In the absence of protic initiators, NHCs
mediate the zwitterionic ring-opening polymerization (ZROP)
of lactones to propagate and cyclize to release macrocycles,
18,19
and therefore it is difficult to prepare block copolymers.
However, by choosing monomers with sufficiently different
reactivity, Waymouth et al. succeeded in the copolymerization
of ε-CL and δ-VL with an NHC in batch conditions and
obtained cyclic gradient copolymers. On the other hand, with
alcohol as initiator, both linear polyester and block copolymer
could be obtained. Although the ZROP of lactones with NHCs
7
7,78
yl-α-methylene-γ-butyrolactone
as well as monomers
bearing the CC−CN functionality, such as 2-vinylpyridine
20
Received: November 4, 2016
Revised: December 9, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.6b02398
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
7
9,80
and 2-isopropenyl-2-oxazoline.
In such polymerization, the
molecular sieves 4 Å. NMR spectra were recorded on a Varian
1
13
19
Inova 300 (300 MHz, H; 75 MHz, C; 282 MHz, F) or a Bruker
cooperativity of the LA and LB sites of Lewis pairs is essential
to achieve an effective polymerization system, which was
demonstrated by the borane/phosphine LPs that showed the
interacting LPs, and even classical Lewis adducts, can be highly
active for the polymerization. This was further nicely
demonstrated by Rieger and co-workers, showing the high
activity and high degree of control over the polymerization of
Michael-type and extended Michael monomer systems by the
Avance II 500 (500 MHz, 1H; 126 MHz, C; 471 MHz, F)
13
19
1
13
instrument at room temperature. Chemical shifts for H and
C
spectra were referenced to internal solvent resonances and are
19
reported as parts per million relative to SiMe , whereas F NMR
81
4
1
13
spectra were referenced to external CFCl . H and C NMR chemical
3
shifts are reported in ppm relative to the residual solvent. Air-sensitive
NMR samples were conducted in Teflon valve sealed J. Young-type
NMR tubes.
Trimethylaluminum, triethylaluminum, bromopentafluorobenzene,
diazabicycloundec-7-ene (DBU), 4-(dimethylamino)pyridine
82
highly interacting organoaluminum and phosphine LPs.
Extending beyond the commonly employed NHC and
phosphine LBs, in 2014, Lu et al. first applied NHOs in the
Al(C F ) -based LPs for the addition polymerization of
The presence of both LA and LB has
proven to be crucial for all the cases to achieve the much
enhanced polymerization activity and selectivity.
The LPP method has also been extended to the ROP of
lactones. In 2012, we attempted the ROP of lactones with
several Al(C F ) -based LPs and found only P Bu /Al(C F )
LP polymerized ε-CL to PCL at room temperature but with a
broad molecular weight distribution of Đ = 2.76 and a low
monomer conversion of only 58% even after 20 h. In 2013,
Buchmeiser and co-workers reported the bulk polymerization
of ε-CL catalyzed by LPs consisting of NHCs and different
Lewis acids, yielding PCL with number-average molecular
(DMAP), ammonium acetate, and sodium bicarbonate were purchased
from J&K. 1,2-Dimethylimidazole and benzil were purchased from
Titan. Iodomethane, boron trichloride (1.0 M solution in hexanes), n-
BuLi (2.5 M solution in hexanes), and isobutyraldehyde were
6
5 3
8
3,84
(
meth)acrylates.
purchased from Energy Chemical. 1,3-Di-tert-butylimidazol-2-ylidene
tBu
(
NHC) was purchased from TCI. Potassium hydride (30 wt %
dispersion in mineral oil) and tris(2,4,6-trimethylphenyl)phosphine
(Mes P) were purchased from Alfa Aesar. All of the chemicals were
3
t
used as received unless otherwise specified as follows. Potassium
hydride was diluted with hexanes in a glovebox, then mixed, and
decanted. This procedure was repeated three times, and the washed
KH was finally dried under vacuum. ε-Caprolactone (ε-CL, J&K) and
δ-valerolactone (δ-VL, Energy Chemical) were dried over CaH2,
distilled under nitrogen, and stored in a glovebox at −35 °C.
Tris(pentafluorophenyl)borane, B(C F ) , was prepared according to
6
5 3
3
6
5 3
78
6
5 3
89,90
literature procedures.
Al(C F ) , as a (toluene) adduct, or in its
6 5 3 0.5
weights (M ) ranging from 9.0 kg/mol (Đ = 1.73) to 16.5 kg/
mol (Đ = 1.69). Bourissou et al. employed Zn(C F ) -based
LPs for the controlled ROP of lactide and ε-CL to afford well-
defined cyclic polyesters, achieving PLA with M = 50.6 kg/mol
n
unsolvated form, was prepared by ligand exchange reactions between
B(C F ) and AlMe3 or AlEt3 (for preparation of the unsolvated
8
5
6
5 2
6
5 3
9
1−93
form).
(Extra caution should be exercised when handling these
, due to its thermal and shock
sensitivity!) 1,3-Dimethyl-2-methylene-2,3-dihydro-1H-imidazole
materials, especially the unsolvated Al(C F )
6 5 3
w
(
Đ = 1.5) and PCL with M = 18.9 kg/mol (Đ = 1.3),
w
86
94
(NHO2) was prepared according to literature procedures.
respectively. Most recently, Dove and co-workers revealed
that LPs consisting of simple LAs, such as MgI and YCl , and
Synthesis of 2-Isopropyl-4,5-diphenyl-1H-imidazole. A 250 mL
2
3
glass vial was charged with NH OAc (7.7 g, 99.9 mmol), MeOH (150
4
LBs, such as 4-(dimethylamino)pyridine (DMAP), can be
highly effective for the ROP of lactones, especially the
challenging macrolactone. For the ROP of ε-CL and δ-VL
by this system, the highest reported molecular weights of the
resulting PCL and PVL were M = 28.7 kg/mol (Đ = 1.33) by
MgI /DMAP and M = 13.2 kg/mol (Đ = 1.5) by YCl /
DMAP. In 2016, Li and co-workers employed the combination
of Zn(C F ) and different LBs for the ROP of lactide,
mL), and a PTFE stirring bar. The mixture was allowed to cool to 0
°
C, and isobutyraldehyde (3.6 g, 49.9 mmol) and benzil (10.5 g, 49.9
mmol) were added. The vial was sealed and submerged in a preheated
87
oil bath at 120 °C for 3 h. The resulting mixture was concentrated
under vacuum. The off-white precipitate was collected by filtration,
n
1
then washed with Et
O, and dried in a vacuum (8.0 g, 61.0%). H
2
n
3
2
NMR (300 MHz, DMSO-d ): δ 11.92 (s, 1H, NH), 7.48−7.16 (m,
6
1
0H, Ph), 3.01 (sept, J = 7.1 Hz, 1H, CHMe ), 1.31 (d, J = 7.1 Hz, 6H,
2
6
5 2
88
CHMe2).
Synthesis of 2-Isopropyl-1,3-dimethyl-4,5-diphenyl-1H-imidazol-
-ium Iodide. 2-Isopropyl-4,5-diphenyl-1H-imidazole (8.0 g, 30.5
affording PLA with M ranging from 2.4 to 28.9 kg/mol. As
n
shown by the above overview, there are still several undressed
key issues in the ROP by LPs. First, the degree of
polymerization control needs significant improvement. Second,
the molecular weight of the polyester is still limited. Third,
detailed mechanistic studies of this polymerization are lacking.
To this end, this contribution reports that NHO/Al(C F )
LPs promote the living/controlled ROP and copolymerization
of δ-VL and ε-CL to produce medium to high molecular weight
polyesters with narrow molecular weight distributions. More
importantly, this LPP system enabled us to isolate key reaction
intermediates and perform kinetic and mechanistic studies,
thereby providing the much-needed insights into the LPP
mechanism for the ROP of lactones.
3
mmol) and NaHCO (10.2 g, 122.0 mmol) were suspended in dry
3
acetonitrile (150 mL). Methyl iodide (46.5 mL, 747.1 mmol) was
added, and the reaction mixture was heated to reflux for 24 h and
filtrated while the solution was hot. The resulting mixture was
concentrated to half volume under vacuum. A pale yellow solid was
6
5 3
collected by filtration, then washed with THF, and dried under
1
vacuum (10.0 g, 78.4%). H NMR (500 MHz, DMSO-d ): δ 7.52−
6
7
.43 (m, 6H, Ph), 7.41−7.39 (m, 4H, Ph), 3.70 (s, 6H, NCH ), 3.84
sept, J = 7.2 Hz, 1H, CHMe ), 1.53 (d, J = 7.2 Hz, 6H, CHMe ).
3
(
2
2
Synthesis of 1,3-Dimethyl-4,5-diphenyl-2-(propan-2-ylidene)-2,3-
dihydro-1H-imidazole (NHO1). 2-Isopropyl-1,3-dimethyl-4,5-diphen-
yl-1H-imidazol-3-ium iodide (5.0 g, 11.9 mmol) was added to a
suspension of KH (0.96 g, 23.9 mmol) in THF (100 mL), and the
mixture was stirred for 24 h at room temperature under exclusion of
light. After filtration and removal of organic solvents in vacuo, the
EXPERIMENTAL SECTION
■
desired product was obtained as air-sensitive, red crystals (2.3 g,
1
Materials, Reagents, and Methods. All syntheses and
manipulations of air- and moisture-sensitive materials were carried
out in flamed Schlenk-type glassware on a dual-manifold Schlenk line,
a high-vacuum line, or an argon-filled glovebox. Toluene, benzene,
diethyl ether, THF, and hexane were refluxed over sodium/potassium
alloy distilled under a nitrogen atmosphere and then stored over
66.3%). H NMR (500 MHz, benzene-d ): δ 7.26−7.23 (m, 4H, o-Ph),
6
7.02−6.99 (m, 4H, m-Ph), 6.95−6.92 (m, 2H, p-Ph), 2.73 (s, 6H,
NCH ), 1.99 (s, 6H, CMe ). C NMR (126 MHz, benzene-d ): δ
13
3
2
6
153.5, 132.0, 128.6, 128.4, 128.0, 126.4, 72.0, 38.9, 20.8.
Isolation of Compound 1. To a solution of NHO2 (0.6 g, 5.4
mmol) in 7 mL of benzene was added δ-VL (0.54 g, 5.4 mmol) at
room temperature with stirring for 2 h. The off-white precipitate was
molecular sieves 4 Å. Benzene-d6 and CD Cl2 was dried over
2
B
DOI: 10.1021/acs.macromol.6b02398
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
obtained by filtration, then washed with benzene (3 × 5 mL), and
(m, 1H, MeCH), 1.38−1.35 (m, 1H, OCHCH ), 1.19−1.17 (m, 1H,
2
1
dried in vacuo (1.0 g, 87.3%). H NMR (500 MHz, benzene-d ): δ 5.38
OCHCH ), 0.91 (d, J = 6.7 Hz, 3H, CHCH ), 0.76−0.66 (m, 2H,
6
2
3
(
s, 2H, NCH), 5.33 (s, 1H, OH), 4.30 (s, 1H, CH), 3.95 (br, 2H,
OCHCH CH ), 0.72 (d, J = 6.7 Hz, 6H, CH(CH ) ). This spectrum
2
2
3
2
CH OH), 2.82 (s, 6H, NCH ), 2.68−2.66 (m, 2H, COCH ), 2.17−
2
CD Cl ): δ 185.3, 151.2, 118.0, 69.8, 61.5, 40.1, 35.5, 32.6, 22.6.
contains a small amount of hexanes (peaks marked with an asterisk).
2
3
2
13
19
.12 (m, 2H, CH ), 1.87−1.82 (m, 2H, CH ). C NMR (126 MHz,
F NMR (471 MHz, benzene-d ): δ −121.68 (d, J = 19.0, 6F, o-F),
2
2
6
F−F
2
2
−157.44 (t, J = 19.7, 3F, p-F), −163.53 to −163.66 (m, 6F, m-F).
F−F
13
Isolation of Compound 2. Compound 2 was isolated as off-white
C NMR (126 MHz, benzene-d ): δ 145.6, 120.4, 101.2, 78.8, 51.4,
6
powders in 88.3% yield using the same procedure as described for the
3
8.0, 37.6, 34.6, 33.9, 29.7, 27.7, 23.5, 18.2, 16.8 (broad resonances for
1
isolate of compound 1. H NMR (500 MHz, benzene-d ): δ 5.38 (s,
6
the C6F5 groups due to C−F coupling omitted). This spectrum
contains a small amount of chlorobenzene (peaks marked with an
asterisk).
2
3
2
1
1
H, NCH), 4.36 (s, 1H, CH), 3.80 (q, J = 6.0 Hz, 2H, CH OH),
2
.53 (t, J = 6.0 Hz, 1H, OH), 2.86 (s, 6H, NCH ), 2.65 (t, J = 6.7 Hz,
3
H, COCH ), 2.07−2.01 (m, 2H, CH ), 1.77−1.72 (m, 2H, CH ),
2
2
2
Synthesis and Isolation of Ring-Opened, Zwitterionic Intermedi-
ate (INT2). To a solution of INT1 (250 mg, 0.3 mmol) in 5 mL of
chlorobenzene was added Al(C F ) (163.3 mg, 0.3 mmol) at −35 °C
13
.71−1.66 (m, 2H, CH ). C NMR (126 MHz, CD Cl ): δ 185.0,
51.2, 118.0, 70.0, 61.9, 41.9, 35.5, 32.9, 27.2, 25.7.
2
2
2
6
5 3
Synthesis of 1-Hydroxynonan-5-one. 1-Hydroxynonan-5-one was
with stirring for 2 h. The resulting mixture was concentrated in vacuo,
and the residue was dissolved in 5 mL of benzene. After filtration, the
organic solvents were removed; the resulting white solid was then
washed with benzene/hexane (1:5) and dried in vacuo (189 mg,
95
prepared according to a modified literature procedure. δ-VL (3.0 g,
0.0 mmol) was dissolved in 75 mL of Et O under argon. The solution
3
2
was brought to −78 °C, where a solution of n-BuLi (1.6 M in hexane,
45.8%). 1H NMR (500 MHz, benzene-d ) INT2: δ 5.27 (s, 2H,
18.8 mL, 30.0 mmol) was added dropwise over a 20 min span. The
6
mixture was stirred for 4 h at this temperature and quenched with
NCHCHN), 4.77 (d, J = 8.4 Hz, 1H, OH), 4.01 (s, 1H, CH),
saturated NH Cl(aq). After allowing the solution to warm to room
3.91−3.87 (m, 1H, CHOH), 2.56 (s, 6H, NCH ), 2.25 (dd, J = 13.2,
4
3
temperature, NaCl was added to saturate the solution. The product
was extracted from the aqueous layer with copious amounts of diethyl
4.1 Hz, 1H, COCH ), 1.85−1.77 (m, 1H, MeCH), 1.65 (dd, J =
2
13.1, 10.3 Hz, 1H, COCH ), 1.44−1.36 (m, 1H, CH(CH ) ), 1.44−
2
3 2
ether. Pure product (3.2 g, 67.5%) was obtained by distillation under
1.36 (m, 1H, CH CHCO), 1.34−1.29 (m, 1H, CH CHCO), 1.14−
2
2
1
reduced pressure as colorless oil. H NMR (500 MHz, CDCl ): δ 3.61
1.07 (m, 1H, CH CH CHCO), 0.84−0.78 (m, 1H, CH CH CHCO),
3
2
2
2
2
(
q, J = 6.1 Hz, 2H, CH OH), 2.44 (t, J = 7.1 Hz, 2H, COCH ), 2.40
0.76 (d, J = 6.5 Hz, 3H, CHCH ), 0.56 (d, J = 6.9 Hz, 6H,
2
2
3
(
2
t, J = 7.5 Hz, 2H, COCH ), 2.19−2.17 (m, 1H, OH), 1.68−1.62 (m,
CH(CH ) ). This spectrum contains a small amount of hexanes
2
3 2
19
H, CH ), 1.59−1.43 (m, 4H, CH ), 1.39−1.18 (m, 2H, CH ), 0.89 (t,
(peaks marked with an asterisk). F NMR (471 MHz, benzene-d ): δ
2
2
2
6
J = 7.4 Hz, 3H, CH ).
−122.98 (dd, J = 26.5, 11.9 Hz, 12F, o-F, Alane), −150.52 (t, JF−F =
F−F
3
Isolation of Adduct Al(C F ) ·VL. To a solution of Al(C F ) (0.5 g,
19.7 Hz, 3F, p-F, HO·Alane), −154.93 (t, J = 19.6 Hz, 3F, p-F, C
6
5 3
6
5
3
F−F
0
.95 mmol) in benzene (5 mL) was added δ-VL (94.8 mg, 0.95 mmol)
CO·Alane), −160.05 to −160.19 (m, 6F, m-F, HO·Alane), −162.29 to
13
at room temperature with stirring for 5 min. The resulting mixture was
concentrated in vacuo, and the residue was washed with hexane (20
mL) and then dried in vacuo to afford Al(C F ) ·VL as a white powder
−162.42 (m, 6F, m-F, CCO·Alane). C NMR (126 MHz, benzene-
d ): δ 170.9, 145.0, 119.6, 91.5, 81.0, 44.9, 34.0, 32.8, 31.6, 29.9, 29.8,
6
18.8, 17.3, 14.7.
6
5 3
1
(
0.57 g, 95.8%). H NMR (500 MHz, benzene-d ): δ 3.08 (t, J = 5.5
Synthesis and Isolation of NHO1-CL-Al(C F ) (INT3). To a
6
6 5 3
Hz, 2H, OCH ), 1.60 (t, J = 6.6 Hz, 2H, COCH ), 0.44−0.32 (m, 4H,
solution of Al(C F ) (0.53 g, 1.0 mmol) in 3 mL of benzene was
2
2
6 5 3
19
CH CH ). F NMR (471 MHz, benzene-d ): δ −123.72 (dd, J =
F−F
added ε-CL (0.11 g, 1.0 mmol) and NHO1 (0.29 g, 1.0 mmol) at
room temperature with stirring for 30 min. After filtration and removal
of the organic solvent in vacuo, the solid was washed with hexane (3 ×
2
2
6
2
7.4, 11.9 Hz, 6F, o-F), −152.60 (t, J = 19.8 Hz, 3F, p-F), −160.27
F−F
13
to −162.53 (m, 6F, m-F). C NMR (126 MHz, benzene-d ): δ =
1
6
85.3, 75.2, 28.9, 20.0, 16.0.
5 mL). The desired product was obtained as colorless crystals (0.80 g,
1
Isolation of Adduct Al(C F ) ·CL. Adduct Al(C F ) ·CL was
86.1%). H NMR (500 MHz, benzene-d ): δ 6.94−6.86 (m, 10H, Ph),
6
5 3
6
5
3
6
isolated as a white powders in 96.2% yield using the same procedure
3.86 (t, J = 12.0 Hz, 1H, OCH ), 3.30−3.28 (m, 1H, OCH ), 3.28 (s,
2
2
1
as described for the isolate of the adduct Al(C F ) ·VL. H NMR (500
6H, NMe), 2.25−2.21 (m, 1H, COCH ), 2.15−2.10 (m, 1H, COCH ),
6
5
3
2
2
MHz, benzene-d ): δ 3.05−3.03 (m, 2H, OCH ), 1.69−1.67 (m, 2H,
1.50−1.45 (m, 2H, CH ), 1.42 (s, 3H, Me), 1.34−1.28 (m, 1H, CH ),
6
2
2
2
COCH ), 0.78−0.73 (m, 2H, CH ), 0.68−0.64 (m, 2H, CH ), 0.58−
1.21−1.13 (m, 2H, CH ), 1.09 (s, 3H, Me), 1.06−1.01 (m, 1H, CH ).
2
2
2
2
2
19
19
0
.54 (m, 2H, CH ). F NMR (471 MHz, benzene-d ): δ −123.50 (dd,
F NMR (471 MHz, benzene-d ): δ −121.44 (d, J = 26.9 Hz, 6F,
F−F
2
6
6
JF−F = 27.6, 11.7 Hz, 6F, o-F), −152.55 (t, J
= 19.8 Hz, 3F, p-F),
o-F), −156.14 (t, J
= 19.9 Hz, 3F, p-F), −162.97 to −163.10 (m,
F−F
F−F
13
13
−
161.42 to −161.55 (m, 6F, m-F). C NMR (126 MHz, benzene-d ):
6F, m-F). C NMR (126 MHz, methylene chloride-d ): δ 152.4,
6
2
δ 189.6, 75.1, 33.6, 27.1, 26.3, 20.4.
132.9, 130.9, 130.0, 128.9, 128.3, 126.3, 107.2, 63.0, 38.8, 37.8, 31.4,
Isolation of Adduct NHO2·Al(C F ) . To a solution of Al(C F )
30.2, 27.6, 27.2, 25.2 (broad resonances for the C F groups due to
6
5 3
6
5
3
6 5
(
0
0.4 g, 0.76 mmol) in 9 mL of benzene was added NHO2 (83.5 mg,
C−F coupling omitted).
X-ray Crystallographic Analysis of Compound 1, Al(C F ) ·CL,
.76 mmol) at room temperature with stirring for 1 min. The resulting
mixture was concentrated under vacuum to give the white precipitate
6
5 3
and INT3. Single crystals were quickly covered with a layer of
Paratone-N oil (Exxon, dried and degassed at 120 °C/10− Torr for 24
h) after decanting the mother liquor. A crystal was then mounted on a
thin glass fiber and transferred into the cold nitrogen stream of a
Bruker APEX-II CCD diffractometer. The structures were solved by
direct methods and refined using the Bruker SHELXTL program
6
(
NHO2·Al(C F ) ). The solid was collected by filtration, then washed
6 5 3
1
with benzene/hexane (1:4), and dried in vacuo (0.29 g, 60.0%). H
NMR (500 MHz, benzene-d ): δ 5.01 (s, 2H, NCH), 2.15 (s, 6H,
6
19
NCH ), 2.01 (br, 2H, CH ). F NMR (471 MHz, benzene-d ): δ
3
2
6
−
122.44 (d, JF−F = 22.3 Hz, 6F, o-F), −154.39 (t, JF−F = 19.7 Hz, 3F,
2
p-F), −161.89 to −162.03 (m, 6F, m-F).
library by full-matrix least-squares on F for all reflections (SHELXTL,
Synthesis and Isolation Zwitterionic, Tetrahedral Intermediate
INT1. To a solution of Al(C F ) (264.1 mg, 0.5 mmol) in 4 mL of
Version 6.12; Bruker Analytical X-ray Solutions: Madison, WI, 2001).
2
The structure was refined by full-matrix least-squares on F for all
6
5 3
chlorobenzene was added MI (85.1 mg, 0.5 mmol) and NHO2 (55.1
mg, 0.5 mmol) in hexane at −30 °C with stirring for 5 min. The solid
was collected by filtration, then washed with hexane (3 × 5 mL), and
reflections. All non-hydrogen atoms were refined with anisotropic
displacement parameters, whereas hydrogen atoms were included in
the structure factor calculations at idealized positions. Selected
crystallographic data for compound 1: C H N O , triclinic, space
dried in vacuo to afford INT1 as a light-yellow powder (279 mg,
11
18
2
2
1
6
9.2%). H NMR (500 MHz, benzene-d ): δ 5.39 (s, 2H, NCH
group P-1, a = 7.5941(13) Å, b = 8.6701(15) Å, c = 9.0386(16) Å, α =
6
3
CHN), 2.88 (dd, J = 8.9, 3.8 Hz, 1H, OCH), 2.70 (s, 6H, NCH ), 2.28
86.210(3)°, β = 77.541(3)°, γ = 76.473(3)°, V = 564.91(17) Å , Z = 2,
3
3
(
d, J = 15.2 Hz, 1H, CN CH ), 2.17 (d, J = 15.1 Hz, 1H, CN CH ),
Dcalcd = 1.248 mg/m , GOF = 1.036, R1 = 0.0562 [I > 2σ(I)], wR2 =
2
2
2
2
2
.12−2.06 (m, 1H, CH(CH ) ), 1.81 (d, J = 13.8 Hz, 1H,
0.1472 (all data). Selected crystallographic data for Al(C F ) ·CL:
3
2
6 5 3
CO CH CH), 1.51 (d, J = 9.0 Hz, 1H, CO CH CH), 1.49−1.39
C H AlF O , orthorhombic, space group Pbca, a = 18.8274(7) Å, b
24 10 15 2
2
2
2
2
C
DOI: 10.1021/acs.macromol.6b02398
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Macromolecules
Article
3
=
12.7040(5) Å, c = 19.8612(7) Å, α = β = γ = 90°, V = 4750.5(3) Å ,
also for the chain-growth addition polymerization of Michael
acceptors, we initially probed the LPP approach by examining
3
99
Z = 8, Dcalcd = 1.791 mg/m , GOF = 1.062, R1 = 0.0387 [I > 2σ(I)],
wR2 = 0.0981 (all data). Selected crystallographic data for INT3:
C52.5H AlClF N O , triclinic, space group P-1, a = 10.9044(8) Å, b =
2.1572(9) Å, c = 20.2479(15) Å, α = 79.430(2)°, β = 84.882(2)°, γ =
7.124(2)°, V = 2569.0(3) Å , Z = 2, Dcalcd = 1.398 mg/m , GOF =
.049, R1 = 0.0716 [I > 2σ(I)], wR2 = 0.2461 (all data).
the cooperative effects of NHC and other common LBs on the
Al(C F ) -based LPs for the ROP of ε-CL in a fixed 400/1/2 ε-
43
15
2
2
6
5 3
1
7
1
3
3
CL/LB/Al(C
F ) ratio (Table S1 and Scheme S1). Based on
6 5 3
the results of this initial screening, the NHO/Al(C F ) pair
6
5 3
stood out as the most active and effective LP for the ROP of ε-
CL (Scheme 1), thanks to the higher nucleophilicity of NHO
Crystallographic data for the structure of compound 1 (CCDC
513721), Al(C F ) ·CL (CCDC 1513741), and INT3 (CCDC
1
1
6
5 3
513717) have been deposited in the Supporting Information.
General Polymerization Procedures. Polymerizations were
Scheme 1. ROP of δ-VL and ε-CL Mediated by NHO/
performed either in 25 mL flame-dried Schlenk flasks interfaced to
the dual-manifold Schlenk line for runs using external temperature
bath or in 30 mL glass reactors inside the glovebox for ambient
temperature (ca. 25 °C) runs. In a typical polymerization procedure, a
predetermined amount of Al(C F ) ·(toluene) was first dissolved in
Al(C F ) LPs
6
5 3
6
5
3
0.5
monomer (456 μL for δ-VL or 529 μL for ε-CL, 200 equiv relative to
the LB) and toluene inside a glovebox. The polymerization was started
by rapid addition of a solution of a LB (1 equiv of an NHO) in 1.0 mL
of toluene via a gastight syringe to the above mixture containing the
LA and monomer under vigorous stirring. The amount of the
monomer was fixed for all polymerization. After the measured time
interval, a 0.2 mL aliquot was taken from the reaction mixture via
syringe and quickly quenched into a 4 mL vial containing 0.6 mL of
(
due to the highly polarized double bond that places a partial
negative charge on the exocyclic carbon) relative to other LBs
investigated (NHCs, phosphines, etc.). Thus, quantitative
monomer conversion was achieved with the NHO/Al(C F )
undried “wet” CDCl stabilized by 250 ppm of BHT-H; the quenched
6
5 3
3
1
LP (18 h for NHO1 and 3.5 h for NHO2), while much lower
aliquots were later analyzed by H NMR to obtain the percent
monomer conversion data. After the polymerization reaction mixture
was stirred for the stated reaction time, then the polymer was
immediately precipitated into 200 mL of hexane, stirred for 1 h,
filtered, washed with hexane, and dried in a vacuum oven at 50 °C
overnight to a constant weight.
and incomplete monomer conversions (17.0−56.2%) were
observed, even after 24 h with the other LB/Al(C F ) LPs.
6
5 3
Encouraged by these promising initial results, we investigated
the ROP of lactones by NHO/Al(C F ) LPs in more detail. At
6
5 3
the outset, we performed a series of control experiments and
found that Al(C F ) itself only produced a trace amount of
Polymerization Kinetics. Kinetic experiments were carried out in
a stirred glass reactor at ambient temperature (ca. 25 °C) inside an
argon-filled glovebox using the polymerization procedure already
described above, with the [Al(C F ) ]/[NHO2] ratio fixed at 2:1, [ε-
6
5 3
polymer products for the ROP of both ε-CL and δ-VL up to 24
h at ambient temperature (Table 1, runs 1 and 2). Without
6
5 3
CL] was fixed at 1.0 M, where NHO2 = 1.25, 2.5, 5.0, 10.0 mM and
Al(C F ) , NHO1 alone yielded a trace amount of PCL but
6 5 3
0
[
Al(C F ) ] = 2.5, 5.0, 10.0, 20.0 mM in 5 mL mixture solutions. At
effectively polymerized δ-VL, achieving 93.8% δ-VL conversion
after 24 h (Table 1, runs 3−5). On the other hand, NHO2
alone is completely ineffective for the ROP of either ε-CL or δ-
VL, and no polymer products were obtained for up to 24 h at
room temperature (Table 1, runs 6 and 7). Noteworthy here is
that although NHO1 alone is effective for the ROP of δ-VL, the
resulting polymers with different [VL]/[NHO1] ratios all
exhibited a bimodal molecular weight distribution (Table 1,
runs 3 and 4). Most recently, Dove and co-workers used the
alcohol initiator in addition to an NHO, 2-isopropylidene-
1,3,4,5-tetramethylimidazoline, the combined system of which
rapidly converted δ-VL (500 equiv) into PVL but with
6
5 3
appropriate time intervals, 0.2 mL aliquots were withdrawn from the
reaction mixture using syringe and quickly quenched into 4 mL
septum-sealed vials containing 0.6 mL of undried “wet” CDCl mixed
with 250 ppm BHT-H. The quenched aliquots were analyzed by H
3
1
NMR for determining the ratio of [ε-CL] at a given time t to [ε-CL] ,
t
0
[
ε-CL] :[ε-CL] . Apparent rate constants (kapp) were extracted from
t 0
the slopes of the best fit lines to the plots of [ε-CL] :[ε-CL] vs time.
t
0
Another set of kinetic experiments were carried out to determine the
kinetic order with respect to [NHO2]. In these experiments, with
[
fixed at 1.0 M for all polymerization, where NHO2 = 10, 7.5, 5.0, 2.5
mM and [Al(C F ) ] = 12.5, 10, 7.5, 5 mM in 5 mL mixture solutions.
The rest of the procedure was same as the described above.
Al(C F ) ]/[NHO2] ratios of 5:4, 4:3, 3:2, and 2:1, [ε-CL] was
6
5
3
0
6
5 3
relatively low molecular weight of M = 11.4 kg/mol and
n
Polymer Characterizations. The weight-average molar masses
M ) and molar mass distributions (M /M ) of the polymer samples
w w n
broad molecular weight distribution of Đ = 3.29. In the absence
(
of the alcohol initiator, this NHO alone produced PVL with
were determined by gel permeation chromatography (GPC) with a
multiangle light scattering detector at 35 °C. THF (HPLC grade) was
used as an eluent with a flow rate of 1 mL/min. The differential
refractive index (DRI) increment (dn/dc) value of 0.084 mL/g was
used for PVL and 0.076 for PCL. Chromatograms were processed with
Waters Breeze2 software.
12
even higher Đ value of 4.26. In contrast, after introducing
Al(C F ) into the NHO1 system, the M value of the resulting
6
5 3
w
PVL increased linearly from 29.5 to 45.6 to 69.2 kg/mol as the
monomer-to-LP ratio increased from 100 to 200 to 400 (Table
1, runs 8−10). However, the Đ value was still relatively broad
The isolated low-MW polymer samples were analyzed by matrix-
(
1.74−1.77). More pronounced synergetic effects rendered by
assisted laser desorption/ionization time-of-flight mass spectroscopy
the NHO1/Al(C F ) system were observed for the ROP of ε-
6
5 3
(
MALDI-TOF MS); the experiment was performed on a BrukerAuto-
CL. Thus, the monomer conversion was drastically enhanced
flex III mass spectrometer in linear, positive ion mode. The matrix was
from essentially zero in the absence of Al(C F ) to 100% in
6
5 3
trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propylidene]malonitrile
(
the presence of Al(C F ) . With increasing the monomer-to-LP
6
5 3
DCTB), and the solvent was THF.
ratio from 100 to 200 to 400, the Mw value of the
corresponding PCL increased linearly from 33.9 to 65.2 to
103 kg/mol while the Đ value remained in a narrow range of
1.27−1.43 (Table 1, runs 11−13). The relatively broad
molecular weight distribution for both PVL and PCL is
presumably related to the existence of different active species in
RESULTS AND DISCUSSION
■
Characteristics of the ROP of Lactones by NHO/
Al(C F ) LPs. As NHCs alone have been shown to be effective
6
5 3
21,96−98
for the ROP of lactones producing cyclic polyesters,
and
D
DOI: 10.1021/acs.macromol.6b02398
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
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a
Table 1. Selected Polymerization Results by the NHO/Al(C F ) System
6
5 3
b
c
c
d
run no.
LA (equiv)
LB
M
time (h)
conv (%)
Mw (kg/mol)
Mn (kg/mol)
Đ
I* (%)
1
2
3
2
2
0
400VL
400CL
200VL
24
24
8
trace
trace
93.8
NHO1
NHO1
32.5
30.0
71.7
32.3
75.2
1.08
1.09
1.06
1.09
7
8.2
34.2
2.0
4
0
400VL
24
93.8
8
5
6
7
8
9
0
0
0
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
NHO1
NHO2
NHO2
NHO1
NHO1
NHO1
NHO1
NHO1
NHO1
NHO2
NHO2
NHO2
NHO2
NHO2
NHO2
NHO2
NHO2
NHO2
NHO2
400CL
400VL
400CL
100VL
200VL
400VL
100CL
200CL
400CL
100VL
200VL
400VL
800VL
100CL
200CL
400CL
800CL
1600CL
3200CL
24
24
24
6
trace
0
0
92.3
91.2
93.7
100
100
100
93.2
94.4
93.9
94.1
100
100
100
100
100
100
29.5
45.6
69.2
33.9
65.2
17.0
25.8
39.1
23.7
51.3
78.0
12.0
18.5
34.5
59.3
14.3
21.5
51.6
96.2
223
1.74
1.77
1.77
1.43
1.27
1.32
1.08
1.06
1.15
1.09
1.03
1.06
1.02
1.06
1.37
1.63
56
72
12
24
6
1
1
1
1
1
1
1
1
1
1
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
97
49
12
18
1.25
2.5
5
45
103
59
13.0
19.6
39.7
64.6
14.7
22.8
52.6
78
102
110
127
81
10
1
2
106
89
3.5
6
102
95
12
24
306
855
82
525
70
a
b
Carried out at ambient temperature (∼25 °C) in toluene (Tol), where [ε-CL] = 1.0 M and LA = Al(C F ) ·(Tol) . Monomer conversions
0
6
5
3
0.5
1
c
measured by H NMR. Absolute molecular weight (M ) measured by GPC using a light scattering detector. Number-average molecular weight
w
d
(
M ) is calculated from M /Đ. Initiator efficiency (I*) = M (calcd)/M (exptl), where M (calcd) = MW(M) × [M]/[I] × conversion (%) +
n
w
n
n
n
MW(chain-end groups).
the NHO1/Al(C F ) LP system as NHO1 itself is also active
for the polymerization of lactones (vide supra).
PCL M vs monomer conversion at a fixed [ε-CL] /[NHO2] /
6
5 3
n
0
0
2
[Al(C F ) ] ratio of 800:1:2 also gave a straight line (R =
6 5 3 0
To avoid the background polymerization initiated by NHO
itself, NHO2 was chosen as the more desired LB in the LPP of
lactones as the above control experiments showed that it is
inactive by itself for the ROP of lactones. Gratifyingly, the
NHO2/Al(C F ) LP system promoted rapid and controlled
0.993), which was coupled with the small Đ values (Figure
S26). Third, chain extension experiments were also carried out
to provide more direct evidence for the living characteristics of
the polymerization. The PCL with M = 22.8 kg/mol and Đ =
w
1.06 was first prepared by polymerizing 200 equiv of ε-CL to
completion without quenching. The polymerization was
resumed by addition of another 200 equiv of ε-CL, affording
6
5 3
ROP of lactones, yielding polymers with narrow molecular
weight distributions. Runs 14−17 (Table 1) summarized
selected ROP results of δ-VL by NHO2/Al(C F ) in toluene
the resulting PCL with M = 58.6 kg/mol and Đ = 1.07 (Table
6
5 3
w
at room temperature. Relative to the NHO1/Al(C F ) system,
S2, run 1). It is also noted that the corresponding initiator
efficiency (I*%) obtained from the polymerization by NHO2/
Al(C F ) was significantly higher compared to that obtained
6
5 3
the polymerization activity of NHO2/Al(C F ) was 4.8 times
6
5 3
higher (Table 1, run 15 vs run 9) and also produced PVL with
much lower Đ values (1.06−1.15). When switching from δ-VL
to ε-CL (Table 1, runs 18−23), NHO2/Al(C F ) exhibited
6
5 3
by NHO1/Al(C F ) ; for example, the I*% for the ROP of ε-
6
5 3
CL in a 200/1/2 ε-CL/LB/Al(C F ) ratio was increased from
6
5 3
6 5 3
significantly enhanced polymerization activity by 6-fold (Table
, run 19 vs run 12), compared with NHO1/Al(C F ) ,
45% for NHO1 to 106% for NHO2.
1
Characterization of Key Intermediates. To gain more
insights into this living/controlled polymerization by NHO2/
Al(C F ) , we endeavored to isolate and characterize the key
6
5 3
achieved quantitative monomer conversion for all ratios
examined, and produced PCL with very low Đ values of
6
5 3
1
.02−1.06 for [ε-CL]/[LP] = 100−800. Even with a high
intermediates of the reaction. NHO2 alone is inactive for the
polymerization of both δ-VL and ε-CL, but it forms cleanly
stable compounds 1 (NHO2-VL) and 2 (NHO2-CL) with δ-
VL or ε-CL at room temperature, respectively. They were
readily characterized by NMR spectra (see Experimental
Section and Figures S3−S6) to be ring-opened products, and
the molecular structure of 1 was also confirmed by single crystal
X-ray diffraction analysis (Figure 1). From the X-ray diffraction
data, the distance for C(5)−C(6) is 1.423(3) Å and that for
C(6)−C(7) is 1.390(3) Å, which is somewhat between a typical
C−C single bond (∼1.53 Å) and CC double bond (∼1.32
monomer to LP ratio of 3200, quantitative ε-CL conversion
was still achieved and produced PCL with a high M of 855 kg/
mol (Table 1, run 23).
Three lines of key evidence summarized below clearly
revealed the living characteristics of the ROP of lactones by
NHO2/Al(C F ) . First, the M value of PVL or PCL
produced by NHO2/Al(C F ) increased linearly (R = 0.998
and 0.995) with an increase in [M] /[NHO2] /[Al(C F ) ]
w
6
5 3
n
2
6
5 3
0
0
6 5 3 0
ratio from 100:1:2 to 800:1:2 (Figure S25), while the Đ value
remained narrow (from 1.02 to 1.15). Second, a plot of the
E
DOI: 10.1021/acs.macromol.6b02398
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101
group. To determine whether the terminal hydroxy group
initiates the polymerization or not, 1-hydroxynonan-5-one
(
Figure S7) with a terminal hydroxy group was chosen as a
model compound to test its catalytic performance. The addition
of 1 or 2 equiv of Al(C F ) to the model compound yielded
6
5 3
only negligible monomer conversion in 1 h, in contrast to the
quantitative monomer conversion achieved by the compound
1
/2Al(C F ) system under the same condition. This
6 5 3
observation provided strong evidence that the enol-form
oxyanion in compound 1 plays an important role in initiating
the polymerization. It is also noted that 2 equiv of Al(C F ) is
6
5 3
required for the enhanced polymerization activity such that 1
equiv of Al(C F ) is utilized for the activation of the monomer,
6
5 3
Figure 1. X-ray crystal structure of compound 1. Ellipsoids are drawn
at 50% probability.
while the other equiv is reacted with compound 1 to generate
the zwitterionic enolaluminate active species as shown in
Scheme 2. The evidence for the activation of monomer by
Al(C F ) was confirmed by the crystal structure of adduct
Å). It is noted that only one hydrogen atom was found on
C(6); while the distance for C(7)−O(1) is 1.269(2) Å,
between a C−O single bond (1.41−1.44 Å) and CO double
bond (1.19−1.23 Å), no hydrogen atom was found on O(1).
On the other hand, the terminal O(2) carries a hydrogen atom,
transferred from the original exocyclic methylene unit of
NHO2. Overall, the spectroscopic analysis and X-ray diffraction
data confirmed the structure of the ring-opened, H-transferred
product 1, which is best represented as resonance contributors
6
5 3
Al(C F ) ·CL (Figure 2). As revealed from the X-ray diffraction
6
5 3
1
a and 1b (Scheme 2). The clean formation of 1 upon mixing
NHO2 with δ-VL can be rationalized via fundamental steps
involving initial nucleophilic attack of the δ-VL carbonyl group
by NHO2, followed by the ring-opening of the lactone to form
a zwitterionic intermediate, in which the acidic H atom on the
exocyclic methylene unit of NHO2 is subsequently transferred
to the terminal alkoxy anion (or oxyanion) to generate
compound 1.
Isolation, characterization, and reactivity check of compound
proved instructive for a mechanistic understanding of this
1
LPP system. Ring-opened product 1 itself does not initiate the
polymerization since it is not nucleophilic enough to attack
monomer. In fact, the formation of ring-opened product 1 was
proposed as a possible deactivation pathway in the ROP of
Figure 2. X-ray crystal structure of adduct Al(C F ) ·CL. Hydrogen
atoms are omitted for clarity, and ellipsoids are drawn at 50%
probability.
6
5 3
12
lactones by NHO. However, addition of 2 equiv of Al(C F )
6
5 3
to 1 rendered a highly active ROP system, consuming all of the
monomer in 1 h, which is even considerably more active (by
data, a coordinated bond is formed between Al(1) and O(1)
(1.8327(13) Å). The distance for C(1)−O(1) (1.252(2) Å)
and C(1)−O(2) (1.294(2) Å) is between a C−O single bond
(1.41−1.44 Å) and a CO double bond (1.19−1.23 Å).
Next, in situ NMR reaction of NHO2 and adduct Al(C F ) ·
>
3×) than the system with direct mixing of NHO2/Al(C F )
6 5 3
in a 1:2 ratio. These results implied that the formation of ring-
opened product 1 is a prerequisite for the formation of active
species in such ROP by LPs. There are two possible oxygen
sites in compound 1 that could initiate the polymerization in
the presence of Al(C F ) : one is from the enol-form
6
5 3
CL was carried out to investigate the synergetic effects of
NHO2 and Al(C F ) ; a complicated NMR spectrum was
6
5 3
obtained. Replacing ε-CL with menthide (MI), a clean,
zwitterionic, tetrahedral intermediate INT1 is generated
6
5 3
1
00
oxyanion, and the other one is from the terminal hydroxy
Scheme 2. Formation of Compound 1 and Its Derived Active Zwitterionic Enolaluminate
F
DOI: 10.1021/acs.macromol.6b02398
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Macromolecules
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Scheme 3. Formation of Intermediate INT1 and Ring-Opened Intermediate INT2
(
Scheme 3). Adding another equiv of Al(C F ) , a ring-opened,
bond, which indicated the activation of the C−O bond.
Previously, Waymouth et al. predicted the formation of such a
zwitterionic tetrahedral intermediate in the NHC mediated
6
5 3
zwitterionic intermediate INT2 was formed (Scheme 3). The
formation of INT2 can be reasoned by following the similar
ring-opening, proton transfer, and enolaluminate formation
reaction sequences as outlined in Scheme 2, followed by the
terminal hydroxyl group coordinated to Al(C F ) . INT1
102,103
ZROP of lactones with the aid of computational studies,
but the isolation and/or characterization of such a tetrahedral
intermediate was not accomplished until this work. The
sufficient stability of INT3 that enabled our isolation, and
characterization can be ascribed to the stabilization of
Al(C F ) on the oxyanion. INT3 itself only converted 9.3%
6
5 3
(
Figures S16−S18) and INT2 (Figures S19−S21) have been
successfully isolated and characterized. INT2 could polymerize
9.1% of ε-CL (400 equiv) into PCL in 10 h, which clearly
9
6
5 3
indicated both INT1 and INT2 are the key intermediates for
the ROP of ε-CL. Attempts to get the crystal structures of
INT1 and INT2 were not successful.
of ε-CL into PCL in 14 h. However, adding 1 equiv of
Al(C F ) to the isolated INT3 enhanced the polymerization
6
5 3
activity drastically, thus achieving 100% monomer conversion
in 8 h. These results indicated that INT3 is an active
intermediate for the ROP of ε-CL, which can be ring-opened
to form the zwitterionic species that attacks the incoming
Al(C F ) -activated monomer to proceed the polymerization
Switching the NHO2 to NHO1 enabled us to generate the
corresponding clean, zwitterionic, tetrahedral intermediate
(
INT3) from the in situ NMR reaction of NHO1, Al(C F ) ,
6 5 3
and ε-CL (Scheme 4). More importantly, we have successfully
6
5 3
19
(
vide infra). The F NMR spectrum of INT3 (Figure S23)
Scheme 4. Formation of Intermediate INT3
closely resembled that of INT1 (Figure S17); therefore, the
structure of zwitterionic tetrahedral INT1 was also indirectly
verified by its analogue INT3.
Kinetics and Mechanism of Polymerization. To
continue the investigation of the mechanistic aspects of the
polymerization, we next examined the kinetics of the ROP of ε-
CL by the NHO2/Al(C F ) system. The kinetic experiments
6
5 3
employed [CL] /[NHO2] with varied ratio of 100, 200, 400,
0
0
and 800 and a fixed [Al(C F ) ] /[NHO2] ratio of 2:1. As can
6
5 3 0
0
isolated and characterized INT3 both spectroscopically by
NMR and structurally by X-ray diffraction analysis (Figure 3).
The diffraction data revealed the formation of a single bond
between C(7) and C(8) [1.534(4) Å], from the previous CC
double bond, and between C(7) and C(1) [1.578(4) Å], for
the newly formed σ bond between NHO1 and ε-CL. It is also
noted the newly formed C−O single bond between C(1) and
O(1) (1.381(3) Å) in INT3 from the previous CO carbonyl
be seen from the representative kinetic plots of [ε-CL] /[ε-
t
CL] vs time, the polymerization clearly followed zero-order
0
kinetics with respect to [ε-CL] concentration for all the ratios
employed. Since the [Al(C F ) ]/[NHO2] ratio was kept at
6
5 3
2:1, the combined kinetic order of [Al(C F ) ] and [NHO2]
6
5 3
can be obtained from the double-logarithm plot (Figure 4) of
the apparent rate constants (K ) obtained from the slopes of
app
the best-fit lines to the plots of [ε-CL] /[ε-CL] vs time as a
t
0
Figure 4. Zero-order kinetic plots for ROP of ε-CL by NHO2/
Al(C F ) in toluene at 25 °C: [ε-CL]0 = 1.0 M; [Alane]
Figure 3. X-ray crystal structure of zwitterionic, tetrahedral
intermediate INT3. Solvent molecules and hydrogen atoms are
omitted for clarity, and ellipsoids are drawn at 50% probability.
=
0
6
5
3
2[NHO2] ; [NHO2] = 0.01 (△), 0.005 (▲), 0.0025 (□), 0.00125 M
0
0
(■). Inset: plot of ln(kapp) vs ln[NHO2].
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function of ln[NHO2], which was fit to a straight line (R =
Article
2
propagation is first order in [NHO2] concentration. In
combination with the first set of kinetic results, the propagation
is zero-order dependence on [Al(C F ) ] concentration, which
0
.999) with a slope of 0.991. Thus, the combined kinetic order
with respect to the [NHO2]/[Al(C F ) ] LP, given by the
6
5 3
6 5 3
slopes of ∼1, reveals that the propagation is first-order in the
concentration of the LP.
thus highlighted that the rate-determining step for the ROP of
ε-CL catalyzed by NHO2/Al(C F ) is only dependent on the
6
5 3
In the second set of kinetic experiments, with a fixed amount
of [ε-CL] , the [Al(C F ) ]/[NHO2] ratio was varied at 5:4,
concentration of [NHO2], not on [Al(C F ) ] and [ε-CL].
6 5 3
Overall, the above kinetic results, coupled with mechanistic
insights obtained through monitoring the polymerization and
identification or characterization of the reaction intermediates
(vide supra), led to a proposed polymerization mechanism for
the ROP of lactones by the [NHO2]/[Al(C F ) ] LP (Scheme
0
6 5 3
4
:3, 3:2, and 2:1 such that at each ratio there was 1 equiv of
Al(C F ) left to activate the monomer upon formation of the
6
5 3
active intermediate that consumes equimolar NHO2 and
Al(C F ) . The same zero-order dependence was observed
6
5 3
6 5 3
for all the ratios investigated in this study. A double-logarithm
5). In this mechanism, initiation involves nucleophilic attack of
plot (Figure 5) of the apparent rate constants (k ), obtained
Al(C F ) -activated monomer by NHO2 to form zwitterionic,
app
6 5 3
tetrahedral intermediate INT4, followed by its ring-opening to
generate active zwitterionic enolaluminate species INT5. In the
propagation cycle, INT5 and its homologues INT6 attack the
incoming Al(C F ) -activated monomer to generate zwitter-
6
5 3
ionic, tetrahedral intermediate INT7, which is then ring-opened
to re-form INT6, entering into the next chain propagation
catalytic cycle. Kinetic studies indicated that the rate-
determining step is only dependent on the concentration of
[NHO2]. As shown in Scheme 5, only the ring-opening of
INT7 to regenerate INT6 does not rely on the concentration
of [Al(C F ) ] and [monomer], and therefore this ring-opening
6
5 3
process is the rate-determining step and also required for each
propagation cycle during the polymerization.
The low-MW oligomer produced by the NHO2/Al(C F )
6
5 3
system was analyzed by MALDI-TOF mass spectroscopy. The
results clearly showed one series of molecular mass ions
(Figures 6 and 7). A plot of m/z values of this series vs the
number of monomer repeat units (n) yielded a straight line
with a slope of 100.12 (mass of δ-VL) or 114 (mass of ε-CL)
and intercept of 211 or 225 corresponding to the sum of
NHO2, monomer (δ-VL or ε-CL) and H moieties,
respectively, indicating PVL or PCL produced in the polymer-
ization is a linear polymer. The imdazolium enol-form chain-end
group formed by the NHO2 and monomer is consistent with
Figure 5. Zero-order kinetic plots for ROP of ε-CL by NHO2/
Al(C F ) in toluene at 25 °C: [ε-CL] = 1.0 M; [NHO2] = 0.01
6
5
3
0
0
(
0
△), 0.0075 (▲), 0.005 (□), and 0.0025 M (■); [Al(C F ) ] =
6 5 3 0
.0125 (△), 0.01 (▲), 0.0075 (□), and 0.005 M (■). Inset: plot of
ln(kapp) vs ln[NHO2].
from the slopes of the best-fit lines to the plots of [ε-CL] /[ε-
t
CL] vs time, as a function of ln[NHO2] was fit to a straight
0
2
line (R = 0.996) with a slope of 0.963, revealing that the
Scheme 5. Proposed Mechanism for ROP of δ-VL or ε-CL by NHO2/Al(C F ) Lewis Pairs
6
5 3
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Article
Figure 6. MALDI-TOF MS analysis for the resulting PVL produced by NHO2/Al(C F ) in toluene at room temperature.
6
5 3
Figure 7. MALDI-TOF MS analysis for the resulting PCL produced by NHO2/Al(C F ) in toluene at room temperature.
6
5 3
that proposed in the mechanism (Scheme 5). Furthermore,
quantitative 13C NMR spectra (Figure 8C), the equal integral
of each type of methylene resonance within the copolymer
[VL−CL (0.23), CL−CL (0.27), VL−VL (0.25), CL−VL
(0.25)] is characteristic of a random copolymer. On the other
hand, sequential block copolymerization by polymerizing ε-CL
such imdazolium enol-form chain-end was also confirmed by
1
H NMR spectra (Figure S27), as evidenced by the resonances
at δ 5.47 [NCHCHN], 4.37 [OCCH], and 2.94 ppm
[
NCH ], which are similar to those observed in compound 1 (δ
3
5
.38 [NCHCHN], 4.30 [OCCH], and 2.82 ppm
first with [ε-CL] /[NHO2] /[Al(C F ) ] = 200/1/2 without
0 0 6 5 3 0
[
[
NCH ]) and in compound 2 (δ 5.38 [NCHCHN], 4.36
quenching, followed by addition of another 200 equiv of δ-VL,
afforded successfully linear diblock copolymer PCL-b-PVL
3
OCCH], and 2.86 ppm [NCH ]), respectively.
3
13
Random and Block Copolymerizations. The living
(Table S2, run 3). As shown in C NMR spectra (Figure 8d),
the methylene signals for the resulting PCL-b-PVL have two
equal integrals, corresponding to the homodyads CL−CL and
VL−VL, and no resonances for the heterodyads VL−CL and
CL−VL. This implies the formation of the well-defined diblock
copolymer, with no occurrence of transesterification side
reactions, which is rather difficult to achieve as commonly
features of this ROP by LPs, as demonstrated by the above-
described chain-extension experiments and mechanistic studies,
also enabled the synthesis of the well-defined block copolymers.
When both monomers were added at the same time,
copolymerization of δ-VL and ε-CL produced randomly
sequenced copolymers (Table S2, run 2). As indicated by
I
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Article
Figure 8. 13C NMR spectra (a) PCL, (b) PVL, (c) linear random PCL-co-PVL, and (d) linear diblock PCL-b-PVL, all produced by NHO2/
Al(C F ) in toluene at room temperature. CL−VL denotes the methylene signal of the δ-VL unit is connected to a CL.
6
5 3
random copolymers with broad dispersities were ob-
underway. On the other hand, through the conventional
sequential monomer addition method for triblock copolymers,
copolymer PCL-b-PVL-b-PCL was also successfully prepared
1
04−113
tained,
due to inter- and intramolecular transesterifica-
tion side reactions.
GPC traces (Figure 9a) provided further evidence for the
well-defined block copolymer formation with NHO2/Al-
with a M value of 72.1 kg/mol (Figure 9a, blue trace) and a Đ
w
value of 1.15 (Table S2, run 5). Therefore, the NHO2/
Al(C F ) LP successfully prevented any transesterification side
6
5 3
reaction and yield well-defined block copolymers, regardless of
the monomer addition order.
CONCLUSIONS
■
Although LPP has been extended to include several classes of
monomers, this work reveals the first LPP system for the living
ROP of lactones and also reports the first comprehensive
mechanistic study of such polymerization including examina-
tion of polymerization characteristics, isolation and structural
characterization of key reaction intermediates, polymerization
kinetics, and analysis of resulting polymer chain structures.
Using NHO/Al(C F ) LPs for the ROP of lactones
6
5 3
including δ-VL and ε-CL in this study, we showed that while
the individual LA or LB exhibited none to negligible activity for
the polymerization or produced polymer products with bimodal
molecular weight distributions, a combination of the two in the
form of a LP mediates the living/controlled ROP of lactones to
produce high molecular weight linear (co)polyesters with Mw
up to 855 kg/mol and narrow molecular weight distributions,
highlighting the unique cooperative and synergistic effects of
the LP in the polymerization. The livingness of the ROP has
been confirmed with multiple protocols, including observations
Figure 9. (a) GPC traces for PCL (black), PCL-b-PVL (red), and
PCL-b-PVL-b-PCL (blue). (b) GPC traces for PVL (black) and PVL-
b-PCL (red) produced by NHO2/Al(C F ) in toluene at room
6
5 3
temperature.
(
C F ) in toluene at room temperature. As can be seen from
6 5 3
Figure 9a, the GPC trace for the PCL produced during the first
ROP shifted to a higher molecular weight region with low
dispersity value of Đ = 1.06−1.09, while the Mw values
increased from 22.8 kg/mol (black trace) for the homopolymer
PCL to 44.6 kg/mol (red trace) for the diblock PCL-b-PVL
produced. In addition, we employed δ-VL as the primary
monomer and ε-CL as the subsequent monomer for the
copolymerization and obtained similar results (Figure 9b and
Table S2, run 4). Worth noting here is the possibility of the
rapidly exchanging Al(C F ) moiety between the two terminal
of the linear increase of the polymer M vs time or conversion
n
and monomer-to-LP ratio while maintaining low Đ values
(from 1.02 to 1.15), no induction or chain termination events
by kinetics, successful chain extension experiments, and the
synthesis of the well-defined di- and triblock copolymers with
narrow molecular weight distribution (Đ = 1.06−1.15),
regardless of the comonomer addition order and without
transesterification side reactions.
6
5 3
alkoxy and alcohol sites (cf. INT6 in Scheme 5), which would
lead to a possible triblock copolymer structure. The validation
of this possibility requires extensive future studies that are
Several key intermediates directly relevant to the ROP have
been successfully isolated and characterized spectroscopically
and structurally, including LA−monomer adduct (i.e., activated
J
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Article
monomer) Al(C F ) ·CL, the LB-monomer reaction product
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5 3
(
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ionic, tetrahedral intermediate (i.e., pre-ring-opening species)
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polymerization kinetics revealed that ROP of lactones catalyzed
(
by NHO2/Al(C F ) is zero-order with respect to both
6
5 3
monomer and Al(C F ) concentrations but first-order to
6
5 3
2
(
NHO2 concentration. Taken in total, the polymerization is
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the zwitterionic, tetrahedral intermediates, followed by its ring-
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5 3
the rate-determining ring-opening step to regenerate the
zwitterionic species, thus re-entering into the next chain
propagation cycle. NMR and MALDI-TOF MS analyses of
the obtained low molecular weight oligomers confirmed the
resulting linear polymer structure with predicted chain-end
groups.
(11) Zhang, X. Y.; Jones, G. O.; Hedrick, J. L.; Waymouth, R. M. Fast
and Selective Ring-Opening Polymerizations by Alkoxides and
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ASSOCIATED CONTENT
Supporting Information
■
M.; Dubois, P. Metal-Free Catalyzed Ring-Opening Polymerization of
β-Lactones: Synthesis of Amphiphilic Triblock Copolymers Based on
Poly(dimethylmalic acid). Macromolecules 2006, 39, 4001−4008.
*
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.6b02398.
(14) Nyce, G. W.; Glauser, T.; Connor, E. F.; Mock, A.; Waymouth,
R. M.; Hedrick, J. L. In Situ Generation of Carbenes: A General and
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NMR spectra, crystal data, bond lengths and angles, and
further tabular data (PDF)
Crystallographic data for the structure of compound 1
(15) Fevre, M.; Pinaud, J.; Gnanou, Y.; Vignolle, J.; Taton, D. N-
Heterocyclic Carbenes (NHCs) as Organocatalysts and Structural
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(
CIF)
Crystallographic data for the structure of Al(C F ) ·CL
6
5 3
(
16) Naumann, S.; Dove, A. P. N-Heterocyclic Carbenes as
(
CIF)
Organocatalysts for Polymerizations: Trends and Frontiers. Polym.
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AUTHOR INFORMATION
Corresponding Author
E-mail: ytzhang2009@jlu.edu.cn (Y.Z.).
■
Free Polymerization Catalysis: An Update. Polym. Int. 2016, 65, 16−
27.
*
(18) Culkin, D. A.; Jeong, W.; Csihony, S.; Gomez, E. D.; Balsara, N.
P.; Hedrick, J. L.; Waymouth, R. M. Zwitterionic Polymerization of
Lactide to Cyclic Poly(Lactide) by Using N-Heterocyclic Carbene
Organocatalysts. Angew. Chem., Int. Ed. 2007, 46, 2627−2630.
(19) Kiesewetter, M. K.; Shin, E. J.; Hedrick, J. L.; Waymouth, R. M.
Organocatalysis: Opportunities and Challenges for Polymer Synthesis.
Macromolecules 2010, 43, 2093−2107.
ORCID
Yuetao Zhang: 0000-0002-6406-1959
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(20) Shin, E. J.; Brown, H. A.; Gonzalez, S.; Jeong, W.; Hedrick, J. L.;
Waymouth, R. M. Zwitterionic Copolymerization: Synthesis of Cyclic
Gradient Copolymers. Angew. Chem., Int. Ed. 2011, 50, 6388−6391.
■
This work was supported by the National Natural Science
Foundation of China (Grant No. 21374040, 21422401), 1000
Young Talent Plan of China funds, the startup funds from Jilin
University to Y.Z., and by the US National Science Foundation
(21) Brown, H. A.; Waymouth, R. M. Zwitterionic Ring-Opening
Polymerization for the Synthesis of High Molecular Weight Cyclic
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