Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides

Article abstract of DOI:10.1021/jacs.7b01462

Poly(α-hydroxy acids) are important biodegradable polymers with wide applications. Attempts to synthesize them from O-carboxyanhydrides with pendant functional groups by various methods, including methods involving organocatalysts or organometallics, have been plagued by uncontrolled polymerization, including epimerization for some monomers, which hampers the preparation of stereoregular high-molecular-weight polymers. Herein we describe an effective protocol that combines photoredox Ni/Ir catalysis with the use of a Zn-alkoxide for efficient ring-opening polymerization, allowing for the synthesis of isotactic polyesters with expected molecular weights (>140 kDa) and narrow molecular weight distributions (M_w/M_n < 1.1). Mechanistic studies indicate that the use of a low temperature (?20 °C) and photoredox Ni/Ir catalysis synergistically accelerates ring-opening and decarboxylation of the monomer for chain propagation while avoiding the formation of the undesired Ni-carbonyl complex.

Full text of DOI:10.1021/jacs.7b01462

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Article
Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides
Quanyou Feng, and Rong Tong
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01462 • Publication Date (Web): 11 Apr 2017
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Controlled Photoredox Ring-Opening Polymerization of O-
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Quanyou Feng,†, ‡ Rong Tong *, ‡
† Center for Molecular Systems and Organic Devices, Key Laboratory for Organic Electronics and Information Displays & Institute of
Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing University of Posts and Teleꢀ
communications, Nanjing, 210023, China
‡ Department of Chemical Engineering, Virginia Polytechnic Institute and State University, 635 Prices Fork Road, Blacksburg, Virginꢀ
ia, 24061, United States
ABSTRACT: Poly(αꢀhydroxy acids) are important biodegradable polymers with wide applications. Attempts to synthesize them from
Oꢀ
carboxyanhydrides with pendent functional groups by various methods, including methods involving organocatalysts or organometallics,
have been plagued by uncontrolled polymerization, including epimerization for some monomers, which hampers the preparation of stereꢀ
oregular highꢀmolecularꢀweight polymers. Herein we describe an effective protocol that combines photoredox Ni/Ir catalysis with the use of a
Znꢀalkoxide for efficient ringꢀopening polymerization, allowing for the synthesis of isotactic polyesters with expected molecular weights
(>140 kDa) and narrow molecular weight distributions (Mw/Mn < 1.1). Mechanistic studies indicate that the use of a low temperature (–20
°C) and photoredox Ni/Ir catalysis synergistically accelerates ringꢀopening and decarboxylation of the monomer for chain propagation while
avoiding the formation of the undesired Niꢀcarbonyl complex.
INTRODUCTION
carboxyanhydrides, structural analogues of OCAs (Figure 1c).11
Historically, nickel catalysts have been used to mediate the crossꢀ
coupling of redoxꢀactive carboxylic acids12or anhydrides13 with
organozinc agents (Figure 1c). On the basis of these previously
reported results, we hypothesized that a Ni/photoredox catalyst
system—to accomplish OCA decarboxylation—could be merged
with a Zn catalyst to mediate ring opening of OCAs in a manner
similar to that observed for anhydrides,10b generating an active Znꢀ
alkoxide chain end for chain propagation, thus enabling controlled
synthesis of highꢀMW polymers (Figure 1d).
Poly(αꢀhydroxy acids) (PAHAs), including polylactide, are
widely used in everyday applications ranging from clothing and
packaging to agriculture and biomedicine.1 However, their utility
for applications that demand physicochemical properties such as
stiffness, ductility, and tensile strength is greatly limited by the lack
of sideꢀchain functionality in PAHAs and in their monomers (lacꢀ
tones),2 which are usually synthesized by multistep methods that
are difficult to scale up.3 Since 2006,
Oꢀcarboxyanhydrides (OCAs),
which can be prepared with a rich variety of sideꢀchain functionaliꢀ
ties,4 have emerged as an alternative class of highly active monoꢀ
mers for polyester polymerization (Figure 1a).4a, 5 Unfortunately,
current synthetic methods, especially those involving organocataꢀ
lysts, result in uncontrolled polymerization including epimerization
(i.e., lack of stereoregularity) for OCAs bearing electronꢀ
withdrawing groups,4c, 6 unpredictable molecular weights (MWs),
or slow polymerization kinetics (Table S1).5a, 7 Recently, we reportꢀ
ed an active organometallic complex, (BDIꢀIE)Znꢀ
OCH(Me)COOMe (BDIꢀIE = 2ꢀ((2,6ꢀdiethylphenyl)amino)ꢀ4ꢀ
((2,6ꢀdiisopropylphenyl)imino)ꢀ2ꢀpentene), that catalyzes wellꢀ
controlled ringꢀopening polymerization (ROP) of OCAs at a low
degree of polymerization (DP ≤ 200) without epimerization (Figꢀ
ure 1b).7 However, this Znꢀalkoxide catalyst cannot efficiently proꢀ
duce polymers with a high degree of polymerization (DP ≥ 300,
Table S1). Because of the aboveꢀdescribed limitations, the develꢀ
opment of a viable catalyst that can mediate controlled polymerizaꢀ
tion to prepare stereoregular, highꢀMW PAHAs is crucial.
RESULTS AND DISCUSSION
Optimization of photoredox polymerization conditions. With
this hypothesis in mind, we began by investigating the ROP of
PheꢀOCA ( ꢀ1, Table 1), which was readily prepared in two steps
from commercially available ꢀphenylalanine on a multigram
Lꢀ
L
L
scale.4f We discovered that within 4 h at room temperature, the
combination of (bpyꢀ1)Ni(COD) (bpyꢀ1 = 2,2 ꢀbipyridyl, COD =
1,5ꢀcyclooctadiene), Zn(HMDS)2 (HMDS = hexamethyldisilazꢀ
ane), and Ir[dF(CF3)ppy]2(dtbbpy)PF6 (Irꢀ1, dF(CF3)ppy = 2ꢀ
(2,4ꢀdifluorophenyl)ꢀ5ꢀ(trifluoromethyl)pyridine, dtbbpy = 4,4 ꢀ
diꢀtertꢀbutylꢀ2,2 ꢀbipyridine) in the presence of blue LEDs (400–
500 nm) and benzyl alcohol (BnOH) as the initiator resulted in
complete monomer consumption and generation of a product with
a numberꢀaverage MW (Mn) of 39.2 kDa and a MW distribution
(
Ð
=
Mw
300/1/1/1/0.1; entry 1). However, at a higher monomer feed ratio
([ ꢀ1]/[Ni]/[Zn]/[BnOH]/[Irꢀ1] = 500/1/1/1/0.1), unconꢀ
trolled polymerization occurred, affording a polymer with a Mn of
38.1 kDa and a broad of 1.71 (entry 2).
/Mn) of 1.10 ([Lꢀ1]/[Ni]/[Zn]/[BnOH]/[Irꢀ1] =
L
Visibleꢀlight photoredox catalysis has recently emerged as a
powerful technique for radical polymerization8 and organic syntheꢀ
sis.9 Photoredox catalysts and nickel catalysts reportedly can be
successfully combined for the decarboxylation of carboxylic acids
and anhydrides via a catalytic cycle involving singleꢀelectron transꢀ
fer (Figure 1c).10 Nickel catalysts also mediate the ROP of
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(Table S2, entries 3 and 5). When the reaction was carried out
without BnOH, the obtained polymer had uncontrolled Mn and
1
Ð
2
3
4
5
6
7
8
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values (entry 8). Additionally, electrospray ionization mass specꢀ
trometry (ESIꢀMS) analysis of oligo(1) at a DP of 5 confirmed the
attachment of the BnO group to the oligomer (Figure 2d; high
resolution ESIꢀMS results in Figure S2), suggesting that a Znꢀ
alkoxide was involved in ringꢀopening and chain propagation. Addiꢀ
tionally, oligo(1) without the attachment of BnO group was not
observed in ESIꢀMS spectrum. To verify the incorporation of the
alcohol initiator in the polymerization, 1ꢀpyrenebutanol was used
to initiate photoredox oligomerization of 1 (DP=15). The incorpoꢀ
ration efficiency was over 95% determined by HPLC (Figure S4),
confirming the necessity of Znꢀalkoxide for efficient initiation.
Moreover, either incomplete monomer conversion or uncontrolled
polymerization occurred in the absence of Irꢀ1 (entry 9), light (enꢀ
try 10), or both (Table S2, entries 8 and 9). We also found that
monomer conversion was only 39% when TEMPO, a radicalꢀ
trapping agent,15 was present in the reaction mixture (Table S2,
entry 10), indicating the indispensability of photoinduced electron
transfer in this photoredox polymerization. Finally, when LED light
was replaced with sunlight, the reaction still proceeded smoothly to
completion within 4 h (Table 1, entry 11).
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Effects of Ni and Zn complex ligands on the polymerization.
During the optimization process, we also investigated the effects of
the Ni and Zn ligands (Tables S3 and S4). We found that the subꢀ
stituents on the bpy ligand of the Ni complex did not markedly
affect the polymerization results (Table S3, entries 1–3) and that
bpy, phenanthroline, and PCy3 (PCy3 = tricyclohexylphosphine)
were better ligands than PPh3 (PPh3 = triphenylphosphine) (enꢀ
tries 1 and 4–6) for the polymerization without irradiation at room
temperature; a Ni(0) complex with a PPh3 ligand gave a polymer
with an extremely high Mn (entry 5). The weaker NiꢀPPh3 bond
strength, compared to that of very basic PCy3 ligand,16 may affect
the polymerization results. The use of Ni(II) complexes failed to
afford polymers (entries 7 and 8). We tested ligands that worked at
Figure 1. Photoredox ringꢀopening polymerization of
Oꢀ
carboxyanhydrides (OCAs). (a) Synthetic routes to poly(αꢀhydroxy
acid). (b) Previously reported catalysts for ringꢀopening polymerizaꢀ
tion of OCAs. (c) Reported uses of Ni/Zn coꢀcatalysts for reactions of
anhydrides, and Niꢀmediated decarbonylation as a potential side reacꢀ
tion. (d) Photoredox ringꢀopening polymerization reported in this
paper.
Surprisingly, optimization of other reaction parameters revealed
that excellent polymerization control was achieved by carrying out
the reaction in a cooling bath at –15 °C for 6 h (Table 1, entry 3);
these conditions afforded a polymer with a Mn of 78.4 kDa, which is
o
room temperature in the photoredox polymerization at ꢀ15 C.
The use of Ni complexes with bpyꢀ2 or phenanthroline ligands
results in much higher MW than expected (entries 9ꢀ10). We also
found that sterically bulky BDIꢀZn complexes did not initiate conꢀ
trolled polymerization at a DP of 300 (Table S4, entries 1–10),
even using photoredox ROP conditions (entry 13); whereas a less
bulky tridentate ligand did (entries 11 and 12).17
close to the expected MW (74.1 kDa), and a narrow
Ð of 1.05.
During the polymerization, the reaction solution turned from orꢀ
ange to light purple, suggesting regeneration of a Ni(0) complex;
(bpyꢀ1)Ni(COD) is a deep purple color (Figure S1). Under these
lowꢀtemperature conditions, Mn of the poly(
linearly with initial ꢀ1]/[Ni]/[Zn]/[Irꢀ1] ratio up to
1000/1/1/0.1, and all of the values were <1.1 (Figure 2a and b),
which indicates that chainꢀbreaking reactions did not occur during
the ROP.14 Notably, even for highꢀMW poly(
ꢀ1) (entry 4, Mn
Lꢀ1) product increased
[
L
Homopolymerization and copolymerization of various OCAs.
To evaluate the generality of our photoredox Ni/Zn catalytic sysꢀ
tem, we carried out ROP reactions of three other OCA monomers
(2–4; Table 2, entries 1–3). In all cases, polymers were obtained
Ð
L
=
140.5 kDa), no epimerization of the αꢀmethine hydrogen was obꢀ
with excellent polymerization control, similar to that for poly(
Lꢀ1);
1
served in the homodecoupling H NMR spectrum (Figure 2c),
all the Mn values were close to the calculated MWs, the values
Ð
indicating that the Ni and Zn complexes did not affect OCA chiraliꢀ
ty during the ROP.
were <1.1, and epimerization at the αꢀmethine hydrogen did not
occur (Figure S5–S7). Diblock and triblock copolymers could also
be readily prepared in one pot by sequential addition of the monoꢀ
We demonstrated the necessity of each of the catalytic compoꢀ
nents by means of a series of control experiments (Table 1, comꢀ
pare entry 5 with entries 6–11; Table S2). At a high DP of 500,
neither (bpyꢀ1)Ni(COD) alone (Table S2, entry 3) nor
Zn(HMDS)2 combined with BnOH (entry 4) was able to initiate
polymerization. Without the Ni complex the polymerization was
uncontrolled, whereas the Zn complex was essential for chain
propagation (Table 1, entries 6 and 7, respectively). Either (bpyꢀ
1)Ni(COD) or Zn(HMDS)2 could initiate polymerization but in
an uncontrolled manner with incomplete monomer conversion
mers, and remarkable control of the Mn and
Ð values was achieved
(entries 4–11; Figure 2e). We also found the random photoredox
copolymerization of 1 and 3 (200/200) resulted in complete monꢀ
omers conversion and generation of a randomꢀsequenced copolyꢀ
mer with a Mn of 76.8 kDa and a
MW (73.7 kDa, Figure S8).
Ð of 1.05, close to the expected
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Figure 2. Controlled photoredox ringꢀopening polymerization of OCAs. (a) Plots of Mn and molecular weight distribution (Mw/Mn) of poly(Lꢀ
1) versus [
chromatography (GPC) traces of the photoredox polymerization reaction in panel (a). (c) Homodecoupling H NMR spectrum of the αꢀmethine
region of poly( ꢀ1) with a degree of polymerization (DP) of 1000. (d) Electrospray ionization mass spectrum (ESIꢀMS) of oligo( ꢀ1)5 formed by
BnOHꢀinitiated polymerization (high resolution ESIꢀMS results in Figure S2). (e) Representative GPC traces of poly( ꢀ1)300 and the block copolyꢀ
mer poly( ꢀ1)300 ꢀpoly( ꢀ2)300 (Table 2, entry 4). (f) Plots of ꢀ1 conversion versus time at various Zn(HMDS)2 concentrations, showing firstꢀorder
kinetics. [ ꢀ1] = 81.1 mM; [(bpyꢀ1)Ni(COD)]/[Zn(HMDS)2]/[BnOH]/[Irꢀ1] = 1/1/1/0.1.
Lꢀ1]/[Zn(HMDS)2] ratio ([(bpyꢀ1)Ni(COD)]/[Zn(HMDS)2]/[BnOH]/[Irꢀ1] = 1/1/1/0.1). (b) Representative gelꢀpermeation
1
L
L
L
L
ꢀb
L
L
L
Kinetics. We then examined the kinetics of the photoredox ROP
ꢀ1 by varying the concentration of each reaction component
and monitoring the monomer conversion by Fourier transform
infrared (FTIR) spectroscopy.7, 18 The reaction was first order with
where kp is the rate constant for chain propagation.
of
L
Mechanistic studies. The success of the photoredox Ni/Zn cataꢀ
lytic system prompted us to explore the mechanism of the reaction.
The ringꢀopening reaction of 1 was examined by means of lowꢀ
temperature NMR analysis of a mixture of (PCy3)2Ni(COD),
Zn(HMDS)2, BnOH, and 1 (1 equiv. of each) with Irꢀ1 (0.1 equiv.)
under irradiation with light at –20 °C; we used the PCy3 ligand so
that we could elucidate the intermediate Ni complexes. No
(PCy3)2Ni(CO) or (PCy3)2Ni(CO2) was observed in the 31P NMR
respect to Lꢀ1 (Figure 2f) and zero order with respect to BnOH and
Irꢀ1, whereas the reaction orders with respect to (bpyꢀ1)Ni(COD)
and Zn(HMDS)2 were 1.84 ± 0.26 and 2.49 ± 0.27, respectively
(Figure S9). These results indicate that the rate of chain propagaꢀ
tion was independent of both BnOH and Irꢀ1. The propagation
rate independence of BnOH indicates that BnOH only involved in
the initiation but not chain propagation. Similarly Irꢀ1 possessing
longꢀlived excited state likely only influenced the Ni complex oxidaꢀ
tion state via single electron transfer cycle,10 not affecting the chain
end reactivity. The fractional reaction orders for the Ni and Zn
complexes were presumably due to the formation of separate agꢀ
gregates of each complex, which was also observed in the ROP of
lactide by Zn or Al complexes.18ꢀ19 To explore whether Ni and Zn
catalysts formed a coordinate complex to mediate the polymerizaꢀ
tion, we fixed the Ni/Zn ratio at 1/1 in the kinetic study, and found
the reaction order was 4.66 ± 0.26 (Figure S9b). The value is close
to the sum of the individual reaction order of Zn(HMDS)2 (2.49 ±
0.27 in Figure S9d) and (bpyꢀ1)Ni (1.84 ± 0.26 in Figure S9f),
suggesting no formation of NiꢀZn complex during the polymerizaꢀ
tion (confirmed by the NMR studies in Figure S10). These experꢀ
13
spectrum (Figure 3a(i)). Both the C NMR results (the disappearꢀ
ance of the carbonyl peak at 149 ppm, Figure 3b(i)) and the FTIR
results (the shift in υ(CO) from 1810 to 1750 cm–1; Figure S11(a5))
confirmed the complete ringꢀopening of 1.
iments suggest that the kinetics of the photoredox ROP of
be described by the following equation:
L
ꢀ1 can
–d[Lꢀ1]/dt = [L
kp [Ni]1.84[Zn]2.49 ꢀ1]1
(1)
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Table 1. Optimization of conditions for ringꢀopening polymerization of L
ꢀ1. a
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
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34
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37
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Entry
Conditions
DP b
Time (h)
Conv. (%) c
Mn (kDa) d
MWcal (kDa) Ð d
1.10
Temp. (°C)
1
As shown
300
500
500
1000
300
300
300
300
300
300
300
r.t.
4
4
6
8
4
4
4
4
4
4
4
100
100
100
100
100
100
20
39.2
38.1
78.4
140.5
47.2
101.8
4.8
44.5
74.1
74.1
148.1
44.5
44.5
44.5
44.5
44.5
44.5
44.5
2
As shown
r.t.
1.71
1.05
1.05
1.01
1.39
4.61
1.73
1.45
1.30
1.06
3
As shown
–15
–15
–15
–15
–15
–15
–15
–15
–15
4
As shown
5
As shown
6
No (bpyꢀ1)Ni(COD)
No Zn(HMDS)2
No BnOH
7
8
97
62.9
54.8
48.9
54.6
9
No Irꢀ1
100
100
100
10
No light
11
Sunlight instead of LED
a
Abbreviations: DP, degree of polymerization; Temp., temperature; Conv., monomer conversion; Mn, numberꢀaverage molecular weight; MWcal,
, molecular weight distribution; r.t.,
molecular weight calculated on the basis of the ratio of the amount of monomer to the amount of the catalyst;
Ð
b
c
room temperature. DP refers to the ratio of the amount of monomer to the amount of the catalyst. Determined from the intensity of the Fourier
transform infrared peak at 1805 cm–1, which corresponds to the OCA anhydride group. d Determined by gelꢀpermeation chromatography.
Table 2. Photoredox polymerization and block copolymerization of various OCA monomers. a
O
O
n
O
O
O
O
O
O
R =
O
O
(bpy-1)Ni(COD), Zn(HMDS)2
R2
O
CH3
O
O
H
O
O
O
H
m
BnO
BnO
BnOH, Ir-1, hv, -15oC
m
n
1
R1
R1
R2
L-PheOCA (1)
3
L-Glu(Cbz)OCA ( ) L-LacOCA (4)
2
L-Ser(Bn)OCA ( )
Entry
Monomer DP b
Time (h)
Conv. (%) c
100
Mn
(
Mnꢀb1) (kDa) d
MWcal (MWcalꢀb1) (kDa) d
Ð (
Ðꢀb1) d
1
2
3
4
2
400
300
300
8
65.7
72.7
24.1
71.3
1.05
3
5
100
66.1
1.04
4
6
100
21.7
1.04
1/2
300/300
4/12
100
112.7 (50.6)
97.9 (44.5)
1.04 (1.05)
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2/1
1/3
3/1
1/4
2/3
3/2
2/3/1
100/200
200/100
300/200
200/100
100/100
300/100
100/100/100
3/4
3/3
5/4
4/3
4/3
5/3
3/3/3
100
100
100
100
100
100
100
63.2 (21.5)
46.7 (25.6)
86.2 (72.7)
37.6 (29.9)
46.7 (25.6)
89.7 (72.7)
66.2 (18.0)
47.5 (17.9)
51.7 (29.7)
95.7 (66.1)
36.9 (29.7)
39.9 (17.9)
83.9 (66.1)
54.7 (17.9)
1.07 (1.05)
1.06 (1.06)
1.06 (1.04)
1.05 (1.06)
1.02 (1.02)
1.05 (1.04)
1.04 (1.02)
1
2
3
4
5
6
7
8
6
7
8
9
9
10
11
10
11
12
13
a
Abbreviations: DP, degree of polymerization; Temp., temperature; Conv., monomer conversion; Mn, numberꢀaverage molecular weight; MWcal,
, molecular weight distribution. Mnꢀb1 and Ðꢀb1
molecular weight calculated on the basis of the amount of monomer to the amount of the catalyst;
Ð
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
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36
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42
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46
47
48
49
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59
60
b
refer to the molecular weight and molecular weight distribution of the first block polymer. DP refers to the ratio of the amount of monomer to the
amount of the catalyst. Determined from the intensity of the Fourier transform infrared peak at 1805 cm–1, which corresponds to the OCA anhyꢀ
c
dride group. d For homopolymers, Mn and
Ð were determined by gelꢀpermeation chromatography (GPC). For copolymers, Mnꢀb1, Ð, and Ðꢀb1 were
determined by GPC; Mn of the block copolymer was calculated on the basis of the 1H NMR integration ratio of the αꢀmethine peaks of the different
blocks.
In contrast, Niꢀcarbonyl complexes were observed in the 31P
NMR spectrum of the same mixture at room temperature
((PCy3)2Ni(CO) at 37.9 ppm, (PCy3)2Ni(CO)2 at 41.0 ppm; Figꢀ
results obtained using Znꢀalkoxide with those obtained with the
13
Ni/Zn dual catalyst (the C NMR spectrum showed an ester peak
at 167 ppm but no C5(O) peak at 178 ppm and no C2(O) peaks
from 155ꢀ145 ppm; Figure 3b(i–iii)) indicates that a Ni complex
likely added oxidatively to 1 before coordination and insertion of
the Znꢀalkoxide.
ure 3a(iv)),20 which was also confirmed by the C NMR spectrum
13
(Figure 3b(iii)),21 suggesting the formation of a nickelacycle interꢀ
mediate by means of an oxidative addition reaction between Ni(0)
and 1 at room temperature (Figure 3a, reaction scheme).11b, 22 In
addition, oxidative addition of a Ni(0) complex to 1 likely occurred
To confirm that a Niꢀmediated decarbonylation side reaction did
not occur in the lowꢀtemperature photoredox ringꢀopening reacꢀ
tion, we carried out the ringꢀopening reaction with Wilkinson’s
catalyst, (PPh3)3RhCl, an extremely effective CO scavenger (Figure
S11).24 Both at room temperature and at –15 °C (1 equiv. of 1,
(bpyꢀ1)Ni(COD), Zn(HMDS)2, BnOH and (PPh3)3RhCl), the
carbonyl stretching vibrations (1976 and 1917 cm–1) of
(PPh3)2Rh(CO)Cl were observed in the FTIR spectrum of the
reaction mixture (Figure S11(b3) and (b4)), but they were not
present in the reaction mixture containing the photoredox catalyst
Irꢀ1 (0.1 equiv.) under irradiation at –15 °C (Figure S11(b5)),
confirming that only decarboxylation occurred in this system. Beꢀ
cause both decarbonylation and decarboxylation reportedly occur
during the oxidative addition of Ni(0) complexes to anhydrides
(Figure 1c),25 our results clearly indicate that irradiation under lowꢀ
temperature conditions in the presence of photoredox catalyst Irꢀ1
accelerated decarboxylation while avoiding the formation of a Niꢀ
carbonyl, which is detrimental to the ROP.10 Note that no reaction
was observed when Zn(HMDS)2 and (bpyꢀ1)Ni(COD) were preꢀ
sent at a 1/1 ratio (Figure S10), ruling out the formation of a Zn–
Ni coordination complex prior to the ROP.26
regioselectively at the O1–C5 bond of 1, similar to the case for
Nꢀ
carboxyanhydride.11 This possibility was supported by the absence
of an anhydride stretching vibration (1820–1800 cm–1) in the FTIR
spectrum of 1/(PCy3)2Ni(COD) (Figure S8(a1)) and by the apꢀ
pearance of a carbonate peak (160 ppm) in the 13C NMR spectrum
of the 1/(PCy3)2Ni(COD)/Zn(HMDS)2/BnOH mixture (Figure
3b(iii)). We also explored the nickelacycle formed from
(PCy3)2Ni(COD) and 1.11,
(PCy3)2Ni(COD) and 1, a hexaneꢀsoluble red oil and a THFꢀ
soluble yellow powder were isolated (Figure S12). The P NMR
(same as in Figure 3a) and FTIR analysis of the oil
(υ(Ni(CO))=1979 and 1940 cmꢀ1) confirmed the presence of
(PCy3)2Ni(CO) and (PCy3)2Ni(CO)2 (Figure S12b and d). The
nickelꢀOCA complex could not be recrystallized or isolated from
the hexane solution but the doublet set in P NMR suggested its
presence (Figure S12b).22d The hexaneꢀinsoluble powder was preꢀ
22d
From the mixture of
31
31
13
sumably oligo(1), as indicated by C NMR (only 168 ppm peak
and no other peaks above 140 ppm, Figure S12c) and FTIR analyꢀ
sis (υ(COO)=1758 cmꢀ1, Figure S12e). The uncontrolled polymerꢀ
ization of 1 by (bpyꢀ1)Ni(COD) was also observed (Table S2,
entry 3), suggesting that oligo(1) was formed via the undesired Niꢀ
mediated ringꢀopening reaction. In addition, the use of low temꢀ
perature but no light could not avoid the Niꢀmediated carbonylaꢀ
tion over the time (Figure 3a, (ii) versus (iii)).
13
Notably, both the C NMR results (C5(O) at 178 ppm, slight
shift of C2 from 147 to 152 ppm; Figure S13b) and the FTIR results
(υ(Zn(CO)) = 1948 cm–1,23 Figure S11(a2)) indicate that the Znꢀ
alkoxide complex likely inserted into the O1–C5 bond, in a way
similar to the Ni(0) complex. However, the insertion of Znꢀ
alkoxide into 1 was not followed by efficient decarboxylation, indiꢀ
cated by the presence of both C2(O)’s monomer peak and carꢀ
bonate peak (147 and 152 ppm, Figure S13b). Comparison of the
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Figure 4. Proposed mechanism of controlled photoredox ringꢀopening
polymerization of OCAs.
CONCLUSIONS
In conclusion, the unprecedented Ni/Znꢀmediated photoredox
polymerization reported herein offers a strategy for achieving rapid,
controlled OCA polymerization to prepare stereoregular PAHAs
and PAHA copolymers bearing various functional sideꢀchain
groups. This strategy can be expected to enable the generation of
PAHAs with desirable macroscopic properties such as rigidity,
elasticity, and biodegradability and has the potential to allow for
new fabrication techniques such as photolithography of biodeꢀ
gradable polyesters.27
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website.
AUTHOR INFORMATION
Corresponding Author
*rtong@vt.edu
Figure 3. Mechanistic studies of the photoredox ringꢀopening reaction
of
ꢀ1. (a) 31P NMR and (b) 13C NMR spectra obtained under various
reaction conditions.
L
Notes
[Lꢀ
The authors declare no competing financial interests. A provisional
patent (U.S. Patent Application No: 62/414,016) has been filed perꢀ
taining to the results presented in this paper.
1]/[(PCy3)2Ni(COD)]/[Zn(HMDS)2]/[BnOH]/[Irꢀ1]
1/1/1/1/0.1. The grey area in (a) indicates the side products.
=
On the basis of our results and by analogy to prior mechanistic
investigations of Niꢀcatalyzed coupling reactions of anhydrides and
alkylzinc agents,26 we propose the following catalytic pathway (Figꢀ
ure 4). First, oxidative addition of a Ni(0) complex to 1 leads to a
nickelacycle intermediate, which undergoes photoinduced decarꢀ
boxylation and transmetalation with a Zn complex to generate a
reactive Znꢀalkoxide terminus for chain propagation. Of note, Niꢀ
mediated decarbonylation happens in the photoredox decarbonylaꢀ
tion reaction in an insignificant way;10b however, such decarbonylaꢀ
tion reaction considerably affects the photoredox ROP results esꢀ
pecially for high DP polymers (Table 1, entries 2 versus 3; Figure
3). Given the wealth of reported Niꢀcatalyzed carbonylation and
decarbonylation,22, 25 the synergistic use of both low temperature
and photoredox catalysts can be suggestive for other organic reacꢀ
tions.
ACKNOWLEDGMENT
This work was supported by startꢀup funding from Virginia Polytechnic
Institute and State University. We thank Dr. N. Murthy Shanaiah for
NMR experiments and Dr. William Bebout for ESIꢀMS experiments.
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