Regioselective Carbyne Transfer to Ring-Opening Alkyne Metathesis Initiators Gives Access to Telechelic Polymers

Article abstract of DOI:10.1021/jacs.7b02225

Regioselective carbyne-transfer reagents derived from (3,3,3-trifluoroprop-1-yn-1-yl)benzene give access to functionalized ring-opening alkyne metathesis polymerization (ROAMP) initiators [R-C₆H₄C-Mo(OC(CH₃)(CF₃)₂)₃] featuring electron-donating or -withdrawing substituents on the benzylidyne. Kinetic studies and linear free-energy relationships reveal that the initiation step of the ring-opening alkyne metathesis polymerization of 5,6,11,12-tetradehydrobenzo[a,e][8]annulene exhibits a moderate positive Hammett reaction constant (ρ = +0.36). ROAMP catalysts featuring electron-withdrawing benzylidynes not only selectively increase the rate of initiation (k_i) over the rate of propagation (k_p) but also prevent undesired intra- and intermolecular chain-transfer processes, giving access to linear poly-(o-phenylene ethynylene) with narrow molecular weight distribution. The regioselective carbyne transfer methodology and the detailed mechanistic insight enabled the design of a bifunctional ROAMP-reversible addition-fragmentation chain-transfer (RAFT) initiator complex. ROAMP followed by RAFT polymerization yields hybrid poly-(o-phenylene ethynylene)-block-poly-(methyl acrylate) block copolymers.

Full text of DOI:10.1021/jacs.7b02225

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Article
Regioselective Carbyne Transfer to Ring-Opening Alkyne
Metathesis Initiators Gives Access to Telechelic Polymers
Stephen von Kugelgen, Renee Sifri, Donatela Bellone, and Felix R. Fischer
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 17 May 2017
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Regioselective Carbyne Transfer to Ring-Opening Alkyne Metathesis
Initiators Gives Access to Telechelic Polymers
Stephen von Kugelgen,† Renee Sifri,† Donatela Bellone† and Felix R. Fischer† *
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∫||
†Department of Chemistry, University of California Berkeley, Berkeley, California 94720, United States
∫Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
||Kavli Energy Nanosciences Institute at the University of California Berkeley and Lawrence Berkeley National Laboratory, Berkeley,
California 94720, United States
ABSTRACT: Regioselective carbyne transfer reagents derived from (3,3,3-trifluoroprop-1-yn-1-yl)benzene give access to functionalized ring-
opening alkyne metathesis polymerization (ROAMP) initiators [R-C6H4CºMo(OC(CH3)(CF3)2)3] featuring electron-donating or with-
drawing substituents on the benzylidyne. Kinetic studies and linear free-energy relationships reveal that the initiation step of the ring-opening
alkyne metathesis polymerization of 5,6,11,12-tetradehydrobenzo[a,e][8]annulene exhibits a moderate positive Hammett reaction constant
(ρ = +0.36). ROAMP catalysts featuring electron-withdrawing benzylidynes not only selectively increase the rate of initiation (ki) over the
rate of propagation (kp) but also prevent undesired intra- and intermolecular chain transfer processes, giving access to linear poly-(o-
phenylene ethynylene) with narrow molecular weight distribution. The regioselective carbyne transfer methodology and the detailed mecha-
nistic insight enabled the design of a bifunctional ROAMP-reversible addition-fragmentation chain-transfer (RAFT) initiator complex.
ROAMP followed by RAFT polymerization yields hybrid poly-(o-phenylene ethynylene)-block-poly-(methyl acrylate) block copolymers.
We recently demonstrated the use of living ring-opening alkyne
metathesis polymerization (ROAMP) of ring-strained cyclic al-
kynes with initiators derived from [RCºMo(OC(CH3)(CF3)2)3]
for the controlled synthesis PPEs featuring either a linear or cyclic
topology.38
INTRODUCTION
Functional materials derived from semiconducting π-conjugated
polymers have become ubiquitous in the field of organic electron-
ics. A unique combination of rationally tunable band structures,
favorable mechanical properties of polymers, and bulk solution-
based synthesis and processing have contributed to the develop-
ment of a wide variety of highly specialized functional materials for
applications in organic light-emitting diodes (OLEDs),1–3 organic
photovoltaics (OPVs),4–6 organic field effect transistors
(OFETs),7,8 and optical9 and molecular sensors.10 The performance
of these polymers relies to a significant extent on the intra- and
intermolecular interactions that span the gap between the nano-
and the microscopic scale.11–13 While the primary sequence of mon-
omers largely determines the band gap and optical absorption, the
preferred secondary structure adopted by polymer chains controls
critical performance parameters like the mean free path of charge
carriers and excitons.14–18 Lastly, the challenge to direct the self-
assembly of polymers into ordered microscopic domains, a tertiary
structure, is a key technology in the fabrication of bulk heterojunc-
tions for OPVs.19–22 Herein we focus on conjugated polymers de-
rived from poly-(phenylene ethynylene) (PPE) featuring a pattern
of alternating aromatic rings and CºC triple bonds. By varying the
substitution pattern (ortho-, meta-, para-) along the aromatic rings
lining the backbone of the polymer the secondary structure of PPEs
can be adjusted from a linear zig-zag to a helically coiled confor-
mation.23–30 Classical syntheses of PPEs have relied on transition
metal catalyzed cross-coupling or alkyne cross metathesis (ACM)
step-growth polymerizations that suffer from unselective chain-
termination, uncontrolled chain-transfer, and result in polymers
with disproportionately broad molecular weight distributions.31–37
In this study we report the first synthesis of telechelic PPEs by
ROAMP using functional Mo carbyne initiators accessed by kinet-
ically-controlled
[EtCºMo(OC(CF3)2CH3)3•DME] (
ethane). Substituted (3,3,3-trifluoroprop-1-yn-1-yl)benzenes (
serve as efficient carbyne transfer agents to yield substituted Mo
benzylidynes [p-X-C6H4CºMo(OC(CF3)2CH3)3•DME] ( ). The
benzylidyne
transfer
to
1
) (DME = 1,2-dimethoxy-
2
)
3
mild conditions and high regioselectivity of this transfer protocol
enable the incorporation of a wide range of functional groups into
the structure of the ROAMP initiator. Linear free-energy relation-
ship analysis reveals that the electronic structure of the ROAMP
initiators controls the rate of initiation in the polymerization of
5,6,11,12-tetradehydrobenzo[a,e][8]annulene ( ). A moderate
8
positive Hammett reaction constant (ρ = +0.36) indicates that
electron-withdrawing substituents stabilize a buildup of negative
charge in the rate-determining transition state, a trend that is con-
firmed by theory. We demonstrate the versatility of our carbyne
transfer protocol that provides access to well-defined PPE-block-
poly-(methyl acrylate) (PMA) copolymers via reversible addition-
fragmentation chain-transfer (RAFT)39 chain extension of
telechelic PPE macroinitiators derived from a bifunctional Mo
ROAMP-RAFT initiator complex.
RESULTS AND DISCUSSION
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Traditional routes toward substituted Mo benzylidyne complex- First, the electron withdrawing CF3 group directs the regioselectivi-
1
2
3
4
5
6
7
8
es have thus far relied on the alkylation of Mo(CO)6 with aryl lithi-
um reagents followed by deoxygenation,40–47 the cross-metathesis of
ty of the cycloaddition to favor the transfer of the electron rich
aromatic carbyne to the Mo metal center. Second, the fluorinated
4
alkyne co-product, 1,1,1-trifluoropent-2-yne ( ), is highly volatile,
readily removed from the reaction mixture by application of a dy-
namic vacuum, and is electronically deactivated towards further
metathesis when compared to unfluorinated 1,2-dialkylethynes.
Finally, the cross-metathesis proceeds at mild temperatures ranging
between –60 to 24 °C and is compatible with a wide variety of func-
tional groups that cannot be introduced through a low-valent route.
As an added bonus, characteristic shifts in the 19F NMR signal asso-
(RO)3MoºMo(OR)3 or NºMo(OR)3 with symmetric/asymmetric
arylethynes,48–50 or the activation of trigonal planar Mo(NRAr)3
complexes.51–64 The harsh reaction conditions associated with low
oxidation state routes65 are incompatible with many polar function-
al groups. On the other hand, a cross-metathesis strategy suffers
from low selectivity, unfavorable thermodynamic equilibria, and
difficult purification of a complex product mixture. While the re-
versible cross-metathesis of Mo alkylidynes accessed by high oxida-
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tion state routes66,67 e.g. [EtCºMo(OC(CF3)2CH3)3 •DME] (
1
)
ciated with the CF3 group in 2a- can serve as a highly sensitive
g
internal reference for the progress of the reaction.
with symmetric 1,2-diarylethynes is facile, shifting the product
mixture to the desired Mo benzylidyne complexes requires a large
excess of reagent that proved challenging to separate by fractional
crystallization. Cross-metathesis with asymmetric 1-aryl-1-alkynes
such as 1-aryl-1-propynes instead leads to a mixture of Mo alkyli-
dyne complexes (Supporting Information Figure S1) and the cor-
responding 1,2-diarylethyne. Our attempts to drive the reaction
towards the desired product by either distilling or trapping the
volatile 1,2-dialkylethyne with molecular sieves led to inseparable
decomposition products.
Figure 1. A) Time resolved 19F NMR spectroscopy of the carbyne
transfer of 2f to
tube). B) 19F NMR spectrum of the equilibrium mixture of the carbyne
transfer reaction of 2f to under thermodynamic control (24 °C in
1
under kinetic control (–30 °C in CDCl3, sealed NMR
Scheme 1. Traditional low oxidation state and cross-metathesis routes
toward substituted [RCºMo(OC(CF3)2CH3)3•DME] complexes
(top). Schematic representation of the regioselective carbyne transfer
1
CDCl3, sealed NMR tube). R = CCH3(CF3)2
The unusual regioselectivity inherent to our benzylidyne transfer
reagents relies on the electron withdrawing CF3 group in (3,3,3-
with substituted (3,3,3-trifluoroprop-1-yn-1-yl)benzenes (2a
-
g
) to give
the corresponding molybdenum benzylidyne complexes (3a
g
- ) (bot-
trifluoroprop-1-yn-1-yl)benzenes (2a
of the CºC bond. The preferred orientation of the carbyne transfer
reagent with respect to the MoºC bond in places the electron-
- ) that induces a polarization
g
tom).
To address this synthetic challenge, we developed a highly selec-
tive and functional group tolerant cross-metathesis protocol. The
necessary requirements for an efficient carbyne transfer reagent are:
i) high regioselectivity for the transfer of only the desired aryl group
to the Mo complex; ii) side-products generated in the cross-
metathesis step are either highly volatile or insoluble in the reaction
mixture, facilitating the purification of the desired complex; and iii)
the reaction conditions are mild and the carbyne transfer reagent is
versatile enough to install a large variety of sensitive functional
1
rich end of the CºC bond bearing the substituted benzene ring
19
next to the electron-deficient Mo center. Time resolved F NMR
spectroscopy (Figure 1A) reveals that under kinetic control (–30
°C, sealed NMR tube) the reaction of one equiv of
trifluoromethyl-4-(3,3,3-trifluoroprop-1-yn-1-yl)benzene
1
with 1-
2f
(
)
leads predominantly to the formation of the desired trifluoro-
methylbenzylidyne complex 3f over the undesired trifluoroethyli-
dyne complex
small amount of
5
. Based on the conservative assumption that the
present in the reaction mixture (Figure 1A, 20h)
groups. Cross-metathesis of
1
with substituted (3,3,3-trifluoroprop-
5
1-yn-1-yl)benzenes (2a ) (Scheme 1) satisfies all three criteria.
-
g
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is directly formed by reaction of
selectivity for the desired product 3f over
ly is produced to a significant extent through a slow cross-
metathesis equilibrium with the coproduct accumulating during
the reaction in a sealed NMR tube. Under thermodynamic control
(24 °C, sealed NMR tube) instead the reaction of one equiv. of
1
with the substrate 2f, the kinetic
exceeds 8/1. More like-
regioselectivity for the formation of 3a- falls below 70% (Support-
g
1
2
3
4
5
ing Information Figure S1). The reaction tolerates nucleophilic,
electrophilic, basic, and potentially coordinating functional groups
on the carbyne transfer reagent that are otherwise incompatible
with the traditional low oxidation state route toward molybdenum
benzylidyne complexes (Scheme 1).
5
4
5
1
6
7
8
9
with 2f reaches equilibrium within < 2 h (Figure 1B). The position
of the equilibrium is driven by the effectively irreversible formation
NMR spectroscopy indicates that the molybdenum benzylidyne
g
complexes 3a- prepared in this series are isostructural in solution.
of 1,2-diarylethyne
°C and favors the formation of
Table 1. ROAMP initiators 3a-g accessed by cross-metathesis of
substituted (3,3,3-trifluoroprop-1-yn-1-yl)benzenes 2a-g
7
that is slow to undergo cross-metathesis at 24
The structural homology is further supported by the crystallo-
graphic analysis of complexes 3a and 3g depicted in Figure 2. Dark
green plates of 3a from Et2O/pentane (1:1) and orange prisms of
3g from toluene/pentane (1:1) suitable for X-ray crystallography
were obtained from saturated solutions at –35 °C. In both com-
plexes the geometry at the Mo center is pseudo-octahedral. X-ray
crystallography of 3a (Figure 2A) confirms the presence of a
C(1)ºMo(1) triple bond with a bond length of 1.761(3) Å and a
C(2)–C(1)–Mo(1) angle of 177.3(2)°. Three hexafluoro-tert-
butoxide ligands adopt a meridional conformation featuring
Mo(1)–O(1), Mo(1)–O(2), and Mo(1)–O(3) distances of
1.968(2) Å, 1.928(2) Å, and 1.975(2) Å. One equiv of DME is
coordinated to the Mo complex in the crystal. The bond distances
are 2.226(2) Å and 2.415(2) Å for the Mo(1)–O(4) cis and
Mo(1)–O(5) trans to the carbyne, respectively. X-ray crystallog-
raphy of 3g (Figure 2B) shows that substitution of the electron
5
over 3f
.
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1
with
.
X =
alkyne
T (°C)
Yield (%)a
Mo
complex
N(CH3)2
OCH3
CH3
2a
2b
2c
2d
2e
2f
24
3a
3b
3c
3d
3e
3f
48 (96)
66 (94)
83 (95)
79
0 to 24
–20 to 24
–20 to 24
–60 to 24
–60 to 24
–60 to 24
H
OAc
CF3
77 (92)
74 (92)
45 (83)
NO2
2g
3g
a Isolated yield after recrystallization. Crude NMR yields in parentheses.
3a
for an electron withdrawing NO2
donating N(CH3)2 group in
group in 3g has only a small effect on the C(1)ºMo(1) bond length
(1.754(7) Å) and C(2)–C(1)–Mo(1) bond angle (174.8(6)°).
More pronounced changes are observed for the bond lengths be-
tween the molybdenum and the three hexafluoro-tert-butoxide
ligands. The weaker π-donating character of the carbyne in 3g is
partially compensated by a contraction of the Mo(1)–O(1),
Mo(1)–O(2), and Mo(1)–O(3) bond lengths, 1.958(4) Å,
1.922(4) Å, and 1.952(5) Å respectively.
Figure 2. Single crystal X-ray structures of A) 3a and B) 3g exhibiting a
meridional, pseudo-octahedral geometry at Mo. Thermal ellipsoids are
drawn at the 50% probability level. Color coding: C (gray), O (red), F
(green), Mo (turquoise). Hydrogen atoms are omitted for clarity.
Taking advantage of the regioselectivity inherent to the benzyli-
g
dyne transfer with 2a- we prepared a series of ROAMP initiators
featuring electron-donating (N(CH3)2, OCH3, CH3) and electron-
withdrawing (OAc, CF3, NO2) substituents on the aromatic ring
(Scheme 1). The precursor molybdenum propylidyne complex
1
can be synthesized on multi gram scale following a procedure pio-
Scheme 2. Schematic representation of the ROAMP initiation step of
neered by Gdula and Johnson.38,67 The substituted carbyne transfer
substituted molybdenum benzylidyne complexes 3a
strained alkyne . R = CCH3(CF3)2
-
g
with ring-
reagents 2a
1–2 steps from simple starting materials. Addition of one equiv of
2a
to in toluene at –60 to 24 °C and warming to 24 °C under
dynamic vacuum (0.1 torr to remove volatile ) leads to the desired
benzylidyne complexes 3a in up to 96% crude yield (NMR), or
- are either commercially available or were prepared in
g
8
With a series of substituted ROAMP initiators 3a
-
g
in hand we
-
g
1
explored the electronic effects on the ROAMP initiation step in the
ring-opening metathesis of 5,6,11,12-
tetradehydrobenzo[a,e][8]annulene ( ). Upon addition of to a
twofold excess of 3a , the ring-strained monomer is consumed in
less than 60 s to yield a mixture of interconverting metallacyclobu-
4
-
g
8
8
45–83% isolated yield following recrystallization (Table 1). A slight
decrease in selectivity (96–83%) is observed as the polarization of
-
g
the alkyne is altered by going from electron-donating 2a
tron withdrawing substituents 2e on the aromatic ring. If the
trifluoromethyl group in 2a is replaced by a methyl group the
- to elec-
c
1
tadienes 10a
observe that the intermediate metallacyclobutadienes decay in a
unimolecular process to give the ring-opened benzylidynes 9a
- and 11a-
g g
(Scheme 2).38 By 19F and H NMR we
-
g
-
g
-
g
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(Figure 3A). As the initial cycloaddition between 3a
-
g
and
8
is al-
metallacyclobutadienes 10a- and 11a- , the barrier is only 1.5 kcal
g g
mol–1 by DFT gas phase calculations. The rate determining step in
the ROAMP initiation is associated with the cycloelimination lead-
1
2
3
4
5
6
7
8
most instantaneous, the observed rate of initiation, ki,obs can be
approximated by the rate limiting step, ki. Fitting the experimental
data to a first-order exponential decay of 10a
unique rate constant ki for the cycloelimination step for each of the
ROAMP initiators. As the isosteric series 3a spans the Hammett
-
g
and 11a
-
g
gives a
ing from Int3 through TS3 to the product . While theory underes-
P
timates the magnitude of the barriers, the relative trends TS3(X =
N(CH3)2) > TS3(X = H) > TS3(X = CF3) faithfully reproduce the
experimental results. Natural population analysis (NPA)69 for X =
H shows a buildup of negative charge at the benzylic α-position
consistent with our Hammett LFER analysis. The charge decreases
-
g
parameter space, a linear free-energy relationship (LFER) analysis
provides further insight into the structure of the rate determining
transition state (Figure 3B). A positive Hammett reaction constant
ρ = +0.36 is indicative of either the buildup of negative charge or
the decrease of positive charge in the benzylic α-position in the rate
determining transition state. The significant rate acceleration of the
ROAMP initiation step upon introducing electron withdrawing
groups on the benzylidyne complex is unusual and has not been
observed for the analogous ring-opening metathesis polymerization
(ROMP) with Mo or Ru carbene complexes.68 This contrast is
likely attributed to contributions from the extended π-conjugation
of the metallacyclobutadiene that mediates resonance stabilization
effects more efficiently than a saturated metallacyclobutane.
from +0.21 on the carbyne carbon atom in Int1 to +0.14 in TS1
,
9
+0.07 in Int2, +0.11 in TS2, +0.03 in Int3, and finally –0.01 in the
rate-limiting transition state TS3. As expected, electron donating
substituents (X = N(CH3)2) lead to an increase of the activation
barrier, while electron withdrawing groups (X = CF3) stabilize the
negative charge buildup in the transition state thereby lowering the
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
energy of TS3
.
Figure 4. Calculated reaction coordinate diagram of the rate determin-
ing step in the ROAMP initiation reaction for three model complexes
representing 3a (X = N(CH3)2), 3d (X = H), and 3f (X = CF3). (DFT
ωB97xD; CHNOF (6-31G+(d,p)); Mo (SDD ECP MWB28); ZPE
corrected) R = CCH3(CF3)2
Figure 3. A) First-order kinetic plot of the rate-limiting ring-opening
cycloelimination of 3a
of initiation ki,X for 3a
-
g
with
8
.
. B) Hammett LFER analysis of the rate
-
g
with
8
To further support our interpretation of the LFER analysis, we
used theory to explore the potential energy surface associated with
the ROAMP initiation reaction. Figure 4 summarizes the results of
density functional theory (DFT) calculations on three model com-
plexes representative for the tetracoordinate Mo complexes result-
ing from the reversible dissociation of DME from 3a,
3d, or 3f
(equilibria involving reversible association of DME were not in-
cluded in an effort to facilitate convergence on a flat potential ener-
Table 2. Molecular weight analysis of poly-
8
X =
[ ]/[ ]
8 3
Mn
Mw
ĐM
cyclic
(SEC)a
(SEC)a
1700
3400
3000
2500
3300
3400
3500
(Mw/Mn) poly-
8
N(CH3)2
OCH3
CH3
H
10/1
10/1
10/1
10/1
10/1
10/1
10/1
800
2.1
1.9
1.6
1.4
1.3
1.3
1.3
60%
1800
1900
1800
2500
2600
2600
20%
<1%b
<1%b
<1%b
<1%b
<1%b
gy surface). The coordination of ring-strained monomer
8
to the
tetracoordinate complex SM is an exothermic process (–24 kcal
mol–1) and leads to intermediate Int1. A nearly barrierless cycload-
dition step (TS1, for X = H) yields the initial metallacyclobutadi-
ene intermediate Int2 that is localized on a rather flat potential
energy surface. Int2 undergoes a double bond isomerization to give
the secondary molybdacyclobutadiene Int3. While F NMR spec-
troscopy clearly indicates an equilibrium between interconverting
OAc
CF3
NO2
a
19
Size exclusion chromatography (SEC) calibrated to polystyrene
standards. b Concentration below detection limit.
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Journal of the American Chemical Society
Page 14 of 18
The observed rate acceleration attributed to the electronic stabi-
lization of the cycloelimination transition state acts exclusively on
the initiation step, ki. The rate of propagation, kp, for complexes 3a
is largely unaffected by substituent effects as the distance between
the end group and the reaction center increases with each mono-
mer in the growing polymer chain. Electron withdrawing substitu-
ents are thus uniquely suited to selectively increase ki over kp, a
crucial requirement for the synthesis of living polymers with narrow
molecular weight distributions. The effect is most apparent at high
accessible (3,3,3-trifluoroprop-1-yn-1-yl)benzenes described above
1
2
3
4
5
6
7
8
gives access to telechelic polymers. Electron-withdrawing substitu-
ents such as the acyloxybenzylidyne 3e (Scheme 1) provide a versa-
tile functional group that serves as an adaptable chemical linker for
post-polymerization functionalization. To further illustrate this
concept, we synthesized (3,3,3-trifluoroprop-1-yn-1-yl)benzene 2h
featuring a trithiocarbonate group that acts as a reversible addition-
fragmentation chain-transfer (RAFT) initiator (Scheme 3).39,71
Benzylidyne transfer from 2h cleanly yields the functionalized Mo
complex 3h in 59% isolated yield (86% by crude NMR). ROAMP
-
g
catalyst loadings (e.g. [ ]/[3a- ] = 10/1, Table 2). The molecular
8 g
9
weight dispersity, ĐM,70 of the resulting polymers derived from size-
exclusion chromatography (SEC) ranges from 2.1 for electron
donating X = N(CH3)2 to 1.3 for electron withdrawing substituents
X = OAc, CF3, NO2. Polymers resulting from ROAMP initiators 3a
and 3b feature an electron-rich activated alkyne as an end group
that promotes intra- and intermolecular chain-transfer reactions. In
of
excess 3-hexyne yields poly-(o-phenylene ethynylene) poly-8h10
The MALDI mass spectrum shows two families of molecular ions
separated by the mass of the monomer
(MW = 200 g mol–1) that
8
with functional initiator 3h followed by chain-termination with
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
.
8
correspond to linear polymers, functionalized with a fragment of
the trithiocarbonate initiator on one end and a butyne group on the
other (Supporting Information Figure S4). Poly-8h10 acts as a func-
tional macroinitiator for RAFT polymerization. Chain extension
with methyl acrylate initiated by AIBN in benzene at 70 °C gives
poly-(methyl acrylate) (PMA) block polymers poly-8h-block-
PMA. The amphiphilic block copolymer can be isolated from a
small amounts of PMA homopolymer either by precipitation in
ethanol, Soxhlet extraction with acetonitrile, or by preparative SEC
depending on the weight of the PMA block. The molecular weight
increases from 2.5 kDa for poly-8h10, to 9.6 kDa (corresponding to
fact, 3a is even a superior catalyst for the synthesis of cyclic poly-
8
(as compared to ), yielding > 60% macrocyclic products in less
1
than 2 h (Supporting Information Figures S2, S3). ROAMP initia-
tors featuring electron-withdrawing groups instead deactivate the
terminal alkyne preventing undesired chain-transfer processes that
contribute to a broadening of the dispersity.
1
poly-8h10-block-PMA90 by H NMR end group analysis) without a
significant broadening of the ĐM = 1.3. While block-copolymers
featuring molecular weights up to 36 kDa, poly-8h10-block-PMA380,
could be prepared by this method a challenging purification of
block copolymers from PMA homopolymers resulting from radical
chain-transfer processes leads to a slight broadening of the molecu-
lar weight distribution (ĐM = 1.5).
CONCLUSION
We herein report a novel highly regioselective carbyne transfer
protocol that gives access to a large number of structurally diverse
ROAMP initiators featuring both electrophilic and nucleophilic
functional groups that are incompatible with traditional low oxida-
tion state routes. Detailed kinetic analysis and Hammett linear free
energy relationships reveal that electron withdrawing substituents
on the benzylidyne effectively increase the rate of initiation ki over
the rate of propagation kp. These kinetic studies are further corrob-
orated by gas phase theoretical calculations that highlight a buildup
of negative charge at the carbyne carbon atom in the rate determin-
ing cycloelimination step. Our regioselective functional group tol-
erant carbyne transfer strategy along with an expanded mechanistic
insight have enabled the design of a bifunctional ROAMP-RAFT
initiator complex that provides access to hybrid poly-(o-phenylene
ethynylene)-block-poly-(methyl acrylate) block copolymers
through a sequential ROAMP-RAFT polymerization protocol.
Scheme 3. Synthesis of telechelic ROAMP-RAFT copolymerization
8
initiator via benzylidyne transfer to 3h, followed by ROAMP of , and
RAFT chain-extension with methyl acrylate to give poly-8h-block-
PMA. R = CCH3(CF3)2
EXPERIMENTAL SECTION
Materials and General Methods. Unless otherwise stated, all manipula-
tions of air and/or moisture sensitive compounds were carried out in oven-
dried glassware, under an atmosphere of Ar or N2. All solvents and reagents
were purchased from Alfa Aesar, Spectrum Chemicals, Acros Organics,
TCI America, Matrix Scientific, and Sigma-Aldrich and were used as re-
ceived unless otherwise noted. Organic solvents were dried by passing
through a column of alumina and were degassed by vigorous bubbling of
N2 or Ar through the solvent for 20 min. Liquid alkynes were dried over 4 Å
molecular sieves. Commercial methyl acrylate was purified by extraction
Figure 5. SEC traces showing the chain extension of macroinitiator
poly-8h (red) to give poly-8h10-block-PMA90 (blue) and poly-8h10
block-PMA380 (green); calibrated to polystyrene standards.
-
Besides the obvious mechanistic advantages provided by elec-
tron withdrawing substituents in a controlled living ROAMP (ki >
kp), the versatile and mild carbyne transfer reaction with readily
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Journal of the American Chemical Society
with aqueous KOH and dried by passing through a column of alumina.
(C6H4)CºMo(OC(CH3)(CF3)2)3(DME)]: C, 33.98; H, 3.09. Found: C,
Flash column chromatography was performed on SiliCycle silica gel (parti-
cle size 40–63 μm). Thin layer chromatography was performed using Sili-
Cycle silica gel 60 Å F-254 precoated plates (0.25 mm thick) and visualized
by UV absorption. All 1H, {1H}13C, and 19F NMR spectra were recorded on
Bruker AV-600, DRX-500, AV-500, and AVQ-400 spectrometers, and are
referenced to residual solvent peaks (CDCl3 1H NMR δ = 7.26 ppm, 13C
NMR δ = 77.16 ppm; C6D6 1H NMR δ = 7.16 ppm, 13C NMR δ = 128.06
ppm;) or hexafluorobenzene (19F NMR δ = –162.90 ppm). High-
resolution mass spectrometry (EI) was performed on an Autospec Premier
(Waters) sector spectrometer in positive ionization mode. ESI mass spec-
trometry was performed on a Finnigan LTQFT (Thermo) spectrometer.
MALDI mass spectrometry was performed on a Voyager-DE PRO (Ap-
plied Biosystems Voyager System 6322) in positive mode using a matrix of
dithranol or dithranol/AgNO3. Elemental analysis (CHN) was performed
on a Perkin Elmer 2400 Series II combustion analyzer (values are given in
%). Size-exclusion chromatography (SEC) was carried out on a LC/MS
Agilent 1260 Infinity set up with one guard and two Agilent Polypore 300 ´
7.5 mm columns at 35 °C. All SEC analyses were performed on a 0.2
mg/mL solution of polymer in chloroform. An injection volume of 25 μL
and a flow rate of 1 mL/min were used. Calibration was based on polysty-
rene standards ranging from Mw = 100 to 4,068,981 Da. X-ray crystallog-
raphy of 3a and 3g was performed on an APEX II QUAZAR, using a Micro-
focus Sealed Source (Incoatec IμS; Mo-Kα radiation), Kappa Geometry
with DX (Bruker-AXS build) goniostat, a Bruker APEX II detector,
QUAZAR multilayer mirrors as the radiation monochromator, and Ox-
ford Cryostream 700 at 100 K. Crystallographic data were solved with
SHELXT, refined with SHELXL-2014, visualized with ORTEP-32, and
1
2
3
4
5
6
7
8
34.27; H, 3.04.
Preparation of [4-CH3-(C6H4)CºMo(OC(CH3)(CF3)2)3(DME)] (3c).
A 10 mL flask was charged under N2 with
1
(77 mg, 0.1 mmol) in toluene
(2 mL, [ ] = 50 mM). The solution was cooled to –20 °C and 2c (22 mg,
1
0.11 mmol) was added to the reaction mixture. The flask was immediately
placed under dynamic vacuum (0.1 torr) and removed from the cooling
bath to warm to 24 °C. The solvent was removed in vacuum at 24 °C and
the residue was recrystallized from pentane/Et2O (4:1, 1 mL) at –30 °C to
yield 3c (69 mg, 0.083 mmol, 83%) as an orange crystalline solid. 1H NMR
(500 MHz, C6D6, 22 °C) δ = 7.05 (d, J = 8.0 Hz, 2H), 6.76 (d, J = 8.0 Hz,
2H), 3.29 (s, 6H), 3.06 (s, 4H), 2.03 (s, 3H), 1.84 (s, 9H) ppm; 19F NMR
(376 MHz, C6D6, 22 °C) δ = –76.85 ppm; HRMS (EI) m/z: [4-CH3-
(C6H4)CºMo(OC(CH3)(CF3)2)3]+ calcd [C20H16F18MoO3] 743.9866;
found 743.9854. Spectroscopic data is consistent with previous reports.76
Preparation of PhCºMo(OC(CH3)(CF3)2)3(DME)] (3d). A 10 mL
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
flask was charged under N2 with (77 mg, 0.1 mmol) in toluene (2 mL, [ ]
1 1
= 50 mM). The solution was cooled to –20 °C and 2d (34 mg, 0.2 mmol, 2
equiv due to its high volatility) was added to the reaction mixture. The flask
was immediately placed under dynamic vacuum (0.1 torr) and removed
from the cooling bath to warm to 24 °C. The solvent was removed in vacu-
um at 24 °C and the residue was recrystallized from pentane/Et2O (1:1, 2
mL) at –30 °C to yield 3d (65 mg, 0.079 mmol, 79%) as a red-orange crys-
talline solid. 1H NMR (400 MHz, C6D6, 22 °C) δ = 7.11 (d, J = 7.4 Hz, 2H),
6.96 (t, J = 7.9 Hz, 2H), 6.75 (t, J = 7.5 Hz, 1H), 3.25 (s, 6H), 3.03 (s, 4H),
1.81 (s, 9H) ppm; 19F NMR (376 MHz, C6D6, 22 °C) δ = –76.67 ppm;
HRMS
(EI)
m/z:
[(C6H5)CºMo(OC(CH3)(CF3)2)3]+
calcd
[C19H14F18MoO3] 729.9710; found 729.9722. Spectroscopic data is con-
finalized with WinGX. 1 38,67
synthesized following literature procedures.
Preparation of
,
2b 72
,
2c 72,73
,
2d 72,73
,
2f 74
,
and 2g72 and 875 were
sistent with previous reports.66
Preparation of [4-(AcO)-(C6H4)CºMo(OC(CH3)(CF3)2)3(DME)]
(3e). A 10 mL flask was charged under N2 with
toluene (2 mL, [ ] = 50 mM). The solution was cooled to –60 °C and 2e
[4-((CH3)2N)-
1
(77 mg, 0.1 mmol) in
(C6H4)CºMo(OC(CH3)(CF3)2)3(DME)] (3a). A 10 mL flask was
charged under N2 with (77 mg, 0.1 mmol) in toluene (2 mL, [ ] = 50
1
1
1
(23 mg, 0.1 mmol) was added to the reaction mixture. The flask was im-
mediately placed under dynamic vacuum (0.1 torr) and removed from the
cooling bath to warm to 24 °C. The solvent was removed in vacuum at 24
°C and the residue was recrystallized from pentane/Et2O (4:1, 1.5 mL) at –
30 °C to yield 3e (67 mg, 0.077 mmol, 77%) as a dark red crystalline solid.
1H NMR (400 MHz, C6D6, 22 °C) δ = 7.12 (d, J = 8.7 Hz, 2H), 6.92 (d, J =
8.7 Hz, 2H), 3.24 (s, 6H), 3.06 (s, 4H), 1.79 (s, 9H), 1.70 (s, 3H) ppm;
13C{1H} NMR (126 MHz. C6D6, 22 °C) δ = 293.17, 168.3, 151.7, 141.1,
mM). 2a (21 mg, 0.1 mmol) was added at 24 °C to the reaction mixture.
The flask was immediately placed under dynamic vacuum (0.1 torr) and
the reaction mixture stirred for 1 h at 24 °C. The solvent was removed in
vacuum at 24 °C and the residue was recrystallized from pentane/toluene
(3:1, 1.5 mL) at –30 °C to yield 3a (41 mg, 0.048 mmol, 48%) as a dark
1
crystalline solid. H NMR (600 MHz, C6D6, 22 °C) δ = 7.12 (d, J = 9.0 Hz,
2H), 6.15 (d, J = 9.0 Hz, 2H), 3.32 (s, 6H), 3.10 (s, 4H), 2.37 (s, 6H), 1.96
(s, 9H) ppm; 13C{1H} NMR (151 MHz. C6D6, 22 °C) δ = 297.7, 150.2,
1
2
124.7 (q, JCF = 290 Hz), 121.6, 84.0 (m, JCF = 29 Hz), 71.6, 63.6, 20.4,
19.0 ppm; 19F NMR (376 MHz, C6D6, 22 °C) δ = –76.70 ppm; HRMS (EI)
m/z: [4-AcO-(C6H4)CºMo(OC(CH3)(CF3)2)3]+ calcd [C21H16F18MoO5]
1
2
134.5, 132.1, 124.9 (q, JCF = 289 Hz), 110.8, 84.1 (m, JCF = 29 Hz), 71.6,
63.7, 39.3, 19.3 ppm; 19F NMR (376 MHz, C6D6, 22 °C) δ = –76.71 ppm;
HRMS (EI) m/z: [4-((CH3)2N)-(C6H4)CºMo(OC(CH3)(CF3)2)3]+
calcd [C21H19F18MoNO3] 773.0132; found 773.0132; Anal. calcd for [4-
((CH3)2N)-(C6H4)CºMo(OC(CH3)(CF3)2)3(DME)]: C, 34.86; H, 3.39;
N, 1.63. Found: C, 35.23; H, 3.41; N, 1.63. Dark green plates suitable for X-
ray diffraction were grown from saturated pentane/Et2O (1:1) solution at –
35 °C. 3a crystallizes in the monoclinic space group P 21/n, a =14.3977(5)
Å, b = 14.4686(6) Å, c = 15.8068(7) Å, β = 100.490(2)°, Z = 4, GOF on F2
= 1.051, R indices (all data) R1 = 0.0426, wR2 = 0.1021.
787.9764;
found
787.9765;
Anal.
calcd
for
[4-AcO-
(C6H4)CºMo(OC(CH3)(CF3)2)3(DME)]: C, 34.26; H, 2.99. Found: C,
34.59; H, 3.34.
Preparation of [4-CF3-(C6H4)CºMo(OC(CH3)(CF3)2)3(DME)] (3f).
A 20 mL flask was charged under N2 with
1
(231 mg, 0.3 mmol) in toluene
(6 mL, [ ] = 50 mM). The solution was cooled to –60 °C and 2f (87 mg,
1
0.36 mmol, 1.2 equiv) was added to the reaction mixture. The flask was
immediately placed under dynamic vacuum (0.1 torr) and removed from
the cooling bath to warm to 24 °C. The solvent was removed in vacuum at
24 °C and the residue was recrystallized from toluene (3 mL) at –25 °C and
washed with cold pentane/Et2O (1:1, 1 mL) to yield 3f (196 mg, 0.22
Preparation of [4-(CH3O)-(C6H4)CºMo(OC(CH3)(CF3)2)3(DME)]
(3b). A 10 mL flask was charged under N2 with
1
(77 mg, 0.1 mmol) in
toluene (2 mL, [ ] = 50 mM). The solution was cooled to 0 °C and 2b (22
1
mg, 0.11 mmol) was added to the reaction mixture. The flask was immedi-
ately placed under dynamic vacuum (0.1 torr) and removed from the cool-
ing bath to warm to 24 °C. The solvent was removed in vacuum at 24 °C
and the residue was recrystallized from pentane/toluene (4:1, 1 mL) at –30
°C to yield 3b (56 mg, 0.066 mmol, 66%) as a red crystalline solid. 1H NMR
(600 MHz, C6D6, 22 °C) δ = 7.07 (d, J = 8.8 Hz, 2H), 6.48 (d, J = 8.8 Hz,
2H), 3.28 (s, 6H), 3.17 (s, 3H), 3.08 (s, 4H), 1.87 (s, 9H) ppm; 13C{1H}
NMR (151 MHz. C6D6, 22 °C) δ = 295.3, 160.5, 137.9, 132.3, 124.8 (q, 1JCF
1
mmol, 74%) as an orange crystalline solid. H NMR (600 MHz, C6D6, 22
°C) δ = 7.12 (d, J = 8.2 Hz, 2H), 6.95 (d, J = 8.2 Hz, 2H), 3.22 (s, 6H), 3.03
(s, 4H), 1.73 (s, 9H) ppm; 13C{1H} NMR (126 MHz. CDCl3, 22 °C) δ =
2
3
291.8, 145.3, 130.7 (q, JCF = 33 Hz), 130.3, 125.5 (q, JCF = 3.7 Hz), 123.9
1
1
2
(q, JCF = 289 Hz), 123.4 (q, JCF = 272 Hz), 83.7 (m, JCF = 29 Hz), 72.0,
64.0, 19.0 ppm; 19F NMR (564 MHz, C6D6, 22 °C) δ = –62.71, –76.63
ppm; HRMS (EI) m/z: [4-CF3-(C6H4)CºMo(OC(CH3)(CF3)2)3]+ calcd
[C20H13F21MoO3] 797.9583; found 797.9583; Anal. calcd for [4-CF3-
(C6H4)CºMo(OC(CH3)(CF3)2)3(DME)]: C, 32.52; H, 2.62. Found: C,
32.39; H, 2.73.
2
= 290 Hz), 113.7, 84.1 (m, JCF = 29 Hz), 63.8, 54.9, 19.2 ppm; 19F NMR
(470 MHz, C6D6, 22 °C) δ = –76.68 ppm; HRMS (EI) m/z: [4-(CH3O)-
(C6H4)CºMo(OC(CH3)(CF3)2)3]+ calcd [C20H16F18MoO4] 759.9815;
Preparation of [4-O2N-(C6H4)CºMo(OC(CH3)(CF3)2)3(DME)]
(3g). A 10 mL flask was charged under N2 with
found
759.9830;
Anal.
calcd
for
[4-((CH3O)-
1
(154 mg, 0.2 mmol) in
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 16 of 18
toluene (4 mL, [
1
] = 50 mM). The solution was cooled to –60 °C and 2g
MHz, CDCl3, 22 °C) δ = 175.0, 132.2, 128.1, 125.7, 92.6, 51.8, 41.6–41.3
(44 mg, 0.2 mmol) was added to the reaction mixture. The flask was im-
mediately placed under dynamic vacuum (0.1 torr) and removed from the
cooling bath to warm to 24 °C. The solvent was removed in vacuum at 24
°C and the residue was recrystallized from pentane/ Et2O (1:1, 1 mL) at –
30 °C to yield 3e (77 mg, 0.89 mmol, 45%) as a dark red crystalline solid.
1H NMR (500 MHz, C6D6, 22 °C) δ = 7.58 (d, J = 8.9 Hz, 2H), 6.79 (d, J =
8.9 Hz, 2H), 3.21 (s, 6H), 3.02 (s, 4H), 1.71 (s, 9H) ppm; 13C{1H} NMR
(126 MHz. C6D6, 22 °C) δ = 290.5, 147.1, 146.3, 130.4, 124.6 (q, 1JCF = 290
(m), 36.3–34.1(m) ppm.
1
2
3
4
5
6
7
8
ASSOCIATED CONTENT
Supporting Information
Figures S1 to S4, methods and instrumentation, synthetic procedures
for 2a
2e, and 2h and characterization, kinetic experiments, computa-
,
tional details (Tables S1 to S2) and X-ray crystallographic data (Tables
S3 to S12), and NMR spectra (Figures S7 to S37), are included in the
Supporting Information. This material is available free of charge via the
WWW at http://pubs.acs.org.
2
Hz), 123.7, 84.0 (m, JCF = 29 Hz), 71.7, 64.4, 18.9 ppm; 19F NMR (376
MHz, C6D6, 22 °C) δ = –76.69 ppm; HRMS (EI) m/z: [4-O2N-
9
(C6H4)CºMo(OC(CH3)(CF3)2)3]+ calcd [C19H13F18MoNO5] 774.9560;
10
11
12
13
14
15
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17
18
19
20
21
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23
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25
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found
774.9560;
Anal.
calcd
for
[4-O2N-
(C6H4)CºMo(OC(CH3)(CF3)2)3(DME)]: C, 32.00; H, 2.69; N, 1.62.
Found: C, 32.25; H, 2.60; N, 1.55. Orange prisms (yellow in transmission)
suitable for X-ray diffraction were grown from saturated pentane/toluene
(1:1) solution at –35 ºC. The compound crystallizes in the monoclinic
space group P 21, a = 9.4048(5) Å, b = 9.2801(5) Å, c = 17.2135(9) Å, β =
98.630(3)°, Z = 2, GOF on F2 = 0.994, R indices (all data) R1 = 0.0544,
wR2 = 0.0820.
AUTHOR INFORMATION
Corresponding Author
* ffischer@berkeley.edu
Author Contributions
All authors approved the final version of the manuscript.
Preparation
(C6H4)CºMo(OC(CH3)(CF3)2)3(DME)] (3h). A 10 mL flask was
charged under N2 with (154 mg, 0.2 mmol) in toluene (4 mL, [ ] = 50
of
[4-(O(C=O)C(CH3)2S(C=S)SCH2CH3)-
ACKNOWLEDGMENT
1
1
Research supported by the National Science Foundation under
contract number CHE-1455289, Berkeley Molecular Graphics and
Computation Facility is supported in part by NSF grant CHE-
0840505, Berkeley NMR Facility is supported in part by NIH grant
SRR023679A, and X-Ray Facility is supported in part by NIH
Shared Instrumentation Grant S10‐RR027172. The authors
acknowledge Dr. Hasan Celik for support with NMR acquisition,
Dr. Antonio DiPasquale for assistance with X-ray analysis, and Dr.
Rita Nichiporuk for assistance with ESI-TOF.
mM). The solution was cooled to –60 °C and 2h (78 mg, 0.2 mmol) was
added to the reaction mixture. The flask was immediately placed under
dynamic vacuum (0.1 torr) and removed from the cooling bath to warm to
24 °C. The solvent was removed in vacuum at 24 °C. The solid residue was
dissolved in minimum pentane and precipitated with perfluoromethylcy-
clohexane (10 mL) at 24 °C to yield 3e (122 mg, 0.12 mmol, 59%) as an
orange powder. 1H NMR (600 MHz, C6D6, 22 °C) δ = 7.05 (a q, J = 8.9 Hz,
2H), 3.23 (s, 6H), 3.05 (s, 4H), 2.87 (q, J = 7.4 Hz, 2H), 1.76 (s, 9H), 1.60
(s, 6H), 0.84 (t, J = 7.4 Hz, 3H) ppm; 13C{1H} NMR (151 MHz. C6D6, 22
1
°C) δ = 293.7, 222.4, 171.3, 152.4, 141.6, 131.9, 125.0 (q, JCF = 289 Hz),
2
122.0, 84.4 (m, JCF = 29 Hz), 71.9, 64.2, 56.2, 31.8, 25.5, 19.4, 13.0; 19F
NMR (376 MHz, C6D6, 22 °C) δ = –76.70 ppm; FTMS (ESI-TOF) m/z:
[4-(EtSCS2C(CH3)2CO2)-(C6H4)CºMo(OC(CH3)(CF3)2)3+AcO]–
calcd [C28H27F18MoO7S3] 1010.9686; found 1010.9653.
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