Synthesis of cis,syndiotactic-A-alt-B Copolymers from Enantiomerically Pure Endo-2-Substituted-5,6-Norbornenes

Article abstract of DOI:10.1021/jacs.7b01747

Cis,syndiotactic A-alt-B copolymers, where A and B are two enantiomerically pure endo-2-substituted-5,6-norbornenes with "opposite" chiralities of the endo-2-substituted-5,6-norbornene skeleton, can be prepared using Mo(N-2,6-Me₂C₆H₃)(CHCMe₂Ph)(OHMT)(pyrrolide) (1) as the initiator (OHMT = O-2,6-Mesityl₂C₆H₃). Formation of a high percentage of A-alt-B dyads is proposed to rely on an inversion of chirality at the metal with each propagating step and a kinetically preferred diastereomeric relationship between a given chirality at the metal in propagating species and the chirality of the endo-2-substituted-5,6-norbornene skeleton. We also demonstrate that A-alt-B copolymers can be modified to give new variations which may not be accessible through direct copolymerization.

Full text of DOI:10.1021/jacs.7b01747

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Communication
Synthesis of Cis,syndiotactic-A-alt-B Copolymers from
Enantiomerically Pure Endo-2-Substituted-5,6-Norbornenes
Eun Sil Jang, Jeremy M. John, and Richard R. Schrock
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01747 • Publication Date (Web): 29 Mar 2017
Downloaded from http://pubs.acs.org on March 29, 2017
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Synthesis of Cis,syndiotactic-A-alt-B Copolymers from
Enantiomerically Pure Endo-2-Substituted-5,6-Norbornenes
Eun Sil Jang, Jeremy M. John, and Richard R. Schrock*
Department of Chemistry 6-331, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
RECEIVED DATE (automatically inserted by publisher); rrs@mit.edu
Supporting Information Placeholder
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stereochemistries (cis vs. trans, and tacticity) are not
fixed. In 2011 we reported3a that polymerization of a
rac-2,3-disubstituted-5,6-norbornene yields a polymer
with a cis,syndiotactic basic structure3,4 in which the two
enantiomers of the monomer are incorporated alternately
to give poly(A-alt-A*), a special type of copolymer in
which A and A* are enantiomers. The most successful
of the initiators we tried was rac-1, a member of the
MAP (MonoAryloxidePyrrolide) family in which the
metal is a stereogenic center.5 A polymerization that
gives cis linkages is attributed to restricted formation of
any trans metallacyclobutane intermediate as a conse-
quence of the presence of the large terphenoxide ligand.
Important requirements for forming poly(A-alt-A*) are
(i) an inversion of the chirality at the metal6 with each
insertion and (ii) a preference for formation of one dia-
stereomeric intermediate in each step of the "cross-
polymerization" (A/A*) step. To our knowledge the
ABSTRACT: Cis,syndiotactic A-altꢀB copolymers,
where A and B are two enantiomerically pure endo-2-
substituted-5,6-norbornenes with "opposite" chiralities
of the endo-2-substituted-5,6-norbornene skeleton, can
be
prepared
using
Mo(N-2,6-
Me2C6H3)(CHCMe2Ph)(OHMT)(pyrrolide) (1) as the
initiator (OHMT = O-2,6-Mesityl2C6H3). Formation of a
high percentage of A-altꢀB dyads is proposed to rely on
an inversion of chirality at the metal with each propagat-
ing step and a kinetically preferred diastereomeric rela-
tionship between a given chirality at the metal in propa-
gating species and the chirality of the endo-2-
substituted-5,6-norbornene skeleton. We also demon-
strate that A-altꢀB copolymers can be modified to give
new variations which may not be accessible through
direct copolymerization.
only other ROMP that gives
a
stereoregular
Copolymers in which monomers A and B are incorpo-
rated in an alternating manner, poly(A-alt-B), are rare.1
A significant number of AB copolymers have been
formed through ring-opening metathesis polymerization
(ROMP),2 although in the
vast majority of cases the
polymers do not have pure
A-alt-B structures, and the
cis,syndiotactic-A-alt-A* copolymer was reported by
Hamilton and coworkers,7 who deduced that inversion of
configuration of the metal site in a heterogeneous cata-
lyst was required for formation of cis,syndiotactic-
poly(A-alt-A*) from rac-1-methylbicyclo[2.2.1]hept-2-
ene.
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Figure 1. The partial 13C NMR (CDCl3, 20 °C, 126 MHz)
spectrum (left) and H NMR (CDCl3, 20 °C, 500 MHz) spectrum
(right) of cis,syndiotactic-poly(ASR-alt-BRS).
We have shown recently that two monomers that have
1
1
2
3
4
5
6
7
8
the same 2,3-disubstituted-5,6-norbornene "chiral mo-
tif," but slightly different R and R' substituents in each
monomer (eq 1) can be polymerized through ROMP
initiated by 1 to give a poly(A-alt-B) copolymer in
which A and B are not strictly enantiomers.8 This struc-
ture is readily confirmed through 13C NMR analysis of
the polymers, which reveals resonances for four differ-
ent olefinic carbons. Proton NMR spectra can be defini-
tive, but are often compromised by overlap of proton
resonances and non-first order olefinic proton resonanc-
es. The non-propagating "errors" in the polymer shown
in eq 1 were proposed8 to be both AA and BB
trans,isotactic dyads3b formed from trans metallacyclo-
butane intermediates that "flip over" before they open to
give a syn propagating species, a process that leads to
retention of configuration at the metal for that step and
consequently an isotactic structure.3a We turned to an
exploration of enantiomerically pure endo-2-substituted-
5,6-norbornenes in order to determine whether only di-
Me
i-Pr
i-Pr
R
R
S
S
R
R
R
O
Me
O
Me
O
O
O
O
i-Pr
CSR
CRS
CRR
O
R
9
O
O
R
10
11
12
13
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60
O
O
R
O
O
O
O
O
O
O
ARR
DR
ER
Figure 2. Six additional monomers explored here.
The search for A-alt-B copolymers was expanded to in-
clude the monomers shown in Fig 2. Copolymerization of
25 equiv of ASR and 25 equiv of CRS (0.1 M in toluene-d8)
was complete in less than one minute to give a toluene-d8-
soluble polymer whose 13C NMR spectrum in CDCl3
showed four different olefinic carbon resonances, con-
sistent with the polymer having a cis,syndiotactic-ASR-alt-
CRS structure (Fig 3, left). The partial proton NMR spec-
trum (Fig 3, right) is not convincing that the degree of or-
der is high, in part because the pantolactone methine reso-
nance (at 5.36 ppm) overlaps with the four olefinic proton
resonances.
substitute monomers can form an A-alt-B copolymer.
R
R'
R
H
H
R'
+
(1)
H
R
R'
H1
R'
H4
A
B
H2
H3
R
H
Copolymerization of a mixture of 25 equiv of ASR
and 25 equiv of BRS (eq 2) with 1 as initiator (0.1 M in
tol-d8) was complete in less than one minute to give a
Me
GPC
analysis
was
performed
on
four
cis,syndiotactic-poly(ASR-alt-CRS) samples made from
M
N
e
O
R
O
O
2
m
o
l
%
1
O
O
n
S
R
O
+
S
O
O
O
O
O
T
o
l
-d
8
,
2
2
°
C
O
O
O
O
A
SR
B
R
S
N
M
O
e
c
S
R
R
S
N
O
O
O
O
O
O
n
R
2 mol% 1
O
O
O
O
S
(2)
+
S
R
O
Tol-d8, 22 °C
O
O
O
O
N
ASR
BRS
O
Me
cis,syndiotactic-poly(ASR-alt-BRS
)
polymer that precipitated out of toluene. The partial 13C
NMR spectrum of this polymer in CDCl3 (Fig 1 left)
showed four olefinic carbon resonances consistent with
it having the cis,syndiotactic,alt structure. Although the
1
partial H NMR spectrum of this polymer showed two
pseudo-triplet olefinic proton resonances at 5.21 and
5.28 ppm having 3JHH = 10 Hz (Fig 1 right), the remain-
ing two olefinic proton resonances overlap with each
other and with methine proton resonances belonging to
the pantolactone (at 5.36 ppm) and the methylsuccin-
imide (at 5.37 ppm). The absence of proton resonances
corresponding to trans,isotactic ASRASR and/or BRSBRS
dyads near 5.46 ppm (estimated8) suggests that
cis,syndiotactic-poly(ASR-alt-BRS) contain few, if any,
trans,isotactic errors.
Figure 3. The partial 13C NMR (CDCl3, 25 °C, 125 MHz) spec-
trum (left) and 1H NMR (CDCl3, 25 °C, 500 MHz) spectrum
(right) of cis,syndiotactic-poly(ASR-alt-CRS).
25, 50, 75, and 100 equivalents of ASR and CRS (each).
The olefinic proton patterns in proton NMR spectra were
all the same as that shown in Fig 3 (right). The GPC
peaks were monomodal, the molecular weights in-
creased steadily, and the Đ values were between 1.35
and 1.55. (See SI for details.)
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1
errors (cf. Fig 5 left). The partial H NMR spectrum of
this copolymer (Fig 6 right) showed two complex pat-
terns that result from overlap of the methine resonances
in the two pantolactone groups with two sets of olefinic
1
2
3
4
5
6
7
8
9
proton resonances.
Formation of cis,syndiotactic-
poly(ASR-alt-ARR) also suggests that it is largely the
"primary" chirality in the monomer, what we call the
"chiral motif," that regulates formation of the highly
structured copolymer.
Figure 4. The partial 13C NMR (CDCl3, 20 °C, 126 MHz) spec-
trum (left) and H NMR (CDCl3, 20 °C, 500 MHz) spectrum
1
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(right) of cis,syndiotactic-poly(CSR-alt-BRS). *Residual catalyst
decomposition peaks.
A third cis,syndiotactic polymer was prepared from
25 equiv of CSR and 25 equiv of BRS. The olefinic re-
13
gion of the C NMR spectrum of the resulting polymer
(Fig 4 left) clearly shows four distinct olefinic carbon
resonances. The resonances at 128.1 and 126.3 ppm are
believed to result from catalyst decomposition products,
while the small resonance at 131.4 ppm we propose aris-
es from an insertion error (exact type unknown). The
olefinic region of the 1H NMR spectrum of
cis,syndiotactic-poly(CSR-alt-BRS) (Fig 4 right) is com-
plicated by overlap and is not itself convincing that the
polymer structure is highly regular.
Figure 6. The partial 13C NMR (CDCl3, 20 °C, 126 MHz)
spectrum (left) and H NMR (CDCl3, 20 °C, 500 MHz) spectrum
1
(right) of cis,syndiotactic-poly(ASR-alt-ARR).
Copolymerization of ASR and DR (25 equiv of each)
produced cis,syndiotactic-poly(ASR-alt-DR), whose car-
bon and proton NMR spectra are similar to those in Fig
5 (see SI).
Copolymerization of ASR and CRR (25 equiv of each)
proceeded smoothly to give a toluene-d8-soluble
copolymer. The partial 13C NMR spectrum (Fig 5 left)
showed primarily four olefinic resonances consistent
with the formation of cis,syndiotactic-poly(ASR-alt-
CRR). However, a significant number of minor olefinic
resonances between 133 and 134 ppm and between 131
and 132 ppm suggest that the structure is not as regular
In order to test whether an ester functionality that
contains a second chiral entity is necessary, we prepared
ER from BRS and paired it with ASR (25 equiv of each).
The 13C NMR spectrum (Fig 7 left) of the resulting
cis,syndiotactic-poly(ASR-alt-ER) is without significant
1
errors. The H NMR spectrum is one of the more con-
as that for cis,syndiotactic-poly(ASR-alt-CRS) (Fig 3
vincing that the structure is highly regular and the ole-
finic proton NMR resonances are first order. The syn-
thesis of cis,syndiotactic-poly(CSR-alt-ER) is nearly as
successful (see SI), while the synthesis of one A-alt-B
copolymer in which one of the monomers is the TMS
derivative of ER was relatively unsuccessful (see SI).
1
left). The partial H NMR spectrum of cis,syndiotactic-
poly(ASR-alt-CRR) (Fig 5 right) is even less informative
than that for cis,syndiotactic-poly(ASR-alt-CRS) (Fig 3).
We conclude that the "second" chirality in the monomer
(here CRR or CRS) is not a major determinant of the
overall polymer structure, at least in this case.
Figure 7. The partial 13C NMR (CDCl3, 20 °C, 126 MHz) spec-
1
trum (left) and H NMR (CDCl3, 20 °C, 500 MHz) spectrum
Figure 5. The partial 13C NMR (CDCl3, 20 °C, 126 MHz) spec-
1
(right) of cis,syndiotactic-poly(ASR-alt-ER). *Residual catalyst
decomposition peaks.
trum (left) and H NMR (CDCl3, 20 °C, 500 MHz) spectrum
(right) of cis,syndiotactic-poly(ASR-alt-CRR).
Cis,syndiotactic poly(ARS-alt-BRS) was hydrolyzed
over the course of 4 days in a mixture of CHCl3, THF,
and water that contained a large excess of LiOH. The
Copolymerization of ASR and ARR in toluene-d8 yield-
ed cis,syndiotactic-poly(ASR-alt-ARR). The partial 13C
NMR spectrum contained four sharp olefinic carbon
resonances (Fig 6 left) with just a hint of resonances (at
~131.5 and ~133.5) that we again ascribe to structural
result is formation of cis,syndiotactic poly(AS-alt-AR)
1
(eq 3) whose 13C and H NMR spectra in DMSO-d6 are
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Page 4 of 6
shown in Fig 8.
We also have shown that We conclude that cis,syndiotactic-A-alt-B copoly-
1
2
3
4
5
6
7
8
cis,syndiotactic-poly(ASR-alt-ER) can be transesterified
as shown in eq 4 to yield cis,syndiotactic-poly(AS'-alt-
ER) (Fig 9). The ability to modify A-alt-B copolymers
to yield others that may not be prepared directly from
the respective monomers adds a significant degree of
flexibility to the method.
mers can be formed from enantiomerically pure endo-2-
substituted-5,6-norbornenes with a substituent at C2 of
the appropriate size that allows the configuration of each
chiral metal in propagating intermediates derived from 1
to distinguish the primary chiral motif of each chiral
norbornene in the mixture. We propose that only two
propagation steps out of a possible 25 (syn or anti alkyli-
denes, cis or trans metallacycles, two chain ends, two
monomers, and two M=C bond faces) dominate to give
the results we have observed here; these two are M=AP
(where monomer A is last inserted and P is polymer)
reacting with B and M*=BP reacting with A (where M
and M* are the two configurations at M), all in a
cis,syndiotactic manner via syn alkylidene intermediates
and with inversion of the metal configuration (M and
M*) in each step. A-alt-B copolymers prepared from
monomers that have a single endo substituent on the
chiral C2 position (eqs 2 and 3) may be more successful
in the long run for preparing alternating copolymers in
large variety than one based on 2,3-disubstituted-5,6-
norbornenes (eq 1). It remains to be seen what other
chiral motifs will allow A-alt-B polymers to be synthe-
sized with high fidelity, what substituents are optimum,
what functionalities can be changed through post
polymerization modification to yield A-alt-B polymers
that cannot be prepared directly from the corresponding
monomers, and what are the errors and how they can be
avoided.
M
N
e
O
O
R
O
2
m
o
l
%
1
O
O
O
n
S
R
O
+
S
O
O
O
O
T
o
l
d
8
,
2
2
°
C
O
A
O
O
O
SR
B
R
S
O
N
M
e
c
S
R
R
S
O
O
O
O
H
O
n
n
9
(3)
LiOH
O
H
O
O
O
O
10
11
12
13
14
15
16
17
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19
20
21
22
23
24
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53
54
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56
57
58
59
60
O
N
cis,syndiotactic-poly(AS-alt-AR)
O
Me
cis,syndiotactic-poly(ASR-alt-BRS
)
Figure 8. The partial 13C NMR (DMSO-d6, 20 °C, 126 MHz)
spectrum (left) and 1H NMR (DMSO-d6, 20 °C, 500 MHz) spec-
trum (right) of cis,syndiotactic-poly(AS-alt-AR).
ASSOCIATED CONTENT
Experimental details for all reactions and all supporting
NMR characterization of polymers. Supporting Infor-
mation is available free of charge via the Internet at
http://pubs.acs.org.
M
N
e
O
R
O
O
2
m
o
l
%
1
O
O
O
O
O
O
n
S
R
O
+
S
O
O
O
O
T
o
l
-d
8
,
2
2
°
C
O
A
SR
B
R
S
O
N
M
e
c
S
R
R
S
O
O
O
O
O
n
n
(4)
NaOMe
MeOH
O
O
Me
Me
Me
O
cis,syndiotactic-poly(ASR-alt-ER)
cis,syndiotactic-poly(AS'-alt-ER)
AUTHOR INFORMATION
Corresponding Author rrs@mit.edu
Author Contributions ESJ and JMJ contributed equally to
this work.
Funding Sources We are grateful to the Department of
Energy (DE-FG02-86ER13564) for research support.
ACKNOWLEDGMENT
The authors thank Y. Jiang in the laboratory of Professor
J. Johnson in this department for GPC analyses of the
cis,syndiotactic-poly(ASR-alt-CRS) samples.
Figure 9. The partial 13C NMR (CDCl3, 20 °C, 126 MHz) spec-
1
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'
(right) of cis,syndiotactic-poly(AS -alt-ER).
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