Bidirectional metal-free ROMP from difunctional organic initiators

Article abstract of DOI:10.1002/pola.28704

Ring-opening metathesis polymerization (ROMP) has enabled access to a diverse array of polymeric materials. Inspired by the breadth of tunability afforded by metal-mediated ROMP, opportunities from an organocatalyzed variant were investigated. We first investigated divinyl ether initiators with different relative orientation of alkoxy moieties. Our intention was to preserve the spacing between reactive sites of the vinyl ethers while varying the nature of the monomer incorporation. Specifically, initiator 1 would incorporate monomers via insertion at the center of the outwardlygrowing polymer, whereas initiator 2 would propagate via an active chain end mechanism. Each initiator was found to undergo metal-free ROMP of norbornene (3) under previously reported conditions using 2,4,6-tris(4-methoxyphenyl)pyrylium tetrafluoroborate (4) as a photoredox catalyst. In comparison with 1, initiator 2 also proceeded with high conversion of monomer although the initiator efficiency was found to be lower than for 1, and more consistent with previous polymerizations using ethyl propenyl ether. Attempts to identify products of deleterious reaction pathways from the initiators were met with limited success. Despite the differences in initiator efficiency, we found that 1 and 2 gave comparable rates of polymerization.

Full text of DOI:10.1002/pola.28704

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Bidirectional Metal-Free ROMP from Difunctional Organic Initiators
Pengtao Lu, Nasser M. Alrashdi, Andrew J. Boydston
Department of Chemistry, University of Washington, Seattle, Washington 98195
Correspondence to: A.J. Boydston (E-mail: ajb1515@uw.edu)
Received 14 March 2017; accepted 18 June 2017; published online 00 Month 2017
DOI: 10.1002/pola.28704
KEYWORDS: block copolymers; metathesis; organocatalysis; photoredox; polymerization
(Fig. 2) using 2,4,6-tris(4-methoxyphenyl)pyrylium tetrafluor-
oborate (4) as a photoredox catalyst. Averaged data from
three parallel runs are summarized in Table 1. Using an ini-
tial monomer to vinyl ether molar ratio ([3]0/[VE]0) of 50:1,
initiator 1 provided polynorbornene 5 with excellent agree-
ment between theoretical and experimental Mn values (entry
1). Notably, these initiator efficiencies, which are near unity,
are considerably higher than observed for organocatalyzed
ROMP using ethyl propenyl ether as initiator under similar
conditions. In comparison with 1, initiator 2 also proceeded
with high conversion of monomer (entry 2) although the ini-
tiator efficiency was found to be lower than for 1, and more
consistent with previous polymerizations using ethyl pro-
penyl ether. Attempts to identify products of deleterious
reaction pathways from the initiators were met with limited
success.
INTRODUCTION Ring-opening metathesis polymerization
(ROMP) has enabled access to a diverse array of polymeric
materials.1,2 The structural control that has been made possible
from well-defined metal-based initiators includes a broad scope
of monomer classes with diverse functional group tolerance,3–9
stereocontrol,2,10–12 sequence specificity,13–15 and copolymer
constructs.16–22 Inspired by the breadth of tunability afforded
by metal-mediated ROMP, we have recently investigated oppor-
tunities from an organocatalyzed variant.23–28
The organocatalyzed approach makes use of vinyl ethers as
initiators, which are activated in situ via one-electron oxida-
tion. We envisioned the use of vinyl ether initiators to pro-
vide unique synthetic routes to multitopic initiators, such as
those that could lead to bidirectional polymer chain growth,
multiarm star polymers, or graft-from brush polymers. Such
architectures have been achieved via metal-mediated ROMP,
but to our knowledge the organocatalyzed method has not
been put to this challenge. A potential caveat of the vinyl
ether initiators is that high effective concentrations of initia-
tor sites could lead to unwanted intramolecular reactions
between oxidized and neutral species. To investigate the fea-
sibility of organocatalyzed ROMP from multitopic initiators,
we evaluated a series of small and macromolecular initiators
bearing two or more vinyl ethers.
Despite the differences in initiator efficiency, we found that
1 and 2 gave comparable rates of polymerization (Fig. 2).
This suggested to us that since 2 suffers from some nonpro-
ductive initiator consumption (lower initiator efficiency), the
rate of monomer incorporation with 2 is likely higher than
for 1. We speculate that disparate steric access to the active
vinyl ether site is the cause of the different propagation
rates.
The vinyl ether moieties in polymer 5 provide opportunities
for postpolymerization modification and controlled degradation.
In our studies, we were able to confirm the bidirectional chain
growth from initiator 1 via hydrolysis experiments. We first
prepared a higher molecular weight sample (Table 1, entry 3,
We first investigated divinyl ether initiators with different
relative orientation of alkoxy moieties (1 and 2, Fig. 1). Our
intention was to preserve the spacing between reactive sites
of the vinyl ethers while varying the nature of the monomer
incorporation. Specifically, initiator
1 would incorporate
-
Mn 5 46.0 kDa, D 5 1.2) such that the daughter fragments
monomers via insertion at the center of the outwardly-
growing polymer, whereas initiator 2 would propagate via
an active chain end mechanism.
would be of sufficient molecular weight for analysis. After sub-
jecting this sample of polymer 5 to vinyl ether hydrolysis using
Amberlyst 15 ion-exchange resin, we found that the resulting
Each initiator was found to undergo metal-free ROMP of
norbornene (3) under our previously reported conditions
-
polynorbornene product had
roughly half that of the initial polymer (Fig. 3). These results
a Mn 5 24.0 kDa (D 5 1.1),
Additional Supporting Information may be found in the online version of this article.
C
V
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FIGURE 3 GPC traces of polymer 5 (Mn 5 45 kDa) as isolated
via precipitation (solid) and after hydrolysis (dashed, Mn 5 24
kDa).
FIGURE 1 Top, small molecule divinyl ether initiators and pyry-
lium photoredox catalyst used in this study. Bottom, organoca-
talyzed ROMP of norbornene.
-
product polymer with Mn 5 5.9 kDa (D 5 1.3), slightly less
than half that of the original polymer (minus the PPG block).
Additionally, a low molecular peak was observed that was
consistent with the retention time of the PPG initiator. These
results were consistent with roughly uniform polymer growth
from each initiator site.
Using a similar approach, we also prepared difunctional mac-
roinitiator 9 based upon poly(ethylene glycol) (PEG). Again,
MF-ROMP provided an ABA triblock copolymer (10) after
moderate (51%) monomer conversion. GPC analysis of 10
(Fig. 5, middle) after isolation by precipitation revealed
-
Mn 5 8.6 kDa (D 5 1.5). After hydrolysis with Amberlyst 15
resin, the resulting polymer was found to have Mn 5 5.1 kDa
-
(D 5 1.6).
FIGURE 2 Conversion versus time for organocatalyzed ROMP
from initiators 1 (black) and 2 (white).
We next turned our attention toward grafting from a statisti-
cal copolymer. Toward this end, macroinitiator 11 was pre-
pared by a sequence of atom-transfer radical polymerization,
dehalogenation of the chain end, and isomerization of the
side chain olefins (see Experimental Section). Analysis of 11
are consistent with about equal chain lengths having been syn-
thesized from each initiator site.
Motivated by the success of the bidirectional chain growth,
we considered broader applications in the synthesis of
degradable triblock copolymers. Toward this end, we prepared
a ditopic macroinitiator (7) from poly(propylene glycol) (PPG)
as shown in Figure 4 (Mn 5 1.0 kDa). Organocatalyzed ROMP
proceeded smoothly with 67% monomer conversion to yield
-
by GPC (Fig. 5, bottom) revealed Mn 5 2.2 kDa (D 5 1.1) and
1H NMR analysis was consistent with an average of three
vinyl ether moieties per chain.
When subjected to MF-ROMP conditions, polymer 12 was
obtained with 64% monomer conversion. The isolated poly-
-
-
triblock copolymer 8 having Mn 5 15.9 kDa (D 5 1.2) based
mer was found to have Mn 5 28.2 kDa (D 5 1.6) by GPC analy-
upon GPC analysis of the polymer after isolation by precipita-
tion (Fig. 5, top). Upon hydrolysis, GPC analysis revealed a
sis. After treatment with Amberlyst 15 ion-exchange resin, the
resulting polymer was found to have a reduced molecular
TABLE 1 Summary of Results for Bidirectional ROMP from 1 and 2
b
c
c
d
Entry Initiator [3]0/[VE]0 Time (min) Conversiona (%) Mn, theo (kDa) Mn, GPC (kDa) Mw, GPC (kDa)
IEe (%)
-
D
1
2
3
1
2
1
50
75
75
35
82 6 4
83 6 4
33%
7.9 6 0.4
8.1 6 0.3
31.2
8.0 6 0.9
12.3 6 2.2
46.0
10.3 6 0.5
14.7 6 2.7
54.4
1.29 6 0.08 98 6 7
1.20 6 0.01 67 6 12
50
500
1.2
68
c
Rows 1 and 2 reflect average of three runs, error 5 1 SD.
Determined by gel permeation chromatography using in-line multian-
gle laser light scattering and RI detection.
a
Monomer conversion as judged by 1H NMR spectroscopy of crude
d
reaction mixture.
Molecular weight dispersity.
b
e
Theoretical number-average molecular weight based upon initial
Initiator efficiency 5 Mn, theo/Mn, GPC.
monomer to initiator loading and monomer conversion.
2
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doublet, dt 5 doublet of triplets, q 5 quartet, m 5 multiplet,
br 5 broad peak), coupling constants (Hz) and integration.
Gel permeation chromatography (GPC) was performed using
a GPC setup consisting of: an Agilent pump, three in-line col-
umns, and light scattering and refractive index detectors
(Wyatt Technology Corp.) with THF as the mobile phase.
Weight average molecular weight (Mw) and number average
molecular weight (Mn) were calculated from light scattering
and refractive index data, respectively, using Astra software
from Wyatt Technology Corp.
Synthesis of Initiator 1
To a suspension of NaH (2.8 g, 69.96 mmol) in DMF (20 mL)
at 0 8C was added a DMF solution of 1,6-hexanediol drop-
wise (20 mL, 1.06 M, 21.2 mmol). The ice bath was then
removed and the reaction mixture was stirred at room tem-
perature for 30 min. Then, the ice bath was returned and
allyl bromide (4 mL, 46.5 mmol) was added dropwise into
the solution at 0 8C. The reaction was then stirred for 17 h,
FIGURE 4 MF-ROMP from macroinitiators.
-
weight of Mn 5 18.3 kDa (D 5 1.7). Assuming complete hydro-
lysis of all vinyl ether moieties, the molecular weight of the
cleaved polynorbornene daughter fragments are consistent
with about 53% of the vinyl ether moieties having initiated
MF-ROMP.
In conclusion, we have demonstrated bidirectional polymer
growth in organocatalyzed ROMP from difunctional initiators.
Evaluation of inwardly-growing polymers versus outwardly-
growing polymers revealed that incorporation of monomers
at chain-end and chain-centered active sites was successful.
The ability to grow polymer chains from internal active sites
enabled rapid access to cleavable copolymers based upon
PPG, PEG, and polystyrene macroinitiators. The relative ease
with which vinyl ether moieties can be installed on potential
initiator structures holds promise for access to a broader
diversity of organocatalyzed ROMP nanostructures. Herein,
we established the feasibility and first steps toward this end.
EXPERIMENTAL
Materials and Methods
Dimethylformamide (DMF), dimethyl sulfoxide (DMSO),
dichloromethane (CH2Cl2), and tetrahydrofuran (THF) were
obtained from a solvent purification system. For polymeriza-
tion of norbornene, CH2Cl2 was dried over 4 Å molecular
sieves before use. Norbornene was sublimed prior to use.
The pyrylium tetrafluoroborate was prepared according to
literature procedure.29 All other reagents and solvents were
obtained from commercial sources and used as received
unless otherwise noted. All polymerizations were carried out
in standard borosilicate glass vials with magnetic stirring.
Irradiation of photochemical reactions was done using a 2 W
Miracle Blue LED indoor gardening bulb purchased from
Amazon. 1H and 13C NMR spectra were recorded on Bruker
AVance 300 MHz or 500 MHz spectrometers. Chemical shifts
are reported in delta (d) units, expressed in parts per million
(ppm) downfield from tetramethylsilane using the residual
protio-solvent as an internal standard (CDCl3, 1H: 7.26 ppm
and 13C: 77.0 ppm). Data are reported as follows: chemical
shift, multiplicity (s 5 singlet, d 5 doublet, dd 5 doublet of
FIGURE 5 GPC traces of macroinitiators (dotted), copolymers
(solid), and hydrolyzed polymers (dashed). Top, GPC traces for
7, 8, and hydrolysis of 8. Middle, GPC traces for 9, 10, and
hydrolysis of 10. Bottom, GPC traces for 11, 12, and hydrolysis
of 12.
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during which time the ice bath expired. Deionized water
(5 mL) was then added dropwise into the reaction mixture
at 0 8C. More deionized water (100 mL) was then poured
into the resulting suspension and the final mixture was
extracted with Et2O (4 3 50 mL). The combined organic
layers were dried with Na2SO4 and decanted. The solvent
was then removed under reduced pressure to give the crude
product as a yellow oil, which was then purified by flash
chromatography on silica gel (10:1 hexanes/ether). The spec-
tral data matched those previously reported.30
1H NMR (500 MHz, CDCl3) d 5 6.27 (d, J 5 12 Hz, 1H, trans)
5.85 (d, J 5 6 Hz, 0.7H, cis) 4.72 (dt, J 5 12 Hz, 7.5 Hz, 1H,
trans) 4.33 (dt, J 5 6 Hz, 7.5 Hz, 0.7H, cis) 3.56 (s, 2H, cis)
3.49 (s, 3H, trans) 2.04 (q, J 5 7 Hz, 1.4H, cis) 1.90 (q, J 5 7
Hz, 2.6H, trans) 1.22–1.35 (m, 16H, cis/trans); 13C NMR (125
MHz, CDCl3) d 5 147.1, 146.1, 107.3, 103.4, 59.5, 56.0, 30.9,
30.0, 29.8, 29.6, 29.4, 29.2, 27.8, 24.0; GC-MS (m/z) calcd. for
C16H30O2, 254.2; found, 254.3.
Synthesis of PPG Divinyl Ether 7
To a suspension of NaH (3.0 g, 75.00 mmol) in DMF (20 mL)
at 0 8C was added a DMF solution of PPG-diol dropwise
(10 mL, 0.50 M, 5.00 mmol). The reaction mixture was then
removed from the ice bath and stirred at room temperature
for 30 min. Then, the mixture was returned to an ice bath
and allyl bromide (4.3 mL, 50 mmol) was added dropwise.
The reaction mixture then stirred for 24 h, during which
time the ice bath expired. Deionized water (5 mL) was then
added dropwise into the reaction mixture at 0 8C. More
deionized water (100 mL) was then poured into the resulting
suspension and the final mixture was extracted with Et2O (4
3 50 mL). The combined organic layers were dried with
Na2SO4 and then decanted. The solvent was then removed
under reduced pressure to give the crude product as a yellow
oil, which was then purified by flash chromatography on silica
gel (20:1 CH2Cl2/methanol) to give a,x-diallyl PPG.
The diallyl ether obtain the previous step (1.1 g, 5.5 mmol)
was dissolved in DMSO (30 mL) at 0 8C and then potassium
tert-butoxide (5.0 g, 40.9 mmol) was added in a single por-
tion. The reaction was stirred for 6 h, during which time the
ice bath expired. Then, deionized water (5 mL) was added
dropwise into the solution at 0 8C. More deionized water
(100 mL) was then poured into the mixture and the result-
ing suspension was extracted with Et2O (4 3 50 mL). The
combined organic layers were dried with Na2SO4 and then
decanted. The solvent was then removed under reduced
pressure to give the crude product as a yellow oil, which
was purified by passing through a silica plug using hexanes
as eluent (75% yield over two steps). The spectral data
matched those previously reported.30
Synthesis of Initiator 2
1H-NMR (500 MHz, CDCl3) d 5 5.93–5.85 (m, 2H) 5.24 (d,
J 5 17.5 Hz, 2H) 5.11 (d, J 5 10.5 Hz, 2H) 4.05–4.02 (m, 4H)
3.75–3.25 (br, 52H) 1.14–1.06 (br, 52H).
To a suspension of 1,12-dodecanediol (2.02 g, 10 mmol) in
CH2Cl2 (30 mL) was added 2,2,6,6-tetramethyl-1-piperidiny-
loxy (TEMPO) (313 mg, 2 mmol) and PhI(OAc)2 (7.09 g, 22
mmol). After 20 h, the reaction mixture was diluted with
CH2Cl2 (30 mL) and the solution was washed with saturated
aqueous Na2S2O3 solution (60 mL). The aqueous phase was
then extracted with CH2Cl2 (3 3 20 mL). The combined
organic layers were dried with Na2SO4 and then decanted.
The solvent was removed under reduced pressure and the
crude product was then purified by flash chromatography on
silica gel (8:1 hexanes/ethyl acetate). The spectral data
matched those previously reported.31
To a solution of the PPG-diallyl ether (2.7 g, 2.7 mmol) in
DMF (20 mL) at 0 8C was added potassium tert-butoxide
(2.0 g, 16.3 mmol) in a single portion. The reaction mixture
was then stirred for 24 h, during which time the ice bath
expired. Then, deionized water (5 mL) was added dropwise
into the solution at 0 8C. More deionized water (100 mL)
was then poured into the solution and the resulting suspen-
sion was extracted with Et2O (4 3 50 mL). The combined
organic layers were dried over Na2SO4 and then decanted.
The solvent was then removed under reduced pressure to
give the crude product as a yellow oil. Purification by flash
chromatography on silica gel (15:1 CH2Cl2/methanol) gave
initiator 7 in 70% yield for the two steps.
A solution of potassium tert-butoxide (2.97 g, 24.3 mmol) in
8
mL of THF was slowly added to a solution of
[Ph3PCH2OCH3]Cl (8.3 g, 24.3 mmol) in 30 mL of THF. After
stirring the solution for 1 h, a solution of the dialdehyde
obtained from the prior step (1.6 g, 8.1 mmol) in 10 mL THF
was added dropwise. The resulting mixture was then stirred
at room temperature for 2 h, and then diluted with ether
and washed with deionized water. The organic phase was
dried with Na2SO4 and the solvent was evaporated. The
crude mixture was then taken up in hexanes and then deca-
nted. The filtrate was then concentrated under reduced pres-
sure. Residual Ph3P was then consumed by stirring the
crude mixture for 12 h with 1.5 mL of CH3I, leading to pre-
cipitation of [Ph3PCH3]I. The mixture was then filtered
through a plug of silica with diethyl ether as eluent. The
product was prepared according to above procedure in 50%
yield over two steps (2:3 cis to trans ratio as judged by 1H
NMR spectroscopy).
1H-NMR (300 MHz, CDCl3) d 5 6.05–5.97 (m, 2H) 4.40–4.29
(m, 2H) 3.89–3.77 (m, 2H) 3.62–3.26 (br, 58H) 1.54 (dd,
J 5 6.9 Hz, 1.8Hz, 6H) 1.19 (dd, J 5 6.6Hz, 2.4Hz, 6H) 1.15–
1.07 (br, 54H).
Synthesis of PEG Divinyl Ether 9
To a suspension of NaH (3.0 g, 75.00 mmol) in DMF (20 mL)
at 0 8C was added a DMF solution of PEG-diol dropwise
(10 mL, 0.50 M, 5.00 mmol). The reaction mixture was
stirred at room temperature for 30 min. Then, allyl bromide
(4.3 mL, 50 mmol) was added dropwise into the solution at
0 8C. The reaction flask was then removed from the ice bath.
After 24 h, the reaction flask was again placed into an ice
bath and then deionized water (5 mL) was added dropwise
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into the solution. Next, more deionized water (100 mL) was
poured into the mixture and the suspension was extracted
with CH2Cl2 (4 3 50 mL). The combined organic layers were
dried with Na2SO4 and then decanted. The solvent was then
removed under reduced pressure to give the crude product
as a yellow solid which was then purified by flash chroma-
tography (20:1 CH2Cl2:methanol).
mmol, 80 equiv.), the monomer obtained in the previous
step (977 mg, 5.6 mmol, 20 equiv.), toluene (2 mL), and a
magnetic stir bar were added into a 20 mL scintillation vial.
A solution of CuBr (20.2 mg, 0.14 mmol, 0.5 equiv.) and
PMDETA (24.2 mg, 0.14 mmol, 0.5 equiv.) in toluene
(1.0 mL) was then added to the reaction mixture. The vial
was sealed with a PTFE-lined screw cap and taken to a
hood. There, the reaction solution was stirred at 90 8C for
3 h. Then, the reaction mixture was opened to air after the
vial cooled to room temperature. The viscous solution was
diluted with CH2Cl2 and the solution was filtered through a
neutral alumina plug. The solution volume was reduced
under vacuum to about 3 mL under reduced pressure. The
solution was then added dropwise into an excess of cold
MeOH causing the polymer to precipitate from solution. The
precipitated polymer was collected by filtration and dried
under reduced pressure. The polymer was isolated in 20%
yield according to above procedure.
1H-NMR (500 MHz, CDCl3) d 5 5.93–5.85 (m, 2H) 5.20 (d,
J 5 17.5 Hz, 2H) 5.11 (d, J 5 10.5 Hz, 2H) 3.98–3.91 (m, 4H)
3.62–3.54 (br, 92H).
To a solution of PEG-diallyl ether (2.7 g, 2.7 mmol) in DMF
(20 mL) at 0 8C was added potassium tert-butoxide (2.0 g,
16.32 mmol) in a single portion. The reaction flask was then
removed from the ice bath. After 24 h, the reaction flask was
again placed into an ice bath and then deionized water
(5 mL) was added dropwise into the reaction at 0 8C. Next,
more deionized water (100 mL) was poured into the mixture
and the suspension was extracted with CH2Cl2 (4 3 50 mL).
The combined organic layers were dried with Na2SO4 and
then decanted. The solvent was then removed under reduced
pressure to give the crude product as a yellow solid which
was purified by flash chromatography (15:1 CH2Cl2:metha-
nol). The initiator 9 was isolated in 80% yield for the two
steps.
1H NMR (300 MHz, CDCl3) d 5 7.35–6.28 (br, 87H) 6.09–5.85
(br, 3H) 5.42–5.12 (br, 6H) 4.60–4.25 (br, 6H) 4.15–3.80 (br,
6H) 2.60–0.90 (br, 54H).
In a nitrogen-filled drybox, the copolymer obtained in the
previous step, dry toluene (2 mL) and a magnetic stir bar
were added into a 20 mL scintillation vial. A solution of
CuBr (7.2 mg, 0.05 mmol) and PMDETA (8.7 mg, 0.05 mmol)
in toluene (1.0 mL) was then added to the reaction mixture.
The vial was then sealed with a rubber septum. Then, tribu-
tyltin hydride (80 mL) was added into the reaction mixture
by syringe. The reaction solution was stirred at 90 8C for
5 h. Then, the vial was opened to air after cooling back to
room temperature. The viscous solution was diluted with
CH2Cl2 and then filtered through a neutral alumina plug. The
solution volume was reduced under vacuum to about 3 mL
under reduced pressure. The solution was then added drop-
wise into an excess of cold MeOH causing the polymer to
precipitate from solution. The precipitated polymer was col-
lected by filtration and dried under reduced pressure. The
polymer was isolated in 90% yield according to above
prodecure.
1H-NMR (300 MHz, CDCl3) d 5 6.01–5.91 (m, 2H) 4.43–4.31
(m, 2H) 3.89–3.82 (m, 6H) 3.66–3.61 (br, 92H) 1.54 (dd,
J 5 6.9 Hz, 1.8 Hz, 6H).
Synthesis of Polystyrene Macroinitiator 11
4-Vinylbenzyl alcohol was first obtained according to litera-
ture procedure.32 To a suspension of NaH (2.7 g, 67 mmol)
in DMF (75 mL) at 0 8C was added a DMF solution of 4-
vinylbenzyl alcohol dropwise (20 mL, 2.23 M, 44.5 mmol).
The reaction mixture was then stirred at room temperature
for 30 min. Then, allyl bromide (4.25 mL, 49 mmol) was
added dropwise into the reaction mixture at 0 8C. The reac-
tion flask was then removed from the ice bath. After 17 h,
deionized water (5 mL) was added dropwise into the reac-
tion at 0 8C. Then, more deionized water (100 mL) was
poured into the solution and the suspension was extracted
with Et2O (4 3 50 mL). The combined organic layers were
dried with Na2SO4 and then decanted. The solvent was then
removed under reduced pressure to give the crude product
as a colorless oil which was purified by flash chromatogra-
phy (10:1 hexane:ether eluent). The product was prepared
according to above procedure in 80% yield.
1H NMR (300 MHz, CDCl3) d 5 7.35–6.28 (br, 87H) 6.09–5.85
(br, 3H) 5.42–5.12 (br, 6H) 4.60–4.25 (br, 6H) 4.15–3.80 (br,
6H) 2.60–0.90 (br, 54H).
The copolymer obtained in the previous step was dissolved
in dry DMF (20 mL) at 0 8C and then potassium tert-butox-
ide (240 mg, 1.96 mmol) was added in a single portion. The
reaction was warmed to room temperature. The reaction
flask was then removed from the ice bath. After 20 h, the
reaction solution was filtered through a neutral alumina
plug. The solvent was then removed under reduced pressure.
Then, the crude copolymer was dissolved in 3 mL of CH2Cl2
and resulting solution was added dropwise into cold MeOH,
causing precipitation of the polymer. The copolymer was col-
lected by filtration and dried under reduced pressure. The
macroinitiator 11 was isolated in 80% yield according to
above prodedure.
1H NMR (300 MHz, CDCl3) d 5 7.42 (d, J 5 8.1 Hz, 2H) 7.33
(d, J 5 8.1 Hz, 2H) 6.74 (dd, J 5 17.4 Hz, 10.8 Hz, 1H) 6.04–
5.92 (m. 1H) 5.76 (dd, J 5 17.7 Hz, 1H) 5.37–5.21 (m, 3H)
4.53 (s, 2H) 4.05 (d, J 5 5.7 Hz, 2H); 13C NMR (75 MHz,
CDCl3) d 5 138.1, 137.1, 136.7, 134.9, 128.0, 126.3, 117.2,
113.8, 71.9, 71.2.
In a nitrogen-filled drybox, initiator 1-bromoethyl benzene
(51.7 mg, 0.279 mmol, 1.0 equiv.), styrene (2.6 ml, 22.7
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1H NMR (300 MHz, CDCl3) d 5 7.35–6.28 (br, 87H) 6.11–5.93
(br, 3H) 4.82–4.58 (br, 6H) 4.51–4.35 (br, 3H) 2.45–1.15 (br,
63H)
7 L. Zhao, X. Liu, L. Zhang, G. Qiu, D. Astruc, H. Gu, Coord.
Chem. Rev. 2017, 337, 34.
8 N. P. Godman, G. J. Balaich, S. T. Iacono, Chem. Commun.
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General Procedure for MF-ROMP
9 V. K. Kensy, G. I. Peterson, D. C. Church, N. A. Yakelis, A. J.
Boydston, Org. Biomol. Chem. 2016, 14, 5617.
To a 2-dram vial containing a magnetic stir bar was added
pyrylium salt 4 (1.3 mg, 0.003 mmol) followed by norbor-
nene. CH2Cl2 was then added, followed by the corresponding
initiator (0.045 mmol). The vial was then sealed with a
PTFE-lined cap and irradiated with blue LEDs (k 5 450–
480 nm, 2 W) at a distance of 2.0 cm. Aliquots were taken
for analysis to determine the conversion of norbornene by
1H NMR spectroscopy. The contents of the vial were then
diluted with CH2Cl2 and filtered over neutral alumina to
remove 4. The filtrate was then partially concentrated and
then added slowly into an excess of cold methanol, causing
the polymer to precipitate. The solids were collected by fil-
tration, washed with methanol, and then dried under
reduced pressure to give the final polymer.
10 L. E. Rosebrugh, V. M. Marx, B. K. Keitz, R. H. Grubbs, J.
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11 R. K. M. Khan, S. Torker, A. H. Hoveyda, J. Am. Chem. Soc.
2013, 135, 10258.
12 R. K. M. Khan, R. V. O’Brien, S. Torker, B. Li, A. H. Hoveyda,
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13 B. R. Elling, Y. Xia, J. Am. Chem. Soc. 2015, 137, 9922.
14 W. R. Gutekunst, C. J. Hawker, J. Am. Chem. Soc. 2015, 137,
8038.
15 R. M. Weiss, A. L. Short, T. Y. Meyer, ACS Macro Lett. 2015,
4, 1039.
16 T.-P. Lin, A. B. Chang, H.-Y. Chen, A. L. Liberman-Martin, C.
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17 M. P. Thompson, L. M. Randolph, C. R. James, A. N.
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General Procedure for Hydrolysis Studies
To a 4-dram vial containing a magnetic stirbar was added
polynorbornene followed by THF (polymer concentration 5
5 mg/mL). Then, Amberlyst 15 ion-exchange resin was
added. The vial was sealed with a PTFE-lined cap and trans-
ferred into an oil-bath preheated to 45 8C. The hydrolysis
progress was monitored by GPC and judged to have reached
maximum conversion when there was further no shift of
retention time for the solution aliquot.
18 S. C. Radzinski, J. C. Foster, R. C. Chapleski, D. Troya, J. B.
Matson, J. Am. Chem. Soc. 2016, 138, 6998.
19 C. Belger, J. G. Weis, E. Egap, T. M. Swager, Macromole-
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20 M. J. Allen, K. Wangkanont, R. T. Raines, L. L. Kiessling,
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21 D. C. Church, S. Nourian, C. -U. Lee, N. A. Yakelis, J. P.
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22 H. -K. Lee, K. -T. Bang, A. Hess, R. H. Grubbs, T. -L. Choi, J.
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ACKNOWLEDGMENTS
23 K. A. Ogawa, A. E. Goetz, A. J. Boydston, J. Am. Chem.
Soc. 2015, 137, 1400.
Financial support of this research by the Army Research Office
(grant no. W911NF-15–1-0139), Washington Research Founda-
tion, and Camille and Henry Dreyfus Foundation.
24 A. E. Goetz, A. J. Boydston, J. Am. Chem. Soc. 2015, 137,
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26 L. M. M. Pascual, D. G. Dunford, A. E. Goetz, K. A. Ogawa,
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