Photoresponsive polymers containing nitrobenzyl esters via ring-opening metathesis polymerization

Article abstract of DOI:10.1021/ma2015529

This paper describes controlled ring-opening metathesis polymerizations (ROMP) of norbornene derivatives that bear photoreactive o-nitrobenzyl ester (NBE) moieties. Termination reactions have frustrated previous reported attempts to execute controlled radical polymerizations with nitrobenzyl ester-containing acrylate derivatives. We therefore prepared new NBE-containing norbornene derivatives, and found that polymerization with Grubbs's ruthenium carbene catalysts led to complete consumption of monomer in minutes to hours and yielded polymers with predictable molecular weights. We also demonstrate the preparation of NBE-containing block copolymers and show through UV irradiation of these polymers in solution and in thin film that the NBE moieties remain photoreactive.

Full text of DOI:10.1021/ma2015529

ARTICLE
pubs.acs.org/Macromolecules
Photoresponsive Polymers Containing Nitrobenzyl Esters via
Ring-Opening Metathesis Polymerization
Patricia Gumbley, Damla Koylu, and Samuel W. Thomas, III*
Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, Massachusetts 02155, United States
S Supporting Information
b
ABSTRACT: This paper describes controlled ring-opening metathesis polymeriza-
tions (ROMP) of norbornene derivatives that bear photoreactive o-nitrobenzyl ester
(NBE) moieties. Termination reactions have frustrated previous reported attempts
to execute controlled radical polymerizations with nitrobenzyl ester-containing
acrylate derivatives. We therefore prepared new NBE-containing norbornene
derivatives, and found that polymerization with Grubbs’s ruthenium carbene catalysts
led to complete consumption of monomer in minutes to hours and yielded polymers
with predictable molecular weights. We also demonstrate the preparation of NBE-
containing block copolymers and show through UV irradiation of these polymers in solution and in thin film that the NBE moieties
remain photoreactive.
’ INTRODUCTION
Although there have been several reports of controlled ATRP of
nitrobenzyl-containing vinyl monomers,3,24,25 these reactions
must be limited to low conversion or small percentage of
NBE groups to prevent broadening of the polydispersity index
(PDI) to values greater than 1.5. As a result, successful inclu-
sion of photoresponsive NBE-functionalized monomers into well-
defined polymer architectures has been limited.
Ring-opening metathesis polymerization (ROMP) is a class of
polymerizations that, depending the monomer and initiator used,
can result in the favorable initiation kinetics and slow termination
reactions required for a controlled living polymerization.26 In
addition, the functional group compatibility of ROMP is excel-
lent, with the exception of Lewis bases such as amines that can
coordinate to the transition metal catalysts.27,28 Although there
are several examples of ROM polymers that contain nitrobenzyl
groups—Meijer used NB ethers to photocage 2-ureido-pyrimi-
dinone dimerization,29 Zojer and co-workers used photodepro-
tection of a di-NBE homopolymer to tune grain sizes in a
pentacene-based transistor30,31—and other photoreactive ROMP-
derived polymers,32 we are unaware of any studies of controlled
polymerizations ofnitrobenzyl-containingmonomersusingROMP.
Herein we report the use of ROMP to perform controlled
polymerizations of NBE-substituted norbornyl monomers, in-
cluding the preparation of block copolymers.
This paper describes new polymers, synthesized from norbor-
nene-based monomers using ring-opening metathesis polymer-
ization (ROMP), which contain photocleavable o-nitrobenzyl
ester side chains. Polymers and other materials that irreversibly
change their chemical structure upon irradiation with light are
critical for technologies such as photolithography, optical sto-
rage, and actuation.1 In addition to more traditional homopoly-
mers and random copolymers, several block copolymers that
have either one photoresponsive block or a photolabile linkage
between two blocks have been reported.2ꢀ9 Such strategies have
taken advantage of block copolymer self-assembly properties to
form light-degradable micelles for drug-delivery10ꢀ12 or photo-
responsive templates for nanoporous materials.5
In the general class of photolabile structural moieties, nitro-
benzyl (NB) groups, including nitrobenzyl esters (NBEs), are
among the most popular. Upon irradiation with UV light, an
NBE undergoes a Norrish-type reaction, breaking its benzylic
CꢀO bond to yield a carboxylic acid and o-nitrosobenzal-
dehyde.13 In addition to the numerous applications in “photo-
caging” properties such as fluorescence14ꢀ16 or biochemical
activity,17ꢀ19 the photolabile nature of NB groups and its relative
ease of functionalization with other moieties has led to a number
of materials-related applications such as polymer coatings for the
micropatterning of proteins,20 NBE-containing polymer brushes
for phototunable wettability,21 and photocleavable NBE linkers
for combinatorial chemistry.22,23
’ EXPERIMENTAL SECTION
The following chemicals were purchased from commercial sources
and used as received: cis-5-norbornene-exo-2,3-dicarboxylic anhydride,
glycine, 2-nitrobenzyl alcohol, N,N0-dicyclohexylcarbodiimide, 4-(dime-
thylamino)pyridine, triethylamine, benzene, 1-(2-chloroethyl)piperidine
Despite their vast utility, incorporation of NB groups into
polymers is not necessarily readily achieved. Fustin, Gohy, and
co-workers recently described the difficulty inherent in polymer-
izing monomers such as nitrobenzyl acrylate and methacrylate
using popular techniques of controlled radical polymerization
such as atom transfer radical polymerization (ATRP), because
of the radical-terminating reactions of nitroaromatic groups.24
Received: July 8, 2011
Revised:
September 7, 2011
Published: October 03, 2011
r
2011 American Chemical Society
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ARTICLE
hydrochloride, 5-hydroxy-2-nitrobenzaldehyde, 1-butanol, pyridine, extra
dry DMF, potassium carbonate, sodium sulfate, sodium borohydride, iodo-
methane, oxalyl chloride, thionyl chloride, dichloro[1,3-bis(2,4,6-trimethyl-
phenyl)-2-imidazolidinylidene](benzylidene)bis(3-bromopyridine)ruthenium-
(II), Grubbs catalyst, second generation, Grubbs catalyst, first generation, and
5-norbornene-2-carboxylic acid. Dry tetrahydrofuran (THF) and dichloro-
methane (DCM) were obtained using a column purification system. The
following compounds were prepared following literature procedures:
N-(Glycine)-cis-5-norbornene-exocarboximide (NB-Gly),33 and 5-nor-
bornyl acid chloride (NB-Cl).34 All synthetic procedures were carried
out under standard air-free conditions unless otherwise indicated.
NMR spectra were acquired on either a Bruker Avance III 500 or a
Bruker DPX-300 spectrometer. Chemical shifts are reported relative to
the solvent (7.27 ppm for CHCl3, 2.50 ppm for DMSO, and 77.23 ppm
for 13CHCl3). High-resolution mass spectra (HRMS) were obtained
at the MIT Department of Chemistry Instrumentation Facility using
a peak-matching protocol to determine the mass and error range of
the molecular ion. Molecular weight distribution measurements of the
polymers were conducted with a Shimadzu gel permeation chromatog-
raphy (GPC) system equipped with a TOSOH TSKgel GMHh-M
mixed-bed column and guard column (5 μm), equipped with both UV
and refractive index detectors. The column was calibrated with low
polydispersity poly(styrene) standards (TOSOH, PSt Quick Kit) with
THF as the mobile phase eluting at 0.75 mL/min.
Silicon wafers were cleaned by rinsing with acetone and lint-free
cleanroom cloths. Chlorobenzene solutions of polymer (5 mg/mL)
were filtered with a 0.4 μm PTFE syringe filter and spun-cast on clean
silicon wafers using a Laurell Technologies Corporation spin coater
(Model WS-400-6NPP-Lite) and then dried in a vacuum oven at 60 °C
at ꢀ14.7 psi (ꢀ30 in. Hg) for 1 h. Program for spin-coater: Spinning at
500 rpm for 3 s (acceleration of 330 rpm/s), followed by spinning at
1500 rpm for 20 s (acceleration of 1650 rpm/s). Irradiation of the films
and solutions was performed with an ozone-free 200W Hg/Xe lamp
(Newport-Oriel) equipped with recirculating water filter and manual
shutter. The dynamic contact angles of sessile water droplets on the
surfaces of polymer films were determined with a Ramꢀe-Hart goniometer
(Model200-F1) atroom temperature bymeasuring the advancing contact
angles by adding water to the droplet with a 30-Gauge needle in stepwise
fashion.
5-Norbornene-2-butyl Ester (1). To a solution of n-butanol
(0.223 mL, 181 mg, 2.44 mmol) and pyridine (0.818 mL, 0.803 g, 10.2
mmol) in 5 mL of dry THF, a solution of NB-Cl (382 mg, 2.44 mmol) in
8 mL of dry THF was added dropwise under an atmosphere of argon.
The reaction mixture was stirred overnight at room temperature. The
solid white precipitate that had formed upon addition of NB-Cl was
removed by vacuum filtration. The filtrate was then concentrated in
vacuo to afford a yellow oil. The crude product was purified via flash
chromatography using hexanes/EtOAc (12:1 v/v) to yield compound 1
as a mixture of the endo and exo stereoisomers. Yield: 341 mg (72%) 1H
NMR (500 MHz, CDCl3): δ 6.22ꢀ6.21 (1H, m), 5.96ꢀ5.94 (1H, m),
4.11ꢀ4.04 (2H, m), 3.23 (1H, s, br), 3.13ꢀ2.93 (2H, m), 1.92ꢀ1.91
(3H, m), 1.60ꢀ1.41 (8H, m). 13C NMR (125 MHz, CDCl3): δ 176.3,
174.8, 138.0, 137.7, 135.8, 132.4, 64.0, 49.6, 46.6, 46.3, 45.7, 43.4, 43.2,
42.5, 41.6, 30.7, 30.3, 29.2, 19.2, 13.7, 13.7. HRMS (m/z): calcd for
C12H18O2 (M+Na)+, 217.1199; found, 217.1207.
NBEꢀNorbornyl Monomer (2). To a solution of 2-nitrobenzyl
alcohol (1.36 g, 8.89 mmol) and pyridine (2.98 mL, 2.93 g, 37.0 mmol)
in 15 mL of dry THF, a solution of NB-Cl (1.39 g, 8.89 mmol) in 15 mL
of dry THF was added dropwise under an atmosphere of argon. The
reaction mixture was stirred overnight at room temperature. The solid
yellow precipitate formed upon addition of NB-Cl was removed by
vacuum filtration. The filtrate was concentrated in vacuo to afford a
yellow oil. The crude product was purified via flash chromatography
using hexanes/EtOAc (6:1 v/v) to yield 2 as a mixture of the endo and
exo stereoisomers. Yield: 2.13 g (88%) 1H NMR (300 MHz, CDCl3): δ
8.12ꢀ8.08 (1H, m), 7.68ꢀ7.57 (2H, m), 7.52ꢀ7.47 (1H, t), 6.16ꢀ6.14
(1H, m), 5.93ꢀ5.92 (1H, m) 5.53ꢀ5.46 (2H, m), 3.10 (1H, br s),
3.08ꢀ3.05 (1H, m), 3.07 (1H, br s), 1.99ꢀ1.91 (2H, m), 1.32ꢀ1.26
(2H, m). 13C NMR (75 MHz, CDCl3): δ 174.1, 147.7, 138.0, 135.7,
133.6, 129.2, 128.8, 125.0, 62.9, 49.7, 46.6, 46.4, 45.9, 43.3, 43.1, 42.6,
41.7, 30.4, 29.2. HRMS (m/z): calcd for C15H15NO4 (M + Na)+,
296.0893; found, 296.0903.
NBE-Substituted Imide Monomer (3). A three-neck round-
bottom flask was charged with N-(glycine)-cis-5-norbornene-exo-car-
boximide (1.0 g, 4.5 mmol), 2-nitrobenzyl alcohol (1.16 g, 7.6 mmol),
DCC (1.57 g, 7.6 mmol), and DMAP (0.92 g, 7.6 mmol). Upon addition
of 35 mL of CH2Cl2, the reaction was stirred at room temperature under
argon overnight. The reaction mixture was filtered to remove the pre-
cipitate formed during the reaction, and the filtrate was concentrated to
dryness to yield a yellow liquid. The residue was dissolved in EtOAc,
washed with 10% HCl (3 ꢁ 30 mL) and then dried over MgSO4.
Recrystallization of the residue from ethyl acetate/hexanes gave 0.83 g
1
(56%) of compound 3 as a pale yellow solid. H NMR (300 MHz,
CDCl3): δ 8.11 (d, J = 8.1 Hz, 1H), 7.68 (t, J = 7.4 Hz, 1H), 7.58 (d, J =
7.0 Hz, 1H), 7.50 (t, J = 7.3 Hz, 1H), 6.28 (s, 2H), 5.59 (s, 2H), 4.34
(s, 2H), 3.30 (s, 2H), 2.76 (s, 2H), 1.61 (d, J = 9.8 Hz, 1H), 1.49 (d, J = 9.9
Hz, 1H). 13C NMR (75 MHz, CDCl3): 177.1, 166.5, 147.3, 138.0, 134.0,
131.2, 129.1, 129.0, 125.2, 64.1, 48.1, 45.4, 42.8, 39.4. HRMS (m/z):
calcd for C18H16N2O6 (M + Na)+, 379.0901; found, 379.0918.
2-Nitro-5-(2-piperidin-1-ylethoxy)benzaldehyde (4). 4 was
synthesized as previously reported.35 1H NMR (500 MHz, CDCl3): δ
10.43 (1H, s), 8.12ꢀ8.22 (1H, d), 7.30ꢀ7.29 (1H, d), 7.15ꢀ7.12 (1H,
dd), 4.22ꢀ4.20 (2H, t), 2.79ꢀ2.77 (2H, t), 2.48 (4H, br s), 1.60ꢀ1.55
(4H, m), 1.44ꢀ1.42 (2H, m). 13C NMR (125 MHz, CDCl3): δ 188.5,
163.4, 142.1, 134.3, 127.2, 118.9, 114.0, 67.4, 55.5, 55.1, 25.9, 24.1.
HRMS (m/z): calcd for C14H18N2O4 (M + H)+, 279.1339; found,
279.1362.
2-Nitro-5-(2-piperidin-1-ylethoxy)benzyl Alcohol (5). So-
dium borohydride (232 mg, 6.13 mmol) was added to a solution of 4
(1.11 g, 3.96 mmol) in 18 mL of MeOH at 0 °C. The ice bath was
removed and the reaction was stirred at room temperature overnight.
The reaction mixture was concentrated in vacuo. The yellow solid was
then redissolved in ethyl acetate, washed with water and brine, and dried
over sodium sulfate to yield 5. Yield: 0.68 g (62%) 1H NMR (300 MHz,
CDCl3): δ 8.10ꢀ8.07 (1H, d, J = 9.3 Hz), 7.29 (1H, d, J = 2.7 Hz),
6.82ꢀ6.78 (1H, m), 4.98 (2H, s), 4.21ꢀ4.17 (2H, t), 2.82ꢀ2.78 (2H, t),
2.54 (4H, br s), 1.66ꢀ1.59 (4H, m), 1.48ꢀ1.46 (2H, m). 13C NMR (125
MHz, CDCl3): δ 163.4, 141.1, 140.0, 127.7, 113.9, 113.4, 66.7, 62.0,
57.6, 55.2, 25.7, 24.0. HRMS (m/z) calcd for C14H20N2O4 (M+H)+,
281.1496; found, 281.1501.
Cationic Monomer 6. First, 0.500 g (1.79 mmol) of compound 5
was dissolved in 5 mL of dry THF. Pyridine (0.173 mL, 0.170 g, 2.14
mmol) was then added to the solution. NB-Cl (0.290 g, 1.79 mmol) in
8 mL of dry THF was subsequently added dropwise to the reaction
mixture, which was then stirred overnight at room temperature. The gray
precipitate that formed upon addition of NB-Cl was removed by vacuum
filtration. The filtrate was then concentrated in vacuo to yield a brown
solid. The crude product was purified via flash chromatography (95%
DCM/5% MeOH) to yield the expected ester product as a mixture of
endo and exo isomers. Yield: 160 mg (22%) 1H NMR (300 MHz,
CDCl3): δ 8.19ꢀ8.15 (1H, m), 7.05ꢀ7.02 (1H, m), 6.91ꢀ6.87 (1H, m),
6.23ꢀ6.11 (1H, m), 5.96ꢀ5.93 (1H, m), 5.53ꢀ5.47 (2H, m),
4.24ꢀ4.20 (2H, t), 3.28 (1H, br s), 3.11ꢀ3.05 (1H, m), 2.94 (1H, br
s), 2.88ꢀ2.84 (2H, m), 2.57 (4H, br s), 2.00ꢀ1.81 (2H, m), 1.66ꢀ1.59
(4H, m), 1.53ꢀ1.33 (2H, m), 1.32ꢀ1.25 (2H, m). 13C NMR (75 MHz,
CDCl3): δ 178.5, 174.0, 162.9, 140.3, 138.2, 138.0, 137.5, 135.7, 135.6,
132.6, 132.3, 127.9, 114.6, 114.5, 113.2, 113.0, 66.5, 63.1, 57.2, 54.8, 49.8,
49.7, 46.7, 46.4, 45.9, 45.8, 43.8, 43.3, 43.1, 42.6, 42.5, 41.7, 30.5, 29.3,
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Scheme 1. Synthesis of Monomers 1 and 2 and Structures of
Corresponding Polymers P1 and P2
Scheme 2. Synthesis of Imide-Based Monomer 3 and of
Corresponding Polymer P3
25.5, 23.9. HRMS (m/z): calcd for C22H28N2O5 (M+H)+, 401.2071;
found, 401.2080.
The unmethylated ester (701 mg, 1.75 mmol) was dissolved in 50 mL
of dry CH2Cl2. Iodomethane (4.80 mL, 10.9 g, 77.1 mmol) was added to
the reaction mixture and stirred overnight at room temperature. The
solvent was removed in vacuo to yield 6, a yellow solid, as a mixture of endo
and exo isomers. 1H NMR (500 MHz, CDCl3): δ 8.18ꢀ8.15 (1H, m),
7.11ꢀ7.10 (2H, m), 6.21ꢀ5.89 (2H, m), 5.48ꢀ5.42 (2H, m), 4.74 (2H,
br s), 4.34 (2H, br s), 3.87ꢀ3.79 (4H, m), 3.51 (3H, s), 3.26 (1H, br s),
3.13ꢀ3.11 (1H, m), 2.93 (1H, br s), 2.04ꢀ2.02 (4H, m), 1.84 (4H, br s),
1.51ꢀ1.25 (2H, m). 13C NMR (125 MHz, CDCl3): δ 174.2, 160.9,
141.5, 138.2, 137.9, 135.7, 132.4, 132.1, 128.2, 115.5, 113.6, 63.0, 62.9,
62.7, 62.2, 49.8, 49.5, 46.6, 46.4, 46.0, 45.7, 43.3, 43.1, 42.9, 42.6, 41.7,
30.5, 29.3, 29.2, 20.6, 20.3. HRMS (m/z): calcd for C23H31N2O5 (M+),
415.2227; found, 415.2237.
Scheme 3. Synthesis of photocleavable cationic polymer P6
General Polymerization Procedure. Dry dichloromethane was
sparged with argon for ∼20 min. Initiator I, II, or III was then added to a
solution of monomer at a concentration between 50 and 125 mg/mL in
CH2Cl2. The reaction mixture was stirred until all monomer was
consumed as shown by TLC. The polymerization was then terminated
by the addition of 5 equiv of ethyl vinyl ether.
1
P1. H NMR (300 MHz, CDCl3): δ 5.49ꢀ5.27 (2H), 4.10ꢀ3.91
(2H), 3.2ꢀ2.4 (3H), 2.04ꢀ1.66 (2H), 1.63ꢀ1.50 (2H), 1.45ꢀ1.19
(4H), 0.98ꢀ0.85 (3H).
1
P2. H NMR (300 MHz, CDCl3): δ 8.11ꢀ7.99 (1H), 7.69ꢀ7.54
(2H), 7.53ꢀ7.39 (1H), 5.56ꢀ5.12 (4H), 3.30ꢀ2.55 (3H), 2.17ꢀ1.86
(2H), 1.37ꢀ1.26 (2H).
1
P3. H NMR (300 MHz, CDCl3): δ 8.12ꢀ8.07 (1H), 7.67ꢀ7.45
drop of ethyl vinyl ether. NMR analysis of each block copolymer showed
the ratio of incorporation of the monomers was approximately 1:1.
(3H), 5.69ꢀ5.50 (4H), 4.26ꢀ4.12 (2H), 3.50ꢀ2.60 (4H), 2.36ꢀ2.03
(1H), 1.65ꢀ1.40 (1H).
P6. 1H NMR (500 MHz, (CD3)2SO): δ 8.27ꢀ8.07 (1H), 7.30ꢀ7.03
(2H), 5.55ꢀ5.03 (4H), 4.72ꢀ4.54 (2H), 3.98ꢀ3.81 (2H), 3.58ꢀ3.39
(4H), 3.22ꢀ3.13 (3H), 3.12ꢀ2.97 (1H), 2.95ꢀ2.61 (2H), 2.09ꢀ1.75
(6H), 1.67ꢀ1.46 (4H).
’ RESULTS AND DISCUSSION
We prepared two types of ester-containing norbornene mono-
mers. The first (Schemes 1 and 3, monomers 1, 2, and 6) were
mixtures of endo and exo (80:20) stereoisomers of 5-norbornene-
2-carboxylic acid esters. Our generally applicable synthetic scheme
was to first prepare the acid chloride NB-Cl from the commercially
available mixture of stereoisomeric carboxylic acids, followed by
esterification with the appropriate alcohol in the presence of
pyridine. The second (Scheme 2, monomer 3) was a norbor-
nene-exo-imide substituted with a carboxylic ester, prepared from
norbornene-exo-anhydride and glycine,33 followed by DCC-
mediated esterification with o-nitrobenzyl alcohol.
Block copolymers were prepared with an identical procedure, except
that upon consumption of the first monomer, an aliquot of another
monomer was added. An example follows:
P2-b-P1. To a solution of monomer 2 (50 mg, 0.18 mmol) in CH2Cl2
was added a solution of catalyst III (6.3 mg, 0.0071 mmol) in CH2Cl2
such that the final concentration of monomer was 50 mg/mL (total
solvent was 1.0 mL). The reaction was stirred at ambient temperature
under argon for 30 min, at which time TLC analysis showed no
monomer remained in solution. A small aliquot was then removed for
GPC analysis, followed by addition of a solution of monomer 1 (36 mg,
0.18 mmol) dissolved in 0.7 mL of CH2Cl2. TLC analysis after 25 min
indicated no monomer remained, so the reaction was terminated with a
We used three different alcohols for esterification of the
norbornene carboxylic acids. Two were commercially available:
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Chart 1. Ruthenium Carbene Initiators Used in ROMP
Reactions
Table 1. Comparison of Initiators I, II, and III for Polymer-
ization of 2 at [M]/[I] = 20
Figure 1. Linear dependence (R = 0.998) of Mn of P2, prepared with
initiator I, on the molar ratio of monomer to initiator, [M]/[I]. [M]0 =
125 mg/mL.
I
II
III
time (min)a
Mn (g/mol)
PDI
120
20
25
7100
68 000
2.16
8300
1.28
1.14
a Approximate time to reach 100% conversion as determined by thin-
layer chromatography.
Table 2. Molecular weight distributions of diblock copoly-
mers with NBE groups with initiator III
n-butanol and 2-nitrobenzyl alcohol, which gave monomers 1
and 2, respectively. Scheme 3 shows the synthesis of photo-
cleavable cationic monomer 6, which we prepared by alkylating
the phenol group of 2-nitro-5-hydroxybenzaldehyde with N-
(2-chloroethyl)piperdine hydrochloride, followed by reduction
with sodium borohydride in methanol to give the benzylic alcohol.
Esterification as described above yielded the amine-substituted
photocleavable norbornene, which we methylated with CH3I
before polymerization because of the known inhibitory effect that
amines have on ROMP reactions initiated with ruthenium
carbene complexes.27,36
1st block
Diblock
1st/2nd block
[M]/[I] per block
Mn
PDI
Mn
PDI
P2/P1
P1/P2
P2/P3
26/26
36/36
30/30
9900
11 500
11 500
1.11
1.08
1.10
16 600
25 800
26 700
1.18
1.16
1.10
Although initiator II polymerized 2 efficiently, it did so in an
uncontrolled fashion, with the resulting P2 having an Mn ten-
times higher than expected for a controlled polymerization, and a
PDI of 2.16. Such a result is characteristic of a chain reaction that
has a high ratio of rate of propagation to rate of initiation. Our
overall results in comparing these three initiators are consistent
with their previously reported kinetics of initiation and prop-
agation.39,40 Monomer 3 was also amenable to homopolymeri-
zation using initiator I or III, each of which yielded a similarly
narrow molecular weight distribution at molecular weights below
20 000 g/mol: Mn = 11 200 ( 2300 (expected Mn = 7100 for
[M]/[I] = 20), PDI = 1.14 ( 0.06. Although we were able to
purify these polymers by precipitation into nonsolvents
(methanol for P2 and P3, diethyl ether for P6), we acquired
all GPC data for these polymers from crude terminated reaction
mixtures to prevent any fractionation of the polymer samples and
subsequent skewing of molecular weight distributions.
As we were able to demonstrate some characteristics of
controlled polymerizations with NBE-containing monomers,
we proceeded to synthesize block copolymers using monomers
1, 2, and 3 in CH2Cl2 with initiator III, which consistently gave
polymers with low PDIs. Addition of a second monomer after
TLC analysis indicated the consumption of the first monomer
(30 min after combining monomer with initiator III) resulted in
A-B diblock copolymers. In all cases, the second monomer was
consumed within 30 min. Table 2 summarizes the molecular
weight distributions (via GPC) of AꢀB diblock copolymers that
have at least one NBE-containing block, as well as the homo-
polymers resulting from quenching a small aliquot of each
polymerization after the first monomer was consumed but before
addition of the second monomer. In each combination listed in
Table 2, the Mn of the sample increased upon addition of the
Upon completing the successful synthesis of NBE-containing
norbornene monomers, our next goal was to determine if they
were amenable to metathesis polymerization, and to determine
whether these polymerizations showed characteristics of living
polymerizations. We therefore initiated polymerizations of 2 in
CH2Cl2 using Grubbs’s catalysts I, II, and III (Chart 1) at a
monomer:initiator molar ratio of 20:1. As Table 1 shows, under
these conditions all three initiators resulted in complete mono-
mer conversion as determined by both TLC and NMR in times
ranging from minutes to hours. NMR spectra of the crude
reaction mixtures were consistent with the expected structure
of P2. Initiator I required at least 4x more time than initiators II
or III to consume all monomer, which is consistent with the more
active nature of second- and third-generation Grubbs initiators.
Samples of P2 prepared with initiators I and III did show
characteristics of controlled polymerizations. Number-average
molecular weights (Mn) of P2 prepared with initiator I depended
linearly on the ratio of monomer to initiator (Figure 1) as
determined by gel permeation chromatography referenced with
low-PDI poly(styrene) standards. These polymers generally had
polydispersity indices (PDIs) generally between 1.2 and 1.3,
although high molecular-weight shoulders on the GPC traces of
P2 tended to broaden the PDI values of these polymers at
[M]/[I] ratios greater than 100. These highest molecular weight
polymers of those we prepared therefore had PDI values that
ranged between 1.2 and 1.8. Others have reported similar high
molecular weight shoulders on GPC traces of ROM polymers
initiated by ruthenium carbene complexes, and have attributed
them to coupling between two growing chain ends.37,38
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Figure 3. Decrease of contact angle of water on a spun-cast thin film of
P2 from before irradiation (top) to after irradiation (bottom).
became soluble after UV irradiation for 30 min, and showed similar
results. Solutions of P1 in CDCl3, which lacks any NBE groups,
showed no change in either NMR spectra or physical appearance
upon exposure to UV irradiation under identical conditions.
Films of the NBE-containing polymers were also photoreac-
tive. We prepared thin films of P2 with Mn ∼ 10 000 g/mol by
spin-casting from chlorobenzene. Before irradiation with UV
light, sessile drops of water on these films had an average advancing
contact angle of 74 ( 2° (Figure 3, top). After irradiation of films
of P2 with the unfiltered output of an ozone-free 200 W Hg/Xe
lamp for 30 min, the advancing contact angle of water was 64 ( 2°
(Figure 3, bottom). In comparison, butyl-ester containing poly-
mer P1 showed no change in advancing contact angle (76 ( 1°)
after identical conditions of irradiation. Other NBE-substituted
materials have shown significant decreases in water contact
angles upon photolysis.42ꢀ44 This observed decrease in hydro-
phobicity of P2, along with the observations of change in
solubility and NMR spectra, is consistent with the photoconver-
sion of hydrophobic NBEs to more hydrophilic carboxylic acids.
In addition, we were able to synthesize homopolymers of
monomer 6 in high yield using ROMP. Polymerization of 6 in
dichloromethane using initiator II or III gave P6 with complete
conversion of monomer within 30 min as determined by NMR
analysis. Although we were unable to obtain GPC data for
cationic P6 because of the known interactions between cationic
polymers and GPC columns,28,45 we made two observations
consistent with a successful polymerization: (i) during the reaction
in CH2Cl2, a solid which was soluble in dipolar aprotic solvents
such as DMF and DMSO precipitated from the reaction, (ii) the
NMR spectrum of the crude reaction mixture in DMSO-d6
(Figure S2 in the Supporting Information) showed no observa-
ble peaks associated with the monomer and was consistent with
the expected structure. This successful polymerization stands in
strong contrast to our earlier attempts to polymerize an metha-
crylate monomer with functionality similar to that of monomer 6
through a variety of radical-initiated methods (AIBN, ATRP,
RAFT), all of which gave low conversion and low molecular
weight polymers even after long reaction times and at high
concentrations of monomer. Photocleavable cationic groups
have been used in several applications that take advantage of
the ability to control formal charge of a material with light,46,47
but we are unaware of any such structure in a polymeric material
with nitrobenzyl groups. Polymerization of the unmethylated
precursor of 6 with initiator III remained incomplete after 3 h,
which suggests inhibition of the polymerization by the Lewis
basic amine.
Figure 2. GPC chromatograms. Top: P1-b-P2 Bottom: P2-b-P3. In
each case, the dashed line is the chromatogram of a terminated aliquot of
the reaction upon consumption of the first monomer; the solid line is the
chromatogram upon consumption of the second monomer.
second monomer while the PDI remained less than 1.3. When 3
was the first monomer used in an attempt to make a block
copolymer, however, not all of the chain ends of P3 added to the
second monomer; a low molecular weight peak that corre-
sponded to unreacted P3 homopolymer remained after con-
sumption of the second monomer. Therefore, although the
ROM polymerization of 3 does show predictable Mn values
up to Mn = 20 000 g/mol with relatively narrow PDI values, it is
not completely living due to termination of some active
chain ends. Nevertheless, we were able to prepare a diblock
copolymer of the two different NBE-containing monomers 2 and
3 with initiator III when monomer 3 was the second monomer
added (Figure 2, bottom). This strategy of adding a monomer
that shows nonliving behavior as the second monomer in a
ROMP-derived diblock copolymer has been used in previous
reports.40,41
The NBE-containing polymers we prepared were photoreac-
tive upon irradiation with UV light. After irradiation of colorless
solutions of P2 or P3 in CDCl3 with a 200 W Hg/Xe lamp for
20 min at wavelengths higher than 295 nm, the solutions
became deeply yellow in color and polymeric material precipi-
tated. NMR spectra of the CDCl3 supernatant showed no poly-
mer remaining dissolved and contained a mixture of aromatic
products. An identical experiment in DMSO-d6, in which P2 is
soluble both before and after irradiation, showed that the poly-
meric NBE groups were cleaved from the polymer after 60 min of
irradiation, while the poly(norbornene) backbone remained intact
(see Supporting Information, Figure S1). In addition, relative to
the rest of the polymeric backbone, the integration of the signals
between 5.0 and 5.5 ppm, which comprised both the benzylic and
olefinic protons of P2, decreased by a factor of 2. This observation
is consistent with loss of the benzylic methylene protons upon
irradiation. P3, which was insoluble in DMSO before irradiation,
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’ CONCLUSION
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Corresponding Author
*E-mail: sam.thomas@tufts.edu.
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