Synthesis and functionalization of ROMP-based gradient copolymers of 5-substituted norbornenes

Article abstract of DOI:10.1021/ma036002w

Gradient copolymers of exo-5-(benzyloxy)norbornene and exo-5-[(4-tert- butyl)benzyloxy]-norbornene were synthesized via ring-opening metathesis polymerization (ROMP). Kinetic studies revealed that the reactivity ratios of both monomers were close to unity. As predicted, this monomer pair did not result in copolymers with significant gradients under batch polymerization conditions. However, semibatch conditions resulted in copolymers with sizable gradients whose shape was independent of the rate of addition. Addition of the monomers simultaneously via a dual and opposite ramping strategy led to a 50/50 copolymer with an exceptionally linear gradient. These gradient copolymers were further functionalized via hydrogenation, bromination, and bromoalkoxylation.

Full text of DOI:10.1021/ma036002w

5504
Macromolecules 2004, 37, 5504-5512
Synthesis and Functionalization of ROMP-Based Gradient Copolymers
of 5-Substituted Norbornenes
Ch r istin e M. Dettm er ,† Ma ish a K. Gr a y,‡ J oh n M. Tor k elson ,†,§ a n d
Son Bin h T. Ngu yen *,†
Department of Chemistry, Department of Materials Science and Engineering, and
Department of Chemical and Biological Engineering, Northwestern University,
2145 Sheridan Road, Evanston, Illinois 60208-3113
Received December 26, 2003; Revised Manuscript Received May 14, 2004
ABSTRACT: Gradient copolymers of exo-5-(benzyloxy)norbornene and exo-5-[(4-tert-butyl)benzyloxy]-
norbornene were synthesized via ring-opening metathesis polymerization (ROMP). Kinetic studies revealed
that the reactivity ratios of both monomers were close to unity. As predicted, this monomer pair did not
result in copolymers with significant gradients under batch polymerization conditions. However, semibatch
conditions resulted in copolymers with sizable gradients whose shape was independent of the rate of
addition. Addition of the monomers simultaneously via a dual and opposite ramping strategy led to a
50/50 copolymer with an exceptionally linear gradient. These gradient copolymers were further
functionalized via hydrogenation, bromination, and bromoalkoxylation.
In tr od u ction
and reversible addition-fragmentation transfer polym-
erization (RAFT).5 Herein, we report the first synthesis
of gradient copolymers by ring-opening metathesis
polymerization (ROMP). exo-5-(Benzyloxy)norbornene
(3a ) and exo-5-[(4-tert-butyl)benzyloxy]norbornene (3b)
have been homopolymerized successfully with Grubbs
catalyst 1, and their copolymerization under controlled
addition rates yielded polymers with highly linear
gradients.
In recent years, there has been a growing interest in
the synthesis of gradient copolymers.1-22 Gradient
copolymers from two monomers A and B constitute a
new class of copolymer where the monomer composition
changes continuously along the backbone from predomi-
nantly A to predominantly B. The composition of an
A-B gradient copolymer is best described as being
between two extremes: those of the corresponding
diblock copolymer and the random copolymer. Several
theoretical studies have been conducted regarding the
properties and applications of gradient copolymers.23-27
Modeling studies by Pakula and Matyjaszewski24 dem-
onstrated that dynamic28 and thermodynamic properties
of gradient copolymers could be shifted continuously
along a temperature regime by synthetically modulating
the composition and shape29 of the gradient.18 The
order-disorder transition and microphase separation
morphology have also been examined in bulk gradient
copolymers.23,24 Hence, one can, in theory, access a wide
range of new polymer materials with programmable
thermal properties from only two monomers simply by
varying the synthetic conditions. A-B gradient copoly-
mers are also of interest because their unusual archi-
tecture could potentially make them excellent compat-
ibilizers in blends of homopolymers A and B.30 Gradient
copolymers are also being applied as the backbone in
usual polymer brushes7,10,21,22,31 so that the density of
grafted side chains varies smoothly along the chain.
Theoretically, monodisperse gradient copolymers can
be synthesized from any controlled/“living” polymeriza-
tion technique which exhibits simultaneous initiation of
all growing chains with negligible chain transfer and easy
cross-propagation.1 To date, gradient copolymers have
been successfully synthesized by nitroxide-mediated
controlled radical polymerization (NM-CRP),9,13,15,16 atom-
transfer radical polymerization (ATRP),6,7,10,12-14,17-19,21,22
When used with the “living” catalyst 1,32,33 ROMP
meets all of the requirements for successful gradient
copolymer synthesis and can provide several advantages
over radical polymerization systems. First, modern
single-component catalysts allow ROMP to be carried
out at room temperature (or below), with significantly
faster rates than those observed for NM-CRP, which
often requires higher temperature for successful initia-
tion. Hence, using ROMP would eliminate undesirable
thermal polymerization and the consequent broadening
of the polydispersity of the resulting polymer.34 Second,
the C-C double bonds in the backbone of the resulting
ROMP polymer can be further derivatized to provide a
facile route to several new functionalized polymers.
Gradient copolymers can be produced under either
batch or semibatch conditions. In batch polymerization,
both monomers are added at the same time to the
reaction flask, and the gradient produced is highly
dependent on the reactivity ratio of the monomer pair
and the monomer feed ratio.1 While gradient copolymers
have been formed under batch conditions for monomer
pairs with very different reactivity ratios, only moderate
changes in chain composition were observed.6,12,14,18,35
In semibatch polymerization, a forced gradient is formed
through the continuous addition of one monomer into a
reaction flask containing the second monomer.1 Semi-
batch polymerization of monomers even with reactivity
ratios close to unity, as in the case for 3a and 3b, has
† Department of Chemistry.
‡ Department of Materials Science and Engineering.
§ Department of Chemical and Biological Engineering.
* To whom correspondences should be addressed: e-mail
stn@northwestern.edu.
10.1021/ma036002w CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/01/2004

Macromolecules, Vol. 37, No. 15, 2004
ROMP-Based Gradient Copolymers 5505
a magnetic stir bar was charged with exo-5-norbornene-2-ol38,39
(2 g, 18 mmol), THF (25 mL), and oil-free sodium hydride (500
mg, 21 mmol). The flask was capped with a rubber septum,
removed from the glovebox, attached to a water-cooled con-
denser and a nitrogen bubbler, heated to reflux for 12 h while
stirring, and then cooled to room temperature. Under nitrogen,
benzyl bromide (2 mL, 17 mmol) in a THF solution (20 mL)
was then added to the reaction mixture via cannula. The
reaction mixture was again brought to reflux and was stirred
for an additional 12 h. After cooling to room temperature, the
reaction mixture was poured into diethyl ether (100 mL) and
washed with water (3 × 100 mL). The organic layer was
collected, dried over anhydrous sodium sulfate, and filtered.
resulted in gradient copolymers with significant changes
in chain composition.7,13,14,16-18,20
To carry out a comprehensive study of the properties
of A-B gradient copolymers, a variety of such copoly-
mers, ranging from a 50/50 copolymer with a linear
gradient to those with less significant gradients or
nonlinear gradients, are needed. To date, only a few
examples of A-B copolymers with cumulative composi-
tions of 50% A and linear gradients have been re-
ported.14,17,18 In fact, the gradient shape for gradient
copolymers of styrene and either acrylonitrile, methyl
acrylate, or n-butyl acrylate was shown to change from
S-like to linear with increasing addition rate. Herein
we report the synthesis of 50/50 A-B copolymers with
linear gradients whose significance can be changed with
increasing addition rate. We also formed A-B gradient
copolymers of 3a and 3b via the use of a forced gradient,
where the two monomers are added at opposite ramping
speeds.
The solvent was removed from the filtrate on
a rotary
evaporator to afford the crude product as a yellow oil. Flash
column chromatography with methylene chloride:hexanes (20:
80) as the eluent gave the desired product as colorless oil (2.54
g, 69.8%). 1H NMR (CDCl3): δ 1.58 (m, 7-norbornenyl-H2, 2H),
1.78 (d, 6-norbornenyl-H2, 2H), 2.96 (m, 4-norbornenyl-H and
1-norbornenyl-H, 2H), 3.62 (d, 5-norbornenyl-H, 1H), 4.56 (q,
benzyl-H2, 2H), 5.94 (m, 3-norbornenyl-H, 1H), 6.21 (m,
2-norbornenyl-H, 1H), 7.30 (m, o- and p-aromatics, 3H), 7.35
(d, m-aromatics, 2H). 13C NMR (CDCl3): δ 34.7 (6-norbornenyl-
C), 40.6 (1-norbornenyl-C), 46.3 (7-norbornenyl-C), 46.7 (4-
norbornenyl-C), 71.4 (benzyl-C), 80.2 (5-norbornenyl-C), 127.8
(o- and p-aromatics-3C), 128.6 (m-aromatics-2C), 133.4 (3-
norbornenyl-C), 139.1 (1-phenyl-C), 140.92 (2-norbornenyl-C).
GC-MS: calcd for C14H16O: 200.28; found: 200.
exo-5-[(4-ter t-Bu tyl)ben zyloxy]n or bor n en e (3b). This
compound was synthesized by the same procedure used for
3a . A colorless oil was obtained (3.61 g, 77.6%). 1H NMR
(CDCl3): δ 1.35 (s, tert-butyl-H9, 9H), 1.58 (m, 7-norbornenyl-
H2, 2H), 1.78 (d, 6-norbornenyl-H2, 2H), 2.85 (d, 1,4-nor-
bornenyl-H, 2H), 3.65 (d, 5-norbornenyl-H, 1H), 4.54 (q, benzyl-
H2, 2H), 5.96 (m, 3-norbornenyl-H, 1H), 6.21 (m, 2-nor-
bornenyl-H, 1H), 7.31 (m, o- and p-aromatics, 3H), 7.40
(d, m-aromatics, 2H). 13C NMR (CDCl3): δ 31.6 (tert-butyl-
3C), 34.7 (6-norbornenyl-C), 40.6 (1-norbornenyl-C), 46.3 (7-
norbornenyl-C), 46.7 (4-norbornenyl-C), 71.2 (benzyl-C),
80.2 (5-norbornenyl-C), 125.5 (m-aromatics-2C), 127.7 (o-
aromatics-2C), 133.4 (1-phenyl-C), 136.1 (3-norbornenyl-C),
140.9 (2-norbornenyl-C), 150.5 (p-aromatics-1C). EIMS: calcd
for C18H24O: 256.38; found: 256.1.
Deter m in a tion of Rea ctivity Ra tios. In an inert-atmo-
sphere glovebox, THF (25 mL) and a mixture of 3a and 3b
(10, 30, 50, 70, or 90% of 3a , with a total of 7 mmol in each
round-bottom) were added to a series of 100 mL round-bottom
flasks equipped with magnetic stir bars. To a series of 5
scintillation vials, Grubbs catalyst 1 (8.0 mg, 9.7 × 10-3 mmol,
0.0014 equiv) and THF (1 mL) were added. While each
monomer mixture was stirred rapidly, the catalyst solution
was injected. The reaction was quenched after 3 min with ethyl
vinyl ether (1 mL).40 Conversion of the monomer into copoly-
mer was determined by GC, and the reactivity ratios were
determined from a Fineman-Ross plot. The final reactivity
ratios were obtained as averages of four runs.
Syn th esis of Hom op olym er s of 3a a n d 3b. In a typical
polymerization, a scintillation vial equipped with a stir bar
was charged with either 3a or 3b (2.5 mmol) and diluted with
THF (10 mL). Another scintillation vial was charged with
Grubbs catalyst 1 (5.1 mg, 6.2 × 10-3 mmol) and diluted with
THF (1 mL). The catalyst solution was injected into the stirring
monomer solution, and the vial was capped. The vial was
removed from the glovebox, and after 40 min the polymeriza-
tion was quenched with ethyl vinyl ether (2 mL). 2,6-Di-tert-
butyl-4-methylphenol (BHT) (30 mg) was added, and the
polymer was precipitated in swirling methanol. The final
polymer was dried for 2 days under vacuum.
Exp er im en ta l Section
Ma ter ia ls. HPLC-grade tetrahydrofuran (THF) was puri-
fied using the Dow-Grubbs purification system,36 collected
under argon, degassed under vacuum, and stored under
nitrogen in a Strauss flask prior to use. Ethyl vinyl ether was
dried over CaH2, vacuum-transferred into an airtight solvent
bulb prior to transfer into an inert-atmosphere glovebox, and
stored at 0 °C. All monomers were synthesized and stored
under nitrogen at 0 °C. Deuterated solvents (Cambridge
Isotope Laboratories), all other solvents, and reagents were
purchased from commercial sources and used without further
purification.
Meth od s. All reactions were carried out under a dry
nitrogen atmosphere using standard Schlenk techniques or in
an inert-atmosphere glovebox, unless otherwise noted. Conver-
sion of the copolymerizations was determined on a Hewlett-
Packard 5890A gas chromatograph equipped with a FID
detector and a 30 m HP-5 capillary column (0.32 mm inner
diameter, 0.25 mm film thickness; method: initial time ) 0
min, initial temperature ) 50 °C, rate ) 10 °C/min, final
temperature ) 250 °C, final time ) 5 min) using undecane as
an internal standard. Molecular weights relative to polystyrene
standards were measured on a Waters gel-permeation chro-
matograph (GPC) equipped with Breeze software, a 717
autosampler, Shodex KF-G guard column, KF-803L and KF-
806L columns in series, a Waters 2440 UV detector, and a 410
RI detector. HPLC-grade THF was used as the eluent at a flow
rate of 1.0 mL/min, and the instrument was calibrated using
polystyrene standards (Aldrich, 15 standards, 760-1 800 000
1
Da). H spectra were recorded on a Varian Mercury 400 FT-
NMR spectrometer (400.178 MHz for 1H). 13C NMR spectra
were recorded on a Varian Inova 500 FT-NMR spectrometer
1
(125.669 MHz for 13C). H NMR data are reported as follows:
chemical shift (multiplicity: br ) broad, s ) singlet, d )
doublet, t ) triplet, q ) quartet, and m ) multiplet), peak
1
assignments, and integration. H and 13C chemical shifts are
reported in ppm downfield from tetramethylsilane (TMS, δ
scale) with the residual solvent resonances as internal stan-
dards, while peak assignments were made using ACD/Labs
software packages (Advanced Chemistry Development, Inc.).
GC-MS experiments were recorded on a Hewlett-Packard 6890
series instrument equipped with a HP-5 column (initial time
) 2 min, initial temperature ) 50 °C, rate ) 10 °C/min, final
temperature ) 280 °C, final time ) 5 min). All flash column
chromatography was performed under a positive pressure of
nitrogen using 230-400 mesh silica gel (56 mm i.d. × 200 mm
L), unless otherwise noted. Monomer addition was carried out
via either a Kd Scientific KDS100 syringe pump or a Sage
Instruments syringe pump model 341B.
Syn th esis of Block Cop olym er s of 3a a n d 3b. In a
typical polymerization, a scintillation vial equipped with a stir
bar was charged with 3a (0.50 g, 2.50 mmol) and diluted with
THF (7 mL). Another scintillation vial was charged with
Grubbs catalyst 1 (13.7 mg, 1.7 × 10-2 mmol) and diluted with
THF (1 mL). The catalyst solution was injected into the stirring
monomer solution, and the vial was capped. Another scintil-
exo-5-(Ben zyloxy)n or bor n en e (3a ). This compound was
prepared using a modified literature procedure.37 In an inert-
atmosphere glovebox, a 100 mL Schlenk flask equipped with

5506 Dettmer et al.
Macromolecules, Vol. 37, No. 15, 2004
Ta ble 1. Mon om er Ad d ition Ra tes (m m ol/h ) of 3a a n d 3b
th r ou gh ou t th e Rea ction for Du a l-Ra m p in g Exp er im en ts
lation vial was charged with 3b (0.65 g, 2.55 mmol) and diluted
with THF (2 mL). After 40 min of polymerization, the 3b
solution was injected into the reaction mixture. The vial was
rinsed with THF (1 mL × 3) and added to the reaction mixture.
The reaction vial was removed from the glovebox, and after
40 min the polymerization was quenched with ethyl vinyl ether
(2 mL). BHT (30 mg) was added, and the polymer was
precipitated in swirling methanol. The final polymer was dried
for 2 days under vacuum.
Syn th esis of Ra n d om Cop olym er s of 3a a n d 3b. In a
typical polymerization a scintillation vial equipped with a stir
bar was charged with 3a (0.25 g, 1.25 mmol) and 3b (0.32 g,
1.25 mmol) and diluted with THF (7 mL). Another scintillation
vial was charged with Grubbs catalyst 1 (8.3 mg, 1.0 × 10-2
mmol) and diluted with THF (1 mL). The catalyst solution was
injected into the stirring monomer solution, and the vial was
capped. The vial was removed from the glovebox, and after 40
min the polymerization was quenched with ethyl vinyl ether
(2 mL). BHT (30 mg) was added, and the polymer was
precipitated in swirling methanol. The final polymer was dried
for 2 days under vacuum.
time (min)
run monomer
0
3
6
12
18 24
30
36
1
2
3
3a
3b
3a
3b
3a
3b
1.8
27.0
0.82 1.2
9.3
1.2
9.4
2.8
3.4 4.0 6.0 10.0
18.0
2.3
12.0
0.84
1.2
17.0 10.0 6.8 5.0
3.4
2.9
5.0
2.9
5.0
3.3 3.7 6.2
2.9 1.7 1.3
3.3 3.7 5.8
2.9 1.7 1.3
9.4
0.84
9.9
7.1
2.1
7.1
0.85
0.85
Ta ble 2. Mon om er Ad d ition Ra tes (m m ol/h ) of 3a a n d 3b
for Syn th esis of a n Un u su a l Gr a d ien t Sh a p e d u r in g th e
Du a l-Ra m p in g Meth od
time (min)
run monomer
0
2
6
12
18
24
30
36
4
3a
3b
0.00 1.80 2.80 3.60 6.00 8.00 12.00 18.00
4.02 4.02 2.54 1.54 0.76 0.50 0.35 0.20
Syn th esis of Gr a d ien t Cop olym er s: Sem iba tch . In an
inert-atmosphere glovebox, compounds 3a (25 µL, 1.2 × 10-1
mmol) and 3b (570 µL, 2.2 mmol), undecane (70 µL, 3.3 × 10-1
mmol), and THF (152 mL) were added to a 500 mL round-
bottom flask containing a magnetic stir bar. The flask was
capped with a rubber septum and attached to a syringe pump
loaded with neat 3a (1 mL) in a 1 mL gastight syringe. Grubbs
catalyst 1 (8.3 mg, 1.0 × 10-2 mmol) was dissolved in THF (2
mL) in a scintillation vial and loaded into a 3 mL disposable
syringe. The catalyst solution was injected into the rapidly
stirring monomer solution, and the syringe pump was started
simultaneously to add 3a at a set rate. During the polymer-
ization, 2 mL aliquots were removed from the reaction flask
and quenched with ethyl vinyl ether (1 mL).40 BHT (5 mg) and
triphenylphosphine oxide (20 mg) were then added to each
aliquot to prevent cross-linking and ease the removal of the
catalyst.41 A portion of each aliquot (0.3 mL) was loaded on a
plug of silica gel (3.55 cm L × 0.55 cm i.d.), eluted with CH2-
Cl2 (8 mL), and analyzed by GC. The amount of monomer
incorporated into the polymer was calculated from the amount
of remaining monomer.1 The remaining portion of each aliquot
was concentrated on a rotary evaporator to about 0.1 mL.
Methanol (2 mL) was then added to the residue to precipitate
the polymer, the mother liquor was decanted, and the polymer
was dried under vacuum overnight before being analyzed by
GPC.
Next, a 500 mL round-bottom flask equipped with
a
magnetic stir bar was charged with 3a (9 µL, 4.3 × 10-3 mmol),
3b (100 µL, 3.8 × 10-1 mmol), undecane (100 µL, 8.6 × 10-1
mmol), and THF (102 mL). The monomer solution flask was
capped with a rubber septum, attached to the loaded syringe
pumps, and allowed to stir rapidly. The catalyst solution was
injected into the reaction flask, and the solutions of 3a and
3b were simultaneously added via syringe pumps. The addi-
tion rate of the 3a solution was ramped up while the 3b
solution was ramped down every 3-6 min (Table 1). Aliquots
(2 mL) were taken 1 min after each change in addition rate
and quenched with ethyl vinyl ether (1 mL).40 BHT (5 mg) and
triphenylphosphine oxide (20 mg) were then added to each
aliquot. A portion of each aliquot (0.3 mL) was loaded on a
plug of silica gel (3.55 cm L × 0.55 cm i.d.), eluted with CH2-
Cl2 (8 mL), and analyzed by GC. The amount of monomer
incorporated into the polymer was calculated from the amount
of remaining monomer. The remaining portion of each aliquot
was concentrated on a rotary evaporator to about 0.1 mL.
Methanol (2 mL) was then added to the residue to precipitate
the polymer, the mother liquor was decanted, and the polymer
was dried under vacuum overnight before being analyzed by
GPC.
The reaction was quenched at 42 min with ethyl vinyl ether
(5 mL). BHT (125 mg) and triphenylphosphine oxide (300 mg)
were then added to the flask. The reaction solution was
concentrated on a rotary evaporator to about 5 mL and
precipitated in rapidly stirred methanol (250 mL). The pre-
cipitated polymer was collected and precipitated a second time
if necessary. The final polymer sample was collected and dried
under vacuum before analysis by GPC.
Syn th esis of Gr a d ien t Cop olym er s w ith Un u su a l Gr a -
d ien t Sh a p es: Du a l Ra m p in g. The same dual-ramping
procedure described above was used. The rates of addition
(Table 2) were adjusted every 6 min, and the reaction was
allowed to polymerize for a total of 42 min.
P olym er Hyd r ogen a tion . This experiment was a modi-
fication of a literature procedure.43 In an inert nitrogen
atmosphere glovebox, the ROMP polymer (630 mg) was
dissolved in THF (9 mL) and methylene chloride (3 mL) and
loaded into a 125 mL Parr reactor equipped with a magnetic
stir bar and a glass liner. To this solution, Grubbs catalyst 1
(25 mg, 0.030 mmol) was added. The reactor was sealed,
removed from the box, pressurized with hydrogen (1000 psi),
and allowed to stir for 1 week at room temperature. The
reaction was monitored by 1H NMR spectroscopy though
opening the Parr reactor in the glovebox, removing an aliquot,
and precipitating the polymer. If the reaction was not com-
plete, the Parr reactor was repressurized with H2 and the
reaction was allowed to continue. Once hydrogenation was
complete, the reaction was transferred into a 500 mL round-
bottom flask, and the reactor was rinsed with methylene
chloride (3 × 2 mL). BHT (50 mg) was then added to the flask.
The combined organics were concentrated on a rotary evapora-
Typical reaction times were 30 min to 1 h, depending on
the rate of addition of 3a . The polymerization was allowed to
proceed until the feed ratio42 was 90% 3a (as determined by
GC) and then was quenched with ethyl vinyl ether (5 mL).40
(At this point, the gradient composition is approximately 3a :
3b 1:1, and the resulting polymer is designated as 3a 1-g-3b1.
To achieve a gradient composition of 3a :3b 2:1 (3a 2-g-3b1), the
polymerization is carried out for about 15-20 min beyond the
initial point of 90% 3a feed ratio.) BHT (125 mg) and
triphenylphosphine oxide (300 mg) were then added to the
flask. The reaction solution was concentrated on a rotary
evaporator to about 5 mL and precipitated in rapidly stirred
methanol (250 mL). The precipitated polymer was collected
and precipitated a second time if necessary. The final polymer
sample was collected and dried under vacuum before analysis
1
by GPC. H NMR (C6D6): δ 1.22 (s, tert-butyl-H9), 1.56 (br s,
cyclopentyl-Hs), 2.06 (br s, cyclopentyl-Hs), 2.82 (br s, cyclo-
pentyl-Hs), 3.30 (br d, cyclopentyl-Hs), 3.71 (br s, cyclopentyl-
H), 4.45 (br d, benzyl-H2), 5.49 (m, olefin-Hs), 7.14-7.37 (m,
aromatic-Hs).
Syn th esis of Gr a d ien t Cop olym er s: Du a l Ra m p in g. In
an inert-atmosphere glovebox, compounds 3a (0.808 g, 4.040
mmol) and 3b (0.517 g, 2.020 mmol) were weighed into a 10
and 50 mL volumetric flask, respectively, and diluted with
THF. Both solutions were taken up into gastight syringes and
loaded onto two different syringe pumps. In a scintillation vial,
Grubbs catalyst 1 (8.3 mg, 0.010 mmol) was dissolved in THF
(2 mL) and loaded into a 3 mL disposable syringe.

Macromolecules, Vol. 37, No. 15, 2004
ROMP-Based Gradient Copolymers 5507
Ta ble 3. Nu m ber -Aver a ge Molecu la r Weigh t, Weigh t-Aver a ge Molecu la r Weigh t, P olyd isp er sity In d ex, a n d Tg,(1/2)∆C Da ta
p
for Hyd r ogen a ted , Br om in a ted , a n d Br om oa lk oxyla ted Gr a d ien t Cop olym er s of 3a a n d 3b
polymer
Mn × 10-3 (g/mol)
Mw × 10-3 (g/mol)
PDI
Tg,(1/2)∆C (°C)
% 3a
p
original 1:1 gradient poly(3a 1-g-3b1)
original 2:1 gradient poly(3a 2-g-3b1)
brominated 2:1 gradient poly(3a 2-g-3b1)
bromomethoxylated 2:1 gradient poly(3a 2-g-3b1)
hydrogenated 1:1 gradient poly(3a 1-g-3b1)
171
164
168
203
177
192
197
210
242
202
1.12
1.20
1.25
1.19
1.14
32.8
30.4
decomp
decomp
21.2
54.1
68.3
68.3
68.3
54.1
Sch em e 1. Syn th esis of Mon om er s
tor to 5 mL and precipitated into rapidly stirred methanol (200
mL). The precipitated polymer was collected and precipitated
a second time. The final polymer sample was collected and
dried under vacuum before analysis by GPC. GPC analysis
was performed on both the hydrogenated and original polymer
1
to verify that no degradation of the polymer had occurred. H
NMR (C6D6): δ 0.72 (br s, cyclopentyl-Hs), 1.22 (s, tert-butyl-
H9), 1.33 (br s, backbone-Hs), 1.65 (br s, cyclopentyl-Hs), 2.17
(br s, cyclopentyl-Hs), 3.63 (br d, cyclopentyl-H), 4.47 (br m,
benzyl-H2), 7.10-7.39 (m, aromatic-Hs).
P olym er Br om in a tion . In a typical experiment, ROMP
gradient polymer (314 mg) was dissolved in methylene chloride
(100 mL) in a 500 mL round-bottom flask equipped with a
magnetic stir bar. 2,6-Lutidine (0.30 mL, 2.6 mmol) was added
via syringe, and the reaction flask was attached to a 25 mL
addition funnel. Bromine (0.10 mL, 1.9 mmol) was then
dissolved in methylene chloride (10 mL) and loaded into the
addition funnel. The bromine solution was added to the rapidly
stirring reaction over 15 min, and the resulting red solution
was allowed to stir for 1 h. The reaction mixture was
concentrated to dryness on a rotary evaporator, redissolved
in a minimal amount of methylene chloride (<5 mL), and
precipitated into rapidly stirring methanol (250 mL). The
precipitated polymer was collected and dried under vacuum
overnight. GPC analysis was performed on both the bromi-
nated and original polymer to ensure that no randomization
of the polymer had occurred (Table 3). 1H NMR (C6D6): δ 1.24
(s, cyclopentyl-Hs), 1.38 (s, tert-butyl-H9), 1.88 (br s, cyclopen-
tyl-Hs), 3.11 (br s, backbone-H2), 4.27 (br s, cyclopentyl-H), 4.44
(br s, benzyl-H2), 7.23-7.39 (m, aromatic-Hs). EA: Theoretical
wt % Br ) 42.5% for full bromination, actual wt % Br )
40.46%, indicating that 95% of the polymer double bond has
been brominated.
was dissolved in CH2Cl2 (2 mL) and precipitated into stirring
methanol (100 mL). The precipitated polymer was collected
and dried under vacuum. The mother liquor was evaporated
on a rotary evaporator and resuspended in CH2Cl2 (3 mL). A
portion of this solution (1 mL) was then eluted through a plug
of slica gel with CH2Cl2 (8 mL) and checked by GC for sign of
residual monomers. When residual monomer was detected, the
precipitated polymers were again redissolved and reprecipi-
tated and the mother liquor was reanalyzed. This process was
repeated until no monomer could be detected in the mother
liquor by GC and required an average of six cycles. The clean
polymer was then dried under vacuum for 2 days.
Th er m a l P r op er ty Ch a r a cter iza tion . The glass transi-
tion temperature, Tg, of each as-precipitated polymer, prepared
in the manner described above, was obtained via differential
scanning calorimetry (DSC) on a Mettler-Toledo DSC 822
instrument. Dry nitrogen was purged through the cell at a flow
rate between 45 and 55 mL/min, and an external refrigeration
unit was used to cool. In a typical experiment, 1-10 mg of
sample in an aluminum pan was heated from -50 to 100 °C
at a rate of 10 °C/ min, held at 100 °C for 10 min, quenched at
a rate of 40 °C/min, and then reheated to 100 °C at 10 °C/
min, unless otherwise noted. The Tg’s are reported as the onset
temperature, Tg,onset, and the temperature at half the change
in heat capacity, Tg,(1/2)∆Cp, for the second heating cycle. All
reported Tg’s have a (0.3 K experimental error.
P olym er Br om oa lk oxyla t ion . In a typical experiment,
ROMP gradient copolymer (0.393 g) was dissolved in methyl-
ene chloride (125 mL) and methanol (70 mL) in a 500 mL
round-bottom flask equipped with a magnetic stir bar. 2,6-
Lutidine (0.30 mL, 2.6 mmol) was added via syringe, and the
reaction flask was attached to a 25 mL addition funnel.
Bromine (0.11 mL, 2.1 mmol) was then dissolved in methylene
chloride (10 mL) and loaded into the addition funnel. The
bromine solution was added to the rapidly stirring reaction
over 15 min, and the resulting red solution was allowed to stir
for 1 h. The reaction mixture was concentrated to dryness on
a rotary evaporator, redissolved in a minimal amount of
methylene chloride (<5 mL), and precipitated into rapidly
stirring methanol (250 mL). The precipitated powder polymer
was collected via vacuum filtration and dried under vacuum
overnight. GPC analysis was performed on both the bromo-
alkoxylated and original polymer to verify that no randomiza-
Resu lts a n d Discu ssion
Monomers 3a and 3b were synthesized in three steps
from norbornadiene (Scheme 1). Their reactivity ratios
for ROMP using catalyst 1 were measured to be 0.99 (
0.06 and 1.13 ( 0.09, respectively. Since the reactivities
of ROMP monomers toward well-defined ROMP cata-
lysts are known to be influenced by the ring strain of
the monomer,44-46 the stability of the propagating
carbene,47 and steric effects,44,48-51 similar reactivities
for 3a and 3b are expected: the discriminating substit-
uents are simply too far from the olefin moiety to have
a large effect. We are aware of only one other example
where reactivity ratios for 5-subsituted norbornenes are
found to be similar and close to unity.52
To date, differences in the reactivities between ROMP
monomers toward a specific catalyst system are typi-
cally framed in terms of the differences in their homo-
polymerization rates and the observed differences in
monomer conversion during copolymerization;53-58 mea-
sured reactivity ratios are rare. Ilker and Coughlin
determined the reactivity ratios for endo-N-ethyl-7-
oxanobornene-2,3-dicarboxylimide and cyclooctene (COE)
1
tion of the polymer had occurred. H NMR (C6D6): δ 1.24 (s,
tert-butyl-H9), 1.88 (br s, cyclopentyl-Hs), 2.78 (br s, methoxy-
H3), 3.28 (br s, backbone ether-H), 4.27 (br s, benzyl-H2), 4.45
(br s, brominated backbone-H), 7.22-7.35 (m, aromatic-Hs).
EA: theoretical wt % Br ) 17.5% for completely selective
bromomethoxylation, actual wt % Br ) 34.31%, indicating that
57% of the polymer double bond has been brominated and 43%
of the double bonds have been bromomethoxylated.
Sa m p le P r ep a r a tion for DSC An a lysis. The unsaturated
ROMP polymers and copolymers described above often contain
a small amount of residual monomers, which plasticize the
polymers and influence their thermal properties. Hence, the
residual monomers must be removed before accurate thermal
data for these materials can be obtained. To accomplish this
objective, a small quantity (270 mg) of the ROMP copolymer

5508 Dettmer et al.
Macromolecules, Vol. 37, No. 15, 2004
F igu r e 1. Fcum of 3a vs normalized chain length for different
addition rates of 3a .
F igu r e 3. Mn and PDI of a 3a 1-g-3b1 copolymer as a function
of polymerization time.
F igu r e 2. Finst of 3a vs normalized chain length for different
addition rates of 3a .
F igu r e 4. Comparison of percentage of 3a incorporated into
the copolymer as determined by GC and 1H NMR.
in the presence of 1 to be 0.04 and 0.08, respectively,
allowing for the formation of an almost perfectly alter-
nating copolymer.59 Balcar and Dosedlova´ found the
reactivity ratios for 5-norbornene-2-yl acetate and COE
and norbornene (NBE) to be r1 ) 1 and r2 ) 132 and r1
) r2 ) 1, respectively, using WCl6/(CH3)4Sn.52 Reactivity
ratios for NBE and COE were determined to be 8.9 and
0.51, respectively, using WCl6/Ph4Sn.60 Monakov et al.
also measured reactivity ratios for NBE with various
7-oxonorbornenes in the presence of RuCl3‚3H2O.61
As expected for monomers with similar reactivity
ratios, mixtures of 3a and 3b gave no gradients in batch
polymerization. However, semibatch polymerizations,
with the addition of 3a to a 1:9 mixture of 3a :3b at
different rates, did produce significant gradients (Figure
These latter monomer pairs have some tendency to
alternate while 3a and 3b tend toward random copoly-
mers. Gradient copolymers of styrene and 4-acetoxy-
styrene, whose reactivity ratios in radical polymeriza-
tion are also close to unity (rS ) 0.887 and rAS ) 1.128),62
have been made via NM-CRP.13,15,16 However, signifi-
cant gradients were only achieved for copolymer com-
positions that are well over 50% styrene16 or 50%
4-acetoxystyrene.13 Our work suggested that, for ROMP
monomer pairs that favor statistical tendencies, con-
sistent linear gradient shape could be obtained over a
range of appropriate addition rates. This is significant
because a plethora of gradient copolymer compositions
can be achieved via semibatch polymerization for a
single pair of monomers simply by changing the addition
rates and times for one monomer.
Figure 3 shows the Mn and polydispersity index of a
3a 1-g-3b1 copolymer as a function of polymerization
time. The linear increase in Mn and the steady low PDI
demonstrate the living nature of catalyst 1 toward 3a
and 3b and the formation of a “controlled” copolymer.
For ease of experiment, we determined the extent of
polymerization and monomer incorporation via GC
quantification of unreacted monomer, an indirect but
facile method. However, the percentage of monomer
incorporated into the copolymer can also be followed by
NMR spectroscopy at long reaction times when suf-
ficient amounts of polymer can be recovered for analysis.
Increasing incorporation of 3a into the copolymer can
be observed, and this data corresponded well with the
GC conversion data (Figure 4).
1). From the monomer conversion data, cumulative
1
(Fcum) and instantaneous compositions (Finst
)
were
calculated from eqs 1 and 2, where X and Y are the
quantities (mol) of monomers x and y in the polymer,
respectively, and F is conversion.
X
X + Y
Fcum,1
)
(1)
(2)
∆Fcum,1
∆F
Finst,1 ) Fcum,1 + F
As seen in Figure 1, 50/50 copolymers of 3a and 3b
with linear gradients were produced at three different
addition rates of 3a . The significance of the gradient,
as expressed by either Fcum,3a (Figure 1) or Finst,3a
(Figure 2), decreases with increasing addition rate of
3a . The shape of the gradient for 3a and 3b ROMP
copolymers does not change with addition rate as is does
in ATRP polymerization with n-butyl acrylate/styrene14
(rS ) 0.70, rnBA ) 0.16), methyl acrylate/styrene,17 and
styrene/acrylonitrile18 (rS ) 0.29, rAN ) 0.02) where the
reactivity ratios of the polymer pairs are less than 1.
Ideally, for a monomer pair with similar and close-
to-unity reactivity ratios, such as 3a and 3b, the feed
ratio for the monomers should be changed linearly from
0 to 1 during the polymerization to achieve the best
gradient (Figure 5). In our case, because of the high

Macromolecules, Vol. 37, No. 15, 2004
ROMP-Based Gradient Copolymers 5509
F igu r e 5. Idealized relationship between feed ratio of 3a and
time. The solid line represents the perfectly ideal gradient
polymerization where the feedstock begins with no 3a . The
dotted line represents the desired change in the feed ratio for
a polymerization setup that begins with a feedstock of 10%
3a .
F igu r e 7. Experimental observation of the change in the feed
ratio of 3a during a dual-ramping polymerization. The solid
line represents the ideal change in the feed ratio for the
polymerization time span.
F igu r e 8. Finst and Fcum of 3a vs normalized chain length for
several different dual-ramping runs. The rates for the runs
are listed in Table 1.
F igu r e 6. Experimental observation of the change in the feed
ratio of 3a during polymerization at three different addition
rates. The solid lines represent the ideal change in the feed
ratio for the polymerization time span.
activity of the ROMP catalyst and the limitation of our
addition equipment, the initial feed ratio needs to be
kept at about 0.1 to avoid making a homopolymer block
at one end of the desired gradient copolymer.
While the gradient shapes shown in Figures 1 and 2
are fairly linear, they begin to deviate near the end of
the polymerization. This is a mathematical consequence
of the change in feed ratio toward the end of the
polymerization for monomer pairs with reactivity ratios
near unity. The feed ratio of 3a is the mole fraction of
3a in the feedstock. At a constant addition rate, the feed
ratio of 3a ceases to be linear and begins to taper off
near 70% (Figure 6) because the relationship between
the change in the feed ratio with respect to time is
“hyperbolic”, asymptotically approaching 100%. To main-
tain a linear change in the feed ratio for the entire
polymerization time span, an increasingly large excess
of 3a would need to be added toward the end of the
polymerization. However, such large and rapid additions
of neat 3a near the end of the polymerization would
result in local concentration gradient problems and may
give rise to a block of 3a at the end of the gradient
copolymer. We hypothesized that a dual-ramping method
where the addition rate of both monomers can be varied
semicontinuously in opposite directions would allow us
to compensate for this problem. This strategy would
allow us to fine-tune the feed ratio as the polymerization
occurred by adjusting the addition rate of both mono-
mers over time.
F igu r e 9. Finst of 3a vs normalized chain length for an
unusually shaped gradient copolymer synthesized via a dual-
ramping strategy.
addition rate of 3a (ramp up) while the concentration
of 3b in the reaction mixture was decreased by reducing
the addition rate of 3b (ramp down). This dual-ramping
strategy allowed us to achieve a near-ideal feed ratio
(Figure 7) and resulted in copolymers with ideal linear
gradients containing about 55% 3a (Figure 8). It also
allowed for the design of novel gradient shapes such as
that shown in Figure 9. The gradient copolymer with
this S-shape19 can be considered as having three blocks.
The first and last blocks have steep gradients while the
middle block has a gradual gradient, with a composition
that is close to that of a random copolymer (Figure 9).
Fu n ction alization of th e ROMP Copolym er . While
NM-CRP can be utilized to make gradient copolymers
of a variety of vinyl monomers, the final polymer chains
are capped with dormant radicals. Unless chemically
deactivated, these radicals can become active upon
heating and the polymer chains may recombine63 which
could limit their application and processing potential.
To this end, we set up a copolymerization with two
syringe pumps, each loaded with a solution of one
monomer. The concentration of monomer 3a in the
reaction mixture was gradually increased by raising the

5510 Dettmer et al.
Macromolecules, Vol. 37, No. 15, 2004
Ta ble 4. Th er m a l P r op er ties of ROMP Gr a d ien t, Block , a n d Ra n d om Cop olym er s of 3a a n d 3b a s Well a s th e Resp ective
Hom op olym er s
polymer
Tg,onset (°C)
Tg,(1/2)∆C (°C)
Mn × 10-3 (g/mol)
Mw × 10-3 (g/mol)
PDI
% 3a
p
poly(3a )
poly(3b)
poly(3a -b-3b)
18.1
45.0
17.9
40.7
23.9
26.1
29.4
28.9
29.9
20.7
48.6
20.8
45.3
28.0
29.5
34.8
32.8
36.0
87
65
110
120
72
148
1.38
1.10
1.35
100
0
46.4
poly(3a -r-3b)
67
161
165
163
151
79
184
191
184
186
1.18
1.14
1.15
1.13
1.23
50.1
58.4
52.9
54.0
46.3
1.1 gradient poly(3a 1-g-3b1) 1
1:1 gradient poly(3a 1-g-3b1) 2
1:1 gradient poly(3a 1-g-3b1) 3
1.1 gradient poly(3a 1-g-3b1) 4
Ta ble 5. Th er m a l P r op er ties of th e Hyd r ogen a ted ROMP Gr a d ien t, Block , a n d Ra n d om Cop olym er s of 3a a n d 3b a s Well
a s th e Resp ective Hom op olym er s
polymer
hydrogenated poly(3a )
hydrogenated poly(3b)
hydrogenated poly(3a -b-3b)
Tg,onset (°C)
Tg,(1/2)∆C (°C)
Mn × 10-3 (g/mol)
Mw × 10-3 (g/mol)
PDI
% 3a
p
6.4
36.4
6.1
9.2
40.0
9.8
125
105
179
156
119
196
1.24
1.13
1.10
100
0
49.5
28.6
16.9
18.9
16.6
35.7
20.1
23.8
20.7
hydrogenated poly(3a -r-3b)
hydrogenated 1:1 gradient poly(3a 1-g-3b1) 2
hydrogenated 1:1 gradient poly(3a 1-g-3b1) 3
74
136
177
86
177
202
1.16
1.30
1.14
61.9
52.9
54.0
Sch em e 2. Hyd r ogen a tion , Br om in a tion , a n d
Br om oa lk oxyla tion of th e Gr a d ien t Cop olym er
of the diimide hydrogenation agent. Established poly-
mer hydrogenation catalysts such as Crabtree’s cata-
lyst66 resulted only in limited success: 39.2% of the
olefins remained even after 24 h under 50 psi of H2. An
additional 25.8% of the olefins were hydrogenated at 66
h after an addition of more catalyst (2 mg, 2.5 × 10-6
mol). Our best strategy took advantage of a procedure
by Drouin et al. where Grubbs catalyst 1 was used in a
tandem ROMP-hydrogenation cycle for the synthesis
of ROMP polymers and hydrogenated ROMP poly-
mers.43 Under moderate hydrogen pressure, Grubbs
catalyst 1 can be converted into a hydrogenation cata-
lyst, which allowed us to carry out polymer synthesis
and the subsequent polymer hydrogenation in a one-
pot manner. Drouin and co-workers reported complete
hydrogenation of poly(cyclooctene) in 24 h; however, our
hydrogenation experiments required up to a week for
complete conversion. The increased reaction time may
result from the more sterically hindered double bond
present in norbornene-based ROMP polymers as well
as the increased chain lengths of our gradient copoly-
mers: on average, our gradient copolymers were 700-
800 repeating units long while Drouin et al. used
polymers with DP of about 200-300.
Electrophilic addition of a halogen to unsaturated
polymers is another well-known method for polymer
modification. For example, chlorination of elastomers
such as butyl rubber is a commercially important
strategy to improve cure properties.67-69 Bromination
of unsaturated polymers is also straightforward but
has been less extensively studied.67,69-73 In our hands,
the gradient copolymers of 3a and 3b were easily
brominated in methylene chloride using a bulky base
(2,6-lutidine) in conjunction with liquid Br2. The result-
ing fully brominated polymer was soluble in most
organic solvents. We also synthesized a soluble bromo-
methoxylated polymer through the controlled bromina-
tion of the 3a and 3b gradient copolymers in methanol.
Other reactive cosolvents such as THF can potentially
be incorporated into this bromination chemistry as
demonstrated by Dreyfuss and co-workers for polybuta-
diene.70 Table 3 lists the characterization data for our
brominated and hydrogenated polymers.
The same problem could potentially occur with gradient
copolymers made from ATRP if the metal catalyst is not
completely removed at the end of the polymerization.
In contrast, gradient copolymers synthesized via ROMP
would not be plagued by such problems as the catalyst
can be cleaved from the end of the polymer via a
quenching agent. Furthermore, the olefins in the back-
bone of ROMP polymers provide the opportunity for
postpolymerization modification. For example, we
have demonstrated that the gradient copolymers of 3a
and 3b can be hydrogenated, brominated, and bromo-
methoxylated (Scheme 2).
In our hands, chemical hydrogenation of our 3a -
and 3b-based polymers with toluenesulfonylhydrazine
(TSH)64,65 in the presence of tributylamine led to an
increase in polymer dispersity and polymer degradation
due to the high temperatures required for the formation
While there is a growing number of reports on the
synthesis of gradient copolymers, there have been few

Macromolecules, Vol. 37, No. 15, 2004
ROMP-Based Gradient Copolymers 5511
(2) Lutz, J .-F.; Pakula, T.; Matyjaszewski, K. In Advances in
Controlled/ Living Radical Polymerization; Matyjazewski, K.,
Ed.; American Chemical Society: Washington, DC, 2003; Vol.
854, pp 268-282.
studies that report on the thermal properties of gradient
copolymers. Farcet and Charleux detected only one Tg
in their styrene/n-butyl acrylate copolymers with weak
gradients. Matyjaszewski et al. demonstrated micro-
phase separation of copolymers with a much stronger
gradient through both thermal and rheological analysis
for styrene/methyl acrylate gradient copolymers and by
small-angle X-ray scattering (SAXS) in styrene/acrylo-
nitrile gradient copolymers. These workers observed a
single Tg in a thermally quenched gradient copolymer
and two Tg’s in an annealed gradient copolymer.
The thermal properties for our norbornene-based
ROMP polymers are shown in Table 4 and the corre-
sponding hydrogenated polymers in Table 5. The benzyl
ether substituents on norbornene raised the Tg’s of
poly3a and poly3b to near and above room temperature,
respectively. Hydrogenation lowered the Tg’s due to
increased freedom of rotation in the backbone. This
decrease was also observed by Hillmyer et al. in their
carboximide-functionalized 7-oxanorbornene polymers.66
The brominated and bromomethoxylated gradient co-
polymers were not thermally stable and decomposed
above 100 °C.
In our studies, the linear gradient copolymers syn-
thesized by the dual-ramping method exhibit only one
Tg, and no microphase separation is detected by pre-
liminary SAXS experiments. This is not surprising since
ø (the Flory-Huggins interaction parameter) is esti-
mated to be low for these copolymers.74 Interestingly,
the monomers are substantial plasticizers, and a com-
bined 5 wt % can depress the Tg by 13-19 °C. The
ROMP gradient, block, and random copolymers of 3a
and 3b as well as the respective homopolymers were
all high molecular weight (Mn ) 80-170 × 103) with
narrow PDI’s, most within 1.11-1.20.
(3) Davis, K. A.; Matyjaszewski, K. Adv. Polym. Sci. 2002, 159,
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(13) (a) Gray, M. K.; Zhou, H.; Nguyen, S. T.; Torkelson, J . M.
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molecules 2004, 37, 1118-1123.
(28) Properties related to motion such as relaxation and diffusion.
(29) The shape refers to the profile of the chain composition as a
function of normalized chain length.
In conclusion, we have synthesized the first gradient
copolymers via ROMP. These norbornene-based gradi-
ent copolymers are among a very few gradient copoly-
mers reported to date where a linear gradient shape can
be obtained in conjunction with a 50/50 monomer
cumulative composition. Furthermore, through a dual-
ramping strategy, we can fine-tune the monomer feed
ratio to produce an exceptionally linear gradient copoly-
mer and manipulate the gradient shape. Our ROMP
gradient polymers can be easily functionalized further
via hydrogenation, bromination, or bromoalkoxylation.
Interfacial segregation studies are being carried in our
laboratory to evaluate the use of these gradient copoly-
mers as compatibilization agents and as model for
systems of copolymers having a low ø parameter.
(30) Numerical self-consistent-field theory was utilized to describe
the diffusion of gradient copolymer to the interface and its
effects on the interfacial width of an immiscible blend. See:
Shull, K. R. Macromolecules 2002, 35, 8631-8639.
(31) Shinoda, H.; Matyjaszewski, K.; Okrasa, L.; Mierzwa, M.;
Pakula, T. Macromolecules 2003, 36, 4772-4778.
(32) Schwab, P.; Grubbs, R. H.; Ziller, J . W. J . Am. Chem. Soc.
1996, 118, 100-110.
(33) Grubbs first-generation catalyst was chosen because of its
high kinitiation/kpropagation ratio, is known to be a living catalyst
for a variety of cyclic olefins, and has excellent monodisper-
sity. See ref 32.
Ack n ow led gm en t. This work was supported by the
NSF-MRSEC program at Northwestern University
(Grant DMR-0076097), the NSF-DMR program (DMR-
CAREER Grant 0094347), and an IMGIP Fellowship
and Ford Foundation Dissertation Year Fellowship (to
M.K.G.). We thank Professor Kenneth Shull and Dr.
Donald Trinite (Waters Corp.) for many helpful conver-
sations and Professor Wesley Burghardt and Ms. Kristin
Brinker for carrying out the SAXS experiment on our
linear gradient copolymer.
(34) Gray, M. K.; Zhou, H.; Nguyen, S. T.; Torkelson, J . M.
Macromolecules 2003, 36, 5792-5797.
(35) It must be noted that Matyjaszewski (Macromolecules 2002,
35, 6773-6781) recently observed from results of computer
modeling the possibility that in “many controlled polymeriza-
tions processes ... rate of consumption of commoners may be
different from conventional systems, and this can result in
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