Controlled/ living reverse atom transfer radical polymerization of a monocyclic olefin, methyl 1-cyclobutenecarboxylate

Article abstract of DOI:10.1021/ma012104f

ATRP of the first cyclic olefin, methyl 1-cyclobutenecarboxylate (MCBE), by addition polymerization in a well controlled/ living manner is described, along with a new practical method to synthesize MCBE. The polydispersity of poly(methyl 1-cyclobutenecarboxylate) (PMCBE) synthesized by a conventional ATRP initiation system was narrow (M_w/M_n = 1.1 - 1.4), but M_n was usually less than half of the theoretical number-average molecular weight. However, the polymerization of MCBE initiated by a reverse ATRP initiation system, AIBN/CuBr₂/dNbpy, was well controlled; the rate of polymerization was first order in monomer, and the molecular weights of the polymers were close to the designed values with very narrow polydispersities (M_w/M_n = ～1.15). The obtained polymer PMCBE can be used as a macroinitiator to further polymerize a second monomer, such as styrene.

Full text of DOI:10.1021/ma012104f

Macromolecules 2002, 35, 4277-4281
4277
Controlled/“Living” Reverse Atom Transfer Radical Polymerization of a
Monocyclic Olefin, Methyl 1-Cyclobutenecarboxylate
Xia o-P in g Ch en , Bila l A. Su fi, An n e Bu yle P a d ia s, a n d H. K. Ha ll, J r .*
C.S. Marvel Laboratories, Chemistry Department, The University of Arizona,
Tucson, Arizona 85721-0041
Received December 3, 2001; Revised Manuscript Received March 22, 2002
ABSTRACT: ATRP of the first cyclic olefin, methyl 1-cyclobutenecarboxylate (MCBE), by addition
polymerization in a well controlled/“living” manner is described, along with a new practical method to
synthesize MCBE. The polydispersity of poly(methyl 1-cyclobutenecarboxylate) (PMCBE) synthesized
by a conventional ATRP initiation system was narrow (Mw/Mn ) 1.1-1.4), but Mn was usually less than
half of the theoretical number-average molecular weight. However, the polymerization of MCBE initiated
by a reverse ATRP initiation system, AIBN/CuBr2/dNbpy, was well controlled; the rate of polymerization
was first order in monomer, and the molecular weights of the polymers were close to the designed values
with very narrow polydispersities (Mw/Mn ) ∼1.15). The obtained polymer PMCBE can be used as a
macroinitiator to further polymerize a second monomer, such as styrene.
In tr od u ction
So far, the monomers subjected to these techniques
have been conventional vinyl monomers, including
styrene, (meth)acrylates, vinylpyridines, etc. More re-
cently, we have reported the first ATRP of a non-olefinic
monomer, methyl 1-bicyclobutanecarboxylate, in a con-
trolled/“living” fashion.12
Methyl 1-cyclobutenecarboxylate readily undergoes
conventional free radical homopolymerization and co-
polymerization with vinyl monomers.13a This polymer-
ization proceeds via addition polymerization of the
double bond in the ring to give a high yield of polymer
containing exclusively 1,2-linked cyclobutane rings in
its backbone. Polymers having ring repeat units in the
backbone often impart desirable physical properties,
e.g., improved thermal, mechanical, optical, and piezo-
electric properties to the materials.13-15 These enhanced
properties may qualify them for specialty, high-tech
applications. In this paper we wish to report the results
of ATRP of the first cyclic olefin, namely methyl 1-cy-
clobutenecarboxylate (MCBE), by addition polymeriza-
tion in a controlled/“living” manner.
Since Szwarc et al.1 first reported the concept of living
polymerization in 1956, a number of living polymeriza-
tion systems have been reported for anionic,1 cationic,2
or group transfer polymerization.3 Generally, free radi-
cal polymerization often yields polymers with a large
variety of monomers under relatively mild experimental
conditions, but it is usually ill-controlled because of the
facile coupling and disproportionation reactions between
the propagating species. In recent years, free radical
polymerization has undergone a revolutionary trans-
formation. By “buffering” the concentration of propagat-
ing free radicals to extremely low concentration, termi-
nation has been effectively eliminated. In this way, clean
polymers of defined molecular weight and low polydis-
persity can be obtained. These methods include atom
transfer radical polymerization (ATRP)4-6 using chloro
compounds and transition metal complex catalysts, the
polymerization of styrene in the presence of the 2,2,6,6-
tetramethylpiperidinyl-1-oxy (TEMPO) stable free radi-
cal,7 and living radical polymerization by the reversible
addition-fragmentation chain transfer (RAFT) process
in the presence of dithioesters.8
Exp er im en ta l Section
Controlled/“living” radical polymerization via ATRP
is an efficient way to synthesize well-defined polymers.
There are two kinds of ATRP initiation systems: (con-
ventional) ATRP and reverse ATRP (RATRP). In
ATRP,4-6 organic halides (RX) are used as initiators,
transition metal compounds in their lower oxidation
state as catalysts, and electron-donating compounds,
Ma ter ia ls. Methyl 2-bromopropionate (MBP) (Aldrich, 98%)
was purified by distillation. Styrene (Aldrich, 99%) was
purified with a silica gel column and stored under an argon
atmosphere at -15 °C. AIBN was recrystallized twice from
ethanol and stored at -15 °C. Ethyl 1-bromocyclobutanecar-
boxylate (Aldrich, 96%), hexamethylphosphoramide (Aldrich,
99%), methyl iodide (Aldrich, 99%), potassium hydroxide
(Aldrich, 85+%), diphenyl ether (Aldrich, 99+ %), CuBr
(Aldrich, 99.999%), CuBr2 (Aldrich, 99.999%) and 4,4′-dinonyl-
2,2′-bipyridyl (dNbpy) (Aldrich, 97%) were used without any
further purification.
such as 2,2′-bipyridine, as ligands (L). In RATRP,9-11
a
radical initiator and a higher oxidation state transition
metal catalyst complex Mn+1XLm are used instead of the
organic halides (usually toxic) and the lower oxidation
state catalyst complex MnLm (easily oxidized by the
oxygen in air). Common free radical initiators, such as
AIBN9 or BPO, are often used for RATRP.10 Recently,
the first noncommon radical initiator, 1,1,2,2-tetraphe-
nyl-1,2-ethanediol (TPED), was successfully used as an
initiator for the RATRP initiation system to polymerize
MMA in a controlled /“living” process.11
Ch a r a cter iza tion . GPC analysis was performed using a
LC-10AT Shimadzu high-performance liquid chromatography
system with
a
set of Phenomenex columns (103-105 Å)
calibrated vs polystyrene standard, THF as eluent, at 25 °C.
1H NMR spectra were recorded on a Bruker DRX-500 MHz
spectrometer in CDCl3 at 25 °C.
Syn th esis of Meth yl 1-Cyclobu ten ecar boxylate (MCBE).
A new practical method for the synthesis of methyl 1-cy-
clobutenecarboxylate is as follows: A mixture of 100 mL of
benzene and 12 g (85%, 0.182 mol) of finely pulverized
potassium hydroxide was refluxed for 2 h in a 500 mL three-
* Corresponding author. e-mail: hkh@u.arizona.edu.
10.1021/ma012104f CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/20/2002

4278 Chen et al.
Macromolecules, Vol. 35, No. 11, 2002
Ta ble 1. ATRP of Meth yl 1-Cyclobu ten eca r boxyla te
Sch em e 1
(MCBE) In itia ted by MBP /Cu Br /d Nbp y in Bu lk a t 100 °Ca
c
no.b time (h) conv (%) Mn(GPC)
Mn(th)
Mw/Mn
f d
1
2
3
3
7
12
6
17
6.5
41.5
79.4
99.8
63
82
87.2
4940
7410
10400
6280
7630
6590
9 310
17 800
22 400
14 100
18 400
20 600
1.22
1.19
1.14
1.36
1.31
1.19
1.88
2.40
2.15
2.25
2.41
3.13
4e
5e
6f
neck-flask. After removing the solvent by distillation, 200 mL
of dry toluene was added to the dry potassium hydroxide.
Under argon atmosphere, at reflux temperature and with
vigorous stirring, ethyl 1-bromocyclobutanecarboxylate (10 g,
96%, 0.046 mol) was slowly added at a rate of one drop every
2 s. A vigorous exothermic reaction ensued, and solid potas-
sium salts separated out. After completion of the addition,
reflux was maintained for one additional hour. Toluene was
thoroughly removed under vacuum followed by addition of 60
mL of water, 120 mL of hexamethylphosphoramide (HMPA),
and 30 g (99%, 0.208 mol) of methyl iodide. The orange
aqueous solution was stirred at room temperature for 24 h
under an argon atmosphere. An aqueous 5% HCl solution was
added until the pH of the mixture was below 7. The solution
was extracted twice with 200 mL of ether. The combined ether
extracts were washed twice with 50 mL of saturated NaCl
solution and dried over anhydrous sodium sulfate, and the
solvent was slowly evaporated at 0 °C under reduced pressure.
The crystalline target product was obtained by distillation at
34-36 °C/12 mmHg while the receiver was chilled with a dry
ice/acetone bath. The overall yield was 85%. 1H NMR (CDCl3)
δ (ppm): 2.48 (2H, m, 3-H), 2.74 (2H, t, 4-H), 3.73 (3H, s,
OCH3), 6.78 (1H, t, 2-H), which correspond to the literature
values.16a
P olym er iza tion . All polymerizations were performed with
a Schlenk vacuum apparatus. A solution of monomer, initiator
(MBP, AIBN, etc.), CuBr or CuBr2, and dNbpy in a glass tube
was sealed under vacuum after three freeze-pump-thaw
cycles. The tube was placed in an oil bath at the desired
temperature. At a specific time, cooling the tube in an ice-
water bath stopped the polymerization. The polymer product
was dissolved in tetrahydrofuran (THF), and the THF solution
was used directly to measure Mn(GPC), number-average molec-
ular weight by GPC, and Mw/Mn, at 25 °C. The THF solution
of the polymer product was precipitated in n-heptane, filtered,
and dried under vacuum at 50 °C. The transition metal
catalyst was removed with a silica gel column. The conversion
of polymerization was determined gravimetrically.
a
[MCBE]0 ) 7.99 mol/L, [MCBE]0/[MBP]0/[CuBr]0/[dNbpy]0 )
b
200:1:1.5:3; MBP ) methyl bromopropionate. Entry number.
c Mn(th) ) ([MCBE]0/[MBP]0) × MWMCBE × conversion. f ) Mn(th)
/
d
Mn(GPC).
e The catalyst was CuCl instead of CuBr. f The initiator
was methyl 1-bromocyclobutanecarboxylate instead of MBP, at 85
°C.
instead of potassium 1-cyclobutenecarboxylate, due to
the presence of water in the potassium hydroxide.18 To
eliminate this side reaction, it is imperative to remove
the water absorbed in the KOH. The water can be
removed as an azeotrope with benzene by distillation.
The new procedure also has good reproducibility in high
overall yield (∼85%).
ATR P of Met h yl 1-Cyclob u t en eca r b oxyla t e
(MCBE). We investigated the ATRP of MCBE initiated
by the methyl 2-bromopropionate (MBP)/CuBr/dNbpy
initiation system, and the results are summarized in
Table 1. The rate of polymerization was found to be low
at 85 °C, so the homogeneous ATRP of MCBE was
carried out in bulk at 100 °C using [MCBE]0/[MBP]0/
[CuBr]0/[dNbpy]0 ) 200:1:1.5:3. The molecular weights
of the polymers linearly increased in the conversion
range from about 40 to 100% (Mn ) 4940-10 400) with
rather narrow polydispersities, Mw/Mn ) 1.14-1.22
(entries 1, 2, and 3 in Table 1). However, the molecular
weights of the obtained polymers were not well con-
trolled, generally about half of the designed values, i.e.,
1
Mn(GPC) ∼ /2Mn(th). As can be seen from entries 4 or 5 in
Table 1, when CuCl was used as a catalyst instead of
CuBr, similar results were observed, but with larger
polydispersities (1.31-1.36). The polymerizations cata-
lyzed by CuBr were thus better controlled than those
using CuCl. When methyl 1-bromocyclobutanecarboxy-
Resu lts a n d Discu ssion
late was used as the initiator instead of MBP, the
Syn th esis of MCBE. According to the previous
literature,16 MCBE was synthesized by the esterification
of 1-cyclobutenecarboxylic acid (CBEA) with diazo-
methane. But CBEA is prone to rapid polymerization
and absorption of atmospheric oxygen.16
1
molecular weight was even lower, Mn(GPC) ∼ /3Mn(th)
,
though with a narrow polydispersity (1.19) (entry 6 in
Table 1). The initiator efficiency values f, computed from
f ) Mn(th)/Mn(GPC), were high, usually >2 (Table 1). This
indicates that more active species were produced in this
particular MCBE polymerization, probably from the
facile auto thermal polymerization of monomer MCBE
at this high temperature (100 °C).13a
Our new procedure to synthesize methyl 1-cyclobuten-
ecarboxylate is shown in Scheme 1. Carboxylic acids can
be easily converted to esters in high yield by reaction
of their salts with alkyl iodides in HMPA at room
temperature.17 The stable potassium 1-cyclobutenecar-
boxylate was produced by the reaction of ethyl 1-bro-
mocyclobutanecarboxylate with excess anhydrous po-
tassium hydroxide. Reaction of potassium 1-cyclobutene-
carboxylate with methyl iodide in HMPA at room
temperature led to MCBE in high yield. An advantage
of this procedure is that the potassium salt can be
directly used for the next reaction, i.e., the esterification,
without isolating the salt. In comparison with the
method mentioned above, the new synthesis procedure
has a relatively simple workup and can be performed
under mild conditions. More importantly, the highly
unstable CBEA is not an intermediate. A possible side
reaction in the first step of the synthesis, i.e., the
dehydrohalogenation/saponification step, is the forma-
tion of 1-(hydroxymethyl)cyclopropanecarboxylic acid,
RATRP of MCBE. In response to the rather unsat-
isfactory ATRP results, we decided to investigate other
initiation systems. We found that RATRP can be
performed effectively at the lower temperature, i.e., 85
°C. The following RATRP initiation system was se-
lected: 2,2′-azobis(isobutyronitrile) (AIBN)/CuBr2/dN-
bpy, in which AIBN was used as an initiator and CuBr2
as catalyst. The polymerization system, with [MCBE]0:
[AIBN]0:[CuBr2]0:[dNbpy]0 ∼ 200:1:2:4, was heteroge-
neous at room temperature but instantly (<20 s) became
homogeneous after the polymerization tubes were placed
in a 85 °C oil bath for polymerization. The results show
that the number-average molecular weight linearly
increased with monomer conversion as shown in Figure
1. Mn(GPC), measured by GPC, was almost equal to Mn(th)
(from 2620 to 9270), while the polydispersity was very

Macromolecules, Vol. 35, No. 11, 2002
Methyl 1-Cyclobutenecarboxylate 4279
F igu r e 1. Dependence of molecular weight and polydispersity on conversion in bulk polymerization of MCBE at 85 °C with
[MCBE]0 ) 7.96 mol/L, [AIBN]0 ) 0.04 mol/L, [CuBr2]0 ) 0.08 mol/L, and [dNbpy]0 ) 0.16 mol/L.
F igu r e 2. Correlation of ln([M]0/[M]) and conversion vs time in bulk polymerization of MCBE at 85 °C with [MCBE]0 ) 7.96
mol/L and [AIBN]0:[CuBr2]0:[dNbpy]0 ) 1:2:4.
Ta ble 2. RATRP of Meth yl 1-Cyclobu ten eca r boxyla te (MCBE) In itia ted by AIBN/Cu Br 2/DNbp y
c
no.a
Cb
temp (°C)
time
7 h
2.3 h
10 min
conv (%)
Mn(GPC)
Mn(th)
Mw/Mn
f d
1
2
3
200/0.61/1.22/2.44
200/1/2/4
200/1/0/0
85
100
85
91
62.4
89
15 500
7 100
86 600
16 700
7 000
1.15
1.17
3.09
1.08
0.99
a
b
c
d
Entry number. C ) [MCBE]0/[AIBN]0/[CuBr2]0/[dNbpy]0. Mn(th) ) ([MCBE]0/2[AIBN]0) × MWMCBE × conversion. f ) Mn(th)/Mn(GPC)
.
narrow, Mw/Mn ) 1.11-1.19, and the monomer conver-
sion increased from 20% to 81% over a 9 h time period.
Mn(th) is the theoretical number-average molecular
weight, computed from Mn(th) ) ([MCBE]0/2[AIBN]0) ×
MWMCBE × conversion. The initiator efficiencies f were
about 1.0 in all cases. These results suggest that the
polymerization is well controlled. A plot of ln([MCBE]0/
[MCBE]) vs time displayed first-order kinetics, as seen
in Figure 2, indicating that the concentration of propa-
gating radicals was almost constant and that the rate
of polymerization was first order in monomer. Therefore,
the polymerization of MCBE was a “living” polymeri-
zation process without any significant irreversible ter-
mination.
Using the AIBN/CuBr2/dNbpy initiation system at 85
°C, but with a higher ratio of [MCBE]0 to [AIBN]0 (200:
0.61 vs 200:1 above), a polymer with higher molecular
weight (Mn ) 15 500) and still very narrow polydisper-
sity (Mw/Mn ) 1.15) was produced (entry 1 in Table 2).
Even at higher temperature, 100 °C, a well-controlled
polymerization of MCBE was still possible. In contrast,
the conventional radical polymerization was not at all
controlled, with broad molecular weight polydispersities,
i.e., Mw/Mn ) 3.09 (entry 3 in Table 2).
P olym er Str u ctu r e a n d P olym er iza tion Mech a -
n ism . The structure of the polymer PMCBE synthesized
by either AIBN or AIBN/CuBr2/dNbpy is identical, as
shown by 1H NMR spectroscopy (Figure 3). The signals

4280 Chen et al.
Macromolecules, Vol. 35, No. 11, 2002
F igu r e 3. 1H NMR of PMCBE (Mn ) 7180, Mw/Mn ) 1.17) synthesized by AIBN/CuBr2/dNbpy initiation system at 85 °C.
Sch em e 2
at chemical shift 1.52-2.92 ppm are due to the five
protons of the cyclobutane ring in the polymer backbone
and those at 3.52-3.71 ppm to the ester methyl group.
This indicates that the polymerization initiated by
AIBN/CuBr2/dNbpy proceeded via addition polymeri-
zation just like a conventional radical polymerization
initiated by AIBN. The signals at 3.79 ppm (b′) are due
to the protons of the methoxy groups of the terminal
MCBE unit capped with an ω-bromine end group,
somewhat similar to those reported by Ando et al.19
The presence of an ω-Br end group demonstrates that
the polymerization proceeds via an ATRP process. The
mechanism of the RATRP of MCBE by the controlled/
“living” addition polymerization of cyclobutene ester is
depicted in Scheme 2.
The concentration of Cu2+ is higher at the beginning
of the polymerization in the RATRP system than in
ATRP system.4,9 Our results showed that the radical
polymerization was better controlled with the AIBN/
CuBr2/dNbpy initiation system than with the MBP/
CuBr/dNbpy initiation system. This is probably due to
the fact that Cu2+ is a strong and efficient inhibitor or
retarder for radical polymerization under normal condi-
tions.20 The spontaneous thermal polymerization and
side reactions through conventional radical polymeri-
zation were effectively eliminated in the AIBN/CuBr2-
catalyzed RATRP system due to several factors: RATRP
proceeds at about double the rate of the ATRP polym-
erization, the concentration of Cu2+ is high at the
beginning of the polymerization when monomer con-
centration is high, and the spontaneous thermal polym-
erization of styrene is very dependent on monomer
concentration because it is second order and therefore
will slow down considerably as monomer is used up.
Ch a in Exten sion Rea ction . A chain extension of
PMCBE synthesized with AIBN/CuBr2/dNbpy to an-
other monomer such as St was carried out in diphenyl
ether at 100 °C. When [St]0 ) 2.88 mol/L, [PMCBE]0 )
3.68 × 10-2 mol/L, [PMBCE]0:[CuBr]0:[dNbpy]0 ) 1:1:

Macromolecules, Vol. 35, No. 11, 2002
Methyl 1-Cyclobutenecarboxylate 4281
Ack n ow led gm en t. The authors gratefully acknowl-
edge funding provided by the Petroleum Research Fund,
administered by the American Chemical Society. Ad-
ditional funding was obtained from the National Science
Foundation, Division of Material Research.
Refer en ces a n d Notes
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2, Mn of PMCBE ) 7610, Mw/Mn ) 1.16, the conversion
of St was 52.2% after 45 h, Mn of the obtained polymer
increased to 17 400, and Mw/Mn decreased to 1.07. The
increasing molecular weight of the resultant polymer
was observed from the GPC curves as shown in Figure
4. The GPC curve (a) for the macroinitiator almost
disappeared in the monomodal GPC curve (b) of the final
polymer. This result indicates that the PMCBE was
used as a macroinitiator to further polymerize the
second monomer, St, thereby forming a polymer with
larger molecular weight in a conventional ATRP system.
It further suggests that the PMCBE synthesized by the
AIBN/CuBr2/dNbpy initiation system has an ω-halogen
(bromine) end group, and the mechanism of polymeri-
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Con clu sion s
In conclusion, ATRP of the first monocyclic olefin,
namely methyl 1-cyclobutenecarboxylate, was success-
fully carried out by addition polymerization with an
RATRP initiation system, AIBN/CuBr2/dNbpy at 85 °C,
in a well-controlled/“living” manner. The rate of polym-
erization was first order in monomer, and the molecular
weights of polymer were almost equal to the theoretical
ones with very narrow polydispersities (Mw/Mn ) ∼1.15).
The obtained polymer PMCBE is used as a macro-
initiator to polymerize a second monomer St in an ATRP
system due to the presence of an ω-bromine end group
in the polymer molecules. The polymerization of MBCE
initiated by a conventional ATRP system, MBP/CuBr/
dNbpy, at 100 °C was not well controlled.
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MA012104F