Synthesis and polymerization of methyl 3-methylcyclobutene-1-carboxylate

Article abstract of DOI:10.1021/ma010683i

A new icyclobutene monomer, methyl 3-methylcyclobutene-1-carboxylate (MMCB), has been synthesized and polymerized by radical and anionic initiators. The synthesis started from a [2 + 2] cycloaddition of N-(1-propenyl)piperidine to methyl acrylate, followed by methylation and treatment with base to yield the monomer. Free radical polymerization of MMCB led to low yields of low molecular weight polymers, probably due to chain transfer at the allytic hydrogen. However, p-methoxystyrene, styrene and methyl methacrylate gave high molecular weight copolymers with MMCB in high yields. Anionic homopolymerization with tert-butyllithium (t-BuLi), t-BuLi/bis(2,6-di-tert-butylphenoxy)ethylaluminum, lithium bis(trimethylsilyl)amide, and potassium bis(trimethylsilyl)amide in toluene at 0 and -78°C proceeded smoothly and gave polymers in high yields. NMR and IR analyses of the polymers suggested that the polymerization proceeds via addition mechanism without ring-opening. Thermal properties of the polymers are also described briefly. In contrast to MMCB, methyl 3,3-dimethylcyclobutene-1-carboxylate MDCB did not polymerize due to excessive steric hindrance.

Full text of DOI:10.1021/ma010683i

Macromolecules 2002, 35, 1591-1598
1591
Synthesis and Polymerization of Methyl
3-Methylcyclobutene-1-carboxylate
Ta tsu k i Kita ya m a ,*,† Ta k eh ir o Ka w a u ch i,† Nor i Ued a ,† Ca r in a S. Kn iep ,‡
Won Su k Sh in ,‡ An n e B. P a d ia s,‡ a n d H. K. Ha ll, J r .*,‡
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka,
Osaka 560-8531, J apan; and Department of Chemistry, University of Arizona, Tucson, Arizona 85721
Received April 20, 2001
ABSTRACT: A new cyclobutene monomer, methyl 3-methylcyclobutene-1-carboxylate (MMCB), has been
synthesized and polymerized by radical and anionic initiators. The synthesis started from a [2 + 2]
cycloaddition of N-(1-propenyl)piperidine to methyl acrylate, followed by methylation and treatment with
base to yield the monomer. Free radical polymerization of MMCB led to low yields of low molecular weight
polymers, probably due to chain transfer at the allylic hydrogen. However, p-methoxystyrene, styrene
and methyl methacrylate gave high molecular weight copolymers with MMCB in high yields. Anionic
homopolymerization with tert-butyllithium (t-BuLi), t-BuLi/bis(2,6-di-tert-butylphenoxy)ethylaluminum,
lithium bis(trimethylsilyl)amide, and potassium bis(trimethylsilyl)amide in toluene at 0 and -78 °C
proceeded smoothly and gave polymers in high yields. NMR and IR analyses of the polymers suggested
that the polymerization proceeds via addition mechanism without ring-opening. Thermal properties of
the polymers are also described briefly. In contrast to MMCB, methyl 3,3-dimethylcyclobutene-1-
carboxylate MDCB did not polymerize due to excessive steric hindrance.
Sch em e 1
In tr od u ction
Polymers containing 1,2- and 1,3-linked cyclobutane
rings carrying a functional group are of interest for their
good physical properties.1-4 They can be considered for
use in specialty high-tech applications. Wider use of
such polymers has been limited because of difficulty in
obtaining sufficient supplies of monomers by easy
syntheses.
The main problem is the formation of a suitably
functionalized cyclobutane. Among the common rings,
the cyclobutane ring is the most difficult to form.
However, Brannock et al.5 reported very promising
results from the [2 + 2]-cycloaddition reactions of
enamines with electrophilic olefins. Cycloaddition of
enamines possessing no â-hydrogen, such as N,N-
dimethylisobutenylamine, to acrylonitrile, methyl acry-
late, and diethyl fumarate led to an extensive series of
functional gem-dimethylcyclobutanes. They were also
able to obtain a second series of monomethyl cyclo-
butane derivatives from cycloaddition reactions with
enamines that did possess a â-hydrogen, such as N,N-
dimethylpropenylamine. Particularly interesting to us
was the reported conversion of these cycloadducts to
cyclobutenes by quaternization with methyl iodide,
followed by treatment with base. Examples (Scheme 1)
are methyl 3-methylcyclobutene-1-carboxylate (MMCB),
methyl 3,3-dimethylcyclobutene-1-carboxylate (MDBC),
dimethyl 4,4-dimethyl-1-cyclobutene-2,3-dicarboxylate,
3,3-dimethylcyclobutene-1-carbonitrile, and 3,3-dimeth-
ylcyclobutene-1-carboxylic acid.
ing polymer chain might be substantial. The monomer
carrying a single methyl group could react with growing
radical or anion on the face away from the methyl
substituent, but steric hindrance in MDCB might be too
much. Accordingly, we set about establishing practical
syntheses of these monomers within the framework of
the reported synthesis5 and studied their polymeriza-
tions using both radical and anionic initiation.
Exp er im en ta l Section
Mea su r em en ts. NMR spectra were recorded on a Varian
Gemini 200 or a Varian Unity Inova 500 spectrometer. The
IR spectra were obtained on a Nicolet Impact 410 infrared
spectrometer or a J ASCO Herschel series FT/IR-410 spectrom-
eter. Mass spectrometric low-resolution data (EI) were re-
corded on a HP 5988A GC/MS system. All high-resolution mass
spectra including fast atom bombardment (FAB) with high-
and low-resolution were taken on a J EOL HX 110A sector
instrument. The elemental analyses were performed by Desert
Analytics, Tucson, AZ. Molecular weight and molecular weight
distribution were determined by size exclusion chromatogra-
phy (SEC) using a J ASCO 880-PU chromatograph equipped
with Shodex SEC columns [KF-806L (30 cm × 0.8 cm) × 2]
using chloroform as an eluent. The SEC chromatograms were
calibrated against standard poly(methyl methacrylate) samples.
Thermal analyses of the polymers were made on a SEIKO DSC
6200 and a SEIKO TG/DTA 6200.
We have been unable to find any reports of polymer-
ization studies of these cyclobutenes. Among the poten-
tial cyclobutene monomers, we selected two monomers
for further study, MMCB, with only one â-methyl group,
and methyl 3,3-dimethyl-1-cyclobutenecarboxylate
(MDCB). Steric repulsions between monomer and grow-
Ma ter ia ls. Methyl acrylate were purified prior to use by
filtering over a column with basic aluminum oxide. MMCB,
styrene, p-methoxystyrene, isobutyl vinyl ether, methyl acry-
late, n-butyl acrylate, and methyl methacrylate were purified
† Osaka University.
‡ University of Arizona.
10.1021/ma010683i CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/23/2002

1592 Kitayama et al.
Macromolecules, Vol. 35, No. 5, 2002
prior to polymerization by distillation over calcium dihydride
under reduced pressure and in a nitrogen or argon atmosphere.
Toluene was purified in the usual manner, mixed with a small
amount of butyllithium, and distilled under high vacuum. 2,6-
Di-tert-butylphenol, obtained commercially (Tokyo Kasei Kogyo
Co., Ltd.), was fractionally distilled, and used as a heptane
solution. tert-Butyllithium (t-BuLi) in pentane (Aldrich Co.,
Ltd.) was used as a heptane solution. The concentration was
determined by titration with butan-2-ol using o-phenanthro-
line as an indicator.6 tert-Butylmagnesium bromide (t-BuMg-
Br) was prepared from magnesium turnings and tert-butyl
bromide in diethyl ether.7 The concentration of the t-BuMg
group was determined by hydrolysis with excess 0.1 M HCl
and back-titration with 0.1 M NaOH using phenolphthalein
as an indicator. Lithium bis(trimethylsilyl)amide [LiN(SiMe3)2]
as a hexanes solution and potassium bis(trimethylsilyl)amide
[KN(SiMe3)2] as a toluene solution were purchased from
Aldrich Co., Ltd. and used as received. LiN(SiMe3)2 concentra-
tion was determined by titration with butan-2-ol and that of
KN(SiMe3)2 by acid-base titration. Bis(2,6-di-tert-butylphen-
oxy)ethylaluminum [EtAl(ODBP)2] was prepared from 2,6-di-
tert-butylphenol and triethylaluminum in heptane at 0 °C,
according to the literature,8 recrystallized from heptane, and
used as a toluene solution.
(m, 1 H, CH(CO2CH3)), 3.37-3.14 (m, 4 H, R-ring-CH2), 3.04
(s, 3 H, N+(CH3)R2), 2.95-2.65 (m, 1 H, CH(CH3)), 2.45-2.23
(m, 2 H, CH2), 1.93-1.35 (br m, 6 H, â,γ-ring-CH2), 1.16 (d, 3J
) 6.2 Hz, 3 H, CH(CH3)). 13C NMR (DMSO-d6): δ 171.9 (CO2-
CH3), 72.9 (CH(N+(CH3)R2)), 58.3, 58.1 (R-ring-CH2), 52.0
(OCH3), 43.4 (CH(CO2CH3)), 35.6 (N+(CH3)R2), 27.7 (CH(CH3)),
25.0 (CH2), 20.2, 18.9 (â,γ-ring-CH2), 19.7 (CH(CH3)). IR (KBr)
1737 (CdO) cm-1. MS (FAB): m/z 226 (MH+(cation)), 185, 93,
75, 57. HRMS (FAB): calcd for C13H25NO2 (MH+(cation)),
226.1807; found, 226.1809. Anal. Calcd for C13H24NO2I: C,
44.21; H, 6.85; N, 3.97. Found: C, 44.09; H, 6.90; N, 3.90. Note:
Usually, crude cyclobutane 2 was used for the preparation of
methiodide 3 without purification.
Meth yl 3-Meth ylcyclobu ten e-1-ca r boxyla te (MMCB).
Under an argon atmosphere, a 60% dispersion of sodium
hydride in mineral oil (11 g, 0.27 mol) was washed twice with
low boiling petroleum ether (25 mL each) and suspended in
dry tetrahydrofuran (THF) (350 mL). Methiodide 3 (79 g, 0.22
mol) was added through a powder funnel in one portion. The
reaction starts immediately as witnessed by evolution of
hydrogen gas and a moderate temperature increase. After the
mixture was stirred at room temperature for 1.5 h, the yellow
color had faded and no more gas evolved. The reaction mixture
was centrifuged, the solution decanted, and the residues were
washed three times with dry THF, each time followed by
centrifuging and decanting. Subsequently, a trace of bis(3-tert-
butyl-4-hydroxy-5-methylphenyl) sulfide (inhibitor) and some
dry ice were added to the combined THF solutions, which were
then poured onto brine containing a small amount of ice. After
separation of the aqueous layer, the organic one was washed
with brine, whereas piperidine was removed by extraction with
6 M HCl. After this was washed with water, the colorless
organic layer was dried over MgSO4, concentrated, and
fractionated over a 6 cm Vigreux column under reduced
pressure to give 71% (20 g, 0.16 mol) of cyclobutene MMCB
as a colorless liquid (bp 70-71 °C, 15 mmHg). If inhibited with
3-tert-butyl-4-hydroxy-5-methylphenyl sulfide, MMCB can be
stored for several months at -50 °C without decomposition.
1H NMR (CDCl3): δ 6.81 (s, 1 H, dCH), 3.73 (s, 3 H, OCH3),
2.92-2.75 (m, 2 H, CH(CH3), CHH), 2.28-2.15 (m, 1 H, CHH),
1.19 (d, 3J ) 7.1 Hz, 3 H, CH3). 13C NMR (CDCl3): δ 163.1
(CO2CH3), 151.5 (dCH), 136.5 (dC(CO2CH3)), 51.2 (OCH3),
36.5 (CH2), 34.9 (CH(CH3)), 18.2 (CH3). IR (neat): 3057 (Cd
C), 1726 (CdO), 1611, 1601 (CdC) cm-1. MS (EI): m/z (rel
intensity) 126 (M+, 34), 111 (36), 95 (24), 67 (100). HRMS
(EI): calcd for C7H10O2, 126.0681; found, 126.0681.
N-(1-P r op en yl)p ip er id in e (1). The enamine was prepared
by a condensation method described in the literature using
anhydrous potassium carbonate.9 The primary amination
product (aminal) synthesized from 1 molar equiv of propion-
aldehyde (70 g, 1.20 mol) and 2 molar equiv of piperidine (204
g, 2.40 mol), was thermally decomposed by distillation. Very
slow fractionation through a 12 cm Vigreux column under an
argon atmosphere in vacuo provided 74% (111 g, 0.89 mol) of
1 as a colorless oil (bp 99-102 °C, 75 mmHg).9,10 1H NMR
3
4
(CDCl3): δ 5.86-5.79 (m, J ) 13.8 Hz, J ) 1.4 Hz, 1 H, CH-
3
(NR2)), 4.38 (dq, J ) 6.4, 13.8 Hz, 1 H, CH(CH3)), 2.75-2.70
3
4
(m, 4 H, R-ring-CH2), 1.62 (dd, J ) 6.4 Hz, J ) 1.4 Hz, 3 H,
CH3), 1.65-1.41 (m, 5 H, â,γ-ring-CH2). 13C NMR: δ 141.6
(CH(NR2)), 94.5 (CH(CH3)), 50.2 (R-ring-CH2), 25.7 (â-ring-
CH2), 24.7 (γ-ring-CH2), 15.8 (CH3). IR (neat): 3052, 3033 (d
CH2), 1694, 1660 (CdC) cm-1. MS (EI): m/z (rel intensity) 125
(M+, 42), 124 (M+ - H, 33), 110 (M+ - CH3, 100), 96 (18), 82
(14), 68 (25), 55 (10), 41 (21). HRMS (EI): calcd for C8H15
N
(M+), 125.1204; found, 125.1202.
1-Met h oxyca r b on yl-3-m et h yl-2-(N-p ip er id in yl)cyclo-
bu ta n e (2). Similar to a procedure described in the literature,5
enamine 1 (63 g, 0.50 mol) and methyl acrylate (43 g, 0.50
mol) in the presence of a trace of bis(3-tert-butyl-4-hydroxy-
5-methylphenyl) sulfide as an inhibitor were combined under
an argon atmosphere and stirred at ambient temperature.
After 3 days, conversion of the starting materials to cyclo-
butane 2 was quantitative. Distillation through a 12 cm
Vigreux column under reduced pressure (bp 92 °C, 1.0 mmHg)
gave 88% (93 g, 0.44 mol) of cyclobutane 2 as a mixture of at
Meth yl 2-(d im eth yla m in o)-3,3-d im eth ylcyclobu ta n e-
ca r boxyla te.5 A mixture of methyl acrylate (156 g, 1.82 mol),
N,N-dimethylisobutylamine12 (60 g, 0.61 mol), and acetonitrile
(250 mL) was refluxed overnight. Solvent and unreacted
starting materials were removed by rotary evaporation. Vacuum
distillation gave 61 g (yield 54%) of 2-(dimethylamino)-3,3-
o
dimethylcyclobutanecarboxylate, bp 60-65 C at 2 mmHg. 1H
1
least 2 isomers (lit.:3 67%). H NMR (CDCl3): δ 3.69, 3.67 (2
NMR(CDCl3): δ 3.60 (s, 3H), 2.74 (q, 1H, J ) ∼9 Hz), 2.43 (d,
1H, J ) 8.7 Hz), 2.02 (s, 6H), 1.69 (dd, 1H, J ) 9.6. 10.5), 1.55
(dd, 1H, J ) 9.6, 10.5), 1.07 (s, 3H), 1.05 (s, 3H) ppm. 13C NMR-
(CDCl3): δ 157.2, 72.3, 51.6, 43.1, 39.3, 35.4, 33.4, 29.8, 21.7
ppm.
s, 3 H, OCH3), 2.88-2.48 (m, 2 H, CH(NR2), CH(CO2CH3)),
2.41-2.08 (m, 6 H, CH2, R-ring-CH2), 1.64-1.30 (m, 7 H,
CH(CH3), â,γ-ring-CH2), 1.13, 1.05 (2 d, 3J ) 6.2 Hz, 3 H, CH3).
13C NMR (CDCl3): main isomer δ 174.8 (CO2CH3), 70.8 (CH-
(NR2)), 51.2 (CO2CH3), 50.7 (R-ring-CH2), 39.8 (CH(CO2CH3),
31.7 (CH(CH3)), 26.2 (CH2), 25.2, 24.0 (â,γ-ring-CH2), 20.7
(CH3). IR (neat): 1732 (CdO) cm-1. MS (FAB) m/z 212 (MH+),
126 (MH+(enamine)), 85. HRMS (FAB): calcd for C12H22NO2
N -(1-Me t h oxyca r b on yl-3,3-d im e t h yl-2-cyclob u t yl)-
N,N,N-tr im eth yla m m on iu m Iod id e. Methyl iodide (63 g,
0.44 mol) was added to a solution of methyl 2-(dimethylamino)-
3,3-dimethylcyclobutanecarboxylate (61 g, 0.33 mol in 300 mL
of diethyl ether), and the mixture was stirred for 6 days. Then
the mixture was filtered and dried in a vacuum to give 89 g
(83%) of N-(1-methoxycarbonyl-3,3-dimethyl-2-cyclobutyl)-
N,N,N-trimethylammonium iodide. 1H NMR (DMSO-d6): δ
4.00 (d, 1H, J ) 10.4 Hz), 3.76 (q, 1H, J ) 10 Hz), 3.64 (s,
3H), 3.09 (s, 9H), 1.91 (t, 1H, 10.2 Hz), 1.50 (t, 1H, J ) 9.9
Hz), 1.36 (s, 3H), 1.24 (s, 3H) ppm. 13C NMR (DMSO-d6): δ
171.9, 74.0, 52.1, 51.7, 40.3, 36.2, 32.8, 29.4, 22.5 ppm.
(MH+): 212.1651; Found: 212.1650. Anal. Calcd. for C12H21
-
NO2: C, 68.21; H, 10.00; N, 6.63. Found: C, 68.00; H, 10.15;
N, 6.68.
N -(1-Me t h o x y c a r b o n y l-3-m e t h y l-2-c y c lo b u t y l)-N -
m eth ylp ip er id in iu m iod id e (3). The quaternization of cy-
clobutane 2 (70 g, 0.33 mol) with methyl iodide (57 g, 0.40 mol)
was carried out according to a method described in the
literature.11 After 4 days at ambient temperature, methiodide
3 solidified and was isolated in 86% yield (100 g, 0.28 mol) by
filtration. Recrystallization from ethanol/petroleum ether
(4:1) gave 68% of 3 (79 g, 0.22 mol) as slightly yellow crystals
(mp 108 °C). 1H NMR [dimethyl sulfoxide (DMSO)-d6]: δ 4.12-
4.03 (m, 1 H, CH(N+(CH3)R2)), 3.66 (s, 3 H, OCH3), 3.62-3.55
Meth yl
3,3-Dim eth ylcyclobu ten e-1-ca r boxyla te
(MDCB). Under an argon atmosphere, a 60% dispersion of
sodium hydride in mineral oil (12 g, 0.30 mol) was washed
twice with pentane (30 mL each) and suspended in dry THF
(250 mL). N-(1-Methoxycarbonyl-3,3-dimethyl-2-cyclobutyl)-

Macromolecules, Vol. 35, No. 5, 2002
Methyl 3-Methylcyclobutene-1-carboxylate 1593
N,N,N-trimethylammonium iodide (65.4 g, 0.20 mol) was
added through a powder funnel in one portion. After the
mixture was stirred overnight at room temperature, anhydrous
ether (200 mL) was added. Then the reaction mixture was
centrifuged and the solution decanted, and the residues were
washed three times with anhydrous ether, each time followed
by centrifugation and decantation. Subsequently, a pinch of
bis(3-tert-butyl-4-hydroxy-5-methylphenyl) sulfide (inhibitor)
and some dry ice were added to the combined organic solutions,
which were poured onto a mixture of ice and brine. After
separation of the aqueous layer, the organic layer was washed
with brine, 5 M HCl and distilled water. The organic layer
was dried over MgSO4, concentrated, and vacuum distilled to
give 18 g of cyclobutene. MDCB contained about 10% of a
methanol adduct, methyl 3,3-dimethyl-2-methoxycyclobutane-
1-carboxylate, which was confirmed by GC-MS, but was used
as such for the polymerization studies. 1H NMR(CDCl3): δ 6.78
(s, 1H), 3.67 (s, 3H), 2.38 (s, 2H), 1.17 (s, 6H) ppm. 13C NMR-
(CDCl3): δ 163.5, 155.4, 134.2, 51.1, 42.5, 40.9, 25.3 ppm. MS
(m/z): 141 (100%, -CO2), 139, 125, 79, 53.
Ra d ica l Hom op olym er iza tion in Bu lk . Freshly distilled
MMCB (0.005 mol), and AIBN (2 mol %) were degassed by
freeze-thawing and kept at 80 °C for 20 h. The clear, colorless
product was highly viscous at 80 °C and a glass at room
temperature. The reaction product was dissolved in a small
amount of dichloromethane and the polymer precipitated into
cold diethyl ether. The colorless polymer was washed repeat-
edly with diethyl ether followed by drying over P2O5 at 80 °C
in vacuo. The poly(MMCB) is soluble in dichloromethane,
chloroform, acetonitrile and acetone.
Ra d ica l Hom op olym er iza tion a n d Cop olym er iza tion
in Solu tion . Solutions of freshly distilled MMCB (2.5 mmol)
and AIBN (2 mol %) in N,N-dimethylformamide (DMF) or
γ-butyrolactone (2.5 mL) were degassed by freeze-thawing.
After 20 h at 80 °C, the reaction mixtures were poured into
chilled diethyl ether. Isolation, purification, and drying of the
polymers were carried out as described above.
P olym er iza tion in Aqu eou s Slu r r y. A mixture of freshly
distilled MMCB (0.005 mol) in 3 mL of distilled water and
aqueous solutions of K2S2O8 (0.1 M) and Na2S2O4 (0.05M) were
degassed separately by freeze-thawing. Subsequently, K2S2O8-
(aq) (0.1 mL, 0.2 mol %) and Na2S2O4(aq) (0.1 mL, 0.1 mol %)
were added to the stirred monomer slurry using a syringe.
After 1.5 h, another 0.1 mL of each solution was added.
Polymerization started after about 10-20 min, and the reac-
tion mixture was stirred at ambient temperature for additional
20 h. The slurry was filtered and washed with water, and the
colorless polymer was treated twice with water in a blender
and one time in methanol, followed each time by filtering and
washing. The polymer was dried over P2O5 at 80-100 °C in
vacuo to give the product in 56% yield.
Sch em e 2
Various methods to convert the salts to the cyclo-
butene monomers were examined. Brannock et al.5 used
potassium hydroxide for this elimination, which resulted
also in saponification of the ester to the carboxylic acid.
In our hands, sodium methoxide gave the monomer
containing a substantial amount of methyl 2-methoxy-
cyclobutane-1-carboxylate, possibly formed by Michael
addition of the methoxide to the formed monomer.
Sodium hydride, a nonnucleophilic non-hydroxylic base,
in THF was used at several temperatures. At room
temperature (28 °C), the best results were obtained, i.e.,
71% yield of MMCB, without hydrolyzing the ester
functionality. It is interesting that the reaction of an
insoluble base with an insoluble salt proceeds so readily.
Ra d ica l Hom op olym er iza tion . The results of the
free radical polymerization studies are summarized in
Table 1. MMCB polymerized with AIBN initiation.
However, neither the yields nor the molecular weights
were high. The difficulty did not appear to be caused
by impurities; rather we suspect that the tertiary allylic
hydrogen acts as a chain transfer agent. In contrast to
the homogeneous radical polymerizations, poly(MMCB)
obtained with K2S2O8 and Na2S2O4 in a slurry polym-
erization in water, was not soluble in common organic
solvents such as acetone, acetonitrile, chloroform, di-
chloromethane, DMF, DMSO, and THF. This is possibly
due to cross-linking.
To ascertain if spontaneous polymerization or isomer-
ization of the monomer occurs under the polymerization
conditions, MMCB was separately heated neat at 80 °C
for 20 h. No polymerization occurred, and the yellow
liquid obtained consisted mainly of the thermally stable
An ion ic P olym er iza tion . All the anionic polymerizations
were carried out in glass ampules, cooled at -78 °C and filled
with dried nitrogen passed through molecular sieves 4A. The
polymerization reaction was initiated by adding the monomer
to the initiator solutions at the polymerization temperature
and terminated by adding methanol containing a small amount
of aqueous HCl.
1
monomer (>95%, H NMR, IR); i.e., no isomerization
to 1,3-diene occurred.
The dimethyl-substituted monomer MDCB did not
polymerize under free radical conditions. In an attempt
to overcome the expected low ceiling temperature, we
tried the homopolymerization at -50 °C using redox
conditions. Although these conditions smoothly polym-
erized methyl acrylate, no polymerization of MDBC was
observed. The difference in reactivity between MMCB
and MDCB is ascribed to the additional methyl group
at the 3-position of MDCB. Whereas MMCB has an
exposed face without steric hindrance, in MDCB both
faces of the cyclobutene ring are sterically hindered by
the two methyl substituents.
Ra d ica l Cop olym er iza tion . Better results were
obtained in copolymerization with electron-rich mono-
mers. When p-methoxystyrene or styrene was copoly-
merized with MMCB, high molecular weight copolymers
were obtained in high yields. Methyl methacrylate also
copolymerized well with MMCB, but isobutyl vinyl ether
gave only a low molecular weight product. The results
Resu lts a n d Discu ssion
Mon om er Syn th esis. The route shown in Scheme 2
illustrates the successful synthesis of methyl 3-methyl-
cyclobutene-1-carboxylate MMCB, which was also uti-
lized for methyl 3,3-dimethylcyclobutene-1-carboxylate
MDCB. N-(1-Propenyl)piperidine (1), from the reaction
of piperidine with propionaldehyde as reported by
Brannock et al.,5 was satisfactory as the enamine
component. Cycloaddition of the enamine to methyl
acrylate proceeded in high yield under moderate heat-
ing. Methyl iodide converted the cycloadducts to the
quaternary ammonium salts. Precipitation of the salt
from diethyl ether solution as it formed gave the pure
salt as a mixture of isomers.

1594 Kitayama et al.
Macromolecules, Vol. 35, No. 5, 2002
Ta ble 1. Ra d ica l P olym er iza tion s of MMCB w ith AIBN a t 80 °C for 20 h a
b
b
run
solvent
initiator
monomer/initiator
yield/%
Mh n
Mh w/ Mh n
1
2
0
20
AIBN
50
7900
58800 (shoulder)
1.34
2.31
3
4
DMF
H2O
AIBN
K2S2O8 + Na2S2O4
100
100
0
56
a
b
Monomer 2.5 mmol; solvent 2.5 mL. Determined by SEC in CHCl3, calibrated against PMMA standards.
Ta ble 2. Ra d ica l Cop olym er iza tion s of MMCB a n d
Sever a l Com on om er s w ith AIBN in Bu lk a t 80 °C for
20 h a
isolated MMCB
run comonomerb monomer/AIBN yield/% compn/%c Mw
d
5
6
7
8
9
IBVE
IBVE
IBVE
pMeOSt
pMeOSt
0
67
56
0
67
56
47
48
25
50
96
87
4900
<4000
25
50
100
50
41
41
40
40
20100
16200
12700
48000
10 pMeOSt
11 St
12 MMA
a
MCBM 2.5 mmol; comonomer 2.5 mmol. b Key: IBVE, isobutyl
vinyl ether; pMeOSt, p-methoxystyrene; St, styrene; MMA, methyl
d
methacrylate. c Based on elemental analysis. Determined by SEC
in CH2Cl2.
Ta ble 3. An ion ic P olym er iza tion s of MMCB w ith Sever a l
In itia tor s in Tolu en ea
b
b
run
initiator
temp/°C time/h yield/%
Mh n
Mh w/ Mh n
13 t-BuMgBr
14
0
-78
0
24
48
24
48
24
48
24
48
24
48
trace
trace
85.2
12.5
55.3
46.6
65.9
74.9
95.5
93.5
15 t-BuLi
16
(2100)d (2.38)d
(2270)d (2.83)c
-78
17 t-BuLi/c
18 EtAl(ODBP)2
19 LiN(SiMe3)2
20
0
4900
9000
1.75
1.26
-78
0
(1280)d (2.80)d
(1280)d (1.69)d
-78
21 KN(SiMe3)2
22
0
4500
6800
1.56
1.34
-78
a
b
Monomer 2 mmol; initiator 0.07 mmol; toluene 1 mL. De-
termined by SEC in CHCl3, calibrated against PMMA standards.
c EtAl(ODBP)2 0.21 mmol. Measured for CHCl3-soluble fraction.
d
F igu r e 1. FT-IR spectra of poly(MMCB)s prepared with AIBN
at 80 °C (a), KN(SiMe3)2 at 0 °C (b), t-BuLi at 0 °C (CHCl3-
soluble fraction) (c), and t-BuLi at 0 °C (CHCl3-insoluble
fraction) (d), measured by the KBr disk method.
are summarized in Table 2. MMCB was also heated at
80 °C for 24 h with equimolar amounts of p-methoxy-
styrene, styrene, or isobutyl vinyl ether, but no spon-
taneous copolymerization was observed.
polymerization has been well demonstrated in living
polymerization of methacrylates and acrylates.13-22 The
present results provide an additional example of the
utility of bulky aluminum compounds to control the
anionic polymerization of carbonyl-containing polar
monomers. Coordination of the bulky Lewis acid to the
propagating anion may stabilize the anion and prevent
it from unfavorable side reactions, and the Lewis acid
coordination to the carbonyl group of MMCB may
enhance its reactivity toward the nucleophilic propagat-
ing anion. In fact the polymer yield at -78 °C in this
case is much higher than that with t-BuLi alone.
Lithium bis(trimethylsilyl)amide [LiN(SiMe3)2] and
potassium bis(trimethylsilyl)amide [KN(SiMe3)2] are
known as strong bases with low nucleophilicity. Never-
theless, the former has been used successfully for the
polymerization of MMA, though the initiator efficiency
was very low.23 Since both the amides are supplied as
solutions in nonpolar solvents, such as hexane and
toluene, the polymerizations with these bases were
examined in toluene under similar conditions as de-
scribed above. The reactions proceeded smoothly and
gave polymers in high yields at -78 and 0 °C. In
An ion ic Hom op olym er iza tion . Table 3 summa-
rizes the results of the polymerizations of MMCB with
several anionic initiators in toluene at 0 and -78 °C.
Polymerization with t-BuMgBr, which is known to
initiate living polymerization of methacrylates,7 did not
give polymers either at 0 or -78 °C. On the other hand,
t-BuLi, a stronger base than t-BuMgBr, afforded poly-
mers, and the yield at 0 °C was quite high. The products
were only partially soluble in common organic solvents
such as acetone, acetonitrile, chloroform, dichloro-
methane, DMF, DMSO, THF, and hexafluoropropan-2-
ol (HFIP), and the molecular weight data were obtained
only for chloroform-soluble fractions. The molecular
weight distribution (MWD) of the soluble fraction was
very broad. When t-BuLi was used in combination
with bis(2,6-di-tert-butylphenoxy)ethylaluminum [EtAl-
(ODBP)2], a bulky aluminum Lewis acid, the polymer-
ization gave soluble polymers in moderate yields, whose
MWDs were narrower than those of the polymers
formed with t-BuLi. In particular, the polymer obtained
at -78 °C showed fairly narrow MWD with an Mw/Mn
value of 1.26. This effect of EtAl(ODBP)2 on the anionic

Macromolecules, Vol. 35, No. 5, 2002
Methyl 3-Methylcyclobutene-1-carboxylate 1595
Ta ble 4. An ion ic Cop olym er iza tion s of MMCB a n d Acr yla tes in Tolu en e a t -78 ˚C for 48 h a
MMCB unit
content/%d
c
c
run
initiator
t-BuLi
comonomerb
yield/%
Mh n
Mh w/ Mh n
23
24
25
26
27
28
29
MA
50
68
72
33
46
10
64
9500
9800
2.71
3.49
9
11
n-BuA
MMA
MA
n-BuA
MMA
MMAg
(15400)e
4400
(4.13)e
2.82
(60)e
9
f
t-BuLi/EtAl(ODBP)2
7200
16700
5300
3.33
2.20
1.24
13
41
89
a
b
MMCB 2 mmol; acrylate 2 mmol; t-BuLi 0.07 mmol; EtAl(ODBP)2 0.21 mmol; toluene 2 mL. Key: MA, methyl acrylate; MMA,
methyl methacrylate; n-BuA, n-butyl acrylate. c Determined by SEC in CHCl3, calibrated against PMMA standards. Determined by H
d
1
NMR. e Measured for CHCl3-soluble fraction. f EtAl(ODBP)2 0.21 mmol. t-BuLi 0.2 mmol; EtAl(ODBP)2 0.6 mmol.
g
particular, the polymerization with KN(SiMe3)2 gave
polymers with fairly narrow MWD almost quantitatively
even at -78 °C. In the polymerization with LiN(SiMe3)2,
the reaction medium became heterogeneous as the
reaction proceeded, and the polymer obtained was partly
insoluble.
To examine the structural difference between the
soluble and insoluble polymers, IR spectra of chloroform-
soluble and -insoluble fractions of the polymer obtained
with t-BuLi at 0 °C were measured as shown in Figure
1. The spectra of the polymers obtained with AIBN and
KN(SiMe3)2,which are completely soluble, are also in-
cluded. The IR spectra of these polymers were almost
identical.
Anionic polymerization of the dimethyl derivative
MDCB with the initiators listed in Table 3 did not give
any polymeric products, similar to the radical polym-
erization case. Thus steric hindrance by the two methyl
groups substantially suppresses the polymerization of
MDCB in both free radical and anionic modes. Lack of
ability to polymerize is not thought to be caused by the
presence of ∼10% methoxy adduct, inasmuch as either
are often used as solvents in anionic polymerizations.
An ion ic Cop olym er iza tion . Table 4 shows the
results of copolymerizations of MMCB and several
acrylates with t-BuLi or t-BuLi/EtAl(ODBP)2 as initia-
tors in toluene at -78 °C. In the case of copolymeriza-
tions with methyl or n-butyl acrylate, the copolymers
formed with both initiators contained less than 10% of
MMCB units. In contrast, copolymerization with MMA
gave copolymers with higher MMCB contents. These
results suggest that the reactivity of MMCB in anionic
copolymerization is lower than acrylate but higher than
methacrylate. The order of reactivity is not altered by
the addition of EtAl(ODBP)2. MMCB can be regarded
as an R,â-disubstituted acrylate monomer. Methyl tiglate,
a typical R,â-disubstituted acrylate, polymerized to a
very small extent under the same conditions, probably
due to steric hindrance.24 The fact that MMCB exhibits
even higher reactivity than MMA may be ascribed to
enhanced reactivity of the CdC bond in the four-
membered ring structure and smaller steric hindrance
F igu r e 2. 500 MHz 1H NMR spectra of poly(MMCB)s
prepared with AIBN at 80 °C (a), t-BuLi at 0 °C (b), LiN-
(SiMe3)2 at 0 °C (c), KN(SiMe3)2 at 0 °C (d), t-BuLi/EtAl-
(ODBP)2 at 0 °C (e), and t-BuLi/EtAl(ODBP)2 at -78 °C (f),
measured in CDCl3 at 55 °C.
at the â-position where the substituent group is fixed
within the ring structure.
NMR Sp ectr a of P oly(MMCB). As described above,
poly(MMCB)s were obtained under several different
1
polymerization conditions. H and 13C NMR spectra of
the polymers were studied in detail to examine to what
extent the structural features of poly(MMCB)s are
controlled in the polymerization reaction. Figure 2
shows 1H NMR spectra of poly(MMCB)s obtained by
radical and anionic polymerizations. As mentioned
above, some of the polymers were partly insoluble, and
the spectra were obtained only for the chloroform-
soluble fractions. None of the polymers showed olefinic
proton signals confirming that the polymerizations
proceeded via addition mechanism. The signals were
assigned as shown in the figure on the basis of chemical
shift consideration, peak intensity, and COSY. The
different spectral patterns observed in each signal

1596 Kitayama et al.
Macromolecules, Vol. 35, No. 5, 2002
F igu r e 4. Carbonyl carbon resonances in 125 MHz 13C NMR
spectra of poly(MMCB)s prepared with AIBN at 80 °C (a),
KN(SiMe3)2 at 0 °C (b), and t-BuLi/EtAl(ODBP)2 at -78 °C
(c), measured in CDCl3 at 55 °C.
Sch em e 3. F ou r P ossible Ster eoisom er ic Mon om er
Un itsa
F igu r e 3. 125 MHz 13C NMR spectra of poly(MMCB)s
prepared with AIBN at 80 °C (a), KN(SiMe3)2 at 0 °C (b), and
t-BuLi/EtAl(ODBP)2 at -78 °C (c) and the DEPT-135 spectrum
(d) of the mixture used in part c where CH3 and CH carbons
are detected upward and CH2 carbons downward, measured
in CDCl3 at 55 °C.
a
The stereochemical configuration in the main chain is
described by erythro/threo notation, and (trans) and (cis)
denote the geometrical arrangement of the 3-methyl group
relative to the ester group.
in Scheme 3. According to the IUPAC nomenclature,25
the main-chain stereoisomerism is denoted as erythro
and threo. To describe the relative configurations at the
3-position, the cis and trans notation is employed in
Scheme 3 for clarity, that is, cis (and trans) means that
3-methyl group [CH3 (a)] is cis (trans) relative to the
ester group in the same cyclobutane ring. Thus the four
isomers are named erythro-(cis), erythro-(trans), threo-
(cis) and threo-(trans). Though the assignments for
these four possible structures are not made, it is evident
that the polymer formed with t-BuLi/EtAl(ODBP)2 is
much simpler than that obtained radically. Further
splittings observed in each peak group should reflect
the stereochemical sequences along the main chain.
Figure 5 displays the expanded spectra of CH3 (a)
carbon at 3-position in the shift range of 20-25 ppm,
which in principle should split into eight peaks. A diad
sequence consists of six sites of stereoisomerism and 32
relative configurations are possible. However, since the
chemical shift of CH3 (a) is expected to be most sensitive
to relative configurations of the carbonyl groups in the
same monomer unit and the nearest neighboring unit,
the diad tacticity of two ester groups is to be taken into
consideration. The eight possible diads are illustrated
in Scheme 4, in which the relative configuration of the
other unit attached at the C (e) side is neglected. Though
the assignments of the eight peaks are not completed,
the spectra clearly indicate the difference in the stereo-
region reflect the difference in the stereochemical
structures, such as cis-trans isomerism within the
cyclobutane ring unit and/or tacticity along the main
chain. Among them, the methoxy proton signals at 3.4-
3.6 ppm, which are free from spin-spin coupling
complexity, showed splittings reflecting such stereo-
chemical structures. Though the assignments are not
made, the polymer obtained with t-BuLi/EtAl(ODBP)2
at -78 °C showed the simplest signal pattern and
probably has the most regular structure.
Figure 3 shows the partial 13C NMR spectra of poly-
(MMCB)s prepared with AIBN at 80 °C, with
KN(SiMe3)2 at 0 °C and with t-BuLi/EtAl(ODBP)2 at
-78 °C, together with the DEPT-135 spectrum of the
latter polymer. From the DEPT spectrum, the CH (b)
and quaternary carbon (e) signals were assigned as
indicated in the figure. Except for the simple OCH3
signal at 51 ppm, all other carbon signals show com-
plicated splitting. Expanded carbonyl carbon spectra are
shown in Figure 4. The spectra seem to consist of three
or four peak groups. Since the monomer unit contains
two sites of stereoisomerism in the main chain and one
at the 3-position in the cyclobutane ring, the latter is
derived from the racemic nature of the starting MMCB.
Since NMR is only sensitive to the relative configuration
of these three sites of stereoisomerism, four kinds of
stereoisomeric monomer units are possible, as shown

Macromolecules, Vol. 35, No. 5, 2002
Methyl 3-Methylcyclobutene-1-carboxylate 1597
F igu r e 6. TGA (solid line) and DTA (dotted line) curves of
poly(MMCB)s prepared with AIBN at 80 °C (a) and t-BuLi/
EtAl(ODBP)2 at -78 °C (b), measured at a heating rate at 20
°C/min under nitrogen.
F igu r e 5. Methyl carbon resonances in 125 MHz 13C NMR
spectra of poly(MMCB)s prepared with AIBN at 80 °C (a),
KN(SiMe3)2 at 0 °C (b), and t-BuLi/EtAl(ODBP)2 at -78 °C
(c), measured in CDCl3 at 55 °C.
Sch em e 4. Eigh t P ossible Ster eoisom er ic Dia d sa
1
F igu r e 7. 500 MHz H NMR spectrum of thermally treated
poly(MMCB) (Table 3, run 18) at 280 °C, measured in CDCl3
at 55 °C.
Th er m a l P r op er ties. DSC measurements were car-
ried out for several polymers obtained by different
methods. In the temperature range from -50 to +150
°C, the homopolymers did not show a clear glass
transition, or a melting endotherm. The copolymer with
MMA (cf. Table 2) showed a Tg at 138 °C, which is
higher than that of PMMA. These results suggest that
inclusion of MMCB units contributes to an increase in
the rigidity of the polymer chain.
Thermal gravimetric analysis (TGA) of the poly-
(MMCB)s revealed that it starts to decompose at about
300 °C and almost completely degrades at about 400
°C as shown in Figure 6. There was no substantial
difference between radical and anionic poly(MMCB).
Some thermal changes were observed in differential
thermal analysis (DTA) measurement in the tempera-
ture range from 200 to 280 °C, at which the weight loss
was negligibly small. When the polymer was recovered
after heating to 280 °C, the1H NMR spectrum of the
product showed olefinic proton signals at 5.4-5.5 and
6.4-6.5 ppm (Figure 7). These should be related to the
a
The terms “isotactic” and “syndiotactic” denote the relative
configurations of two C-1 carbons bearing ester groups. See
also Scheme 3.
chemical structure between the polymers obtained by
radical and anionic processes. It should be mentioned
that the monomer itself is a racemic mixture, and thus
the presence of 3-methyl group may cause further
complications in stereochemical features of this polymer.

1598 Kitayama et al.
Macromolecules, Vol. 35, No. 5, 2002
a detailed investigation both of the stereochemical
structure and the isomerization reaction.
Ack n ow led gm en t. We are deeply indebted to the
National Science Foundation, Division of Materials
Research, and the donors of the Petroleum Research
Fund, administered by the American Chemical Society,
for generous support of this work. We also want to thank
Dr. Bilal Sufi for help with the synthesis of the
monomers.
Refer en ces a n d Notes
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Collette, J . W. Appl. Polym. Symp. 1974, 25, 113.
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F igu r e 8. SEC curves of poly(MMCB) prepared with t-BuLi/
EtAl(ODBP)2 at -78 °C (a) and thermally treated at 280 °C
(b) (1.0 wt % loss), calibrated against PMMA standards.
(10) Fleming, I.; Rowley, M. Tetrahedron 1986, 42, 3181.
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Sch em e 5
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ring opening of the cyclobutane units and the formation
of CdC bonds between successive monomer units as
shown in Scheme 5. The polymer thus formed contains
units hypothetically derived from 2-methoxycarbonyl-
penta-1,3-diene through 1,4-addition polymerization.
Though the synthesis of this compound was recently
reported,26 there has been no report of its polymeriza-
tion, and thus the thermal treatment described above
might provide an alternative route to the diene polymer
with functional carboxylate group. The SEC curve of the
heated polymer showed tailing toward the low-molec-
ular-weight side (Figure 8), showing that the thermal
reaction is accompanied by main-chain scission to some
extent but the scission did not cause degradation by
unzipping to cause substantial weight loss. Further
investigation is needed to clarify this phenomenon. We
are currently investigating the polymerization of methyl
cyclobutene-1-carboxylate, a simple analogue of MMCB,
the polymer of which is expected to be better suited for
(16) Kitayama, T.; Hirano, T.; Zhang, Y.; Hatada, K. Macromol.
Symp. 1996, 107, 297.
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(26) Uemura, K.; Inoue, Y. Appl. Organomet. Chem. 2000, 14, 8.
MA010683I