Anionic cyclopolymerization of methacrylic anhydride with the aid of bulky aluminum Lewis acid

Article abstract of DOI:10.1016/j.polymer.2012.12.078

Methacrylic anhydride was anionically polymerized for the first time by a lithium ester enolate with the aid of the bulky aluminum bisphenoxides in dichloromethane at -78 °C to afford polymers consisting of six-membered cyclized units as confirmed by NMR and IR spectroscopy, while a less bulky trialkylaluminum was ineffective. At higher temperatures, no polymeric product formed partly due to the carbonyl addition reaction of the alkyl group in the aluminum compound to the monomer. From the model reactions in the presence and absence of the aluminum compounds, importance of formation of an enolate-aluminate type species is suggested. Tacticity of PMMA derived from poly(MAAn) is rich in syndiotacticity in comparison to radically prepared one.

Full text of DOI:10.1016/j.polymer.2012.12.078

Polymer 54 (2013) 1987e1992
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Anionic cyclopolymerization of methacrylic anhydride with the aid
of bulky aluminum Lewis acid
*
Takehiro Kitaura, Naoko Moroi, Tatsuki Kitayama
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Machikane-yama 1-3, Toyonaka, Osaka 560-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Methacrylic anhydride was anionically polymerized for the first time by a lithium ester enolate with the
aid of the bulky aluminum bisphenoxides in dichloromethane at ꢀ78 ꢁC to afford polymers consisting of
six-membered cyclized units as confirmed by NMR and IR spectroscopy, while a less bulky tri-
alkylaluminum was ineffective. At higher temperatures, no polymeric product formed partly due to the
carbonyl addition reaction of the alkyl group in the aluminum compound to the monomer. From the
model reactions in the presence and absence of the aluminum compounds, importance of formation of
an enolateealuminate type species is suggested. Tacticity of PMMA derived from poly(MAAn) is rich in
syndiotacticity in comparison to radically prepared one.
Received 19 November 2012
Received in revised form
27 December 2012
Accepted 28 December 2012
Available online 8 January 2013
Keywords:
Anionic polymerization
Cyclopolymerization
Divinyl monomer
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
These precedent examples of the use of MAAn in the polymer
syntheses imply that the reactivity of MAAn against nucleophiles is
Methacrylic anhydride (MAAn) is known to undergo cyclo-
polymerization by radical polymerization to form soluble polymers
with six-membered anhydride units as a major structural unit [1,2],
while five-membered unit formation is favored at high tempera-
tures [3]. Since MAAn can be categorized in polar vinyl monomers
carrying electron withdrawing carbonyl function, it is potentially
polymerizable by anionic mechanism; i.e., NMR chemical shift
dominantly as a carbonyl compound but not as a vinyl one.
Herein we report the first successful anionic polymerization of
MAAn by using appropriate additives, bulky aluminum Lewis acids
in particular. Selectivity in the reaction of MAAn with nucleophilic
initiating and propagating species could be controlled in a way
favorable to the polymer formation. The soluble polymers with
cyclic units were obtained almost exclusively in moderate yields.
The role of the additives is discussed in some detail.
value of b-methylene carbon of MAAn (128.84 ppm) as a measure of
electron density at the carbon atom is in between those of methyl
methacrylate (MMA) (125.23 ppm) and methyl acrylate
(130.56 ppm) [4], suggesting enough high reactivity in anionic
polymerization. However, there have been no reports on the
anionic polymerization of MAAn, while its amide analogs,
N-substituted diacrylamides and dimethacrylamides, are known to
undergo anionic cyclopolymerization; the diacrylamides form
polymers with five-membered cyclic units [5,6], and the dimetha-
crylamides afford polymers with six-membered ring units [7],
while the difficulty of anionic polymerization of the latter was
noted later by Kodaira et al. [8]. In the anionic polymerization
chemistry, MAAn has been used as an acylating agent for end-
functionalization of living polymer anions such as poly(ethylene
oxide) anion [9], polystyrene anion [10], and PMMA anion [11].
2. Experimentals
2.1. Materials
Methacrylic anhydride (Aldrich) was dried over CaH2 and dis-
tilled under reduced nitrogen pressure just before use. Dichloro-
methane (Wako, super dehydrated grade) was dried over CaH2 and
distilled under high vacuum just before use. Me3SiOLi (Aldrich) was
dried at 100 ꢁC in vacuum for several hours and used as dry toluene
solution [12]. Isopropyl a-lithioisobutyrate (Li-iPrIB) was prepared
and recrystallized in toluene by a procedure slightly modified from
the ones reported previously [13e15]. Alkylaluminum bisphen-
oxides (RAl(ODBP)2; R ¼ Me, Et) were prepared from 2,6-di-tert-
butylphenol (2 equiv.) (Tokyo Chemical Industry) and the respec-
tive trialkylaluminum (1 equiv.) (Kanto Chemical) in heptane at
0
ꢁC, and purified by recrystallization according to References
[16,17]. Tributylaluminum (Kishida Chemical) was used as a toluene
* Corresponding author. Tel.: þ81 6 6850 6230; fax: þ81 6 6841 0104.
E-mail address: kitayama@chem.es.osaka-u.ac.jp (T. Kitayama).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.12.078

1988
T. Kitaura et al. / Polymer 54 (2013) 1987e1992
solution. Trimethylsilyldiazomethane (2.0 M in Et2O (Aldrich)),
LiAlH4 (Aldrich), CaH2 (Nacalai Tesque), and butyllithium (n-BuLi)
(Kanto Chemical) were used as received. Toluene, benzene (Aldrich,
anhydrous grade), and heptane (Wako, super dehydrated grade)
were mixed with a small amount of n-BuLi and distilled under high
vacuum just before use. Diethyl ether (Aldrich, anhydrous grade)
was dried over LiAlH4 and distilled under high vacuum just
before use.
2.6. NMR spectroscopic coordination experiment of MAAn with
MeAl(ODBP)2
Coordination to MAAn by MeAl(ODBP)2 was confirmed by 13C
NMR analysis of a mixture of MAAn and MeAl(ODBP)2 in toluene-
d8. Toluene-d8 was distilled under high vacuum directly to sealable
NMR tubes. MeAl(ODBP)2 was added via hypodermic syringes at an
ambient temperature, and after cooled to ꢀ78 ꢁC, MAAn was added,
where [MAAn]/[MeAl(ODBP)2] ratios of 1/0.5, 1/1, and 1/2, where
[MAAn] ¼ 0.34 ([MAAn]/[Al] ¼ 1/0.5, 1/1), 0.17 mmol ([MAAn]/
[Al] ¼ 1/2) in 0.6 mL toluene-d8 solution. The measurements were
performed at ꢀ78, ꢀ60, ꢀ40, ꢀ20, 0 ꢁC, and then at ꢀ78 ꢁC again.
2.2. Polymerizations
All the anionic polymerizations were carried out in glass am-
poules filled with dried nitrogen which was passed through a Mo-
lecular Sieves-4A cooled at ꢀ78 ꢁC. A typical polymerization
procedure is as follows; an initiator solution was prepared by
adding CH2Cl2 (5 mL), additive (1 mmol) and Li-iPrIB (0.2 mmol,
0.4 mL of benzene solution) via hypodermic syringes at an ambient
temperature, successively. The polymerization reaction was ini-
tiated by adding MAAn (5 mmol, 0.74 mL, 771 mg) to the initiator
solution at ꢀ78 ꢁC. After 24 h of polymerization period, the reaction
was quenched by adding a small amount of acetic acid (AcOH) in
toluene (3 M). The reaction mixture was then poured into diethyl
ether (300 mL), and the precipitate was collected by filtration and
dried under vacuum at an ambient temperature for 4 h.
2.7. Characterizations
1H NMR spectra of poly(MAAn)s were recorded in dimethyl
sulfoxide-d6 (DMSO-d6) at 30 ꢁC on a Jeol JNM-ECS 400 spec-
trometer. The chemical shifts were referred to the signal due to
residual hydrogens in the solvent (at 2.49 ppm). 1H NMR spectra of
PMMAs derived from poly(MAAn)s were recorded in chloroform-
d at 55 ꢁC on the same spectrometer. The chemical shifts were
referred to the signal due to CHCl3 in the solvent (at 7.24 ppm).
Molar masses and its distributions of the polymers were
determined at 40 ꢁC by size-exclusion chromatography (SEC) using
a Jasco model GPC-900 chromatograph equipped with two Polymer
Laboratories SEC columns [PL-gel, Mixed C (300 mm ꢂ 7.5 mm)],
using tetrahydrofuran (THF) as an eluent, and calibrated against
standard PMMA samples (Shodex, MW: 1.25 ꢂ 106, 6.59 ꢂ 105,
1.95 ꢂ 105, 4.96 ꢂ 104, 2.06 ꢂ 104, 6.82 ꢂ 103, 2.00 ꢂ 103). IR spectra
were recorded in KBr disc (for succinic anhydrideand glutaric an-
hydride) or in liquid paraffin mull (for poly(MAAn) and MAAn) with
a Jasco FT/IR-410 Fourier-transform infrared spectrophotometer.
Gas chromatographyemass spectrometry (GCeMS) was performed
on a Jeol JMS-700 mass spectrometer.
2.3. Recovery of polymers under anhydrous conditions
The polymers obtained by the procedure described in Section
2.2 were partially hydrolyzed through quenching with the acid and
work-up. In order to recover the polymeric product without hy-
drolysis, the polymerization was carried out at ꢀ78 ꢁC using the
procedure described above at a molar ratio of [MAAn]0/[Li-iPrIB]0/
[MeAl(ODBP)2] to be 100/1/10; [MAAn]0
¼
1.0 M, [Li-
iPrIB]0 ¼ 10 mM in CH2Cl2 (15 mL) solution. After 48 h, the solvent
was removed in vacuo at 0 ꢁC, where the polymerization did not
proceed. The residue was washed with dried diethyl ether, and then
dried in vacuo; yield of poly(MAAn) 1.07 g (45.9%).
3. Results and discussion
3.1. Anionic polymerization of MAAn
2.4. Derivatization of poly(MAAn) to PMMA
Anionic polymerization of MAAn was attempted in CH2Cl2
at ꢀ40 or ꢀ78 ꢁC with a lithium ester enolate, Li-iPrIB, as an ini-
tiator in the presence or absence of additives which are known to
be effective for stereospecific living polymerizations of methacry-
late monomers [12,17]. In the absence of additives, the reaction did
not give any polymeric products. Me3SiOLi has been known to be an
effective additive in combination with Li-iPrIB for highly isotactic-
specific living polymerization of MMA [12]. The polymerization of
MAAn in the presence of Me3SiOLi (five-fold excess to the initiator)
gave a soluble polymer in a low yield at ꢀ78 ꢁC, while the reaction
at elevated temperature (ꢀ40 ꢁC) did not afford any polymeric
product.
The derivatization was performed according to the procedure
for that of silyl methacrylate polymers to PMMA [18] with slight
modification. The obtained poly(MAAn)s (20 mg) were dispersed in
CH3OH (2 mL) containing a small amount of conc. H2SO4 (2 drops),
and the solution was kept at 65 ꢁC for 24 h. After concentrating it by
evaporation, the product was dispersed in CHCl3 (1 mL), to which
acetylacetone (0.1 mL) and trimethylsilyldiazomethane (2.0 M in
Et2O, about 1 mL) were added. After overnight, the reaction mixture
was quenched by adding a small amount of acetic acid, poured into
hexane (100 mL), and the precipitate was collected by decantation
and then washed with distilled water.
Several organoaluminum compounds are also known effective
as additives for controlled anionic polymerizations via stabilization
of propagating species [17,19e21] and/or monomer activation
through coordination at the carbonyl group of acrylic monomers
[17,22e25]. While the addition of n-Bu3Al, an effective additive for
syndiotactic-specific living polymerization of methacrylates [26],
brought about no positive effect, the polymerization at ꢀ78 ꢁC with
bulky aluminum bisphenoxides, ethyl- and methylaluminum
bis(2,6-di-tert-butylphenoxide)s, EtAl(ODBP)2 and MeAl(ODBP)2,
respectively, afforded soluble polymers in moderate yields. The
polymerizations at ꢀ40 ꢁC did not proceed as in the case of
Me3SiOLi, suggesting the occurrence of serious side reactions at
elevated temperature.
2.5. Equimolar reaction of MAAn and Li-iPrIB
The equimolar reactions were carried out in glass ampoules
filled with dried nitrogen at ꢀ78 ꢁC with a similar procedure to the
polymerization at a molar ratio of MAAn/Li-iPrIB/MeAl(ODBP)2 to
be (i) 1/1/0 or (ii) 1/1/5 (MAAn 0.50 mmol (77.1 mg) in CH2Cl2
(5.0 mL)). After 1 h of reaction period, the reaction was quenched by
adding 1M HCl aq. (1.5 eq.). The solvent was removed in vacuo.
The products were analyzed by GC, GCeMS, and 1H NMR. The
yields in case (i) and case (ii) were 32.7 mg and 217 mg, respec-
tively, the latter contained 2,6-di-tert-butylphenol derived from
MeAl(ODBP)2.

T. Kitaura et al. / Polymer 54 (2013) 1987e1992
1989
Since the polymerization in the presence of MeAl(ODBP)2
showed highest polymer yield, effects of the reaction conditions
were studied in some detail. The ratio of MeAl(ODBP)2 to Li-iPrIB
was found critical for the polymerization as seen in Table 1 (runs
10e14), that is, the excess amount of the aluminum additive is
necessary for the successful polymerization and the larger amount
of the aluminum compound enhances the polymer yield but in
a less drastic manner when the ratio exceeds five. The polymer-
ization was also examined at different monomer-to-initiator ratio
up to 100. However, the yield was limited at higher monomer-to-
initiator ratio, probably due to slow propagation of the system, so
that the preparation of higher molar-mass polymer is still difficult.
Tacticity of the polymer was estimated by 1H NMR analysis for
PMMA samples derived from the products through complete
cleavage of anhydride group to carboxylic group followed by
methylation with trimethylsilyldiazomethane. While the PMMA
derived from radically prepared poly(MAAn) is rich in heterotactic
(mr) and syndiotactic (rr) triads as reported previously [27], those
from poly(MAAn)’s obtained with the aluminum bisphenoxides are
rich in syndiotacticity (rr) (Table 1, supplementary Fig. S1). The mr
sequence formation in the radical cyclopolymerization is rational-
ized by different stereospecificity in the intramolecular (cycliza-
tion) and intermolecular addition processes; the former prefers
meso addition while the latter favors racemo addition. The stereo-
chemical difference in the radical and anionic processes may be
most evident in the intramolecular, cyclization process, that is, the
anionic process favors racemo additions both in the intramolecular
(cyclization) and intermolecular additions. The detailed study on
the stereochemical aspect of this anionic cyclopolymerization is
now under way.
3.2. Structure of polymer
The structure of the obtained polymers was examined by IR and
NMR analyses. Normally, anionic polymerization is terminated by
adding protic terminator such as acidified alcohol. In the ordinary
polymerization procedure, the reaction was quenched with acetic
acid in toluene. Through this procedure, acid anhydride units in
either pendant group or cyclized unit might be cleaved so that the
structural information of the polymer formed would be lost. To
prevent such difficulty, the polymer formed with MeAl(ODBP)2 was
recovered without quenching the reaction with acidic terminator
under anhydrous conditions as described in the Experimental
Section. IR spectrum of the polymer shows absorptions due to
C]O stretching at 1759 and 1801 cmꢀ1, which is close to those for
3.3. Role of the aluminum bisphenoxide
Nucleophilic species may react with MAAn at C]O and C]C
double bonds. If all the nucleophiles (initiator) are consumed by the
C]O attack, the initiation reaction should not proceed. Thus the
suppression of the C]O attack is inevitable for the success of the
anionic polymerization of MAAn. To investigate the effect of the
added aluminum bisphenoxide in this regard, an equimolar reac-
tion of MAAn and Li-iPrIB was examined in CH2Cl2 at ꢀ78 ꢁC in the
presence or absence of MeAl(ODBP)2. The product without the
aluminum compound consisted of two major components [A] and
[B] in the ratio of 60:36 (GC peak area), the structures of which
were estimated from GCeMS and 1H NMR analyses (supplementary
Fig S2 and S3) as shown in Scheme 1. The compound [A] consists of
one methacrylic acid unit and one initiator fragment, isopropyl
isobutyrate unit, and should be derived from a one-to-one adduct
a
six-membered cyclic model, glutaric anhydride (1758,
1808 cmꢀ1), rather than those of MAAn (1730, 1789 cmꢀ1) or those
of succinic anhydride (1785, 1869 cmꢀ1), a five-membered cyclic
model (Fig. 1). 1H NMR spectrum of the polymer did not show
olefinic proton signals due to pendant methacryloyl group, though
the signals of unreacted MAAn remained (Fig. 2a). Absence of C]C
stretching vibration (1637 cmꢀ1) observed in the IR spectrum of the
monomer also confirms the absence of methacryloyl side group.
Table 1
Anionic polymerization of MAAn with Li-iPrIB in CH2Cl2 for 24 h.a
b, c
Run
Additive
Temp., ꢁ
C
MAAn/additive/Li-iPrIB
Yield%
Mn/103
Mw
Mn
Tacticity/%b, d
SECb, c
1H NMRb, d
mm
mr
rr
1
2
3
4
5
6
7
8
None
ꢀ40
ꢀ78
ꢀ40
ꢀ78
ꢀ40
ꢀ78
ꢀ40
ꢀ78
ꢀ40
ꢀ78
25/0/1
25/0/1
25/5/1
25/5/1
25/5/1
25/5/1
25/5/1
25/5/1
25/5/1
25/5/1
25/0.5/1
25/1/1
25/2/1
25/10/1
50/5/1
50/10/1
100/5/1
100/10/1
25/5/1
1000/0/1
Trace
Trace
Trace
8.9
e
e
e
e
e
e
e
e
e
e
e
e
Me3SiOLi
n-Bu3Al
e
e
e
e
e
e
3.8
e
e
144
e
23.5
e
42.5
e
34.0
e
Trace
Trace
Trace
44.3
Trace
61.5
Trace
Trace
31.7
68.1
42.9
53.5
6.4
e
e
e
e
e
e
e
EtAl(ODBP)2
MeAl(ODBP)2
e
e
e
e
e
e
2.3
e
3.9
e
1.06
e
3.2
e
27.2
e
69.6
e
9
10
11
12
13
14
15e
16e
17e
18e
19f
20g
3.1
e
5.4
e
1.18
e
2.4
e
23.8
e
73.8
e
e
e
e
e
e
e
2.4
3.6
4.9
5.0
4.4
5.6
3.1
77.8
3.3
5.8
9.6
9.8
9.4
15.4
5.0
e
1.08
1.12
1.28
1.27
1.13
1.36
1.22
1.91
2.2
1.1
1.1
1.3
0.8
0.8
1.0
12.5
24.0
27.7
24.7
26.1
22.6
24.7
26.1
44.1
73.8
71.1
74.2
72.7
76.6
74.5
72.9
43.4
23.2
55.6
100
<Radical>
60
a
MAAn 5.0 mmol, CH2Cl2 5.0 mL.
b
c
d
e
f
Measured for PMMAs derived from poly(MAAn)s.
Determined by SEC in THF (against PMMA standards).
Determined by 400 MHz 1H NMR (CDCl3, 55 ꢁC).
For 48 h.
For 72 h.
g
MAAn 15 mmol, AIBN 2.3 mg, benzene 18.5 mL, 24 h.

1990
T. Kitaura et al. / Polymer 54 (2013) 1987e1992
probably due to stabilization through the intramolecular coordi-
nation of methacryloyl carbonyl group to lithium cation.
The equimolar reaction of MAAn and Li-iPrIB in the presence of
MeAl(ODBP)2 gave a product mixture consisting of [A] (38%) and
higher molar-mass components but not of [B]. The absence of [B] in
the product suggests that the addition of the aluminum Lewis acid
suppresses the carbonyl addition reaction. Moreover, the formation
of higher molar-mass products indicates that the monomer anion
[C] should be activated enough to add MAAn possibly by the action
of the added aluminum Lewis acid.
Schlaad and Müller proposed structure models of ester enolate-
aluminum alkyl complexes based on NMR and quantum chemical
calculations [28]. They suggested that, in a one-to-one complex
model, aluminum alkyl is coordinated to the carbonyl oxygen
(anionic oxygen in the enolate form) instead of ether oxygen, while
the dimer and tetramer complex are energetically more stable.
Chen and his coworker reported an X-ray structure of an estere
enolate complex of an aluminum bisphenoxide, in which the alu-
minum center also locates at the anionic oxygen of the enolate,
while Li cation is linked to the two oxygen atoms of the enolate unit
[15]. If this type of structure model is adopted to the MAAn anion
formed in the presence of MeAl(ODBP)2, one can postulate the
terminal structure [D] in which acyl carbonyl group is free from the
coordination to Li cation and a rotation of the acyl group can give
rise to a conformation favorable to the intramolecular addition [E]
(Scheme 2).
Fig. 1. IR spectra of MAAn (a), poly(MAAn) obtained with Li-iPrIB/MeAl(ODBP)2 (b),
glutaric anhydride (c), and succinic anhydride (d).
3.4. Coordination of aluminum bisphenoxide with MAAn
Aluminum Lewis acids are known to act as a monomer activator
in anionic polymerization of carbonyl-containing monomers
including methacrylates [22e25]. In order to examine effects of
MeAl(ODBP)2 in the polymerization of MAAn, 13C NMR coordina-
tion experiments were conducted in toluene-d8 at MAAn/MeAl(-
ODBP)2 ratios of 1/0.5, 1/1 and 1/2 (Fig. 3) at ꢀ78 ꢁC. At the ratio of
1/0.5, the carbonyl region spectrum showed signal of the free
MAAn at 162.83 ppm along with a pair of signals at 160.84 ppm and
178.84 ppm due to a coordinated MAAn carrying MeAl(ODBP)2 at
one of the carbonyl oxygens, resulting in the large lower field shift.
At the ratio of 1/1, the free MAAn signal disappeared and the two
of Li-iPrIB and MAAn formed through the addition of Li-iPrIB to C]
C bond of MAAn followed by cleavage of methacryloyl group upon
acidic work-up. The compound [B] consists of two initiator frag-
ments attached to one methacrylate unit, which formed through
the addition of Li-iPrIB to C]C and C]O bonds of MAAn. No higher
molar-mass products were detected. The results suggest that the
carbonyl addition takes place in the initiation process of MAAn
polymerization as a side reaction but the normal initiation through
C]C attack is also a major process. Nevertheless, only the mono-
meric product [A] formed, indicating the anionic species formed in
the latter process ([C] in Scheme 1) is too stable to add MAAn,
Fig. 2. 1H NMR spectra of poly(MAAn)s obtained with Li-iPrIB/MeAl(ODBP)2 (a), and
that of MAAn (b) (DMSO-d6, 30 ꢁC, 400 MHz). Asterisked signal (*) is due to 2,6-di-tert-
butylphenol derived from MeAl(ODBP)2, and olefinic signals due to remained MAAn
and methacrylic acid (MAA).
Scheme 1. Equimolar reaction of MAAn and Li-iPrIB in the presence or absence of
MeAl(ODBP)2.

T. Kitaura et al. / Polymer 54 (2013) 1987e1992
1991
Fig. 4. Temperature-dependent changes of carbonyl region 13C NMR spectra of a 1/1
mixture of MAAn and MeAl(ODBP)2 in toluene-d8 ( ꢂ : unidentified signals). The
spectrum at the bottom (*) was measured at ꢀ78 ꢁC, after the measurement at 0 ꢁC.
Scheme 2. Plausible mechanism of the effect of MeAl(ODBP)2 on the initiation.
peaks due to the coordinated monomer prevailed. When the
MeAl(ODBP)2 was added two-fold of MAAn, the signals observed
were almost identical to those of the spectrum at 1/1 ratio, sug-
gesting that the excess aluminum compound did not coordinate to
the remaining carbonyl group of the 1/1 coordinated monomer. The
suggest that methyl groups of MeAl(ODBP)2 reacts with MAAn at
elevated temperatures to form methyl isopropenyl ketone. In other
words, MAAn monomer acts as an acylating agent toward the
aluminum compound. This may be one of the reasons why the
polymerization at higher temperature was unsuccessful (see
Table 1), since the resulting aluminum carboxylate should be
ineffective as Lewis acid.
b
-methylene carbon of the 1/1 coordinated monomer showed also
two peaks at 132.0 and 128.9 ppm assignable to the coordinated
methacryloyl group and non-coordinated one, respectively. The
larger chemical shift value for the former indicates the higher
reactivity of the coordinated methacryloyl group toward nucleo-
philic attack by the initiator.
4. Conclusions
Methacrylic anhydride was anionically polymerized for the first
time with the aid of the bulky aluminum Lewis acids in
dichloromethane at ꢀ78 ꢁC. The product polymers consisted of
six-membered cyclized units exclusively, as in the reported cases
of N-methyldimethacrylamide [7]. Based on the fact that the
model reaction of MAAn with the ester enolate initiator did not
afford an oligomeric product due to the formation of unimeric
anion stabilized by the coordination with the remaining meth-
acryloyl carbonyl group, it is proposed that the formation of the
enolate-aluminate type species prevents such coordinative stabi-
lization so that the remaining methacryloyl C]C bond becomes
accessible for intramolecular addition to form the cylized unit.
Tacticity of PMMA derived from poly(MAAn) is rich in syndiotactic
triad in contrast to the cases of radically prepared one, suggesting
the different stereochemistry in the intramolecular (cyclization)
process between anionic and radical mechanisms. At higher
temperatures than ꢀ78 ꢁC, no polymeric product formed partly
due to the carbonyl addition reaction of MeAl(ODBP)2 to MAAn.
Though the rate of polymerization is so low that the monomer
conversion is still moderate and the attained molar mass is limited
less than 104, the polymerization system used is known to be
effective for living anionic polymerizations of methacrylate and
acrylates so that the MAAn can readily be incorporated in the block
copolymer syntheses with these monomers as a rigid segment and
reactive precursor block for the materials applications of acrylic
polymers.
The temperature-dependent change of the spectrum was also
studied for the case of 1/1 ratio (Fig. 4). At ꢀ78 ꢁC, the two paired
signals were observed as described above. The spectrum at ꢀ40 ꢁC
did not show the signals responsible to the monomer carbonyls
probably due to rapid exchange of the species formed. At 0 ꢁC,
a new peak appeared at 202.72 ppm assignable to ketonic carbonyl,
which remained when the sample solution was cooled to ꢀ78 ꢁC.
Alkylaluminums are known to react with acid anhydride as an
alkylating agent to form ketonic compounds [29]. Thus the results
Acknowledgment
Fig. 3. Carbonyl region 13C NMR spectra of MAAn (a), mixtures of MAAn and MeAl(-
ODBP)2 with the ratio of 1/0.5 (b), 1/1 (c) and 1/2 (d) measured in toluene-d8 at ꢀ78 ꢁ
(ꢂ: unidentified signals).
C
This work was supported by JSPS KAKENHI Grant Number
24750109.

1992
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