New degradable alternating copolymers from naturally occurring aldehydes: Well-controlled cationic copolymerization and complete degradation

Article abstract of DOI:10.1021/ma3004828

Three naturally occurring conjugated aldehydes, (1R)-(-)-myrtenal, (S)-(-)-perillaldehyde, and β-cyclocitral, were cationically copolymerized with isobutyl vinyl ether using the EtSO₃H/GaCl₃ initiating system in the presence of 1,4-d

Full text of DOI:10.1021/ma3004828

Article
pubs.acs.org/Macromolecules
New Degradable Alternating Copolymers from Naturally Occurring
Aldehydes: Well-Controlled Cationic Copolymerization and
Complete Degradation
Yasushi Ishido, Arihiro Kanazawa, Shokyoku Kanaoka, and Sadahito Aoshima*
Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
*
S Supporting Information
ABSTRACT: Three naturally occurring conjugated alde-
hydes, (1R)-(−)-myrtenal, (S)-(−)-perillaldehyde, and β-
cyclocitral, were cationically copolymerized with isobutyl
vinyl ether using the EtSO H/GaCl initiating system in the
3
3
presence of 1,4-dioxane as an added Lewis base. Alternating
copolymerization proceeded exclusively via 1,2-carbonyl
addition of the aldehydes. In addition, controlled alternating
copolymerization was achieved under appropriate reaction
conditions, producing copolymers with controlled molecular
weights and narrow molecular weight distributions. The relationships between the copolymerization behaviors and the cyclic side
group structures of the aldehydes suggested that conjugated and bicyclic structures were important factors for controlled
alternating copolymerization. However, too much bulkiness around the carbonyl group resulted in termination of
copolymerization. The resulting alternating copolymers were stable under neutral and basic conditions. In sharp contrast,
mild acidic conditions degraded the alternating copolymers almost selectively to conjugated aldehydes with low molecular
weights as nearly single products.
INTRODUCTION
molecular weights and narrow molecular weight distributions
MWDs) were obtained from the appropriate combinations of
monomers, i.e., VEs with lower reactivity and aldehydes with
■
(
Polymer synthesis from naturally occurring renewable resources
has been driven by the urgent need to conserve petroleum
resources, leading to a “carbon neutral” and sustainable
2
7
larger basicity (Scheme 1). The resulting alternating
1
society. In fact, various biomass-based polymers have recently
Scheme 1. Alternating Copolymerization of Benzaldehydes
been synthesized as alternatives for petroleum-based poly-
2
−7
(
BzAs) with Vinyl Ethers (VEs) and Selective Acid
mers.
Naturally occurring compounds often contain specific
27
Hydrolysis of Product Copolymers
structures such as mono- or bicyclic structures, olefinic
moieties, or chirality, some of which are not easily obtained
from petroleum-derived molecules. Typical examples are
terpenes, the polymers of which exhibit various characteristics
such as a very high glass transition temperature, high
8
−10
transparency, and/or unique monomer sequences.
Lactic
1
1
12
13
acid, its derivatives, and some lactones have also drawn
attention as biomass-based materials for degradable polymers.
The recent success of controlled/living polymerizations of such
monomers has allowed additional functionalization by block
1
4−18
copolymerization or chain-end modification.
Potential natural resources for cationic polymerization are
aldehydes, which are abundant in nature. For example,
1
9
20
benzaldehydes (BzAs) are found in almond or cherry, and
copolymers possessed acetal linkages in their main chains at
regular intervals; thus, acid hydrolysis selectively yielded
cinnamaldehyde derivatives (CinAs), used in perfumes, which
can also be copolymerized with a VE monomer.
p-methoxyBzA (also called anisaldehyde) can be extracted from
anise and fennel oils. However, the cationic polymerization
of these compounds has rarely been studied due to difficulty in
their polymerization as reported decades ago.
2
1
22
23,24
We recently reported the successful controlled cationic
copolymerization of BzAs with alkyl vinyl ethers (VEs) using
weak Lewis base-assisting initiating systems for the first
Received: March 8, 2012
Revised: April 19, 2012
Published: May 1, 2012
2
5,26
time.
Nearly alternating copolymers with controlled
©
2012 American Chemical Society
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Scheme 2. Cationic Copolymerization of Naturally Occurring Aldehydes with Isobutyl Vinyl Ether (IBVE) and Acid Hydrolysis
of Resulting Copolymers
Since CinAs could be copolymerized with VEs into
alternating copolymers, other naturally occurring cyclic
conjugated aldehydes became candidates as comonomers for
the synthesis of new selectively degradable copolymers with
VEs. Three terpenoids, (1R)-(−)-myrtenal (myrtenal), (S)-
EXPERIMENTAL SECTION
■
Materials. Isobutyl VE (IBVE; TCI, >99.0%) was distilled twice
over calcium hydride before use. Toluene-d8 (Wako, 99.6%) was
distilled once over calcium hydride before use. (1R)-(−)-Myrtenal
(
>
(
>
myrtenal; Aldrich, >97%), (S)-(−)-perillaldehyde (PA; Aldrich,
92%), β-cyclocitral (CC; Aldrich, >90%), cyclohexanecarbaldehyde
CHA; TCI, >98%), and 2,6-di-tert-butylpyridine (DTBP; Aldrich,
97%) were distilled over calcium hydride at least twice under reduced
pressure before use. Diethyl ether (Wako, >99.5%) and 1,4-dioxane
Wako, >99.5%) were distilled over calcium hydride and then lithium
(−)-perillaldehyde (PA), and β-cyclocitral (CC), are the most
readily available (Scheme 2). Myrtenal, a main component of
the essential oils of asteraceae plants, is a chiral bicyclic
28
conjugated aldehyde. PA, a chiral monocyclic conjugated
(
aldehyde with an olefinic moiety, is abundantly extractable from
aluminum hydride. Toluene and hexane were dried by passage through
solvent purification columns (Glass Contour). The adduct of IBVE
2
9
perilla. CC is another monocyclic conjugated aldehyde with
three methyl substituents in the proximity of the polymerizable
with HCl [IBVE−HCl; CH
addition reaction of IBVE with HCl according to literature methods.
Ethanesulfonic acid (EtSO H; Aldrich, 95%) and an EtAlCl solution
CH(OiBu)Cl] was prepared from
3
38
3
0−32
carbonyl group and is convertible from citral,
an acyclic
33
3
2
conjugated aldehyde existing in lemongrass oil. However,
there are no examples of the addition polymerization of these
monomers, although a polycondensation using myrtenal was
(
Wako, 1.0 M in hexane) were used as received. For FeCl , a stock
solution in diethyl ether was prepared from commercial anhydrous
3
FeCl (Aldrich, 99.99%). For GaCl , a stock solution in hexane was
3
3
34
recently reported.
prepared from commercial anhydrous GaCl3 (Aldrich, 99.999+%).
Hydrochloric acid (Nacalai Tesque) and tetrahydrofuran (THF;
Wako, >99.5%) were used as received. All reagents except for toluene,
hexane, THF, and hydrochloric acid were stored in brown ampules
under dry nitrogen.
Polymerization Procedure. Polymerization was carried out under
a dry nitrogen atmosphere in a glass tube equipped with a three-way
stopcock. The tube was carefully dried using a heat gun (Ishizaki; PJ-
In this study, we investigated cationic copolymerizations of
the above-mentioned naturally occurring aldehydes, myrtenal,
PA, and CC, with isobutyl VE (IBVE) using the ethanesulfonic
acid (EtSO H)/GaCl initiating system in the presence of weak
3
3
Lewis bases (Scheme 2). The effects of the side group
structures such as mono- or bicyclic structures, bulkiness
around carbonyl groups, and olefin structures on the
copolymerization behaviors are of great interest. Addition
modes of the enal (α,β-unsaturated carbonyl) moiety, i.e., 1,2-,
2
06A; blow temperature ∼450 °C) under dry nitrogen before
polymerization reactions. A prechilled initiator solution (0.40 mL)
was added to a prechilled mixture solution of a monomer, an added
base, and a solvent (3.20 mL) at 0 °C by a dry medical syringe. The
polymerization reaction was started by the addition of a prechilled
Lewis acid solution (0.40 mL) to the prechilled mixture (3.60 mL) at
1
,4-carbonyl, and vinyl additions, may vary depending on the
35,36
catalyst and reaction conditions,
while complete control of
the addition mode in conjugated aldehyde (co)polymerizations
has never been achieved except for exclusive vinyl addition
−
78 °C. The reaction mixture was stirred with a magnetic stir bar
throughout the polymerization. The polymerization was quenched
with prechilled methanol containing a small amount of aqueous
ammonia solution (2.50 mL, 0.1%). The quenched reaction mixture
was diluted with dichloromethane and then washed with water to
remove the initiator residues. The volatiles were evaporated under
reduced pressure, and the residue was vacuum-dried for at least 6 h at
37
(
co)polymerization via a radical process. Thus, we also
focused on the microstructure of the product copolymers as
well as the optimization of the reaction conditions for
controlled cationic copolymerization of the three aldehydes
bearing conjugated cyclic structures with VEs. The present
study not only develops degradable polymers from naturally
occurring compounds but also establishes a new strategy for
polymerization of conjugated aldehydes.
Under appropriate reaction conditions, the controlled
alternating copolymerizations successfully proceeded, yielding
copolymers with narrow MWDs. Surprisingly, NMR analysis of
the product copolymers revealed the exclusive 1,2-carbonyl
addition reactions of the aldehydes. The effects of the side
group structures of the aldehydes on the copolymerization
behavior were also examined. Moreover, the resulting
alternating copolymers could be completely degraded via acid
hydrolysis to single aldehydes with extended conjugated
structures without any oligomeric residues.
6
0 °C. For copolymerization with CC, the quenched mixture was
analyzed directly by NMR and gel permeation chromatography (GPC)
due to the instability of the product copolymers to the purification
process. The monomer conversion was determined by the gravimetric
1
method and/or H NMR analysis.
Hydrolysis. Obtained copolymers were purified by reprecipitation
in alcohols at least twice to remove low molecular weight oligomers.
The purified copolymer (30 mg) was dissolved in THF (5.0 mL), and
then the reaction was started by the addition of an aqueous HCl−THF
(
1.0 M, 5.0 mL) solution at 30 °C. The reaction mixture was stirred
with a magnetic stir bar throughout the hydrolysis. The hydrolysis was
quenched with aqueous sodium hydroxide (1.0 M). The quenched
reaction mixture was diluted with dichloromethane, washed with water
to remove the resulting salt, and evaporated under reduced pressure.
The residue was vacuum-dried at room temperature.
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Figure 1. (A) Time−conversion curves, (B) M and M /M for polymer peaks, and (C) molecular weight distribution curves of products obtained
n
w
n
by copolymerization of (1R)-(−)-myrtenal with isobutyl vinyl ether (IBVE) {[(1R)-(−)-myrtenal] = 0.60 M, [IBVE] = 0.60 M, [EtSO H] = 4.0
0
0
3
0
mM, [GaCl ] = 4.0 mM, [1,4-dioxane] = 1.0 M, in toluene at −78 °C; M : number-average molecular weight, M /M : polydispersity ratio (weight-
3
0
n
w
n
a
b
c
average molecular weight/number-average molecular weight), by GPC (polystyrene calibration)}. For polymerization. For polymer. Cyclic trimer
ratio in products.
Table 1. Cationic Copolymerization of (1R)-(−)-Myrtenal and Isobutyl Vinyl Ether (IBVE) Using Various Lewis Acids and
a
Initiators
entry
Lewis acid
initiator
time (h)
conv (%)
Mn
M /Mn
w
polymer/oligomer
aldehyde content (%)
1
2
3
4
5
GaCl3
GaCl3
GaCl3
FeCl3
EtSO H
2
2
92
86
17
20
6
22100
16300
10700
4100
1.12
1.61
1.74
1.81
1.51
98/2
97/3
94/6
94/6
81/19
48
47
48
45
−
3
IBVE−HCl
none
2
EtSO H
48
48
3
EtAlCl2
EtSO H
2200
3
a
[
n
(1R)-(−)-myrtenal] = 0.60 M, [IBVE] = 0.60 M, [initiator] = 4.0 mM, [Lewis acid] = 4.0 mM, [1,4-dioxane] = 1.0 M, in toluene at −78 °C.
0
0
0
0
M : number-average molecular weight, M /M : polydispersity ratio (weight-average molecular weight/number-average molecular weight), by GPC
w
n
(
polystyrene calibration).
3
9
Characterization. The MWD of the polymers was measured by
than 90% conversion in 2 h (Figure 1A). The M s of the
n
GPC in chloroform at 40 °C with three polystyrene gel columns (TSK
product copolymers increased linearly in direct proportion to
the myrtenal conversion, and their MWDs were very narrow
throughout the reaction (M /M = 1.09−1.16; Figure 1B). The
MWD curves of the copolymers clearly shifted to the higher
molecular weight region, indicating the generation of long-lived
growing species (Figure 1C). Our previous study showed that
gel G-4000H , G-3000H , and G-2000H ; 7.8 mm i.d. × 300 mm;
XL
XL
XL
flow rate = 1.0 mL/min or TSKgel MultiporeH -M; 7.8 mm i.d. ×
XL
w
n
3
00 mm; flow rate = 1.0 mL/min) connected to a Tosoh DP-8020
pump, a CO-8020 column oven, a UV-8020 ultraviolet detector, and
an RI-8020 refractive index detector. The number-average molecular
weight (M ) and polydispersity ratio [weight-average molecular
n
weight/number-average molecular weight (M /M )] were calculated
the copolymerizations of aldehydes with VEs by the EtSO H/
3
w
n
from the chromatographs with respect to 16 polystyrene standards
GaCl initiating system produced not only copolymers but also
3
6
(
Tosoh; 292−1.09 × 10 , M /M ≤ 1.1). NMR spectra were recorded
cyclic trimers (9%−56%) consisting of two aldehydes with one
w
n
2
6,27
at 30 °C in CDCl or CD Cl using a JEOL ECA 500 spectrometer
(
3
2
2
VE.
However, the cyclic oligomer was scarcely produced
1
13
500.16 MHz for H, 125.77 MHz for C and DEPT). Differential
(
1−2%) in the copolymerization of myrtenal (Figure 1C).
The copolymerization was also examined using other Lewis
scanning calorimetry (DSC EXSTER-6000, Seiko Instruments Inc.)
was used to determine the glass transition temperature (T ) in the
g
acid catalysts and initiators. Although polymers were obtained
range from −100 to 120 °C for product copolymers. The heating and
in each case as shown in Table 1, the M values of the product
cooling rates were 10 °C/min. The T of the copolymers was defined
n
g
copolymers and their copolymerization behaviors were
markedly different depending on the Lewis acid and the
initiator. When IBVE−HCl/GaCl3 (entry 2) was used, a
copolymer with a MWD broader than that by EtSO H/GaCl
as the temperature of the midpoint of a heat capacity change on the
second heating scan. Optical rotations were measured in hexane with a
1
0 mm quartz cell on a JASCO J-720WO spectropolarimeter and
calculated on the basis of the aldehyde units.
3
3
(
entry 1) was produced. Without any initiator, the monomer
RESULTS AND DISCUSSION
■
conversions were much smaller than that with an initiator,
1
. Precision Synthesis and Selective Acid Hydrolysis
although a copolymer was produced (entry 3). FeCl , a very
active Lewis acid catalyst for the polymerization of VEs,
showed poor monomer conversion (entry 4) compared to the
3
40
of Alternating Copolymer from Myrtenal with IBVE.
1
.1. Controlled Copolymerization of Myrtenal and IBVE.
Cationic copolymerizations of myrtenal with IBVE were first
reaction with GaCl , although a long-lived growing species
3
carried out using the EtSO H/GaCl initiating system in the
existed. With EtAlCl , a Lewis acid that induced only
3
3
2
presence of 1,4-dioxane as an added Lewis base in toluene at
cyclotrimerization in the copolymerizations of BzAs with
26
−
78 °C ([myrtenal] = 0.60 M, [IBVE] = 0.60 M, [EtSO H]
VEs, the monomer conversion was very low and a polymer
with a low molecular weight was obtained (entry 5). These
results demonstrate that the proper choice of the Lewis acid
and initiator is critical for controlled copolymerization of
myrtenal and IBVE.
0
0
3
0
=
4.0 mM, [GaCl ] = 4.0 mM, [1,4-dioxane] = 1.0 M). These
3 0
were the most appropriate conditions for the copolymerizations
26,27
of BzAs with VEs.
The copolymerization proceeded
smoothly, and each monomer was consumed to reach more
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.2. Structure Analysis of Product Copolymers. 13C NMR
measurements of the product copolymers were conducted after
cyclic oligomers were removed by reprecipitation from a
methanol/ethanol mixture. As shown in Figure 2, the resonance
Article
1
Resonance integral ratios of the olefin peaks (g) and the
acetal peaks (b) were almost the same as in the copolymeriza-
tion of IBVE and myrtenal. These results support that the olefin
moieties in the polymer side groups did not react with
propagating chain ends at all through the copolymerization.
This is also confirmed in that no shoulder peaks resulting from
cross-linking reactions were observed in the higher molecular
weight regions in the MWD curves (Figure 1C). This is
probably because the olefin moieties in the side groups are
sterically hindered and possess less electron density compared
with the carbon−carbon double bond of IBVE and the carbonyl
group of myrtenal.
Figure 2. Overlaid DEPT 135 (upper) and 13C NMR (lower) spectra
of poly[(1R)-(−)-myrtenal-co-isobutyl vinyl ether] [M (GPC) = 2.23
n
4
1
×
10 , M /M (GPC) = 1.12, aldehyde content: 48%; 125.77 MHz in
Figure 3. H NMR spectrum of poly[(1R)-(−)-myrtenal-co-isobutyl
w
n
4
CDCl at 30 °C].
vinyl ether] [M (GPC) = 2.23 × 10 , M /M (GPC) = 1.12, aldehyde
3
n
w
n
content: 48%; 500.16 MHz in CDCl at 30 °C].
3
peaks derived from the myrtenal units were observed. The
absence of a carbonyl resonance ruled out polymerization
through vinyl addition with myrtenal. Furthermore, olefin
carbons adjacent to the oxygen atom, which would arise from
1.3. Acid Hydrolysis and Characteristic Properties of
Resulting Alternating Copolymers. As described above, the
resulting copolymers had alternating structures, which means
that the acetal linkages of the IBVE−myrtenal sequences
existed in the polymer backbone at regular intervals. The acid
hydrolysis of poly(myrtenal-co-IBVE) was conducted in a
THF−aqueous HCl mixture at 30 °C for 2 h. The reaction
mixture was homogeneous throughout the reaction. The MWD
curves of the original copolymer and the hydrolysis product are
shown in Figure 4A. After the hydrolysis, the peak of the
copolymer disappeared quantitatively and a sharp peak was
1,4-addition, were also not observed, and 15 resonance peaks
were consistent with the structures derived from 1,2-carbonyl
addition. These results demonstrated the incorporation of
myrtenal into the copolymer through only the 1,2-carbonyl
addition mode.
This highly selective addition mode is attributed to the
following two effects: electron densities of the polymerizable
groups and steric hindrance around the double bonds. The
carbonyl group in the enal moiety withdraws electrons strongly
from the adjacent carbon−carbon double bond; hence, the
growing carbocation reacts with the electron-rich carbonyl
group selectively. In addition, the carbon−carbon double bond
of the conjugated aldehyde is trisubstituted in a bulky six-
membered ring, whereas the carbonyl group is monosubsti-
tuted. Therefore, the vinyl addition reaction is also unfavorable
in terms of steric hindrance. These effects are most likely to
contribute to the exclusive 1,2-carbonyl addition reaction.
1
observed in the low molecular weight region instead. The H
NMR spectrum of the hydrolysis product, measured without
further purification, showed that the major product was an
aldehyde with an extended conjugation system [Figure 4B; the
13
structure of the aldehyde was also confirmed by C NMR
analysis (Figure S1)]. The almost exclusive production of an
aldehyde with an extended conjugated structure without
oligomeric products also supports the alternating structures of
the copolymers resulting from the 1,2-additions of myrtenal
1
The H NMR spectra of the product copolymers were then
(
Scheme 3).
measured to determine the contents of the myrtenal units
We further examined the characteristics of the new
(
Figure 3). The contents of the myrtenal units were calculated
alternating copolymer obtained in this study. The polymer
was stable in the bulk state and in neutral and basic solutions,
although it was easily degraded under acidic conditions. The
polymer did not decompose at all after storage in a freezer for
to be 47−48% based on the resonance integral ratios of the
acetal peaks (b) and the aliphatic proton peaks (e, j, k, n).
These values and no homopolymerizability of myrtenal indicate
that the resulting copolymers had nearly alternating structures.
one year. The glass transition temperature (T ) of the
g
13
Similar to the C NMR spectrum, there were no resonance
copolymer was 56 °C, which was higher than those of
1
26,27
peaks in the H NMR spectrum assignable to the olefin protons
alternating copolymers from BzAs with VEs.
The
adjacent to the oxygen atom or the aldehyde proton, which
again indicates that the copolymers had no 1,4-carbonyl and
vinyl addition structures.
copolymer showed a specific optical rotation derived from
the chiral side groups of the myrtenal units {[α] = 52 (at 25
D
4
°C); M (GPC) = 1.9 × 10 , M /M (GPC) = 1.09, aldehyde
n
w
n
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Figure 4. (A) Molecular weight distribution (MWD) curves of original poly[(1R)-(−)-myrtenal-co-isobutyl vinyl ether] (upper) and the hydrolysis
1
product (lower) and (B) H NMR spectrum of the hydrolysis product [500.16 MHz in CDCl at 30 °C; hydrolysis conditions: 0.50 M aqueous
3
4
HCl−tetrahydrofuran (THF) at 30 °C for 2 h, 0.33 wt % polymer solution; original copolymer: M (GPC) = 2.23 × 10 , M /M (GPC) = 1.12,
aldehyde content: 48%; #: peaks of THF and its ring-opening product; M : number-average molecular weight, M /M : polydispersity ratio (weight-
n
w
n
n
w
n
average molecular weight/number-average molecular weight), by GPC (polystyrene calibration)].
Scheme 3. Controlled Alternating Copolymerization of
structure. Cyclohexanecarbaldehyde (CHA), a petroleum-based
unconjugated aldehyde possessing a similar structure with the
three naturally occurring aldehydes, was also copolymerized
with IBVE.
(1R)-(−)-Myrtenal with Isobutyl Vinyl Ether and Selective
Acid Hydrolysis of the Product Copolymers
2.1. Copolymerization of PA with IBVE. The copolymeriza-
tion of PA with IBVE using EtSO H/GaCl gave polymers with
3
3
relatively high molecular weights and narrow MWDs. This
system was examined in detail and the addition of 2,6-di-tert-
41
butylpyridine (DTBP), a proton trap, was found to be crucial
for controlling the copolymerization of PA with IBVE (Figure 5
and Figure S2; reactions with and without DTBP, respec-
42
tively). The M s of the product copolymers increased in
n
direct proportion to PA conversion, and their MWDs were
relatively narrow (Figure 5B). This is in sharp contrast to the
reaction without DTBP, in which the Mn values clearly
decreased and the MWD became broader in the later stage
of the copolymerization due to frequent occurrences of proton
transfer reactions (Figure S2). Another noticeable difference in
the case of myrtenal is the larger amount of the cyclic trimer
byproduct as shown in Figure 5C.
content: 47%}. The solubility of the copolymer was also
4
examined for 1.0 wt % solutions [M (GPC) = 2.23 × 10 , M /
n
w
M (GPC) = 1.12, aldehyde content: 48%]. The copolymer
n
showed good solubility in hexane, toluene, dichloromethane,
chloroform, diethyl ether, and THF. Water, methanol, ethanol,
and dimethyl sulfoxide did not dissolve the copolymer.
2. Cationic Copolymerization of Other Aldehydes
with IBVE: Effects of Cyclic Side Group Structures.
Copolymerizations of other naturally occurring aldehydes, PA
and CC, with IBVE were examined to elucidate the relationship
between the copolymerization behavior and the aldehyde
The structure of the product copolymer was examined by
C NMR analysis. No resonance peaks derived from 1,4-
13
carbonyl or vinyl addition reactions were observed, and the
Figure 5. (A) Time−conversion curves, (B) M and M /M for polymer peaks, and (C) molecular weight distribution curves of products by
n
w
n
copolymerization of (S)-(−)-perillaldehyde (PA) with isobutyl vinyl ether (IBVE) {[PA] = 0.60 M, [IBVE] = 0.60 M, [EtSO H] = 4.0 mM,
0
0
3
0
[
GaCl ] = 4.0 mM, [1,4-dioxane] = 1.0 M, [2,6-di-tert-butylpyridine] = 4.0 mM, in toluene at −78 °C; M : number-average molecular weight, M /
3
0
n
w
a
M : polydispersity ratio (weight-average molecular weight/number-average molecular weight), by GPC (polystyrene calibration)}. For
polymerization. For polymer. Cyclic trimer ratio in products.
n
b
c
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Article
selective 1,2-carbonyl addition reaction was confirmed as in the
case with myrtenal (Figure S3). The PA contents of the
1
resulting copolymers calculated by H NMR analysis were 44−
4
6% (Figure S4). The cyclohexene and the isobutylene
moieties in the side group also remained intact throughout
the copolymerization.
2.2. Copolymerization of CC with IBVE. The monomer
conversions of the copolymerization of CC with IBVE were
very low in a reaction time similar to those for myrtenal and
PA. With longer reaction time, the conversions increased
slightly, but leveled off after 48 h (Figure 6A), and oligomers
Figure 7. Molecular weight distribution (MWD) curves of (A) original
poly((S)-(−)-perillaldehyde-co-isobutyl vinyl ether) (upper) and
hydrolysis products (lower) and (B) original poly[β-cyclocitral-co-
isobutyl vinyl ether] (upper) and hydrolysis products (lower)
{
hydrolysis conditions: 0.50 M aqueous HCl−tetrahydrofuran at 30
°
C for 2 h, 0.33 wt % polymer solution; original copolymer: (A)
4
M (GPC) = 1.39 × 10 , M /M (GPC) = 1.21, aldehyde content: 46%,
(
n
w
n
3
B) M (GPC) = 3.4 × 10 , M /M (GPC) = 1.17, aldehyde content:
n w n
4
1%}; M : number-average molecular weight, M /M : polydispersity
n w n
ratio (weight-average molecular weight/number-average molecular
weight), by GPC (polystyrene calibration).
under similar conditions (Figure 7B). The copolymer was
degraded into oligomers with various molecular weights due to
its partially random sequence. As a matter of course, the
alternating sequence of a copolymer is crucial for the selective
degradation to conjugated aldehydes.
Figure 6. (A) Time−conversion curves and (B) molecular weight
distribution curves of products by copolymerization of β-cyclocitral
(
CC) with isobutyl vinyl ether (IBVE) {[CC] = 0.60 M, [IBVE] =
0
0
0
1
.60 M, [EtSO H] = 4.0 mM, [GaCl ] = 4.0 mM, [1,4-dioxane] =
3 0 3 0
.0 M, in toluene at −78 °C; M : number-average molecular weight,
n
2.4. Relationships between Copolymerization Behaviors
M /M : polydispersity ratio (weight-average molecular weight/
w
n
and Side Group Structures. To study the effects of the
conjugated structures of aldehydes on copolymerization, CHA
was used as a comonomer for IBVE. The copolymerization of
CHA with IBVE with EtSO H/GaCl was almost completed in
number-average molecular weight), by GPC (polystyrene calibra-
a
b
tion)}. For polymerization. For polymer.
3
3
3
with a broad MWD were obtained [M (GPC) = 1.7 × 10 , M /
n
4 h (>90% total monomer conversion; Figure 8). While a small
amount of polymers was produced, low molecular weight
n
w
M (GPC) = 1.49]. This indicates that side reactions in the
1
copolymerization of CC were not transfer, but termination,
compounds were the major components. The H NMR
reactions in contrast to the copolymerization of PA. The 13
C
spectrum of the polymer after the removal of the oligomeric
parts by preparative GPC was similar to that of a homopolymer
of IBVE but also included some weak resonance peaks derived
from CHA units (∼9%, Figure S9).
The copolymerization behaviors of the aldehydes with IBVE
and the properties of the product copolymers are summarized
in Table 2 to illustrate the effects of the side group structures
on the copolymerizations. Three naturally occurring conjugated
aldehydes were incorporated into the product copolymers with
higher aldehyde contents (38−48%) than CHA (9%), an
aliphatic aldehyde. This is probably caused by the difference in
stability among the intermediate cations.
NMR spectra of the products purified by reprecipitation from
methanol showed that repeating CC units in the copolymer
chains formed via the exclusive 1,2-addition reactions of the
1
enal group (Figure S5). The H NMR analysis revealed that the
resulting copolymers had 38−41% CC content (Figure S6).
2.3. Acid Hydrolysis of Poly(PA-co-IBVE) and Poly(CC-co-
IBVE). Acid hydrolysis of poly(PA-co-IBVE) was examined
under the conditions similar to those for poly(myrtenal-co-
IBVE) (Figure 7A). The acid hydrolysis yielded a nearly single
compound, a conjugated aldehyde, as shown in Figure 7A (see
Figures S7 and S8 for the identification), although slight
amounts of products derived from undegraded VE−VE
sequences also existed. Poly(CC-co-IBVE) was also hydrolyzed
Judging from the copolymerization results using myrtenal
and PA, aldehydes with a bulkier side group were favorable for
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suppresses both addition reactions: a CC monomer to an IBVE
44
growing chain end and a CC-derived growing cation to IBVE.
CONCLUSION
■
Cationic copolymerizations of naturally occurring aldehydes,
myrtenal, PA, and CC, with IBVE were examined using the
EtSO H/GaCl initiating system with 1,4-dioxane as an added
3
3
Lewis base. Myrtenal and PA were successfully copolymerized
into nearly alternating copolymers with narrow MWDs under
appropriate reaction conditions, although copolymerization of
CC did not reach quantitative conversion and produced only
oligomers. Interestingly, the three aldehydes reacted with the
growing carbocations selectively via 1,2-carbonyl addition. By
comparison with the case of CHA, an unconjugated aldehyde, it
was found that a conjugated structure was crucial for achieving
higher aldehyde contents in the product copolymers and that a
bulkier side group was also preferred for controlled
copolymerization. In contrast, increased bulkiness around the
carbonyl groups resulted in slower polymerization and
termination reactions.
The obtained alternating copolymers have unique properties
such as highly selective acid-degradability. The copolymer of
myrtenal with an almost perfect alternating sequence can be
degraded quantitatively and selectively to an aldehyde with an
extended conjugated structure. Furthermore, the copolymers
have the potential not only as selectively degradable polymers
but also as recyclable polymers, since the conjugated aldehydes
obtained as the hydrolysis products are also polymerizable.
These features will contribute to the industrial use of these
copolymers as photoresistors or temporary adhesive materials
with low environmental loads.
Figure 8. Molecular weight distribution curve of product by
copolymerization of cyclohexanecarbaldehyde (CHA) with isobutyl
vinyl ether (IBVE) {[CHA] = 0.60 M, [IBVE] = 0.60 M, [EtSO H]
0
0
0
3
=
4.0 mM, [GaCl ] = 4.0 mM, [1,4-dioxane] = 1.0 M, in toluene at
3 0
−
78 °C; M : number-average molecular weight, M /M : polydisper-
n
w
n
sity ratio (weight-average molecular weight/number-average molecular
a
b
weight), by GPC (polystyrene calibration)}. For polymerization. For
polymer. Cyclic trimer ratio in products.
c
controlled reactions. The controllability of the copolymeriza-
tion of myrtenal, an aldehyde with a bicyclic side group, was
better than the copolymerization of PA, a monocyclic aldehyde.
In the latter case, DTBP was required for suppressing proton-
transfer reactions. The bulky cyclic side groups both in
monomers and polymer chains probably suppress the access
of basic counteranions and/or carbonyl groups to β-hydrogen
atoms of the propagating carbocation. Since a monocyclic side
group has less steric bulk, the proton elimination reactions
seem to occur more frequently in the copolymerization of PA
without DTBP compared to the case of myrtenal. CC also has a
bulky side group with three methyl substituents at the β-
positions of its carbonyl carbon. The copolymerization behavior
of CC, however, was distinctly different from those of myrtenal
and PA. In past research, it was found that alkyl substitutions
43
on the α-carbon of aldehydes prevented their polymerization,
most likely due to the steric hindrance of the substituents.
Thus, the steric bulk of the three methyl substituents of CC
Table 2. Copolymerization Behaviors of Aldehydes with Isobutyl Vinyl Ether and Properties of Product Copolymers
a
b
4
c
4
Mearsured in hexane at 25 °C. M (GPC) = 1.90 × 10 , M /M (GPC) = 1.09, aldehyde content 47%. M (GPC) = 1.91 × 10 , M /M (GPC) =
n
w
n
n
w
n
1.20, aldehyde content 45%.
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(
18) Miyake, G. M.; Newton, S. E.; Mariott, W. R.; Chen, E. Y.-X.
ASSOCIATED CONTENT
■
Dalton Trans. 2010, 39, 6710−6718.
*
S
Supporting Information
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Figures showing time−conversion curves, M and M /M −
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w
n
2011, 4, 199−202.
conversion plots of copolymerization, MWD curves, and NMR
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at http://pubs.acs.org.
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́
chez-
AUTHOR INFORMATION
■
*
́
Corresponding Author
E-mail: aoshima@chem.sci.osaka-u.ac.jp.
(23) Aso, C.; Tagami, S.; Kunitake, T. J. Polym. Sci., Part A 1969, 7,
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Notes
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25) Aoshima, S.; Kanaoka, S. Chem. Rev. 2009, 109, 5245−5287.
26) Ishido, Y.; Aburaki, R.; Kanaoka, S.; Aoshima, S. J. Polym. Sci.,
The authors declare no competing financial interest.
(
(
ACKNOWLEDGMENTS
Part A: Polym. Chem. 2010, 48, 1838−1843.
27) Ishido, Y.; Aburaki, R.; Kanaoka, S.; Aoshima, S. Macromolecules
010, 43, 3141−3144.
28) Burgueno-Tapia, E.; Zepeda, L. G.; Joseph,-Nathan, P.
Phytochemistry 2010, 71, 1158−1161.
29) Partal Urena, F.; Aviles Moreno, J. R.; Lop
Phys. Chem. A 2008, 112, 7887−7893.
■
(
2
(
This research was supported in part by a Grant-in Aid for
Scientific Research (No. 22107006) on Innovative Areas of
̃
“
Fusion Materials: Creative Development of Materials and
Exploration of Their Function through Molecular Control”
No. 2206), and a Grant-in-Aid for JSPS Fellows (No. 23-
924) for Y. Ishido from the Ministry of Education, Culture,
Sports, Science and Technology, Japan (MEXT). Y. Ishido
thanks The JSPS Research Fellowships for Young Scientists and
The Global COE Program “Global Education and Research
Center for Bio-Environmental Chemistry” of Osaka University.
We thank Prof. T. Inoue group (Osaka University) for DSC
experiments and Prof. T. Sato group (Osaka University),
especially Dr. K. Terao and Mr. T. Yoshida, for optical rotation
measurements and helpful discussions.
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1
(
7
(
(
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(44) To investigate the polymerization behavior of CC, a three-
component copolymerization of IBVE, myrtenal, and CC was carried
(
(
2
(
5
out ([IBVE]
[EtSO H] = 4.0 mM, [GaCl ]
3 0
= 0.60 M, [myrtenal]
= 0.30 M, [CC] = 0.30 M,
0
0
0
́
e
̌
3
0
= 4.0 mM, [1,4-dioxane] = 1.0 M, in
toluene at −78 °C; Figure S10). The IBVE and myrtenal conversions
were 70% and 69% in 48 h, respectively (CC conversion: 38%; Figure
S10). The three-component copolymerization is much slower than the
copolymerization of myrtenal and IBVE without CC as described
before (more than 90% for both monomers in 2 h; Figure 1A). This
(
́ ̌
ek, J.; Yoon, J. A.; Juhari, A.; Koynov, J.; Matyjaszewski,
(
4
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slower reaction is probably due to the steric hindrance of CC around
the carbonyl group.
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