Renewable biobased polymeric materials: Facile synthesis of itaconic anhydride-based copolymers with poly(l -lactic acid) grafts

Article abstract of DOI:10.1021/ma300387j

This paper reports a new synthesis of biobased polymers by using itaconic anhydride (IAn) and lactic acid (LA) as renewable starting materials. Poly(lactic acid) (PLA)-graft copolymers were synthesized via two approaches. First, the macromonomer approach utilized IAn for Sn-catalyzed synthesis of PLA-containing macromonomers (IAn-PLA Macro). The macromonomer was radically copolymerized with n-butyl methacrylate (BMA), n-butyl acrylate (BA), methyl methacrylate (MMA), and ethyl methacrylate (EMA) to give efficiently graft copolymers (PLA-Graft copolymer (I)) with molecular weight M_n up to 1.61 × 10⁵ having biomass content higher than 34 wt %. Second, the copolymer approach employed first IAn as comonomer for radical copolymerization with BMA, giving rise to IAn-BMA copolymer with M_n higher than 5.76 ×10⁴. Then, Sn-catalyzed grafting of PLA onto IAn moiety of the copolymer produced PLA-Graft copolymer (II) with M_n higher than 5.88 × 10⁴, showing biomass content ≥29 wt %. In addition, radical homopolymerization of IAn was examined to give polyIAn. By using these two approaches employing IAn as a starting reactive material, PLA-graft copolymers were obtained as biomass-plastics. Properties of PLA-Graft copolymers (I) were also examined, which revealed possible applications for coatings and plastics. Furthermore, the IAn-containing graft copolymers will be a convenient starting biomass polymer having reactive IAn moiety in the main chain for further grafting or various functional group-introducing reactions.

Full text of DOI:10.1021/ma300387j

Article
pubs.acs.org/Macromolecules
Renewable Biobased Polymeric Materials: Facile Synthesis of Itaconic
Anhydride-Based Copolymers with Poly(L-lactic acid) Grafts
Tomoya Okuda,† Kiyoaki Ishimoto,† Hitomi Ohara,*,† and Shiro Kobayashi*,‡
†Department of Biobased Materials Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
‡Center for Nanomaterials and Devices, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
S
* Supporting Information
ABSTRACT: This paper reports a new synthesis of biobased
polymers by using itaconic anhydride (IAn) and lactic acid
(LA) as renewable starting materials. Poly(lactic acid) (PLA)-
graft copolymers were synthesized via two approaches. First,
the macromonomer approach utilized IAn for Sn-catalyzed
synthesis of PLA-containing macromonomers (IAn-PLA
Macro). The macromonomer was radically copolymerized
with n-butyl methacrylate (BMA), n-butyl acrylate (BA),
methyl methacrylate (MMA), and ethyl methacrylate (EMA)
to give efficiently graft copolymers (PLA-Graft copolymer (I))
with molecular weight Mn up to 1.61 × 105 having biomass
content higher than 34 wt %. Second, the copolymer approach employed first IAn as comonomer for radical copolymerization
with BMA, giving rise to IAn-BMA copolymer with Mn higher than 5.76 × 104. Then, Sn-catalyzed grafting of PLA onto IAn
moiety of the copolymer produced PLA-Graft copolymer (II) with Mn higher than 5.88 × 104, showing biomass content ≥29 wt
%. In addition, radical homopolymerization of IAn was examined to give polyIAn. By using these two approaches employing IAn
as a starting reactive material, PLA-graft copolymers were obtained as “biomass-plastics”. Properties of PLA-Graft copolymers (I)
were also examined, which revealed possible applications for coatings and plastics. Furthermore, the IAn-containing graft
copolymers will be a convenient starting biomass polymer having reactive IAn moiety in the main chain for further grafting or
various functional group-introducing reactions.
chemicals derived from biomass.33−35 Accordingly, itaconic
INTRODUCTION
■
anhydrides (IAn) belongs to a renewable resource. In polymer
chemistry, radical reactivity of IAn was examined,36−39 and in
polymer synthesis IAn was utilized for radical copolymerization
with methacrylate-terminated PLA macromonomer and with
N-vinyl-2-pyrrolidone for the preparation of a polymeric
surfactant.40,41 The present paper reports a new synthesis
strategy of biobased graft copolymers starting from IAn and LA,
both being typical renewable resources.1−16,33−35
Currently, biobased materials have been paid much attention
due to environmental problems, typically for reducing the
carbon dioxide emission. Consuming fossil fuel resources like
petroleum and coal increases the net amount of carbon dioxide
in the atmosphere and effects the global warming eventually.
To mitigate this tendency, the concept of “carbon neutral” was
created; even though the biobased materials are burned, the
carbon dioxide amount is not increased, because the carbon
source of biobased materials are virtually from the atmosphere
via photosynthesis by plants. In the polymeric materials field, it
is considered very important to perform research on polymer
synthesis using renewable resources as starting materials.1
Poly(lactic acid) (PLA) is one of the most well-known and
extensively studied polymers prepared from renewable
resources.2−16 Synthesis of PLA has been achieved mainly via
two routes; ring-opening polymerization of lactide with a
variety of metal or nonmetal catalysts and direct polyconden-
sation of lactic acid (LA) with acid catalysts.2−16 As part of
investigations in this direction to conduct “green polymer
chemistry”,17−29 we have recently developed a new mini-
emulsion system of PLA-graft copolymers for coating and
elastic materials,30 and a novel oligomerization of alkyl
lactates.31,32 Itaconic acid (IA) is produced in a large scale by
fermentation process and regarded as one of key platform
EXPERIMENTAL SECTION
■
Materials. Itaconic anhydride (IAn, Aldrich Inc.), L-lactide (Purac
Biochem bv, Gorinchem, Holland), n-butyl alcohol (BuOH) (Kanto
Chemical Co.), n-butyl methacrylate (BMA), n-butyl acrylate (BA),
methyl methacrylate (MMA), ethyl methacrylate (EMA), and tin
octoate (Sn(Oct)2) (Nacalai Tesque Inc., Kyoto) were commercial
reagents and used as received. A radical initiator, azobis-
(isobutyronitrile) (AIBN) (Nacalai Tesque Inc.) was a commercial
reagent. Toluene, 1,4-dioxane (DON), chloroform, deuterochloroform
(CDCl3), and other solvents and reagents were commercially available
and used without further purification.
Received: February 23, 2012
Revised: April 10, 2012
Published: April 23, 2012
© 2012 American Chemical Society
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Scheme 1. Whole Reaction Processes via Two Approaches
Model Reactions of IAn with n-Butyl Alcohol. For the 1.0:1.0
molar reaction: A mixture of IAn (1.0 g, 8.9 mmol) and BuOH (0.66 g,
8.9 mmol) with Sn(Oct)2 catalyst (0.073 g, 0.18 mmol) in 1.0 mL of
toluene solvent in a 50 mL round-bottom flask with a refluxing cooler
was reacted at 80 °C for 3 h under nitrogen. Then, the reaction
mixture was dissolved in chloroform, washed with 1 N hydrochloric
acid to remove the catalyst. The product was isolated by drying under
reduced pressure to give 1.50 g of an oily product (90% yields).
Furthermore, IAn is so reactive, and hence, without the catalyst the
1.0:1.0 molar reaction proceeded to give the product in close yields via
similar reaction procedures.
For the 1.0:3.0 molar reaction: A mixture of IAn (1.0 g, 8.9 mmol),
BuOH (1.98 g, 26.7 mmol), and Sn(Oct)2 (0.066 g, 0.16 mmol) in 3.0
mL of toluene solvent in a 50 mL round-bottom flask with a refluxing
cooler was reacted at 60 °C under a reduced pressure from 100 kPa
to10 kPa for 45 min and reacted further at 60 °C for 2 h for giving the
1.0:2.0 adduct. The reaction mixture was dried under reduced pressure
to produce 1.55 g of the product (52% yields based on the total
reactants).
group of methacryloyl-type (IAn-PLA Macro) was synthesized via
one-pot, two-stage reaction. At the first stage, ring-opening polymer-
ization of L-lactide (6.5 g, 45 mmol) by using BuOH (1.1 g, 15 mmol)
initiator and Sn(Oct)2 (0.030 g, 0.075 mmol) catalyst in toluene
solvent (1.5 mL) was carried out in a 50 mL flask at 110 °C for 3 h
under nitrogen. The lactide was consumed quantitatively, evidenced by
1
the H NMR analysis of the reaction mixture.
Subsequently, at the second stage, IAn (5.04 g, 45 mmol) was added
to the reaction mixture to allow the reaction of IAn with the in situ-
formed PLA at 90 °C for 3 h. The product was then dissolved in
chloroform and washed with 1N-hydrochloric acid and further washed
with distilled water. The macromonomer was obtained after
1
evaporation of chloroform to give 8.4 g (92% yields). The H NMR
analysis of the product gave IAn-PLA Macro with m = 5.8 value and
functionality of 98.2%.
Synthesis of PLA-Graft Copolymer (I) via Copolymerization
of IAn-PLA Macro with BMA, BA, MMA or EMA. A typical run
(code 3, Table 2) is given as follows. A mixture of 1.8 mmol (1.0 g) of
IAn-PLA Macro (m = 5.0) and 4.8 mmol (0.68 g) of BMA in 0.66 mL
of toluene was reacted with AIBN initiator (5.0 mol % for the total
monomer) at 70 °C for 24 h under nitrogen. The resulting polymer
Synthesis of Macromonomer (IAn-PLA Macro). A typical run
(code 3, Table 1) is shown.4 Macromonomer having a polymerizable
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was dissolved in chloroform and the solution was poured into n-
hexane to precipitate PLA-Graft copolymer (I), giving 1.48 g (88%
yields) after drying.
IAn-PLA Macro, and then, the synthesis of PLA-Graft
copolymer (I) is accomplished by radical copolymerization of
the Macro with a vinyl monomer via reaction (3). (II)
Copolymer approach employs first radical copolymerization
reaction (4) between the vinylidine group of IAn and BMA to
give IAn-BMA copolymer, followed by grafting reaction (5) of
PLA onto the copolymer to perform the synthesis of PLA-Graft
copolymers (II).
I. Macromonomer Approach. To our best knowledge,
IAn has not been employed for the synthesis of macromonmers
so far. Therefore, we designed to prepare an IAn-PLA
macromonomer by utilizing the reactive nature of IAn with
ring-opening.
Synthesis of IAn-BMA Copolymer. A typical run (code 1, Table
5) is given. A mixture of IAn (0.50 g, 4.5 mmol) and BMA (1.9 g, 13.4
mmol) in 5.0 mL of DON in a two necked 100 mL round flask was
subjected to copolymerize by AIBN initiator (0.03 g, 1.0 mol % for the
total monomer) at 70 °C for 24 h under nitrogen. The DON solution
was poured into a large amount of methanol to precipitate the product
IAn-BMA copolymer, to give 2.1 g (87% yields) after drying.
Synthesis of PLA-Graft Copolymer (II) via Grafting of PLA
onto IAn-BMA Copolymer. A typical run (code 1, Table 6) is shown
as follows. A mixture of IAn-BMA copolymer (1.0 g, 1.9 mmol of IAn
unit), PLA (m = 5.0, 2.41 g, 5.6 mmol units), and Sn(Oct)2 catalyst
(0.030 g, 4 mol % for IAn unit) in DON solvent (3.7 mL) was allowed
to react at 90 °C for 24 h under nitrogen. Then, the reaction mixture
was cooled by using a cold water bath, and the DON solution was
poured slowly to diethyl ether to precipitate the polymeric materials to
give 0.73 g (40% yields) after drying in vacuo.
Model Reaction. Prior to the macromonomer synthesis, an
Sn-catalyzed reaction of IAn with n-butyl alcohol (BuOH) as
shown in Scheme 2 was examined as a model reaction of (2),
Homopolymerization of Itaconic Anhydride (IAn). To a
solution of IAn (2.0 g, 17.8 mmol) in toluene (2.0 mL), AIBN (14.5
mg, 0.09 mmol) was added and the mixture was allowed to react at 70
°C with stirring for 24 h under nitrogen. White powdery precipitates
were formed during the reaction. Then, 20 mL of ethyl acetate was
added to the reaction mixture and the powdery material was obtained
by filtration of the solution with removing the unreacted IAn to give
1.73 g (87% yields) of polyIAn. PolyIAn was soluble in methanol
whereas insoluble in ethyl acetate and chloroform.
Scheme 2. Model Reaction of IAn with BuOH for
Macromonomer Synthesis
Preparation of Cast Film of PLA-Graft Copolymer (I). The
graft copolymer (1.0 g) obtained from BA was dissolved in 14 mL of
chloroform and the solution was placed in a Tefron shale (d = 75
mm). The shale was covered with an aluminum foil and kept standing
for 2 days at room temperature under atmospheric pressure, and the
film was further dried in vacuo to give a cast film of the graft
copolymer.
Analytical Methods. 1H NMR measurements were recorded on a
spectrometer ARX-type (300 MHz, Bruker Co.). CDCl3 was normally
used as solvent.
where BuOH is a model of PLA. In the products of IAn-BuOH,
there are three possibilities, monoesters a and b, and diester c,
depending on the attacked carbonyl position by the BuO
nucleophile. It was also reported previously that the IAn versus
methanol reaction afforded a conjugate carboxylic acid type
product (a) as major component.42
ESI−TOF−MS analysis was performed by using a microTOF
instrument (ESI−TOF−MS) (BRUKER DALTONICS, Germany).
Molecular weight of polymers was measured by a gel permeation
chromatography (GPC) instrument (GL-7400 Series, GL Science Inc.,
Japan) with a refractive index (RI) detector using chloroform eluent at
a column temperature 40 °C, in which polystyrene standards
(molecular weight =2.2 × 103 − 6.5 × 105) were employed. For the
polyIAn sample measurement, DMF solvent containing LiBr (10
mmol/L) was employed with the sample concentration of 10 mg/mL
after membrane-filtration. Measurement conditions: column of 7.8
mm o.d. × 30 cm (TGK-GEL G3000HHR, Tosoh Co., Japan),
Hitachi RI Detector (L-7490, Hitachi Hi-technologies Co.), PMMA
standards, and eluent flow rate of 1.0 mL/min at 40 °C, were used.
Differential scanning calorimetric (DSC) analysis was carried out
using a DSC-50 (Shimadzu Co., Japan) under N2 flow (20 mL/min)
with increasing the temperature at a rate of 10 °C/min with a
temperature range from −90 to +200 °C.
In Figure 1, parts A and B show 1H NMR spectra of IAn and
of the reaction mixture (IAn:BuOH = 1.0:1.0 in the molar
ratio) together with the peaks assignment. Very specific peaks
due to vinylene protons of IAn are observed; Ha proton at δ
6.40 and Hb proton at δ 5.80 in part A. In addition, ring-
methylene protons appear at δ 3.62. In spectrum B of the IAn-
BuOH product, the larger peaks at δ 6.46 and 5.83 are assigned
to Ha and Hb, respectively, ascribed to structure a. The smaller
peaks at δ 6.36 and 5.74 are due to Ha and Hb, respectively, of
product structure b. The peaks intensity ratio was 87:13, which
corresponds to the ratio of product a and product b. Very tiny
peaks at δ 6.50 and 5.88 are due to free IA produced as a side-
product.
Physical properties of the film were measured on an Autograph
CATY500BH (Yonekura Co., Japan) with a strain application change
rate of 2 mm/s at room temperature for the film length of 35 mm and
width of 5 mm.
In addition, the reaction without catalyst occurred in a similar
way to give similar products, whose peak intensity ratio. i.e., the
product ratio, was a:b = 89:11.
When the reaction of IAn and BuOH was carried out with
the 1.0:3.0 molar ratio, H NMR spectrum of the product
1
RESULTS AND DISCUSSION
■
Itaconic anhydride (IAn) contains two reactive groups, an acid
anhydride ring group and a vinylidine group. The present study
utilizes these natures, and thus, the outline is shown by five
reactions (1)−(5) in Scheme 1. The outline is constituted from
two approaches. (I) Macromomomer approach involves a one-
pot, two-stage reaction method utilizing first the BuOH−lactide
reaction (1) to give PLA, followed by reaction (2)of PLA with
the acid anhydride group of IAn via ring-opening to lead to
showed new small peaks at δ 6.32 and 5.68, in addition to the
above similar major peaks (Figure 1C). The peaks were
reasonably assigned to Ha and Hb of a diester species of
structure c, whose content is 4%. The product ratio is a:b:c =
85%:11%:4%.
Instead of BuOH, EtOH was employed with the 1.0:1.0
molar reaction, and an adduct product ratio of 89:11 was
observed, indicating a close ratio to that of BuOH.
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Figure 2. 1H NMR spectra of (A) PLA and (B) IAn-PLA Macro (m =
5.8).
Macro) involving ring-opening of IAn. Using 2−3 molar excess
1
amount of IAn was preferred. Figure 2B shows H NMR
spectrum of the product IAn-PLA Macro. Similarly to the
model reaction in Scheme 2, vinylidine protons showed two
peaks. Taking the results of the model reactions into account,
larger peaks at δ 6.48 and 5.89 were ascribed to Ha and Hb,
respectively, of the conjugate carboxylic acid type a in Scheme
2. Smaller peaks at δ 6.46 and 5.83 were assigned at Ha and Hb,
respectively, of the nonconjugate type b. The ratio of a and b is
87:13. IAn-PLA Macro has the structure corresponding to type
a as the main (at least, > 85% content) and hence its structure
is hereafter formulated as in reaction 2. The terminal-methine
peak b at δ 4.35 in Figure 2A completely disappeared after the
reaction in Figure 2B, indicating a quantitative reaction to give
functionality of 100%, which means the vinylidine group
located quantitatively at the PLA end. In addition, −CO2H
protons of the macromonomer appear very broadly at δ 8−12.
Synthesis results of IAn-PLA Macro under various reaction
conditions are given in Table 1. By varying the feed ratio of
BuOH and L-lactide, m value can be tuned. The reaction yields
were almost quantitative and functionalty of ∼100% was
realized in all cases. For reference, biomass content (weight of
PLA and IAn components) × 100/(total weight of the Macro)
is also recorded.
ESI−TOF−MS trace of IAn-PLA Macro (sample code 3 in
Table 1, m = 5.8) is given in Figure 3, indicating that the Macro
contains the PLA oligomers with a peak top at m = 6 in every
72 m/z interval of the repeat mass. Not only even numbered
oligomers and polymers (m = 2, 4, 6, 8, 10, 12, 14 and 16) but
also odd numbered ones (m = 3, 5, 7, 9, 11, 13 and 15) are
formed via the Sn-catalyzed ring-opening polymerization of L-
lactide. These results show that the Sn-catalyzed polymerization
of lactide induced the extensive occurrence of transesterification
during the reaction, as discussed on the mechanistic
aspects.43−45 In particular, such transesterification became
noticeable after a relatively longer reaction time which was
enough to reach the monomer-polymer equilibrium concen-
1
Figure 1. H NMR spectra of (A) IAn, (B) the reaction product of
IAn:BuOH = 1.0:1.0, and (C) the reaction product of IAn:BuOH =
1.0:3.0.
From these results, the reaction of IAn and an alcohol in the
1.0:1.0 molar ratio produces a type adduct as the major and b
type as the minor product. But c type diester adduct, a
transesterification product, was not detected via the 1.0:1.0
reaction.
ESI−TOF−MS (negative) analysis of the 1.0:1.0 reaction
product showed the main peak at m/e 184 corresponding to the
value of an adduct of IAn-BuOH (186 − H+), supporting a
clean reaction of Scheme 2.
Synthesis of IAn-PLA Macromonomer (IAn-PLA Macro).
IAn-PLA Macro was synthesized via one-pot, two-stage reaction
method. At the first stage, according to reaction (1) in Scheme
1, poly(L-lactic acid) (PLA) was prepared by the well-known
procedure; Sn-catalyzed ring-opening polymerization of L-
lactide with using BuOH as initiator.2−16,30 The m value of
PLA, degree of polymerization, can be tuned with varying the
molar ratio of BuOH and L-lactide in the feed.
In the case of feed ratio of BuOH: L-lactide =1.0:3.0, after 3 h
at 110 °C, a small potion of the reaction mixture was taken out
and the m value of the product PLA was determined by H
1
NMR analysis as m = 5.8 from the integrated peak ratio of (a +
b)/b as demonstrated in Figure 2A.30
At the second stage, reaction (2) was consecutively allowed
to proceed by adding IAn to the reaction mixture, in which IAn
was reacted with the in situ-produced PLA catalyzed by
Sn(Oct)2 at 90 °C for 3 h to give a macromonomer (IAn-PLA
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Table 1. Synthesis of IAn-PLA Macromonomer (IAn-PLA Macro)
a
reaction feed
product IAn-PLA Macro
b
b
b
code
BuOH:lactide:IAn (molar ratio)
catalyst (mmol)
toluene (mL)
yield (%)
m value
functionality (%)
biomass content (wt %)
1
2
3
4
5
1.0:2.5:3.0
1.0:3.0:2.0
1.0:3.0:3.0
1.0:3.0:3.0
1.0:6.0:3.0
0.075
0.075
0.075
0.075
0.15
1.5
5.0
1.5
1.5
1.5
96
90
92
92
95
5.0
5.6
100.1
99.8
86
87
88
88
5.8
98.2
6.0
100.2
12.2
100.3
93
a
b
Amount of BuOH was 1.1 g (15 mmol). Reaction at the first stage at 110 °C for 3 h and at the second stage at 90 °C for 3 h. Determined by 1H
NMR.
support this speculation, in the ESI−TOF−MS trace of PLA
the corresponding similar small peaks due to the dimer
formation were not observed, since PLA molecules do not
contain −CO2H group.
Synthesis of PLA-Graft Copolymer (I). According to reaction
(3) of the graft copolymer synthesis, n-butyl methacrylate
(BMA) was first employed as comonomer.30 The PLA-graft
copolymer (I) was produced by radical copolymerization of
1
IAn-PLA Macro with BMA. H NMR spectrum of product
copolymer I (code 3, Table 2) is demonstrated in Figure 4.
Figure 3. ESI−TOF−MS (negative mode) trace of IAn-PLA Macro
(m = 5.8).
1
Figure 4. H NMR spectrum of PLA-Graft copolymer (I) produced
tration under the reaction conditions like code 3 in Table 1.46
In addition, very small ten peaks (m/e from 874 to 1522 at
every 72 interval) are due to the dimer formation of the each
molecule of IAn-PLA Macro during the measurement, probably
because the molecule has −CO2H group to form a dimer. To
via radical copolymerization from combination of IAn-PLA Macro (m
= 5.0) and BMA.
Table 2. Synthesis of PLA-Graft Copolymer (I) via Copolymerization of IAn-PLA Macro with Comonomer BMA
a
copolymerization feed
product PLA-Graft copolymer (I)
a
Macro, mmol
(m value)
BMA
reaction
solvent
amount of
AIBN
Macro:BMA
biomass content
b
b
c
c
code
(mmol)
solvent (mL)
(mol %)
yield (%)
Mn
PDI
5.0
(molar ratio)
(wt %)
1
2
0.90 (5.0)
0.90 (5.0)
1.80 (5.0)
1.80 (5.0)
1.80 (5.0)
1.80 (5.0)
1.80 (5.0)
2.60 (5.8)
0.94 (12.2)
0.94 (12.2)
0.90 (5.0)
0.90 (5.0)
0.90 (5.0)
3.20 (6.0)
2.8
4.7
4.8
8.0
8.0
4.8
8.0
7.8
2.8
4.7
2.8
4.5
4.5
9.6
bulk
0
5.0
5.0
5.0
5.0
1.0
1.0
1.0
1.0
5.0
5.0
2.5
2.5
5.0
5.0
80
83
88
87
83
84
90
92
72
90
67
42
62
79
35 500
1.0:2.4
1.0:4.0
1.0:3.4
1.0:5.9
1.0:5.5
1.0:2.3
1.0:4.2
1.0:2.5
1.0:2.5
1.0:4.5
1.0:2.5
1.0:4.7
1.0:4.4
1.0:2.9
53
42
46
34
36
54
41
55
70
58
52
39
40
53
d
d
bulk
0
110 000
2.1
2.0
1.9
2.3
3.2
2.2
3.8
1.7
3.1
2.2
2.2
2.2
2.0
3
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DON
0.66
0.98
0.98
2.64
3.92
1.04
0.75
1.12
0.37
1.08
0.54
1.28
35 000
28 600
45 200
36 100
29 600
71 000
60 700
54 700
11 800
4
5
6
7
8
9
10
11
12
13
14
d
d
DON
14 600
DON
11 100
37 600
toluene
a
Copolymerization at 70 °C with AIBN initiator in bulk, in toluene or in 1,4-dioxane (DON) for 24 h under N2. Amount of AIBN for the total
b
c
d
monomer in mol %. Determined by GPC; PDI (polydispersity index). Determined by 1H NMR. GPC trace is given in Supporting Information.
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Table 3. Synthesis of PLA-Graft Copolymer (I) via Copolymerization of IAn-PLA macro with Comonomer BA
a
copolymerization feed
product PLA-Graft copolymer (I)
a
Macro, mmol
(m value)
BA
(mmol)
reaction
solvent
amount of
solvent (mL)
AIBN
Macro:BA
(molar ratio)
biomass content
b
b
c
c
code
(mol %)
yield (%)
Mn
PDI
(wt %)
1
2
3
4
5
6
7
8
1.60 (5.8)
1.60 (5.8)
1.60 (5.8)
1.60 (5.8)
1.60 (5.8)
0.94 (12.2)
0.94 (12.2)
3.20 (6.0)
5.3
8.9
5.3
5.3
8.9
3.1
5.2
9.6
bulk
0
5.0
1.0
79
56
78
89
60
65
85
96
14 000
79 600
2.7
3.3
1.0:2.5
1.0:4.1
1.0:2.0
1.0:2.3
1.0:4.0
1.0:2.4
1.0:3.7
1.0:2.4
56
46
62
57
46
70
62
59
bulk
0
d
d
toluene
toluene
toluene
toluene
toluene
toluene
0.69
0.69
1.05
0.51
0.77
1.28
10.0
5.0
21,900
3.7
2.0
2.4
2.2
4.6
4.8
40 600
25 000
5.0
1.0
72 000
1.0
102 000
161 000
1.0
a
Copolymerization at 70 °C with AIBN initiator in bulk or in toluene for 24 h under N2. Amount of AIBN for the total monomer in mol %.
b
c
d
1
Determined by GPC; PDI (polydispersity index). Determined by H NMR. GPC trace is given in the Supporting Information.
Table 4. Synthesis of PLA-Graft Copolymer (I) via Copolymerization of IAn-PLA Macro with Comonomers MMA and EMA
a
copolymerization feed
product PLA-Graft copolymer (I)
a
a
b
b
c
c
code M(E)MA (mmol) toluene (mL) AIBN (mol %) yield (%)
Mn
MMA
PDI
Macro:M(E)MA (molar ratio)
biomass content (wt %)
1
2
0.83
1.66
0.83
1.66
0.83
1.66
0.83
1.66
1.66
1.66
0.33
0.50
0.66
1.00
0.33
0.50
0.66
1.00
0.50
1.00
1.0
1.0
1.0
1.0
2.5
2.5
2.5
2.5
5.0
5.0
77
79
73
87
79
62
75
77
79
78
50 000
27 000
35 000
19 300
34 200
21 300
16 200
15 000
20 100
13 000
3.1
3.5
2.8
2.2
3.5
2.7
1.7
1.8
2.5
1.9
1.0:1.2
1.0:2.1
1.0:1.0
1.0:2.0
1.0:1.3
1.0:1.4
1.0:1.2
1.0:2.0
1.0:1.9
1.0:2.2
73
65
75
66
72
71
73
66
67
64
3
4
5
6
7
8
9
10
EMA
11
12
2.49
4.15
0.66
1.00
1.0
1.0
88
80
35 200
27 400
4.8
3.0
1.0:2.3
1.0:3.5
61
53
a
Copolymerization at 70 °C with AIBN initiator in toluene for 24 h under N2. Feed amount of 0.83 mmol of IAn-PLA Macro (m = 5.8) was used in
b
c
1
all runs. Amount of AIBN for the total monomer in mol %. Determined by GPC; PDI (polydispersity index). Determined by H NMR.
From the integration ratio of specific peak b (δ 4.14) and peak
c (δ 3.94), the Macro unit and BMA unit are contained in the
ratio of 1.0:3.4, which indicates little higher radical reactivity of
BMA than the Macro. In Figure 4, assignments of other signals
of the copolymer are also given.
regard all the PLA-Graft copolymers possess biomass content
higher than 34 wt %, and hence, they are categorized as
“biomass-plastics”.30
Further, PLA-Graft copolymer (I) synthesis via copolymer-
ization of IAn-PLA Macro with BA as comonomer according to
reaction (3) has been carried out; the results are given in Table
3. In bulk copolymerization of BA, 5.0 mol % of AIBN gave a
lower molecular weight PLA-Graft copolymer (code 1), while
1.0 mol % of AIBN produced higher molecular weight
copolymers (code 2). Then, the copolymerization was
performed in toluene solution (codes 3−7). The macro-
monomer with m = 12.2 exhibited a similar reactivity to that
with m = 5.8.
Likewise, copolymerization of PLA-Graft Macro (m = 5.8)
with MMA and EMA was performed to produce PLA-Graft
copolymer (I) with feed molar ratio from 1.0:1.0 to 1.0:5.0
(Table 4). In all runs for BA, MMA, and EMA, the graft
polymers having Mn = 13 000 (MMA) to 161 000 (BA) were
obtained in good yields.
Results of synthesis of PLA-Graft copolymer (I) under
various conditions are given in Table 2 using BMA as
comonomer. In all runs, PLA-Graft copolymers were obtained
in high yields in bulk or in toluene having higher molecular
weight (Mn > 2.8 × 104, up to 1.1 × 105), while in DON
resulting in lower copolymer yields having lower molecular
weight. Both IAn-PLA Macro and BMA are of methacryloyl-
type structure; the former showed little less radical copoly-
merization reactivity toward BMA in all reactions, judging from
the feed monomer ratio and the product copolymer
composition ratio. It is to be noted that all the final isolated
copolymers (I) showed a unimodal peak and were not
contaminated with other homo- or copolymers by GPC
analysis. Two examples of GPC traces are found in Supporting
Information.
1H NMR spectra of PLA-Graft copolymers of BA (code 6, m
= 12.2, Table 3), MMA (code 8, m = 5.8, Table 4), and EMA
(code 12, m = 5.8. Table 4) are found in the Supporting
Information as parts 1−3, respectively. These spectra fully
support the structures of the copolymers, the peak assignment
of which are indicated in the figures.
Values of biomass content (wt %) of PLA-Graft copolymers
was calculated from the relationship; (weight of LA and IAn
components) × 100/(total weight of the copolymer).
According to the definition of Japan BioPlastics Association
proposed in 2006, “biomass plastics” denote the plastics
containing the biomass content higher than 25 wt %. In this
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The macromonomer is of methacryloyl-type structure and
BA is, of course, of acryloyl-type, and hence, the former showed
a little more reactive in radical copolymerizability, speculating
from the feed ratio and the copolymer composition ratio. In the
copolymerization between the Macro and MMA, both being of
methacryloyl-type, MMA is higher in copolymerizability.
The biomass content of PLA-Graft copolymers (I) is all
higher than 46% with the BA graft-copolymers and also higher
than 64 wt % with MMA graft-copolymers. Therefore, it is to
be stressed that graft copolymers of Tables 3 and 4 are all
belonging to “biomass-plastics”.
II. Copolymer Approach. By using IAn as comononer, it is
also possible to synthesize a similar type of PLA-Graft
copolymer.
Synthesis of IAn-BMA Copolymer. According to radical
copolymerization (4), an IAn-BMA copolymer was first
prepared with varying the feed ratio. Some results of the
copolymerization are given in Table 5. IAn-BMA copolymers
were obtained in good yields and the molecular weight Mn
reaching to 1.1 × 105 with PDI value of 1.8.
1
The H NMR spectrum of product (code 1, Table 5) is
shown in Figure 5A. A peak at δ 4.1−3.9 is ascribed to −OCH2
Table 5. Synthesis of IAn-BMA Copolymers via
Copolymerization of IAn with BMA
Figure 5. 1H NMR spectra of (A) IAn-BMA copolymer and (B)PLA-
Graft copolymer (II).
a
copolymerization feed
product IAn-BMA copolymer
IAn:BMA
IAn
BMA
DON yield
code (mmol) (mmol) (mL)
(molar
ratio)
b
b
c
(%)
Mn
PDI
at δ 4.36 due to the terminal methine proton of PLA (Figure
2A) disappeared completely, indicating no starting PLA
remained in the isolated graft copolymer. This was also
confirmed by observing a unimodal peak by GPC analysis (see
Supporting Information).
These graft copolymers (II) belong to biomass plastics,
because of biomass content ≥29 wt %.
1
2
3
4.5
4.5
13.4
22.3
5.0
5.0
87
95
88 800
1.6
1.8
1.0:3.5
1.0:5.4
1.0:2.9
110 000
d
e
e
53.6
161
30.0
54
57 600
1.8
a
Copolymerization at 70 °C with AIBN initiator (amount = 1.0 mol %
for the total monomer) in 1,4-dioxane (DON) for 24 h under N2.
b
c
Determined by GPC:PDI (polydispersity index). Determined by 1H
d
NMR. For the precipitation of the product, diethyl ether was used
instead of methanol. GPC trace is given in the Supporting
Information.
e
So far, grafting reaction (5) was less effective in terms of the
reaction selectivity for the grafting; under the reaction
conditions the grafting at 90 °C took place in 33 mol % for
the IAn unit to give PLA-G unit in the PLA-Graft copolymer
(II). At a lower reaction temperature of 70 °C, only 18 mol %
of IAn unit was grafted (code 2). This relatively low efficiency
of the grafting is probably due to the following reason as
generally observed. The grafting is a polymer−polymer reaction
like reaction (5) between IAn-BMA copolymer and PLA, which
is normally harder than a polymer-monomer reaction like
reaction (3) between IAn-PLA Macro and a monomer.
It is to be noted that IAn-BMA copolymers contain a reactive
anhydride moiety in the main chain and hence will be a good
starting polymer candidate for introduction of various graft
chains as well as functional groups. Present graft copolymers
are soluble in an organic solvent and form a film, while their
detailed property study needs further examinations.
Synthesis of Poly(itaconic anhydride). In relevant to the
main subject of the present study to synthesize the graft
copolymers by using IAn, radical homopolymerization of IAn
was examined (Scheme 3). To date, polymerization of IAn has
been performed mainly from the viewpoint of radical
polymerizability, and product polyIAn itself was not well
characterized but methylated or modified polyIAn was
mentioned.36−39 The reaction of Scheme 3 proceeded
homogeniously in DON, whereas precipitates were formed in
toluene or in ethyl acetate. In toluene white powdery polyIAn
in BMA unit and a peak at δ 2.8−2.6 is to −CH2C(O) group
in IAn ring unit. From the integration ratio of these peaks,
molar ratio in the copolymer was determined as 1.0: 3.5. Other
peaks assignments are indicated in the spectrum, supporting the
copolymer structure. Radical copolymerizability between IAn
and BMA is comparable judging from the molar ratio value of
the feed and of the product.
Synthesis of PLA-Graft Copolymer (II). Grafting the PLA
chain onto IAn-BMA copolymer of code 3 (Table 5) was
performed by Sn-catalyzed reaction of PLA to afford PLA-Graft
copolymer (II) via reaction (5). The grafting reaction results
are given in Table 6. The copolymer yield was moderate, which
was partly due to the difficulty of the complete separation of the
starting PLA and the product graft copolymer. The molecular
weight of Mn value 5.76 × 104 was little increased to 5.88 × 104
after the grafting.
The 1H NMR spectrum of the copolymer (II) (code 1, Table
6) is shown in Figure 5(B). The grafting content was evaluated
by a peak δ 5.16 due to methine proton in PLA-G unit and by a
peak δ 2.8−2.6 due to −CH2 in IAn unit to be 33 mol % of IAn
unit of the starting copolymer. Other peak assignments are
given in the spectrum, reasonably supporting the graft
copolymer structure. In the spectrum a quartet peak centered
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Table 6. Synthesis of PLA-Graft Copolymer (II) via Grafting of PLA onto IAn-BMA Copolymers
a
grafting reaction of PLA
reaction (°C)
product PLA-Graft copolymer (II)
b
b
c
code
Sn(Oct)2 catalyst (mmol)
yield (%)
Mn
PDI
PLA- G:IAn:BMA (molar ratio)
biomass content (wt %)
d
d
1
2
90
70
0.075
0.075
40
54
58 800
59 600
1.4
1.3
0.33:0.67:2.9
0.18:0.82:2.9
35
29
a
Grafting reaction of PLA (m = 5.0, 2.41 g, 5.6 mmol) onto IAn-BMA copolymer of code 3 in Table 5 (1.0 g, containing 1.9 mmol of IAn units,
b
c
IAn:BMA = 1.0:2.9) in 3.7 mL of DON with Sn(Oct)2 catalyst for 24 h under N2. Determined by GPC; PDI (polydispersity index). Determined
by H NMR. GPC trace is given in the Supporting Information.
d
1
III. Properties of PLA-Graft Copolymers. PLA-Graft
copolymers (I) and (II) are all soluble in an organic solvent like
chloroform. As a typical example, a transparent film was
prepared by casting from PLA-Graft copolymer (I) sample
derived from BA (m = 6.0, code 8 of Table 3) of a chloroform
solution. The sample is of high molecular weight (Mn = 161
000) having Tg value of 11.2 °C with high biomass content of
59 wt % and showed a very good elastic property as shown by
the following data: Young’s modulus, 316 kgf/cm2; tensile
strength, 33.7 kgf/cm2; and elongation at break, 496.1%. These
results suggest that the copolymers can be applied for usages of
coatings, soft films, etc.30
Scheme 3. Synthesis of Poly(itaconic anhydride)
was obtained as isolated yields of 87%. Molecular weight was
relatively lower as Mn 1,400, showing Tm = 146.8 °C. A part of
IAn monomer remained unreacted in DON and ethyl acetate.
The synthesis results are given in Table 7. A previous paper
With using BMA as comonomer, on the other hand, the graft
copolymer (I) (code 14 of Table 2), having Mn = 37 600 with
biomass content of 53 wt %, showed a Tg value of 27.0 °C. The
copolymer sample is very hard while very brittle as
demonstrated by a value of elongation at break, 101.3%. It is
understandable from the monomer structure that both the
Macro and BMA have an α,α-disubstituted structure of CH2
CRR′, and moreover, the Macro contains a bulky PLA group of
R = CH2C(O)PLA, and then, the resulting copolymer
should have a main-chain with condensed packing, bringing
about nonflexible polymeric materials. Therefore, the copoly-
mers from BMA may find applications in hard plastic materials
area requiring a tough nature which is probably accomplished
via cross-linking reaction.
Table 7. Synthesis of polyIAn via Radical
Homopolymerization
product polymer of
a
feed for homopolymerization of IAn
polyIAn
IAn g
(mmol)
AIBN
(mmol)
yield
(%)
b
b
code
solvent (mL)
Mn
PDI
1.2
1
2
3
2.0 (18)
10.0 (89)
3.0 (27)
0.09
0.90
0.27
toluene (2.0)
DON (22.0)
87
28
10
1400
−
−
−
−
ethyl acetate
(6.7)
a
b
Reaction at 70 °C for 24 h under N2 in all runs. PDI (polydispersity
index) determined by GPC.
Further, graft copolymers (I) and (II) possess a −CO2H
group in the main chain, and hence, they are regarded as
derivative polymers of methacrylic acid. Their applications to
this direction will also be conceivable.
described that bulk polymerization gave polyIAn up to 49%
yields with higher molecular weight after methylation (∼2.0 ×
104), whereas solution polymerization gave a lower molecular
weight polymer.37
1H NMR spectrum of polyIAn has not been shown to date,
while the present polyIAn clearly exhibits a structure of vinyl
type polymer from IAn, in which two methylene peaks of the
ring and the main chain broadly appear centered at around δ
2.9 and δ 2.2, respectively as shown in Figure 6.
CONCLUSION
■
We have demonstrated for the first time that itaconic anhydride
(IAn) is conveniently used as renewable monomer for the
synthesis of biobased polymeric materials of poly(lactic acid)
(PLA)-graft copolymers. For accomplishing the purpose, two
methods, Macromonomer Approach and Copolymer Ap-
proach, were introduced. First, the methacryloyl-type polymer-
izable PLA macromonomer (IAn-PLA Macro) was derived
from IAn and its copolymerization with BMA, BA, MMA or
EMA gave PLA-Grafted copolymers (I) with biomass content
higher than 34%. It is possible that the macromonomers can be
copolymerized not only with these monomers but also with
other various vinyl monomers for developing the PLA graft
copolymers depending on the usage of the products. Second,
IAn was copolymerized with a vinyl monomer to provide
various copolymers like IAn-BMA copolymer. Grafting of PLA
onto the copolymer gave PLA-graft copolymer (II) having
biomass content higher than 29%. These two approaches
employing IAn as a starting reactive material produced a variety
of PLA-graft copolymers as “biomass-plastics” possessing
various potential applications. Actually, a typical example
copolymer demonstrated an excellent elastic nature. Further
Figure 6. 1H NMR spectrum of polyIAn in methanol-d4. Peaks with an
asterisk indicate those due to ethyl acetate remaining in the sample.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Three 1H NMR spectra of PLA-Graft copolymers (I) produced
via radical copolymerization are given for combinations: (1)
IAn-PLA Macro (m = 12.2) and BA, (2) IAn-PLA Macro (m =
5.8) and MMA, and (3) IAn-PLA Macro (m = 5.8) and EMA,
along with (4) five GPC traces of copolymers. This material is
available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ohara@kit.ac.jp (H.O.); kobayash@kit.ac.jp (S.K.).
Notes
The authors declare no competing financial interest.
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