Synthesis and characterization of copolyanhydrides of carbohydrate-based galactaric acid and adipic acid

Article abstract of DOI:10.1016/j.carres.2014.07.009

A series of copolyanhydrides, consisting of 2,3,4,5-tetra-O-acetylgalactaric acid (AGA) and adipic acid (AA) as monomer units, was polymerized. Synthesis of AGA monomer consisted of two steps. First, O-acetylation of galactaric acid secondary hydroxyl groups was performed using acetic anhydride as a reagent. Acetic anhydride was then further used as a reagent in the synthesis of diacetyl mixed anhydride of AGA. Polymerizations were conducted as bulk condensation polymerization at 150 °C. Thermal properties of the copolymers varied depending on monomer composition. Increase in the AGA content had a clear increasing effect on the T_g. A similar increasing effect was observed in T_m. The degree of crystallinity decreased as AGA content increased. There was a slightly lowering tendency in the molecular weights of the obtained polymers when the AGA content in the polymerization mixtures increased. The described synthesis route shows that bio-based aldaric acid monomers are potential candidates for the adjustment of thermal properties of polyanhydrides.

Full text of DOI:10.1016/j.carres.2014.07.009

Accepted Manuscript
Synthesis and Characterization of Copolyanhydrides of Carbohydrate-based
Galactaric Acid and Adipic Acid
Tuomas Mehtiö, Leena Nurmi, Virpi Rämö, Hannu Mikkonen, Ali Harlin
PII:
S0008-6215(14)00286-9
DOI:
http://dx.doi.org/10.1016/j.carres.2014.07.009
CAR 6802
Reference:
To appear in:
Carbohydrate Research
Received Date:
Revised Date:
Accepted Date:
13 May 2014
26 June 2014
11 July 2014
Please cite this article as: Mehtiö, T., Nurmi, L., Rämö, V., Mikkonen, H., Harlin, A., Synthesis and Characterization
of Copolyanhydrides of Carbohydrate-based Galactaric Acid and Adipic Acid, Carbohydrate Research (2014), doi:
http://dx.doi.org/10.1016/j.carres.2014.07.009
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Synthesis and Characterization of Copolyanhydrides of Carbohydrate-based
Galactaric Acid and Adipic Acid
1
Tuomas Mehtiö , Leena Nurmi , Virpi Rämö , Hannu Mikkonen , Ali Harlin
1
1
1
2
1
VTT Technical Research Centre of Finland, Patruunantie 19, FI-05200, Rajamäki,
Finland.
2
VTT Technical Research Centre of Finland, Biologinkuja 7 P.O Box 1000, FI-
2044, VTT, Espoo, Finland.
0
corresponding author: tuomas.mehtio@vtt.fi
ABSTRACT
A series of copolyanhydrides, consisting of 2,3,4,5-tetra-O-acetyl-galactaric acid
(
AGA) and adipic acid (AA) as monomer units, was polymerized. Synthesis of AGA
monomer consisted of two steps. First, O-acetylation of galactaric acid secondary
hydroxyl groups was performed using acetic anhydride as a reagent. Acetic anhydride
was then further used as a reagent in the synthesis of diacetyl mixed anhydride of
AGA. Polymerizations were conducted as bulk condensation polymerization at 150
°
C. Thermal properties of the copolymers varied depending on monomer
composition. Increase in the AGA content had a clear increasing effect on the T
Similar increasing effect was observed in T . The degree of crystallinity decreased as
g
.
m
AGA content increased. There was a slightly lowering tendency in the molecular
weights of the obtained polymers when the AGA content in the polymerization
mixtures increased. The described synthesis route shows that bio-based aldaric acid
monomers are potential candidates for the adjustment of thermal properties of
polyanhydrides.
Keywords: polyanhydrides, biodegradable, polycondensation, renewable resources,
synthesis, biopolymers
1

1. Introduction
The past decade has seen a dramatic increase in the research of bio-based materials.
Diminishing fossil fuel resources and growing environmental concerns have increased
the interest towards biorefinery concepts, where renewable resources offer a wide
1, 2
variety of molecules that can be further upgraded into useful compounds.
Development steps taken in the field of industrial biotechnology have provided access
3
-6
to new types of monomeric carbohydrate derivatives. One particularly interesting
class is aldaric acids, as they contain two terminal carboxylic acid functionalities,
which make them suitable for use in condensation polymerization as rigid monomers.
Galactaric acid, often also called mucic acid, (Figure 1) is an aldaric acid that can be
7
produced from D-galacturonic acid by engineered filamentous fungi. D-galacturonic
acid is abundant e.g. in pectin, which is common in primary cell wall and middle
lamella of all higher plants. Pectin-rich residues are obtained as side streams e.g. from
sugar beet processing as well as from apple and orange juice production. These
residues are commonly utilized as animal feed and it would be beneficial to find
8
higher-value products for these side streams.
Many types of polymers have already been synthesized from aldaric acid based
monomers. Production of polyamides from aldaric acids and alkylenediamines has
9
-11
12-14
been studied extensively by Ogata et al.
and more recently by Kiely et al.
In
these studies the aldaric acids monomers were applied as such, without protection of
the secondary hydroxyl groups. On the other hand, Muñoz-Guerra and Galbis et al.
have extensively applied synthesis routes where the secondary hydroxyl groups were
transformed to e.g. methyl ethers or acetals to ensure production of linear polymers.
1
5-18
19-21
They have synthesized aldaric acid based polyesters,
24
polyamides,
and polyester amides. The utilization of aldaric acids in synthesis
2
2, 23
polyurethanes
2
5
26
of polydimethylsiloxanes and polycarbonates has also been reported. However,
according to our knowledge, preparation of aldaric acid based polyanhydrides has not
been previously reported.
Figure 1. Galactaric acid.
Polyanhydrides are extensively studied materials for clinical applications. Due to
their rapid hydrolytic degradation their main application is on controlled short-term
2
7, 28
delivery of bioactive agents.
from diacids is to first produce a diacetyl mixed anhydride of the diacid,
The most common way to synthesize polyanhydrides
7, 28
2
which is
then further polymerized through polycondensation. Properties of polyanhydrides
highly depend on the nature of the repeating units. Short aliphatic dicarboxylic acids,
e.g. adipic acid, provide polyanhydrides with low melting points and relatively rapid
2
9-31
hydrolytic degradation rates.
moieties into the polyanhydride structure provides increased melting points and
decreased degradation rates.
On the other hand, incorporating rigid aromatic
32
In this article we describe the synthesis of O-acetylated galactaric acid (AGA), and its
utilization in the polymerization of a series of copolyanhydrides with adipic acid
(
AA). The O-acetylation step was performed through reaction of the galactaric acid
2

secondary hydroxyl groups with acetic anhydride to give 2,3,4,5-tetra-O-
acetylgalactaric acid. Through a synergistic fashion, acetic anhydride was then further
used in the transformation of the AGA diacid to the diacetyl mixed anhydride
monomer. Our target was to observe how the rigid AGA moiety affects the
copolymerization reaction and the properties of the obtained copolymers. Based on
the study, we regard bio-based aldaric acid monomers as potential candidates for the
adjustment of polyanhydride properties.
2. Results and discussion
A series of copolyanhydrides, consisting of 2,3,4,5-tetra-O-acetyl-galactaric acid
AGA) and adipic acid (AA) as monomer units, was synthesized according to Scheme
. The monomer unit composition was varied throughout the series in order to observe
(
1
how the rigid AGA moiety affects the polymerization reaction and the properties of
the obtained copolymers. As presented in scheme 1, the acetylation of secondary
hydroxyl groups of galactaric acid (1) was performed using acetic anhydride as a
reagent. Acetic anhydride was further used as a reagent in the synthesis of diacetyl
mixed anhydride (3). Utilization of acetic anhydride in both of the monomer synthesis
steps offers synergistic advantages to the synthesis route, and these reaction steps
could be even considered to be combined into a more continuous process in future.
Scheme 1. Reaction scheme from galactaric acid to poly(2,3,4,5-tetra-O-acetyl-
galactaric-co-adipic anhydride).
2.1. Monomer synthesis
2,3,4,5-tetra-O-acetyl-galactaric acid (AGA) (2) was synthesized from commercially
available galactaric acid (1) in acidic conditions through reaction with acetic
3
3-34
anhydride.
The reaction was performed under relatively mild conditions using
acetic acid as a reaction medium. As both galactaric acid and AGA were poorly
soluble in the reaction medium, the reaction proceeded as a two phase system. After
recrystallization from water the desired product was obtained as needle-like white
1
13
crystals. H and C NMR characterizations were conducted in DMSO-d
6
at 70 °C and
the obtained spectra are presented in Figure 2. Peaks shown at 2.14 ppm and 2.26 ppm
1
in the H NMR spectrum as well as two peaks at 20.6 ppm in the C NMR spectrum
13
1
3
are distinctive to O-acetyl CH
3
groups, indicating successful acetylation. The
C
NMR spectrum also shows peaks at 169.0 ppm and 169.9 ppm distinctive O-acetyl
carbonyls. As neither spectra show signals from OH-bound backbone CHs it can be
stated that pure and fully O-acetylated product was obtained. Partial O-acetylation
would lead into peak splitting which is not observed in the obtained spectra.
1
13
Figure 2. H NMR (above) and C NMR (below) spectra of 2,3,4,5-tetra-O-acetyl-
galactaric acid (2).
3

Diacetyl mixed anhydride of AGA (3) was synthesized from 2 by refluxing in excess
of acetic anhydride. As the reaction proceeded, the insoluble starting material 2
dissolved into the reaction mixture. After purification the product was obtained as
white powder. Solubility of the compound organic solvents such as chloroform, THF
and acetone was significantly improved when compared to starting material (2). The
1
13
H NMR and C NMR spectra of 3 with assigned peaks are presented in Figure 3.
1
The acetyl anhydride end-group signals are observed at 2.22 ppm in the H NMR
1
3
spectrum and at 22.2 ppm in the C NMR spectrum. It is known from the literature
that preparation of diacetyl mixed anhydrides often yields oligomeric products,
31
1
consisting of acetyl end-capped short polyanhydrides. The H NMR integral areas of
AGA backbone CH signals at 5.22 ppm and 5.76 ppm were compared to the
anhydride acetyl end-group signal at 2.22 ppm. As the ratio was corresponding to
monomeric structure (3), no oligomerization was found to have occurred during the
synthesis.
1
13
Figure 3. H NMR (above) and C NMR (below) spectra of diacetyl mixed
anhydride of 2,3,4,5-tetra-O-acetyl-galactaric acid (3) measured in CDCl
3
.
Diacetyl mixed anhydride of adipic acid (AA) (4) was prepared as previously reported
3
by Domb et al. The H NMR and C NMR spectra of 4 with assigned peaks are
1
1
13
presented in Figure 4. The acetyl anhydride end-group signals are observed at 2.23
1
ppm in the H NMR and 22.3 ppm in the C NMR spectrum. Through comparison of
13
1
backbone methylene signals in H NMR at 1.74 ppm and 2.51 ppm, and acetyl
anhydride end-group signal at 2.23 ppm, the mixed anhydride was found to have 1.8
adipoyl units on average, indicating that partial AA oligomerization had occurred.
1
13
Figure 4. H NMR (above) and C NMR (below) of diacetyl mixed anhydride of
adipic acid (4) measured in CDCl
3
.
The IR spectra of compounds 1 to 4 are presented in Figure 5 from the region between
-1
-1
500 cm and 2100 cm . The spectrum of galactaric acid (1) shows one absorbance
1
signal at 1725 cm , which is related to carboxylic acid carbonyls. In the acetylated
-
1
-1
AGA product (2), also another signal can be seen at 1758 cm , most likely from the
O-acetyl carbonyls. According to literature, aliphatic anhydrides commonly give two
-1
-1 28
separate stretching absorbances at about 1720 cm and 1810 cm . Therefore, in the
-
spectrum of diacetyl mixed anhydride of AGA (3), adsorption peak at 1837 cm is
1
expected to be distinctive to anhydride carbonyls whereas absorption peak at 1755
-1
cm is expected to be a combined peak from carbonyls of anhydride bonds and O-
-
1
acetyl groups. The absorption at 1725 cm , distinctive to carboxylic acid carbonyls of
compounds 1 and 2, is absent from the spectrum of 3. The IR spectrum of diacetyl
-
mixed anhydride of AA (4) also shows two signals, at 1743 cm and 1812 cm , as is
1
-1
typical to anhydride compounds.
4

Figure 5. Area of IR spectra distinctive to stretching absorbances of carbonyl groups.
Galactaric acid (1), 2,3,4,5-tetra-O-acetyl-galactaric acid (2), diacetyl mixed
anhydride of 2,3,4,5-tetra-O-acetyl-galactaric (3) and diacetyl mixed anhydride of
adipic acid (4).
2.2. Synthesis and characterization of polymers
Poly(2,3,4,5-tetra-O-acetyl-galactaric-co-adipic anhydride)s (PAGAA) with varying
monomer composition as well as poly(2,3,4,5-tetra-O-acetyl-galactaric anhydride)
(
PAGA) and poly(adipic anhydride) (PAA) homopolymers were successfully
synthesized by melt condensation polymerization at 150 °C under reduced pressure.
The synthesized polymers are presented in Table 1.
Table 1. The conducted polymerizations, NMR characterization results, SEC results,
and thermal properties of the obtained polymers.
1
The chemical structures of the prepared polymers were analyzed with H NMR and
1
3
1
13
C NMR. The H NMR and C NMR spectra with assigned peaks of representative
copolymer sample PAGAA3 are presented in Figure 6. The O-acetyl CH peaks of
AGA units are seen at 2.08 ppm and 2.22 ppm in the H NMR spectrum as well as at
3
1
1
3
2
0.3 ppm in the C NMR spectrum, indicating that O-acetyl groups have remained
intact during the polymerization. This is essential when targeting in linear polymer
structure. AGA backbone signals can be seen in the area between 5.19 and 5.83 ppm
1
of H NMR spectrum as well as at 67.2 ppm and 69.2 ppm in C NMR spectrum.
13
1
Similarly AA backbone signals appear at 1.74 ppm and 2.51 ppm in the H NMR
1
spectrum and at 23.2 ppm and 34.7 ppm in the C NMR spectrum. Carbonyl carbon
3
1
3
1
signals can be seen in the area between 162-170 ppm of the C NMR spectrum. H
NMR spectra were used to calculate the monomer compositions of copolymers.
Composition calculations were based on integral areas of peaks distinctive to each
monomer unit. Calculated compositions are presented in Table 1. The results show
that the monomer unit compositions in the copolymers were very close to the
monomer compositions in feed.
1 13
Figure 6. H NMR (above) and C NMR (below) spectra of sample PAGAA3.
1
3
Molecular weights of the synthesized polymers were studied with quantitative
NMR spectroscopy and size exclusion chromatography (SEC). Obtained results are
C
1
3
presented in Table 1. C NMR spectra were used to calculate the degree of
polymerization (DP) and M of the polymers. Peak at 20.3 ppm related to O-acetyl
CH groups of AGA moieties, peaks at 23.2 and 34.7 ppm related to CH backbone of
AA moieties as well as peak at 22.2 ppm related to anhydride CH end groups were
n
3
2
3
used in the calculations. The end group signals of PAA and PAGAA1 samples were
too small for DP calculations, indicating that they had higher DP than the other
samples. The DP of samples PAAGA2-4 (AGA content 9-75 mol%) were similar to
each other (DP = 19-21). The PAGA homopolymer had the lowest molecular weight
5

(
DP = 5). These values are comparable to molecular weights of previously reported
polyanhydrides prepared by melt polycondensation. M of polyanhydrides typically
although also higher molecular weights (M
n
3
0, 32, 35
vary between 1 000-14 000 g/mol,
n
3
1
up to 26 900 g/mol) have been obtained with optimized process conditions. It is
possible that optimization, e.g. through further reduction in polymerization pressure
or more efficient mixing, might have increased the molecular weights also in this
study.
The lowering tendency in DP values from PAA homopolymer to PAGA
homopolymer could be due to increase in the reaction mixture viscosity with
increasing AGA content (especially in the case of AGA homopolymerization), which
could hinder the removal of the acetic anhydride during the polymerization. In
general, viscosity increase has been mentioned to be one of the most significant
3
6
factors leading to low molecular weights in melt polycondensations.
The SEC results in Table 1 also suggest that increasing AGA content would have a
diminishing effect on the molecular weight of the copolymers. However, it should be
noted that it is not possible to compare the SEC results through varying monomer
compositions accurately. As the monomer composition varies throughout the
polymerization series, also solution properties of the copolymer are affected, which
affects the SEC results.
As the molecular weights of the copolymers presented in Table 1 were relatively low,
a new set of polymerizations was performed with added catalyst. Cadmium acetate
dihydrate (CdOAc) and calcium carbonate (CaCO
are commonly used in polyanhydride synthesis. Based on previous literature, CdOAc
3
) were tested as catalysts, as they
2
8
is one of the best performing catalysts in synthesis of polyanhydrides. However, its
possible toxicity sets limitations to its use in biomedical applications, and therefore
CaCO
polymerizations were performed for selected monomer compositions containing 25-
5 mol% of each monomer unit, applying 2 wt% of catalyst. Obtained molecular
weights are collected in Table , including non-catalyzed corresponding results from
Table 1 as references. CaCO had clear increasing effect on the molecular weights of
3
with no reported toxic effects was chosen as the second catalyst. The
7
3
all samples. The addition of CdOAc seemed to have less effect. The best
improvements with both tested catalysts were obtained for the copolymers with the
largest AGA content.
Table 2. Comparison of molecular weights obtained with different catalysts.
Thermal transition temperatures of the synthesized copolymers were studied with
differential scanning calorimetry (DSC). Results are collected in Table 1. The glass
o
) of the PAA homopolymer was measured to be -25 C.
transition temperature (T
g
Increasing AGA content in the copolymers had an increasing effect on the T
g
, and the
of the PAGA homopolymer was already 60 C, despite low molecular weight of the
homopolymer sample. AGA has multiple O-acetyl substituents resulting in more rigid
o
T
g
3
7
g
structure when compared to AA, which generally have tendency to increase T .
6

The polymers that had up to 50 mol% of AA (PAA and PAGAA1-4) had melting
peaks in the DSC curves. No melting peak was observed when AGA was main
polymer component (PGA and PAGAA5), indicating that only AA units had tendency
m
to crystallize in the copolymers. The melting enthalpies (∆H ) (Table 1) also
decreased as the AGA content increased, suggesting decrease in the degree of
crystallinity. AA units have structural simplicity beneficial for crystal formation that
AGA units do not have. It was also observed that increase in the AGA content to
above 25 mol% had an increasing effect on T
incorporation of rigid aromatic moieties into aliphatic structure can increase the T
copolyanhydrides.
m
. It has been previously observed that
of
m
3
1, 32
Thermal degradation of the synthesized polymers was studied with thermogravimetric
analysis (TGA). The Td10% values of the polymers are presented Table 1. Slightly
decreasing trend can be seen in Td10% values as AGA content of the polymer increases.
Thermogravimetric (TG) and differential thermogravimetric (DTG) curves of PAA,
PAGA, and PAGAA3 samples are presented in Figure 7. As can been seen from DTG
curve of PAGAA3, the copolymer sample possessed two-stage degradation behavior.
The first stage is clearly related to the degradation of AGA moieties and the second
stage to the degradation of AA moieties of the copolymer. Thermal degradation of
samples containing AGA moieties started at around 160 °C. This result shows that
polymerization temperature cannot be further increased from the 150 °C, which was
the temperature at which polymerizations were conducted in this study.
Figure 7. TG (above) and DTG (below) curves of PAA (dotted line), PAGA (dashed
line) and PAGAA3 (solid line).
3
. Experimental
.1. Materials
3
Adipic acid (>99%) was purchased from Fluka and was recrystallized from ethanol
before use. Galactaric acid (97%, mucic acid), p-toluene sulfonic acid (≥98.5%),
cadmium acetate dihydrate (>98%), calcium carbonate (>99%), acetic acid (analytical
grade), toluene (analytical grade), chloroform (anhydrous grade), and diisopropyl
ether (anhydrous grade) were purchased from Sigma-Aldrich and used as received.
3.2. Characterization
1
13
H NMR and C NMR measurements were conducted with a Bruker 500 MHz
spectrometer at 22 °C when measuring in deuterated chloroform (CDCl
3
) or at 70 °C
when measuring in deuterated dimethyl sulfoxide (DMSO-d ). Chemical shift scale
6
was calibrated to tetramethylsilane (TMS). Chromium(III)acetylacetonate was used as
a relaxation reagent to ensure that quantitative spectra were obtained. Distinctive
peaks of each monomer unit were integrated and monomer unit compositions of
synthesized polymers as well as degrees of polymerization (DP) of diacetyl mixed
anhydrides and polymer samples were calculated based on integral areas of repeating
units and end groups.
7

FT-IR measurements were conducted from KBr tablets with a Perkin Elmer Spectrum
BX FT-IR System.
Elemental analyses were conducted with Elementar Analysensysteme GmbH
Variomax CHN analyzer.
Molecular weights and molecular weight distributions were determined with size
exclusion chromatography (SEC) at 30 ºC. The system was equipped with Waters
Styragel columns and Waters 2414 Refractive Index Detector. Chloroform (CHCl )
3
was used as an eluent and was delivered at a rate of 1.0 ml/min. Results were
calculated with respect to polystyrene standards.
Differential scanning calorimetry (DSC) was used to determine the glass transition
g m
temperatures (T ) and melting peak temperatures (T ) of the prepared polymers using
e
Mettler Toledo DSC820 STAR SW 9.20 instrument under nitrogen atmosphere. In
the first scan samples were heated from -50 °C to 170 °C at a rate of 10 °C/min and
cooled to -50 °C at a rate of 10 °C/min. Results were collected from the second
heating scan where samples were heated from -50 °C to 200 °C at a rate of 10 °C/min.
Temperatures of 10 wt% decomposition (Td10%
)
were measured with
thermogravimetric analysis (TGA) utilizing TG/DTA320 Seiko Instrument. Analyses
were performed from 25 ºC to 550 ºC with a heating rate of 20 ºC/min under argon
flow.
3.3. Synthesis of 2,3,4,5-tetra-O-acetyl-galactaric acid (2)
Galactaric acid 1 (200.0 g, 0.95 mol), acetic anhydride (728.0 g, 7.13 mol), acetic acid
312.0 g) and p-toluene sulfonic acid (3.0 g, 1.5 wt% of galactaric acid) were charged
(
into Juchheim Laborgeräte GmbH two-liter reactor. Nitrogen flow was led into the
reactor for 1h. The reactor was sealed and nitrogen flow was stopped. Stirring was
continued at room temperature for 24 h after which the temperature was increased to
85 °C. Reaction was continued at 85 °C for 48 h. Heating was turned off and reaction
continued at room temperature for 24 h. Insoluble fraction containing the product was
filtered off from the reaction mixture and washed twice with toluene. The desired
product was further purified by recrystallizing from ion-exchanged water, filtered and
dried in vacuum oven at 40 °C for at least 24 h. The product was obtained as needle-
-
1
shaped white crystals (103.4 g, 29% yield). mp (DSC): 248 °C dec. IR: υ (cm ) 1757
1
(
C=O OAc), 1734 (C=O COOH). H NMR (DMSO-d
.02 (s, 6H, OAc CH ), 2.10 (s, 6H, OAc CH ), 5.06 (s, 2H, CH), 5.58 (s, 2H, CH).
, 70 °C, 125 MHz): δ (ppm) 20.6 (2x2C, OAc CH ), 68.6 (2C,
CH), 70.0 (2C, CH), 168.1 (2C, COOH), 169.0 (2C, OAc C=O), 169.9 (2C, OAc
C=O). Anal. Calcd. for C14 12: C; 44.45, H; 4.80, O; 50.75. Found: C; 44.6, H;
6
, 70 °C, 500 MHz): δ (ppm)
2
1
3
3
3
C NMR (DMSO-d
6
3
18
H O
4.8, O; 50.6.
3.4. Synthesis of diacteyl mixed anhydride of 2,3,4,5-tetra-O-acetyl-galactaric
acid (3)
2,3,4,5-tetra-O-acetyl-galactaric acid 2 (50.0 g, 0.132 mol) and acetic anhydride
(
132.94 g, 1.32 mol) was charged into two-neck round bottom flask equipped with
8

magnetic stirrer and reflux condenser. The mixture was refluxed under nitrogen flow
for 30 min. 2 was completely dissolved into acetic anhydride after 15 min of refluxing
forming clear colorless solution. Reaction mixture was filtered when still hot to
remove all unreacted 2 and cooled down in freezer. Recrystallized product 3 was
filtered, washed with diisopropyl ether twice and dried in vacuum oven at ambient
temperature for at least 24 h. Product was stored for further investigations at -20 °C in
vacuum desiccator to prevent hydrolytical degradation. Desired product was obtained
-1
as white powder (39.2 g, 64% yield). mp (DSC): 142 °C. IR: υ (cm ) 1837 (C=O
1
anh.), 1755 (C=O comb. OAc and anh.). H NMR (CDCl
s, 6H, OAc CH ), 2.22 (s, 6H, anh. CH ), 2.27 (s, 6H, OAc CH
.76 (s, 2H, CH). C NMR (CDCl3, 125 MHz): δ (ppm) 20.3 (4C, OAc CH
2C, anh. CH ), 67.3 (2C, CH), 69.3 (2C, CH), 162.3 (2C, anh. CH C=O), 164.3 (2C,
anh. C=O), 169.1 (2C, OAc C=O), 169.9 (2C, OAc C=O). Anal. Calcd. for
14: C; 46.76, H; 4.80, O; 48.45. Found: C; 46.4, H; 4.8, O; 48.8.
3
, 500 MHz): δ (ppm) 2.08
), 5.23 (s, 2H, CH),
), 22.2
(
3
3
3
1
3
5
(
3
3
3
18 22
C H O
3.5. Synthesis of diacetyl mixed anhydride of adipic acid (4)
Diacetyl mixed anhydride of adipic (AA) was prepared based on procedure described
3
1
by Domb et al. Adipic acid was refluxed in excess of acetic anhydride for 20 min.
After the complete reaction time the reaction mixture was cooled down to 50 °C and
excess acetic anhydride was evaporated under vacuum. The product was washed
several times with diisopropyl ether to remove remaining acetic anhydride. Product
was stored for further investigations at -20 °C in vacuum desiccator to prevent
-1
hydrolytical degradation. mp (DSC): 67 °C. IR: υ (cm ) 1812 (C=O anh.), 1743 (C=O
1
anh.). H NMR (CDCl
3
, 500 MHz): δ (ppm) 1.74 (s, 4H, CH
2
), 2.22 (s, 6H, anh.
). C NMR (CDCl3, 125 MHz): δ (ppm) 22.2 (2C, anh. CH ),
), 34.7 (2C, CH ), 166.5 (2C, anh. CH C=O), 168.9 (2C, anh. C=O).
Anal. Calcd. for C10 : C; 52.17, H; 6.13, O; 41.70. Found: C; 54.7, H; 6.2, O;
1
3
CH
3
), 2.51 (s, 4H, CH
2
3
2
3.3 (2C, CH
2
2
3
14 6
H O
39.1.
3.6. Polymerization
Polyanhydrides were synthesized by melt condensation polymerization. A series of
polyanhydrides were synthesized from diacetyl mixed anhydrides of 2,3,4,5-tetra-O-
acetyl-galactaric acid (3) and adipic acid (4) varying monomer unit compositions. For
example, polymerization containing 10 mol% in feed of 3 was conducted as follows:
Diacetyl mixed anhydrides of 2,3,4,5-tetra-O-acetyl-galactaric acid (3) (0.672 g, 1.5
mmol) and adipic acid (4) (2.00 g, 7.2 mmol) were charged into round bottom flask
equipped magnetic stirrer and distillation apparatus. Nitrogen flow was led into the
flask at room temperature for 60 min. The flask was immersed into Radleys heat-on
block preheated to 150 °C. After 30 min the pressure was reduced to 0.5-0.9 mbar
within 5 min during which the continuous nitrogen flow was stopped. The reaction
was continued for 45 min. During the reaction flask was purged with nitrogen for 15 s
every 20 min. After the complete reaction time atmospheric pressure was returned to
reaction flask with nitrogen and the reaction was stopped by immersing in liquid
nitrogen. Obtained product was dissolved in chloroform, precipitated from
diisopropyl ether and dried under vacuum at ambient temperature. Products were
stored for further investigations at -20 °C in vacuum desiccator to prevent
hydrolytical degradation.
9

In addition, a smaller polymerization series was conducted with added catalyst. Two
tested catalysts were cadmium acetate dehydrate (CdOAc) and calcium carbonate
(
conducted similarly to polymerizations with no catalyst.
CaCO
3
). Catalysts were used with 2 wt% dosage and polymerizations were
4. Conclusions
In this study we describe the synthesis of O-acetylated galactaric acid (AGA), and its
utilization in the polymerization of a series of copolyanhydrides with adipic acid
(
AA). The AGA monomer synthesis consisted of two steps, where the first step was to
acetylate the secondary hydroxyl groups of starting material galactaric acid, and the
second step was the preparation of diacetyl mixed anhydride used in the
polymerizations.
In the copolymerization studies it was observed that there was a slightly lowering
tendency in the molecular weights of the obtained polymers when AGA content in the
polymerization mixtures increased. Thermal properties of the copolymers also varied
depending on monomer composition. Increase in the AGA content had clear
g m
increasing effect on the T . Similar increasing effect was observed in T . However,
the degree of crystallinity decreased as AGA content increased, and the polymers with
high AGA contents (over 50 mol%) were completely amorphous.
According to our knowledge this is the first time when carbohydrate derived aldaric
acids were incorporated into polyanhydride. The work was conducted to assess the
exploitability of galactaric acid in the synthesis of polyanhydrides. In addition, we
wanted to study the effect of acetylated galactaric acid on e.g. thermal properties of
the copolymers. Galactaric acid can be produced by engineered filamentous fungi
7
from D-galacturonic acid which is a major component of e.g. pectin. Pectin is
currently abundant and underutilized in many industrial side-streams, such as sugar
beet processing, and apple and orange juice production.
Acknowledgements
This work was financially supported by the Academy of Finland and is part of the
Finnish Centre of Excellence in White Biotechnology - Green Chemistry, Project No.
118573.
Riku Talja, Harri Heikkinen, Liisa Mansukoski, Elina Paasonen and Jaana Korhonen
are acknowledged for their assistance in analytical work. Eija Silvasti is
acknowledged for her contributions in practical work.
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12

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13

OH OH O
HO
OH
O OH OH
14

15

16

17

18

19

20

OH OH
O
HO
1
OH
O
OH OH
O
O
Acetic Acid
PTSA
O
1
2
3
. RT, N , 24 h
2
. 85°C, N , 48 h
2
O
. RT, N , 24 h
2
OH
OAc OAc O
OAc OAc
2
HO
OH
O
O
O
O
Reflux, 30 min
O
OH
OAc OAc O
O
3
H C
3
O
O
CH3
O
O
OAc OAc
O
O
4
H C
3
O
O
CH3
ni
O
O
1
1
50°C, 30 min
50°C, 0.5-0.9 mbar, 45 min
O
O
O
OAc OAc O
OAc OAc
O
H C
3
O
O
CH3
O
m
n
O
O
O
O
21

Table 1. The conducted polymerizations, NMR characterization results, SEC results, and thermal properties of the obtained polymers.
1
AGA content in polymer ( H NMR)
13
AGA content in feed
C NMR
SEC
DSC
(°C)
TGA
d(10%) (°C)
223
(
mol%)
0
(mol%)
M
n
(g/mol)
a
n.d.
DP
M
w
(g/mol)
M
n
(g/mol)
5600
4500
3200
1800
1350
960
M
w
/M
n
T
g
(°C)
T
m
∆H
m
(J/g)
T
a
PAA
0
5
n.d.
n.d.
20
20
21
19
5
10700
9200
7000
3500
2500
1300
1200
1.91
2.04
2.19
1.94
1.85
1.35
1.25
-25
73
88
63
55
5
a
a
PAGAA1
PAGAA2
PAGAA3
PAGAA4
PAGAA5
5
n.d.
-28
-27
-18
17
67
60
219
10
9
3000
3800
5300
5800
1900
206
25
25
53
76
100
97
195
50
106
12
186
b
b
b
75
29
n.d.
n.d.
n.d.
n.d.
195
b
PAGA
100
960
60
199
a
Not determined due to too small polymer end group signal.
Not detected.
b
22

Table 2. Comparison of molecular weights obtained with different catalysts.
AGA content in feed
catalyst
(2 wt%)
-
SEC
(
mol%)
25
M
w
(g/mol)
M
n
(g/mol)
1800
1350
960
w n
M /M
PAGAA3
3500
2500
1300
3300
2000
3200
4400
1.94
1.85
1.35
2.06
1.33
1.78
1.69
PAGAA4
50
-
PAGAA5
75
-
PAGAA3-2
PAGAA4-2
PAGAA5-2
PAGAA3-3
25
CdOAc
CdOAc
CdOAc
CaCO3
1600
1500
1800
2600
50
75
25
PAGAA4-3
PAGAA5-3
50
75
3600
3500
2100
2100
1.71
1.67
CaCO
3
CaCO
3
23

Highlights
•
A series of meso-galactaric acid based copolyanhydrides was polymerized
•
Copolyanhydrides consisted of O-acetylated meso-galactaric acid (AGA) and
adipic acid moieties
•
•
•
Thermal properties of the copolymers varied depending on monomer composition
g m
Increase in the AGA content had clear increasing effect on the T and T
Results offer utilization route for meso-galactaric acid producible from pectin-rich
industrial side-streams
24

Graphical abstract
25