Synthesis and properties of alternating copolymers of 3-hydroxybutyrate and lactate units with different stereocompositions

Article abstract of DOI:10.1021/ma501783f

Alternating copolymers of (R)-3-hydroxybutyrate ((R)-3HB) and lactate (2-hydroxypropionate: 2HP) units were synthesized by polycondensation reaction of preprepared dimeric monomers, (R)-3HB-(R)-2HP and (R)-3HB-(S)-2HP, in the presence of condensation agent. On the basis of the NMR analyses, it was confirmed that the obtained copolymers had an alternating sequence of (R)-3HB and 2HP units. In contrast to random copolymers of (R)-3HB and 2HP units, the repeating sequence of alternately connected (R)-3HB and 2HP units formed crystalline region. The copolymer with alternating sequence of (R)-3HB and (S)-2HP units had a melting temperature at 83 °C. On the other hands, the melting temperature of copolymer of (R)-3HB and (R)-2HP units was quite higher than those of the corresponding homopolymers (around 180°C) and reached to 233 °C. When the alternating copolymers were prepared from a mixture of stereoisomeric dimers, both the melting temperature and crystallinity varied in the wide ranges depending on the composition of stereoisomeric dimers.

Full text of DOI:10.1021/ma501783f

Article
pubs.acs.org/Macromolecules
Synthesis and Properties of Alternating Copolymers of
3‑Hydroxybutyrate and Lactate Units with Different
Stereocompositions
Yuta Tabata†,‡ and Hideki Abe*,†,‡
†Department of Innovative and Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama,
Kanagawa 226-8502, Japan
‡Bioplastic Research Team, Biomass Engineering Program Cooperation Division, RIKEN Center for Sustainable Resource Science,
2-1 Hirosawa, Wako, Saitama 351-0198, Japan
S
* Supporting Information
ABSTRACT: Alternating copolymers of (R)-3-hydroxybuty-
rate ((R)-3HB) and lactate (2-hydroxypropionate: 2HP) units
were synthesized by polycondensation reaction of preprepared
dimeric monomers, (R)-3HB-(R)-2HP and (R)-3HB-(S)-
2HP, in the presence of condensation agent. On the basis of
the NMR analyses, it was confirmed that the obtained
copolymers had an alternating sequence of (R)-3HB and
2HP units. In contrast to random copolymers of (R)-3HB and
2HP units, the repeating sequence of alternately connected
(R)-3HB and 2HP units formed crystalline region. The
copolymer with alternating sequence of (R)-3HB and (S)-2HP
units had a melting temperature at 83 °C. On the other hands,
the melting temperature of copolymer of (R)-3HB and (R)-2HP units was quite higher than those of the corresponding
homopolymers (around 180 °C) and reached to 233 °C. When the alternating copolymers were prepared from a mixture of
stereoisomeric dimers, both the melting temperature and crystallinity varied in the wide ranges depending on the composition of
stereoisomeric dimers.
to 230 °C, and the value is the highest ones, along with
poly(glycolate), in the aliphatic polyesters.21
INTRODUCTION
■
Nowadays, aliphatic polyesters have attracted an industrial
attention as environmentally degradable thermoplastics to be
used for a wide range of applications.1−21 Among a family of
aliphatic polyesters, poly[(R)-3-hydroxybutyrate] (P[(R)-
3HB]),1−4 poly(L-lactate) (PLLA, poly[(S)-2-hydroxypropio-
nate]; P[(S)-2HP]),5−8 and their copolymers9−18 have been
most extensively investigated. Bacterially synthesized P[(R)-
3HB] has a melting temperature around 180 °C, and the value
is very close to that of polypropylene which is a commodity
plastic. The introduction of second monomer units into P[(R)-
3HB] molecules by copolymerization varies their melting
temperatures in the range from 50 to 180 °C, depending on
both the chemical structure of second monomer and the
composition, and P[(R)-3HB]-based copolymers offer a wide
variety of polymeric materials in properties, from hard
crystalline plastic to elastic rubber.9−14 Optically pure PLLA
also show a melting temperature at around 180 °C. Since the
PLLA has a glass-transition temperature of 60 °C, the
transparent materials can be provided. It has been found that
the PLLA molecules form a stereocomplex crystalline structure
in the presence of stereoisomeric poly(D-lactate) segments.19,20
The melting temperature of the stereocomplex crystal reaches
For copolymerization, the regulation of sequential structure
is powerful tool to design the polymeric materials with
appropriate properties.22−25 Various types of copolymers with
statistically random,26,27 blocking tendency,28,29 or alternative
distributions30,31 have been produced, and the structure and
physical properties were characterized. We previously inves-
tigated the effects of sequential structure on the thermal
properties by preparing the copolymers of (R)-3HB and 2HP
units with various sequential distributions from random to
blocking tendency.32 The obtained results indicate that the
melting temperature of copolymers be tuned to the averaged
sequential length of crystallizable monomer units. However, in
each case, the second monomer units introduced with statistical
regularity act as defects of crystalline state consisting of
repeating crystallizable monomer units and induce the
inhibition of crystallization and the depression of melting
temperature.
Received: August 28, 2014
Revised: October 11, 2014
Published: October 27, 2014
© 2014 American Chemical Society
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1H and 13C NMR analyses of copolymer samples were carried out
on a Varian NMR System 500 spectrometer. The samples (2 mg/mL)
were dissolved in CDCl3 or 7.5% HFIP in CDCl3 and the 500 MHz
1H NMR spectra were recorded at 25 °C with a 5.45 μs pulse width
With the progresses in polymer synthesis techniques,
approaches to the precise regulation of monomer sequence in
copolymer molecules have been attempted, instead of usual
statistical regulation of sequential structure.33−39 For olefinic
monomers, Wagener et al. synthesized the copolymers
possessing an ethyl branch on every ninth, 15th and 21st
carbon along the backbone of polyethylene via acyclic diene
metathesis.33 Satoh et al. succeeded in 1:2-sequence-regulated
radical copolymerization of maleimide and terpenes in the
presence of fluorinated alcohol.34−36 In the case of polyesters,
Meyer et al. synthesized the various types of copolymers with
precise repeating sequence of lactate and glycolate units by
condensation reactions of oligomeric precursors of two
monomers.37−39 They investigated the effects of sequence
structure in the copolymers containing both α-hydroxyl acid
monomeric units on the physical properties and biodegrad-
ability. Kramer et al. succeeded in syntheses of sequence-
regulated copolymers consisted of β-hydroxyl acid monomeric
units by polymerization of a mixture of enantiopure two β-
lactone monomers as pairing the opposite configuration using
syndiospecific catalysts.40 They reported that the melting
temperatures of the synthesized copolymers with alternating
sequence structure were strongly influenced by characteristics
of side-chain groups.
The simplest type of sequence-regulated copolymer is an
alternating copolymer. In this study, we focused the structure
and physical properties of alternating copolymers consisting of
α-hydroxyl acid monomeric unit of lactate (2HP) and β-
hydroxyl acid monomer unit of (R)-3HB. Taking into account
the chiral structure of monomeric units, two types of
stereoisomeric dimers ((R)-3HB-(R)-2HP and (R)-3HB-(S)-
2HP) were respectively prepared, and the alternating
copolymers with different stereocompositions were synthesized
from the dimeric monomers by condensation reaction. The
structure and physical properties of obtained copolymers were
characterized by NMR, DSC, and WAXD. On the basis of the
obtained results, the relationships among the molecular
structure, thermal properties, and crystalline structure in the
alternating copolymers were discussed.
(45° pulse angle), 3.2 s pulse repetition, 8000 Hz spectral width, and
16 K data points. The 125 MHz 13C NMR spectra were recorded at 25
°C in a CDCl3 solution of polymer (30 mg/mL) with a 4.15 μs pulse
width (45° pulse angle), 1.3 s pulse repetition, 30 000 Hz spectral
width, and 32 K data points. Data were calibrated by tetramethylsilane
(TMS, δ 0.00) Data were analyzed by JEOL ALICE 2 software.
High-resolution solid-state 13C NMR measurements were carried
out by using a Chemagnetics CMX Infinity 400 spectrometer in under
a static magnetic field of 9.4 T. Both the 1H and 13C radio field
strength γB1/2π were 52.0 kHz. The magic angle spinning (MAS) rate
was set to 9.0 kHz to avoid the overlapping of spinning side bands on
other resonance lines. The contact time for the cross- polarization
(CP) process was 3.0 ms throughout this work. 13C chemical shifts
were expressed as values relative to tetramethylsilane (TMS) by using
the CH3 line at 17.36 ppm of hexamethylbenzene as an external
reference.
Differential scanning calorimetry (DSC) data of copolymer samples
were recorded in the temperature range of −120−250 °C on a Perkin-
Elmer DSC 8500 equipped with a liquid nitrogen cooling accessory
under a helium flow of 20 mL/min. Samples of 3−5 mg were
encapsulated in aluminum pans and heated from −100 to +250 °C at a
rate of 20 °C/min (first heating scan). The melting temperature (Tm)
and enthalpy of fusion (ΔHm) were determined from the DSC
endotherms of first heating scan. The Tm value was taken as the peak
temperature of the main endothermal peak. For measurement of the
glass-transition temperature (Tg), the samples were maintained at 250
°C for 1 min, and then rapidly quenched at −120 °C. They were then
reheated from −100 to +250 °C at a heating rate of 20 °C/min
(second heating scan). The Tg was taken as the midpoint of the heat
capacity change.
Thermogravimetric (TG) analysis was performed on a Seiko
instruments TG/DTA 220U using nitrogen as a purge gas. About 3.0
mg of each sample was heated from room temperature to 500 °C at 20
°C/min. Tdmax is the temperature corresponding to the inflection point
of the thermodegradation curves that corresponds to the maximum
reaction rate.
Wide angle X-ray diffraction (WAXD) patterns of copolymer
samples were recorded at 23 °C on a Rigaku RINT 2500 system using
a nickel-filtered Cu Kα radiation (λ = 0.154 nm; 40 kV; 100 mA) in
the 2θ range of 6 to 60° at a scanning speed of 2.0°/min. The
crystallinity of copolymer samples was calculated from diffracted
intensity data according to Vonk’s method.42
EXPERIMENTAL SECTION
■
Materials. Methyl (R)-3-hydroxybutyrate, methyl (R)-lactate, tert-
butyldiphenylchlorosilane (TBDPSCl), benzyl bromide (BnBr), and
N,N′-diisopropylcarbodiimide (DIC) were purchased from Tokyo
Chemical Industry Co. (S)-Lactic acid, benzyl alcohol (BnOH), 1,8-
diazobicycloundec-7-ene (DBU), and N,N-dimethyl-4-aminopyridine
(DMAP) were purchased from Wako Chemical Industry Co. p-
Toluenesulfonate (TsOH), dimethylformamide (DMF), triethylamine
(TEA), lithium hydroxide monohydrate (LiOH·H2O), and N,N′-
dicyclohexylcarbodiimide (DCC) were purchased from Kanto
Chemical Co. The 5% Pd on silica sample was purchased from
Strem Chemicals, Inc. All of these chemical reagents were used as
supplied without further purification. 4-(Dimethylamino)pyridinium p-
toluenesulfonate (DPTS) was prepared from TsOH and DMAP
according to the reported procedure.41 Dichloromethane (DCM) and
THF (Kanto Chemical Co, super dehydrated grade) were passed
through activated alumina using the SPS 800 (Innovative Technol-
ogy).
Synthesis of Dimeric Monomers. Benzyl (R)-Lactate (1: (R)-
2HP-Bn). Methyl (R)-lactate (22.6 mL, 20.8 g, 200.0 mmol) was added
to a solution of benzyl alcohol (104.0 mL, 108.0 g, 1.0 mol) and p-
toluenesultanate (0.2 g, 1.0 mmol). The reaction mixture was stirred at
80 °C for 2 h, and the pressure was reduced subsequently at 9 kPa for
10 h. Diethyl ether (400 mL) was added, and an organic extracts were
successively washed with saturated sodium hydrogen carbonate (2 ×
200 mL), 5% sodium carbonate (4 × 50 mL), saturated sodium
chloride (6 × 50 mL), dried over anhydrous magnesium sulfate, and
evaporated. The crude product was distilled, to give a colorless oil (bp
1
1 kPa, 125−130 °C, 25.2 g, yield = 70%). H NMR (CDCl3, 500
MHz): δ 7.4−7.2 (m, 5H, ArH), δ 5.21 (s, 2H, PhCH2O), δ 4.31 (m,
1H, CH), δ 2.90 (d, J = 4.9 Hz, 1H, OH), δ 1.43 (d, J = 5.5 Hz, 3H,
CH3). Anal. CHN Calcd: C10H12O3: C, 66.7; H, 6.7; N, 0.0. Found: C,
65.7; H, 6.7; N, 0.0.
Benzyl (S)-Lactate (2: (S)-2HP-Bn). Benzyl (S)-lactate was prepared
following Wolfe et al.43 1,8-Diazobicyclo[5.4.0]undeca-7-ene (DBU)
(90.0 mL, 94.5 g, 600.0 mmol) was added slowly with stirring to a
solution of 90% (S)-lactic acid (59.4 mL, 54.0 g, 600.0 mmol) in
methanol (250 mL). The solvent was removed under reduced pressure
at 70−80 °C, and the resulting oil, in DMF (250 mL), was cooled to
15 °C. Benzyl bromide (59.5 mL, 85.5 g, 500.0 mmol) was added
dropwise, and the reaction mixture was stirred at room temperature for
30 h. After removal of approximately 200 mL of solvent by vacuum
Analytical Procedures. All molecular weight data were obtained
by gel-permeation chromatography at 40 °C, using a Shimadzu 10A
GPC system and 10A refractive index detector with Shodex K-806 M
and K-802 columns. Chloroform was used as an eluent at a flow rate of
0.8 mL/min, and sample concentration of 1.0 mg/mL was applied.
Polystyrene standards with a low polydispersity were used to make a
calibration curve.
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Scheme 1. Synthetic Procedures of Dimeric Monomers and Polymers with Alternating Sequences of (R)-3HB and 2HP Units
distillation, ethyl acetate (500 mL) was added, followed by water (150
mL). The aqueous layer was washed with ethyl acetate (3 × 150 mL),
and the combined organic extracts were washed successively with
water (150 mL), 5% citric acid (150 mL), water (150 mL), saturated
sodium chloride (2 × 150 mL), dried over anhydrous magnesium
sulfate, and evaporated. The crude product was distilled, to give a
ArH), δ 5.14 (m, 2H, PhCH2O), δ 5.03 (m, 1H, CH), δ 4.31 (m, 1H,
CH), δ 2.55 (m, 2H, CH2), δ 1.36 (d, J = 7.1 Hz, 3H, CH3), δ 1.13 (d,
J = 6.2 Hz, 3H, CH3), δ 1.03 (s, 9H, CH3). Anal. CHN Calcd:
C30H36O5Si: C, 71.4; H, 7.2; N, 0.0. Found: C, 71.3; H, 7.2; N, 0.0.
(S)-2-[(R)-3-(tert-Butyldiphenylsilanyloxy)butoxy]propionic Acid
Benzyl Ester (5: TBDPS-(R)-3HB-(S)-2HP-Bn). In the same way as
TBDPS-(R)-3HB-(R)-2HP-Bn (4), TBDPS-(R)-3HB-(S)-2HP-Bn
(5) was prepared from TBDPS-(R)-3HB and (S)-2HP-Bn (2). The
1
colorless oil (bp 1 kPa, 125−130 °C, 91.8 g, yield = 85%). H NMR
(CDCl3, 500 MHz): δ 7.4−7.2 (m, 5H, ArH), δ 5.21 (s, 2H,
PhCH2O), δ 4.32 (m, 1H, CH), δ 2.89 (d, J = 5.9 Hz, 1H, OH), δ 1.43
(d, J = 7.1 Hz, 3H, CH3). Anal. CHN Calcd: C10H12O3: C, 66.7; H,
6.7; N, 0.0. Found: C, 65.5; H, 6.7; N, 0.0.
1
product was obtained as colorless oil (15.2 g, yield = 83%). H NMR
(CDCl3, 500 MHz): δ 7.7−7.6 (m, 4H, ArH), δ 7.5−7.2 (m, 11H,
ArH), δ 5.14 (m, 2H, PhCH2O), δ 5.06 (m, 1H, CH), δ 4.31 (m, 1H,
CH), δ 2.55 (m, 2H, CH2), δ 1.46 (d, J = 7.1 Hz, 3H, CH3), δ 1.12 (d,
J = 6.1 Hz, 3H, CH3), δ 1.02 (s, 9H, CH3). Anal. CHN Calcd:
C30H36O5Si: C, 71.4; H, 7.2; N, 0.0. Found: C, 71.3; H, 7.3; N, 0.0.
(R)-2-[(R)-3-Hydroxybutoxy]propionic Acid Benzyl Ester (6: (R)-
3HB-(R)-2HP-Bn). The diprotected dimeric monomer (4) (4.0 g, 8.0
mmol) and acetic acid (1.5 g, 25.0 mmol) were combined under
nitrogen with 60 mL of THF. TBAF (1 M in THF) (12.0 mL) was
added, and the reaction mixture was allowed to stir for 3 day at 50−60
°C. Brine (30 mL) was added and extracted with 2 × 40 mL with
ether. The ethereal phase was washed with 3 × 30 mL saturated
NaHCO3, 2 × 30 mL H2O, and dried with MgSO4. The crude oil was
chromatographed over silica gel (10% ethyl acetate in hexanes) to give
a colorless liquid (5.8 g, yield = 72%). 1H NMR (CDCl3, 500 MHz): δ
7.4−7.2 (m, 5H, ArH), δ 5.19 (m, 2H, PhCH2O), δ 5.19 (m, 1H, CH),
δ 4.19 (m, 1H, CH), δ 3.22 (s, 1H, OH), δ 2.52 (m, 2H, CH2), δ 1.50
(d, J = 7.1 Hz, 3H, CH3), δ 1.24 (d, J = 6.4 Hz, 3H, CH3). Anal. CHN
Calcd: C14H18O5: C, 63.2; H, 6.8; N, 0.0. Found: C, 62.1; H, 6.8; N,
0.0.
(S)-2-[(R)-3-(Hydroxybutoxy)]propanoic Acid Benzyl Ester (7: (R)-
3HB-(S)-2HP-Bn). In the same way as (R)-3HB-(R)-2HP-Bn (6), (R)-
3HB-(S)-2HP-Bn (7) was prepared from TBDPS-(R)-3HB-(S)-2HP-
Bn (5). The product was a clear liquid (4.6 g, yield = 89%). 1H NMR
(CDCl3, 500 MHz): δ 7.5−7.2 (m, 5H, ArH), δ 5.20 (m, 2H,
PhCH2O), δ 5.17 (m, 1H, CH), δ 4.26 (m, 1H, CH), δ 3.03 (s, 1H,
OH), δ 2.54 (m, 2H, CH2), δ 1.52 (d, J = 13.5 Hz, 3H, CH3), δ 1.24
(d, J = 12.0 Hz, 3H, CH3). Anal. CHN Calcd: C14H18O5: C, 63.2; H,
6.8; N, 0.0. Found: C, 62.4; H, 6.8; N, 0.0.
(R)-3-(tert-Butyldiphenylsilanyloxy)butanoic Acid (3: TBDPS-(R)-
3HB). Methyl (R)-3-hydroxybutyrate (11.8 g, 100.0 mmol), triethyl-
amine (20.2 g, 200.0 mmol), DMAP (6.1 g, 50.0 mmol), and
dichloromethane (400 mL) were added under nitrogen to an 1 L
oven-dried two-neck eggplant-shaped flask. After cooling the reaction
mixture to 0 °C, TBDPSCl (30.2 g, 110.0 mmol) was added by
syringe. The ice bath was removed and the reaction was stirred at
room temperature overnight (24 h). The reaction mixture was filtered,
and the filtrate was washed with 2 × 300 mL 10% HCl, 2 × 200 mL
H2O, and dried with MgSO4. Removal of the solvent under vacuum
gave a colorless oil (37.4 g). The resultant oil (37.4 g) was dissolved in
THF (1500 mL) and cooled in an ice bath. LiOH·H2O (9.4 g, 200.0
mmol) in 600 mL of H2O was added dropwise for over 15 min. The
ice bath was removed, and the reaction was stirred for 10 min. Water
(100 mL) was added and the THF was removed under vacuum. The
aqueous phase was extracted with 2 × 150 mL of ether to remove
starting material, acidified using 1.0 M HCl, and then extracted with 2
× 150 mL of ether. The second ethereal phase was dried with MgSO4
and the solvent was removed to give a colorless oil, and recrystallized
from hexane solution under isothermal condition at 4 °C (20.6 g, yield
1
= 60%). H NMR (CDCl3, 500 MHz): δ 7.7−7.6 (m, 4H, ArH), δ
7.5−7.2 (m, 6H, ArH), δ 4.27 (m, 1H, CH), δ 2.49 (m, 2H, CH2), δ
1.13 (d, J = 6.4 Hz, 3H, CH3), δ 1.03 (s, 9H, CH3). Anal. CHN Calcd:
C20H26O3Si: C, 70.1; H, 7.7; N, 0.0. Found: C, 70.1; H, 7.7; N, 0.0.
(R)-2-[(R)-3-(tert-Butyldiphenylsilanyloxy)butoxy]propionic Acid
Benzyl Ester (4: TBDPS-(R)-3HB-(R)-2HP-Bn). TBDPS-(R)-3HB (3)
(34.3 g, 100.0 mmol), (R)-2HP-Bn (1) (18.0 g, 100 mmol), and
DMAP (6.1 g, 50 mmol) were combined under nitrogen with 600 mL
of methylene chloride. DCC (20.6 g, 100 mmol) was added, and the
reaction was stirred for 18 h. The reaction mixture was filtered to
remove DCU, and the filtrate was concentrated under vacuum. The
crude oil was chromatographed over silica gel (5% ethyl acetate in
hexanes) to give a colorless oil (37.8 g, yield = 75%). 1H NMR
(CDCl3, 500 MHz): δ 7.7−7.6 (m, 4H, ArH), δ 7.5−7.2 (m, 11H,
(R)-2-[(R)-3-(Hydroxybutoxy)]propanoic Acid (8: (R)-3HB-(R)-
2HP). The benzyl dimeric monomer (6) (266.0 g, 10.0 mmol) was
hydrogenated by a flow reactor, using an EYELA CCR-1000G system
with 10% Pd on silica and an Air-tech NM-H-100. Hydrogen gas was
used at a flow rate of 10.0 mL/min, ethanol was used as an eluent at a
flow rate of 0.5 mL/min, and a concentration of 0.1 M was applied.
After reaction, the solvent was removed to give a colorless oil (1.75 g,
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Table 1. Molecular Weights of Alternating Copolymers of (R)-3HB and 2HP Units
dimeric monomer composition
a
b
dimeric monomer feed ratio (mol %)
(mol %)
molecular weight
yield
(%)
Mn
sample
(R)-3HB-(R)-2HP (R)-3HB-(S)-2HP (R)-3HB-(R)-2HP (R)-3HB-(S)-2HP
(×103 g/mol) Mw/Mn
P[(R)-3HB-alt-(S)-2HP]
(R/S = 0/100)
0
33
100
67
50
33
25
0
0
37
100
63
46
36
28
0
41
42
37
78
57
77
72
54
46
32
20
7
1.5
1.3
1.5
1.8
1.7
2.0
P[(R)-3HB-alt-(R,S)-2HP]
(R/S = 33/67)
P[(R)-3HB-alt-(R,S)-2HP]
(R/S = 50/50)
50
54
P[(R)-3HB-alt-(R,S)-2HP]
(R/S = 67/33)
67
64
P[(R)-3HB-alt-(R,S)-2HP]
(R/S = 75/25)
75
72
P[(R)-3HB-alt-(R)-2HP]
(R/S = 100/0)
100
100
a
b
1
Determined from H NMR spectra. Determined by GPC measurements.
1
yield = 99%). H NMR (CDCl3, 500 MHz): δ 5.19 (m, 1H, CH), δ
solidified polymer of (R)-3HB-(R)-2HP was dissolved in a
mixture solution of chloroform with 7.5% HFIP. The number-
average molecular weights (Mn) and polydispersities (Mw/Mn)
of the samples were determined from GPC analysis and ranged
from 7000 to 72 400 g/mol and from 1.3 to 2.0 by using a
polystyrene calibration, respectively. For the polymer obtained
from (R)-3HB-(S)-2HP dimeric monomer, the sample was
subjected to GPC analysis without precipitation treatment in
cool 2-propanol. Both the Mn and Mw/Mn values of crude
sample were almost identical with those of purified sample by
precipitation. This result indicates that the polymerization of
(R)-3HB-(S)-2HP dimeric monomer facilely gives product with
higher molecular weight than that of (R)-3HB-(R)-2HP
dimeric monomer. The Mn values of samples tended to
decrease with an increase of feed ratio of (R)-3HB-(R)-2HP
dimeric monomer.
4.27 (m, 1H, CH), δ 2.56 (m, 2H, CH2), δ 1.56 (d, J = 7.0 Hz, 3H,
CH3), δ 1.28 (d, J = 6.5 Hz, 3H, CH3). Anal. CHN Calcd: C7H12O5:
C, 47.7; H, 6.9; N, 0.0. Found: C, 45.5; H, 6.9; N, 0.0.
(S)-2-[(R)-3-(Hydroxybutoxy)]propanoic Acid (9: (R)-3HB-(S)-
2HP). In the same way as (R)-3HB-(R)-2HP (8), (R)-3HB-(S)-2HP
(9) was prepared from (R)-3HB-(S)-2HP-Bn (7). The product was a
1
colorless oil (2.29 g, yield = 100%). H NMR (CDCl3, 500 MHz): δ
5.19 (m, 1H, CH), δ 4.28 (m, 1H, CH), δ 2.54 (m, 2H, CH2), δ 1.52
(d, J = 7.5 Hz, 3H, CH3), δ 1.24 (d, J = 4.0 Hz, 3H, CH3). Anal. CHN
Calcd: C7H12O5: C, 47.7; H, 6.9; N, 0.0. Found: C, 46.8; H, 6.9; N,
0.0.
Condensation of Dimeric Monomers. 3HB-2HP dimeric
monomer (176.0 mg, 1.0 mmol) was added to 1 mL of methylene
chloride. DPTS (64.7 mg, 0.2 mmol) was added, and the reaction
mixture cooled to 0 °C. Then, DIC (189.3 mg, 232 μL, 1.5 mmol) was
added slowly for over 1 min. The reaction mixture was allowed to stir
for 5 h at room temperature (25 °C). The urea was removed by filter,
then taken up in methylene chloride and precipitated in cool 2-
propanol (IPA).
Sequence structure of the obtained copolymers was
1
examined by H and 13C NMR analyses. Figures 1 and 2
1
show H and 13C NMR spectra of the polymers obtained from
RESULTS AND DISCUSSION
■
3HB-2HP dimeric monomers, respectively. As previously
Synthesis of Alternating Copolymers of P[(R)-3HB-alt-
2HP]. The dimeric monomers of (R)-3HB and 2HP were
synthesized by coupling reaction of benzyl lactate with silyl
protected 3-hydroxybutanoic acid in the presence of DCC and
DMAP, according to the method reported by Meyer et al.,37
with modification of the conditions as necessary (see Scheme
1). Two types of stereoisomeric dimers ((R)-3HB-(R)-2HP
and (R)-3HB-(S)-2HP) were obtained by selecting the
enantiomer of the lactate monomer. By mixing the stereo-
isomers, dimeric monomers with different stereocompositions
were prepared.
reported,32 the majority of both proton and carbon resonances
Polymerization of dimeric monomers was carried out by
using condensation reaction in the presence of DIC/DPTS as a
condensation agent.41 As the condensation reaction progressed,
solidified urea was generated in the reaction mixture. The
polymer products were still soluble in methylene chloride after
the removal of urea by filtration. The polymer solution was
poured into cold 2-propanol to precipitate as a solid product.
Table 1 lists the polymerization results of stereoisomeric (R)-
3HB-2HP dimeric monomers. Polymer yields were varied from
37 to 78%. Except for the product from (R)-3HB-(R)-2HP
dimeric monomer, the obtained polymers were soluble in
methylene chloride or chloroform. However, the polymer from
(R)-3HB-(R)-2HP dimeric monomer became to be insoluble in
methylene chloride or chloroform once it was solidified. The
Figure 1. 500 MHz 1H NMR spectra of P[(R)-3HB-alt-2HP]
alternating copolymer in CDCl3 containing 7.5% HFIP at 25 °C.
Key: (a) P[(R)-3HB-alt-(R)-2HP], (b) P[(R)-3HB-alt-(S)-2HP], and
(c) P[(R)-3HB-alt-(R,S)-2HP] (R/S = 50/50).
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Figure 2. 125 MHz 13C NMR spectra of P[(R)-3HB-alt-2HP] alternating copolymer in CDCl3 containing 7.5% HFIP at 25 °C. Key: (a) P[(R)-
3HB-alt-(R)-2HP], (b) P[(R)-3HB-alt-(S)-2HP], and (c) P[(R)-3HB-alt-(R,S)-2HP] (R/S = 50/50).
for the random copolymers of 3HB and 2HP units were split
into multiple peaks due to diad or triad sequences of 3HB and
2HP units (see Figures S1 and S2 in the Supporting
Information). On the other hand, the polymers obtained
from 3HB-2HP dimeric monomers in this paper exhibited the
simple signals in each resonance. This result indicates that the
polymers have an alternating sequence of (R)-3HB and 2HP
units. Here, it is of important to note that the chemical shifts of
signals arising from the identical functional group were different
between the polymers obtained from (R)-3HB-(R)-2HP and
(R)-3HB-(S)-2HP dimeric monomers. Especially, significant
differences in the chemical shifts were detected at the signals
from methyl group of 2HP units in proton resonance and
signals from methylene group of (R)-3HB units in carbon
resonance, and the gaps between chemical shifts of two
stereoisomeric polymers were 0.04 ppm in proton resonance of
methyl group of 2HP units and 0.25 ppm in carbon resonance
of methylene group of (R)-3HB units, respectively. In the
NMR spectra for the polymers obtained from a mixture of (R)-
3HB-(R)-2HP and (R)-3HB-(S)-2HP dimeric monomers, the
characteristic signals from both stereoisomers were detected.
From the peak intensities of the signals, it was confirmed that
the stereoisomeric compositions of the obtained polymers were
in good agreement with the dimeric monomer feed ratios. The
signals from methylene group of (R)-3HB units in carbon
resonance were split into totally four peaks, and it was
attributed to the difference in stereosequence of 2HP units
adjoining both sides of (R)-3HB unit.
Physical Properties of Alternating Copolymers of
P[(R)-3HB-alt-2HP]. Thermal properties of alternating copoly-
mers of (R)-3HB and 2HP units were determined by DSC.
Figure 3 shows typical DSC thermograms of P[(R)-3HB-alt-
2HP] samples. All of the copolymers with alternating sequence
structure showed melting behavior during the first heating scan.
The values of melting temperatures (Tm) and heat of fusion
(ΔHm) are summarized in Table 2. The Tm value of P[(R)-
3HB-alt-(S)-2HP] was detected at 83 °C. On the other hand,
the P[(R)-3HB-alt-(R)-2HP] had the Tm at 233 °C, and the
value was much higher than those of homopolymers of (R)-
Figure 3. DSC thermograms of alternating copolymer samples at a
heating rate of 20 °C: (A) first heating scans and (B) second heating
scans after rapid quenching from the melt at 250 °C. Key: (a) P[(R)-
3HB-alt-(R)-2HP], (b) P[(R)-3HB-alt-(S)-2HP], and (c) P[(R)-3HB-
alt-(R,S)-2HP] (R/S = 50/50).
3HB and 2HP and was comparable to those of poly-
(glycolate)21 and stereocomplex poly(lactate).19 It was found
that the random copolymers of (R)-3HB and 2HP units with
almost equimolar composition were completely amorphous.
Therefore, the precise regulation of sequential structure as
alternating manner brought about the crystallization of
copolymer chains. For the samples prepared from a mixture
of (R)-3HB-(R)-2HP and (R)-3HB-(S)-2HP dimeric mono-
mers, the Tm values increased linearly from 53 to 174 °C as the
(R)-3HB-(R)-2HP dimeric monomer feed ratio was increased
from 33 to 75 mol % (see Figure 4A). The ΔHm values also
increased with an increase in the (R)-3HB-(R)-2HP dimeric
monomer feed ratio (Figure 4B). These results indicate that the
incorporation of stereoisomeric dimer inhibits the formation of
crystalline state composed of the repeating sequences of the
other stereoisomeric dimers. Thus, by selecting the combina-
tions of the enantiomeric 3HB and 2HP monomers, the
melting temperatures of the copolymers with alternating
sequence structure varied in the wide range.
For the samples with (R)-3HB-(R)-2HP dimeric monomer
contents less than 67 mol %, the change in heat capacity
corresponding to the glass transition phenomenon could be
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Table 2. Thermal Properties and X-ray Crystallinities of Alternating Copolymer Samples
thermal properties
a
b
c
d
e
sample
Tg (°C)
Tm (°C)
ΔHm (J/g)
Tdmax (°C)
crystallinity, Xc (%)
P[(R)-3HB-alt-(S)-2HP] (R/S = 0/100)
P[(R)-3HB-alt-(R,S)-2HP] (R/S = 33/67)
P[(R)-3HB-alt-(R,S)-2HP] (R/S = 50/50)
P[(R)-3HB-alt-(R,S)-2HP] (R/S = 67/33)
P[(R)-3HB-alt-(R,S)-2HP] (R/S = 75/25)
P[(R)-3HB-alt-(R)-2HP] (R/S = 100/0)
P[(R)-3HB-co-47 mol %(R)-2HP]
P[(R)-3HB]
18
14
19
21
84
41
2
300
296
298
299
301
304
300
299
57
7
5
53
5
126
151
174
233
n.d.
176
176
14
42
78
116
0
24
37
48
78
n.m.
74
47
5
5
5
5
f
n.d.
n.d.
15
4
g
96
43
5
5
P[(S)-2HP]
60
378
a
b
Glass-transition temperature; measured by DSC (second scan) from −100 to 250 °C at a rate of 20 °C/min. Melting temperature; measured by
c
d
DSC (first scan) from −120 to 250 °C at a rate of 20 °C/min. Enthalpy of fusion; measured by DSC (first scan). Thermal degradation; measured
by TG analysis from 25 to 500 °C at a rate of 20 °C/min. Degree of crystallinity; determined from X-ray diffraction patterns. Not detected. Not
e
f
g
measured.
from a mixture of (R)-3HB-(R)-2HP and (R)-3HB-(S)-2HP
dimeric monomers was inclined to have blocking tendency.
The thermal degradation profiles of alternating copolymers
of P[(R)-3HB-alt-2HP] were examined by TG analysis. The
temperatures of maximum degradation rate (Tdmax) were in the
range from 296 to 304 °C for the alternating copolymers of
P[(R)-3HB-alt-2HP]. It is well-known that the degradation of
P[(R)-3HB] occurs exclusively via random chain scission
reaction (cis-elimination), which has a six-membered ring ester
intermediate.44 On the other hand, the dominant thermal
decomposition of PLLA (P[(S)-2HP]) progresses via cyclic
rupture of polymer molecules by unzipping reaction from the
Figure 4. Effect of stereocomposition of dimeric monomer on the
melting temperature (A) and the heat of fusion (B) of alternating
copolymer samples.
hydroxyl chain-end or/and random intramolecular trans-
esterification at the main-chain.45 The Tdmax values of the
alternating copolymers of P[(R)-3HB-alt-2HP] were very close
to that of P[(R)-3HB] (299 °C), rather than P[(S)-2HP] (374
°C). Therefore, it was expected that the chain scission by cis-
detected in the DSC endotherms of second heating scan. The
Tg values of copolymers were in the range from 14 to 21 °C.
elimination occurred at the (R)-3HB units in the alternating
copolymers molecules during thermal degradation.
However, the Tg could not be detected in the DSC
Figure 5 shows the X-ray diffraction patterns of alternating
thermograms for the samples with (R)-3HB-(R)-2HP dimeric
monomer contents over than 75 mol %, since the crystallization
took place during the cooling process from the melt state at 250
°C.
copolymer samples, together with those of P[(R)-3HB] and
P[(S)-2HP]. The diffraction patterns of P[(R)-3HB] and
As well as the random copolymers of (R)-3HB and 2HP
units mentioned above, the most of copolymers with a random
sequence of almost equimolar two monomeric units become to
be amorphous. However, the polymer obtained from the
equimolar mixture of (R)-3HB-(R)-2HP and (R)-3HB-(S)-
2HP dimeric monomers formed crystalline state and showed
relative high melting temperature. As shown in Figure 2, in the
13C NMR spectrum for the polymer obtained from the
equimolar mixture of (R)-3HB-(R)-2HP and (R)-3HB-(S)-
2HP dimeric monomers, the methylene carbon signals of (R)-
3HB units were split into four peaks corresponding to the
different stereosequence of 2HP units adjoining both sides of
(R)-3HB unit. Since the peak intensities of each sequence were
not equivalent, the obtained polymer rather had blocking
tendency repeating the identical stereoisomeric dimers than
random sequence. Taking account of the difference in solubility
of precipitated polymers in methylene chloride, it was expected
that the dispersibility of resultant oligomers in the reaction
mixture changed during the polymerization dependent on both
the degree of condensation and the sequence of stereoisomeric
dimers. As a result, the sequential distribution of the polymers
Figure 5. Wide-angle X-ray diffraction patterns of alternating
copolymers.
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P[(S)-2HP] showed typical reflections arising from the α-forms
of the P[(R)-3HB] and P[(S)-2HP] crystalline lattice,
respectively. The alternating copolymer samples revealed
different patterns from neither P[(R)-3HB] nor P[(S)-2HP].
In addition, the diffraction patterns were quite different
between the two types of copolymers composed of (R)-3HB-
(R)-2HP and (R)-3HB-(S)-2HP dimeric monomers (see the
calculated d-spacing values listed in Table S2 of Supported
Information). Since the crystalline structure of these alternating
copolymers have not been characterized, further investigation
on the relationship between the thermal properties and
crystalline structure of these samples is necessary. However, it
is concluded that both the (R)-3HB and 2HP units participate
into the formation of crystalline region by repeating the
alternating sequence. The degrees of X-ray crystallinity (Xc) of
alternating copolymers samples are also listed in Table 2. The
Xc values of P[(R)-3HB-alt-2HP] samples decreased from 78%
to 7% as the fraction of (R)-3HB-(S)-2HP dimeric monomers
was increased from 0 to 67 mol %. The Xc value of P[(R)-3HB-
alt-(S)-2HP] sample was 73%.
the chemical shifts in the solution state were quite smaller than
those in the solid-state. This result suggests that the differences
in the chemical shifts in the solid-state CP/MAS 13C NMR
spectra are predominantly attributed to the conformational
structure of monomeric units in the crystalline states.
CONCLUSION
■
Novel alternating copolymers of (R)-3HB and 2HP units were
synthesized by condensation reaction of stereoisomeric dimers
of (R)-3HB-(R)-2HP and (R)-3HB-(S)-2HP. The effects of
enantiomeric structure of 2HP units and stereocomposition of
dimeric monomers on the thermal properties and crystalline
structure of alternating copolymers were investigated. Precisely
regulated alternating sequence of (R)-3HB and 2HP units were
crystallizable, and the formed crystals had different structure
from both P[(R)-3HB] and P[(S)-2HP] crystals. The melting
of the crystalline region for alternating copolymers varied in the
wide range of temperatures depending on both the
enantiomeric structure of 2HP units and stereocomposition
of dimeric monomers. In particular, the melting temperature of
P[(R)-3HB-alt-(R)-2HP] was reached to 233 °C, and the
copolymers was one of the aliphatic polyesters with highest
melting temperature. Further work to obtain details in
crystalline structure of alternating copolymers is currently
underway.
Figure 6 shows high-resolution solid-state CP/MAS 13C
NMR spectra of P[(R)-3HB-alt-(R)-2HP] and P[(R)-3HB-alt-
ASSOCIATED CONTENT
* Supporting Information
■
S
1H and 13C NMR spectra and tables of chemical shifts of
alternating copolymers and interplanar d-spacing values
measured from X-ray diffraction patterns of copolymer samples.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*(H.A.) E-mail: habe@riken.jp.
■
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Figure 6. Cross-polarization magic-angle spinning 13C NMR spectra of
alternating copolymers of P[(R)-3HB-alt-(R)-2HP] (a) and P[(R)-
3HB-alt-(S)-2HP] (b).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(S)-2HP]. In both of the P[(R)-3HB-alt-(R)-2HP] and P[(R)-
3HB-alt-(S)-2HP], the carbonyl signals from (R)-3HB and
2HP units were overlapped, and the single peak was detected.
The chemical shifts of carbonyl resonances were differed
P[(R)-3HB-alt-(R)-2HP] and P[(R)-3HB-alt-(S)-2HP] sam-
ples. The carbonyl signal of P[(R)-3HB-alt-(R)-2HP] was
shifted around 2 ppm toward lower magnetic field, compared
with that of P[(R)-3HB-alt-(S)-2HP]. The peak of methylene
group of (R)-3HB units also appeared at different chemical
shifts between the P[(R)-3HB-alt-(R)-2HP] and P[(R)-3HB-
alt-(S)-2HP]. In contrast to the carbonyl groups, the chemical
shifts of methylene group were shifted 3 ppm toward higher
magnetic field as changing from connecting with (R)-2HP to
with (S)-2HP units. The differences in chemical shifts of
resonance from the identical functional groups were also
detected in the 13C NMR spectra of solution dependent on
configuration of 2HP units (see Figure 2), however, the gaps in
This work supported by the Grant-in-Aid for Scientific
Research (C) from Japan Society for the Promotion of Science
(JSPS) (No. 26410231) (to H.A.).
REFERENCES
■
(1) Doi, Y. Microbial Polyesters; VCH Publishers: New York, 1990.
(2) Sudesh, K.; Abe, H.; Doi, Y. Prog. Polym. Sci. 2000, 25, 1503−
1555.
(3) Lenz, R. W.; Marchessault, R. H. Biomacromolecules 2005, 6, 1−8.
(4) Chen, G.-Q. Chem. Soc. Rev. 2009, 38, 2434−2446.
(5) Kulkarni, R. K.; Moore, E. G.; Hegyeli, A. F.; Leonard, F. J.
Biomed. Mater. Res. 1971, 5, 169−181.
(6) Kricheldorf, H. R.; Berl, M.; Scharnagl, N. Macromolecules 1988,
21, 286−293.
(7) Dubois, P.; Jacobs, C.; Jerome, R.; Teyssie, P. Macromolecules
1991, 24, 2266−2270.
(8) Auras, R.; Harte, B.; Selke, S. Macromol. Biosci. 2004, 4, 835−864.
7360
dx.doi.org/10.1021/ma501783f | Macromolecules 2014, 47, 7354−7361

Macromolecules
Article
(9) Findlay, R. H.; White, D. C. Appl. Environ. Microbiol. 1983, 45,
71−78.
(10) Doi, Y.; Kunioka, M.; Nakamura, Y.; Soga, K. Macromolecules
1986, 19, 2860−2864.
(11) Nakamura, S.; Doi, Y.; Scandola, M. Macromolecules 1992, 25,
4237−4241.
(12) Saito, Y.; Doi, Y. Int. J Biol. Macromol. 1994, 16, 99−104.
(13) Doi, Y.; Abe, C. Macromolecules 1990, 23, 3705−3707.
(14) Doi, Y.; Kitamura, S.; Abe, H. Macromolecules 1995, 28, 4822−
4828.
(15) Taguchi, S.; Yamada, M.; Matsumoto, K.; Tajima, K.; Satoh, Y.;
Munekata, M.; Ohno, K.; Kohda, K.; Shimamura, T.; Kambe, H.;
Obata, S. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 17323−17327.
(16) Shozui, F.; Matsumoto, K.; Motohashi, R.; Sun, J.; Satoh, T.;
Kakuchi, T.; Taguchi, S. Polym. Degrad. Stab. 2011, 96, 499−504.
(17) Sodergarda, A.; Stolt, M. Prog. Polym. Sci. 2002, 27, 1123−1163.
(18) Abe, H.; Doi, Y.; Hori, Y.; Hagiwara, T. Polymer 1997, 39, 59−
67.
(19) Ikada, Y.; Jamshidi, K.; H. Tsuji, K.; Hyon, S. H. Macromolecules
1987, 20, 904.
(20) Tsuji, H. Macromol. Biosci. 2005, 5, 569.
(21) Tsuji, H.Polyesters III: Applications and Commercial Products
In Biopolymers; Doi, Y., Steinbuchel, A., Eds.; VCH: Weinheim, 2002;
Vol. 4, 129.
(22) Ueda, M. Prog. Polym. Sci. 1999, 24, 699−730.
(23) Badi, N.; Lutz, J. F. Chem. Soc. Rev. 2009, 38, 3383−3390.
(24) Thomas, C. M. Chem. Soc. Rev. 2010, 39, 165−172.
(25) Ouchi, M.; Badi, N.; Lutz, J. F.; Sawamoto, M. Nat. Chem. 2011,
3, 917−924.
(26) Wu, T. K. J. Phys. Chem. 1969, 73, 1801−1805.
(27) Bluhm, T. L.; Hamer, G. K.; Marchessault, R. H.; Fyfe, C. A.;
Vergin, R. P. Macromolecules 1986, 19, 2871−2876.
(28) Lenz, R. W.; Go, S. J. Polym. Sci. Polym. Chem. Ed. 1974, 12, 1−
10.
́
(29) Fernandez, J.; Meaurio, E.; Chaos, A.; Etxeberria, A.; Alonso-
Varona, A.; Sarasua, J. R. Polymer 2013, 54, 2621−2631.
(30) Hirooka, M.; Yabuuchi, H.; Iseki, J.; Nakai, Y. J. Polym. Sci., Part
A-1 1968, 6, 1381−1396.
(31) Shirota, Y.; Yoshimura, M.; Matsumoto, A.; Mikawa, H.
Macromolecules 1974, 7, 4.
(32) Tabata, Y.; Abe, H. Polym. Degrad. Stab. 2013, 98, 1796−1803.
(33) Sworen, J. C.; Smith, J. A.; Berg, J. M.; Wagener, K. B. J. Am.
Chem. Soc. 2004, 126, 11238−11246.
(34) Satoh, K.; Matsuda, M.; Nagai, K.; Kamigaito, M. J. Am. Chem.
Soc. 2010, 132, 10003−10005.
(35) Satoh, K.; Ozawa, S.; Mizutani, M.; Nagai, K.; Kamigaito, M.
Nat. Commun. 2010, 1, 1−6.
(36) Matsuda, M.; Satoh, K.; Kamigaito, M. J. Polym. Sci. Part A.
Polym. Chem. 2013, 51, 1774−1785.
(37) Stayshich, R.; Meyer, T. J. Polym. Sci., Part A: Polym. Chem.
2008, 46, 4704−4711.
(38) Stayshich, R.; Meyer, T. J. Am. Chem. Soc. 2010, 132, 10920−
10934.
(39) Li, J.; Stayshich, R.; Meyer, T. J. Am. Chem. Soc. 2011, 133,
6910−6913.
(40) Kramer, J. W.; Treitler, D. S.; Dunn, E. W.; Castro, P. M.;
Roisnel, T.; Thomas, C. H.; Coates, G. W. J. Am. Chem. Soc. 2009,
131, 16042−16044.
(41) Moore, J. S.; Stupp, S. I. Macromolecules 1990, 23, 65−70.
(42) Vonk, C. G. J. Appl. Crystallogr. 1973, 6, 148−152.
(43) Wolfe, S.; Wilson, M. C.; Cheng, M. H.; Chustov, G. V.;
Akuche, C. I. Can. J. Chem. 2003, 81, 937−961.
(44) Aoyagi, Y.; Yamashita, K.; Doi, Y. Polym. Degrad. Stab. 2002, 76,
53−59.
(45) Kopinke, F. D.; Remmler, M.; Mackenzie, K.; Moder, M.;
Wachsen, O. Polym. Degrad. Stab. 1996, 53, 329−342.
7361
dx.doi.org/10.1021/ma501783f | Macromolecules 2014, 47, 7354−7361