Synthesis and property of novel amino acid-based polymers by self-polyaddition of monomers containing both oxetanyl and carboxyl groups

Article abstract of DOI:10.1002/pola.25053

A new synthetic strategy for polymers containing amino acids in the main chain was developed. Monomers N-oxetanylalanine (N-Oxe-Ala-COOH), N-oxetanylglutamic acid (N-Oxe-Glu-COOH), and N-oxetanyllysine (N-Oxe-Lys-COOH) containing both oxetanyl and carboxyl groups were synthesized, and self-polyaddition and self-copolyaddition of these monomers afforded the corresponding polymers containing amino acids in the main chain [poly( _OxAla), poly(_OxLys), poly(_OxGlu), poly( _OxAla-co-_OxGlu), poly(_OxGlu-co- _OxLys), and poly(_OxAla-co-_OxLys)] with molecular weight in the range of 920-6620, in satisfactory yields. The physical properties, such as solubility, glass transition temperature, and thermal stability, were consistent with the amount of carboxyl groups at the chain ends. Biodegradability of the polymers was examined by the biochemical oxygen demand method; the decomposition ratios of poly(_OxAla) and poly( _OxAla-co-_OxGlu) were about 60%, whereas that of poly( _OxGlu) was nearly 100% after 28 days. 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011 This article reports the development of new strategy for polymers containing amino acids in the main chain. Monomers containing both oxetanyl and carboxyl groups were synthesized derived from amino acids, and self-polyaddition and self-copolyaddition of these monomers afforded the corresponding polymers containing amino acids in the main chain. The obtained amino -acid- based polymers had good solubility, high- glass transition temperature, high thermal stability, and good biodegradability. Copyright

Full text of DOI:10.1002/pola.25053

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Synthesis and Property of Novel Amino Acid-Based Polymers by
Self-Polyaddition of Monomers Containing both Oxetanyl and
Carboxyl Groups
1
1
1
1
Hiroto Kudo, Toshiki Ohshiro, Hiroyuki Atsumi, Ayako Kamio,
1
2
Tadatomi Nishikubo, Saeko Murakami
1
Department of Materials and Life Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1, Rokkakubashi, Kanagawa-ku,
Yokohama 221-8686, Japan
2
Kanagawa Industrial Technology Center, 705-1, Shimoimaizumi, Ebina-Shi, Kanagawa 243-0435, Japan
Correspondence to: T. Nishikubo (E-mail: nishikubot@kanagawa-u.ac.jp)
Received 4 August 2011; accepted 8 October 2011; published online 7 November 2011
DOI: 10.1002/pola.25053
ABSTRACT: A new synthetic strategy for polymers containing
amino acids in the main chain was developed. Monomers
N-oxetanylalanine (N-Oxe-Ala-COOH), N-oxetanylglutamic acid
solubility, glass transition temperature, and thermal stability,
were consistent with the amount of carboxyl groups at the
chain ends. Biodegradability of the polymers was examined by
the biochemical oxygen demand method; the decomposition
ratios of poly(OxAla) and poly(OxAla-co-OxGlu) were about 60%,
whereas that of poly(OxGlu) was nearly 100% after 28 days.
VC 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
(
N-Oxe-Glu-COOH), and N-oxetanyllysine (N-Oxe-Lys-COOH)
containing both oxetanyl and carboxyl groups were synthe-
sized, and self-polyaddition and self-copolyaddition of these
monomers afforded the corresponding polymers containing
amino acids in the main chain [poly(OxAla), poly(OxLys), poly
50: 458–465, 2012
(
OxGlu), poly(OxAla-co-OxGlu), poly(OxGlu-co-OxLys), and poly
OxAla-co-OxLys)] with molecular weight in the range of 920–
(
KEYWORDS: amino acid; biodegradable; hyperbranched; self-
6620, in satisfactory yields. The physical properties, such as
polyaddition; synthesis
INTRODUCTION Amino acid-based polymers, that is, poly
amino acid)s, are commonly synthesized by means of ani-
onic ring-opening polymerization of N-carboxyanhydrides
reported the certain polymers containing amino acid in their
main chain by the polyaddition of amines and acrylamides
and showed that physicochemical and biological properties
of the obtained polymers were examined. Endo and co-
(
1
2
(
NCAs) derived from amino acids. Deming reported new
7
living polymerization of NCA by the transition metal initia-
tors system using cobalt or iron and developed the func-
workers reported the synthesis of various polymers con-
taining amino acid moieties in the main or side chains using
a wide variety of polymerization methods and suggested that
the products could find application as optically active, bio-
compatible, or biodegradable materials. Recently, they also
reported that NCAs could be synthesized through N-phenyl-
carbamate amino acids as intermediates, that is, NCAs could
3
tional well-defined block copolypeptides. However, this syn-
thetic strategy has some problems. NCAs are usually
synthesized via the noncatalytic reaction of amino acids with
phosgene in an aprotic solvent, and the reaction proceeds
with release of HCl gas. As phosgene (triphosgene) and HCl
are both poisonous, great care is needed. Bases such as pyri-
dine and triethylamine cannot be used to trap HCl in situ
during the synthesis, because NCAs are highly reactive and
8
be synthesized without the use of phosgene. In spite of
these attempts at the synthesis of amino acid-based poly-
mers, NCA is still the only general monomer for the synthe-
sis of amino acid-based polymers. This is presumably
because direct condensation of amino acids is unsatisfactory
to obtain the corresponding polymers, and only a cyclic com-
pound, 2,5-piperazinedione, is formed selectively.
readily polymerize in the presence of weak bases or H O as
2
initiators, that is, NCAs are not easy to handle.
4
Sanda and coworkers reported various polymers containing
amino acids in the side chain and showed that the secondary
structure of the obtained polymers could be controlled.
Furthermore, they succeeded in developing an alternating
ring-opening metathesis copolymerization by using the inter-
We have examined the synthesis and self-polyaddition of
9
monomers containing both oxetanyl and carboxyl groups.
The self-polyaddition proceeded in the presence of onium
salts as catalysts to give the corresponding linear hetero-
5
6
action between acid and base moieties. Ferruti et al.
V
C
2011 Wiley Periodicals, Inc.
458
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telechelic polymers with oxetanyl and carboxyl groups at the
chain ends, with satisfactory molecular weights and yields.
As the reaction of oxetane and carboxylic acid can proceed
only in the presence of quaternary onium salts as catalysts,
the monomers containing both oxetanyl and carboxyl groups
are stable and easy to handle. In the course of our study on
the self-polyaddition, we considered that these monomers
might be suitable for developing a general synthetic strategy
for amino acid-based polymers.
washed with ethyl acetate (300 mL) thrice. The aqueous
phase was separated and acidified to pH ¼ 3.0 with concen-
trated H SO . The resulting aqueous mixture was extracted
2
4
with ethyl acetate (300 mL) five times. The organic phase
was dried over MgSO and concentrated in a rotary evapora-
4
tor. The residue was purified by recrystallization from ethyl
ꢀ
acetate at ꢁ20 C to obtain 3-carboxy-3-ethyloxetane (CEO)
as a colorless solid.
ꢀ
ꢁ1
Yield ¼ 2.41 g (86%). Mp ¼ 31.0–31.9 C. IR (film, cm ):
3400 (m COOH), 2886 (m CH
In this article, we examined the synthesis and self-polyaddi-
tion reaction of oxetanyl and carboxyl functionalized mono-
mers derived from alanine, glutamic acid, and lysine. Fur-
thermore, the biodegradability of the products was
examined.
2
), 1724 (m C ¼¼ O carbonyl), 970
1
(m CAOAC of oxetane). H NMR (600 MHz, DMSO-d , TMS) d
6
(ppm): 0.83 (t, J ¼ 7.2 Hz, 3.0H, ACH ), 1.93 (q, J ¼ 7.2 Hz,
3
2.0H, ACH A), 4.33 and 4.70 (AB quartet, J ¼ 6.0 Hz, 4.0H,
2
1
3
ACH A of cyclic ether), 12.64 (s, 1.0H, ACOOH). C NMR
2
(
150 MHz, DMSO-d , TMS) d (ppm): 8.76 (CH ), 28.3
6
3
EXPERIMENTAL
(ACH
2
CH
3
), 48.4 (>C<), 76.9 (AOACH
2
A), 175.2 (>C ¼¼ O).
Elemental analysis data for CEO. Calcd. for C H O C,
5
Measurements
Infrared (IR) spectra were measured on a Thermo Electron
Model NICOLET 380 spectrometer. The H NMR spectra were
recorded on JEOL Model JNM a-500 (500 MHz for H NMR)
instruments in DMSO-d and CDCl using Me Si (TMS) as an
internal standard reagent for H NMR. The number-average
molecular weight (M ) and molecular weight distribution
14 20 5
5.37; H, 7.74. Found: C, 55.32; H, 7.91.
1
1
Synthesis of Monomers Containing both Oxetanyl and
Carboxyl Groups
Synthesis of N-Oxetanylalanine Methyl Ester
6
3
4
1
(
N-Oxe-Ala-COOMe)
n
EDC HCl (5.75 g, 30 mmol) was added to a solution of CEO
(
M /M ) of the polymers were estimated by size-exclusion
w n
ꢀ
(
3.52 g, 27 mmol) in CH Cl (27 mL) at 0 C. The resulting
2
2
chromatography (SEC) with a Tosoh model HLC-8120 GPC
equipped with refractive index and ultraviolet detectors and
TSK gel columns (eluent dimethylformamide (DMF), cali-
brated with narrow-molecular weight polystyrene standards).
Thermal analysis was performed on a Seiko Instruments ther-
mogravimetric analyzer Model EXSTAR6000/TG/DTA6200
and differential scanning calorimetry Model EXSTAR6000/
solution was stirred for 2 h. NEt3 (4.17 mL, 30 mmol) and
L-alanine methyl ester hydrochloride (4.19 g, 30 mmol) were
added, and the mixture was stirred at 25 C for 24 h, then
washed successively with 1 N hydrogen chloride solution,
saturated aqueous sodium hydrogen carbonate, and water.
4
The organic phase was dried over MgSO and concentrated
in a rotary evaporator to obtain N-oxetanylalanine methyl
ester (N-Oxe-Ala-COOMe) as a colorless oil, which was used
for the next step without further purification.
ꢀ
ꢀ
DSC6200 at a heating rate of 10 C/min under nitrogen.
Materials
1
-Methyl-2-pyrrolidinone was dried over CaH2 and purified
in the usual way before use. 3-Ethyl-3-hydroxymethyloxetane
EHO) was a gift from Toa Gosei Company.
ꢁ
1
Yield ¼ 2.46 g (42%). IR (KRS, cm ): 3306 (m NH), 2965
and 2880 (m CH CH ), 1746 (m C ¼¼ O of ester), 1650 (m C ¼¼ O
2
3
(
1
of amide), 982 (m CAOAC of oxetane). H NMR (500 MHz,
DMSO-d , TMS) d (ppm): 0.78 (t, J ¼ 7.5 Hz, 3.0H, ACH ),
Jones reagent was prepared from a mixture of CrO (3.80 g)
and concentrated H SO (3.25 mL) in H O (19.7 mL) accord-
3
6
3
2
4
2
1.29 (d, J ¼ 7.5 Hz, 2.0H, ACH A), 1.95 (q, J ¼ 7.5 Hz, 2.0H,
2
1
0
ing to the usual method. 1-Ethyl-3-(3-dimethylaminopro-
pyl)carbodiimide hydrochloride (EDC HCl), isopropyl alcohol,
NEt , CH Cl , L-alanine methyl ester hydrochloride, L-glutamic
ACH CH ), 3.63 (s, 3.0H, AOCH ), 4.21–4.24 (m, 1.0H,
2
3
3
>
CHA), 4.27 and 4.66 (AB quartet, 4.0H, AOACH A), 8.20
2
3
2
2
(s, 1.0H, ANHA).
acid dimethyl ester hydrochloride, L-lysine methyl ester
hydrochloride, and tetraphenylphosphonium bromide (TPPB)
were commercial-grade products and were used without fur-
ther purification. o-Dichlorobenzene was distilled over P2O5
before use.
Synthesis of N-Oxetanylalanine (N-Oxe-Ala-COOH)
A solution of N-Oxe-Ala-COOMe (2.46 g, 11.42 mmol) in 6 M
ꢀ
NaOH (13 mL) was stirred at 25 C. After 12 h, the aqueous
solution was acidified to pH 3.0 with concentrated HCl. The
resulting mixture was extracted with ethyl acetate (100 mL)
Synthesis of 3-Carboxy-3-ethyloxetane
1
0 times. The organic phase was dried over MgSO and con-
4
A solution of Jones reagent (22.8 mL) and acetone (180 mL)
centrated in a rotary evaporator. The residue was poured
into large amount of ethyl ether to precipitate N-Oxe-Ala-
COOH as a white solid.
ꢀ
was stirred at 0 C. After 1 h, a solution of EHO (2.78 g, 24
mmol) in acetone (180 mL) was added slowly and stirring
ꢀ
ꢀ
was continued at ꢁ5 C for 2 h, then at 25 C. After 4 h, iso-
ꢀ
ꢁ1
propyl alcohol (5 mL) was added to the mixture at below 0
C. The resulting suspension was filtered through Celite to
Yield ¼ 1.33 g (54%). Mp ¼ 146.0–146.6 C. IR (KRS, cm ):
3299 (m NH), 1731 (m C ¼¼ O), 1616 (m C ¼¼ O of amide),1220
ꢀ
1
remove insoluble products, and the filtrate was concentrated
in a rotary evaporator. Then, NaOH aqueous (6 M) (300 mL)
was added to the residue, and the resulting mixture was
and 1186 (m CAOAC), 968 (m cyclic ether). H NMR (600
MHz, DMSO-d , TMS) d (ppm): 0.77 (t, J ¼ 6.5 Hz, 3.0H,
6
ACH ), 1.26 (d, J ¼ 6.5 Hz, 3.0H, >CHCH ), 1.92 (q, J ¼ 6.5
3
3
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Hz, 2.0H, ACH A), 4.25–4.65 (m, 5.0H, AOACH A and
Synthesis of N-Oxetanyllysine (N-Oxe-Lys-COOH)
N-Oxe-Lys-COOH was synthesized by the same method
as described for the synthesis of N-Oxe-Ala-COOH, but using
N-Oxe-Lys-COOMe.
2
2
1
3
>
CHA), 8.07 (s, 1.0H, ANHA), 12.04 (s, 1.0H, ACOOH).
C
NMR (150 MHz, DMSO-d , TMS) d (ppm): 8.49 (ACH CH ),
6
2
3
1
6.93 (>CHACH ), 29.37 (ACH CH ), 47.57 (quaternary car-
3 2 3
bon of cyclic ether), 48.96 (quaternary carbon of alanine),
7.27 (AOCH A), 174.21 (ACOOH), 181.2 (ACONHA).
Elemental analysis data for N-Oxe-Ala-COOH. Calcd. for
ꢁ1
Yield ¼ 76% (viscous oil). IR (KRS, cm ): 3,305 (m NH), 2,969
7
2
and 2,888 (m CAH), 1,724 (m C ¼¼ O of ester), 1649 (m C ¼¼ O of
1
amide), 982 (m cyclic ether). H NMR (500 MHz, DMSO-d ,
6
C H NO C, 53.72; H, 7.51; N, 6.96. Found: C, 53.48; H, 7.43;
9
15
4
TMS) d (ppm): 0.76 (t, J ¼ 7.5 Hz, 6.0H, ACH ), 1.27–1.42 (m,
3
N, 6.71.
4
.0H, ACH CH ACH<), 1.54–1.76 (m, 2.0H, ANHCH CH A),
2 2 2 2
Synthesis of N-Oxetanylglutamic Acid Methyl Ester
1.96 (q, J ¼ 7.5 Hz, 4.0H, ACH
2
CH
3
), 3.06 (t, J ¼ 4.0 Hz, 2.0H,
ANHACH A), 4.18 (q, J ¼ 5.0 Hz, 1.0H, >CHA), 4.25 and 4.64
(N-Oxe-Glu-COOMe)
2
(
AB quartet, 8.0H, AOACH A), 5.74 (s, 1.0H, >CHA), 7.77 (t, J
N-Oxe-Glu-COOMe was synthesized by the same method as
described for the synthesis of N-Oxe-Ala-COOMe, but using
L-glutamic acid dimethyl ester hydrochloride.
2
¼
4.0 Hz, 1.0H, AC(O)NHA), 8.11(d, J ¼ 5.0 Hz, 1.0H,
13
AC(O)NHA), 12.4 (s, 1.0H, ACOOH). C NMR (150 MHz,
DMSO-d , TMS) d (ppm): 8.48 and 8.63 (CH A), 23.13
ꢁ
1
6
3
Yield ¼ 47%. IR (KRS, cm ): 3308 (m NH), 2,959 and 2,880
(
CH
3
CH
2
A), 28.66, 29.40, and 38.37 (AC
4
H
8
A), 49.1 (quater-
(
m CH CH ), 1,741 (m C ¼¼ O of ester), 1,653 (m C ¼¼ O of amide),
2
3
nary carbon), 51.80 (>CHA), 77.30 and 79.20 (ACH OA),
1
2
9
83 (m CAOAC of oxetane). H NMR (500 MHz, DMSO-d ,
6
173.24 (ACOOH), and 173.73 and 173.81 (ACONHA). matrix
TMS) d (ppm): 0.78 (t, J ¼ 7.5 Hz, 3.0H, ACH ), 1.95–2.03
3
assisted Laser desorption and ionization time of flight mass
(
m, 4.0H, ACH A), 2.40 (q, J ¼ 7.5 Hz, 2.0H, ACH CH ), 3.63
þ
2
2
3
spectroscopy (MALDI-TOF MS) m/z (M þ Na) Calcd. for
(
s, 1.0H, >CHA), 4.28 and 4.67 (AB quartet, 4.0H,
A), 8.40 (s, 1.0H, ANHA).
(
C H N O þ Na): 393.54. Found: 393.43.
1
8 30 2 6
AOACH
2
Self-Polyaddition of N-Oxe-Ala-COOH
Synthesis of N-Oxetanylglutamic Acid (N-Oxe-Glu-COOH)
N-Oxe-Glu-COOH was synthesized by the same method as
described for the synthesis of N-Oxe-Ala-COOH, but using
N-Oxe-Glu-COOMe.
A typical procedure for the self-polyaddition of N-Oxe-Ala-
COOH was as follows: N-Oxe-Ala-COOH (0.1 g, 0.5 mmol),
TPPB (0.021 g, 0.05 mmol), and o-dichlorobenzene (0.5 mL)
were charged into a glass tube, which was evacuated and
ꢀ
ꢁ1
Yield ¼ 92%. Mp ¼ 144.3–145.6 C. IR (KRS, cm ): 3,343
m NH), 2,968 (m CAH), 1,732 (m C ¼¼ O of ester), 1,615 (m
sealed using a gas torch. The reaction was performed at
(
ꢀ
1
30 C for 24 h in the sealed tube with stirring. The polymer
C ¼¼ O of amide), 1,220 and 1,186 (m CAOAC), 968 (m cyclic
solution was then poured into a large amount of ethyl ether
1
ether). H NMR (500 MHz, DMSO-d , TMS) d (ppm): 0.78 (t,
6
to precipitate poly( Ala), which was dried in vacuo at room
ox
J ¼ 7.5 Hz, 3.0H, ACH ), 1.81–1.94 (m, 4.0H, ACH CH A),
3
2
2
temperature for 24 h. The yield of poly( Ala) was 0.089 g
ox
2
.40 (q, 2.0H, ACH CH ), 4.21–4.24 (m, 1.0H, >CHA), 4.27
2 3
(89%). The number-average molecular weight (M ) of the
n
and 4.67 (AB quartet, 4.0H, AOACH A), 8.07 (s, 1.0H,
2
polymer estimated by SEC was 5100 (M /M ¼ 1.70).
w
n
1
3
ANHA), 12.40 (s, 2.0H, ACOOH).
C NMR (150 MHz,
ꢁ
1
IR (KRS, cm ): 3,374 (m OAH), 3,340 (m ANHA), 1740
DMSO-d , TMS) d (ppm): 8.52 (ACH CH ), 29.42 (ACH CH ),
6
2
3
2
3
1
(
m C ¼¼ O ester), 1,226 (m CAOAC ester). H NMR (600 MHz,
DMSO-d TMS) (ppm): 0.75 (s, ACH CH ), 1.25
), 1.47–1.53 (m, ACH CH ), 3.49–3.60 (m, >CHA),
2
5.98 and 30.27 (ACH CH A), 49.13 (quaternary carbon of
2 2
6
,
d
2
3
cyclic ether), 51.22 (quaternary carbon of alanine), 77.34
(
s, >CHCH
3
2
3
(
AOCH A), 172.44 (>CHCOOH), 173.82 (ACH COOH),
2
2
4
.03–4.26 (m, ACH OH and ACOOCH A), 4.65 (s, AOH),
2
2
173.85 (ACONHA). Elemental analysis data for N-Oxe-Glu-
7.81–8.16 (m, ANHA).
COOH. Calcd. for C H NO C, 50.96; H, 6.61; N, 5.40. Found:
1
1
17
6
C, 50.94; H, 6.34; N, 5.11.
Self-Polyaddition of N-Oxe-Glu-COOH
The self-polyaddition of N-Oxe-Glu-COOH was performed by
the same method as described for N-Oxe-Ala-COOH. Yield of
poly(oxGlu) was 85%. Poly(oxGlu) was insoluble in common
organic solvents, and SEC and NMR measurements could not
be done.
Synthesis of N-Oxetanyllysine Methyl Ester
(N-Oxe-Lys-COOMe)
N-Oxe-Lys-COOMe was synthesized by the same method as
described for the synthesis of N-Oxe-Ala-COOMe, but using
L-lysine methyl ester hydrochloride.
ꢁ
1
ꢁ
1
IR (KBr, cm ): 3420 (m OAH), 2,974 (m ACHA), 1,742 (m C ¼¼ O
ester), 1,651 (m C ¼¼ O amide), 1,206 (m CAOAC ester).
Yield ¼ 28%. IR (KRS, cm ): 3305 (m NH), 2969 and 2888
(
m CH CH ), 1724 (m C ¼¼ O of ester), 1649 (m C ¼¼ O of amide),
2
3
1
9
82 (m CAOAC of oxetane). H NMR (500 MHz, DMSO-d ,
6
Self-Polyaddition of N-Oxe-Lys-COOH
TMS) d (ppm): 0.76 (t, J ¼ 7.5 Hz, 6.0H, ACH ), 1.27–1.40
3
The self-polyaddition of N-Oxe-Lys-COOH was performed by
the same method as described for N-Oxe-Ala-COOH. The
yield of poly( Lys) was 89%. M ¼ 6620, M /M ¼ 3.81.
(m,
4.0H,
ACH CH ACH<),
1.61–1.73
(m,
2.0H,
2
2
ANHCH CH A), 1.94 (q, J ¼ 6.0 Hz, 4.0H, ACH CH ), 3.06
2
2
2
3
ox
1
n
w
n
(
q, J ¼ 4.0 Hz, 2.0H, ANHACH
2
A), 3.61 (s, 3.0H, AOCH
3
),
ꢁ
4
.25 and 4.64 (AB quartet, 4.0H, AOACH A), 5.74 (s, 1.0H,
IR (KBr, cm ): 3,345 (m OAH), 2,965 and 2,937 (m ACHA),
2
>
CHA), 7.77 (d, J ¼ 5.5 Hz, 1.0H, AC(O)NHA), 8.11(d, J ¼
1,740 (m C ¼¼ O ester), 1,650 (m C ¼¼ O amide), 1,267 (m CAOAC
1
5.5 Hz, 7.5H, AC(O)NHA).
ester), 980 (m CAOAC oxetane). H NMR (600 MHz, DMSO-d ,
6
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SCHEME 1 Synthesis of N-oxetanylamino acids.
TMS)
d
(ppm): 0.76 (broad s, ACH CH ), 1.27–1.98
oxetanylamino acid methyl ester. The corresponding products
were treated with alkali aqueous solution to give the amino
acid derivatives containing both oxetanyl and carboxyl groups,
as shown in Scheme 1. The structures of the synthesized
2
3
(m, >CHCH ACH ACH ACH ANHA, and ACH CH ), 3.06 (broad
2
2
2
2
2
3
s, >CHCH A), 3.43–3.58 (m, >CHA), 4.15–4.28 (C( ¼¼ O)OCH A
2
2
and ACH OH), 4.64 (s, ACH OH), 7.71–8.07 (m, ANHA).
2
2
1
13
amino acid derivatives were confirmed by IR, H NMR, and
C
Biodegradability of the Polymers
NMR spectroscopy, as well as elemental analysis or MALDI-
TOF mass spectrometry.
The biodegradability of the synthesized polymers was exam-
ined by the biochemical oxygen demand (BOD) method using
activated sludge according to the Japanese Industrial Stand-
ards method, as follows. A mixture of polymer (100 mg) and
activated sludge (30 mg) in a culture medium was stirred
In the case of L-alanine methyl ester hydrochloride, the cor-
responding N-oxetanylalanine (N-Oxe-Ala-COOH) containing
an oxetanyl group and a carboxyl group could be synthe-
sized. N-Oxe-Glu-COOH, containing an oxetanyl group and
two carboxyl groups, and N-Oxe-Lys-COOH, containing two
oxetanyl groups and a carboxyl group, were similarly synthe-
ꢀ
slowly at 25 C for 28 days. The value of biodegradability
(
%) was calculated as the ratio of the values of BOD and the-
oretical oxygen demand.
sized from glutamic acid and lysine, respectively. These
0
RESULTS AND DISCUSSION
amino acid derivatives can be regarded as AB, ABB , and A
type monomers for self-polyaddition.
B
2
Monomer Synthesis
Careful design is needed to synthesize monomers containing
two different reactive groups, which can undergo self-polyad-
dition. Here, we employed N-oxetanylamino acids as mono-
mers, because the reaction of oxetane and carboxylic acid to
afford the corresponding polyester can only proceed in the
Self-Polyaddition
9
In our previous report, the self-polyaddition of monomers
containing both oxetanyl and carboxyl groups proceeded using
onium salts as catalysts, affording hetero telechelic polyesters
with pendant primary hydroxyl groups in the side chains, and
oxetanyl and carboxyl groups at the chain ends. In this system,
TPPB showed the highest catalytic activity among the quater-
nary onium salts in o-dichlorobenzene as a reaction solvent,
and the reaction did not occur in the absence of quaternary
onium salt. This means that the monomers containing both
oxetanyl and carboxyl groups are stable at room temperature,
so they are easy to handle and should be useful for self-poly-
addition. This time, we examined the self-polyaddition and
1
1
presence of an appropriate catalyst. We carried out the oxi-
dation reaction of EHO using Jones reagent. When this reac-
tion was carried out using H SO , only hydroxyl groups of
2
4
EHO were converted to carboxyl groups, and the oxetane moi-
ety remained intact, that is, the corresponding 3-carboxyl-3-
ethyloxetane (CEO) could be synthesized. Next, the reaction of
CEO and amino acid methyl ester hydrochloride was carried
out using EDC HCl as a condensation agent, affording the N-
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SCHEME 2 Self-polyaddition of N-oxetanylamino acids.
1
self-copolyaddition of N-Oxe-Ala-COOH, N-Oxe-Glu-COOH, and
N-Oxe-Lys-COOH using TPPB as a catalyst in o-dichloroben-
zene at 130 C for 48 h (Scheme 2). Table 1 summarizes the
appeared after the reaction. Figure 1 depicts the H NMR
spectra before and after the self-polyaddition of N-Oxe-Ala-
COOH: the peaks at 4.28 and 4.68 ppm assignable to the oxe-
tane moiety disappeared, and we observed new peaks at
ꢀ
reaction conditions and results.
4.03–4.26 ppm and 4.65 ppm, which are assignable to meth-
In the case of N-Oxe-Ala-COOH, polymer with Mn ¼ 5100
ylene and hydroxyl protons of the polymer formed by addi-
tion reaction of the oxetanyl and carboxyl groups. These
findings indicate that the self-polyaddition of N-Oxe-Ala-
COOH proceeded to afford poly(OxAla) with pendant hydroxyl
groups and alanine moieties in the main chain, as shown in
Scheme 2. The yield was 89%.
(
M /M ¼ 1.70) was obtained. Its structure was confirmed
w
n
1
by IR and H NMR spectroscopy. In the IR spectrum, a peak
ꢁ
1
at 982 cm assignable to the oxetane moiety disappeared,
ꢁ
1
ꢁ1
and new peaks at 3374 cm and 1740 cm assignable to
hydroxyl group and carbonyl group of an ester moiety
a
TABLE 1 Self-Polyaddition of N-Oxe-Amino Acids
b
c
c
d
ꢀ
e
ꢀ
Run
Monomer
Yield (%)
M
n
M
w
/M
n
T
g
( C)
T ( C)
d
1
2
3
a
N-Oxe-Ala-COOH (AB type)
89
85
89
5,100
1.70
75
289
227
289
0
f
f
f
N-Oxe-Glu-COOH (ABB type)
–
–
–
N-Oxe-Lys-COOH (A
2
B type)
6,620
3.81
63
e
Reaction conditions; monomer: 0.5 mmol, o-dichlorobenzene
5 wt % loss of thermal decomposition temperature was deter-
ꢀ
(
0.5 mL) at 130 C for 24 h.
mined by thermogravimetric analyzer.
b
f
Ethyl ether-insoluble part.
Estimated by SEC (eluent; DMF).
Poly(Glu) was insoluble in common organic solvents.
c
d
g
Glass transition temperature (T ) was determined by differen-
tial scanning calorimetry.
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1
FIGURE 1 H NMR spectra of N-oxe-Ala-COOH (A) and poly(OxAla) (B).
Self-polyaddition of N-Oxe-Glu-COOH and N-Oxe-Lys-COOH
mon organic solvents, presumably because poly( Glu) has
Ox
afforded the corresponding polymers poly( Glu) and poly
(OxLys) in 85% and 89% yields, respectively (runs 2 and 3
branched structures and many carboxyl groups at the chain
ends which would form strong intermolecular hydrogen
Ox
in Table 1). The obtained poly( Glu) was insoluble in com-
Ox
bonds. The structure of poly( Glu) was confirmed only by
Ox
a
TABLE 2 Self-Copolyaddition of N-Oxe-Ala-COOH/N-Oxe-Glu-COOH
b
c
c
d
ꢀ
e
ꢀ
Run
Monomer Composition
Yield (%)
M
n
M
w
/M
n
T
g
( C)
T ( C)
d
0
f
f
1
2
3
a
N-Oxe-Ala-COOH/N-Oxe-Glu-COOH (AB/ABB ¼ 1/1)
N-Oxe-Ala-COOH/N-Oxe-Lys-COOH (AB/A
B ¼ 1/1)
N-Oxe-Glu-COOH/N-Oxe-Lys-COOH (AB /A
B ¼ 1/1)
91
91
90
–
–
130
47
236
289
273
2
4,070
f
2.36
f
2
2
–
–
119
e
Reaction conditions; total monomer: 0.5 mmol, o-dichloro-
5 wt % loss of thermal decomposition temperature was deter-
ꢀ
benzene (0.5 mL) at 130 C for 24 h.
b
mined by thermogravimetric analyzer.
f
Ethyl ether-insoluble part.
Estimated by SEC (eluent; DMF).
Polymer was insoluble in common organic solvents.
c
d
g
Glass transition temperature (T ) was determined by differen-
tial scanning calorimetry.
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a
TABLE 3 Self-Copolyaddition of N-Oxe-Ala-COOH/N-Oxe-Glu-COOH
N-Oxe-Ala-COOH/N-Oxe-
0
b
c
c
d
ꢀ
e
ꢀ
Run
Glu-COOH (AB/ABB )
Yield (%)
M
n
M
w
/M
n
T
g
( C)
T ( C)
d
f
f
1
2
3
4
5
6
7
8
a
1/1
2/1
5/1
6/1
7/1
8/1
9/1
10/1
91
95
75
85
90
88
91
90
–
–
130
89
88
78
83
84
84
75
236
235
280
281
281
281
282
284
f
f
f
–
–
–
f
–
1,600
920
3.91
1.81
1.79
1,120
2,870
1,480
12.8
21.6
e
Reaction conditions; total monomer: 1.0 mmol, o-dichloro-
5 wt % loss of thermal decomposition temperature was deter-
ꢀ
benzene (0.5 mL) at 130 C for 24 h.
b
mined by thermogravimetric analyzer.
The obtained polymer was insoluble in common organic
f
Ethyl ether-insoluble part.
c
Estimated by SEC (eluent, DMF).
solvents.
d
g
Glass transition temperature (T ) was determined by differen-
tial scanning calorimetry.
ꢀ
IR spectroscopy. Poly( Lys) also has branched structure, but
sized polymers were in the range between 227 and 289 C,
Ox
0
is soluble in common organic solvents, and its M and M /M
n
and polymers containing ABB type monomer tended to have
lower Td . Thus, the thermal properties of the polymers
n
w
5
%
were 6620 and 3.81, respectively. The degrees of branching of
poly(OxGlu) and poly(OxLys) could not be evaluated from the
IR and NMR results. Furthermore, the optical activity of the
monomers was not examined, because racemization is
expected to occur readily both during synthesis of the mono-
mers and during polymerization (aqueous alkaline degrada-
tion reaction was used during the synthesis of monomers and
self-polyaddition was performed at the high reaction tempera-
were consistent with the structures of the monomers.
Next, self-copolyaddition was carried out at feed ratios of
N-Oxe-Ala-COOH/N-Oxe-Glu-COOH ¼ 1/1 to 10/1 under the
same conditions as used for the self-polyaddition described
above. The results and conditions are also summarized in
Table 3. At monomer feed ratios of N-Oxe-Ala-COOH/N-Oxe-
Glu-COOH ¼ 1/1 to 5/1, the obtained polymers were
insoluble in common organic solvents, presumably due to
strong hydrogen bonding of the many carboxyl groups at the
chain ends (runs 1–3 in Table 3). Self-copolyaddition at feed
ratios of N-Oxe-Ala-COOH/N-Oxe-Glu-COOH ¼ 6/1 to 10/1
gave poly( Ala-co- Glu) with M ’s ¼ 920–2870 and M /M ’s
ꢀ
ture of 130 C).
Self-Copolyaddition of N-Oxe-Ala-COOH (AB Type),
0
N-Oxe-Glu-COOH (ABB Type), and N-Oxe-Glu-COOH
(
A
2
B Type)
Ox
Ox
n
w
n
0
Self-copolyaddition of N-Oxe-Ala (AB type), N-Oxe-Glu (ABB
¼
1.81–21.6 in 85–91% yields. The large values of M /M
w n
type), and N-Oxe-Lys (A B type) was carried out at the same
2
were observed presumably due to adhesive effect on polysty-
rene gel column caused from many carboxyl groups at the
ends and side chain of the obtained polymers. The values of
molar feed ratios, affording the corresponding polymers in
satisfactory yields. The conditions and results are summar-
0
0
ized in Table 2. Self-copolyaddition of AB þ ABB and ABB
5%
ꢀ
T and T
were in the ranges of 75–130 C and 236–284
g
d
þ A
2
B gave polymers poly(OxAla-co-OxGlu) and poly(OxGlu-
co-OxLys) that were insoluble in common organic solvents
ꢀ
5%
C, respectively. Decrease of Tg and increase of Td
were
observed with increase of the feed ratio of AB type monomer.
These results indicate that the amount of terminal carboxyl
groups can be controlled by adjusting the feed ratio in the
self-copolyaddition. Consequently, it is expected that physical
properties such as solubility, glass transition temperature, and
thermal stability of the copolymers can also be controlled by
the adjusting the amount of terminal carboxyl groups.
(
runs 1 and 3 in Table 2). However, these polymers showed
ꢀ
higher T ’s compared with those of poly( Ala) (T ¼ 75 C)
g
Ox
g
ꢀ
and poly( Lys) (T ¼ 63 C). This is presumably because
Ox
g
poly( Ala-co- Glu)
and
poly( Glu-co- Lys)
contain
Ox
Ox
Ox
Ox
branched structures and have so many terminal carboxyl
groups that form intermolecular and intramolecular hydro-
gen bonds. In the case of self-copolyaddition using AB and
A B, the corresponding polymer poly( Ala-co- Lys) could
Biodegradability of Polymers
2
Ox
Ox
be obtained with Mn ¼ 4070 and M /M ¼ 2.36 in 91%
Biodegradable polymers such as poly(amino acid)s, poly
w
n
1
2
13
yield (run 2 in Table 2). The unit ratio of alanine and lysine
(lactic acid), and polycaprolactone can be decomposed
by aerobic or anaerobic bacteria, affording H O and CO . We
moieties of poly( Ala-co- Lys) was almost agree with that
Ox
Ox
2
2
of feed ratios, that is, alanine:lysine ¼ 1:1, which was con-
examined the biodegradability of the synthesized polymers
1
ꢀ
firmed by H NMR spectroscopy. Its value of T (47 C) was
poly( Ala), poly( Glu), and poly( Ala-co- Glu) (N-Oxe-Ala-
g
Ox
Ox
Ox
Ox
the lowest among the copolymers obtained. Furthermore, the
values of 5% weight-loss temperature (Td ) of the synthe-
COOH/N-Oxe-Glu-COOH ¼ 1/1) by the BOD method using
5
%
activated sludge. Figure 2 shows the biodegradability of the
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57.8%, >99%, and 65.3%, respectively. The synthesis and
properties of other amino acid-based polymers are under
investigation.
REFERENCES AND NOTES
1
1
For example: (a) Berger, A.; Katchalski, E. J. Am. Chem. Soc.
951, 73, 4084–4088; (b) Daly, W. H.; Poche, D. Tetrahedron
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2
(a) Deming, T. J. Nature 1997, 390, 386–389; (b) Deming, T.
J. J. Am. Chem. Soc. 1998, 120, 4240–4241; (c) Deming, T. J.
J. Polym. Sci. Part A: Polym. Chem. 2000, 38, 3011–3018.
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For example: (a) Kramer, J. R.; Deming, T. J. J. Am. Chem.
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J. A.; Chang, C. B.; Graves, A. M.; Li, Z.; Manson, T. G.;
Deming, T. J. Nature 2008, 455, 85–88.
FIGURE 2 Biodegradation ratios of poly(OxAla), poly(OxGlu),
and poly(OxAla-co-OxGlu) and alanine as a reference. h: Alanine
4
For example: (a) Hiromitsu S.; Shiotsuki, M.; Matsuoka, H.;
Sanda, F. Macromolecules 2011, 44, 3338–3345; (b) Shirakawa,
Y.; Suzuki, Y.; Terada, K.; Shiotsuki, M.; Masuda, T.; Sanda, F.
Macromolecules 2010, 43, 5575–5581; (c) Sutthasupa, S.; Shiot-
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2010, 43, 1815–1822.
(
reference), ~: poly(OxAla), n: poly(OxGlu).l: poly(OxAla-co-Ox-
Glu) (N-Oxe-Ala-COOH/N-Oxe-Glu-COOH ¼ 1/1).
polymers and alanine as a reference. Alanine was quantita-
tively degraded after 28 days. This confirms that the
activated sludge is appropriate for the examination of
biodegradability. The degradation ratios of poly(OxAla) and
poly( Ala-co- Glu) reached 57.8% and 65.3%, respectively.
5
Sutthasupa, S.; Shiotsuki, M.; Masuda, T.; Sanda, F. J. Am.
Chem. Soc. 2009, 131, 10546–10551.
(a) Emilitri, E.; Ferruti, P.; Annunziata, R.; Ranucci, E.; Rossi,
6
M.; Falciola, L.; Mussini, P.; Chiellini, F.; Bartoni, C. Macromole-
cules 2007, 40, 4785–4793; (b) Ferruti, P.; Manzoni, S.; Richard-
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Ox
Ox
Furthermore, poly( Glu) could show excellent biodegrad-
Ox
ability, amounting to >99% after only 14 days. Thus, all
these polymers had good biodegradability, and this might be
further improved by increasing the composition ratio of glu-
tamic acid moieties in the main chain.
7
For example; (a) Review: Sanda, F.; Endo, T. Macromol.
Chem. Phys. 1999, 200, 2651–2661; (b) Kudo, H.; Sanda, F.;
Endo, T. Macromolecules 1999, 32, 8370–8375; (c) Kudo, H.;
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CONCLUSIONS
3
9, 23–31; (d) Kudo, H.; Nagai, A.; Ishikawa, J.; Endo, T. Macro-
molecules 2001, 34, 5355–5357; (e) Sanda, F.; Yokoi, M.; Kudo,
H.; Endo, T. J. Polym. Sci. Part A: Polym. Chem. 2002, 40,
A novel general method was developed for the synthesis of
polymers containing amino acids in the main chain, via self-
polyaddition of amino acid-based monomers containing
oxetanyl and carboxyl groups. N-Oxetanylalanine (N-Oxe-Ala-
COOH), N-oxetanylglutamic acid (N-Oxe-Glu-COOH), and
N-oxetanyllysine (N-Oxe-Lys-COOH) were synthesized from
alanine, glutamic acid, and lysine, respectively. These synthe-
sized monomers were stable and easy to handle. Self-polyad-
dition reactions of N-Oxe-Ala-COOH, N-Oxe-Glu-COOH, and
N-Oxe-Lys-COOH were carried out using TPPB as a catalyst
2
782–2788.
8
(a) Koga, K.; Sudo, A.; Endo, T. J. Polym. Sci. Part A: Polym.
Chem. 2010, 48, 4351–4355; (b) Koga, K.; Sudo, A.; Nishida, H.;
Endo, T. J. Polym. Sci. Part A: Polym. Chem. 2009, 48,
3
839–3844; (c) Kamei, Y.; Sudo, A.; Nishida, H.; Kikukawa, K.;
Endo, T. J. Polym. Sci. Part A: Polym. Chem. 2008, 46,
2525–2535; (d) Kamei, Y.; Nagai, A.; Sudo, A.; Nishida, H.;
Kikukawa, K.; Endo, A. J. Polym. Sci. Part A: Polym. Chem.
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008, 46, 2649–2657; (e) Fujita, Y.; Koga, K.; Kim, H. K.; Wang,
X. S.; Sudo, A.; Nishida, H.; Endo, T. J. Polym. Sci. Part A:
Polym. Chem. 2007, 45, 5365–5369.
ꢀ
in o-dichlorobenzene at 130 C for 48 h, to afford the corre-
sponding polymers poly( Ala), poly( Glu), and poly( Lys)
Ox
Ox
Ox
9
Kudo, H.; Atsumi, H.; Sato, M.; Nishikubo, T. J. Polym. Sci.
Part A: Polym. Chem. 2008, 46, 7835–7842.
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1 Nishikubo, T.; Kameyama, A.; Suzuki, A. React. Funct.
Polym. 1998, 37, 19–25.
2 For example; Review: S o¨ derg a˚ rd, A.; Stolt, M. Prog. Polym.
with pendant hydroxyl groups in satisfactory yields. Further-
more, self-copolyaddition reactions of these monomers were
also performed to give the corresponding polymers, and it
was found that the physical properties, such as solubility,
glass transition temperatures, and thermal stability, of the
polymers reflected the amount of the terminal carboxyl
groups. The biodegradability of the polymers was examined
by the BOD method, and that the decomposition ratios of
poly(OxAla), poly(OxGlu), and poly(OxAla-co-OxGlu) were
1
0
1
1
Sci. 2002, 27, 1123–1163.
13 For example; Review: Sinha, V. R.; Bansal, K.; Kaushik, R.;
Kumria, R.; Trehan, A. Int. J. Pharm. 2004, 278, 1–23.
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