Controlled radical polymerization of an acrylamide containing l-alanine moiety via ATRP

Article abstract of DOI:10.1007/s00726-015-2097-8

Homopolymerization of an optically active acrylamide having an amino acid moiety in the side chain, N-acryloyl-l-alanine (AAla) was carried out via atom transfer radical polymerization (ATRP) at room temperature using 2-hydroxyethyl-2′-methyl-2′-bromopropionate (HMB) or sodium-4-(bromomethyl)benzoate (SBB) as initiator in pure water, methanol/water mixture and pure methanol solvents. The polymerization reaction resulted in the optically active biocompatible amino acid-based homopolymer in good yield with narrow molecular weight distribution. The number average molecular weight increased with conversion and polydispersity was low. The structure and molecular weight of synthesized polymer were characterized by ¹H NMR, FT-IR spectroscopic techniques and size-exclusion chromatography.

Full text of DOI:10.1007/s00726-015-2097-8

Amino Acids
DOI 10.1007/s00726-015-2097-8
ORIGINAL ARTICLE
Controlled radical polymerization of an acrylamide containing
l‑alanine moiety via ATRP
Zahra Rafiee1
Received: 14 July 2015 / Accepted: 8 September 2015
© Springer-Verlag Wien 2015
Abstract Homopolymerization of an optically active
acrylamide having an amino acid moiety in the side chain,
N-acryloyl-l-alanine (AAla) was carried out via atom
transfer radical polymerization (ATRP) at room tempera-
ture using 2-hydroxyethyl-2′-methyl-2′-bromopropionate
(HMB) or sodium-4-(bromomethyl)benzoate (SBB) as ini-
tiator in pure water, methanol/water mixture and pure meth-
anol solvents. The polymerization reaction resulted in the
optically active biocompatible amino acid-based homopol-
ymer in good yield with narrow molecular weight distribu-
tion. The number average molecular weight increased with
conversion and polydispersity was low. The structure and
molecular weight of synthesized polymer were character-
ized by 1H NMR, FT-IR spectroscopic techniques and size-
exclusion chromatography.
PS
Polystyrene
Alkyl halide
Sodium-4-(bromomethyl)benzoate
Size exclusion chromatography
Oxidation state
RX
SBB
SEC
z
Introduction
The synthesis and application of optically active polymers
have received extensive interest owing to their chiral struc-
ture which is common to naturally occurring polymers
(Okamoto and Nakano 1994; Pu 1998; Zhi et al. 2009;
Sogawa et al. 2013; Huang et al. 2015; Song et al. 2013).
Most of the naturally occurring polymers are optically
active and some of them indicate characteristic function-
alities such as molecular recognition ability and catalytic
activity, due to their specific chiral structure as denoted
by genes and proteins. In synthetic polymer chemistry, it
seems that one of the most challenging tasks is to make
functional polymeric systems that will be as effective as
those in living systems (Han et al. 2013; Mallakpour and
Soltanian 2012; Takeoka et al. 2013; Sogawa et al. 2011;
Mallakpour and Rafiee 2008, 2011; Mallakpour and Rafiee
2009; Miyake et al. 2010; Tsuji et al. 2010; Kolitz et al.
2009; Raus et al. 2014).
Keywords Atom transfer radical polymerization ·
Optically active · Homopolymerization
Abbreviations
AAla
ATRP
bpy
N-Acryloyl-L-alanine
Atom transfer radical polymerization
2,2′-Bipyridine
HMB
2-Hydroxyethyl-2′-methyl-2′-bromopropionate
1H-NMR Proton nuclear magnetic resonance
L
Ligand
Mt
Metal ion
Amino acids are the constitutional units of peptides and
proteins, which are able to produce highly ordered hierarchi-
cal structures, organized on a length scale ranging from a few
nanometers to several microns. Their secondary structures and
higher order structures are highly packed through intra- and
interchain associations by noncovalent forces, such as hydro-
gen bonding, hydrophobic stacking, electrostatic, and dipolar
interactions. The primary structures of the biomacromolecules
are created by covalent bond of the building blocks, and the
Handling Editor: D. Tsikas.
* Zahra Rafiee
z.rafiee@mail.yu.ac.ir; zahrarafiee2004@yahoo.com
1
Department of Chemistry, Yasouj University,
Yasouj 75918-74831, Islamic Republic of Iran
1 3

Z. Rafiee
molecular information such as amino acid sequence, chain
chirality, and amphiphilicity encoded in the primary struc-
tures determines their highly ordered structures. Incorporation
of amino acid residues into synthetic polymers is of attention
because these combinations may produce nonbiological mac-
romolecules with biomimetic structures and properties (Sanda
and Endo 1999; Fox et al. 2015; Rafiee and Zare 2015).
Atom transfer radical polymerization (ATRP) is a controlled
radical polymerization technique that is used for the synthesis
of functional macromolecules with controlled and complex
architectures because of its versatility and compatibility with a
range of monomers (Matyjaszewski and Xia 2001; Tsarevsky
and Matyjaszewski 2007; Ran et al. 2014; Shi et al. 2014; Ran
et al. 2014; Cho et al. 2013; Maria et al. 2005).
This method is as an effective synthetic way to produce
functional polymers with controlled molar mass, narrow
molecular weight distribution, and well-defined architectures
and functionalities. ATRP is based on the reversible reaction
of a low oxidation state metal complex, MtzLm (Mtz repre-
sents the metal ion in oxidation state z, and L is a ligand),
with an alkyl halide (RX). This reaction produces radicals
and the corresponding high oxidation state metal complex
with a coordinated halide ligand, XMtz+1Lm. ATRP involves
the reactivation of the alkyl halide adduct of the unsaturated
compound (monomer) and the further reaction of the formed
radical with the monomer (propagation). The livingness of
this polymerization process can be determined from a linear
first-order kinetic plot, accompanied by a linear increase in
polymer molecular weights with conversion, with the value of
the number-average degree of polymerization determined by
the ratio of reacted monomer to initially introduced initiator.
In this study, ATRP of a monosubstituted acryla-
mide with an amino acid moiety, N-acryloyl-l-alanine
(AAla) was investigated in the presence of two initiators,
2-hydroxyethyl-2′-methyl-2′-bromopropionate (HMB) and
sodium-4-(bromomethyl)benzoate (SBB), separately at
room temperature.
Chemical Co. l-Alanine, acryloyl chloride, 2,2′-bipyridine
(bpy), 2-bromo-2-methylpropanoyl bromide, ethylene gly-
col and SBB were purchased and used as received without
further purification. CuBr was purified as reported in the lit-
erature (Keller and Wycoff 1946). It was stored under nitro-
gen and weighed in the open air prior to polymerization.
Techniques
Number-average molecular weight (Mn) and molecular
weight distribution (Mw/Mn) were estimated by size-exclu-
sion chromatography (SEC), the elution rate was 1 mL/min
and standard polystyrene (PS) was used for calibration.
Proton nuclear magnetic resonance (1H-NMR, 500 MHz)
and 13C NMR (125 MHz) spectra were recorded in DMSO-
d6 solution using a Bruker (Germany) Avance 500 instru-
ment. FT-IR spectra were recorded on (Jasco-680, Japan)
spectrophotometer. The spectra of solids were obtained
using KBr pellets. Inherent viscosities were measured
using a Cannon–Fenske Routine Viscometer (Germany)
at concentration of 0.5 g dL−1 at 25 °C. Specific rotations
were measured by a Jasco Polarimeter (Japan).
Synthesis of initiator
2-Hydroxyethyl-2′-methyl-2′-bromopropionate (HMB) was
synthesized as described previously (Scheme 1) (Yin et al.
2001; Haddelton et al. 1997). Bromoisobutyryl bromide was
reacted with cold anhydrous ethylene glycol (excess) under
stirring for 4 h. The molar ratio of ethylene glycol and bro-
mobutyryl bromide was 25 to avoid coupling reaction. The
reaction mixture was added to deionized water, and the reac-
tion product was extracted by dichloromethane. The organic
solution was washed with water and sodium carbonate and
dried over anhydrous magnesium sulfate. The final product
was isolated as a colorless liquid upon removal of the sol-
vent, and it was vacuum-distilled. The chemical structure
1
and purity of HMB were proven with FT-IR and H-NMR
spectroscopic techniques. 1H NMR (500 MHz, CDCl3,
Experimental
Materials
ppm): δ 4.4 (t, 2H), 3.83 (t, 2H), 3.2 (s, 1H); 1.96 (s, 6H).
Synthesis of monomer
All chemicals were purchased from Fluka Chemical Co.
(Buchs, Switzerland), Aldrich Chemical Co. (Milwaukee,
WI), Riedel–deHaen AG (Seelze, Germany) and Merck
The vinyl monomer N-acryloyl-l-alanine (AAla) was syn-
thesized by the general techniques previously reported
(Bueno et al. 1991). As shown in Scheme 2, the acylation
Scheme 1 Synthesis of
2-hydroxyl-2′-ethyl-2′-
bromopropionate (HMB)
O
O
0 oC
, 4h
HO
+
HO
Br
Br
OH
O
Br
1 3

Controlled radical polymerization of an acrylamide containing l-alanine moiety via ATRP
Scheme 2 Synthesis of
poly(N-acryloyl-l-alanine)
[poly(AAla)] via ATRP using
two initiators
n
NH2
Initiator, CuBr/bpy
O
O
Cl
NH
NH
O
O
OH
Water, Methanol or
H3C
Water/methanol mixture
OH
OH
H3C
H3C
O
O
O
L-Ala
AAla
Poly(AAla)
Initiators:
O
NaOC
HO
CH2Br
,
O
Br
SBB
HMB
Fig. 1 1H-NMR (500 MHz)
spectrum of AAla in DMSO-d6
at room temperature
reaction of l-alanine with acryloyl chloride was performed
in the presence of alkali at 0 °C. Treatment with concen-
trated hydrochloric acid provided a white solid. Recrys-
tallization from ethanol gave white crystals with good
yield and purity. The chemical structure and purity of the
FT-IR (KBr, cm−1): 3422–2760 (COOH stretch), 3323
(N–H stretch), 3029 (=C–H stretch), 2928 (–C–H stretch),
1729 and 1647 (C=O stretch), 1210 (C–O stretch). 1H NMR
(500 MHz, DMSO-d6, ppm): δ 1.29–1.30 (d, J = 7.3 Hz,
3H), 4.26–4.32 (m, 1H), 5.60–5.62 (d, J = 10.2 Hz, 1H),
6.09–6.12 (d, J = 17.1 Hz, 1H), 6.26–6.32 (dd, J1 = 10.2 Hz,
J2 = 17.1 Hz, 1H), 8.38-8.39 (d, NH, J = 7.0 Hz, 1H), 12.54
(s, COOH, 1H); 13C NMR (125 MHz, DMSO-d6, ppm): δ
1
optically active monomer were proven with FT-IR, H-
NMR and 13C-NMR spectroscopic techniques (Figs. 1, 2).
[α]2D5 = +21.48 (0.05 g in 10 mL ethanol).
1 3

Z. Rafiee
Fig. 2 13C-NMR (125 MHz)
spectrum of AAla in DMSO-d6
at room temperature
18.06 (CH3), 48.37 (CH chiral center), 126.47 (=CH2),
132.18 (=CH), 165.14 (C=O amide), 174.92 (C=O acidic).
Anal. Calcd for C6H9NO3: C, 50.35; H, 6.34; N, 9.78.
Found: C, 50.30; H, 6.57; N, 9.90.
homopolymer in high yield. The monomer conversion was
determined gravimetrically.
Result and discussions
ATRP
Controlled radical polymerization of amino acid-containing
monomers is promising for producing tailored polymers
and copolymers for biomaterials and medical applications.
The amino acid-based polymers are expected to be non-
toxic, as an extensive variety of polymers bearing amino
acid moieties either in the main chains or in side chains,
such as poly(glutamic acid) and polyleucine, have been
prepared, and they showed biocompatibility and biodeg-
radability similar to polypeptides (Sanda and Endo 1999).
The well-defined poly(AAla) was prepared via ATRP
using CuBr/bpy catalyst system and HMB or sodium-
4-(bromomethyl)benzoate (SBB), separately as an ini-
tiator in pH = 8 at room temperature. Bpy was chosen as
ligand since it is commercially available and readily forms
a water-soluble complex with CuI. The route of the syn-
thesis of poly(AAla) is outlined in Scheme 2. The water-
soluble initiator, HMB was synthesized from the reaction
of 2-bromoisobutyryl bromide with ethylene glycol. The
1H-NMR spectrum of poly(AAla) is shown in Fig. 3. The
1H-NMR of this polymer shows the characteristic reso-
nance of acidic and amidic protons at 12.68 and 8.08 ppm,
C–H chiral center at 4.27 ppm. The disappearance of vinyl
General procedure, mixture of HMB (0.018 g,
0.087 mmol), AAla monomer (1.00 g; 6.99 mmol) were
dissolved in 2 mL of a 50:50 v/v % methanol/double-dis-
tilled water mixture at room temperature, then 5 % NaOH
was added to achieve pH = 8. This solution was degassed
by passing dry nitrogen gas, then 2,2′-bipyridine (bpy)
ligand (0.033 g; 0.21 mmol) and CuIBr catalyst (0.012 g;
0.087 mmol) was added rapidly. Upon addition of these
reagents a dark brown solution was formed. An exothermic
polymerization reaction started immediately leading to an
increase in viscosity of the mixture and it was observed by
sensing the vessel of reaction. After certain period of reac-
tion time (Table 1) the mixture was exposed to air. The dark
brown reaction solution turned blue, indicating aerial oxi-
dation of Cu(I) to Cu(II). The polymer solution was diluted
with 5 mL of water and was mixed with about 1 g of sil-
ica gel and was stirred for 1 h. Then it was filtered off and
was acidified with a solution of concentrated HCl (12 M)
to provide white solid polymers. The resulting polymers
were washed with cold THF to remove traces of residual
AAla monomer and/or unreacted initiator, affording AAla
1 3

Controlled radical polymerization of an acrylamide containing l-alanine moiety via ATRP
Table 1 Summary of conversion, molecular weight and polydispersity data for homopolymerization of AAla using two initiators in various
ratios of water and methanol at room temperature
Conversiona (%)
Molecular weight
Mbn (theory)
Mw/Mn
25, c
Initiator
Solvent H2O:MeOH
Time (h)
[α]
Na, 589
Mn (SEC)
Mn (titration)
SBB
SBB
SBB
SBB
SBB
SBB
SBB
SBB
SBB
HMB
HMB
HMB
HMB
HMB
HMB
HMB
50:50
50:50
50:50
50:50
50:50
50:50
50:50
100:0
0:100
50:50
50:50
50:50
50:50
50:50
50:50
50:50
1
2
84.5
99.1
99.2
99.2
99.4
99.5
100.0
98.9
84.7
84.9
86.8
87.1
87.9
89.7
90.5
92.3
–
–
–
–
+21.78
+26.18
+27.14
+27.64
+27.57
+26.94
+26.83
+22.77
+23.06
+23.17
+24.02
+21.69
+22.53
+25.68
+25.94
+24.47
11,630
12,489
12,317
1.1
–
4
–
–
–
6
–
–
–
–
8
–
–
–
–
10
12
2
–
–
–
–
11,734
12,834
12,711
1.2
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2
–
1
–
2
–
4
–
6
–
8
–
10
12
–
–
[AAla]o = 3.495 M and the molar ratio of initiator:CuBr:bpy was 2:2:5
a
Calculated from: [Wp/(Wi + Wm)] × 100 %, where Wp, Wi, and Wm were the weights of the polymer produced and the initial weights of the
related initiator and monomer, respectively
b
Mn (theory) = 143.1([AAla]o/[initiator]) × %Conversion/100 + Mw(initiator)
Measured at a concentration of 0.5 g/dL in DMF at 25 °C
c
Fig. 3 1H-NMR (500 MHz)
spectrum of poly(N-acryloyl-
l-alanine) [poly(AAla)] [pre-
pared via the polymerization
of N-acryloyl-l-alanine (AAla)
in the presence of sodium-4-
(bromomethyl)benzoate (SBB)
as an initiator in 50/50 metha-
nol/water mixture as a solvent
at room temperature at pH = 8]
in DMSO-d6 at room tem-
perature. Monomer concentra-
tion = 3.495 M and the molar
ratio of initiator:CuBr:bpy was
2:2:5
peaks at 5.60–5.62, 6.09–6.12 and 6.26–6.32 ppm con-
firmed complete conversion of monomer to poly(AAla)
after 2 h reaction time. In the first step, we examined the
homopolymerization of AAla in double-distilled, deion-
ized water at room temperature under a nitrogen atmos-
phere. Exceptional rate acceleration in polymerization was
1 3

Z. Rafiee
96 % is achieved within 12 h. In this case, the polymeri-
zation is first order with respect to AAla monomer up to
around 85.6 % of conversion. Chiral poly(AAla)s with
Mn = 12,489 gmol−1 (Mw/Mn = 1.1) and Mn = 12,834
gmol−1 (Mw/Mn = 1.2) were obtained, so molecular weight
distribution value remained low (Mw/Mn = 1.2). Molecu-
lar weight of poly(AAla)s was also determined by titration,
and it gave a result which is close to one that was deter-
mined by SEC (Table 1). Since the reactions with SBB ini-
tiator gave lower molecular weight distribution compared
to HMB initiator, this initiator was chosen to be more suit-
able for the synthesis of these homopolymers. But it is
important to note that HMB initiator from environmental
point of view is friendlier since it is biocompatible.
Fig. 4 Kinetic plot of ATRP of AAla in 50/50 methanol/water mix-
ture as a solvent at room temperature at pH = 8. Monomer concentra-
tion = 3.495 M and the molar ratio of initiator:CuBr:bpy was 2:2:5
Summary
The ATRP of AAla was successfully carried out using
CuCl/bpy catalyst with HMB and SBB water-soluble ini-
tiators in pure water, 50/50 methanol/water mixture or pure
methanol. The controlled character of the polymerization
was confirmed by the formation of narrow polydispersity
products and the linear relationship between the molecu-
lar weight and conversion. Since the resulting homopoly-
mer has amino acid moieties in the backbone; it exhibits
biocompatibility and biodegradability. In this work, well-
defined chiral poly(AAla) having amino acid l-alanine
in the side chain was obtained directly by ATRP, without
requiring protection chemistry for the carboxylic acid resi-
dues, under mild conditions.
Fig. 5 Kinetic plot of ATRP of AAla in methanol as a solvent at
room temperature at pH = 8. Monomer concentration = 3.495 M and
the molar ratio of initiator:CuBr:bpy was 2:2:5
Compliance with ethical standards
observed, so high conversion was obtained in short period
of time. The same experiments were also conducted in the
presence of 50:50 methanol/water mixture or pure metha-
nol as a solvent. Rates of polymerization were noticeably
slower in methanol compared to 50:50 methanol/water,
probably owing to the more active nature of the Cu(I) cat-
alyst in the presence of water. The results of a particular
set of experiments are summarized in Table 1. As predict-
able for a living polymerization, the molecular weight of
the two homopolymers enhanced with conversion. Figure 4
represents typical kinetic data for the homopolymeriza-
tion of AAla in 50:50 methanol/water mixture at room
temperature. Conversion of more than 96 % is attained
within 100 min and the plot of ln([M]o/[M]) versus time
was linear up to 89.8 % conversion, but after this time a
deviation from first-order kinetics is observed. Figure 5
depicts kinetic data for the homopolymerization of AAla
in pure methanol. Under these conditions, the polymeri-
zation is rather slower than in the presence of water and
50:50 methanol/water mixture, so conversion of more than
Conflict of interest The authors have no conflicts of interest with
regard to this work.
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