Chiroptical properties of homopolymers and block copolymers synthesized from the enantiomeric monomers N-acryloyl-L-alanine and N-acryloyl-D-alanine using aqueous RAFT polymerization

Article abstract of DOI:10.1071/CH06264

Chiral homo- and block copolymers based on the enantiomeric monomers N-acryloyl-l-alanine (ALAL) and N-acryloyl-d-alanine (ADAL) were prepared directly in water using controlled reversible addition?fragmentation chain transfer (RAFT) polymerization. The polymerization of the chiral monomers proceeded in a controlled fashion producing the respective homopolymers, block copolymers, and a statistical copolymer with targeted molecular weights and narrow molecular weight distributions. The chiroptical activity of these biomimetic polymers and their analogous model compounds was investigated using circular dichroism (CD). P(ALAL) and P(ADAL) were shown to be optically active exhibiting mirror image CD spectra. In addition, statistical and enantiomeric block copolymers prepared at 1:1 stochiometric ratios exhibited virtually no optical activity. CSIRO 2006.

Full text of DOI:10.1071/CH06264

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2006, 59, 749–754
Chiroptical Properties of Homopolymers and Block Copolymers
Synthesized from the Enantiomeric Monomers N-Acryloyl-l-Alanine
and N-Acryloyl-d-Alanine Using Aqueous RAFT Polymerization∗
Brad S. Lokitz,A Jonathan E. Stempka,A Adam W. York,A Yuting Li,A Hitesh K. Goel,B
G. Reid Bishop,B and Charles L. McCormickA,C,D
A Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406, USA.
B Department of Chemistry and Biochemistry, Mississippi College, Clinton, MS 39058, USA.
C Department of Chemistry and Biochemistry, University of Southern Mississippi, Hattiesburg, MS 39406, USA.
D Corresponding author. Email: charles.mccormick@usm.edu
Chiral homo- and block copolymers based on the enantiomeric monomers N-acryloyl-l-alanine (ALAL) and
N-acryloyl-d-alanine (ADAL) were prepared directly in water using controlled reversible addition–fragmentation
chain transfer (RAFT) polymerization. The polymerization of the chiral monomers proceeded in a controlled fash-
ion producing the respective homopolymers, block copolymers, and a statistical copolymer with targeted molecular
weights and narrow molecular weight distributions. The chiroptical activity of these biomimetic polymers and their
analogous model compounds was investigated using circular dichroism (CD). P(ALAL) and P(ADAL) were shown
to be optically active exhibiting mirror image CD spectra. In addition, statistical and enantiomeric block copolymers
prepared at 1:1 stochiometric ratios exhibited virtually no optical activity.
Manuscript received: 26 July 2006.
Final version: 26 September 2006.
Introduction
chains are of interest because their hydrogen bonding ability
can lead to the formation of higher ordered structures, for
example α-helices and β-sheets.
Optically active synthetic polymers have been the subject
of a large number of scientific investigations,[1–10] some
beginning as early as the 1950s because chiral structures
which mimic attributes of naturally occurring polypetides,
nucleic acids, and polysaccharides have many potential appli-
cations including chiralamplification, molecularrecognition,
liquid crystalline formation, supermolecular assembly, and
polymeric catalysis at asymmetric sites. Chiral polyolefins
generally fall into two major catergories—those in which
asymmetry is induced due to tacticity during the polymer-
ization and those with inherent asymmetry resulting from
incorporation of monomers with chiral side groups. Con-
trol over the polymer tacticity is typically achieved through
Ziegler–Natta polymerization or the anionic polymerization
of bulky substituents with chiral initiators. Some degree
of stereoregularity can also be achieved using coordinating
species such as Lewis acids in conventional or controlled free
radical polymerization techniques.[11–14] Polymers with chi-
ral side chains are usually synthesized from vinyl monomers
with pendant chiral groups. Recent advances in amino acid
synthesis and isolation have increased the availability of
optically pure amino acid enantiomers for synthesis of chi-
ral monomers.[5,15–18] Polymers containing amino acid side
The advent of controlled/‘living’ radical polymerization
(CRP) techniques such as nitroxide mediated polymerization
(NMP),[15] atom transfer radical polymerization (ATRP),[19]
and reversible addition–fragmentation chain transfer
(RAFT)[20] polymerization have allowed for the synthesis
of polymers with controlled molecular weights, low molec-
ular weight distributions, and complex architectures. Due to
these attributes, CRP techniques have become quite attractive
for the preparation of well defined optically active polymers.
Recent reports of the preparation of optically active polymers
using acyclic diene metathesis (ADMET),[5] ATRP,[11,21–25]
and RAFT[11,12] have shown the synthesis of well con-
trolled optically active polymers with chiral side chains to
be possible. All controlled or controlled/‘living’ free radi-
cal polymerizations of optically active monomers reported to
date have been carried out in organic solvents. To the best of
our knowledge, the polymerization of such chiral monomers
and the formation of enantiomeric block copolymers directly
in water have not been previously reported. Our group has
a long-standing interest in the aqueous RAFT polymer-
ization of hydrophilic (meth)acrylamido monomers[26] and
∗ Paper number 126 in a series on Water Soluble Polymers.
© CSIRO 2006
10.1071/CH06264
0004-9425/06/100749

RESEARCH FRONT
750
B. S. Lokitz et al.
the self-assembly behavior of block copolymers in aqueous
solutions.[27,28] Herein we report the RAFT polymeriza-
tion of the enantiomers N-acryloyl-l-alanine (ALAL) and
N-acryloyl-d-alanine (ADAL) directly in water without the
necessity of protecting groups. As well, we compare the chi-
ropticalpropertiesofthehomopolymersandthe1:1statistical
and 1:1 block copolymers with those of the respective model
compounds.
septa. The initial monomer to CTA ratios ([M]0/[CTA]0) were var-
ied between 140:1 and 225:1 while the initial CTA to initiator ratio
([CTA]0/[I]0) was held at a constant ratio of 5:1. Absolute molecular
weightsweredeterminedfromaliquots(0.5 mL)takenatpre-determined
time intervals and subsequently quenched via rapid cooling and expo-
sure to oxygen.The products were purified by dialysis against deionized
water and isolated by lyophilization.
Aqueous Size Exclusion Chromatography
Polymers were analyzed directly by aqueous size exclusion chro-
matography (ASEC) using an aqueous eluent of 20%/80% acetonitrile/
0.5 M Na2SO4. A flow rate of 0.5 mL min−1 at 25◦C, TOSOH Bio-
sciences TSK-GEL columns (G3000 PWXL, <50000 g mol−1, 200 Å)
and G4000 PWXL(2000–300000 g mol−1, 500 Å), a Polymer Laborato-
ries LC 1200 UV/vis, Wyatt Optilab DSP interferometric refractometer,
and a Wyatt DAWN EOS multiangle laser light scattering detector
(690 nm) were employed in our analysis.
Experimental
Materials
All reagents were purchased fromAldrich at the highest purity available
and used as received unless otherwise stated. 2-Ethylsulfanylthio-
carbonylsulfonyl-2-methyl-propionic acid (EMP) was synthesized
according to literature procedures.[29] 4,4ꢀ-Azobis(4-cyanopentanoic
acid) (V-501) was donated by Wako Chemicals and was recrystallized
twice from methanol before use. ALAL and ADAL were synthesized
by the drop-wise addition of acryloyl chloride to an aqueous solution
of l-alanine and d-alanine and recrystallized from water before use.
mp 162–164◦C. δH 6.33 (m, CH2CHCO), 5.71 (d, CH2CHCO), 4.45
(m, HNCH(COOH)CH3), 2.17 (d, HNCH(COOH)CH3). Model com-
pounds N-acetyl-l-alanine (mp 124–125◦C) and N-acetyl-d-alanine
(mp 124–125◦C) were purchased from Aldrich.
Nuclear Magnetic Resonance Spectroscopy
13C NMR spectra were recorded with a temperature controlled Mercury
Innova 500 MHz spectrometer. Samples were prepared as 20% w/w
solutions in D2O (HOD internal standard). The tacticity for the methine
peak (δC 44–47 ppm) was determined based on triad assignments from
a previous literature report for acrylamide.[13]
Circular Dichroism Spectroscopy
General Procedure for Aqueous RAFT Polymerization
Samples were prepared by directly dissolving the dried sample in HPLC
grade water (pH ≈ 6.5) at concentrations of 50 and 1.5 µM for the
monomers and polymers, respectively, such that an absorbance value
of 0.5 was obtained at 210 nm using a HP 8453 diode array spectropho-
tometer in a 1 cm quartz cuvette. CD spectra were collected at room
temperature using a Jasco J-815 spectropolarimeter in a low volume
rectangular 0.5 cm path length cell. An ‘ozone free’ lamp was used to
RAFT-mediated polymerizations of ALAL and ADAL were conducted
at 70◦C, employing V-501 as the primary radical source and EMP or
P(ALAL) as the RAFT CTA and macroCTA, respectively. Polymeriza-
tions were performed directly in water (pH 6.5) with an initial monomer
concentration ([M]0) of 1.0 M under a nitrogen atmosphere in round-
bottomed flasks equipped with magnetic stir bars and sealed with rubber
S
S
S
O
S
Z
HO
R
x
S
V-501, 70°C
pH ϭ 6.5
O
O
NH
NH
O
O
OH
ADAL
OH
P(ADAL)
S
S
S
O
Z
S
S
Z
V-501, 70°C
pH ϭ 6.5
R
HO
R
y
x
x
S
S
V-501, 70°C
pH ϭ 6.5
O
O
O
O
NH
NH
NH
NH
O
O
O
O
OH
OH
OH
ALAL
OH
P(ALAL)
P(ALAL)-b-P(ADAL)
S
S
S
O
R
Z
S
ϩ
HO
x
y
O
S
O
V-501, 70°C
pH ϭ 6.5
O
O
NH
NH
NH
NH
O
O
O
O
OH
ALAL
OH
ADAL
OH
OH
R ϭ (CH3)2COOH
Z ϭ SCH2CH3
P(ALAL)-s-P(ADAL)
Scheme 1. Pathway for aqueous RAFT synthesis of acryloyl-d- and l-alanine homopolymers, block copolymers, and a statistical copolymer.

RESEARCH FRONT
Chiroptical Properties of Copolymers Synthesized Using Aqueous RAFT Polymerization
751
limit exposure of the optical bench to ozone generated by far ultravio-
let (UV) radiation, eliminating the need to purge the optical bench. The
response time used was 16 s with a data pitch of 0.1 nm and a scan speed
ASEC trace (RI detector) to higher elution volume and the
shift of the near-monodisperse distribution to higher molecu-
lar weight.
of 10 nm min−1
.
Circular dichroism (CD) spectroscopy was used to assess
the optical activity of each of the homo- and copolymers
(Fig. 2) and the respective enantiomorphic model compounds
(Figs 3a and 3b). The CD spectra of the homopolymers
P(ALAL) and (PADAL) shown over the 240–190 nm range
are near mirror images with respective molar elipticity val-
ues of about 5000 and −5000 Θ at 220 nm decreasing to zero
at the crossover of 211 nm and rapidly increasing in magni-
tude with opposite sign at increasingly lower wavelengths.
Maximum negative and positive values of molar eliptic-
ity of approximately −35000 and +40000 Θ are reached
in the 190 nm range for P(ALAL) and P(ADAL), respec-
tively. The statistical 1:1 copolymer synthesized with equal
molar amounts of the d- and l-monomers in the feed shows
no optical activity and the block copolymer prepared by
sequential addition of the d-monomer to the preformed
poly(ALAL) block shows only slight activity, presumably
from the slightly larger block (two or three units) of the
d-enantiomer. The block copolymer containing a roughly 2:1
d to l ratio exhibits intermediate behavior as might be
expected. Examination of the CD spectra for the respective l-
and d-homopolymers and their small molecule counterparts
reveals that the amplitude of the CD maxima for each
homopolymer is around two to four times that of its con-
stitutive monomer. Since the molar concentrations of model
compounds and repeating units of the homopolymers are
equivalent in our experiments, the optical activity of the
homopolymers is greater than that of the summation of the
amino acid repeat units alone; therefore, additional inter-
actions appear to contribute in a synergistic fashion to the
optical activity.[4,30]
Results and Discussion
The enantiomeric monomers ALAL and ADAL were
selected for this study based on their facile synthe-
sis from readily available and optically pure amino acid
precursors. Additionally, the amphoteric nature of the
monomers allows polymerization, purification, and char-
acterization directly in aqueous solution. In order to pre-
pare the precisely defined polymers for circular dichroism
studies, we employed a recently communicated[29] aqueous
RAFT polymerization method using the chain transfer agent
2-ethylsulfanylthiocarbonylsulfonyl-2-methyl-propionicacid
(EMP) and the initiator 4,4ꢀ-azobis(4-cyanopentanoic acid)
(V-501) at 70◦C. In the current study, we have prepared
homopolymers of ALAL and ADAL, as well as the 1:1
statistical and 1:1 and 1:2 block copolymers of these two
enantiomers (Scheme 1).
By controlling the [monomer]/[EMP] ratio and the con-
version, molecular weights of the homopolymers were tar-
geted for ∼34000 g mol−1. Table 1 lists the number aver-
age molecular weights and molecular weight distributions
as determined by aqueous size exclusion chromatography
(ASEC) using mutli-angle laser light scattering (MALLS)
and refractive index (RI) detectors.
ShowninFig. 1aarethenormalizedASECchromatograms
and Fig. 1b cumulative weight fractions for the P(ALAL)
macroCTA and the resultant P(ALAL)118-b-P(ADAL)119
copolymer. The efficient blocking (percentage of macroCTA
converted to block copolymer) expected when using
the trithiocarbonate EMP[29] is confirmed by the shift in the
Table 1. Experimental data for homo- and copolymerA synthesis based on the chiral monomers ADAL and ALAL
B
B
t [min]
Conv
Mn [g mol−1
]
Mn,th [g mol−1
]
Mw/Mn
Tacticity (rr/mr/mm)C
P(ALAL)118 macroCTA
P(ALAL)215
P(ADAL)201
P(ALAL)118-b-P(ADAL)119
P(ALAL)118-b-P(ADAL)217
P(ALAL)121-s-P(ADAL)121
30
45
45
45
45
45
0.75
0.91
0.87
0.93
0.89
0.91
16900
30800
28700
33900
47900
34600
15000
27500
26300
32500
44400
29300
1.09
1.03
1.05
1.04
1.05
1.07
42/42/16
40/48/12
37/44/19
43/43/13
—
—
A Abbreviations: P poly, b block, s statistical. B As determined using aqueous size exclusion chromatography, see Experimental. C Measured using
500 MHz 13C NMR. Triad assignments based on ref. [20].
1.6
0.8
0.0
1.0
0.8
0.6
0.4
0.2
0.0
P(ALAL) macroCTA
P(ALAL)-b-P(ADAL)
P(ALAL) macroCTA
P(ALAL)-b-P(ADAL)
3.0
3.5
4.0
4.5
5.0
5.5
1000
10000
100000
1000000
Ve [mL]
log Mw
(a)
(b)
Fig. 1. (a) RI traces and (b) cumulative weight fractions for the P(ALAL) macroCTA (Mn 16900 g mol−1, PDI 1.09)
and P(ALAL)-b-P(ADAL) (Mn 33900 g mol−1, PDI 1.04).

RESEARCH FRONT
752
B. S. Lokitz et al.
As early as 1961, Kulkarni and Morawetz,[4] in a seminal
contribution, explored the possible reasons for optical activ-
ity enhancements of the amphoteric polymer formed from
classical free radical polymerization of N-acryloyl glutamic
acid as compared to its low molecular weight analog. They
observed substantial changes in both the magnitude and sign
of the specific optical activity as a function of pH of the
polymerization medium. Since the CD behavior of molecules
containing asymmetric centers is due to statistical averag-
ing of all conformations, Morawetz reasoned that the steric
restrictions of the side chains of the polymer as compared to
the low molecular weight analogue ‘should result in a corre-
sponding change in the contribution of the asymmetric center
to optical activity’. He discussed optical activity as a function
of conformational restrictions dependent on solvent quality,
hydrogen bonding, and possibly by helix formation.
In our work and that reported by Morawetz, water-soluble
polymers with chiral, amino acid-based pendant groups
were obtained by free radical polymerization techniques.
Since these were neither conducted at low temperature with
constrained monomers nor with catalysts or coordinating
species, highly stereoregular polymers would not be pre-
dicted. However, considering the somewhat bulky nature
of the enantiomorphic monomers, one might expect the
energetically more favorable syndiotactic propagation which
results in long sequences of trans and occasional pairs of
gauche conformations as compared to isotactic propagation
yielding alternating trans and gauche conformations.[4,10]
Indeed, evaluation of the triad content of the d- and l-
homopolymers and the 1:1 block using 13C NMR using
resonance assignments reported in the literature revealed a
syndiotactic tendency of our RAFT polymerizations con-
ducted in water at 70◦C. The backbone methine resonances
located between 44 and 48 ppm (Fig. 4) yield rr/rm/mm ratios
that are shown inTable 1. Syndiotactic propagation is favored
for both hompolymers and the block copolymer over isotactic
propagation. Interestingly, the statistical copolymer displays
a single broad peak which cannot be resolved, likely indi-
cating a larger number of conformational interactions of the
methine carbon and possibly a completely atactic structure.
rr rm mm
42/42/16
P(ADAL)
40/48/12
37/44/19
43/43/13
P(ALAL)
50000
P(ALAL)215
P(ADAL)201
P(ALAL)118-b-P(ADAL)119
P(ALAL)121-s-P(ADAL)121
P(ALAL)118-b-P(ADAL)217
40000
30000
20000
10000
0
P(ALAL) macroCTA
P(ALAL)-b-P(ADAL)
Ϫ10000
Ϫ20000
Ϫ30000
Ϫ40000
N/A
46
P(ALAL)-s-P(ADAL)
48
44
42
40
38
36
34
200
220
λ [nm]
240
260
ppm
Fig. 4. 13C NMR spectra for methylene (34–40 ppm) and methine
(44–47 ppm) backbone carbons for P(ADAL), P(ALAL), P(ADAL)
macroCTA, P(ALAL)-b-P(ADAL), and P(ALAL)-s-P(ADAL). See
Table 1 for structural designations.
Fig. 2. CD spectra of P(ALAL)215, P(ADAL)201, P(ALAL)118
-
b-P(ADAL)119 P(ALAL)118-b-P(ADAL)217 and P(ALAL)121-s-
,
,
P(ADAL)121. See Table 1 for structural data and designations.
10000
0
50000
N-Acetyl-D-alanine
P(ADAL)201
40000
30000
20000
10000
0
Ϫ10000
00
Ϫ200
N-Acetyl-L-alanine
P(ALAL)215
Ϫ30000
Ϫ10000
190 200 210 220 230 240 250 260
190 200 210 220 230 240 250 260
λ [nm]
λ [nm]
(a)
(b)
Fig. 3. CD spectra for (a) P(ALAL)215 and the model compound N-acetyl-l-alanine and (b) P(ADAL)201 and the model compound
N-acetyl-d-alanine.

RESEARCH FRONT
Chiroptical Properties of Copolymers Synthesized Using Aqueous RAFT Polymerization
753
50000
40000
30000
20000
10000
0
10000
0
25°C
95°C
Ϫ10000
Ϫ20000
Ϫ30000
Ϫ40000
Ϫ50000
25°C
35°C
45°C
65°C
75°C
95°C
25°C
35°C
45°C
65°C
75°C
95°C
95°C
25°C
Ϫ10000
190
200
210
220
230
240
250
190
200
210
220
230
240
250
λ [nm]
λ [nm]
(a)
(b)
Fig. 5. CD spectra at selected temperatures for (a) P(ALAL)215 and (b) P(ADAL)201
.
Whether the apparently favored syndiotactic structure in
concert with the chiral side chains is totally responsible for
the enhanced optical behavior of the homopolymers is not
clear. The mirror image CD spectra of the respective d- and
l-homopolymers, the loss of optical activity with the sta-
tistical copolymer and the near cancellation of amplitude
with the nearly balanced block copolymer (Fig. 2) suggest
similar, but opposite ‘sense’ interactions. However, since the
amphoteric chiral side chains are capable of multiple hydro-
gen bonding interactions, it is possible that these interactions
might also play a role in inducing additional order either dur-
ing or after polymerization. For example, hydrogen bonding
influenced by solvent, pH, and temperature can affect orien-
tation and local dielectric constants. In preliminary studies,
we find significant changes in the CD spectra with increas-
ing temperature of the respective homopolymers P(ALAL)
and P(ADAL) (Fig. 5). The molar elipticity values decrease
steadily with increasing temperature over the 25 to 95◦C
range examined. Also changes are observed in CD spectra
with changes in pH (data not shown). However, a residual
enhancement in CD activity over that predicted by simple
summation of chiral units appears to remain in both cases.
hydrogen bonding are operable are yet to be determined, but
it is clear that temperature and pH affect chiroptical behavior.
We believe the technological importance of the work
reported here lies in the possibility of employing RAFT poly-
merization directly in water using non-protected chiral amino
acid-based monomers to yield block copolymers with ‘tun-
able’ optical activity. This technique will allow extension of
the early work of Kulkarni and Morawetz[4] which demon-
strated the pH-dependent optical response for homo- and
random copolymers but which was limited by the lack of
structural and molecular weight control inherent to classical
free radical polymerization. Now, copolymers with precise
amphoteric, enantiomeric content can easily be tailored by
a simple, facile polymerization technique. Additionally, it is
anticipated that issues of conformational and configurational
effects on structural organization of chiral centers in aqueous
media can now be addressed directly. In turn, such studies of
optical activity can then be compared to those reported for
protected (or substituted) amino acid derivates polymerized
in organic media.
Acknowledgements
We gratefully acknowledge The Department of Energy
(DE-FC26-01BC15317), Genzyme, MRSEC program of the
National Science Foundation (DR-0213883), and the Robert
M. Hearin Foundation as well as Mississippi College and the
W. M. Keck Foundation for financial support. We would also
like to thank Wako Chemicals for donating V-501.
Conclusions
In this paper we present a facile synthetic route for the for-
mation of optically active, enantiomeric (co)polymers based
on l- and d-alanine directly in water, requiring no protect-
ing group chemistry. Homo- and copolymers with targeted
molecular weights and low molecular weight distributions
have been synthesized via aqueous RAFT polymerization.
The resulting P(ALAL) and P(ADAL) homopolymers were
shown to be optically active and exhibit mirror image CD
spectra. 13C NMR studies indicate chain end control slightly
favoring syndiotactic placement as elucidated by the mea-
sured ratios of rr/rm/mm sequences. In addition, 1:1 statis-
tical and block copolymers were synthesized and showed
little optical activity attributed to CD amplitude cancelling
by the respective enantiomeric repeat units. Whether the
slightly favored syndiotactic placement is responsible for the
‘enhanced’ elipticity values or whether other factors such as
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