Detection of living anionic species in polymerization reactions using hyperpolarized NMR

Article abstract of DOI:10.1021/ja4001008

Intermediates during the anionic polymerization of styrene were observed using hyperpolarized NMR. Dissolution dynamic nuclear polarization (DNP) of monomers provides a sufficient signal-to-noise ratio for detection of ¹³C NMR signals in real t

Full text of DOI:10.1021/ja4001008

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Communication
Detection of Living Anionic Species in
Polymerization Reaction using Hyperpolarized NMR
Youngbok Lee, Gyu Seong Heo, Haifeng Zeng, Karen L. Wooley, and Christian Hilty
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja4001008 • Publication Date (Web): 05 Mar 2013
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Detection of Living Anionic Species in Polymerization Reaction using
Hyperpolarized NMR
Youngbok Lee, Gyu Seong Heo, Haifeng Zeng, Karen L. Wooley, and Christian Hilty
Department of Chemistry, Texas A&M University, College Station, TX 77843, United States
Received March 4, 2013; E-mail: chilty@chem.tamu.edu
†
†
†,‡
†
∗,†
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previously synthesized polymer of large molecular weight without
specific isotope enrichment, it can be expected that most of the ob-
servable signal would decay before initiation of the NMR measure-
ment. Hyperpolarized signals from polymers can nevertheless be
observed, if a polymerization reaction is carried out in-situ in the
NMR instrument starting from hyperpolarized monomers.8 Since
polarization is continuously incorporated at the site of monomer
addition, the active site of the growing polymer chain can be selec-
tively enhanced. Here, we use this unique feature of a hyperpolar-
ized polymerization reaction to enable the detection of intermediate
species as they arise during the synthesis of polystyrene by anionic
polymerization of hyperpolarized styrene monomer.
Abstract: Intermediate species during an anionic polymerization
of styrene are observed by hyperpolarized nuclear magnetic res-
onance (NMR). Dissolution dynamic nuclear polarization (DNP) of
1
3
monomers provides a sufficient signal-to-noise ratio to detect
C
NMR signals in real time while the reaction progresses. Due to a
large chemical shift dispersion, 13C is well suited to distinguish and
characterize the chemical species that arise during the reaction.
At the same time, incorporation of hyperpolarized small-molecule
monomers represents a unique way of generating polymer that ex-
hibits a transient signal enhancement at the active site. This strat-
egy is applicable despite the rapid spin-lattice relaxation in the final
product that would exacerbate the direct hyperpolarization of the
macromolecule using dissolution DNP. In the study of polymerization
reactions, the real-time measurement gives access to both mecha-
nistic and kinetic information. Further, it does not require stable
isotope labeling of the molecules of interest. These capabilities are
orthogonal to currently established methods that separate synthesis
and analysis into two steps, making dissolution DNP an attractive
method for the study of polymerization reactions.
Polystyrene can be synthesized by several different routes, in-
cluding radical, anionic, cationic, and metallocene, or Ziegler-Natta
9
catalyzed polymerizations. Styrene polymerization by the radical
mechanism was recently studied in miniemulsions using hyperpo-
129
10
larized Xe NMR. Reaction progress was monitored by observ-
1
29
ing a change of Xe chemical shift. For the present work, where
we aim to directly observe hyperpolarized products, we chose the
anionic polymerization of styrene as a model reaction. Rich in-
sights into the mechanism and kinetics of anionic polymerization
1
1–15
reactions of styrene are available,
from extensive studies that
NMR has long played a pivotal role in studying reaction mecha-
have been conducted since the seminal work of Szwarc and cowork-
1
6,17
nisms in the field of polymer chemistry, since the technique reveals
ers in 1956.
This data provides an ideal background for com-
1
,2
molecular structures and interactions with atomic resolution. In
the overwhelming majority of uses, NMR is applied as a steady-
state method for analyzing products after completion of a reaction.
It is then often necessary to use sophisticated synthetic strategies
such as the selective incorporation of stable isotopes, to infer in-
parison. An absence of a radical site in the living species ensures
that it can be observed without reduction in NMR signal due to
paramagnetic relaxation. At the same time, this reaction requires
rigorous conditions, which is suitably challenging the protocol of
the DNP-NMR experiment.
3,4
formation about the reaction mechanism. A more direct and po-
Scheme 1 represents a mechanism for the living anionic poly-
16,17
tentially more powerful approach to characterizing a reaction is the
monitoring of species that arise as the reaction occurs. Dissolution
dynamic nuclear polarization,5 a hyperpolarization technique that
can enhance NMR signal by several orders of magnitude, facili-
tates the observation of species that arise in chemical reactions in
merization of styrene.
Styrene was polymerized in dioxane,
18
with sodium naphthalenide as an initiator. The generation of a bi-
functional initiator takes place via single electron transfer from the
sodium naphthalenide to a styrene monomer, followed by a dimer-
ization between two of the resulting styryl radical anions. Initiation
and propagation of styrene monomers then proceeds bidirectionally
to generate telechelic polymer species, and growth continues until
the monomer supply is exhausted.
6
real time.
Dissolution DNP is a two-step process, where a frozen aliquot
of the analyte is hyperpolarized, and then injected into an NMR
spectrometer for analysis. DNP provides sufficient sensitivity to
1
3
Scheme 1. Proposed mechanism of living anionic polymerization of
styrene using sodium naphthalenide as an initiator.
enable C NMR spectroscopy at natural isotope abundance even
at millimolar to sub-millimolar analyte concentration. The use of
1
3
C with its large chemical shift range further facilitates resolution
Initiation:
radical-anion
of the resonances of interest. Most typically, dissolution DNP has
been applied to small molecules containing nuclei with spin-lattice
relaxation times of at least several seconds to avoid prohibitive re-
laxation losses during sample injection. Under certain conditions, it
is possible to obtain hyperpolarized spectra of macromolecules, for
Na
dimerization
Na
Na
+
Na
THF
dark green
Propagation:
electron
transfer
bifunctional initiator
(dark red)
2
example proteins that are biosynthetically labeled with H to coun-
teract spin relaxation.7 However, if attempting to hyperpolarize a
+
2n
Na
Na
Na
n-1
n-1
Na
dioxane, rt
†
Texas A&M University
Current address: Department of Radiology and Radiological Science, Johns
‡
For the DNP experiment, an aliquot of styrene monomers was
hyperpolarized in the solid state at 1.4 K, then rapidly dissolved in
Hopkins University School of Medicine, Baltimore, MD 21205, United States
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dioxane heated to ∼ 200 °C under pressure, and mixed with a solu-
signals are observed in the range between 90 and 140 ppm (sup-
porting information). These signals arise from the C nuclei in
the phenyl ring (#3–5), which are each bonded to a single proton.
A group of singlet signals is observed near 150 ppm, which, due
to the absence of a directly bonded proton, can be assigned to the
quaternary aromatic carbon #1.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
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3
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3
3
3
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4
4
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4
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5
5
5
5
5
5
5
5
5
6
13
tion of sodium naphthalenide in tetrahydrofuran in the NMR spec-
trometer. The sample injection was driven using inert gases, he-
19
lium and nitrogen. The progress of the polymerization reaction was
monitored for a duration of 13 s through a series of small flip an-
gle excitations (Figure 1a; for methods see supporting information).
Due to a signal enhancement of more than 4000-fold compared to
a conventional single scan NMR spectrum, predominantly the res-
onances of styrene were observed. These resonances decay due to
spin relaxation, as time progresses. In addition, however, the spec-
tra contain several smaller signals, which were not detected in ref-
(
a)
x 1
x 2
x 2
x 2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
3
erence C NMR spectra of the final reaction product polystyrene
Figure 1c) or the reactant styrene (Figure 1d). These peaks, high-
151 149 147
105 103 96
13
94
92 65 64 63
13
1
3
13
δ
C / ppm
δ
C / ppm
δ
C / ppm
δ
C / ppm
(
lighted in green in Figure 1a, therefore apparently belong to reac-
tion intermediates that are present only during the reaction progress.
Here, based on characteristic chemical shifts, we may directly pos-
tulate that these resonances stem from the di-anionic polystyryl in-
termediate. The identity of each individual peak would in general,
(
b)
x 1
x 2
x 2
x 4
1
51 149 147
105 103 96
13
94
92 65 64 63
13
however, be difficult to determine simply based on observation in a
1
3
13
13
δ
C / ppm
δ
C / ppm
δ
C / ppm
δ
C / ppm
1
D
C spectrum.
*
(c)
(a)
12.6
0.2
.8
.4
.0
.6
3,4
*
6
5
4
3
2
1
6'
1
6
(...)
2
1
2'
1'
7
1'
1
,2
5
3
5
5'
5'
3'
3'
5
3
5
4
4'
3
3'
6
0
2'
5'
5'
160 140 120 100
80
60
40
20
4'
δ13C / ppm
(
b)
c)
reaction
hyperpolarized)
160 140 120 100
80
60
40
20
1
3
(
δ C / ppm
1
13
Figure 2. a) Expanded views of H decoupled C spectrum of the reac-
1
tion of hyperpolarized styrene. b) As in (a), except without H decoupling.
*
c) Hyperpolarized correlation experiment with a selective inversion on res-
onance #5 at the beginning of the reaction. Positive and negative signals are
represented in blue and red, respectively. Scaling factors were indicated in
each section in a), and b). (*) designates the resonances from 1,4-dioxane,
and THF.
polystyrene
non-polarized reference)
(
* (
*
Although important information can be extracted from chemi-
cal shifts and coupling constants, this approach usually requires
a certain amount of prior knowledge about the molecules under
study. An inherent limitation of one-dimensional spectroscopy is
the absence of direct correlations between different atoms, which
in conventional NMR would be most readily solved by acquiring
multi-dimensional spectra. Here, using hyperpolarized NMR of the
dynamic system, we exploit a different type of correlation that acts
over time. The Zeeman population difference giving rise to the sig-
nals in an entire time series of spectra is generated by DNP at the
beginning of the experiment. Therefore, a correlation between two
chemical species that are transformed into each other can be es-
tablished by selective manipulation of the spin state of one of those
(d)
styrene
(non-polarized reference)
*
160 140 120 100
80
60
40
20
13
δ
C / ppm
Figure 1. a) Series of 13C NMR spectra recorded from a single sam-
ple of hyperpolarized styrene mixed with sodium naphthalenide. b) Hy-
1
3
perpolarized C NMR spectrum for the reaction between styrene and
1
3
sodium naphthalenide. c) Non-hyperpolarize1d3 C NMR spectrum of syn-
thesized polystyrene. d) Non-hyperpolarized C NMR spectrum of styrene
monomer. (*) designates the resonances from 1,4-dioxane, and THF. The
resonance from 1,4-dioxane in the polystyrene spectrum was suppressed us-
ing a selective 90° pulse and randomized pulsed field gradients.
2
0,21
species, which then transfers to the other.
In Figures 2c and S2,
a selective inversion was applied to the resonance of the ortho car-
bon (#5) in styrene. In the subsequent reaction, negative (red) sig-
nals were observed for the peaks at 112 ppm and 104 ppm, which
In order to identify such intermediates, peak splittings in an un-
′
can therefore be unambiguously assigned to #5 in the polystyryl
1
3
decoupled C spectrum can be useful, since these can be directly
obtained from a sequence of one-dimensional spectra acquired from
a hyperpolarized sample. In Figure 2a and b, individual peaks from
anion. All of the other, non-inverted spins on the reactant and in-
termediate yielded positive signals (blue). The two peaks observed
for #5’ are due to the partial double bond character between #2 and
1
3
a proton decoupled and a proton coupled (i.e. undecoupled)
C
22
#
1, which hinders free rotation of the phenyl ring. The result-
spectrum are illustrated (see also Figure S1). Despite the on-going
reaction, the resolution in the spectra obtained is sufficient to re-
solve the expected peak splittings. For example, groups of doublet
2
ing partial sp hybridization character of the alpha carbon (#2) also
23
manifests itself in the scalar coupling constant of JCH = 152Hz.
′
Close inspection of Figure 2c reveals that the peaks #5 also show
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positive signals for a short time at the beginning of the reaction.
tion discussed here is a second-order reaction, and its propagation
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Inevitably, the polymerization reaction started during sample in-
jection, before the selective inversion pulse could be applied. The
initial positive intensity in the product signal was generated from
non-inverted monomers. In this reaction, the initiation step is much
faster than the propagation step.24 The signal change from posi-
tive to negative therefore confirms that the observed product signals
during the course of the reaction were due to fresh additions of in-
verted monomers to the living intermediate, and not simply due to
the initial dimer.
rate is v = kp ⋅C
−
⋅CM = kp⋅I ⋅CM, where C
−
and CM represent
the concentration of the propagating polystyryl anion and styrene
P
0
P
monomer, respectively. We assumed that kp is independent of the
length of polymer chains. Since there is no termination step, CP
is constant, and equal to the concentration of the initiator I . A
−
0
′
′
pseudo first order reaction rate constant k is defined as k = kp⋅I .
0
Taking the kinetics and spin relaxation rM into account, the time
evolution of signal intensities of styrene monomer SM follows (See
supporting information for detail).
In order to provide direct verification for the origin of the other
′
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
�k +rMꢀ⋅t
−
peaks in the intermediate, the hyperpolarized 13C correlation NMR
S (t) = S (0)⋅e
(1)
M
M
experiments were repeated by changing the selective inversion to
different spins of styrene (Supporting Information). Inverted peaks
were detected at 146.9 ppm, 64.5 ppm, 130.7 ppm and 94.0 ppm
As the living polystyrene possesses negative charges at the chain
ends, the anionic terminal carbon chemical environments are differ-
ent from those of the interior backbone structure, resulting in dis-
tinguishable chemical shifts. The signal intensities of the anionic
chain end SP can be calculated using a model where fresh polar-
ization is added through addition of monomers, and polarization is
lost both due to spin relaxation and due to transitioning of a given
moiety to the interior of the polymer. A differential equation for
SP is
′
′
for carbons #1 –#4 . Selective inversion of #6 did not yield an ob-
servable inverted signal, likely due to increased spin relaxation in
the polystyrene backbone. Based on a set of such inversion exper-
iments, it is possible to quite generally and unambiguously assign
the observed peaks from the reaction intermediate. Here, the deter-
mined chemical shifts of the active site in the growing polystyryl
chain are in good agreement with the values of a mono-anionic
polystyryl molecule determined by conventional NMR under equi-
−
−
′
d
′
−(k +r )⋅t
M
SP
−
(t) = −rP ⋅S
−
(t)+k ⋅SM ⋅e
P
0
2
2
dt
librium conditions. A difference of 5.2 ppm was observed at the
′
−k′⋅t
′
alpha carbon (peak #2 ), which is likely due to the effect of the
counter cation (δ(Na) > δ(Li)).
k ⋅C ⋅e
0
22
−
⋅SP− (t) (2)
I
0
Apart from the resonances discussed above, the spectra in Fig-
ures 1 and 2 contain peaks from minor species. These peaks are
not the focus of the present study, but could be analyzed to gain
more information about the reaction conditions and possible side
reactions. A sharp singlet signal at 143 ppm is noted. This chemi-
cal shift, along with 128.7, 36.1 and 31.6 ppm was also determined
for synthesized 1,4-diphenylbutane (Figure S3 in supporting infor-
mation), and in the reaction could belong to styrene oligomer that
has been quenched by residual water. Additional peaks are also ob-
served near 150 ppm (Figure S4). Based on chemical shift, these
peaks may appear to be related to #1, however, they do not show
inversion in the experiment of Figure S2.
where C is an initial concentration of styrene, and rP is the spin
0
relaxation rate of the anionic chain end, which is assumed to be
independent of the length of polymer chains. In order to find a so-
lution to this equation, it is further necessary to specify a boundary
condition; here, SP
the signal of the dimer is negligible due to fast relaxation of its pre-
cursors, two radical anions.
−
(0) = 0 is used based on the assumption that
The unknown reaction and relaxation rates can be obtained from
fitting these models to time-resolved DNP-NMR data sets, such
as shown in Figure 1a. In order to obtain these data, a series of
fixed small flip angle pulses was applied to acquire NMR spectra
at equal time intervals. To account for the depletion of polariza-
tion by small-flip angle pulses, equation 1, as well as the numeri-
-1
x 10
0.6
#1'
#2'
#3'
#4'
#5'
−
λ⋅t
(a)
#1
#2
1.0
.8
(b)
cal solution of equation 2 was multiplied with factor of e
prior
#3
#4
#5
#6
0
to calculating the root mean square difference. In the exponential,
λ = −ln(cos(α))/Δt depends on the small flip angle α and the time
interval between NMR acquisitions Δt.
0.4
1
2
0.6
0.4
0.2
0.0
#5'
0
.2
.0
′
The pseudo-first order reaction rate constants k were obtained
from fit of equation 1 to the thus normalized signal intensities of
styrene monomer (Figure 3a). This procedure requires knowledge
of the relaxation rates of styrene monomer rM, which were deter-
mined from a DNP experiment in the absence of a reaction (with-
out addition of sodium naphthalenide). The propagation rate con-
stant (kp) was finally calculated with knowledge of the initiator
0
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Time / s
Time / s
Figure 3. a) Fit of the relative signal intensity of styrene with equation
1
, using SM(0) and rM as parameters. b) Fit of signal intensities of living
polystyril ends. The fit was performed by minimizing the root-mean-square
2
5,26
concentration,
which was determined from the chain length
difference between experimental data and the numerical solution of equa-
tion 2, using rP as parameter and S
−
(0) = 0 as boundary condition.
distribution obtained by matrix assisted laser desorption/ionization
(MALDI) mass spectrometry of the reaction product (see Figure
P
In addition to the information pertaining to reaction mechanism,
any real-time measurement also contains kinetic information. A
benefit of the DNP-NMR technique is the capability to determine
reaction mechanism and kinetics simultaneously from a single hy-
perpolarized sample. In order to extract kinetic information, it is,
however, necessary to consider the process of spin-relaxation along
with the reaction, since both give rise to changes in signal intensity.
The DNP experiments start with large initial polarization, which
decays towards the thermal equilibrium. The latter is negligible,
therefore the only source of observable signals stems from the hy-
perpolarized styrene monomers. The living anionic polymeriza-
27,28
S5).
The average propagation rate constant based on three in-
−1 −1
dependent data sets was 5.6±0.6M
s
(Table 1). This value is
−1 −1
in excellent agreement with the literature (3.4 ∼ 6.5M
s
; mea-
sured under similar conditions, but without the additional 5% (v/v)
11
of THF present in the DNP experiments).
Although the reaction kinetics can unambiguously be determined
from the monomer signals, we tested the proposed model for ki-
netics and spin relaxation in the styrene polymerization reaction
by also fitting the signal intensities of the living end intermediate
(Figures 3a and S6). This fit was calculated by adjusting a single
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Page 4 of 5
unknown parameter, the relaxation rate rP of the intermediate to
minimize the root mean square difference between experimental
data and the numerical solution of equation 2. The numerical so-
diate. The living anionic polymerization reaction studied here is
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
sensitive to water and air. Nevertheless, the DNP-NMR technique
in combination with a closed rapid sample injection system could
provide a sufficiently inert environment for the successful study of
the reaction. Systematic modifications, which allow for loading
the active initiator and dried solvents while excluding air contact,
may further improve the experimental results. On this premise, the
method presented here represents an attractive means for funda-
mental studies of polymerization reactions, where alternative tech-
niques would involve cumbersome isotope labeling and synthesis
strategies. Having verified the overall DNP-NMR approach to the
study of the well-known highly reactive polymerization intermedi-
ates involved during the living anionic polymerization of styrene,
we are currently applying this powerful technique to the investiga-
tion of lesser studied polymerization systems.
lution was calculated in each iteration for rP using a variable step
′
Runge-Kutta method with given parameters (k , rM, C ). In addi-
tion, specification of initial conditions was required, which were
0
taken as SP
−
(0) = 0 under the assumption that signals from the ini-
tial dimer carries no signal due to rapid relaxation in its precursors,
the styryl radicals.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 1. Determination of the propagation rate constant, kp
′
a
D.P.b
c
k
s
I
0
kp
s
−
1
−1 −1
(
)
(mM) (M
)
Reaction 1 0.45 ± 0.03
6.3
92
54
65
4.9
5.6
6.2
Acknowledgement The authors thank Dr. Yohannes Rezenom
for assistance with the mass spectra. Support from the Ameri-
can Chemical Society Petroleum Research Fund (Grant 501813-
ND7) and the National Science Foundation (Grant CHE-0840464)
is gratefully acknowledged (Y. L., H. Z. and C. H.). The Welch
Foundation is gratefully acknowledged for partial support through
the W. T. Doherty-Welch Chair in Chemistry, Grant No. A-0001
Reaction 2 0.30 ± 0.02 13.6
Reaction 3 0.40 ± 0.03 10.8
a
From equation 1, average from six different carbon
atoms of styrene. For individual values, see table S1.
Degree of polymerization; determined from the
MALDI mass spectrum of reaction product.
b
c
Determined from comparison of the D.P. obtained from
MALDI data with the known monomer concentration.
(G. S. H. and K. L. W.).
Supporting Information Available: Experimental section, Sup-
plementary calculations, DNP correlation experiments, Hyperpolarized
1
13
H decoupled and undecoupled C spectra, MALDI-TOF Mass spec-
Figure 3b shows signal intensities of the living intermediate with
their corresponding numerical fit lines, and the obtained rP values
from the three independent data sets are summarized in table S2
tra of polystyrene, kinetic results, 13C spectra of 1,4-diphenylbutane.
This material is available free of charge via the Internet at http:
/
/pubs.acs.org/.
(
supporting information). Despite the use of only one fit parame-
ter, the curves agree remarkably well with the experimental data.
Small differences, in particular at the beginning of the reaction,
may be attributed to a certain amount of remaining signal on the
initial styrene dimers, which, however, cannot be modeled reliably
based on the available data. The amount of NMR signal lost due
to the presence of a short-lived radical is expected to depend on the
distance of the observed site to the radical center. In the future, it
would be interesting to determine whether such distance informa-
tion could be extracted from data measured of reactions following
a radical polymerization mechanism.
In the present reaction, the relaxation rates of all of the non-
overlapping carbon atoms in the anionic chain ends rP determined
from the hyperpolarized data agree to within 12 % with reference
rates obtained from a standard of polystyrene (Polymer Laborato-
ries, Church Stretton, UK; average molecular weight Mn = 1360
g/mol), the measurement of which is, however, not selective for
chain ends. Since the rP value from the fit is obtained based on
analysis of the kinetic parameters, the coincidence of the relaxation
rates suggests overall validity of this method for analysis of the
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mixing and turbulence2
conditions.
1,29
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