Time-resolved EPR studies of main chain radicals from acrylic polymers. Dynamic effects due to conformational motion

Article abstract of DOI:10.1021/ma047801x

The temperature dependencies of the time-resolved electron paramagnetic resonance (TREPR) spectra of five main chain acrylic free radicals are presented and discussed in terms of conformational dynamics. The radicals are produced in liquid solution at temperatures ranging from 25 °C to over 100°C by direct excitation (248 nm) of the ester group in the polymers leading to Norrish I α-cleavage of the side chain ester moiety. Restricted rotational motion near the radical center leads to modulation of the β-hyperfine coupling constants, which manifests itself in some of the spectra as an alternating line width effect near room temperature. A standard hyperfine modulation model (two-site exchange) is proposed and has been added to the simulation routine. The model works especially well for two of the polymers (poly(ethyl acrylate) and polydnethyl d₃-methacrylate), for which activation barriers for rotation are extracted from Arrhenius plots. The model is somewhat successful for poly(ethyl cyanoacrylate) but fails for poly(ethyl methacrylate) and polydnethyl methacrylate). This failure is discussed in terms of radical structure and dynamics and the possibility that jumping between more than two low-energy conformational sites is involved. Disruption of the symmetry of the hyperfine couplings at intermediate temperatures supports this explanation.

Full text of DOI:10.1021/ma047801x

3342
Macromolecules 2005, 38, 3342-3350
Time-Resolved EPR Studies of Main Chain Radicals from Acrylic
Polymers. Dynamic Effects Due to Conformational Motion
Vanessa P. McCaffrey, Elizabeth J. Harbron, and Malcolm D. E. Forbes*
Venable and Kenan Laboratories, Department of Chemistry, CB #3290, University of North Carolina,
Chapel Hill, North Carolina 27599
Received October 25, 2004; Revised Manuscript Received February 9, 2005
ABSTRACT: The temperature dependencies of the time-resolved electron paramagnetic resonance
(TREPR) spectra of five main chain acrylic free radicals are presented and discussed in terms of
conformational dynamics. The radicals are produced in liquid solution at temperatures ranging from 25
°C to over 100 °C by direct excitation (248 nm) of the ester group in the polymers leading to Norrish I
R-cleavage of the side chain ester moiety. Restricted rotational motion near the radical center leads to
modulation of the â-hyperfine coupling constants, which manifests itself in some of the spectra as an
alternating line width effect near room temperature. A standard hyperfine modulation model (two-site
exchange) is proposed and has been added to the simulation routine. The model works especially well for
two of the polymers (poly(ethyl acrylate) and poly(methyl d3-methacrylate), for which activation barriers
for rotation are extracted from Arrhenius plots. The model is somewhat successful for poly(ethyl
cyanoacrylate) but fails for poly(ethyl methacrylate) and poly(methyl methacrylate). This failure is
discussed in terms of radical structure and dynamics and the possibility that jumping between more
than two low-energy conformational sites is involved. Disruption of the symmetry of the hyperfine couplings
at intermediate temperatures supports this explanation.
Scheme 1
Introduction
In two previous papers in this series, we presented
conclusive spectroscopic evidence and product analysis
results for side chain cleavage as the primary mecha-
nistic event in the photodecomposition of acrylic poly-
mers.1,2 The process is illustrated for poly(ethyl acrylate)
in Scheme 1. Simulation of the time-resolved electron
paramagnetic resonance (TREPR) spectra of main chain
polymeric radicals such as 1a was straightforward at
high temperatures, using standard techniques and
motionally averaged hyperfine coupling constants. All
of the data in ref 2 were obtained in the “fast-motion”
regime of the polymer.
For most of the acrylic radicals we have studied using
TREPR, significant line broadening effects appear as
the temperature drops below 90 °C. These effects are
present for some transitions but not others, and in
several cases the familiar alternating line width pattern
is present at lower temperatures. This phenomenon
generally appears when hyperfine coupling constants
are being modulated, in this case by temperature-
dependent conformational motion of the polymer main
chain near the radical center. In this paper, these
temperature effects will be presented and discussed in
terms of motional dynamics of the polymer backbone
for five different acrylic main chain polymers, all of
which have previously been unambiguously character-
ized at fast motion.
Dynamic effects have been observed previously in
steady state EPR (SSEPR) studies of acrylic polymers.
However, it is typically the propagating radical (1c in
Scheme 1) rather than the main chain radical that is
observed in this experiment because of the â-scission
rearrangement process. (As explained previously, the
TREPR experiment is fast enough to detect radical 1a
before this rearrangement takes place.) In initial studies
of the photodegradation of these polymers, SSEPR
spectra exhibited an unusual nine-line spectrum with
alternating line widths of five sharp and four broad
lines.3-6 The spectrum was assigned to the propagating
radical 1c. As a function of temperature, this spectrum
changed reversibly to a 13- or 16-line spectrum depend-
ing on the available resolution. Four theories have been
proposed to account for these findings: One is that there
exists a superposition of two or more static conforma-
tions of the radical.7-9 A second theory proposes that
the propagating radical exists with a Gaussian distribu-
tion of dihedral angles, centered about a single preferred
conformation.10
A third and most recent theory comes from Matsu-
moto and Giese, who relied on small molecule model
compounds to simulate the propagating radical.11,12
They suggested that the observed steady state (SSEPR)
spectrum was due to a superposition of two conforma-
tions of the same radical, one of which would convert
reversibly into the other at a given temperature. How-
ever, they also assumed that the radical center was
* To whom correspondence should be addressed. E-mail:
mdef@unc.edu.
10.1021/ma047801x CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/22/2005

Macromolecules, Vol. 38, No. 8, 2005
Main Chain Radicals from Acrylic Polymers 3343
pyramidalized, i.e., that it was pure sp3-hybridized
rather than sp2-hybridized due to steric hindrance. Such
an assumption is not supported by the literature on free
radicals. There are numerous reports of sterically
hindered radicals that are still sp2-hybridized.13,14 Only
strongly electronegative substituents like fluorine have
been found to cause significant deviations toward sp3
hybridization,15-17 and there are solid arguments for
this phenomenon based on modern electronic structure
theory. Extreme pyramidalization of radicals containing
only C, H, and O atoms generally occurs only in rigid
systems like the adamantyl radical.18
Chart 1
Iwasaki19 invoked a fourth model, hyperfine modula-
tion, to explain the spectra of the propagating radical
and were able to simulate their observed 9- and 13-line
SSEPR spectra using a set of modified Bloch equations
for a two-site exchange model between two conforma-
tions. By varying the correlation times of the conforma-
tions over an order of magnitude, reasonable fits to the
experimental data were obtained.
For main chain polymeric radicals studied here, which
are created in solution at room temperature and above,
the presence of a superposition of conformations or
Gaussian distributions is unlikely. Polymers are known
to undergo conformational jumps on the submicrosecond
time scale, even in bulk at room temperature.20-22 Both
of the first two theories above require that the radicals
be fairly rigid with little (Gaussian distribution) or no
(superposition of static conformations) movement around
the CR-Câ bond. The polymeric radical studied here is
sterically hindered but still conformationally quite
flexible; therefore, such a dramatic change in hybridiza-
tion is not likely. For these reasons we have approached
our simulations with the hyperfine modulation model,
the details of which we now briefly present.
an important requirement of this model is that the
hyperfine couplings be anticorrelated; i.e., the sum of
the hyperfine coupling constants always remains con-
stant.27
The appearance of the spectrum depends on whether
the system is undergoing fast or slow motion or is in
some intermediate state. These regimes are defined by
the correlation times for the two conformations and the
difference of the resonance frequencies, ∆ω, of the two
transitions:
τ∆ω < 1 (fast motion)
τ∆ω g 1 (slow motion)
(4)
(5)
In the region of fast motion, the correlation time is
shorter than the inverse of the difference in resonance
frequencies. This will result in one average transition
for the two conformers. The opposite holds true for slow
motion, where separate and sharp lines for each of the
two conformations are observed. We note that in Sakai
and Iwasaki’s work on the propagating radical10 the
opposite trend was observed. This was interpreted as a
crystalline transition that affected the energy barrier.
If the motion of an acrylic polymer radical about the
CR-Câ is hindered, then changing the temperature
should lead to changes in the appearance of the TREPR
spectrum, and this is indeed observed for all of the
polymers studied here. Simulation of the complete
temperature dependence of TREPR spectra of acrylic
polymer main chain radicals should allow information
regarding the conformational motion of the polymer in
solution to be extracted. By careful analysis of the
experimental data, information such as rotational cor-
relation times, spin-lattice relaxation times (T1), and
activation energies for conformational transitions can
be derived.
The alternating line width effect in EPR spectroscopy
was discovered independently in the early 1960s by
Bolton and Carrington and by Freed, Bernal, and
Fraenkel.23,24 The physical origin of this effect is that
the hyperfine couplings fluctuate due to some inter- or
intramolecular process. Selective broadening of the lines
in the spectrum (in the condition of fast exchange)
occurs according to the equation25
2
-1
T2-1 ) γe τ (δa)2 (Ma - Mb)2 + T2,0
(1)
where T2-1 is the line width after broadening, γe is the
gyromagnetic ratio of the electron, Ma and Mb are the
nuclear spin quantum numbers of the transitions, and
-1
T2,0 is the contribution to the line width from other
mechanisms. The term τ is the correlation time for the
exchange process and can be calculated from
τ1τ2
τ )
(2)
Results and Discussion
τ1 + τ2
The polymers studied in this work are shown in Chart
1. They are all polymers for which the high-temperature
(fast-motion) TREPR spectra are simulated to a high
level of confidence. The synthesis, sample preparation,
polymer characterization, and spectroscopic methods are
all described in our earlier publication.2
where τ1 and τ2 are the inverse of the rate constants for
jumping the barrier to rotation. The mean-square
deviation in the hyperfine splitting is
(δa)2 ) (1/4)(a1 - a2)2
(3)
It is instructive to briefly outline our simulation
routine: a two-site jump model for hyperfine modulation
is incorporated into our standard chemically induced
electron spin polarization (CIDEP) simulation program.
The spectroscopic and physical features of the model are
illustrated in Figure 1. The methyl group hyperfine
where (a1 - a2) is the difference between the fast-motion
hyperfine coupling constants. An excellent example of
a two-site exchange process leading to hyperfine cou-
pling constant modulation is the vinyl radical studied
by Sullivan et al.26 With regard to radicals such as 1a,

3344 McCaffrey et al.
Macromolecules, Vol. 38, No. 8, 2005
Figure 1. Streamlined view of the hyperfine modulation model used in the simulation program. The methyl group hyperfine
couplings have been removed for clarity. The conformational change shown converts one triplet of triplets from the equivalent
â-methylene protons on one side of the radical center (Ha, Hb) into their symmetric counterparts on the other side (Ha′, Hb′), and
vice versa. The nuclear spin quantum numbers representing each transition in each state (A and B) are shown below the transitions
in R and â notation. Dashed lines drawn between thick lines show transitions that exchange due to the motionsthese lines
broaden in the TREPR spectrum. The transitions represented with thin lines remain sharp through the modulation process as
their total nuclear spin quantum number does not change during a jump between sites.
coupling constant in PMMA radical 1a is assumed to
be constant for all conformations and therefore is
omitted from the figure. It is assumed that one of the
two triplets from the â-hyperfines (two diastereotopic
H’s on either side of the radical center) is being
modulated into the second triplet on some time scale,
τ. The hyperfine values for Ha and Ha′ are exchanging
into those for Hb and Hb′, and vice versa. This process
is shown graphically on the right-hand side of Figure
1. This clearly indicates the anticorrelation of the two
triplets. In the first conformation shown in Figure 1A,
the stick plot expected from this triplet of triplets is
shown with a given set of hyperfine couplings. Below
this stick plot, the total nuclear spin quantum numbers
for each transition are listed (our notation reads that R
represents mS ) +1/2 and â represents mS ) -1/2). The
lines that do not change total spin quantum numbers
upon interconversion to the new triplet of triplets are
the transitions that remain sharp in the TREPR spec-
trum. These are indicated with thin lines in the stick
plots. Transitions exchanging different total spin quan-
tum numbers are broadened as predicted by eq 1.
Specifically exchanged lines are connected with dashed
horizontal lines in Figure 1.
Three sets of hyperfine coupling constants were input
to the simulation program. The methyl (or R-coupling
for PEA) hyperfine coupling constant is assumed to be
the same in all conformations and is not considered
further. The program is able to calculate the amount of
line width added to each transition from â-hyperfine
modulation separately for each pair of protons according
to eq 1. Values for the correlation time (τc) and the
hyperfine difference (a1 - a2) were input as fitting
parameters. In all the simulations shown here, the (a1
- a2) value used was 15.6 G. The program calculates
the amount of additional line width based on the change
in nuclear spin quantum numbers for each transition.
Some of the lines in the spectrum received no additional
broadening (M1 - M2 ) 0) as per eq 1. As was noted in
the discussion on hyperfine modulation, one of the
requirements for the hyperfine modulation mechanism
to be active is that the sum of the hyperfine couplings
must remain constant. A value of 28 G (approximately
the sum of the two fast motion (average) coupling
constants) was used for this sum.
Conformational Analysis of Main Chain Acrylic
Radicals. With the exception of the R-hyperfine inter-
action of the backbone H in PEA, all of the hyperfine
couplings in the polymeric radicals above result exclu-
sively from â-hyperfine interactions; i.e., they arise from
hyperconjugation between the C-H σ-bond and the
unpaired electron in the p-orbital. The magnitude of the
coupling depends on the dihedral angle θ between the
σ-bond and the p-orbital and can be determined quan-
titatively.28,29 Scheme 2 shows Newman projections of
the polymer down each of the CR-Câ bonds of the radical
produced upon side chain cleavage. The spectral simu-
lations presented below will reveal that at high tem-
peratures the two pairs of methylene proton coupling
constants merge to the same values in PMMA and that
both of the methylene hyperfine coupling constants
differ significantly from the methyl group coupling
constant. From the large difference between the PMMA
and PEA spectra and the strong tacticity dependence
of the PMMA spectra established in our previous paper,
it is possible to establish a connection between the
stereochemical configuration closest to the radical center
and the resulting symmetry relationships in the free
radicals.

Macromolecules, Vol. 38, No. 8, 2005
Main Chain Radicals from Acrylic Polymers 3345
Scheme 2
Figure 2. Experimental (left) and simulated (right) TREPR
spectra of polymeric radical 1a from poly(ethyl acrylate) (PEA)
in propylene carbonate solution at the temperatures indicated.
Sweep width of all spectra is 200 G. The simulation parameters
used in each simulation are given in Table 1.
polymers are somewhat similar but not identical. In fact,
our TREPR spectra exhibit a remarkable and unex-
pected sensitivity to the long-range effects of the side
chain substituents.
The difference in the fast-motion average hyperfine
couplings in the simulations of the TREPR spectra of
the polymeric radicals from PEMA (4a) and PMMA (5a)
can be qualitatively assessed by considering the relative
conformational energies of the polymers in solution. The
three staggered Newman projections along one of the
Câ-Cγ bond of the polymeric radical of either PEMA or
PMMA are shown in Scheme 2 and are labeled as
conformations I, II, and III. While the absolute energies
are not known, for qualitative discussion we estimate
their relative energies based on the steric bulk of the
substituents as EII . EIII > EI. From some preliminary
efforts to model these energetics using force field
calculations on short oligomers, we have concluded that
long-range macromolecular effects on the energies of the
conformers in Scheme 2 may be significant, and a large
amount of work remains to be done to accurately model
the motional dynamics of these structures.
For the present work we assume that the two site
model will work and most likely consists of jumping
between conformers I and III in Scheme 2, with con-
former II being too high in energy to contribute ef-
fectively to the mole fraction xII in eq 7 above. High-
level computational work at the ab initio level with
geometry optimization is being carried out at present
on PMMA oligomers in an attempt to model these
conformational dynamics quantitatively. Once a more
precise model is in place we will use it, together with
our TREPR data, to determine the relative energies of
the polymer conformations. It will be of great interest
to determine the degree of long-range vs short-range
structural influence on these dynamics.
A. Poly(ethyl acrylate). Figure 2 shows the tem-
perature dependence of the TREPR spectra of the PEA
radical 1a in propylene carbonate at a variety of
temperatures. As the temperature is increased from 25
to 125 °C, the broad inner lines become sharp doublets.
The doublets are resolved for the innermost lines at a
temperature of 79 °C. At lower temperatures, a singlet
slightly to the right of center is observed and is assigned
to the oxo-acyl radical 1b. The rapid decay of this signal
is most likely due to fast spin relaxation30 and perhaps
also some chemical decomposition of the radical. The
delay time after the laser flash at which these data were
collected is 0.9 µs, which is where the maximum
intensity of the polymer radical signal is usually ob-
served. The signal from the oxo-acyl radical reaches a
maximum ∼0.3 µs. As the temperature is increased, the
signal from the oxo-acyl radical gradually decreases in
intensity, and at 109 °C its signal has disappeared
almost completely at this delay time.
Continuing qualitatively for the present discussion,
each of the conformations in Scheme 2 will be populated
at a specific temperature by some mole fraction xI, xII,
or xIII of the polymer. The observed fast motion hyper-
fine coupling constant aH can be calculated from the
equation below if the relative energies (and therefore
mole fractions) are known:
aH ) xIaHI + xIIaHII + xIIIaHIII
(6)
where aI, aII, and aIII are the hyperfine coupling
constants of either Ha or Hb in each of the three
conformations in Scheme 2. When the temperature is
raised or lowered, the relative populations of the three
conformations are altered, which in turn changes the
mole fractions of the conformations of interest. This
ultimately affects the observed hyperfine coupling con-
stant in the TREPR spectrum. However, in the process
we are observing we assume only two conformations
participate and that they are equally populated. Below
we will present a discussion of the justification of this
statement. When the polymer is changed from PMMA
to PEMA, the relative energies of the three conforma-
tions (and possibly also the barrier heights) are changed.
We believe this to be the main reason why the hyperfine
coupling constants change from one polymer to the next.
In the simulations presented for PMMA, PEMA, and
the other polymers studied in this work, the hyperfine
couplings remain fairly constant at high temperatures,
indicating that the conformational energies of the
The simulations produced for the photolysis of PEA
using this modified program are shown on the right side
of Figure 2 with the corresponding experimental spectra
on the left. The parameters used are given in Table 1.
Included in the simulations of the lower temperature
spectra, the oxo-acyl radical 1b can be observed as a
sharp singlet. A line width of 3.2 G and a g-factor of
2.0008 were used for the simulations of the oxo-acyl
radical.
By visual inspection, the simulations are well matched
to the experimental data. This is particularly true at
the higher temperatures. As the temperature decreases,
there are minor deviations in the line shape of the

3346 McCaffrey et al.
Macromolecules, Vol. 38, No. 8, 2005
Table 1. Parameters Used in the Simulation of the
Temperature Dependence of TREPR Spectra of Radical
1a (from PEA) Shown in Figure 2
temp
(°C)
line width
(G)
τc ( 0.2
(s)
aH (G)
aCH (G)
aCH (G)
2
2
25
53
21.5
21.5
21.5
21.5
21.7
23.3
23.3
23.5
23.5
23.5
24.0
23.9
24.1
24.0
23.8
3.0
2.5
2.0
1.5
1.7
3 × 10-10
2 × 10-10
1 × 10-10
9 × 10-11
79
109
125
Figure 4. Experimental (left) and simulated (right) TREPR
spectra of the polymeric radical 2a from poly(methyl d3-
methacrylate) in propylene carbonate solution at the indicated
temperatures. Sweep width for all spectra is 200 G. Simulation
parameters are listed in Table 2.
Table 2. Parameters Used in the Simulation of the
Temperature Dependence of TREPR Spectra of Radical
2a (from d3-PMMA) Shown in Figure 4
temp
(°C)
line width
(G)
aD (G) aCH (G) aCH (G)
τc ( 0.2 (s)
2
2
33
50
3.5
3.5
3.5
3.5
3.5
3.5
15.7
15.9
16.1
16.3
16.3
16.3
10.3
10.5
10.7
10.9
10.9
10.9
2.7
2.2
1.7
1.5
1.4
1.3
2.5 × 10-10
2.0 × 10-10
1.0 × 10-10
7.0 × 10-11
71
90
110
119
is 15 ( 2 kJ/mol. This value is comparable to those
measured for poly(methyl acrylate) by Bullock using
spin-labeling methods (19 ( 3 kJ/mol). However, both
the value found here and Bullock’s are somewhat lower
than a value of 23 kJ/mol determined by North using
dielectric measurements.34 This discrepancy is probably
a function of the small difference in side chain and
solvent. It is known that addition of larger alkyl groups
on the ester tail will affect the dynamics of the polymer
in solution from NMR experiments. The solvent system
is also different for all three of the previous Ea deter-
minations, and activation energies have been shown to
vary over 5 kJ/mol in chlorinated solvents alone.35
Variation in the molecular weight of the three different
samples is not likely to be the cause of major differences
in the Ea values. In fact, the TREPR spectra of all of
these acrylic polymer radicals have, to date, not been
dependent on the molecular weight of the polymer.
Relaxation of the polymer is not due to end-over-end
motions but instead to segmental motions which are not
very sensitive to variations in the molecular weight.32,33
Rotation of the polymer around the CR-Câ bond may
also be affected by the presence of the radical center
and might be different from that of the nonradical
polymer. Overall, the Ea value obtained is certainly of
the correct order of magnitude compared to other
studies on similar polymer structures.
B. Poly(methyl d3-methacrylate). The temperature
dependence of the TREPR spectra from the deuterated
PMMA polymer radical 2a is shown in Figure 4. The
differences between the spectra at the various temper-
atures are less dramatic than that of the nondeuterated
polymer due to the smaller spectral width of the spectra.
The spectra again show the alternating line width effect
at the lower temperatures. The simulations are shown
to the right of the experimental data in Figure 4, and
the simulation parameters are presented in Table 2. In
all simulations, a value of 3.5 G was used for the
hyperfine coupling constant of the CD3 group because
this group is always in fast rotation about the CR-Câ
Figure 3. Arrhenius plot for rotational correlation time data
from TREPR spectra of polymeric radical 1a. Dots are the
experimental data, and the solid line is the linear fit with Ea
) 15 ( 2 kJ/mol.
central two lines of the polymer radical. This is probably
due to differences in the hyperfine values used for the
â-methylene protons. In these simulations, it was as-
sumed that the hyperfine values remained constant for
both pairs of protons. This may not always be the case,
as will be seen below for the poly(alkyl methacrylates).
Minor variations were necessary to maintain the correct
spectral width. The hyperfine coupling constant for the
R-proton remained the same at all temperatures, except
for the spectrum at 125 °C, where the value increased
by 0.2 G. These slight variations most likely reflect the
varying contributions of the polymer conformations to
the faster motion spectra and therefore slightly different
average hyperfine coupling constants. A small amount
of increasing natural line width was needed for each
line, independent of the modulation process, to best fit
these data. This suggests that hyperfine anisotropy may
be slightly greater for PEA than for the methacrylates.
The rotational correlation times used in the simula-
tions are also listed in Table 1. The τc values are
comparable to those found by SSEPR and fluorescence
depolarization methods for PMMA.31-33 The correlation
times were used to determine the activation energy of
the conformational changes in the polymeric radical of
PEA using the Arrhenius equation with the rate con-
stant k equal to 1/τc. The slope of a plot of ln(τc) vs 1/T
will be positive and give the activation energy directly.
Figure 3 shows the Arrhenius plot of the correlation
times found for the PEA polymeric radical, 1a, at
different temperatures in propylene carbonate. The
activation energy determined from the slope of this plot

Macromolecules, Vol. 38, No. 8, 2005
Main Chain Radicals from Acrylic Polymers 3347
Figure 6. Experimental (left) and simulated (right) TREPR
spectra of the polymeric radical 3a from poly(ethyl cyanoacry-
late) (PECA) in propylene carbonate solution at the indicated
temperatures. Sweep width of all spectra is 150 G. Simulation
parameters are listed in Table 3.
Table 3. Parameters Used in the Simulation of the
Temperature Dependence of TREPR Spectra of Radical
3a (from PECA) Shown in Figure 6
temp
(°C)
line width
(G)
aN (G) aCH (G) aCH (G)
τc (s)
2
2
25
51
3.3
3.3
3.3
3.3
16.5
16.5
16.5
16.3
14.6
14.6
14.6
14.8
6.0
4.2
2.3
2.0
3.0 × 10-10
2.0 × 10-10
3.0 × 10-11
Figure 5. Arrhenius plot for rotational correlation time data
from TREPR spectra of polymeric radical 2a. Squares are the
experimental data, and the solid line is the linear fit with Ea
) 22 ( 2 kJ/mol.
90
109
â-methylene coupling constants, the two packets im-
mediately adjacent to the center of the spectrum are
split into multiplets. As the temperature is decreased,
all of the lines in the spectra begin to broaden equally.
At room temperature, the signal due to the polymeric
radical is little more than an emissive background, and
the signal from the oxo-acyl radical appears as a sharp
singlet.
It is difficult to conclude whether the TREPR spectra
of this polymer are exhibiting hyperfine modulation.
There are no lines in the lower temperature spectra that
remain sharp throughout the entire temperature range.
The right-hand side of Figure 6 shows simulations of
the experimental data with the parameters given in
Table 3. At 25 and 51 °C, the spectrum of the oxo-acyl
radical was included in the simulation. Visually, the
simulations of the data obtained at 90 and 109 °C
represent good fits to the experimental results. How-
ever, at the lower temperatures, the simulated spectra
are not fit with unique sets of coupling constants. Large
amounts of natural line width had to be added in order
to broaden the spectra enough so that the triplet due
to the nitrogen atom would not be resolved. This made
it difficult to gauge the correct value for τc. The error in
the rotational correlation time is very large, at least 50-
75% of the value listed in Table 3. Because of this large
estimated error, a determination of the activation
energy for this polymer was not made.
D. Poly(ethyl methacrylate). Figure 7 shows the
temperature dependence of the TREPR spectra of the
polymeric radical 4a obtained by photolysis of PEMA
in propylene carbonate solution. The lower temperature
spectra show the same alternating line width effect at
low temperatures as observed with PMMA. The lines
in the TREPR spectrum at 42 °C are much broader than
in PMMA. This is expected because the PEMA polymer
backbone in solution is much stiffer and has slower
dynamics than PMMA. It is well-known from NMR37,38
and fluorescence studies39 that the dynamics of the ester
group (â-relaxation) can couple to the motion of the
polymer backbone (R-relaxation) and influence the
bond. It would be unexpected for this value to change
drastically in our systems. For the spectra obtained at
temperatures of 90, 110, and 119 °C, the hyperfine
coupling constants are the same. The only difference is
the amount of natural line width added into each of the
spectra and the presence of hyperfine modulation at 90
°C. Increases in the natural line width of the spectrum
will broaden all of the transitions uniformly, while
including hyperfine modulation in the simulation pro-
gram only broadens selected transitions as described
above.
For spectra obtained at 110 and 119 °C, additional
line width to each transition equally is necessary to
simulate the experimental data. As the temperature
drops below 100 °C, incorporation of hyperfine modula-
tion becomes necessary to simulate the data. Simply
increasing the natural line width for all transitions
without addition of hyperfine modulation does not lead
to good fits of the experimental data. As in the TREPR
of the PEA radical, as the temperature drops the overall
line width of all of the transitions increases, and also
slight changes in the average hyperfine coupling con-
stants from 90 to 30 °C were observed. The simulated
data in Figure 4 give values of τc that closely fit a
modified Arrhenius plot shown in Figure 5. The activa-
tion energy determined from this plot is 22 ( 2 kJ/mol.
This value is somewhat higher than that found for the
PEA radical, which is not surprising because the addi-
tion of a methyl group on the polymer backbone should
change the conformational energies and particularly
increase the barriers to rotation of the polymer chain
in solution.36
C. Poly(ethyl cyanoacrylate). To study the effect
of main chain substituents on the appearance of the
TREPR spectrum, poly(ethyl cyanoacrylate) (PECA) was
photolyzed. Figure 6 shows the temperature dependence
of the TREPR spectrum of polymeric radical 3a in
propylene carbonate. The high-temperature spectrum
(109 °C) was successfully simulated in ref 2. Because
of the small hyperfine coupling to the CN nitrogen on
the side chain, and the nearly equal values of the

3348 McCaffrey et al.
Macromolecules, Vol. 38, No. 8, 2005
Figure 7. Experimental (left) and simulated (right) TREPR
spectra of the polymeric radical 4a from poly(ethyl methacry-
late) (PEMA) in propylene carbonate solution at the indicated
temperatures. Sweep width for all spectra is 200 G. Simulation
parameters used in each simulation are listed in Table 4.
Figure 8. Experimental (left) and simulated (right) TREPR
spectra of the polymeric radical 5a from a-PMMA in propylene
carbonate solution at the indicated temperatures. Sweep width
of all spectra is 200 G. Simulation parameters are given in
Table 5.
Table 4. Parameters Used in the Simulation of the
Temperature Dependence of TREPR Spectra of Radical
4a (from PEMA) Shown in Figure 7
Table 5. Parameters Used in the Simulation of the
Temperature Dependence of TREPR Spectra of Radical
5a (from a-PMMA) Shown in Figure 8
temp
(°C)
line width
(G)
line
aCH (G) aCH (G) aCH (G)
τc (s)
temp
width
3
2
2
(°C) aCH (G) aCH (G) aCH (G) aCH (G) aCH (G) (G)
τc (s)
3
2
2
2
2
42
99
108
119
23.1
23.1
22.9
22.9
15.2
16.0
16.0
15.8
11.7
10.9
11.4
11.2
1.5
1.5
1.5
1.3
3.0 × 10-10
7.0 × 10-11
5.0 × 10-11
85
112
134
22.9
22.9
23.0
16.5
16.5
16.4
10.0
10.6
11.3
10.8
11.15
15.7
15.95
1.3 4 × 10-11
1.0 2 × 10-11
1.3
that are encountered in the TREPR spectrum of the
PMMA radical.
R-relaxation rates. Specifically, it has been shown by
2
exchange H NMR studies that the relaxation time of
E. Atactic Poly(methyl methacrylate) (a-PMMA).
Figure 8 shows the simulated data for selected TREPR
spectra of a-PMMA. The parameters used in the simu-
lations are shown in Table 5. The most striking differ-
ence in the simulations of the TREPR spectra of the
a-PMMA radical as compared to the other polymeric
radicals shown above is the necessity of four different
â-hyperfine coupling constants to simulate the spectrum
obtained at 112 °C. In other words, the hyperfine
coupling constants of the â-methylene protons are no
longer the same on each side of the radical. Simulations
carried out with only two values of â-hyperfine couplings
failed to reproduce the experimental spectrum. What
this must mean is the nuclear spin symmetry relation-
ships that work well at high temperatures are disrupted
at lower temperatures. More details on this point will
be given below.
The simulation of the topmost experimental spectrum
shown in Figure 8A is not a good fit. Most noticeably,
the central line of the simulation is a narrow doublet,
while that in the experimental spectrum is a broad
singlet. Attempts to correct this by varying the hyper-
fine couplings caused larger changes in other parts of
the simulation. Increasing the τc value had the effect of
increasing the intensity of the central line by an
unacceptable amount. The line width could not be
increased further, as this caused the other peaks in the
spectrum to become too broad. At temperatures lower
than 80 °C, the simulations break down completely and
are not shown. The two-site jump model may not
accurately reflect the motion of the polymeric radical
in lower temperature solutions. The polymer might be
settling into more than one preferred conformation at
lower temperatures, causing the TREPR spectrum to
become better represented by a superposition of several
conformations of the radical.
PEMA in the solid state above the glass transition
temperature is indeed longer than that of PMMA
samples.40 In PMMA, the smaller methyl side chain is
highly mobile and cannot couple to the R-relaxation
processes of the polymer, and the relaxation of the
polymer backbone remains fast. However, the larger
ethyl group in PEMA makes the â-relaxation processes
of the side chains slower by an order of magnitude and
induces anisotropy in the motion of the polymer back-
bone, decreasing the R-relaxation processes.
Simulations of the TREPR data can be seen in Figure
7 along with the corresponding experimental data using
the parameters listed in Table 4. At higher tempera-
tures the simulations reproduce the experimental data
very well. However at 42 °C, there are major differences
between the experimental and simulated spectra. In the
experimental spectrum at this temperature the center
line is a strong singlet. This could not be simulated
without disrupting the fit of other transitions in the
spectrum to unacceptable levels of confidence.
In the experimental spectra, the sharp doublets
denoted at the bottom of each experimental data of
Figure 7 with brackets have a decreased separation with
decreasing temperature. This presented further difficul-
ties in the simulation. As can been seen in Table 4, the
hyperfine coupling constants required to reproduce this
decreased difference in the 42 °C simulation of Figure
7 are very different from the values used at higher
temperatures. While the separation in the doublet in
the simulation does not match that of the experimental
data exactly, further changes to the hyperfine values
result in a central line that was much too intense. When
an appropriate value of τc is added into the simulation,
the resulting spectrum does not resemble the experi-
mental data. The center line of the simulation is more
intense than the doublet, which is not observed in any
of the room temperature spectra presented to this point.
As will be elaborated upon below, the simulations of the
PEMA radical 4a present many of the same difficulties
Scheme 3 helps to amplify this point regarding
disruption of symmetry. Here the stereochemistry of the
individual radicals produced from isotactic, syndiotactic,
and atactic PMMA are shown, as well as the resulting

Macromolecules, Vol. 38, No. 8, 2005
Main Chain Radicals from Acrylic Polymers 3349
Scheme 3
symmetry relationships connecting the nuclei â to the
radical center. Particular emphasis is placed on the
atactic system, where two different diastereotopic radi-
cals are shown at the bottom of Scheme 3 and enlarged
for clarity. Our previous work has shown that the fast
motion coupling constants for these two species are
identical within experimental limits. However, as the
temperature is lowered there is no reason to expect the
hyperfines to remain equal. Another way of saying this
is that at all times with atactic polymer we are dealing
with a superposition of diastereotopic radicals such as
the two shown at the bottom of Scheme 3. It is simply
fortuitous that they overlap at fast motion. At lower
temperatures they would be expected to diverge. In fact,
if one considers longer range macromolecular effects,
e.g., by considering the stereochemistry of the next two
stereogenic centers (one in each direction away from the
radical center), it is clear that more than two diaste-
reotopic structures can contribute to the overall TREPR
spectrum. This problem does not exist for highly syn-
diotactic or highly isotactic material, and a detailed
study of their temperature dependencies and appropri-
ate molecular modeling may prove more successful. This
is the subject of current investigations.
a large effect on the TREPR spectrum. The rotational
correlation times would be the same only if cleavage of
the ester side chain occurred near the center of the
polymer chain. If the cleavage site was near the end of
the polymer chain, then minor differences in the rota-
tional correlation times could have a large effect on the
appearance of the TREPR spectrum. The polymer
chains are also quite polydisperse, and if the radicals
were produced on chains of widely different molecular
weight, then the τ values of the two radicals would be
different. It is only fortuitous that the hyperfine cou-
pling constants of the high-temperature TREPR spectra
are the same on either side of the radical. This is due
to the high temperatures and the fast interconversion
between the different conformations of the PMMA
radical and the fact that the stereochemical differences
are manifested at carbon atoms well removed from the
radical center.
It is also possible that the wrong dynamic model is
being used or that there is combination of models that
must be used to fit the experimental data at lower
temperatures. It seems likely that, even without con-
sidering the stereochemical issues discussed above,
there is a superposition of radical conformations that
exists in equilibrium at lower temperatures. This is
supported by results we have obtained for the polymeric
radical from PFOMA at high temperatures and also at
low temperatures for poly(adamantyl methacrylate). In
these systems broad line widths in the TREPR transi-
tions for the main chain polymers were observed.41
Another indication that there is a superposition of
Another possible reason for failing to simulate all of
the polymer radicals/spectra/TREPR with the same
model is the assumption that the rotational correlation
times on either side of the radical center are the same.
Although the segmental motion of the polymer has little
dependence on molecular weight, there are minor dif-
ferences with molecular weight, and these could have

3350 McCaffrey et al.
Macromolecules, Vol. 38, No. 8, 2005
Supporting Information Available: Summary of experi-
mental details. This material is available free of charge via
the Internet at http://pubs.acs.org.
conformations comes from the TREPR spectra of the
main chain polymeric radical of PMMA measured at
close to 0 °C. These spectra are also very broad and
featureless. However, previous work from our laboratory
has shown that averaging multiple simulations without
hyperfine modulation does not reproduce the experi-
mental spectra.42 Future work will include combining
the two models and investigating other models (e.g.,
three- or four-site jump models) in order to produce
accurate simulations.
Finally, it is worthwhile to note that there is no
reason to expect PMMA and d3-PMMA to have exactly
the same conformational dynamics. There are many
reports in the literature of substantial isotope effects
on polymer physical properties. In particular, the works
of Bates et al.43 and Cukier et al.44 show that rotational
dynamics in polybutadienes and polyethylenes, respec-
tively, can show significant changes upon deuteration.
This can be simply due to the larger van der Waals radii
of the methyl groups in d3-PMMA, although there may
be more subtle stereoelectronic effects as well. The
samples used here were of similar chain length, and
although there was a slight difference in polydispersity
between the protonated and deuterated samples, we
have never observed differences in any of our spectra
that could be attributed solely to molecular weight
changes (down to about Mw ) 10K for PMMA). The
dynamic effects we observe are clearly long range, but
not long enough for molecular weight to play a large
role. Currently, we are attempting to address this issue
by selectively synthesizing known-chain length oligo-
mers of both polymers. This will be the subject of a
future publication.
References and Notes
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Summary and Outlook
Simulations of the temperature dependence of the
TREPR spectra for PEA and d3-PMMA radicals using
a two-site jump model for hyperfine modulation have
returned reasonable values for the activation param-
eters for conformational motion in these polymers. The
numbers compare favorably with values determined by
NMR and SSEPR methods. For PMMA and PEMA our
model was not successful. While the line positions and
general line width features could be reproduced, the
relative intensities of the transitions were not. It is clear
that the simulations of the TREPR spectra shown for
the photolysis of PMMA and PEMA do not accurately
reflect the experimental data.
Future work to determine whether the position of the
cleavage site along the polymer chain has an effect on
the rotational correlation time will include photolysis
of low molecular weight and oligomeric PMMA. The
TREPR spectra of these systems will be compared to
those of the large molecular weight polymer. If the
spectra of the low molecular weight and oligomeric
PMMA are sharp and easily simulated, then differences
in the rotation correlation times on either side of the
chain will have to be incorporated into the simulation
program.
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Spiess, H. W. Macromolecules 1997, 30, 6597.
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Experimental Section
A complete description of the TREPR apparatus, polymer
synthesis, and sample descriptions is given in our previous
papers,1,2 and a summary is provided as Supporting Informa-
tion to this paper.
Acknowledgment. We thank the Rohm and Haas
Co. for polymer samples and the National Science
Foundation for their continued strong support of our
program (Grant CHE-0213516).
MA047801X