Time-resolved EPR studies of main chain radicals from acrylic polymers. Structural characterization at high temperatures

Article abstract of DOI:10.1021/ma047857r

Main chain radicals from several acrylic polymers are characterized in liquid solution at high temperatures (～100 °C) using time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The radicals are produced by laser flash photolysis (248 nm) and subsequent loss of the side chain ester functionality by Norrish I α-cleavage. At these temperatures fast-motion spectra with conformationally averaged hyperfine interactions are observed. The spectra are strongly spin polarized from the triplet mechanism (TM) of chemically induced dynamic electron spin polarization (CIDEP). Computer simulation of the TREPR spectra leads to unambiguous structural characterization of both the main chain radical and the side chain oxo-acyl radical. Hyperfine coupling constants, g-factors, and line widths are reported and discussed for radicals produced from five different acrylic polymers. Variation of the side chain on the polymer backbone leads to significant changes in the observed hyperfine coupling constants. For the main chain radical from poly(fluorooctyl methacrylate) (PFOMA), fast motion of the polymer chain is not accessible at these temperatures. The side chain oxo-acyl radical from PFOMA exhibits long-range ¹⁹F hyperfine interactions. Additional TREPR experiments on small molecule model compounds and gel permeation chromotography results of the photolyzed polymers support the conclusion that the primary photodegradation mechanism proposed in this paper is general for acrylic polymers.

Full text of DOI:10.1021/ma047857r

3334
Macromolecules 2005, 38, 3334-3341
Time-Resolved EPR Studies of Main Chain Radicals from Acrylic
Polymers. Structural Characterization at High Temperatures
Vanessa P. McCaffrey 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 17, 2004; Revised Manuscript Received January 7, 2005
ABSTRACT: Main chain radicals from several acrylic polymers are characterized in liquid solution at
high temperatures (∼100 °C) using time-resolved electron paramagnetic resonance (TREPR) spectroscopy.
The radicals are produced by laser flash photolysis (248 nm) and subsequent loss of the side chain ester
functionality by Norrish I R-cleavage. At these temperatures fast-motion spectra with conformationally
averaged hyperfine interactions are observed. The spectra are strongly spin polarized from the triplet
mechanism (TM) of chemically induced dynamic electron spin polarization (CIDEP). Computer simulation
of the TREPR spectra leads to unambiguous structural characterization of both the main chain radical
and the side chain oxo-acyl radical. Hyperfine coupling constants, g-factors, and line widths are reported
and discussed for radicals produced from five different acrylic polymers. Variation of the side chain on
the polymer backbone leads to significant changes in the observed hyperfine coupling constants. For the
main chain radical from poly(fluorooctyl methacrylate) (PFOMA), fast motion of the polymer chain is not
accessible at these temperatures. The side chain oxo-acyl radical from PFOMA exhibits long-range 19F
hyperfine interactions. Additional TREPR experiments on small molecule model compounds and gel
permeation chromotography results of the photolyzed polymers support the conclusion that the primary
photodegradation mechanism proposed in this paper is general for acrylic polymers.
Chart 1
Introduction
Photodegradation of polymers is a subject of intense
interest, particularly in the areas of lithographic pho-
toresists and the weathering of architectural coatings.
The successful development of more robust coatings and
the controlled degradation of photoresists both rely on
a detailed understanding of the degradation mechanism.
In 2000, we reported the first structural characteriza-
tion of main chain polymeric free radicals formed by UV
photolysis of acrylic polymers in liquid solution using
time-resolved electron paramagnetic resonance spec-
troscopy (TREPR).1 The focus of our previous paper was
on the symmetry properties of these free radicals as a
function of polymer tacticity under fast-motion (high-
temperature) conditions for poly(methyl methacrylate)
(PMMA) and poly(ethyl acrylate) (PEA), structures 1
and 2 in Chart 1, respectively.
We have recently expanded the scope of this research
to the study of several additional acrylic polymers and
related structures, also shown in Chart 1. Structural
modification of acrylates can be achieved at two sites
which are important to distinguish. The first modifica-
tion is that of the substituent of the polymer backbone,
and the second is that of the ester side chain. We will
refer to these modifications using the more common
polymer nomenclature for main chain and side chain
sites as R substitution and â substitution, respectively.
The most structurally simple acrylic polymer we have
studied is poly(ethyl acrylate), PEA (2), which has
hydrogen as the R-substituent. Of the remaining five
acrylic polymers we have studied, four are methacry-
lates (1, 3-5). The last of these methacrylates is poly-
(fluorooctyl methacrylate) (PFOMA, 5), which has very
different physical properties than the first four meth-
acrylates due to the bulky (and more rigid) â-substituent
(see discussion below for more details on PFOMA). The
remaining acrylic polymer in Chart 1 is poly(ethyl
cyanoacrylate) (PECA, 6) where the R-substituent is a
nitrile group.
* To whom correspondence should be addressed. E-mail:
mdef@unc.edu.
10.1021/ma047857r 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 3335
Scheme 1
Polymer 7 has an unusual structure; it is a branched
polyethylene with about 10% methyl methacrylate
incorporation.2,3 The ester functionalities in this polymer
are present at the end of the branches of the polymer
and not on the main polymer chain. This polymer is of
interest in investigating the effect of proximity of the
ester chromophores in the photodegradation process.
Nonacrylic polymers 8, poly(vinyl acetate) (PVA), and
9, poly(caprolactone) (PCL), were also studied to see
whether other ester functionalities are as photolabile
as acrylic structures at 248 nm.
Propylene carbonate is the most commonly used
solvent in our laboratory for TREPR experiments on
nonfluorinated acrylic polymers. It is an excellent
solvent for such polymers at room temperature and
above. It boils at 240 °C, which allows access to high-
temperature spectra even with recirculating samples,
and it shows no TREPR signal when run as a blank
under irradiation at 248 nm. To obtain the high-
temperature TREPR spectrum of PFOMA, a solvent
mixture consisting of different fluorinated hydrocarbons
(FC-70, 3M Corp.) was used. This solvent system has a
boiling point of 201 °C, dissolves PFOMA easily, and is
also transparent at 248 nm.
The proposed mechanism for the photochemical deg-
radation of acrylic polymers, as shown in Scheme 1 for
PMMA, has been studied by steady-state EPR spectros-
copy (SSEPR) for more than 50 years.4-6 However, it
was not until TREPR was applied to the problem that
main chain acrylic radicals such as 1a and the corre-
sponding oxo-acyl counter-radicals 1b could be observed
directly and characterized. The reason for this is that
â-scission of the main chain radical is sufficiently rapid
that a large concentration of the so-called propagating
radical 1c builds up; therefore, this is the intermediate
most often detected in SSEPR experiments. The time
response of the X-band TREPR apparatus used in our
laboratory is about 3 orders of magnitude faster than
for the conventional SSEPR experiment (50 ns vs 40 µs).
For this reason we can easily detect radical 1a in the
first few microseconds after its creation.
In our previous work we photolyzed polymer solutions
with a pulsed 248 nm excimer laser, and intense TREPR
signals were obtained.1 However, because of the high
degree of substitution on these polymers, especially the
methacrylates, the spectra were very broad at room
temperature and difficult to simulate. Alternating line
widths indicative of hyperfine modulation were observed
in nearly all cases near room temperature. By heating
the samples in a variable temperature flow system, the
Figure 1. TREPR spectra of (A) polymeric main chain radical
from PEA in propylene carbonate at 125 °C. Delay time is 0.9
µs. (B) polymeric main chain radical from PMMA in propylene
carbonate at 94 °C. Delay time is 0.8 µs. The TREPR intensity
(arbitrary units) and magnetic field axis shown here are the
same for all spectra and are omitted in the remaining figures.
In this and all subsequent spectra, lines below the baseline
are in emission while those above the baseline are in enhanced
absorption. Intensities of all spectra are normalized.
EPR transitions of the polymeric radicals underwent
motional narrowing to give a spectrum defined by an
average set of coupling constants. The observation of
motionally narrowed spectra led immediately to precise
simulations and therefore to high-resolution structural
characterization of these intermediates for the first
time.
Because of the simplicity of the fast-motion TREPR
spectra, we have continued to perform our experiments
at high temperatures whenever possible. In other
forthcoming publications in this series we will address
other issues such as the analysis of these spectra in
regard to polymer chain stiffness and the transition
from slow- to fast-motion EPR spectra as the temper-
ature is raised. The goals of the work presented here
are (1) to establish the generality of the proposed
photochemical degradation mechanism, (2) to investi-
gate the overall applicability of this spectroscopic tech-
nique to other polymer degradation mechanisms, (3) to
investigate the onset temperature for fast motion for
different acrylic main chain radicals, and (4) to unam-
biguously determine the structure of several polymeric
main chain radicals at fast motion as a function of both
main chain (R) and side chain (â) substitution.
Results and Discussion
A. General Features of TREPR Spectra of Acrylic
Radicals: PMMA (1) and PEA (2). It is instructive
to review the general features of main chain acrylic
radical TREPR spectra at high temperatures. Figure 1
shows spectra obtained from photolysis of PEA and
highly isotactic PMMA in propylene carbonate solution
above 90 °C. (Except for the PMMA sample, all polymers
used in this study are atactic materials.) Simulations

3336 McCaffrey and Forbes
Macromolecules, Vol. 38, No. 8, 2005
and characterization data for these radicals (1a and 2a
in accordance with Chart 1 and Scheme 1) are given in
ref 1. Nuclei on the R-substituent (main chain) have
hyperfine interactions with the radical center, whereas
those on the â-substituent (side chain) do not. It is
immediately evident from Figure 1 that R-substituents
have a large effect on the appearance of the TREPR
spectrum. For example, the spectrum of the radical from
PEA is a well-resolved 10-line spectrum. However, the
presence of a methyl group in the R-position of PMMA
results in the appearance of many more lines (21 total)
in the spectrum. In both radicals the â-methylene
protons are diastereotopic and therefore show a splitting
pattern of a triplet of triplets. In the PMMA radical
spectrum there are accidental degeneracies which lead
to a smaller number of observable transitions than the
maximum expected (4 × 3 × 3 ) 36). Analysis of the
fast-motion hyperfine interactions as a function of
pseudo-symmetry and stereochemistry is explained in
detail in the first paper of this series.
The spectra in Figure 1 show strong net emission from
the triplet mechanism (TM) of chemically induced
electron spin polarization (CIDEP).7-9 It is revealing
that the dominant polarization mechanism is the TM
and not the radical pair mechanism (RPM)10,11 or spin
correlated radical pair mechanism (SCRP).12,13 The
magnitudes of both SCRP and RPM polarization pat-
terns depend on the rate of encounters taking between
the two radical centers. If the mechanism of degradation
of acrylic polymers is correct as shown in Scheme 1, we
are creating two species, polymeric radical 1a and oxo-
acyl radical 1b, which have drastically different diffu-
sional properties in solution. Radical 1b is small and
will undergo much more rapid diffusion than 1a. An
immediate consequence of this is that RPM and SCRP
polarization mechanisms will be quenched by relatively
slow reencounter rates and can therefore be obscured
in this case by the very strong TM. In all TREPR spectra
detected here, emissive TM is always observed, which
is unusual for aliphatic carbonyl compounds, although
not unprecedented for esters.14 We note here that in all
spectra of polymeric radicals reported in this paper the
TREPR transitions are emissive and therefore are
recorded as transitions pointing below the baseline.
There are some other important points to be made
regarding the relative intensities of the observed spin
polarization mechanisms. In our previous papers it was
pointed out that the â-hyperfine interactions are con-
formationally modulated. This can be an additional
means by which the RPM polarization pattern is
quenched. To understand this qualitatively, the modu-
lation process can be thought of as a relaxation mech-
anism that exchanges magnetization between different
nuclear spin orientations or hyperfine lines. Since these
different orientations can have opposite polarizations
from the RPM, the exchange of emissive and absorptive
polarization cancels the intensity of the transitions in
question. This is a very interesting topic in its own right
and will be the subject of further quantitative study.
It is also of interest to consider why the TM polariza-
tion is so strong. We know it must be of very high
magnitude because the absorbance of the polymers is
quite low at 248 nm (this wavelength is just barely
inside the lowest energy electronic transition, presum-
ably n-π*, for the ester moiety), and the quantum yield
for R-cleavage is also quite small (∼0.05).15 It is well
understood from CIDEP theories that slow rotational
Figure 2. (Left) experimental and (right) simulated high-
temperature (fast-motion) TREPR spectra of acrylic polymeric
main chain radicals form the following polymers: (A) PEMA
at 120 °C and delay time of 0.8 µs; (B) d3-PMMA at 120 °C
and delay time of 0.6 µs; (C) PECA at 120 °C and delay time
of 0.3 µs; (D) PFOMA at 110 °C and delay time of 0.3 µs.
Simulation parameters are listed in Table 1, and polymer
structures are shown in Chart 1.
correlation times of a photoexcited state will lead to
more selective intersystem crossing at the X-band field
(3380 G), and this will lead to larger TM polarization.
This strongly suggests that rotational motion of the
ester functional group in these polymers may be quite
restricted in at least one direction.
An oxo-acyl radical similar in structure to radical 1b
is also formed upon photolysis of PEA (it has an ethyl
rather than a methyl alkyl group). However, transitions
representative of this radical were not observed in our
experiments. Radicals such as 1b are expected to exhibit
a single emissive peak slightly to the right of the center
of the spectrum (its g-factor is slightly lower than that
of the polymeric radical). The g-factor of most carbon-
centered alkyl radicals is around 2.0026.16-19 Literature
values for the g-factors of oxo-acyl radicals are ap-
proximately 2.0009.20-23 Oxo-acyl radicals have two
unimolecular decomposition pathways available: loss
of CO and loss of CO2. Previous studies on the photo-
ablation of PMMA using high-resolution MS has de-
tected both gaseous products.24 Griller and Roberts have
determined the decarboxylation rate of an oxo-acyl
radical of similar structure to be 2.1 × 106 s-1 at 373
K.25 This corresponds to a decomposition time of 500
ns, which is well within the resolution time of our
instrument, but the radical was not observed at any
delay time from 0.1 to 10 µs.
The most likely reason for the absence of an oxo-acyl
signal in the spectra from Figure 1 is fast spin relax-
ation. Paul has shown by line shape analysis that
relaxation of an acyl radical (RC•dO) is on the order of
10 ns or faster in solution,26 and this has recently been
confirmed by the kinetic analysis acyl-alkyl biradicals
by Tsentalovich et al.27 The mechanism of this relax-
ation is through spin-rotational interaction in the radi-
cal, which in this case is equal to the longitudinal
relaxation time T1. Electron spin relaxation is much
slower due to the hindered rotation of the polymeric
radical, and therefore radical 1a and its analogues
remain spin polarized long enough to be observed by
TREPR.
B. TREPR of Polymers 3-6. The TREPR spectra
obtained after 248 nm photolyis of these four polymers
are shown on the left side of Figure 2, along with

Macromolecules, Vol. 38, No. 8, 2005
Main Chain Radicals from Acrylic Polymers 3337
Table 1. Hyperfine Couplings (in G) Used in the
Simulations in Figure 2a
polymer precursor
δ aH(CH3)
δ aH(CH2)
δ aH(CH2)
PMMA
PEMA
d3-PMMA
PECA
22.9
22.9
3.5
16.7
15.8
16.3
16.3
11.2
11.2
10.9
14.8
3.3 (N)
a PFOMA; aF(CF2) ) 3.2 G, aF(CF2) ) 0.8 G. All polymers above
lead to main chain radicals of type a in Scheme 1, while PFOMA
leads to radicals of type b.
simulations on the right side computed using magnetic
parameters listed in Table 1. In Figure 2A, a TREPR
spectrum is shown for the main chain radical from
photolysis of PEMA, 3a. The spectrum no longer con-
sists of 21 lines as in Figure 1B for PMMA. Some of the
previously described accidental degeneracies have been
lifted to give a total of 27 lines, and the total spectral
width has decreased slightly. The new lines are assumed
to arise from the main chain radical and not a new
signal carrier because their kinetics are identical to all
the other lines and their intensity ratios are constant
with temperature. These differences in hyperfine cou-
pling constants are somewhat unexpected because the
only structural difference between these radicals is the
identity of the â-substituent in which the ester R group
changes from methyl to ethyl.
To simulate the spectrum in Figure 2A, a hyperfine
coupling constant of 22.9 G was used for the protons on
the R-methyl group. Values of 15.8 and 11.2 G were used
for the two sets of diastereotopic â-methylene protons.
Table 1 shows how these values compare with those
used for the PMMA radical 1a. As expected, the hyper-
fine values for the R-methyl protons remain the same
for both methacrylate polymers. The methyl group has
free rotation about the C-C bond from the backbone to
the methyl group, even at temperatures as low as -100
°C.28 The additional splittings in the TREPR spectrum
of radical 6a can be simulated by changing the hyperfine
values for one set of the â-methylene protons (the largest
coupling constant). The value used for the radical from
PEMA is approximately 1 G smaller than that from
PMMA. This suggests that adding an additional meth-
ylene group to the ester group changes the relative
energies of the conformations of the polymer near the
radical center, which in turn affects the dihedral angles
and therefore the hyperfine values. This will be dis-
cussed in more detail below.
Figure 2B shows the experimental and simulated
TREPR spectra from the photolysis of d3-PMMA (4) in
propylene carbonate at 120 °C. The spectrum is much
more narrow, and there are many more transitions than
in its protonated analogue. There are many overlapping
lines, and only the outermost lines of the spectrum are
clearly resolved. In Table 1, it is seen that the value of
the hyperfine coupling constant for the R-methyl protons
has decreased from 22.9 to 3.5 G, exactly as expected
for this isotopic substitution.29 The values for the
â-methylene protons used in the simulation are 16.3 and
10.9 G, which are close to the values used in the
simulations of the nondeuterated polymer. The fact that
they are slightly different suggests that deuteration of
the polymer backbone also has an effect on the confor-
mational energies of this polymer in solution.
Figure 3. Experimental (top) and simulated (bottom) TREPR
spectra of the oxo-acyl radical from PFOMA in the perfluori-
nated solvent FC-70. Sweep width is 50 G. Simulation
parameters: aF(γ-CF2) ) 3.2 G, aF(δ-CF2) ) 0.8 G, LW ) 0.8
G.
from coupling to the â-methylene protons. Each of
these packets of lines is split into an additional three
lines from the γ-nitrogen (I ) 1) in the nitrile group.
The larger width of the second and fourth packet of
lines can be explained by the diastereotopicity of the
â-methylene protons. The hyperfine values used in the
simulation are 3.3 G for the γ-nitrogen and 16.3 and
14.8 G for the â-methylene protons (Table 1). The
difference between the hyperfine coupling constants for
each of the â-methylene protons is much smaller for this
polymer than for the other acrylic polymers.
An interesting feature of Figure 2C is the line width
of the transitions. Line widths of 1-2 G are common in
TREPR spectra of small molecules at room temperature.
Small molecules have shorter rotational correlation
times than larger molecules. As the motion of a molecule
is restricted, either by cooling or by adding bulk, the
line width in the TREPR spectrum begins to increase
due to faster relaxation. If the line shape is Lorentzian,
then 1/T2 ) 0.5 × x3 × fwhm (full width at half-
maximum). The line width in the simulations of Figures
1 and 2A,B does not exceed 1 G, but the line width
needed to simulate the TREPR spectrum of 6a was
almost twice that value (1.8 G). This indicates qualita-
tively that the polymer may have a stiffer backbone
than the other polymers studied here.
Figure 2D shows the experimental TREPR spectrum
of the radicals from the photolysis of PFOMA (5). In this
experiment, propylene carbonate was not used as the
solvent, as it does not dissolve fluorinated polymers very
well. A solvent consisting of a mixture of fluorinated
hydrocarbons (FC-70, a gift from the 3M Corp.) was
used. The spectrum of the polymeric radical is broad
and somewhat featureless compared to other methacry-
late radicals. The signal from the fluorinated oxo-acyl
radical 5b dominates the spectrum; however, it is not a
singlet as observed for its protonated analogue at lower
temperatures. Instead, the signal is a broad intense
triplet. Upon expansion of the spectrum to a sweep
width of 50 G, it can be seen that the signal is actually
a triplet of triplets (Figure 3A). A simulation of this
spectrum is shown in Figure 3B. For this simulation
fluorine hyperfine couplings of 3.2 and 0.8 G were used.
Figure 2C shows the experimental high-tempera-
ture TREPR spectrum obtained during photolysis of
PECA (6) in propylene carbonate solution, along with
a computer simulation. There are five packets of lines

3338 McCaffrey and Forbes
Macromolecules, Vol. 38, No. 8, 2005
Table 2. GPC Values Determined for Polymers before
and after Photolysisa
tively, the symmetry of the chromophore’s triplet state
and/or its rotational correlation time may not be in favor
of strong TM polarization generation.
before photolysis
after photolysis
C. GPC Results. Table 2 shows typical results for
the GPC analysis of some acrylate polymers before and
after photolysis, which were typically run with concen-
trations of 15 mg/mL against a polystyrene standard.
Photolysis at 248 nm results in a large reduction of the
molecular weight of the polymer, as in the case of d3-
PMMA, shown in entry 4 in Table 2. Polymers that are
initially fairly monodisperse (polydispersity index
(PDI) < 2) show an increase in PDI. Those with broad
polydispersities (PDI > 2) show a decrease after pho-
tolysis. These results are in accordance with accepted
models for chain scission processes.38 The only exception
in our data is the PCL sample (9), which because of its
additional failure to give a TREPR signal, we cannot
comment on further.
Entries 1-3 in Table 2 show GPC results for PMMA
photolyzed at two different wavelengths: 248 nm, where
strong a TREPR signal is observed, and 193 nm, where
there was no TREPR signal. In both cases, the molecular
weight of the polymer decreases upon photolysis. This
is strong evidence that main chain scission is occurring
upon photolysis at both wavelengths. It cannot be stated
that irradiation at 248 nm causes more chain cleavage
simply because the drop in molecular weight is greater.
The GPC samples for each of the polymers were
prepared after different times of exposure to UV light
of 248 nm. From the results in Table 2, it can be
concluded that for all the polymers studied here main
chain cleavage or â-scission is occurring after side chain
cleavage. This is to be expected as the time scale for
side chain scission is less than 100 ns from these
experiments. It has been determined experimentally for
PMMA that the half-life for â-scission is greater than
20 µs.39
Poly(vinyl acetate) (8), which did not show a TREPR
spectrum upon photolysis, shows a large drop in mo-
lecular weight when studied by GPC analysis. This is
in line with results by Buchanan and McGill.40 They
found that upon exposure to UV light from a Hg lamp
for 30 min the molecular weight of this polymer de-
creased by half from 73K with a PDI of 1.83 to 36K with
a PDI of 4.74. In our experiments, the photolysis time
of the polymer in the EPR cavity was approximately the
same time period. The polymer samples in Buchanan
and McGill’s work also decreased in molecular weight
by about half but with a much larger increase in the
PDI. The lack of a TREPR signal from the photolysis of
PVA is therefore perplexing, as a reasonable amount of
spin polarization from the TM would be expected from
these structures. The degradation mechanism of this
polymer is the subject of ongoing investigations.
In NMR experiments (1H at 400 MHz, Varian),
PMMA was photolyzed for 30-90 min in propylene
carbonate at 100 °C and at room temperature, and then
the photolyzed solution was distilled to collect any
volatiles that might have formed. The colorless distillate
was analyzed by NMR and found to contain methyl
methacrylate monomer. This compound could only have
arisen from depolymerization (main chain scission) of
the propagating radical 1c. To ensure that that the
distillate collected was not residual monomer from the
original unphotolyzed polymer sample, solid PMMA was
placed under vacuum for over an hour and no visible
amount of distillate was collected. Because there is no
polymer
MW
PDI
MW
PDI
a-PMMA
a-PMMAb
s-PMMA
d3-PMMA
PEMA
PCN
52K
52K
7K
266K
144K
22K
93K
41K
41K
1.53
1.53
1.16
3.89
2.15
3.73
3.01
1.76
1.76
19K
34K
5K
1.96
2.01
1.27
1.29
1.62
1.26
3.62
1.55
1.59
33K
32K
4K
PVA
50K
47K
47K
PCL
PCLc
a All polymers photolyzed at 248 nm unless otherwise noted.
b Polymer photolyzed at 193 nm. c Polymer photolyzed at 308 nm.
Krusic and co-workers have shown that γ- and even
δ-hyperfine couplings from fluorinated radicals can be
observed in SSEPR spectra due to spin polarization
through C-F bonds. The coupling constants for long
range fluorine hyperfine interactions are typically small,
from 0.1 to 2.5 G. Our values agree well with the values
of 2.6 and 0.1 G reported by Krusic et al. for γ- and
δ-hyperfine 19F couplings in perfluoralkyl radicals.30-32
In all of the spectra reported above, the polymeric
main chain radical generally exhibits a very intense
signal, and the oxo-acyl signal is weak or absent from
the TREPR spectrum at high temperatures. The op-
posite relative intensities are observed for PFOMA
photolysis. Oxo-acyl radical 5b is superimposed on a
large, broad signal which has some similar features to
the signal from the polymeric radical of PMMA at lower
temperatures (spectrum not shown). The signal carrier
is most likely the main chain polymeric radical from
PFOMA. The polarization from radical 5b is much
stronger than that from the polymeric radical for two
reasons: One is that the same magnitude of polarization
is carried by fewer spectral lines. But it is also highly
likely that the fluorinated oxo-acyl radical 5b is bulkier
and more rigid than its alkyl analogues and therefore
will have a much longer rotational correlation time,
leading to longer spin relaxation times at X-band. This
will be discussed in more detail in a subsequent paper
in this series.
Polymers 7-9 failed to show any TREPR signals at
any accessible temperature. For branched polymer 7,
it is possible that degradation is occurring, but the
amount of MMA incorporation is very low in this
structure so there may only be a small amount of
radicals being produced. The experiment may not be
sensitive enough to detect them. Poly(vinyl acetate) (8)
has been studied previously by SSEPR and did show a
signal in those experiments, so our negative result is
somewhat surprising. It is possible that the radicals
produced are not spin polarized to any great extent
(weak TM), or they are formed in very low concentra-
tions because the quantum yield of radical formation is
low. At lower temperatures (77 K) both methyl radicals
and the polymeric radical from side chain cleavage of
PVA have been observed.33,34
No TREPR signal was observed on photolysis of PCL
(9). From the literature on poly(esters) it is believed that
the degradation mechanism for this polymer proceeds
via one or more radical intermediates,35-37 so this result
is also somewhat surprising. However, as a percentage
of the total molecular weight, the ester chromophores
are present to a much higher degree in acrylic structures
than in polymer 9, so the lack of signals may simply be
due to a low concentration of chromophore. Alterna-

Macromolecules, Vol. 38, No. 8, 2005
Main Chain Radicals from Acrylic Polymers 3339
Scheme 2
Figure 4. Experimental (top) and simulated (bottom) TREPR
spectra from 248 nm excitation of small molecule model
compounds: (A) dimethyl-2,2-dimethyl glutarate (0.94 M) in
methanol at a time delay of 0.3 µs. Simulation parameters;
aH(CH3) ) 23.1 G, aH(CH2) ) 17.8 G, line width ) 1.0 G. (B)
Benzophenone-d10 (0.5 M) in neat squalane at a delay time of
0.8 µs. Simulation parameters: Radical 11a: aH(CH3) ) 22.9
G, 2aH(CH2) ) 16.9 G, 2aH(CH2) ) 11.8 G, line width ) 3 G.
Radical 11b: 6aH(CH3) ) 22.9 G, 2aH(CH2) ) 17.5 G. Radical
12: aH(COH) ) 2.2 G, line width ) 3 G. Intensity ratio of
radical 11a to radical 11b to radical 12 is 4/2/3. Substrates
and radical structures are shown in Scheme 2. Transitions due
to radical 11b are indicated by arrows.
way to produce MMA monomer other than â-scission
followed by depolymerization, this further supports the
mechanism shown in Scheme 1, i.e., that main chain
scission is occurring during our experiments. Experi-
mental evidence for all the steps in the mechanism in
Scheme 1 has been presented.
The second model system explored was a solution of
benzophenone-d10 in pure squalane 11 (Scheme 2B). The
viscosity of the squalane solution is similar to that of
the polymer solutions at room temperature. The pho-
toexcited triplet state of benzophenone will abstract
hydrogen from squalane. There are several possible
abstraction sites on the squalane chain. The excited
benzophenone will abstract preferentially from the those
sites leading to tertiary alkyl radicals. There are two
types of tertiary radical possible. The first is an internal
structure, 11a, and the other is a terminal radical, 11b
(Scheme 2B). The major difference in the two radicals
is the total number of hyperfine couplings. The terminal
radical (11b) will have eight hyperfine couplings, while
the internal radical (11a) will only have seven. To a first
approximation, radical 11a is identical to those formed
by photolysis of PMMA. In this case, the radical has a
R-methyl group and two â-methylene groups. The
counter-radical formed is a deuterated hydroxydiphen-
ylmethyl radical, which gives a broad singlet in the
center of the TREPR spectrum.
Figure 4B shows the experimental TREPR spectrum
when deuterated benzophenone is photolyzed in the
presence of squalane. The most obvious difference
between the TREPR spectra of PMMA and this model
compound is the different CIDEP polarization, which
is predominantly from the radical pair mechanism. The
RPM polarization is strong when the two radicals
undergo several diffusive encounters, which is expected
here due to the increased viscosity of the solutions. A
simulation of the experimental TREPR data in Figure
4B is shown below the experimental spectrum. From
the relative intensity of the emissive and absorptive
lines, both the TM and RPM polarization mechanisms
are concluded to be active. About 30% TM polarization
was added into the simulations. From the wide sweep
width of the outermost lines, it can be concluded that a
very small amount of the terminal tertiary radical 11b
is being produced, as indicated by arrows in Figure 4B.
Both tertiary radicals produced by hydrogen abstraction
from squalane were included in this simulation. The
hyperfine couplings used for the internal radical 11a
D. Studies of Model Compounds. To further sup-
port the structural characterization of the acrylic main
chain radicals, a series of model compounds were also
studied by TREPR. Dimethyl-2,2-dimethyl glutamate,
(Scheme 2A), is the most common small molecule used
for modeling PMMA because it structurally approxi-
mates a dimer. Product analyses shows that this
compound is photochemically active at 248 nm. The
mechanism of decomposition proceeds through side
chain cleavage of one of the ester groups.24,41 We used
the dimethylated ester instead of the tetramethylated
one to control the site of side chain cleavage, for which
there are two possible sites. If cleavage occurred on the
left side of the diester, the resulting radical would be a
less stable primary radical. This reaction produces a
tertiary radical 10a that approximates the polymeric
radicals formed by photolysis of PMMA, PEMA, and
others. The more probable cleavage reaction and the
resulting radicals are therefore shown in Scheme 2A.
Figure 4A shows the TREPR spectrum of a 0.94 M
solution of dimethyl 2,2-dimethylglutamate in methanol
photolyzed at 248 nm at room temperature. This
spectrum exhibits strongly emissive TM polarization
similar to that observed from photolysis of the polymers
presented earlier. Absent from this spectrum is the
signal from the oxo-acyl radical, 10b, again presumably
due to fast spin relaxation. A simulation is shown below
the experimental spectrum. Hyperfine couplings of 22.9
G for the methyl group and 17.8 G for the â-methylene
group were used.
The SSEPR spectra of the diacid analogue of 10 has
been measured by Fischer et al. in water and methanol
solutions at room temperature.42,43 They were able to
observe the presence of the acid analogue of 10a, the
oxo-acyl radical, and an adduct due to impurities from
succinic acid. To simulate the spectrum from 10a, they
used values of 23.5 G for the protons of the methyl
groups and 17.5 G for the â-methylene protons. These
compare favorably to the values used to simulate the
polymeric radical spectra in Figure 2.

3340 McCaffrey and Forbes
Macromolecules, Vol. 38, No. 8, 2005
are 22.9 G for the three â-protons of the methyl group
and 16.9 and 11.8 G for the two sets of methylene
protons. For the terminal radical 11b, coupling con-
stants of 22.9 G for the methyl group and 17.5 G for
the â-methylene protons were used. The counter-radical
12, a protonated benzophenone ketyl radical, was
simulated as single peak with g-factor of 2.0032 and a
line width of 3 G. The positions of the transitions are
accurately reproduced; however, the individual intensi-
ties are not. This is especially true of the most central
lines in the spectra, which is a common problem in
CIDEP simulation routines. Despite this limitation, we
can use the line positions to estimate the hyperfine
coupling constants in this radical to a high degree of
certainty.
The hyperfine coupling constants used in the simula-
tion of the squalane model compound are extremely
close to those used to simulate the high-temperature
TREPR data of the radical from PMMA listed in Table
1 and used to generate the simulations in Figure 2 (16.7
and 11.2 G). These additional data further support the
assignment. In an upcoming paper we will use these
data to test the temperature dependence of the squalane-
derived radicals to see whether this can be used to
model the polymer chain dynamics in solution. The
slight differences between the squalane radical hyper-
fine coupign constants and the polymer values are most
likely due to slight differences in averaging at fast
motion because of chain length differences.
Some additional comments should be made regarding
the squalane experiments. First, the line width is
significantly broader than in the polymer experiments
due to the lower temperature. This is most likely just a
viscosity effect on T2. But there exists a very interesting
symmetry issue in this molecule with respect to radical
structure. The ground state squalane molecule has four
stereogenic centers but is optically inactive due to point
symmetry about the center of the C11-C12 bond (i.e., it
is a meso compound). Since the creation of free radical
centers at any carbon atom along the chain disrupts this
symmetry relationship, we might expect similar dia-
stereotopicity issues in squalane radicals as observed
in our polymeric structures. Therefore, it will be very
interesting to see whether higher temperature spectra
lead to additional splittings as in Figure 1A for PEA.
This problem, and the physical modeling of it, are
outside the scope of the present paper and will not be
considered further at present.
A second feature of the squalane spectrum is that the
predominant spin polarization mechanism is RPM
rather than TM. The TM dominates the TREPR spectra
of polymers 1-6 as well as that from model compound
10. The major difference for squalane is of course the
method of free radical production, which takes place by
H atom abstraction rather than R-bond cleavage. The
typical triplet state lifetime for benzophenone in alkane
solutions is about 300 ns,44 whereas bond cleavage is
nominally an order of magnitude or more faster. Since
spin relaxation in the triplet state is competitive with
bond cleavage, strong TM polarization is observed. This
spin polarization mechanism is expected to be very weak
in the squalane system due to the slow rate of reaction
to produce radicals. It is interesting to note that while
the RPM polarization is strong here due to the high
viscosity, it may well be quenched by conformational
modulation as discussed above. This is further motiva-
tion to extend our future work to the temperature
dependence of the TREPR spectrum from photolysis of
the benzophenone/squalane system.
Conclusions
The simulations of the high-temperature TREPR
spectra presented here, coupled with the symmetry
relationships given in our previous paper, lead to an
unambiguous identification of the radicals produced
from the primary photochemical events of acrylic poly-
mer degradation under direct UV irradiation at 248 nm.
The excellent fit of the simulations allows for assign-
ment of the spectra with a high degree of confidence.
Results from small molecule model compounds strongly
support the assignments. As expected, the spectra are
strongly influenced by substitution on the main chain
of the polymer. A surprisingly fine sensitivity to the
ester side chain structure was observed, with hyperfine
couplings changing even when the â-substituent is
changed from methyl to ethyl. Another example of the
influence of the side chain appears to be a control of
the degree of chain stiffness in PFOMA, and this will
be discussed further in a subsequent publication in this
series.47
The general photochemical mechanism for degrada-
tion of acrylic polymers appears to be the same regard-
less of polymer or ester side chain structure. Further
spectroscopic analysis has shown some evidence for all
the steps in the mechanism of acrylic polymer degrada-
tion illustrated in Scheme 1. GPC analysis of the
photolyzed polymers provides proof that main chain
cleavage is occurring upon irradiation, even in polymers
where no TREPR signal is observed. Product analysis
using NMR shows evidence of monomer formation,
which can only come from â-scission of the main chain
polymeric radical.
Experimental Section
A. TREPR. Continuous wave TREPR experiments were
performed as previously described.48 Briefly, all experiments
were performed on a JEOL USA Inc. JES-RE1X X-band EPR
spectrometer equipped with a wide bandwidth preamplifier
and a low-noise GaAsFET microwave amplifier. Solutions of
polymer (2.5 g) in solvent (60 mL) were circulated through a
flat cell (0.4 mm path length) positioned in the center of a
Varian TE103 optical transmission cavity. The solutions were
photolyzed using a Lambda Physik Compex 120 excimer laser
(248 nm, KrF) running at 60 Hz with a pulse energy of 100
mJ (∼20 mJ per pulse hitting the sample) and a pulse width
of 17 ns. Spectra were collected in the absence of field
modulation at a fixed delay time after the laser flash using a
two-gate boxcar integrator (Stanford, 100 ns gates), while the
external magnetic field was swept, typically over 2 or 4 min.
B. High-Temperature Flow System. For high-tempera-
ture experiments, the deoxygenated sample solution was
circulated using a micropump (Micropump model 000-405)
through Teflon PFA 1/8 in. tubing insulated with polyurethane
foam tape. From the pump, the sample tubing passed through
a copper coil wrapped with heating tape (Omega, Inc.), which
was controlled using a feedback circuit between a variable
power temperature controller and thermocouples placed at the
entrance and exit of the quartz flow cell. The sample was
recirculated between the flat cell and the reservoir. Reported
temperatures are the average of three measurements from the
top and bottom of the flat cell and the reservoir. The maximum
temperature gradient at the highest temperature was 10 °C;
therefore, all temperatures are reported as (5 °C.
C. Commercial Samples. Samples of poly(ethyl acrylate)
were a gift from the Rohm and Haas Co. and were used as
received. Poly(ethyl cyanoacrylate) was a gift from the Kodak
Corp. and also used as received. The fluorinated solvent FC-

Macromolecules, Vol. 38, No. 8, 2005
Main Chain Radicals from Acrylic Polymers 3341
(12) Closs, G. L.; Forbes, M. D. E.; Norris, J. R. J. Am. Chem.
Soc. 1987, 91, 3592-3599.
70 was a gift from the 3M Corp. Propylene carbonate,
squalane, and atactic PMMA were purchased from Aldrich and
used as received. Poly(fluorooctyl methacrylate) was gener-
ously provided by Dr. J. M. DeSimone and used as received.
The ethylene-acrylate copolymer was generously provided by
Dr. M. L. Brookhart and was purified by column chromatog-
raphy before use.
D. Synthesis. Unless indicated below, molecular weight
data for the polymers studied are listed in Table 2.
Poly(methyl d3-methacrylate). The deuterated monomer was
synthesized via the three step method of Keah and co-
workers.49 The polymer was then synthesized by radical
polymerization. GPC showed MN ) 266K, MW ) 1037K, and
PDI ) 3.89.
Dimethyl-2,2-dimethyl Glycidate. 25.33 g (0.16 mol) of 2,2-
dimethylglycidic acid was dissolved in 150 mL of methanol,
and approximately 0.5 mL of concentrated sulfuric acid was
added. The reaction flask was equipped with a reflux con-
denser and heating mantle and set to reflux. The reaction was
monitored by NMR. After 48 h, the methanol was removed by
rotary evaporation. The product was dissolved in diethyl ether
and dried over magnesium sulfate. The ether was removed by
rotary evaporation, yielding a colorless oil (29.26 g, 98.41%
yield). 1H NMR (200 MHz, CDCl3): δ ) 3.65 (s, CH3, 6H), 2.27
(m, CH2, 2H), 1.85 (m, CH2, 2H), 1.97 (s, CH3, 6H).
Highly Isotactic Poly(methyl methacrylate). Highly isotactic
poly(methyl methacrylate) was synthesized by the method of
Zundel.50 GPC showed MN ) 38K, MW ) 88K, and PDI ) 2.31.
1H NMR (200 MHz, CDCl3): δ ) 1.20-93% isotactic (mm),
1.00-5% atactic (mr), 0.82-2% syndiotactic (rr). Total meso
dyads: 96%.
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Acknowledgment. We thank the Rohm and Haas
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program (Grant CHE-0213516). We also thank Dr. N.
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