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Acrylic Polymer Radicals: Structural Characterization and Dynamics

Acrylic Polymer Radicals: Structural Characterization and Dynamics

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326



ACRYLIC POLYMER RADICALS: STRUCTURAL CHARACTERIZATION



14.2 THE PHOTODEGRADATION MECHANISM

The photodegradation of acrylic polymers, illustrated in Scheme 14.1 for a generic

structure, has been extensively studied by photochemists and polymer scientists for

many years.14,15 Despite some contrary interpretations,21 there is a general consensus

that Norrish I a-cleavage of the ester side chain is the first bond breaking event after

absorption of UV light by the ester chromophore. The result of side-chain cleavage is a

main-chain polymeric radical a and a smaller oxo-acyl radical b as a primary radical

pair. Radical a has available to it a unimolecular decomposition pathway, namely, bscission,22 to give a terminal alkene and the so-called propagating radical c.23,24 This

reaction is analogous to the reverse of the free radical polymerization reaction, that is,

free radical addition to the alkene monomer.25

R

C



CH2



R



R



C



CH2 C







CH2



R

CH2



C



R



R



C



CH2 C



CH2



C O



C O



C O



C O



C O



OR′



OR′



OR′



OR′



OR′



1a–8a



1–8



O

C



β–scission



+



C

C



R



R



R



O



OR′



H2 C



C



CH2



Cross-linking



Insoluble

products



C



CH2



C



O



+



OR′



1b–8b

Recombination



Starting

material



OR′



1c–8c

SCHEME 14.1 The general structure of an acrylic polymer and the established photodegradation mechanism via Norrish I a-cleavage of the carbonyl side chain, leading to main-chain

polymeric radical a and oxo-acyl radical b. The secondary b-scission rearrangement reaction

leading to the propagating radical c is also shown.



Because of the rapid b-scission reaction, steady-state electron paramagnetic resonance (SSEPR) investigations carried out since 1951 on acrylate and methacrylate

degradation were unable to confirm the chemistry shown in Scheme 14.1 as the first step.

In 1982, Liang et al. suggested the existence of a main-chain acrylate radical in a cold

SSEPR experiment,26 but the spectrum contained many overlapping lines from other

radicals, so a definitive conclusion regarding its structure could not be reached. In fact,

main-chain polymeric radical such as a had not been unambiguously characterized in

any work except in that reported here. The TREPR experiments described below will

provide unambiguous spectral assignment of both radicals a and b from many acrylic



327



POLYMER STRUCTURES



polymers, confirming the mechanism shown in Scheme 14.1. In addition, our results

show novel features beyond the original mechanistic problem, in particular regarding

nuclear spin symmetry relationships in these polymeric free radicals, their chain

dynamics and spin relaxation processes in solution, and the anomalous intensities

associated with chemically induced electron spin polarization (CIDEP) mechanisms27

that almost always accompany TREPR spectra.

14.3 POLYMER STRUCTURES

The photochemistry and ensuing free radical chemistry of the eight acrylic polymers

shown in Chart 14.1 have been studied intensely over the past decade in our

laboratory.16–20 Structural modification of acrylates can be achieved by modifying

either the substituent on the polymer backbone (a-substitution), or the ester side chain

(b-substitution). Poly(methyl methacrylate) (PMMA), 1, was the starting point for our

investigations as it is the most ubiquitous acrylic polymer. Common commercial

names for PMMA include Lucite, Perspex, and Plexiglass; in many applications,

PMMA is exposed to UV light where radiation damage can occur. In EPR spectroscopy, it is often desirable to confirm spectral assignments by isotopic labeling, and

therefore the deuterated analog of PMMA was also studied. Here, the deuterium

substitution is only on the backbone methyl group and the polymer is abbreviated as

d3-PMMA, structure 2 in Chart 14.1.

R

CH2 C

C



O



OR′



n



Acronym



R



R′



1 Poly(methyl methacrylate)



PMMA



CH3



CH3



2 Poly(methyl d 3-methacrylate)



d 3-PMMA



CD3



CH3



3 Poly(ethyl methacrylate)



PEMA



CH3



CH2CH3



4 Poly(ethyl cyanoacrylate)



PECA



CN



CH2CH3



5 Poly(ethyl acrylate)



PEA



H



CH2CH3



6 Poly(fluorooctyl methacrylate)



PFOMA



CH3



CH2(CF2)x CF3



7 Poly(acrylic acid)



PAA



H



H



8 Poly(methacrylic acid)



PMAA



CH3



H



Polymer



CHART 14.1 Structures of all polymers investigated in this work, numbered with their

common acronyms from the polymer literature.



328



ACRYLIC POLYMER RADICALS: STRUCTURAL CHARACTERIZATION



The next three polymers in this series are all ethyl acrylates, meaning that while the

backbone (a) substituent is different in all three structures, the ester side-chain group

(b) is the same for all of them (ÀCH2CH3). Polymer 3 is poly(ethyl methacrylate)

(PEMA), and 4 is poly(ethyl cyanoacrylate) (PECA), which may be recognizable as a

primary component of the so-called “superglues.” Polymer 5 is poly(ethyl acrylate)

(PEA), with H on the backbone a-position. From structure 3 to 4 to 5, the a-substituent

becomes simpler in structure and this will be reflected in the observed TREPR spectra

below in terms of the number of observed transitions, and in some cases the linewidths

as well.

Poly(fluorooctyl methacrylate) (PFOMA, structure 6) has very different physical

properties from the other four methacrylates shown in Chart 14.1. Primarily this is

due to the bulky (and more rigid) b-substituent, which is actually a mixture of

straight chain and branched perfluorooctyl groups. The rigidity of the main chain

enforced by these bulky groups has drastic consequences on the appearance of the

TREPR spectra of the resulting radicals. PFOMA was the first polymer to be

synthesized in liquid CO2 and was the basis of several new surfactant technologies

developed for recyclable solvents.

The last two polymer structures listed in Chart 14.1 are poly(acrylic acid) (PAA, 7)

and poly(methacrylic acid) (PMAA, 8), which exhibit interesting properties due to

their water solubility and ionic strengths. From a structural perspective, PAA and

PMAA are the simplest polyelectrolytes, and they are of significant interest as

biodegradable materials for wound dressings28 and other biomedical and bioanalytical29 applications. The main difference between the polyacrylic acids and their ester

counterparts is the presence of charges on the carboxyl side chains in solution. The

degree of ionization of these functional groups can influence the morphology of

the polymer chains in solution (coiled versus stretched),30 the redox properties of the

carboxylate groups,31 and the nature of the excited states involved in the photodegradation process.32 The water solubility of polymers 7 and 8 has some experimental

ramifications: EPR in aqueous solution presents some technical challenges due to the

high dielectric constant of the sample. In addition, there is the possibility of pH and/or

ionic strength dependences of their photochemical reactivity and their TREPR

spectral features.

The polymers selected for these studies are structurally similar in that the primary

photophysics and chemistry are not expected to change; that is, we do not expect a

major deviation from the mechanism shown in Scheme 14.1 in terms of UV photodegradation for these macromolecules. However, the small structural variations from

one polymer to another have been chosen to allow structural, dynamic, solvent, and pH

effects in the ensuing free radicals to be probed as systematically as possible. It should

be noted that all of the polymers under investigation are, to the best of our knowledge,

homopolymers of high molecular weight (Mw > 10,000), high purity (>95% by NMR

and GPC), and unless otherwise indicated, atactic in terms of macromolecular

stereochemistry. Tacticity is an important factor in magnetic resonance of polymers,

a fact long recognized in the NMR research community33 but, as we will show below, it

is an issue that was somewhat underappreciated by EPR spectroscopists because of the

lack of clean, well-resolved spectra of main-chain macromolecular free radicals.



THE TIME-RESOLVED EPR EXPERIMENT



329



14.4 THE TIME-RESOLVED EPR EXPERIMENT

In routine steady-state EPR spectroscopy, the transitions are detected by sweeping an

external magnetic field B0 through each resonance at a constant microwave frequency

w0. The external field is modulated, usually at a frequency of 100 kHz, so that phasesensitive detection can be used to increase the signal to noise (S/N) ratio and the

spectral resolution.34 The resulting spectra have first-derivative line shapes, and care is

usually taken to keep the amplitude of the field modulation smaller than the linewidth

to avoid line shape distortions. A consequence of the 100 kHz field modulation is that

the time response of the spectrometer becomes limited to, at best, the inverse of the

modulation frequency. Practically, for good S/N, three or four cycles of modulation are

necessary, which means that species with chemical lifetimes less than about 40 ms

become difficult to detect. Since most organic radicals have lifetimes in solution on the

order of 10–100 ms, their detection can be problematic in SSEPR. Lower temperatures

(below À50 C) can help with this problem. Continuous, intense light or heat can be

used to generate large steady-state concentrations of radicals.35

The CIDEP enhancements of 1–100 above the Boltzmann population differences

are common to TREPR spectra and decay with T1 values on the order of 1 to 10 ms. This

makes CIDEP difficult (but not impossible if strong enough) to observe at steady state.

Historically, it was quickly recognized that a significant amount of kinetic and

magnetic information could be obtained by studying the CIDEP mechanisms, and

therefore an EPR experiment with submicrosecond time resolution and response

became highly desirable. The earliest attempts to build such an apparatus were coupled

to pulse radiolysis instrumentation by Avery and Smaller36 and by Fessenden,37 who

also made seminal contributions to the analysis of TREPR data.38 The apparatus and

methodology used in the authors’ laboratory couples laser flash photolysis to EPR,

which was first developed by Trifunac and coworkers39 and used widely by others such

as McLauchlan,40 Levanon,41 van Willigen,42 and Hirota.43 The experiment found

great utility in photosynthesis research, most notably in the research groups of Hoff,44

Mobius,45 Lubitz,46 and Norris.47

In our X-band TREPR apparatus, temporal resolution is achieved by pulsing the

production of the radicals, and then gating the detection system. Pulsed production of

radicals is typically accomplished using an excimer or YAG laser. The EPR signal

from the microwave bridge preamplifier is passed directly to a boxcar gated integrator

or to a transient digitizer, which allows the signal to be trapped and stored on the

submicrosecond timescale. Figure 14.1a shows how the apparatus is connected, and

Fig. 14.1b shows the timing sequence. The phase-sensitive detection system of

the SSEPR system is bypassed (hence, the experiment is sometimes called “direct

detection”). The only additional components to the TREPR spectrometer are the

timing electronics (usually a Stanford DG535 delay generator or equivalent), a boxcar

signal averager, and the laser (excimer or Nd3 ỵ :YAG). A computer is used to store and

display the data as an xy array (B0, TREPR intensity). Quartz flow cells are used to

avoid heating and sample depletion, and either holes or slots are cut into the sides

of the resonators to allow light access. The microwave excitation is continuous

wave throughout the experiment, even during the production of the radicals, as



330



ACRYLIC POLYMER RADICALS: STRUCTURAL CHARACTERIZATION



FIGURE 14.1 The Time–resolved EPR experiment. (a) The apparatus. (b) The timing

sequence (see text for scale and definitions).



opposed to pulsed microwave methods such as electron spin echo or Fourier transform

(FT) EPR. Significant advantages in sensitivity with similar time response are

available with FT-EPR,48 but there are also disadvantages in terms of spectral width

of excitation that limit the application of this technique. The TREPR (CW) method is

the most facile and cost-effective method for the observation of field-swept EPR

spectra of organic radicals on the submicrosecond timescale.

There are two modes in which the experiment can be run. The preferred mode is to

fix the delay time between the two boxcar gates. The first gate opens before the laser

flash to sample the dark signal and provide a baseline; the second gate opens at a set

delay time t after the flash and samples both light and dark signals. Gate widths are 100

to 300 ns, which defines the time resolution of the experiment. Typical delay times (t)

are 0.1 to 10 ms. A processor in the boxcar unit provides the difference between the

gates (the light-induced EPR signal) as an output voltage, which is passed to a

computer for storage at each field point. Time constants in the boxcar charging circuit

are adjusted to give an exponentially averaged output after 5 to 10 laser flashes at a

single magnetic field value. The external magnetic field sweep is usually divided into

1000 data points. Clock pulses are generated during the field sweep to control the rate

at which data are downloaded to the computer. In many spectrometers, there exists an

option to program the data collection software to provide a DC voltage ramp that can

sweep the external magnetic field externally, providing complete computer control of

the scan. The laser repetition rate ranges from 10 to 100 Hz, with 60 Hz being nominal.

The microwave power in most TREPR experiments is 2–20 mW, but it is essential to

vary this parameter during the experiment to investigate whether the line shapes and/or

intensities change with it.

The second mode is to run the experiment at a fixed magnetic field and sweep the

second boxcar gate over time to collect kinetic information. There are two problems

with this approach. First, the experiment must be repeated several times with a slow

scan rate in order to get satisfactory S/N. To extract the EPR kinetic curve, the

experiment is repeated off resonance and the two curves subtracted. Kinetics are more

easily obtained using a high-bandwidth transient digitizer instead of a boxcar, and

many researchers perform TREPR in this fashion.49,50 It is important to note here two



THE TIME-RESOLVED EPR EXPERIMENT



331



disadvantages of the TREPR technique. It is not generally possible to observe a

Boltzmann (equilibrium) population of spin states using the boxcar method because of

1/f noise. Also, lifetime broadening effects are observed when the second boxcar gate

is placed close in time to the laser flash. This is a consequence of having the microwave

excitation running continuously. Near the laser flash, and during the photochemical

events that produce radicals, the apparatus is attempting to excite spin states that are

still in the process of forming. In other words, small interactions such as hyperfine

couplings take time to evolve and may not be visible in the TREPR spectrum for

several hundreds of nanoseconds after the laser flash.

Since a Boltzmann population is not generally detectable using the boxcar

method, it is then logical to ask why the experiment works at all? The answer lies in

the fact that in most, if not all, photochemical reactions that produce radicals, radical

ions, or biradicals, CIDEP phenomena are observed. It is here that much of the

sensitivity is gained back that was lost in bypassing the phase-sensitive detection

unit (100 kHz field modulation). A smaller improvement in sensitivity comes from

the use of the boxcar to signal average. In all of the TREPR spectra shown in this

proposal, transitions below the baseline are in emission (E), while those above the

baseline are in enhanced absorption (A). This is different from most conventional

EPR spectra that are displayed as first derivative curves representing the change in

detected intensity with the external field.

A detailed description of CIDEP mechanisms is outside the scope of this chapter.

Several monographs27,51,52 and reviews53,54 are available that describe the spin

physics and chemistry. Briefly, the radical pair mechanism (RPM) arises from

singlet–triplet electron spin wave function evolution during the first few nanoseconds

of the diffusive radical pair lifetime. For excited-state triplet precursors, the phase of

the resulting TREPR spectrum is low-field E, high-field A. The triplet mechanism

(TM) is a net polarization arising from anisotropic intersystem crossing in the

molecular excited states. For the polymers under study here, the TM is net E in all

cases, which is unusual for aliphatic carbonyls and will be discussed in more detail in a

later section. Other CIDEP mechanisms, such as the radical–triplet pair mechanism55

and spin-correlated radical pair mechanism,56 are excluded from this discussion, as

they do not appear in any of the systems presented here.

The photochemistry taking place in these polymers is destructive. It is therefore

essential to flow or recirculate samples during the experiment. Flowing through the

microwave resonator also prevents excessive heating of the samples by the laser flashes.

To obtain high-temperature TREPR spectra of polymer radicals, a special insulated flow

apparatus has been constructed in our laboratory that provides stable laminar flow of

liquids through the EPR resonator at temperatures up to 150 C. The choice of solvent is

critical for the success of high-temperature experiments. The solvent must (1) dissolve

the polymer to concentrations of several grams per 100 mL, (2) have a high enough

boiling point that it can withstand our reservoir temperature without evaporating or

decomposing, and (3) be optically transparent at 248 nm. To date, we have found only

one solvent, propylene carbonate, shown below, which fits all of these criteria. It is an

excellent solvent for PMMA at room temperature and above, boils at 240 C, and

shows no TREPR signal when run as a blank under 248 nm irradiation. It is used for all



332



ACRYLIC POLYMER RADICALS: STRUCTURAL CHARACTERIZATION



acrylates with ester side chains except for PFOMA, where specialized fluorinated

solvents must be used. For the polyacids PAA and PMAA, aqueous solutions (neutral

or basic) are appropriate, but if temperatures above 100 C are desired, small amounts

of ethylene glycol should be added to raise the boiling point.

O

O



O



propylene carbonate



14.5 TACTICITY AND TEMPERATURE DEPENDENCE

OF ACRYLATE RADICALS

Acrylic polymers have stereogenic centers on every other carbon atom. As a result, the

polymers can be classified as atactic (random stereochemistry), isotactic (the same

configuration at each stereogenic center), or syndiotactic (alternating configurations at

each stereogenic center).57 For PMMA, these are denoted as a-PMMA, i-PMMA, and

s-PMMA, respectively. The tacticity of the polymers can be controlled during

synthesis to some extent. While highly syndiotactic or isotactic acrylic polymers

are reasonably straightforward to synthesize, it is rather difficult to generate highly

atactic material. In general, samples of acrylic polymers purporting to be completely

atactic have large sections of the polymer chain that are syndiotactic. We will see

below that polymer tacticity plays a large role in the appearance of the TREPR spectra

of acrylic main-chain radicals.

Figure 14.2 shows the temperature dependence of the TREPR spectra obtained upon

photodegradation of all three tacticities of PMMA in propylene carbonate. Near room

temperature, an alternating linewidth pattern indicative of conformationally induced

hyperfine modulation is observed (Fig. 14.2, top). Upon heating, a large transformation

takes place. At high temperatures (at or over 100 C), all three samples reveal

motionally narrowed spectra, simple in appearance and with sharp linewidths

(<1 G). Even more remarkable is that at the highest temperature recorded for each

sample, all three spectra are quite similar in appearance. The convergence temperature

to the fast motion spectrum is different for radicals from all three tacticities of PMMA,

with the i-PMMA radical converging at the lowest temperature. This result is expected

as i-PMMA has been found in many studies to be the least rigid of the three polymers.58–

60

Computer simulations of the fully converged, high-temperature limit spectra are

shown at the bottom of each data set in Fig. 14.2. The hyperfine coupling constants used

for each simulation are nearly identical and will be discussed in the next section.

The fast motion spectrum of the i-PMMA radical consists of 21 lines attributed to

three separate isotropic hyperfine coupling constants. There is coupling to the methyl

group to form a quartet (22.9 G) that is then split further into a triplet from one set of bmethylene protons (16.4 G) and another triplet from the other set (11.7 G).

Theoretically, this should lead to 36 lines (4 Â 3 Â 3), but a fortuitous degeneracy

exists because one of the fast motion b-methylene coupling constants is almost exactly



TACTICITY AND TEMPERATURE DEPENDENCE OF ACRYLATE RADICALS



333



FIGURE 14.2 X-band TREPR spectra of main-chain polymer radical 1a produced from

248 nm laser flash photolysis of atactic, isotactic, and syndiotactic PMMA in propylene

carbonate at 0.8 ms delay time. The temperature for each spectrum is shown in  C, and the

magnetic field sweep width is 150 G for all spectra, which exhibit net E CIDEP in all cases.

Simulations of each fast motion spectrum (highest temperature) are shown at the bottom of each

data set. Hyperfine values for each simulation are 3 aH(CH3) ¼ 22.9 G, 2aH(CH2) ¼ 16.4 G,

2aH(CH2) ¼ 11.7 G for isotactic PMMA; 3aH(CH3) ¼ 22.9 G, 2aH(CH2) ¼ 16.2 G,

2aH(CH2) ¼ 11.7 G for syndiotactic PMMA; and 3aH(CH3) ¼ 23.0 G, 2aH(CH2) ¼ 16.4 G,

2aH(CH2) ¼ 11.3 G for atactic PMMA.



half the value of the methyl proton coupling constant. The syndiotactic and atactic

polymers give rise to radicals with 27 lines, due to a lifting of this degeneracy (or

perhaps an incomplete high-temperature averaging process). The slight changes in

coupling constants from 16.4 to 16.2 G for s-PMMA, and from 11.7 to 11.3 G for

a-PMMA, are minor but clearly observable at 0.8 ms delay time.

A special consequence of the stereoregularity of these polymers is the pseudosymmetry relationships between the b-methylene protons of the main-chain radicals.

The concept is briefly reviewed here by introducing Fig. 14.3, which shows the

possible radicals formed by loss of the ester side-chain moiety from PMMA by

the Norrish I a-cleavage reaction from the first excited triplet state. Because of the

repetition pattern of stereogenic centers in isotactic and syndiotactic materials, these

two tacticities are required to lead to the same free radical, which has a mirror plane of

symmetry. We call this the “meso” radical. The mirror plane establishes magnetic

equivalence in each of the two sets of b-methylene protons and is the reason why these

protons show a triplet of triplets in the TREPR spectrum. Note that on each

nonstereogenic center, the two protons are chemically and magnetically inequivalent

because of the adjacent stereogenic centers.

In the “meso” and “racemic” radical structures, the hyperfine coupling constants to

the b-methylene protons are not perturbed greatly by the change in stereochemistry on



334



ACRYLIC POLYMER RADICALS: STRUCTURAL CHARACTERIZATION



σv



Isotactic



CH3







Ha

Syndiotactic



Hb Ha′



Hb′







“Meso” radical



C2

Atactic



CH3





Ha



= H, CH3



= COOH



Hb Hb′



Ha′



= radical center



“Racemic” radical

FIGURE 14.3 Nuclear spin symmetry relationships for main-chain radical 1a from PMMA,

as a function of polymer tacticity. The two radicals shown have sv mirror plane (“meso”) or

C2 axis (“racemic”) symmetry elements establishing magnetic equivalence of each set of

b-methylene protons.



the next carbon further down the chain. For this reason, the observed spectra for each

type of radical are very similar. The difference in coupling constant for a methylene

proton pair in either the meso or the racemic radical can be quite large. In PMMA, for

instance, the methylene couplings on the same carbon atom differ by about 5 G. In

contrast, for an acrylic ester such as PEA, the difference is less than 1 G (see below).

The methylene proton inequivalencies are a function of polymer stereochemistry and

therefore cannot be removed by fast rotation.



14.6 STRUCTURAL DEPENDENCE

The left side of Fig. 14.4 shows TREPR spectra obtained 1.0 ms after 248 nm laser flash

photolysis of eight acrylic polymers, with computer simulations on the right side.

All these spectra were acquired at elevated temperatures ($100 C), that is, where

the observation of fast motion spectra is expected. In Fig. 14.4A, the TREPR spectrum

of the main-chain polymer radical from photolysis of i-PMMA is repeated from the

bottom left side of Fig. 14.2, as it is the starting point for comparisons of spectral

features such as linewidths and hyperfine coupling constants. The nomenclature used

throughout this section is derived using the notations indicated in Scheme 14.1 and

Chart 14.1. For example, a main-chain radical from PMMA will be denoted 1a,

whereas the oxo-acyl radical from PFOMAwill be designated as radical 6b, and so on.

For all radicals simulated, the parameters used are listed in Table 14.1.



STRUCTURAL DEPENDENCE



335



FIGURE 14.4 Experimental TREPR spectra (left) and simulated spectra (right) for radicals

observed at 0.8 ms after 248 nm laser flash photolysis of the following polymers (see Chart 1

for acronym definitions): (A) PMMA, (B), d3–PMMA, (C) PEMA, (D) PECA, (E) PEA,

(F) PFOMA, (G) PAA, (H) PMAA. Except for the spectrum obtained from PFOMA (6), the

simulation unambiguously assigns the signal carrier to main-chain polymer radical a. In the

case of PFOMA, oxo-acyl radical 6b is the dominant signal carrier. For PMMA (spectrum A),

the material is isotactic (91% by NMR), but all other polymer samples are atactic material.

Simulation parameters are listed along with the radical structures in Table 14.1.



14.6.1 d3-Poly(methyl methacrylate), d3-PMMA

Figure 14.4B shows the experimental and simulated TREPR spectra acquired during

the photolysis of d3-PMMA (radical 2a) in propylene carbonate at 120 C. The spectral

width is quite narrow, and there are more transitions observed here than for the

protonated analog. There are many overlapping lines and only the outermost lines of

the spectrum are clearly resolved. In Table 14.1, it is seen that the value of the hyperfine

coupling constant for the a-methyl protons has decreased from 22.9 to 3.5 G, exactly

as expected for this isotopic substitution.61 The values for the b-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 substitutent has a small but

observable effect on the conformational energies of this polymer in solution.



336

TABLE 14.1



ACRYLIC POLYMER RADICALS: STRUCTURAL CHARACTERIZATION



Magnetic Parameters Used in the Simulations in Fig. 14.4



Acronym



Structure



Hyperfine Constant



PMMA

radical

1a



CH3 ¼ 22.9 G

Hb ¼ 16.7 G

Hb0 ¼ 11.2 G



d3-PMMA

radical

2a



CD3 ¼ 3.5 G

Hb ¼ 16.3 G

Hb0 ¼ 10.9 G



PEMA

radical

3a



CH3 ¼ 22.9 G

Hb ¼ 15.8 G

Hb0 ¼ 11.2 G



PECA

radical

4a



N ¼ 3.3 G

Hb ¼ 16.3 G

Hb0 ¼ 14.8 G



PEA

radical

5a



Ha ¼ 21.7 G

Hb ¼ 23.5 G

Hb0 ¼ 23.8 G



PFOMA

radical oxo-acyl

6b



CH2 ¼ 3.2 G

CF2 ¼ 0.8G



PAA

radical

7a



Ha ¼ 21.5 G

Hb ¼ 23.7 G

Hb0 ¼ 23.8 G

Hg ¼ 0.9 G



PMAA

radical

8a



CH3 ¼ 23.1 G

Hb ¼ 27.3 G

Hb0 ¼ 11.0 G



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