Time-Resolved EPR Studies of Main Chain ... - ACS Publications

10686

J. Phys. Chem. B 2005, 109, 10686-10694

Time-Resolved EPR Studies of Main Chain Radicals from Acrylic Polymers. Effects of Tacticity, Solvent, and Side Group Structure on Chain Stiffness 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: January 10, 2005; In Final Form: April 1, 2005

Main chain polymeric radicals from several acrylic polymers, produced by laser flash photolysis at 248 nm in liquid solution, have been studied using direct detection time-resolved electron paramagnetic resonance (TREPR) spectroscopy at 9.5 GHz. Highly isotactic poly(methyl methacrylate) (i-PMMA) shows a sharp, well-resolved spectrum at about 95 °C. Using synthetic methodology to disrupt the tacticity of i-PMMA, we observed different fast-motion hyperfine coupling constants for the main chain radicals. By raising the temperature of observation, we returned the coupling constants to the same value as those in the highly isotactic sample. This result is related qualitatively to the degree of stiffness of the polymer chains as a function of tacticity. The concept is tested further by comparison to two other acrylic polymers with bulky side chains: poly(fluorooctyl methacrylate) (PFOMA) and poly(adamantyl methacrylate) (PAMA), whose main chain radicals show significant line broadening even at 110 °C. Solvent effects on both spectral appearance (the alternating line-width effect) and kinetic decays (attributed to T1 relaxation) are also presented and discussed in terms of main chain conformational motion.

Introduction

SCHEME 1

In 2000 we reported the first high-resolution magnetic resonance data of main chain free radicals from acrylic polymers.1 The radicals were produced in liquid solution (usually in propylene carbonate solvent) by laser flash photolysis at 248 nm and were detected on the microsecond and submicrosecond time scales using time-resolved electron paramagnetic resonance (TREPR) spectroscopy. This work conclusively demonstrated that Norrish I R-cleavage of the ester side chain is the primary step of the mechanism in the photodegradation of acrylate and methacrylate polymers, as shown in Scheme 1. Although some temperature- and solvent-dependence data were collected and reported in our earlier paper, that work focused mainly on the symmetry relationships of the nuclear spin states of the radicals and the effect of these relationships on the magnitudes of the electron-nuclear hyperfine coupling constants. Recently, we expanded the scope of this work to other polymers, presenting detailed analyses of the fast-motion hyperfine coupling constants for several acrylic radicals in solution at high temperatures, allowing for their unambiguous characterization for the first time.2 These experiments also established the photodegradation mechanism as a general one for acrylic polymers. We have also examined in detail the alternating line-width effect observed in the TREPR temperature dependence of the polymeric radicals, which arises from conformational modulation of some of the hyperfine coupling constants.3 Although a two-site jump model was successful in simulating this effect for some polymers, it did not work well for others. This led us to believe that not all acrylic polymers follow the same conformational motions, and new experiments were developed to probe the chain rigidity in such radicals and its manifestation * To whom [email protected].

correspondence

should

be

addressed.

E-mail:

in TREPR spectroscopy. Chain stiffness in methacrylate polymers varies significantly with tacticity, and is also a very strong function of both the ester side chain and substitution on the main chain. Here we have chosen to study the three acrylic polymers shown in Scheme 1. Poly(methyl methacrylate) (PMMA, 1) was modified synthetically to disrupt its tacticity; this is known to be a method for manipulating chain stiffness in such structures. Two other polymers, poly(fluoroalkyl methacrylate) (PFOMA, 2) and poly(adamantyl methacrylate) (PAMA, 3), were studied to show how TREPR spectra are affected when conformational motion is influenced by a bulky side chain moiety. It should be noted that the PFOMA polymer has a mixture of side chains with different numbers of CF2 groups in the side chain. We want to specifically consider situations in which the fast-motion

10.1021/jp0501401 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/07/2005

Time-Resolved EPR Studies of Main Chain Radicals spectrum is perturbed or perhaps not even observed because the polymer chains become too rigid. As will be shown in detail below, we have had some success with these two strategies. Chain stiffness has a strong effect on the TREPR spectral appearance, and in the course of this work we have also determined that many acrylic polymeric radicals show a significant solvent dependence in their TREPR spectra as evidenced by large changes in the spin-lattice relaxation time of the radicals in solvents of different polarities and viscosities. The TREPR experiment is ideal for the observation of radicals such as 1a-3a due to their transient nature and the high structural resolution of the technique. In addition to our three previous papers on main chain acrylic radicals, this method has been used in a mechanistic study of the photodegradation of 2-norbonyl-CO copolymers in solution and to the study of the initial stages of polymerization of methyl methacrylate.4,5 The time response of the experiment at 9.5 GHz is about 3 orders of magnitude faster than that for steady-state EPR (60 ns vs 40 µs), which means that we are almost always observing radicals from the primary photochemical events after the initial excitation. This is an important point because the β-scission reaction of the main chain radical, which leads to the acrylic propagating radical, is so fast at the temperatures used here that the slower steady-state methods have always detected the rearranged species rather than the one created initially.6,7 Experimental Section TREPR. Our apparatus is described in detail in previous publications.30 Samples were flowed through a 0.4-mm path length quartz flat cell using a micropump. Nitrogen was bubbled through all of the samples for 10 minutes before and during all of the experiments. Laser pulse energies hitting the sample were nominally 10-20 mJ. The signals were independent of laser power and microwave power. All of the solvents were reagent grade and were used as received. The FLUORINERT solvent FC-70 was generously supplied by the 3M Company. Highly Isotactic Poly(methyl methacrylate). Highly isotactic poly(methyl methacrylate) was synthesized by the method of Zundel.31 GPC showed MN ) 38 K, MW ) 88 K, 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 is 96%. Moderately Isotactic Poly(methyl methacrylate). Moderately isotactic poly(methyl methacrylate) was synthesized via living anionic polymerization. A 250-mL round-bottom flask was flame dried under nitrogen and charged with 100 mL of toluene (distilled from calcium hydride) and cooled to -78 °C with a dry ice/isopropyl alcohol bath. Methyl methacrylate (8 mL, 75 mmol) was added to the flask via syringe, and the solution was degassed with nitrogen for 25 min before the addition of 2 mL (1.3 M solution in cyclohexane, 2.6 mmol) of sec-butyllithium. The reaction was quenched with degassed methanol and precipitated into methanol. The resulting white solid (2.45 g, 33% yield) was dried in vacuo. GPC showed MN ) 26 K, MW ) 51 K, and PDI ) 1.98. 1H NMR (200 MHz, CDCl3): δ ) 1.20-72% isotactic (mm), 1.00-19% atactic (mr), 0.82-9% syndiotactic (rr). Adamantyl Methacrylate. Adamantyl methacrylate was synthesized by the procedure of Eckl and co-workers.32 1-Adamantol (Aldrich; 2.5 g ) 0.016 m) and phenothiazine (Aldrich; 0.03 g) were added to a three-necked round-bottomed flask equipped with a stirring bar, thermometer, and an addition funnel. The setup was flushed with nitrogen for 15 min, and triethylamine (Aldrich; 3.3 mL) was syringed into the flask.

J. Phys. Chem. B, Vol. 109, No. 21, 2005 10687 Methylene chloride (freshly distilled from calcium hydride) was added to the flask (45 mL) and to the addition funnel (5 mL), and the flask was then cooled to 0 °C. Methacryloyl chloride (Aldrich; previously distilled; 2.3 mL) was syringed into the addition funnel, and the methacryloyl chloride/methylene chloride solution was added dropwise to the flask. The reaction was allowed to warm to room temperature and was stirred for 21 h. The product solution was washed with 0.1 M HCl, a saturated aqueous solution of sodium bicarbonate, and water. The methylene chloride was removed by vacuum distillation, and 3.5 g crude product was isolated (100% crude yield). GCMS showed 94% adamantyl methacrylate and 6% of an unknown side product. The product was combined with 1.6 g of product obtained from a previous batch and was eluted from a short alumina column with benzene. The benzene was removed by vacuum distillation, and a yellow oil (5 g) was isolated. GCMS showed 100% adamantyl methacrylate. 1H NMR (200 MHz, CDCl3): δ 1.56 (br, CH2, 6 H), 1.77 (s, CH3, 3 H), 2.04 (br, CH, 9 H), 5.35 (m, CdH, 1 H), 5.89 (m, CdH, 1 H). Poly(adamantyl methacrylate). Adamantyl methacrylate was polymerized using free-radical methods. Adamantyl methacrylate (4.8 g ) 0.022 mol), AIBN (Aldrich; previously recrystallized; 0.06 g), and benzene (Aldrich, 10 mL) were placed in a round-bottomed flask, which was capped with a septum. After degassing with nitrogen for 25 min, the flask was placed in a 60 °C water bath, and the solution was stirred for 48 h. After 18 h, the solution gelled. After 24 h, additional benzene was added, and the gelled mixture was stirred for 60 h at 60 °C. After all of the polymer had dissolved, the solution was precipitated into methanol, the methanol was filtered, and the polymer was dried under vacuum. The total yield was 3.8 g (79%). Results and Discussion The TREPR spectra of polymeric radicals such as 1a are strongly temperature-dependent from room temperature to about 100 °C. At room temperature, an alternating line-width pattern indicative of hyperfine modulation is observed (Figure 1A). At about 100 °C (higher for some polymers) the spectra converge to a simple 21-line pattern attributed to isotropic β-hyperfine couplings from the three methyl and four methylene protons adjacent to the tertiary radical center (Figure 1B-D). We have shown previously that the convergence temperature is different for radicals from all three tacticities of PMMA, with the i-PMMA radical converging at the lowest temperature. This result is expected because i-PMMA has been found in many studies to be the least rigid of the three polymers.8-10 An important issue in the spectral analysis of these radicals at high temperatures is the diastereotopicity of the β-methylene protons. This is shown more clearly using the Newman projections of the polymeric radicals in Scheme 2. Looking down the indicated bond and performing two 120° rotations gives three possible conformations. The β-methylene protons on the front carbon atom clearly experience different magnetic environments in each conformation and can therefore never reach magnetic equivalence no matter how fast the rotation. Of course, accidental equivalence is possible but unlikely. A similar Newman projection exists for the β-methylene protons on the other side of the radical center, which are also therefore diastereotopic. It might be expected then, that four different coupling constants are expected for these four β-methylene protons. We will see below that this may not always be the case, but the possibility needs to be considered when attempting to simulate the TREPR data at certain temperatures.

10688 J. Phys. Chem. B, Vol. 109, No. 21, 2005

Figure 1. X-band TREPR spectra of PMMA radical 1a in propylene carbonate solution at the following temperatures: (A) 25 °C, (B) 52 °C, (C) 70 °C, and (D) 94 °C. In this and all of the subsequent spectra, the lines below the baseline are in emission and those above the baseline are in absorption. The sweep width is 150 G in all spectra.

We have found in most high temperature TREPR spectra of methacrylate polymers that the fully converged, fast-motion spectrum consists of 21 lines attributed to three separate isotropic hyperfine coupling constants. These lines arise from coupling to the methyl group to form a quartet, which then split further into a triplet from one set of β-methylene protons and another triplet from the other set. Theoretically this should lead to 36 lines (4 × 3 × 3), but a fortuitous degeneracy exists because one of the fast-motion β-methylene coupling constants is almost exactly half of the value of the methyl proton coupling constant. Whether or not this is completely coincidental will be discussed in more detail below. SCHEME 2

McCaffrey et al. Another special consequence of the stereoregularity of these polymers is the pseudosymmetry relationship between the β-methylene protons of the main chain radicals. One of our earlier papers was devoted to an explanation of that principle and its manifestation in the high-temperature spectra as a function of polymer tacticity. It is relevant here because we will use the disruption of taciticity in PMMA to study chain stiffness as it relates to the high-temperature TREPR spectrum. For this reason, the concept is briefly reviewed here by introducing Scheme 3, which shows the possible radicals formed by loss of the ester side chain moiety from PMMA by the Norrish I R-cleavage reaction from the first excited triplet state. Because of the repetition pattern of the stereogenic centers in isotactic and syndiotactic material, these two tacticities are required to lead to the same free radical, which has a mirror plane of symmetry. This is the reason that the β-methylene protons show a triplet of triplets in the TREPR spectrum. For atactic material, the situation in Scheme 3 is slightly different depending on the dyad and triad closest to the radical center (i.e., whether they are m or r dyads, and mm, mr, or rr triads). It is possible for atactic material to lead to a mirror plane radical similar to the other two tacticities, but it is also possible for a main chain radical to be produced that has a C2 axis of symmetry. It is very important to note that the C2 radical is a diastereomer of the mirror plane radical, and as such the two cannot interconvert by rotation of bonds. This has interesting consequences in the TREPR spectra, which we now present. Disruption of Tacticity. The sample of PMMA used to generate the TREPR spectrum in Figure 1 was highly isotactic with 91% mm, 7% mr, and 2% rr triads, and the temperature at which the TREPR spectrum of this radical converges to its fastmotion limit is quite low (94 °C). This spectrum is repeated in Figure 2A for comparison to a spectrum from a polymeric radical created from less isotactic PMMA (Figure 2B). This polymer sample contained only 70% mm triads. It is immediately obvious that the two spectra are different, especially when expansions of one section of transitions are shown below the full spectra (e.g., Figure 2C and D). For the spectrum of the radical from the less stereoregular polymer, many of the lines are broader and some of the lines are split into well-resolved doublets. If the temperature of the less-isotactic polymeric radical is raised by 40 °C (Figure 2E-G), then the spectra converge once again to the 21-line spectrum of the highly isotactic i-PMMA radical. It is instructive to present the origin of the hyperfine interactions to relate them to macromolecular structure and chain stiffness. In all of the polymers presented here, the hyperfine

Time-Resolved EPR Studies of Main Chain Radicals

J. Phys. Chem. B, Vol. 109, No. 21, 2005 10689

SCHEME 3

interactions result entirely from β-hyperfine couplings, arising from a hyperconjugation mechanism between the proton and the electron in the p orbital. The magnitude of the hyperfine coupling depends on the dihedral angle of the β C-H σ bond for the proton of interest and the p orbital containing the unpaired electron. If the dihedral angle is known, then the coupling constant can be determined quantitatively.11 The dependence on this dihedral angle makes the coupling constant a function of the chain dynamics or rotational motion. The data in Figure 2 clearly show that disruption of the tacticity changes the fast-motion coupling constants, which are average values weighted by the conformational energies of the rotamers shown in Scheme 2 as Newman projections. This is an interesting result, especially in light of the discussion above regarding the diastereomeric relationship between the two possible radicals from atactic material (Scheme 3). If two different radicals are produced, then we would expect to see a superposition of spectra, but our ability to simulate the spectrum with only one set of coupling constants suggests that only one radical is formed. Table 1 shows the hyperfine coupling constants used for each simulation. The β-methylene couplings are clearly more sensitive to the temperature effects than the methyl group couplings, which is expected because the methyl group should always be rotating freely at these temperatures. There are several possible explanations for the tacticity and temperature dependencies illustrated in Figure 2. One is that the synthetic disruption of tacticity is leading not to truly atactic regions of the polymer chain but to alternating isotactic and syndiotactic regions. This would mean that radicals created in a syndiotactic region would be required to exhibit the same fast-

motion coupling constants, according to Scheme 3. This also means that the “disrupted” TREPR spectrum has different coupling constants not because different radicals are formed but rather because the same radicals are formed and the rotational dynamics (i.e., the convergence temperatures) are affected by long-range macromolecular conformational motion (i.e., the degree of chain stiffness). Another possibility is that the photochemical cleavage reaction in different regions of the polymer chain takes place at different rates, leading to a preference for formation of one radical over the other. A detailed analysis of the photochemical kinetics is outside the scope of the present paper and is the subject of work currently in progress in our laboratory. The dynamics of the different tacticities of PMMA have been studied using several techniques: IR, NMR, and fluorescence depolarization. Speva´cek and Schneider have shown by 13C NMR that the rotational correlation time of s-PMMA is an order of magnitude longer than that of i-PMMA.12 Ono and coworkers found a similar increase in the relaxation time of the polymers, but the ratio was only about 2:1 for s-PMMA to i-PMMA.13 Grohens and co-workers14 found by IR that if the interactions between the side chains and the polymer backbone were suppressed through appropriate temperature and solvent choice then this resulted in an increase of the number of trans bonds in s-PMMA and therefore a decrease in the mobility of the polymer backbone. Conversely for i-PMMA, this resulted in more gauche bonds, and a higher mobility of the polymer chain. The measurement of the glass transition temperature (Tg) of a polymer is also indicative of the freedom of movement of the


McCaffrey et al.

Figure 2. Temperature dependence of the TREPR spectra of the polymeric radical of PMMA with differing degrees of isotacticity. Left hand side: Experimental spectra: (A) 91% mm, 7% mr, and 2% rr dyads at 94 °C, delay time of 0.6 µs. (B) 72% mm, 19% mr, and 9% rr dyads at 90 °C, delay time of 0.3 µs. (C) and (D): expansion of boxed regions of (A) and (B), respectively. (E) Same spectrum as D but at 110 °C, (F) at 120 °C, (G) at 130 °C. Right-hand side: simulations of experimental data. The simulation parameters are given in Table 1. The sweep widths of the spectra in Figure 2A and B are both 150 G.

TABLE 1: Hyperfine Couplings Used in the Simulations in Figure 2 of the Polymeric Radical of Isotactic PMMAa tacticity (%mm)

temperature

β aH(CH3)

β aH(CH2)

β aH(CH2)

91% 70% 70% 70% 70%

94 °C 90 °C 110 °C 120 °C 130 °C

22.9 G 23.0 G 23.0 G 23.0 G 22.9 G

16.7 G 17.0 G 16.75 G 16.65 G 16.6 G

11.2 G 10.9 G 11.15 G 11.35 G 11.3 G

a The line width of all of the simulations is 1.3 G, except in the first entry where it is 1.2 G.

backbone. For bulk syndiotactic material (s-PMMA), this temperature is about 70 °C higher than that of bulk isotactic material (i-PMMA) measured at 115 °C and 45 °C, respectively.9 This is evidence that in bulk material the backbone of s-PMMA is much more rigid than that of i-PMMA. Although these numbers are for bulk samples, similar inferences can be made for the polymers in solution. Our results follow these established trends qualitatively. Bulky Side Chains. Because of the importance of fluorinated polymers in a wide range of new industrial applications, from

dry cleaning to microlithography,15,16 the photochemistry of PFOMA (2) was investigated. Fluorination of the alkyl tail of the ester group increases the rigidity of the polymer chain, decreasing the internal motion of the polymer.17,18 It should certainly be expected that the TREPR spectra of the radical from the photolysis of PFOMA will be much broader than the radical from PMMA. The room-temperature spectrum of radicals formed from PFOMA in solvent FC-70 (a fluorinated hydrocarbon mixture) is shown in Figure 3A. The main feature of this spectrum is the large emissive triplet superimposed on a broad emissive background. The intense triplet comes from the γ- and η-hyperfine couplings of the unpaired electron to the fluorines in the alkyl tail of the oxo-acyl radical, 2b. The broad emissive signal in the background is from the main chain polymeric radical of PFOMA, 2a. As the temperature is increased, features familiar to the polymeric radicals of methacrylates become apparent. As the temperature is raised to 110 °C (Figure 3B-E) the signal from the polymeric radical is not fully converged to the fast-motion spectrum. The background signal in Figure 3B shows more similarity to the room-temperature spectra collected


J. Phys. Chem. B, Vol. 109, No. 21, 2005 10691

Figure 3. TREPR spectra of the polymeric and oxo-acyl radical of PFOMA (2) in FC - 70 (1.72 g in 40 mL). The delay time is 0.3 µs for all spectra. The sweep width is 150 G. (A) 25 °C, (B) 51 °C, (C) 65 °C, (D) 80 °C, (E) 110 °C.

for the PMMA radical 1a (Figure 1A) than to the sharp 21-line spectrum observed at high temperatures. The slow dynamics of the PFOMA main chain radical is reflected in the TREPR spectra by lines that are broadened because of slower motion around the CR-Cβ bond. Clearly, the rigidity of this polymer chain is apparent in the TREPR spectra at all temperatures. We have observed similar effects in the TREPR spectra of the polymeric radical of poly(adamantyl methacrylate) (PAMA), which is shown for comparison to the PFOMA radical at two different temperatures in Figure 4. Large ester side chains such as the adamantyl and fluorinated alkyl group experience a larger amount of hydrodynamic friction than smaller side chains such as methyl and ethyl groups.19 The polymer then undergoes slower internal rotations, and the TREPR spectrum of the main chain radical is broadened. The similarity in these two spectra suggests that the conformational mobility of the polymeric radical in solution plays a large role in the intensity and spectral shape of the TREPR signal from these polymeric radicals and that side chain size and structure can completely prevent access to the fast-motion limit, at least at temperatures below 135 °C. Higher temperatures are not currently available to us because we do this experiment with a high-temperature flow system that, for safety reasons, is limited to a maximum reservoir temperature of 150 °C. The interesting structure and hyperfine couplings of the oxoacyl radical, 2b, in Figures 3 and 4 can be simulated, and this is part of our separate publication on structural characterization of methacrylate radicals with different side chains.2 Briefly, for most acrylic polymers the main chain radical generally exhibits a very intense signal, and the oxo-acyl signal is absent from the TREPR spectrum because of a short spin relaxation time, particularly at high temperatures. However, the opposite relative intensities are observed here. The signal from the oxo-acyl radical, 2b, is much more intense than the polymeric radical, 2a. We suggest two reasons for this anomaly in the intensities of the two radicals from PFOMA: one is that the relaxation time of radical 2b is longer than usual for an acyl radical. This

Figure 4. TREPR spectra of the polymeric and oxo-acyl radical of PFOMA (2) in FC - 70 (1.72 g in 40 mL). The delay time is 0.3 µs for all spectra. The sweep width is 150 G. (A) 51 °C, (B) 110 °C. (C) TREPR spectrum of the main chain polymeric radical of PAMA (3) in methylene chloride at 25 °C. The delay time is 1.0 µs, and the sweep width is 150 G.

is to be expected because the major relaxation mechanism in such radicals is from the spin-rotation interaction. This process will be less effective in structures such as 2b because the radical is larger in size than typical acyl radicals observed in organic photochemical reactions of carbonyl compounds; it will certainly undergo slower molecular rotations than the oxo-acyl radical observed from side chain cleavage of the ester group in PMMA. Another reason for the strong intensity is based on the number of total transitions for the two radicals. Because both radicals acquire the same initial spin polarization from the triplet mechanism, more intensity has to be packed into fewer lines for radical 2b than for the polymeric radical. Below we will present a detailed discussion of the origin of the triplet mechanism polarization and other possible polarization mechanisms because this also offers some insight into the molecular motion of both the parent polymeric structure and the two free radicals produced from it by photolysis. Kinetic Profiles. Kinetic data can be acquired in TREPR experiments by holding the magnetic field constant at a particular resonant transition and measuring the EPR signal as a function of time using a transient digitizer. Figure 5 shows a kinetic decay trace obtained for the polymeric radical of PMMA at room temperature, as well as a trace for the oxo-acyl counter radical. The field-swept spectrum at the top of Figure 5 shows the magnetic field positions at which the kinetic data were collected. There are two noteworthy features of these kinetic traces. One is that the rise times are different for each radical. This seems at first to be contradictory, because two radicals produced from the same bond cleavage reaction should have the same rise time. However, the rise time of a TREPR signal depends not only on the lifetime of the excited-state precursor but also


McCaffrey et al.

Figure 6. TREPR data obtained during photolysis of atactic PMMA in different solvents at ambient temperature. The delay time is 0.6 µs for all of the spectra, and the sweep widths are all 150 G. (A) 1.61 g in 20 mL dichlormethane. (B) 1.53 g in 20 mL chloroform. (C) 1.04 g in 20 mL acetonitrile. (D) 1.20 g in 20 mL THF. (E) 1.49 g in 25 mL dioxane. Figure 5. Kinetic traces obtained during photolysis of atactic PMMA in chloroform (1.5 g in 20 mL), maintining the external magnetic field at the positions indicated in the field-swept spectrum shown at the top. The solid curve represents the decay of the signal from 1a measured at the most intense line right of center field, whose field position is denoted in the spectrum by a solid arrow. The broken line is the decay of the signal from oxo-acyl radical 1b, measured at the field position indicated by the broken arrow.

on the T2 relaxation time of the radical and the B1 field strength used in the experiment. Because the oxo-acyl radical and polymeric radical have different line widths (proportional to T2-1), it is reasonable that for a given microwave power (proportional to B11/2) different rise times are observed and the oxo-acyl structure has the faster rise time. As metioned above, acyl radicals in general have short T2 values due to the spinrotation interaction. A second feature of note in Figure 5 is the faster decay for the oxo-acyl radical, which is also expected because this species should be at the fast-motion limit for all of the temperatures studied here. Chemical reaction is on a much slower time scale than we can typically observe in the TREPR experiment and can be ignored. The curves in Figure 5 have a shape usually described by the expression t × exp(-t/τ) (i.e., where τ ) T1 ) T2). Differences in decay curves therefore generally reflect differences in T1 values, but it should be stressed that this is for the fast-motion limit only. We collected data only for the sharpest and most intense transition of the polymeric radical in Figure 5. These are transitions for which dynamic effects from conformational motion are minimized, but we are almost certainly not in the fast-motion limit for this polymer, and we cannot expect T1 to be equal to T2. This will become very apparent when the solvent effects are presented below. Solvent Effects. The dynamics of polymer chains in solution depend greatly on the solvent.20,21 Unfavorable polymer-solvent interactions cause the polymer to exist in a collapsed form, slowing down its internal motion. Favorable solvent-polymer interactions have the opposite effect, and the polymer will adopt a more extended conformation allowing for greater mobility.22,23

Also, NMR studies of stereoregular polymers in various solvents demonstrate that the conformation adopted by the polymer is highly solvent-dependent.12,13 It is therefore of interest to see if similar effects are observed in the TREPR spectra of acrylic main chain radicals. Figure 6 shows TREPR field-swept spectra of the PMMA polymeric radical, 1a, in five different solvents. At first glance, these spectra look very similar, but there are subtle differences, especially in the broader packets of lines from the polymeric radical. Even the lines not broadened by hyperfine modulation show slightly different line widths and positions. For kinetic measurements, it was desirable to examine only the sharpest lines for the polymeric radical, as discussed above. In Figure 7 kinetic profiles of the TREPR signal of the main chain polymeric radical for each solvent are presented, obtained for the same transition indicated in Figure 5. There are large differences in the decay rates of these signals at room temperature. The decay is fastest in dioxane (A), whereas in methylene chloride (B) and chloroform (C) it appears to be the slowest. Analysis of the decay of TREPR signals can be complicated and has been the subject of many previous investigations.24,25 Qualitatively, we can say the following about the kinetic profiles in Figure 7: (1) the transitions, for symmetry reasons, are not greatly affected by dynamic effects from hyperfine modulation, that is, there are no exchange broadening effects for this line, (2) chemical decay is unimportant on this time scale, and (3) if the decay rates are governed mostly by T1 then we can conclude that we are not in the fast-motion regime. The line widths are almost the same, yet the decay rates are drastically different, that is, T1 * T2. The fact that we do not see the spectrum of the propagating radical from β-scission of the main chain radical or alkyl radicals from decarbonylation or decarboxylation of the acyl fragment is good evidence that we can ignore chemical decay processes. This, and the fact that the shape of the traces is independent of the microwave power, means that T1 relaxation is indeed the most dominant mechanism for the TREPR signal decay.


Figure 7. Kinetic traces of the TREPR signal from polymeric radical 1a in various solvents. (A) Methylene chloride, (B) Chloroform, (C) Acetonitrile, (D) Tetrahydrofuran, (E) Dioxane. The dashed line represents the baseline for each kinetic trace.

The decay time constants for the polymeric radicals clearly vary from solvent to solvent, ranging over an order of magnitude from about 2 µs in tetrahydrofuran (THF) to 200 ns in methylene chloride. A similar solvent dependence of 13C NMR T1 values has been reported by Spyros and co-workers.19 They studied poly(naphthyl methacrylate) using the 13C inversion recovery technique and found that the spin-lattice relaxation time of the polymer varied from 1 µs in chloroform to 5 µs in pentachloroethane. The solvents used to collect the data in Figure 7 represent a much wider range of viscosities and dielectric constants, so it is not surprising that we observe a much larger variation in T1 here. If the mechanism of relaxation is hyperfine anisotropy modulation, as is typical for heavily substituted alkyl radicals, then our results suggest that these polymers experience faster motion in methylene chloride versus THF. It is a bit perplexing that there appears to be no direct correlation between the decay constants and either the viscosity or the dielectric constant. Solvent effects on these spectra continue to be a subject of current interest to us. Spin Polarization Mechanisms and Polymer Motion. In all of our TREPR spectra on acrylic main chain radicals to date, we see only emissive spin polarization from the triplet mechanism (TM) of CIDEP. It is of interest to learn if this is due to many radicals with a small polarization or relatively few radicals but with a very intense TM polarization. Although we cannot directly correlate signal intensities to radical populations because of the polarization, we know that the photodegradation of our samples over the time taken to collect our data is extremely

J. Phys. Chem. B, Vol. 109, No. 21, 2005 10693 slow. This suggests that either the polymers do not absorb very much light or the quantum yield for the production of radicals is quite low. In fact, both situations exist for our samples. The polymer solutions begin to absorb in the UV at about 250 nm, and we are exciting them at 248 nm. We are therefore just on the edge of the n-π* excitation of the ester carbonyl group. Guillet and co-workers26 and others27-29 have measured overall quantum yields of about 0.1 for degradation of acrylic polymers in solution. We conclude that we are not creating very many radicals with each laser flash, but the triplet polarization they carry is extremely intense. For TM polarization to be strong, several physical and magnetic parameters must fall in certain ranges. For example, the cleavage rate should be faster or comparable to the electron spin relaxation time in the excited triplet state, which is typically 1-10 ns for carbonyl triplet states. However, a major factor is the orbital symmetry of the triplet state and its tumbling rate with respect to the external magnetic field. If an excited state tumbles quickly, the intersystem crossing process loses selectivity and all three triplet levels are populated more or less equally from the singlet excited state. The symmetry of the orbitals is not really an issue for carbonyl n-π* states, but the tumbling rate can be very important. We suggest that (1) the polymer rotational motion along the main axis occurs on a slower time scale than rotation of the individual ester side chain groups, (2) this asymmetric motion leads to higher selectivity in the intersystem crossing process, (3) a somewhat slower relaxation time of both the excited triplet state and the ensuing polymeric radical exists for the same reasons, and (4) the overall effect leads to an optimal situation for the creation of TM polarization. A final noteworthy feature of these spectra is the lack of radical pair mechanism (RPM) spin polarization, which for these radicals would appear as low-field emissive, high-field absorptive transitions. It is interesting that such polarization never develops at any delay time, even out to 20 µs where we have observed only TM polarization. The creation of RPM polarization requires re-encounters of radicals on a suitable time scale, and modulation of the exchange interaction between the unpaired electrons. This is normally accomplished by diffusion of the radials between weak and strong exchange regions. The fact that it never develops indicates that either these radicals do not make a significant number of re-encounters or perhaps it is due to the fast spin relaxation in the oxo-acyl radical. It may also be due to the fact that the TM is simply so dominant that the RPM intensity is always much weaker and is never observed. At lower temperatures (cf. Figure 1, top spectrum at 25 °C), there does appear to be a slight superposition of an E/A pattern on top of the emissive TM polarization, but it is a very small effect. This phenomenon is under further investigation at the present time in our laboratory. Summary The TREPR spectra presented in this paper are rich in information about the degree of stiffness of the polymers in solution. Manipulation of polymer tacticity has a subtle but quantifiable effect on chain rigidity in terms of fast-motion hyperfine coupling constants. Bulky side chains have a much larger effect, preventing access to the fast-motion regime for the PFOMA and PAMA polymers. The dynamics of the PMMA in different solvents can be observed by comparing the approximate spin-lattice relaxation rates measured by obtained kinetic profiles at a constant magnetic field. These profiles show a pronounced solvent dependence for the polymeric radical, although the lack of a clear correlation between these decay

10694 J. Phys. Chem. B, Vol. 109, No. 21, 2005 curves and a single solvent parameter suggests that there are competing effects (e.g., viscosity and polarity) in determining the overall conformational energies of the polymeric radicals. The intense triplet mechanism spin polarization is attributed to the strong anisotropic motion of the ester chromophore on the side chain. Acknowledgment. We thank the National Science Foundation for continued support of this work (grant no. CHE-0213516) and the Rohm and Haas Company for their early support of this project. E.J.H. thanks the National Science Foundation for a predoctoral fellowship. References and Notes (1) Harbron, E. J.; McCaffrey, V. P.; Xu, R.; Forbes, M. D. E. J. Am. Chem. Soc. 2000, 122, 9182. (2) McCaffrey, V. P.; Forbes, M. D. E. Macromolecules 2005, 38, 3334-3341. (3) McCaffrey, V. P.; Harbron, E. J.; Forbes, M. D. E. Macromolecules 2005, 38, 3342-3350. (4) Forbes, M. D. E.; Barborak, J. D.; Dukes, K. E.; Ruberu, S. R. Macromolecules 1994, 27, 1020. (5) Sluggett, G. W.; McGarry, P. F.; Koptyug, I. V.; Turro, N. J. J. Am. Chem. Soc. 1996, 118, 7367-7372. (6) Beck, G.; Lindenan, D.; Schnabel, W. Macromolecules 1977, 10, 135. (7) Liang, R. H.; Tasy, F.-D.; Gupta, A. Macromolecules 1982, 15, 974. (8) Apel, U. M.; Hentschke, R.; Helfrich, J. Macromolecules 1995, 28, 1778. (9) Johnson, J. F.; Porter, R. S. In The Stereochemistry of Macromolecules; Ketley, A. D., Ed.; Marcel Dekker: New York, 1968; Vol. 3, Chapter 5. (10) O’Reilly, J. M.; Teegarden, D. M.; Wignall, G. D. Macromolecules 1985, 18, 2747.

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