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VI. Factors Controlling the Rate of Polymer Photochemical Degradation in the Solid State

VI. Factors Controlling the Rate of Polymer Photochemical Degradation in the Solid State

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274



Polymers with Metal-Metal Bonds as Models

O

O C

C

Mo

C

O C C

O O



CO



Mo



h␯



C-Cl bond in

polymer 2

Mo

C

O C C

O O



2



Mo Cl

C

O C C

O

O



3



SCHEME 7. Photochemical reaction of a polymer with metal-metal bonds along its

backbone.

1.4

1.2



Cp'2Mo2(CO)6 dispersed in PVC

Polymer 5

Cp'2Mo2(CO)6 in hexane/CCl4



Quantum Yield



1.0

0.8

0.6

0.4

0.2

0.0

10



20



30



40



50



60



Temperature, °C



FIGURE 3. Plots of the quantum yields for disappearance of the Cpu2Mo2(CO)6 unit

in polymer 5, Cpu2Mo2(CO)6 dispersed in PVC, and Cpu2Mo2(CO)6 in hexane/CCl4

(Cpu 5 η5-C5H4CH3).



The temperature dependence of the quantum yields for the degradation

of polymer 5 could depend on (1) the inherent temperature dependence of

the photolysis and radical trapping reaction of the Cp2Mo2(CO)6 unit, (2) the

temperature-dependent behavior of the polymer morphology, or (3) a

temperature-dependent dynamical property of the photogenerated radicals in

the polymer. To differentiate among these possibilities, two control experiments were carried out—namely, the photolysis of Cpu2Mo2(CO)6 (Cpu 5 η5C5H4CH3) dispersed in a PVC polymer matrix and the photolysis of

Cpu2Mo2(CO)6 in hexane/CCl4 solution. The quantum yields for the disappearance of the Cpu2Mo2(CO)6 unit in polymer 5, for Cpu2Mo2(CO)6

dispersed in PVC, and for Cpu2Mo2(CO)6 in hexane/CCl4 solution are plotted

versus temperature in Figure 3. Note that all of the solid-state data were collected below the glass-transition temperatures of the polymer films (Tg 5 65À72  C).

The plots showed there is a significant increase in the quantum yields for



Factors Controlling the Rate of Polymer Photochemical Degradation



275



polymer 5 with increasing temperature. In contrast, for Cpu2Mo2(CO)6 dispersed in PVC and for Cpu2Mo2(CO)6 in hexane/CCl4 solution (in which the

Mo-Mo chromophores are unattached to the polymer chains) there are only

slight increases in the quantum yields over this temperature range.

An immediate conclusion from the preceding data is that the large

increase in the quantum yield with temperature for polymer 5 is not attributable to an inherent temperature dependence of the photolysis and subsequent

radical trapping reaction of the Cpu2Mo2(CO)6 unit. (Otherwise, the quantum

yields for Cpu2Mo2(CO)6 in the hexane/CCl4 solution would also show a sizable

temperature dependence.) Also, because the quantum yields for Cpu2Mo2(CO)6

dispersed in PVC showed only a slight temperature dependence, the temperature dependence observed for polymer 5 cannot be ascribed solely to changes in

PVC morphology. (Otherwise, the Cpu2Mo2(CO)6 dispersed in PVC and

polymer 5 would show a similar temperature dependence because the

morphologies of PVC and polymer 5 are similar in regard to crystallinity,

modulus (1300 6 100 vs. 1200 6 50 MPa), and glass-transition temperature

(65 6 4 vs. 72 6 3 C).) As an aside it is noted that, for the Cpu2Mo2(CO)6 dispersed in PVC, the small increase in quantum yields with increasing temperature

was attributed to an increase in the free-volume.2 This explanation was based on a

suggestion by Guillet to explain a similarly small temperature dependence in the

quantum yields for degradation of poly(vinyl ketone), PVK.53

To get better insight into what parameter is controlling the temperature

dependence of Φ in polymer 5, it was necessary to look in more detail at

the mechanism of Cpu2Mo2(CO)6 photolysis and the subsequent radical capture

reaction. (Recall from the discussion above that the increase in Φ with increasing

temperature in polymer 5 cannot be attributed to either an intrinsic temperaturedependent reactivity property of the Cpu2Mo2(CO)6 molecule or to changes in

PVC morphology.) At 25.4 C, the quantum yields for disappearance of the MoMo chromophore were reported as follows: polymer 5, Φ 5 0.20; Cpu2Mo2(CO)6

dispersed in PVC, Φ 5 0.07; and Cpu2Mo2(CO)6 in hexane/CCl4 solution,

Φ 5 0.35. As is normal in such comparisons, the quantum yield in solution is

considerably higher than that for Cpu2Mo2(CO)6 dispersed in the PVC polymer

because the solution state is considerably less viscous than the solid state. Perhaps

surprising, however, is the much larger quantum yield for polymer 5 (0.20)

compared to Cpu2Mo2(CO)6 dispersed in the PVC polymer (0.07). The substantial difference in the two quantum yields was proposed to be attributable to a

difference in the radical-radical recombination efficiencies (the “cage effect”)54 in

the two polymers. The cage effect is illustrated in Scheme 8, which shows the



M M



h␯

kc



M , M



kd



2



M



2 RCl



2



M Cl ϩ 2 R



radical cage pair



SCHEME 8. Reaction of a photoreactive species to form a caged radical pair followed

by a radical trapping reaction.



276



Polymers with Metal-Metal Bonds as Models



elementary steps involved in the photochemical generation of metal radicals and

their subsequent capture reactions with a trapping molecule.

It was proposed that the temperature dependence of polymer 5 arises from

the temperature dependence of the kd step. Specifically, it was suggested that the

polymer segments to which the radicals are attached are conformationally

stressed. There are two possible modes for the newly formed radicals to relax and

become separated: They can rotate or recoil away from each other (Scheme 9).

These secondary motions of the polymer arise from the relaxation of unfavorable

bond conformations that are formed during the polymer casting process. The

increased thermal energy facilitates the rotation and recoil relaxation processes,

which effectively increases the rate constant for diffusion of the radicals out of the

cage, kd. This leads to decreased radical-radical recombination and consequently

an increase in photodegradation efficiency.

The quantum yield data for polymer 5 in Figure 3 has an exponential

dependence on the inverse temperature, and activation parameters were



O

O



O



O

C



O

C

Mo Mo CO



OC

C

O C

O



O

C



O

C

Mo Mo CO



O



h



OC

C

O C

O



O

O



Radical separation by

secondary chain movements

(recoil or rotation)



Mo

O

O



O

O



kd



CO

CO

CO

O

C



O

C

Mo CO



O

O



SCHEME 9. Pathway for the increased separation efficiency of the radicals formed by

irradiation of polymer 5. A rotation process is shown, but radical recoil will also lead to

increased radical-radical separation. Relaxation of the polymer chains leads to an

increase in kd and a subsequent increase in the quantum yield for degradation.



Factors Controlling the Rate of Polymer Photochemical Degradation



277



0.4



0.0



ln⌽



Ϫ0.4



Ϫ0.8



Ϫ1.2



Ϫ1.6

3.12



3.16



3.20



3.24



3.28



3.32



3.36



1000/T (KϪ1)



FIGURE 4. Plot of ln Φ versus T 21 for polymer 5.



extracted from the natural log plots of quantum yield versus inverse temperature (Fig. 4). The relationship between the temperature and activation

parameters in a photochemical reaction is a complex one,55 and the apparent

activation energies thus obtained were interpreted with care. The activation

energy obtained from the lnΦ versus T 21 plot (Fig. 4) was 14.1 6 0.3 kcal

mol21. This value is typical for secondary relaxation chain movements in

polymers (which generally fall in the range of 10À20 kcal mol21) 56À58 and is

consistent with the proposal that the temperature dependence of Φ results from

chain movements involved in recoil and rotation processes.

In summary, the quantum yields for degradation of the Cp2Mo2(CO)6

unit in a polymer chain are strongly temperature dependent. When a polymer

chain is cleaved at the Mo-Mo bond, the chains relax by secondary chain

movements. It was proposed that increased thermal energy facilitates

the rotation and recoil relaxation processes, which effectively increases the rate

constant for diffusion of the radicals out of the cage, kd. In effect, the cage

recombination efficiency is decreased, and this leads to an increase in the efficiency of degradation. The apparent activation energy obtained from the

temperature dependence of the quantum yield of polymer 5 (14.1 6 0.3 kcal

mol21) is consistent with secondary relaxation chain movements in polymers.



B. Interpreting the Kinetics of Polymer Degradation

in the Solid State

Concentration versus time data for the photodegradation of polymer 7 as

a function of irradiation time are shown in Figure 5.59 (The data in the figure

were normalized by dividing the concentration values by the initial



278



Polymers with Metal-Metal Bonds as Models



concentration.) Note that the traces in the figure exhibit biphasic character,

showing a relatively fast rate during the first 1À2% of the reaction but a slower

rate at longer times. Typically, photochemical reactions are simple zero-order

reactions. (The reaction is zero-order after the initially high reaction rate

during the first 1À2% of the reaction.) The plot in Figure 5 does not fit

first-order kinetics (C/C0 5 Ae2kt), second-order kinetics, or the kinetics for

any of several diffusion models. Instead, the decay data were shown to fit a

model based on so-called Perrin kinetics.60,61 The Perrin kinetics model was

originally proposed to explain the nonexponential phosphorescence decay in

solid polymers, but the kinetics also apply to the trapping reactions of radicals

in the solid state. The key feature of the model is that, for phosphorescence

decay, when an acceptor is in the quenching sphere of an electronically excited

donor molecule, the fluorescence will be quenched. The observed rate of

phosphorescence decay is therefore a combination of the decay rate of excited

molecules in the presence of the quencher and the natural decay rate of

molecules in the absence of the quencher. A mechanistic analogy can be made

for photogenerated radical species in solid-state polymers: The observed rate of

radical decay will be the combination of the rate when a radical trapping agent

is in the reactive sphere of the radical and the rate when no radical trapping

agent is present. (The term reactive sphere is equivalent to the term quenching

sphere used in the case of the original Perrin model.) Under such conditions,

the concentration of reactive species is given by equation 27.

O H

H O

C N R' N C O R" O



n



7

CH2Cl

R' =



O

O

N C OCH2CH O C N

m

H

H

CH3



and H3C



R" =



and



CH3



(CO)3Mo Mo(CO)3



ẵA ẳ X0 ỵ k1 t ỵ Y0 ek2 t



27ị



The Perrin-like expression in equation 27 was used to fit the kinetic decay trace

of polymer 7; the fit is illustrated in Figure 5.

The biphasic kinetics curve for the reactions of polymers is very typical and is

found frequently in the polymer literature. Daglen and Tyler showed62 that equation

27 gave excellent fit to these systems as well, which suggests the presence of reaction

spheres is common in the mechanism of solid-state polymer photodegradation.



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