Ultraviolet Light Induced Reactions in Polymers

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Downloaded by UNIV OF MASSACHUSETTS AMHERST on October 13, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch020

Photodegradation of Styrene-Vinyl Ketone Copolymers M. HESKINS EcoPlastics Ltd., 201 Consumers Rd., Willowdale, Ontario, Canada M2J 4G8 T. B. McANENEY* and J. E. GUILLET Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1A1

The photolysis of copolymers containing ketone groups has both academic and practical interest since the way in which the polymeric environment affects the photochemical pathways leads to an understand ing of the photodegradation of polymers in which the ketone group is present as an adventitious or intended impurity. Copolymers with vinyl ketones also provide a practical means for preparing plastics with controlled lifetimes as a means of combatting litter problems (1-3). In this report we examine the effects of several vinyl ketone monomers on the photodegradation of polystyrene in the solid phase. Previous work (4, 5) has indicated that copolymers containing vinyl ketones undergo photolysis by the Norrish type I and type II primary reactions. Studies by Golemba and Guillet (6) and by Kato and Yone shiga (7) have shown that these processes also occur in copolymers of styrene with methyl vinyl ketone and with phenyl vinyl ketone.

It can be seen that the type Π reaction will break a C—C bond in the chain backbone, while the type I reaction will produce two free radicals without primary main chain scission. In the presence of air these radi cals initiate photooxidation and therefore cause chain scission by sec* Present address: INCO Research, Sheridan Park, Ontario, Canada. 281 In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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ondary reactions. The effect of ketone structure on the relative rates of these reactions is the subject of this investigation.


Experimental Preparation of Monomers. Methyl vinyl ketone (MVK) was ob tained from Pfizer Chemical Division, New York, and distilled to r e move the inhibitor. Methyl isopropenyl ketone (MIPK) was prepared by the aldol condensation of methyl ethyl ketone and formaldehyde, accord ing to the method of Landau and Irany (8). The major impurity in this monomer is ethyl vinyl ketone (5%). The monomer was redistilled be fore use. 3 Ethyl 3 buten 2 one (EB) was prepared by the aldol conden sation of methyl propyl ketone and formaldehyde. Ethyl vinyl ketone (EVK) was prepared by a Grignard synthesis of the alcohol, followed by oxidation to the ketone. t-Butyl vinyl ketone (tBVK) was prepared from pinacolone and formaldehyde by the method of Cologne (9). Phenyl vinyl ketone (PVK) was prepared by the dehydrochlorination of β chloro propiophenone (Eastman Kodak). Phenyl isopropenyl ketone (PPK) was prepared by the Mannich reaction using propiophenone, formaldehyde and dimethylamine HC1. These monomers have a tendency to dimerize on standing at room temperature and were kept at 0°F. A l l the monomers were at least 95% pure, except the MIPK which contained 5% EVK. The purity of this monomer was 93%. Copolymers. The copolymers were prepared by emulsion polym erization in a "pop bottle" polymerizer at 80° C using sodium aryl alkyl sulphonate (Ultrawet K) as emulsifier and ammonium persulphate as catalyst. The polymerizations were carried to greater than 95% con version (4-8 hr) and the feed concentrations of the vinyl ketones were taken as their concentrations in the polymer. Copolymers containing 1% and 5% by weight of vinyl ketones were prepared. The copolymers were not homogeneous in ketone concentration since the reactivity ratios indicated that the vinyl ketones would be used up before the end of the polymerizations. Thin films were prepared for irradiation by compres sion molding in a Carver Press at 150° C at 20,000 psi. The films were usually 0.22 mm thick. Blends were prepared by solution blending the copolymers with a Dow general purpose molding grade (667) polystyrene, casting from solution and then compression molding film as for the copolymer. Irradiation. The films were irradiated in air in an American Ultraviolet Co. UV Accelerometer. This uses a medium pressure mercury arc lamp enclosed in Pyrex and its major emission is at 313 nm together with less intense light at 298 and 366 nm. The lamp runs at about 40° C. The amount of near ultraviolet light absorbed by the

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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films during the tests is estimated to be in the order of 4 χ 10"' einstein/ sq cm/hr. The degradation of the copolymers or blends was followed by the change in their solution viscosity in toluene at 30° C. The viscosity average molecular weights were calculated from the equation (10), 0

7 2 5

[η]= l . l x l O ^ M The number of breaks/chain is given by [(M )/(M )] - 1, where M ^ is the initial number average molecular weight and M the number average molecular weight after irradiation. The total number of chain breaks is then


n

n

n

n

where g is the weight of irradiated sample. The results are reported here as [(1/M ) - (1/M ^)] against time of exposure. Although a stand ard size sample is used, there is an error involved in using viscosity average rather than number average. However, since it was not pos sible to measure the light absorbed, we have not attempted to measure quantum yields. The expression is used to counteract the effect of dif fering initial molecular weights of the copolymers. V

V

Results and Discussion The photodegradation of 0. 22 mm thick films of styrene/1% MVK and styrene/1% MIPK copolymers in the Accelerometer are shown in Fig. 1. The MVK copolymer degrades significantly faster than the MIPK copolymer. The difference in structure in the ketone units of the copolymer is that the MIPK copolymer has the acetyl group attached to a tertiary carbon on the chain, while in MVK this carbon is secondary. Studies with simple ketones of precisely this structure, i . e., acetyl group attached to secondary or tertiary carbon which can also undergo a type H reaction, have not been made. However, studies with related methyl ketones (Π, 12) lead to the expectation that the MIPK copolymer will have a somewhat higher type I yield and lower type II. In the i r radiation with high intensity artificial UV sources it is expected that any photooxidation will be minimized and chain scission will be principally due to the type II reaction. Figure 2 shows the effect of outdoor ex posure where, although the breakdown of the MVK copolymer is initially faster, it does not maintain its rate and the MIPK copolymer eventually achieves a greater degree of chain scission. This can be ascribed to the continuing effect of photooxidation initiated by the type I free radicals in the latter case.



ι


τ

Hrs

UV

ι

Γ

Accelerometer

Figure 1. Photolysis of PS containing 1% MVK and 1% MIPK

τ

3

1

1

Γ

6 9 Exposure, weeks

12

Figure 2. Weathering of copolymer of 5% MVK and 5% MIPK with styrene in Arizona



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tert-Butyl vinyl ketone copolymers would be expected to show a low type Π yield since this is observed for simple ketones of analogous struc ture. The 1% tBVK copolymer does indeed give a low rate of degradation compared to a similar MVK copolymer, as shown in Fig. 3. The other two monomers chosen have an ethyl group replacing a methyl group, either in the acyl group (EVK) or on the a carbon (EB). The photolysis of these comonomers, when copolymerized at the 1% level with styrene, are intermediate between the methyl substituted analogs MVK and MIPK, as shown in Fig. 4. The photodegradation of a 1% PVK copolymer is compared with the 1% MVK copolymer in Fig. 5. Previous studies (6,1) have indicated that the PVK copolymer has a significantly higher quantum yield for type II reaction than MVK and the ketone group has a higher extinction coefficient in the UV region. It would therefore be expected that a faster rate of chain scission would be obtained for the PVK copolymer than the MVK. This is indeed true during the early stages of the reac tion, but the rate of degradation slows down rapidly at longer exposure times. The reactivity ratios for PVK-styrene are not favourable for a random distribution of the phenone groups. In fact, calculations show that, with all monomer charged initially, nearly all the PVK is incor porated in the first 20% of polymer produced. Since chain scission is concentrated in this fraction of the copolymer, viscosity will give a poor indication of the total number of chain scissions in this case. The PIPK copolymer showed a somewhat similar effect. At high degrees of de gradation there is also a color-forming reaction (related to photooxida tion) which makes the polymer opaque to UV radiation. This also con tributes to the retardation of the photolytic process. Blends. The type I reaction produces free radicals which, in the presence of oxygen, initiates photooxidation which also results in a de crease in the polymer molecular weight. An indication of the relative importance of the type I reaction in these systems can be estimated from the amount of chain scission induced in a blend of the copolymers with homopolymer polystyrene. For these experiments, one part of 5% vinyl ketone copolymer was blended with four parts of styrene homopolymer to retain an overall ketone monomer concentration of 1%. Figure 6 shows the rates of degradation for four of the copolymers. This figure shows that the overall rate is greater for MIPK than for MVK copolymer blends. It should be noted that for all these blends, the rate of degradation is greater than can be attributed to the break down of the copolymer in the blend. Under the conditions of the test the value of [(1/M) - (1/MQ)] for the MVK copolymer blend would not rise above 1 χ 10" if no sensitization had taken place. This was de termined by irradiating the 5% MVK copolymer and the homopolymer and then preparing a solution of the degraded polymers in the correct 6




Hrs

UV

Accelerometer

Figure 3. Photolysis of 1% PVK copolymer andl% MVK copolymer

Figure 4. Photolysis of 1% tBVK copolymer andl% MVK copolymer




HESKiNs E T A L .

Figure 6. Photolysis of 4:1 blends of polystyrene with polystyrene 5% vinyl ketone copolymers




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proportions prior to measuring the solution viscosity. The greater total degradation in the case of the MIPK copolymer is attributed to the i n creased photooxidation initiated by a higher type I in this system than in the MVK system. This affect is even more marked for the tBVK co polymer where the rate is comparable to that of MVK, even though the molar concentration of ketone groups is only one-half as great. The PVK copolymer blend shows a lower rate and less sensitization of degradation in the homopolymer. The PVK copolymer is known to have a low yield of type I reaction (6) and this supports the idea that the sensitization of photooxidation is due to free radical forma tion rather than more elaborate mechanisms such as via formation of singlet oxygen. It also appears that the hydrogen abstraction reaction of excited phenone groups (13) which is the basis of the use of benzophenone as a prodegradant in polystyrene (14), is not as important in this system. The determination of quantum efficiency of the photochemical reactions has not been attempted because it is difficult to separate definitely the effects of the type I and type II reactions. However, on the assumption that the type II dominates in copolymer photolyses and that the type I dominates in the photolyses of blends, the results appear to be in accord with expectations based on the photochemistry of related simple molecules. Acknowledgements The authors wich to acknowledge the financial support for this r e search by EcoPlastics Limited and from the National Research Council of Canada under a P.R.A.I, grant number S-22. We also wish to thank Dr. H. G. Troth and members of the staff of Van Leer Research Labora tories, Passfield, England, for helpful discussions.

Literature Cited 1. Guillet, J. Ε., in "Polymers and Ecological Problems", J. Guillet, ed., Plenum Publishing Corp., New York, 1973. 2. Guillet, J. E. and Troth, H. G., U.S. Patent 3, 860, 538. 3. Guillet, J. Ε., U.S. Patent 3, 753, 953. 4. Guillet, J. E. and Norrish, R. G. W., Proc. Roy.Soc.,(1955) A233, 153. 5. David, C., Demarteau, W. and Geuskens, G., Polymer, (1967) 8, 497. 6. Golemba, F. J. and Guillet, J. E., Macromolecules, (1972) 5, 212. 7. Kato, M. and Yoneshige, Υ., Makromol. Chem., (1973) 164, 159. 8. Landau, E. F. and Irany, E. P., J. Org. Chem., (1947) 17, 422. 9. J. Colonge, Bull. Soc. Chim., (1936) 3, 2116.


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10. Danusso, F. and Moraglio, G., J. Polym. Sci., (1957) 24, 161 11. Golemba, F. J. and Guillet, J. E., Macromolecules, (1972) 5, 63. 12. Nicol, C. H and Calvert, J. G., J. Amer. Chem.Soc.,(1967) 89 1790. 13. Turro, N. J., "Molecular Photochemistry", W. A. Benjamin, New York, 1965, Ch. 6. 14. Ohnishi, Α., Japanese Patent 71 38, 687.


Ultraviolet Light Induced Reactions in Polymers

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