Investigation of the photolysis of polyurethanes based on 4,4′-methylene bis(phenyldiisocyanate) (MDI) using laser flash photolysis and model compounds

Polymer Degradation and Stability 25 (1989) 325-343

Investigation of the Photolysis of Polyurethanes Based on 4,4'-Methylene Bis(phenyldiisocyanate) (MDI) Using Laser Flash Photolysis and Model Compounds C. E. Hoyle, K. S. Ezzell, Y. G. No, K. Malone & S. F. Thames Department of Polymer Science, University of Southern Mississippi, Southern Station Box 10076, Hattiesburg, MS 39406-0076, USA (Received 18 October 1988; accepted 24 October 1988)

ABSTRACT Mechanistic evidence is given for the primary pathways leading to the primary photochemical reactions in aromatic diisocyanate based polyurethanes. Laser flash photolysis studies on 4.4'-methylene bisi phenyldiisocyanate ) ( M DI) based polyurethanes and appropriate model compound analogs are presented. Transient species are produced by both direct photolysis (248 nm ) and indirect photolysis (351 nm ) via tert-butyl peroxide generated radicals in solution. Results present strong evidence supporting a dual mechanism for photodegradation involving both hydrogen peroxide formation and photo-Fries rearrangement.

INTRODUCTION The photolytic decomposition of polyurethanes based on aromatic diisocyanates is a quite complicated subject which has received a considerable amount of attention spanning almost three decades. 1 - 2 1 To attempt to ascribe the photodegradation processes to a single, concise pathway is impossible due to the many photochemical processes available to excited state urethane moieties. In addition, the resultant primary photoproducts are themselves subject to photochemical reactions. In a series of insightful mechanistic reports Gardette and Lemaire 1 S - 1 7 presented convincing evidence for a dual mechanism leading to the photodecomposition of aromatic diisocyanate based polyurethane elastomers (Scheme I). By combining UV and FT-IR analysis of a photolyzed 4,4'-methylene 325 Polymer Degradation and Stability 0141-3910/89/$03'50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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Laser flash photolysis studies on M Dl-based polyurethanes

327

bis(phenyldiisocyanate) (MDI) based polyurethane, they concluded that both a primary photooxidation process to yield a hydrogen peroxide (Step A) as well as a photo-Fries rearrangement (Step B) were in part responsible for the degradation process. (In addition to the hydrogen peroxide shown in Scheme I, Gardette and Lemaire identified other hydroperoxides (not shown) centered on the aliphatic ether or ester portion of the polyurethanes.) The distribution between the photo-Fries product and the hydrogen peroxide products was found to be dependent upon the wavelength of excitation employed for photolysis, the photo-Fries product being dominant when irradiation was less than 33Q-340nm. This paper is designed to present mechanistic evidence for the primary pa thways leading to the initial photolysis reactions in aromatic diisocyanate based polyurethanes. In order to accomplish this task we have employed laser flash photolysis to aid in the elucidation of some of the mechanistic details associated with polyurethane photochemistry. In this report, we limit ourselves to a basic study of the urethane moiety rather than focusing on the polymer structure in its entirety. We do, however, address to a limited extent some of the complicating reactions which may be interjected by the presence of impurity chromophores in the polymer.

EXPERIMENTAL Materials

4,4'-Methylene bis(phenyldiisocyanate) (MDI, Mobay) was recrystallized from cyclohexane prior to use. 4,4'-Methylene bis(cyclohexyl-diisocyanate) (S-MDI, Mobay) and aniline (V) (Aldrich) (for structures see Results and Discussion below) were purified by vacuum distillation. The MDI/S-MDI polyurethane (I) was synthesized by reacting a mixture of MDI and S-MDI in a 10/90 ratio with 1,4-butanediol. Syntheses of propyl N-phenyl carbamate (II), the bis(propyl carbamate) of MDI (III) and propyl N-tolyl carbamate (VI) have been described previously.r? Synthesis of bis(4,4'-ethyl N-phenyl carbamate)-2,2-propane (IV) was accomplished as follows: 75 g (0'579mol) of aniline hydrochloride was placed in a 250 ml round bottom flask equipped with mechanical stirrer, addition funnel and condenser. The aniline .hydrochloride, was heated to 2300 e and 5·6 g (0'0966 mol) acetone was added dropwise over O' 5 h. The temperature was allowed to decrease to 1900 e as the reflux of acetone and water began. The reaction was allowed to run at 190-200°C for 5 h. The molten reaction mixture was poured into 600ml of 45% NaOH solution. The organic layer was separated, washed twice with water and excess aniline

328

C. E. Hoyle, K. S. Ezzell, Y. G. No, K. Malone, S. F. Thames

removed in vacuo to yield a brown solid. This was subsequently recrystallized from benzene at 8°C and sublimed at lOO°C/O'l mm to yield 16'2g (74%) of a white solid identified as bis(4,4'-aniline)-2,2-propane (melting point = 133-134°C, Lit. reports 1332 2 ) . To 1·89g (0'008 36 mol) of bis(4,4'-aniline)-2,2-propane in a 15ml round bottom flask, 3 m1 of dioxane was added. Then 2 ml ofethyl chloroformate was added dropwise, forming a precipitate. The solution was refluxed under N 2 for 2 h at which time the precipitate had dissipated. After cooling, the solution was poured into 20 ml of water to yield a precipitate. This was recrystallized from EtOH at - 5°C to yield 2·6 g (85%) of (IV) (melting point = 169-170°C). Instrumentation

The laser flash photolysis unit consists of a Lumonics HyperEx 440 Excimer Laser, an Applied Photophysics Xenon Iamp/monochromator/Plvl"T/autooffset probe, a Tektronix 7912 transient digitizer, a micro-PDP 11 computer (Digital Equipment Corporation), and an Applied Photo physics control unit. The laser was operated in the charge on demand mode. The laser was operated at either 248 nm (KrF) or 351 nm (XeF). Nominal outputs were 80mJ/pulse at 248 nm and 60mJ/pulse at 351 nm. Absorption spectra of transient intermediates were constructed point-by-point from decay plots taken at specified wavelength intervals. In the laser flash experiments, all samples were contained in a 1-cm x 1em x 3-cm quartz fluorescence celL In order to ensure oxygen-free samples, solutions were bubbled with nitrogen prior to the experiment. In the oxygen quenching experiments, solutions were bubbled with oxygen before the experiment. The concentrations of all solutions, with absorbances of about 1·2 at 248 nm, were determined by a Model 20 Perkin-Elmer UV/Visible spectrophotometer. In the case of indirect photolysis, the concentration of tert-butyl peroxide was adjusted to provide an absorbance of 0·4 at 351nm. The photolysis ofpropyl N-phenyl carbamate (II) was accomplished using a Southern New England Ultraviolet Company RPR-100 Rayonet'" photochemical reactor with 253'7 A lamps. RESULTS AND DISCUSSION We have recently tentatively identified (Table 1) several of the transient species present in the laser flash photolysis of an MDI based polyurethane in solution. 2 3 ,2 4 The present paper provides corroborative evidence for the earlier assignments and extends our previous laser flash photolysis investigation. Before turning to model compound studies, we will first

Laser flas h photolysis s tudies on M Dl-based polyu rethanes

329

TABLE 1 Proposed Transient Species Formed by Laser Fla sh Photolysis of an MDI Based Pol yurethane in Solution Peak wavelength

Proposed transient

maximum 310nm 350nm 370nm

390-4lOnm ",,440nm

present spectra of a polyurethane conta ining the arylcarbamate moiety unde r consideration. Structures of the primary polyurethanes and model carbamates used in this study are shown below with appropriate designations.

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330

C. E. Hoyle, K. S. Ezzell, Y. G. No, K. Malone, S. F. Thames

Laser flash photolysis of an MDI/S-MDI based polyurethane (I) In our previous study, we presented the transient absorbance spectrum of a

polyurethane elastomer based on MOl and a combination of a low molecular weight diol and a high molecular weight polyol.24 A complicating factor was the necessity for running laser flash photolysis of this system in tetrahydrofuran which has a finite absorbance at 248nm where the KrF excimer laser emits. This, of course, limited our analysis. We therefore, synthesized a polyurethane (I) based on a 10/90 mixture of MDl, methylene bis(4-cyclohexyl-isocyanate) (S-MOl), and l,4-butanediol, which is soluble in dichloromethane in low concentrations. The transient absorption spectrum of (I)is shown in Fig. 1in nitrogen saturated dich1oromethane. On the 2 fJS time scale of the spectrum in nitrogen, peaks at '" 310 nm, '" 340nm, '" 370 nm, and >- 430 nm are prominent. The spectrum in the present case in dichloromethane is comparable to that previously reported for an MOl based polyurethane elastomer in tetrahydrofuran except for a red-shift of about 20-30 nm of the peak above 400 nm in dichloromethane. This most likely represents a contribution from a radical cation species which is also listed in Table 1. The 340 nm peak in Fig. 1 is somewhat difficult to separate on this time scale (2 J1.s) since it is masked by the larger peaks at 310 om and 370 nm. Although this peak (340nm) was noted previously, positive identification was not provided. This will be accomplished by the model compound study of an appropriate aryl carbamate in the following section.

300

350

400 WAVELENGTH [nm]

450

Fig. 1. Transient absorption spectrum (2'0 IlS) of MDlJS-MDl polyurethane (I) in nitrogen saturated CH 2C1 2 (;lex = 248nm).

Laser/lash photolysis studies on M'Dl-based polyurethanes

331

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Fig. 2. Steady state UV spectra of 5 x 1O-4M propyl-N-phenyl carbamate (II) in CH 2C12 : (A) before photolysis, (B) after photolysis (Rayonere, 254 nrn, 15min, air).

Laser flash photolysis of propyl-N-phenyl carbamate (II)

Figure 2 shows the steady state UV spectra of a 5 x 10 -4 M dichloromethane solution of compound (II) before (Curve A) and after (Curve B) photolysis. The band with a maximum at about 340nm in Fig. 2, Curve B, is well established to be due to an ortho photo-Fries product. 8,9,18.20 Figures 3 and 4 show the transient absorption spectrum (50 /-lS after laser pulse) of (II) (5'0 x 1O-4M in dichloromethane) in both nitrogen and oxygen atmospheres. Although the peak at 340 nm is clearlypresent in the nitrogen saturated sample (Fig. 3), it is not as distinct as it appears in Fig. 4 for the oxygen saturated sample. Apparently oxygen is effective in quenching the transient spectrum of the species responsible for absorbance between 300 and 330nm, but does not quench the peak at 340 nm. This is consistent with the formation of a stable product at 340 nm at very early times which is therefore unaffected by the introduction of oxygen, i.e. it is a stable photoproduct which is formed on a time scale much shorter than 50 J1.S. This presumption is readily verified by examination of Figs 5 and 6 which show transient spectra of (II) (nitrogen and oxygen saturated solutions) on four time scales spanning two orders of magnitude. These spectra were reconstructed from decay curves taken every 5 nm. From Fig. 5 (nitrogen saturated solution), one can readily see the decay with time of the peaks at ""' 300-320 nm and above 400 nm. The peak at 340 nm does not show such a marked decrease,

C. E. Hoyle, K. S. Ezzell, Y. G. No, K. Malone, S. F. Thames

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Laser flash photolysis studies

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MDI-based polyurethanes

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C. E. Hoyle, K. S. Ezzell, Y. G. No, K. Malone, S. F. Thames

334

and in fact the loss in intensity of this peak is probably due to the reduction in the tail of the peak at 30G-320nm. The transient spectra in Fig. 6 are perhaps even more revealing of the species responsible for the peak at 340 nm. It is quite obvious that the peak at 340 nm is insensitive to oxygen and has effectively the same intensity at 5 us and 500 us, indicative of stable product formation. By comparison with the UV absorption spectrum of the ortho photo-Fries product in Fig. 2, Curve B, it is obvious that the 340 nm peak in the transient spectrum of (II) (Figs 5 and 6) and the polyurethane in Fig. 1 results from a photo-Fries product which is formed on a sub-microsecond time scale. Indeed, it has been reported that the ortho photo-Fries product forms on a nanosecond time scale and is insensitive to oxygen.i" Laser flash photolysis ofthe bis(propyl carbamate) of MDI (III) and bis(4,4'ethyl-N-phenyl carbamate)-2,2-propane (IV) Having provided evidence for the structure responsible for the 340 nm peak, we turn to a more detailed analysis of the 370 nm peak which appears in Fig. 1 for polyurethane (I) in dichloromethane. Figures 7 and 8 show transient spectra for compound (III) in nitrogen and oxygen saturated solutions at several time intervals after laser firing. The obvious difference in the two sets of spectra is the quenching by oxygen of the 370 nm peak, which

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Fig. 7. Transient absorption spectra of 2 x 1O-4M bis(propyl carbamate) of MDI (III) in nitrogen saturated CH 2Cl 2 (A.x = 248 nm): (A) 5 ps, (B) 50 j.ls, (C) 100/ls, (D) 400 ps.

Laser flash photolysis studies on M Dl-based polyurethanes

335

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in the presence of oxygen (Fig. 8) is absent even at the earliest time (S !is). Apparently, as already reported for MDI based polyurethanes.Pv'" oxygen rapidly reacts with the diarylmethyl radical to yield the peroxy radical which abstracts a hydrogen atom from the medium to give the hydrogen peroxide shown in Scheme 1. One can also see, if comparison is made of the transient spectra recorded at identical times in oxygen and nitrogen, that the region between 300 and 330 nm is also quenched by oxygen, but certainly to a lesser degree. The results in Figs 7 and 8 are confirmed by direct comparison of the transient decay curves recorded at 310nm (Fig. 9(a) and (b)) and 370nm (Fig. lO(a) and (b)) in nitrogen and oxygen saturated dichloromethane. Even though oxygen results in shorter lifetime(s) for the transienus) absorbing at 310nm, the contrast with the dramatic effect of oxygen on the decay of the transient absorbing at 370 nm (Fig. 10) is striking, once again underscoring the rapidity with which oxygen reacts with the diarylmethyl radical. One final note concerning the transient decay curves in Figs 9 and lOis in order. A critical analysis of the kinetics involved in the decay of the transients recorded at 310 nrn reveals a very complex decay curve, i.e. it is not single exponential (first order) nor can it be fitted to a second order decay function. It apparently comprises three components, each decaying by an exponential function, the longest of which has a decay constant well above 100,us. In addi tion, the deca y of the transient at 310 nrn does not return to the original baseline after 400 us, indicating the formation of an extremely long lived transient and/or a stable product. Of course the ortho photo-Fries product

C. E. Hoyle, K. S. Ezzell, Y. G. No, K. Malone, S. F. Thames

336

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would contribute to the absorbance at 310 nm, but at this point it is unclear whether other permanent products or very long lived transients are also responsible for an apparent baseline shift. In view of the number of transient (Table 1) and permanent species which may well absorb at 31Onm, it is not surprising that the decay kinetics are quite complex. A complete analysis of the decay curves at 31Onm, 348 nm, 370nm, and 420nm will be performed, the results of which will be published in a forthcoming paper. 0'025 "

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Laserflash photolys is studies on MDI-ba sed polyurethanes

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338

C. E. Hoyle, K. S. Ezzell, Y. G. No, K. Malone, S. F. Thames

Finally, before concluding this section, we offer additional evidence for assignment of the 370 nm peak to a diarylmethyl radica l which we feel is even more compelling than our original argument.P v'" The transient spectra of compound (IV), for which diarylmethyl radical formation is precluded by the presence of two methyl groups on the central methylene carbon, are shown in Figs 11 and 12 in nitrogen and oxygen saturated dichloromethane solutions. (A similar model to compound (IV) containing a silicon moiety in place of the central carbon was used in our previous study, leaving a finite but nonetheless existing possibility for questioning the conclusions reached.) The primary feature of both sets of spectra is the absence of a peak at 370nm, thus providing strong indirect evidence for the assignment of the 370nm peak in Figs 1 and 7 to the diarylmethyl radical. This also adds credence to the evidence presented in earlier papers for diarylmethyl radical formation and sustains the mechanism proposed by Gardette and Lemaire 15 - 17 in Scheme I. Indirect generation of transients using tert-butyl peroxide

Tert-butyl peroxide (TBP) is an extremely useful compound for generating tert-butoxy radicals in solution which can abstract hydrogen atoms from labile compounds as depicted in Scheme II. Figures 13a-13c show transient

Scheme II

spectra of three compounds, aniline (V) (Fig. 13a), compound (VI) (Fig. 13b), and the model biscarbamate (III) (Fig. Be) produced by excitation of TBP (60/40 by volume mixture of TBP and benzene) at 351 nm (absorbance = 0'4). The transient spectrum in Fig.13a is identical to that previously reported for the anilinyl radical produced in a similar experiment. 25 Since only TBP absorbs light at 351 nm, laser flash photolysis (A. = 351 nm) of aniline (2,8 x 10- 3 M) in benzene with no TBP present yields no detectable transient. The transient spectrum of (VI) in Fig.13b shows a peak absorbance at about 430 nm which is due to either a benzyl radical or an N-substituted anilinyl radical which is red-shifted about 25 nm from the simple anilinyl radical. As previously reported.i" the laser flash photolysis of a BP-MDI solution in a 60/40 ratio TBP/benzene solvent mixture yields a transient spectrum (Fig. Bc) having a distinct maximum at 370nm due to the substituted diarylmethyl radical. By contrast, the laser flash photolysis of a similar solution of compound (IV) exhibits no peak at 370 nm. The

Laser flash photolysis studies on M Dl-based polyurethanes

339

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Fig. 13. Transient absorption spectra. (a) Transient absorption spectrum (2 ~s) of 3·0 x 10- 3M aniline (V) in nitrogen saturated TBP/C6H 6 (A ex = 351 nm). (b) Transient absorption spectrum (12~s) of 7·0 x to- 2 M propyl N-tolyl carbamate (VI) in nitrogen saturated TBP/C 6H 6 (A e x = 351nm),(c)Transient absorption spectrum (10 ~s) of3·5 x 10- 3 M bis(propyl carbamate) of MDI in nitrogen saturated TBP/C6H 6 solution (A ex = 351nm), (Reproduced with permission from Ref. 24. Copyright 1988 ACS.)

340

C. E. Hoyle , K. S. Ezzell, Y. G. No, K. Malone, S. F. Thames

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absorbance above 400 nm results from the N-propyl carboxyl substituted anilinyl radical. It is possible to monitor directly the formation of the substituted diarylmethyl radical in Fig. 13c by recording the absorbance as a function of time. Figure 14 shows a distinct buildup in the absorbance at 368 nm which is complete by about 6 us after firing the laser. This rise represents the time required for abstraction of the central methylene hydrogen by the tertbutoxy radical. Figure 15, obtained by monitoring the BP-MDI transient at 416nm (generated by excitation ofTBP at 351 nm), shows that abstraction of the hydrogen on the nitrogen atom of the carbamate chrornophore is much slower than that of the hydrogen on the central methylene carbon. By comparing the time required for the hydrogen abstraction process to give the substituted diarylrnethyl radical (370nm) and the N-propyl carboxyl substituted anilinyl radical (416 nm), a comparative kinetic analysis of the hydrogen abstraction rates is obtained. Since it is expected that the growth of the transient intermediates will obey first-order kinetics and thus allow calculation of relative rate constants for hydrogen abstraction, the rise portions of the kinetic traces in Figs 14 and 15 were analyzed by applying a linear least-squares fit to plots (Fig. 16) of eqn (1) (see Ref. 25), where A co is the transient absorbance measured at the plateau, At is the absorbance at time t, and kOBs is the rate constant for radical abstraction: (1)

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342

C. E. Hoyle, K. S. Ezzell, Y. G. No, K. Malone, S. F. Thames

The results in Fig. 16show that ko Bs, 368 om is four times larger than kOBs. 416 om' i.e. the hydrogen on the methylene carbon is readily available for hydrogen abstraction by peroxy radicals , These results are quite interesting and have practical significance , since one fully expects that actual polyurethanes employed in industrial applications would have a certain degree of peroxide or hydrogen peroxide impurities in the polymer as a result of mechanical/thermal induced oxidation during synthesis/processing. Thus, excitation of such impurities would result in generation of the diarylmethyl radicals and subsequent peroxy radical formation in accordance with Scheme T. Incidentally, the transient at 370 nm shown in Fig, 15 is almost completely extinguished, as was the case for direct photolysis (Fig. 8), by the introduction of oxygen, thereby confirming the above postulation, CONCLUSIONS This report has extended the spectral characterization of the transient intermediates ultimately responsible for the photodegradation/photooxidation of polyurethanes based on MOl. Strong evidence for the presence of both the ortho photo-Fries product and a diarylmethyl radical in the laser flash photolysis of model arylcarbamates has been presented. Our results support the photolysis scheme proposed by Gardette and Lemaire 15 - 1 7 for the dual degradation process for photolytic decomposition of MOl based polyurethanes and the critical role of oxygen and peroxide type impurities. Future reports will include a detailed kinetic investigation of the decay kinetics of arylcarbamates and the laser flash photolysis of MDI based polyurethane films. ACKNOWLEDGEMENTS This work was sponsored in part by the Office of Naval Research, In addition, acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Acknowledgement is also made to NSF for assistance in purchasing the laser flash photolysis unit (Grant CHE8411829, Chemical Instrumentation Program). REFERENCES 1. Schollenberger, C. S. & Dinsbergs, K, SPE Trans " 1 (1961) 31. 2. Schollenberger, C. S. & Stewart, F , D., J. Elastoplast., 4 (1972) 294.

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3. Schollenberger, C. S. & Stewart, F. D., In Advan ces in Urethane Science and Technology, Vol. II , ed. K. C. Frisch & S. L. Reegen, Technomic Publishing, Connecticut, USA, 1973. Tarakanov, O. G . & Beljakov, V. K., Soviet Plastics, 7 (1966) 45. 4. Nevskii, L. 5. Nevskii, L. V. & Tarakanov, O. G., Soviet Plast ics, 9 (1967) 47. 6. Nevskii , L. V., Tarakanov, O . G . & Beljakov, V. K., So viet Plastics, 10 (1967)23. 7. Nevskii, L. V., Tarakanov, O. G . & Beljakov, V. K., J. Polym. Sci., Part C, 23

v.

(1968) 193. 8. 9. 10. 11.

12. 13. 14. 15. 16. 17. 18.

19. 20. 21.

22. 23. 24. 25.

Beachell, H. C. & Chang, 1. L., Polym. S ci; Part A, 10 (1972) 503. Hoyle, C. E. & Herweh, J. E., J. Org. Chem., 11 (1980) 2195. Allen, N. S. & McKellar, J. F ., J. Appl. Polym. Sci., 20 (1976) 1441. Osawa, Z., Cheu, E. L. & Ogiwara, Y., J. Polym. Sci., Letters Ed., 13 (1975) 535. Chell, E. L. & Osawa, Z., J. Appl. Polym. Sci., 19 (1975) 2947. Osawa, Z., Chell, E. L. & Nagashima, K., J. Polym. Sci., 15 (1977) 445. Schultze, H., Die Makromol. Chem., 172 (1973) 57. Gardette. J, L. & Lemaire, J., Die Makromol. Chem ., 182 (1981) 2723. Gardette,1. L. & Lemaire, J., Die Mak rom01. Chem., 183 (1982) 2415. Gardette, J. L. & Lemaire, J., Poly. Deg. & Stab., 6 (1984) 135. Schwetlick, K., Noack, R. & Schmieder, G., Z. Chem., 12 (1972) 107. Saunders, J. H. & Frisch, K. C, Polyurethanes: Chemistry and Technology, Vol. XVI , Interscience Publishers, New York, 1962, p.273. Hoyle, C. E. & Kim, K. J., J. Polym. Sci., Chern. Ed., 24 (1986) 1879. Osawa, Z., In Developments in Polymer Photochemistry, Vol. 3, ed. N. S. Allen , Applied Science Publishers, London, 1982, p. 209. Krimm, H., Betta, A. & Schnell, H., German Patent No . 1220863, 1964. Hoyle, C. E., No, Y. G., Malone, K. G., Thames, S. F. & Creed, D., Macromole cules, 21 (1988) 2727. Hoyle, C. E., No, Y. G. & Ezzell, K., In The Effects of Radiation on High~ Technology Polymers, ACS Symposium Series 381, ed. E. Reichmanis & 1. H. O'Donnell, ACS, Washington, DC , 1989, 43-56. Platz, M. S., Levya, E., Niu, B. & Wirz, 1., J. Pilys. Chem., 91 (1987) 2293.