June, 1!)61
DEGRAD.LTIOS OF P O L Y M E T H Y L M E T H A C R Y L A T E BY U L T R A V I O L E T LIGHT
967
DEGRADATION OF POLYMETHYL METHACRYLATE BY ULTRAVIOLET LIGHT BY ALLANR. SHULTZ Contribution K O . 191 from the Central Research Department, Minnesota Mining and Manufacturing Company, St. Paul 19, Minn. Receaved November 7, 1960
Four polymethyl methacrylate films, having thickness 0.021, 0.107, 0.220 and 0.465 cm., were degraded by 2537 A. wave length light. The irradiations were conducted in air a t 26". Polymer chain scissions produced by the absorbed photons were computed by a plication of theory to the benzene and 50: 50 butanone:isopropyl alcohol solution viscosities,of the original and irradiated firms. The quantum yield for random chain scission under the above conditions is rpd (2537 A.) = 2.3 X 10-3 (scissions/absorbed photon). The extent of agreement with theory and the nature of deviations from theory of,the experimental dose-effect relations observed are analyzed. A change of the light absorption coefficient with ultraviolet llght exposure is a complicating factor.
Introduction Photolytic degradation of polymer films is of considerable practical interest. Published quantitative studies in this area, however, are not numerous. Quantum yield determinations have been made for ultraviolet light-induced changes in natural rubber,' c e l l ~ l o s e , ~ -cellulose ~ acetate,j polyethylene,6 polymethyl metha~rylate,?-~ polyet'hylene terephthalate, lo polymethyl vinyl ketone" and polymethyl isopropenyl ketone. l1 The changes measured were gas evolution, monomer production,7-9v11new functional group f ~ r m a t i o n and ~,~ degree of polymerization decrease.2-5J-11 Sensit,ized crosslinking of polymer films by using ketones and other photo-labile molecules as radical sources, is receiving increasing attent'ion with some quantitative investigations evolving. This latter area involves radical attack by external agents rather than direct photolytic action by polymer unibabsorbed quanta. The present investigation was undertaken t'o determine the quantum yield for random scission of pure polymethyl methacrylate by 2537 A. light. The change in viscosity-average molecular weight as a function of film thickness is compared with theory.20 (1) L. Bateman, J . Polymer Sci., 2, 1 (1947). (2) H. F. Sauer and W. K. Wilson, J . Am. Chem. Soc., 71, 958
(1949). (3) J. H. Flynn, W. K. W-ikon and W. L. Morrow J . Research Null. Bur. Standards ( U . S.), 6 0 , 229 (1958). (4) J. H. Flynn, J . Polvmer Sei., 27, 83 (19.58). ( 5 ) A. Sippel. 2. Elektrochem., 56, 775 (1992). (6) 4. R. Burgess, Chemistry and Industry, 78 (1952); N.B.S. Cirr. 626, 149 (1953). 17) P . R . E. .J. Conley and H. V. Melville, Proc. Row. Soe. (London), ZlOA, 461 (1952). (8) P. R. E. J. Cowley and H. W. Melville, ibid.. 2 1 t A , 320 (1952). (9) P. R. E. J. Conley and H. W. Melville, N.B.S. Ciro. 626, 59 (19,531. (10) K. R. Osborn, J . Polymrr Sci., 38, 357 (1959). i l l ) K. F. Wissbnin, J . Am. Chem. Soc., 81, 58 (1959). 112) 11. J. Roedel, U. S. Patent 2,484,529, October, 1949. { I S ) G. Oeter, Belgian Patent 553,516,.January, 19.57. (14) G. Oster, G. IC. Oster and H. Uoroson, J . Poiymrr Sci., 34, 671
(1939). (15) H. Moroson. Ph.D. Thesis, Polytechnic Institute of Brooklyn, 19.59. L.C. Card X o . Mic 59-1775. (16) A. Charlesby, C . S. Grace and L. G. Penhale, J . Polymar Sci., 34, 681 (1959). (17) C. 5.Grace and A. Charlesby, Paper No. IC7, IUPAC Polymer
Symposium, Wiesbaden, October, 1959. (18) E. B. Atkinson, A. E. Hoisfield and hl. R. Pettit. Paper No. IV C9, IUPAC Polymer Symposium, Wiesbaden, Oetober, 1959. (19) H. Wilski, Angew. Chem., 71, 612 (1959). (30) A. R. Shultz, J. Chem. Phys., 29, 200 (1958).
Experimental Monomer Purification and Polymerization.-Inhibited methyl methacrylate monomer (Monomer-Polymer Corp .) was washed successively with 5% NaN02, 5% NaHSOa, 5% NaOH and water. After drying over MgSO,, 0.2% by weight of phenyl-p-naphthylamine was added and the monomer was distilled through a short glass helices-packed column a t 100 mm. and 46" with nitrogen bleeding. The central cut, consisting approximately 80% of the initial monomer, was retained for polymerization. Forty-five thousandths g. of lauroyl peroxide was added per 100 ml. of monomer and the solution was saturated with Nt by sparging. Four pairs of lass plates (30 X 30 cm.) were spaced with stainless s t e 3 rod or shim Separ&OrB, their edges were taped together with "Scotch" Brand Polyester Film Tape (#853) having solvent-resistant adhesive, and the se arators were removed in the final stages of closure, T i e monomer-peroxide solution was syringed into these four spaced-plate pairs a t room tem erature, allowance in volume being made for complete 11 of the enclosures at 40e. Polymerization was accomplished in 64 hours a t 40.0 =t0.02 in a water-bath, then 24 hours at 55" in an air oven followed by 18 hours at 100" in an air oven. The oven stages were employed to assure complete decomposition of the peroxide and essentially 100% conversion of monomer to polymer. The films were removed from the glass plates by cooling from 100" under a water tap and stripping the tape edgings. Despite reasonable flexibility of the tape binding, lack of any release agent and the normal 20-21 yo contraction during polymerization caused some irregular surface patterning in the four polymethyl methacrylate films. Large areas of apparently smooth, parallel-surfaced films were selected for the irradiation studies. Micrometer measurements on ten samples of each of the four films gave: film A, 0.0207 Jc 0.0018 cm.; film B, 0.1070 f 0.0023 cm.; film C, 0.2200 f 0.0013 cm.; film D , 0.4650 f 0.0007 cm. The similar absolute thickness variations in the first three films suggest that deviations of the glass plate surfaces from planarity may have been the controlling factor. Film D exhibits a surprisingly small thickness variation. Light Absorption Measurements.-One 2.5 X 2.5 cm. sample of each film 8,B and C was measured against air for apparent optical density, (O.D.)spp,,in a Cary Model 11Spectrophot$meter. The wave length region covered was 2400 to 4000 A. Correction for a single reflection at both the incident and excident film-air interfaces was calculated from (O.D.)spp for films A and C using the relation O.D. = (O.D.)app. 2 log (I - r ) . r is the fraction of light lost t o specular reflection at an interface. Assuming the absorption to obey Lambert's law, Z = l o e - k L , the absorption coefficient, k , may be calculated as k = 2.303 [(O.D.),,, 2 log (1 - r ) ] L-l. At 2537 A. we obtained k = 18.4 cm.-' and (1 - r ) = 0.944. This k agrees fairly well with k = 19.6 cm.-1 determined in chloroform solutions for a thermally-initiated polymethyl methacrylate which had been carefully precipitated, washed and vacuum dried to remove monomer . 2 1 A- benzoyl peroxideinitiated polymethyl methacrylate ( M , = 125,000) was reported7 to have SA k = 2.303 eC = 2.303 X 1.32 X 12.1 = 36.8 ern.-'.
R
+
+
(21) Unpublished data, this Laboratory.
ALLANR. SHULTZ
968
Vol. 65
This high value might be nearly accounted for by the contriTABLE I bution of 1/1250 mole fraction concentration of benzoate INTRINSIC VISCOSITIES AND CALCU1,ATED WEIGHTLAVERAGE end-groups, but not quite on the basis of the same mole W-EICIITS OF ORIGINALPOLI-METIIYL METE fraction of phenyl groups. In the present study lauroyl MOLECULAR peroxide was purposely chosen to avoid the complicating ACRYLATE FILUS effects of excessive initiator fragment light absorption. -2.8 x lo--[?] (dl. g.-I)Further sources for reference on the ultraviolet light abBenzene CCM CCM Benzene Film sorption of polymethyl methacrylate are available.22~23 8 1 8.3 1.60 8.7 A Changes in ultraviolet absorption of films A and B were 5.4 5.9 1.35 6.5 B followed as a function of 2537 A. irradiation exposure by comparison with initially 0.D .-matched non-irradiated 8.2 9.3 1.70 8.8 C films in the Cary spectrophotometer split beam. 7.4 7.7 1.55 8 . 2 D Irradiation Apparatus and Procedure .-The ultraviolet light irradiation facility consists of seven 30-watt General Equation 1 was derived from a light-scattering Electric low-pressure mercury vapor lamps (germicidal) mounted in an aluminum reflector with parabolic cross- and viscosity study of nonfractionated PMMA. section. Positioned in the center of this horizontal lamp Equation 2 is the relation determined in ref. 26 bank is a vertical 16 x 16 x 26 em. cardboard box with non- modified by the logarithm of Mw/Mv assuming gloss black interior. A 16 X 16 X 3 mm. Corning #9863 the fractions studied in ref. 26 to be "monodisglass filter is interposed between the lamps and the sample platform. The average lamp-to-sample platform distance perse" in mol. wt. and assuming the present samples is about 22 cm. KO collimation of the incident light other to possess a most probable distribution of molecuthan that imposed by the apparatus geometry was at- lar weights. The general agreement in molecular tempted. Maximum possible angle of incidence of light weights calculated from the data in these two systo the film surface is 32". The corresponding angle of refraction in the film is about 21'. Amming an optically tems is good. If an allowance had been made for smooth incident surface, the maximum ratio of light path appreciable residual heterogeneity in the ref. 26 travelled in the film to the normal path is 1.07. fractions the agreeme$ would be even better. The lamp and filter combination chosen assures that ap- The 12% difference in M,(calcd.) values for film C proximately 1 0 0 ~ oof the light energy striking the films is of 2537 A. wave length. Radiation flux was determined may reside in filtering difficulty in the CCM soluwith an Eppley thermopile a t sample position using a Leeds tion, allowing evaporation losses prior to viscosity and Northrup portable potentiometer aa the voltage sensing measurement. A calculated value of [v](CCM) = instrument. The thermopile had been previously Cali- 1.62 would place film C in better relation to the bratedS4 against a N.B.S. standard light source (C-744) which gave a radiant energy flux of 45 p watts/cm.2 at 2 other films. Higher concentrations of PMMA in meters when operating a t 0.3 amp. The incident flux at the CCM exhibited peculiar flow memory in the sample position in the irradiation facility was found to be capillary viscosimeter, suggesting, possibly, phasc 580 p watts/cm.2. The entering intensity in the PMMA separation incidence. film is therefore lo = 0.944 X 580 = 550 p watts/cm.* = Exposure times and intrinsic viscosities of the 7.0 X lor4(2537 A. photons) cm.-Zsec.-'. Insertion of the thermopile into the sample position at irregular intervals irradiated films are given in Table 11. Radiation revealed less than &IO% variation in the incident light absorption in the upper surface of the films, based intensity during the irradiation period. upon the original absorption coefficient, is RO = The PMMA films were cut into rectangular strips of ap- k01,t. k = 18.4 cm.-l, D = 0.844 ~ m g.-I, . ~IO = proximately 0.2 g. wei ht (less for the thinnest films) and were irradiated in air fR.H.-25%, temp.-26') for times 2.52 X 1018 photons cm.-2 hr.-l, and 1 = irradiation time in hours. Of somewhat more_ intrinsic equally spaced logarithmically, up to 192 hours. Solution Viscosity Measurements.-Solution viscosjties theoretical interest is the quantity 0.5hfwoRofl-', on the original and irradiated PMMA were measured in a where Mw0is the weight-average molecular weight Cannon-Fenske capillary viscosimeter at 25.0 f 0.02'. Reagent grade benzene (flow time 79.0 sec.) and a 50:50 (g./mole) of the non-irradiated polymer and fii is by volume mixture of reagent grade methyl ethyl ketone: Avogadro's number. o.5i$fw~ofl-'is the number isopropyl alcohol (flow time 86.4 sec.) were the solvent sys- of photons absorbed a t the upper film surface per tems. No kinetic energy or shear corrections were made. Intrinsic viscosities, [ a ] , of the original PMMA films were original number-average molecule assuming an obtained by extrapolating (7) = (In qrel)/c and tsp/c initial most probable distribution of molecular against c plots of a t least four concentrations to c = 0. weights. Introducing the quantum yield for chain Single concentration determinations on irradiated film scigion, 'pd, we arrive a t the quantity 0.5(pdMWosolutions were made (relative viscosity range 1.1 to 1.4) RON-' -Ro/R*, the number of scissions per using the relations original number-average molecule a t the upper benzene: [7] = (7) 0.13(q)2c surface. MEK-isopropyl alc.: [VI = { q ) 0.40(7)2c Theoryz0predicts that the number of new viscos-
=
+
-
Results and Discussion Table I lists the intrinsic viscosity of the nonirradiated PMMA films in benzene and in the MEK-isopropyl alcohol mixture (hereafter called the critical consolute mixture, CCM). -Weightaverage molecular weights (g./mole), Mw, were calculated by the relations benzeneg5: log IC?, = (4.103 + log [7])/0.73 ( 1) CCMZ6:log A?,,
= 2(3.228
+ log [ q ] ) + 0.053
(2)
(22) J . W. Goodeve. Trans. Faraday Soc., 34, 1239 (1938). (23) Per Olof Kinell, "Spectrophotometric Study of Polymethyl Mc=tharrrlate," Almqvist and Wiksells Roktryckeri AB, Uppeala, 1953.
(24) Rirhnrd I,. n'6hf.r. this Laboratory.
ity-average molecules produced per original viscosity-average molecule, ( [v]o/[v])i/a - 1, (where a is the viscosity-molecular weight exponent in [TI = KM") is related to -Rc/R* byz7 ([7IOl[?l)"
- 1 = (kL)"-
1[
n=0,1,2---
(a
+ n)-'
Experimental plots of ( [ v ] o / [ v ] l ) ' a- 1 us. O.LVwo. (25) E. F. Casassa, C. E. Hecht and R. L. Cleland, unpublished data, Massachusetts Institute of Technology, eo. 1951. (26) 5. N. Chinai and C. W. Bondurant, Jr., J . Polymer Sci., 22,565 (1956). (27) Note: +(Ro/R*)g inadvertently appeared in eq. 12 of ref. 20 in place of the correct quantity -(Ra/R*)y.
June, 1961
DEGR.4DATIO;h; OF POLYMETHYL METHACRYLATE BY ULTRAVIOLET
hr.
0 0.375 .75 1.5 3 6 12 24 48 96 192
--[VI
(Benzene)-
A
R
8.7 6.3 6.9 5.9 4.73 3.04 1.67
6.5
..
5.9 5.8 4.80 4.08 2.92 1.06 2.25 0.62 1.99 0.493 1.73 1.03
...
C 8.8
8.2
..
..
..
D
..
7.5 .. 7.2 .. 6.2 7.2 4.84 6.9 . 6.7 3.44 6 . 9 2.70 6.1 2.08 6.1
.
IS[-
CCMC
A
B
1.60 1.58 1.46 1.28 1.06 0.69 ,481 .330 .257 ,220 .132
1.35
...
1.32 1.25 1.11 1.00 0.68 .66 .50 ,360 ,346
..
.. 1.62
.. 1.19 1.02 0.88 .74 .65
.57
6 4
2
D
1.70 1.55
..
2 10’
..
-5
.. .. 1.44 1.39 1.35 1.32 1.17 1.11
on a log-log scale should therefore superpose upon corresponding theoretical log-log plots of ( [v’JO/[v]) l I a - 1 us. - Ro/R” by a simple translation along the -Ro/R* axis. The series in eq. 3 converges very slowly. Alternative forms, 2o applicable for a = 0.50 and a = 1.00,are more convenient for numerical evaluation. Figure 1 shows the benzene solution viscosity data for the light-irradiated films A, B, C and D superposed upon the theoretical curves for LL = 0.38, 1.97, 4.05 and 8.56, respectively. The limiting curves for a = 0.50 and a = 1.00 are drawn for each film. A good fit of the data to theory is found for films A, B and C over a large exposure range. The abscissa translation giving the correspondence is revealed by the vertical arrow yhich- indicates the exposure a t which 0.5 p d ~I~,GRGN-’ = 1. It should be remarked that the same translation was made for all four films. From the exposure indicated by the arrow we calculated p d = 2.3 X 10-3 scissions/photon as the quantum yield for random fracture of the polymer chains by the absorbed 2537 A. light. It is difficult to assess the uncertainty in this quantum yield arising from the several uncertainties in the measurements involved. An estimate of *25Oj, uncertainty in pa is not unreasonable. Figure 2 is a plot of the CCM solution viscosity dat,a superposed on t,he corresponding a = 0.50 theoretical curves. The fit is much poorer than for the benzene solution data. The axis translation chosen fits the film A data (upper curve) t.o theory a t ([7!0/[77])2 - 1 = 1 and gives a good fit of the film B data. The calculated quantum yield from t,he plot is (pd = 2.0 X scissions/photon, but, its reliability is not equal to that of the benzene solution data result. The film C data (second from bottom in Fig. 2) do not match theory even though an adjusted [710 = 1.62 was used in the construction. A viscosity-molecular weight exponent’ of a = 0.60 would give a fairly good fit of the film A and B data to theory, but t,here is no justification for belief in such a high exponent.Z6 At present we must assume that experimental difficulties, especially in tlie viscosity measurements, are the principal sources of the greater CCM-data divergencies from theory. Figure 3 compares the film A and D solution viscosity data for both the benzene and CCM systems with the corresponding theoretical curves. Although in the above paragraph we emphasized the lack of fit of the CCM data with theory, it
969
102 8
TABLE I1 INTRINSIC VISCOSITY DATAFOR 2537 A. LIGHT-IRRADIATED POLYMETHYL METH.4CRYLATE FILMS Irrad. time,
LIGHT
4
h
P I
k c
2
v
00
Y
8 6
4 2 10-1 4
102
2
8
G
2 103
4
G 104 8
4
2
8 G 10‘
.
0.5MwoR ~ z - lPho ( tons/molecule ) Fig. I.-The number of new viscosity-average moleculw per original viscosity average molecule, ( [ ~ l ] o / [ q)l.n ) - 1, plotted against the number of photons absorbed per origizd number-average molecule in the incident surface, 0.5 il.1 M,o R z - 1 . The solid lines are theoretical curves. Benzene solution-viscosity data for films A( o), B( e), C((3) and D(@)are superposed.
2
.
+ ’
2 -e
10’ h 6
z - 1 P
-
2 100 8 6
4 2 IO-’
4
8
2 G 104 4 8 IO3 4 8 2 6 IO’S 0.5&fdR0-% (photons/molecule). Fig. 2.-Same plot as Fig. 1 but for the butanone:isopropyl alcohol solution viscosity data. The viscositymolecular weight exponent is a = 0.50. As in Fig. 1 the theoretical curves were calculated, top-to-bottom, for k L = 0.38, 1.97, 4.05 and 8.56, respectively.
I02
2
6
should be noted that considerable correspondence does exist. Real and understandable systematic deviations of experimental data from simple theory are apparent in Figs. 1, 2 and 3. Focussing attention upon Fig. 1 we note that the experimental curve for film A develops a gradual “positive” deviation from theory as exposure progresses, but finally exhibits “negative” deviation at the highest exposure. The film B and C data do not show deviations from
ALLANR. SHULTZ
970
T‘ol. 6;
of the film B sample has the values 1.00, 1.28, 1.63 ” ” and 2.03 a t 0, 50, 96 and 192 hours exposure, re-
lo; 6 ’
4 ’ 2 .
10-1 I 102
I
6 lo4 4 8 4 8 2 6 105 O.5%,&%-1 (photons/molecule). ~ 1 os. 0.5pd. Fig. 3.-A combined plot of ( [ q ] o / [ q ] ) ”.Ww&y(theoretical) 1 and us. 0.5M~voR~y-l (experimental) for films A and D benzene viscosity data (open circles) and butanone: isopropyl alcohol viscosity data (solid circles). The same horizontal translation of axes was made for all the data. The arrow indicates qYd = 2.3 X scissions/abwrhed photon.
a
4
2
8
6
lo3
1
FILM A
h
2 0.5
Ri
i
v
c
’\-
2600
2800
3000
3200
3400
3600
A&.).
Fig. 4.-Original apparent optical density (broken line) and increases in optical density (solid lines) for a,film A *ample as functions of wave length and total 2537 A. wave length light exposure times.
theory until high exposures are reached; they then deviate in a “negative” sense. Film D data exhibit “negative” deviations from theory a t nearly all exposures studied. The cause of these systematic deviations is discovered by studying the changes in optical density occurring during the irradiation. Figure 4 shows the original apparent optical density curve (broken line) for a film A sample and the increases in optical density (solid lines) observed after chosen total irradiation times. Figure 5 is a similar plot for a thick film B sample over the same wave length region (2480-3640 A.). A broad absorption peak centered a t -2850 A. and having an approximate width a t half height of 900 A. is produced by the 2537 A. light irradiation (cf. Fig. 5 ) . Exact peak location ie prevented by the tail of the strong shorter wave length absorption. At 2537 A. the apparent optical density
spectively. Apparent O.D. values a t 2537 A. of the film A sample are 0.22, 0.35, 0.48 and 0.76, a t 0, 50, 96 and 192 hours exposure, respectively. An average film A sample therefore progressed from 32% absorption of the entering 2537 A. light in the initial stages of irradiation to 80% adsorption of this light after 192 hours exposure. The near coincidence of the data at higher exposures with the theoretical curves (Fig. 1) is thus somewhat of an artifact. The light absorbed by the new chromophores is effective in polymer chain scission, thus producing an apparent increase in the degradation effectiveness with increasing exposure of the thin film A. The scission yield of this new absorption relative to the original absorbing structure is not, under present mathematical construction, calculable. It appears to be roughly comparable. Film D, which initially absorbed nearly 100% of the entering 2537 A. light exhibits an apparent decrease in scission efficiency with increasing exposure. No sensible increase in total absorbed quanta could be caused by the new absorption. The more precipitous attenuation produced further skews the scission distribution in depth and decreases the number of scissions per viscosity-average molecule in the total polymer film. This effect, which is most readily discernible in the film D sample, becomes evident in all the films studied a t high exposure. The curve fitting method employed in Fig. 1 constitutes, in essence, a double extrapolation of the experimental results to the limit of zero film thickness and zero exposure, This minimizes the error introduced by the new absorbing groups into the calculated scission efficiency. Especially when the film thickness range and new absorber scission efficiency embrace the positive-to-negative apparent efficiency contribution is the introduced uncertainty minimized. Inspection of the form of the new ultraviolet absorptions produced in films A and B reveals some interesting, although not definitive, characteristics. Film A does not show the formapon of the well defined absorption peak near 2850 A. which is noted in film B. The rise in absorption below 2700 A. which is common to both films is more pronounced, relatively, in the thinoner film and masks the 2850 A. peak. The 2850 A. peak may be associated with a ketonic or aldehydic carbonyl grouping in a volatile product. The broken dotdash curve in Fig. 5 represents the 192 hr.-exposure film B sample after subsequent 70” heating for 13 hours under vacuum. The form of the resultant absorption curve approaches that of the thinner film. The nature and origin of the new chromophores is open to speculation. If they are photooxidation products of the polymer structural units (this is possible since the film irradiations were conducted in air) they might or might not be necessary precursors to chain scission. The latter is believed true since no definite induction period is observed in the degradation. Oxygen might enter into the photochemical reaction which produces stabilized chain
ruptures from initial light-absorption activated chain units. This is neither proven nor disproven by the limited chemical evidence a t hand. The possibility that the new absorptions might be due to methyl pyruvate produced by photooxidation of residual monomer in the polymer films was explored.28 Although this source of the chromophore seemed logical, subsequent experiments proved that neither the magnitude nor the position of the new absorption could be explained on this basis. The increased absorption by ultraviolet-irradiated PMMA in the near ultraviolet and blue region of the spectrum and the effect of certain additives upon it has been the subject of an optical and chemical analytical study.29 This visible "yellowing" may be similar in chemical origin to that caused by high-energy electron irradiation. 30 The quantum yield for random scission, p d = 2.3 X l O - j , found in the present study is comparable to quantum yields determined for ultravioletdegradation of other solid polymers a t or near room ternperature.'O It is in no manner comparable to the quantum yield, 0.1-0.2, reported for photpinitiation of PMMA depolymerization by 2537 A. light in the 150-195" temperature If these two processes originate in the same photochemical act, the greater mobility of the polymericand low molecular weight-scission fragments a t the elevated temperatures must be credited with cagerecombination prevention. There is for belief that the photo-initiation of depolymerization occurred principally a t some terminal end-groups of the PMMA chains. This belief is questionable when scrutinized in the light of the reported 10-1 quantum yield. These investigators suggested reasonable end groups formed by disproportionation between two polymer radicals. One of the two terminal end groups (the saturated, hydrogenatom acceptor) thus formed would have an ultraviolet-absorption coefficient indistinguishable from the repeating units in the chains. The other terminal end group (unsaturated, hydrogen-atom donor) should have an absorption coefficient at 2537 8. essentially equal to monomeric methyl methacrylate which we measured for ovr monomer a t 23" in chloroform to be k ~ n ~ ( 2 5 A.) 3 7 = 1.71 X lo3 ern.-' Considering, e.g., a P M N A having a number-average degree of polymerization 6Pn= 1250 only 1/2500 of the polymer units would be of this conjugated unsaturated ester structure. Since the L contributions of units in a mixture may be satisfactorily summed on a volume-fraction basis, the contribution of this terminal unit would be (1/2500) x 1.71 X lo3 = 0.68. This is only 0.0185 of the over-all PMMA k(2537 A.) = 36.8 value reported.8 Even if the quantum yield for depolymerization initiation were unity for photons absorbed by such sites the observed quantum yield would be
