Light-induced reactions in polymethylmethacrylate involving a chlorine atom

Light-induced reactions in polymethylmethacrylate

2797

13. R. HOUWINK and A. STAVERMAN, Khimiya i tekhnologiya polimerov (Chemistry and Technology of Polymers). Izd. "Khimiya", 1965 (Russian translation) 14. C. H. BAM~ORD, W. G. BARB, A. D. JENKINS and P. F. ONYON, Kinetika radikal'noi polimerizatsii vinflovykh soyedinenii (The Kinetics of Vinyl Polymerization by Radical Mechanisms). Izd. inostr, lit., 1961 (Russian translation) 15. W. V. SMITH, J. Amer. Chem. Soc. 71: 4077, 1949 16. D. ROBB, J. Polymer Sei. 7, A-I: 417, 1969

LIGHT-INDUCED REACTIONS IN POLYMETHYLMETHACRYLATE INVOLVING A CHLORINE ATOM* Y~. A. MIKHEYEV, T. S. POPRAVKO, L. L. YAsr~x and D. YA. TOPTYGIN Chemical Physics Institute, U.S.S.R. Academy of Sciences

(Received 21 February 1972) I R spectrometry and masss pectrometry have been used to study the photo-transition of PMMA containing ferric chloride. A mechanism is suggested for the rupture of ester groups and the degradation of PMMA that takes into account the photo-decomposition of intermediate middle macroradieals. IT IS well k n o w n t h a t t h e d e g r a d a t i o n of P M M A m a c r o m o l e c u l e s u n d e r t h e a c t i o n o f U V l i g h t is a c c o m p a n i e d b y r u p t u r e o f t h e ester g r o u p s of t h e p o l y m e r . I t h a s r e c e n t l y b e e n e s t a b l i s h e d t h a t t h e g e n e r a t i o n of a c t i v e chlorine a t o m s in P M M A b y t h e p h o t o - r e d u c t i o n of ferric chloride leads t o t h e d e g r a d a t i o n o f t h e m a c r o m o l e c u l e s [1, 2]. I n c o n n e c t i o n w i t h this t h e question n a t u r a l l y arises o f w h e t h e r t h e r u p t u r e of t h e ester g r o u p s occurs o n l y in t h e p r i m a r y p h o t o c h e m i c a l act, as is c u r r e n t l y considered [3], or w h e t h e r it c a n also occur in t h e s e c o n d a r y r a d i c a l reactions. A s t u d y o f t h e t r a n s i t i o n s o f t h e f u n c t i o n a l g r o u p s of t h e p o l y m e r u n d e r t h e a c t i o n of specially g e n e r a t e d a c t i v e radicals should b e c a p a b l e of giving a m o r e c o m p l e t e u n d e r s t a n d i n g of t h e d e c o m p o s i t i o n process in P M M A . EXPERIMENTAL

I R spectroscopy and mass spectrometry were used for the investigation. The specimens for the study of the I R spectra were prepared by slowly withdrawing salt windows (NaC1) from a solution of PMMA (LSO-N grade, M ~ = 150,000) with FeC18 in methylene chloride. The thickness of the polymer layer on one side of the window was 1.5-3 ~m. The specimens obtained with a concentration of 0-30 wt. Yo FoCls were held for 2 days in vacuum to remove traces of the solvent. Specimens of pure PMMA were prepared similarly. Irradiation was * Vysokomol. soyed. A15: No. 11, 2470-2476, 1973.

Yu, A. MIKHEYEV d aZ.

.2798

carried out at room temperature with a parallel beam of light from a DRSh-500 lamp with an intensity of a p p r o x i m a t e l y 10 le em -~ see -1. The I R spectra of the specimens were recorded with a UR-20 spectrophotometer. I n order to investigate the composition of the volatile products found in the system, a film specimen of PMMA containing 0.30 w t . ~ FeC13 (15-17 /ira thick) was irradiated with light from a DRSh-1000 lamp in a quartz cell connected to the ion source of a MKh-1303 mass spectrometer. The specimen was held as a preliminary at 90°C in a high vacuum (2 × 10 -~ torr) to establish a constant background. Under these conditions, the height o f the mass peak is proportional to the rate of evolution of the volatile product. DISCUSSION OF RESULTS T h e a c t i o n o n s p e c i m e n s c o n t a i n i n g FeC13 o f U V l i g h t ( 7 > 260 n m ) t h a t is n o t absorbed by the initial PMMA leads to a change in their IR absorption spectra that indicates rupture of ester, methyl and methylene groups. Figure 1 shows kinetic curves for the decrease in optical density calculated from the intensity TABLE 1. MASS SPECTRA OF THE PRODUCTS OF THE PHOTOLYSIS OF PMMA V~TH 0.3~,~ FoC18

role 100 84 82 69 52 50 44 41 39 38 36 35 32 31 29 28 18 16 15

BS-8 Filter, 2 >360 nm 20 o 350 900 1.3

- - 4

9"5 81.0 0"5 30-2 100 14.2 1-3 2.85 8.1 25.6 13.4 3-6 3.2

10-2 63.5 0.42 0.85 32.5 100 14.8 3.0 5.65 6.3 13.0 11.5 2.1 5.1

18"1 12"7 18"5 22"0 21"0 68"0 19"0 28"10 9"5 33"5 100 14"4 12.2 17.5 10.6 19.0 21.2 28-5

BS-3 Filter, 2 >260 n m 780 20 ° 50 ° 0.97

1-5 5"3 17.6 84.0 1.1 --

35.0 100 14.6 7.0 8.7 7.5 10.0 --

3.0 7.4

2"5

3"4 9"2 23"0 57-5 5.7 1.9 32.5 100 13.9 6.3 18.5 12.0 5-0 10.6 2.5 12.6

48.0 22.6 34.6 71.0 5.2 20.5 36.3 88.0 33.5 34.0 100 16.0 6.2 23-0 17.2 7-6 0.9 21.2

o f a b s o r p t i o n b y - - C H ~ - - (1485 c m - i ) a n d b y - - C H s (1452 e m - i ) , a s w e l l a s t h e c o r r e s p o n d i n g f i g u r e s f o r e s t e r g r o u p s ( b a n d s a t 1275, 1245, 1195 a n d 1152 e m -1 c o n n e c t e d w i t h t h e e x i s t e n c e o f i n t e r m o l e c u l a r a s s o c i a t e s [3]). T h e s e d a t a i n d i cate that not only are hydrogen atoms torn from the macromolecules (consumption of --CH,-- groups), but also point to the decomposition of ester groups under the action of chlorine atoms.

Light-induced reactions in polymethylmethacrylato

2799

The results of the mass spectrometric analysis of the volatile products formed directly during irradiation (Table 1) also give evidence of the rupture of ester groups in PMMA even under conditions when the polymer itself hardly absorbs any light (2~260 nm). I t m a y be seen that, during irradiation with filtered light, apart from the evolution of hydrogen chloride (mass spectrum of HCl: 36]100, 38/32, 35/17, 37/5, where the numerator gives the mass number and the denominator the relative intensity of the corresponding peak*), products are also formed from the transformation of the ester group: COs (44/100, 16/9), CO (28/100), CH3C1 (50/100, 15/72, 52/31), methanol (31/100, 32/67,

29/65, 28/6).

lOI

~1

'

W~T

x

I

I

I

3

x

I

Time,

hp 5

I 7

1. Curves for the consumption of functional groups in PMMA with FeC18 under the action of light with ~>260 nm: 1--1485; 2--1195; 3--1152; 4--1275; 5--1245 and 6--1452 cm -1. Z=0"95X 1016 cm -~ see -z. FIG.

I t m a y be seen from the Table that the ratio between the yields of products varies somewhat as the temperature of the specimen and the spectral composition of the light are altered. In particular, an increase in temperature leads to a reduction in the relative yield of COs and to a definite increase in the yields of methanol and methyl chloride. The rate of evolution of MMA (41/100, 100/30, 69/62, 39/46, 29112, 15/17), which is hardly evolved at all at 20°C, is found to depend most markedly on temperature. A certain rise in the rate of yield of methanol and methyl chloride is also found on going from light with 2 > 360 nm to light with 2 > 260 nm as the spectral composition of the light is changed (Table 1). The data from I R measurements also indicate that the rate of decomposition of ester groups depends on the spectral composition of the light. The transition from light with 2 > 2 6 0 nm (Fig. 1) to unfiltered light from a D R S h lamp (Fig. 2) (that is, with light having 2 < 2 6 0 nm present) considerably aepe!erates this process, a comparitively small increase in the integral intensity of the light I (0-95 × 1 0 t l l . 2 5 × 10le em -2 sec -t) leading to an increase in the decomposition * The mass spectra of the individual substances are cited from the data in [4].

Y u . A. M i x ~ Y ~ V

2800

¢¢ ag.

rate of the ester groups b y a factor of 5-10 (according to the initial sections of the kinetic curves). I t should be noted t h a t the ratio between the rates at which ester and --CHz-- groups are used up also depends on the spectral composition of the light. I t m a y be seen t h a t a change to unfiltered light leads to a more marked increase in the rate of decomposition of ester groups relative

2"0

/'8

/'6

1"2

2

~

T/me ~hr

6

FIG. 2. Curves for the breakdown of functional groups in PMMA under irradiation with unfiltered light from a DRSh-500 lamp (1-6) in the presence of FeCls and (7-12) without it with I0=1"25× 10" cm -s sec-x. 1 and 11--1485; 2 and 8--1195; 3 and 10--1152; 4 and 7--1275; 5 and 9--1245; 6 and 12--1452 cm-x. to - - C ~ - - groups. I t m a y be seen from Fig. 2 t h a t the addition of FeC1s eonsiderably increases the rate of decomposition of the functional groups. Similar data have also been obtained as a result of mass spectrometric measurements. As an example, Table 2 shows the intensities of the characteristic peaks for the principal products from the photolysis of PMMA when FeCl a is present or absent. I t should be noted that, when these systems are irradiated with unfiltered light, methyl formate (MF) (31/100, 29/63, 32/34, 60/28) is one of the main products from the decamposition of the ester groups whereas, under the action of light with 2 ~ 2 6 0 nm, methyl formate is hardly found at all. The results in Table 2 shows t h a t destructive processes are considerably accelerated when the initiator is present.

Light-induced reactions in polymethylmothacrylate

2801

The breakdown of the ester groups of the polymer in the system PMMA-FeC1 a under the action of light points to the occurrence of photochemical reactions involving free PMMA radicals formed intermediately. This conclusion follows from the fact t h a t breakdown of the ester group does not occur even at 210°C in the absence of light in the usual thermal reactions between PMMA and the chlorine atom, which lead to the formation of HC1 and MMA [5]. TABLE 2. I N T E N S I T I E S O F T H E P E A K S O F T H E P R I N C I P A L P R O D U C T S F R O M Tliw. PHOTOLYSIS OF PMMA IN THe. PHES~NC~ (÷) ~ D IN THE ~S~.NCE (--) OF FoC13F O R I R R A D I A T I O N W I T H U N F I L T E R E D L I G H T F R O M A DRSh-1000 LAMP

m/e

I + 44 41 100 31 60

Temperature, °C 50 78 ÷ -I

20 17 0 0

12 1

0 19 11

9

6

-i 33"0 2

32 4 3 110 54

1

50 35

47 31 17 100 65

90"5

i +

40 82 47 200 100

52 180 110 200 100

46 630 390 400 180

Product of photolysis CO2 MMA MMA MF MF

The ways in which both the rates of consumption of the ester groups and the relative yields of the volatile products depend on the wavelength of the light also point to the occurrence of secondary photo-reactions involving intermediate radicals and leading to the breakdown of the ester group. I t m a y be suggested on the basis of existing data [1, 6] t h a t the photo-active macroradieals in the PMMA-FeC13 system are most probably CHa

CHs

(I)

~C

ooo .

ooo..

which are considerably less reactive in thermal transitions t h a n CH,

CHs

|

~ C---CH, ~ ~OOOH,

(II)

and

I

~ C--OH, ~

(HI)

~OOOH,

which are also formed in this system. The thermal stability of the radicals (I) leads to the fact t h a t t h e y can exist for a comparatively long time at T ~<0°C even in the presence of a considerable amount of the monomer (MMA) which readily accepts free radicals [7]. The high photochemical activity of radicals I has recently been observed b y the E P R method at 77°K. I t was convincingly demonstrated t h a t their photo-decomposition leads to the formation of methyl radicals [6]. The following has been,

2502

Yu. A. ~nrm~.yEv et a/.

however, considered as the only possible path: CHs

CHs

CHs (1)

ooo.. 'ooo..

ooo.. ooo..

Nevertheless, the results obtained in the present work make it necessary to assume that effective shedding of the ester group occurs: CHs I

CH, .

I

CH, 4,

I

CH, I

~C--CH---C ~ ---> ~G---CH----C~ -~COOCHs, ~OOCH~ COOCHs I I COOCH3

(s)

which can then undergo decomposition, radical substitution or recombination: (~OOCI~3-~COg+(3H s or C 0 + C I ~ s O PH CH,(R)

HCOOCH8+ P" *CHsCOOCH3(RCOOCHs),

where PM is the polymer molecule. I t is interesting to note t h a t the methyl radicals thus formed are less active in removing hydrogen atoms from PMMA compared with the removal of chlorine from FeC18. I n fact, the intensity of the peak with a mass number of sixteen, corresponding to methane, is very much less than the intensity of the CHsC1 peaks (Table 1). The considerable increase in the rate of decomposition of the ester groups in the PMMA-I~eC1 s system when filtered light is replaced by the complete light from a DRSh lamp (Fig. 1, 2) can be explained i n terms of the photochemical reactions of the radicals I. Radicals I have most probably the greatest absorption efficiency for light in the region of the spectrum with 2<260 nm. The preponderance of MF in the volatile products on going from light with 2>260 n m to the complete light (Tables 1 and 2) m a y be connegted with the fact that, when radicals I absorb photons of the short-wavelength region, the •COOH 8 group carries a greater kinetic energy than when photons of lower energy are absorbed. The lM;~or circumstance could have been conducive to the fact that the removal of an H atom from PMMA occurs even before the decomposition of the "COOCHs radical. Such an explanation is hypothetical. The effect of excess kinetic energy on the occurrence of radical substitution is known, in particular, for methyl radicals formed in the photolysis of solid solutions of CHsI and azomethane [8]. As distinct from "hot" radicals, "thermal" "CHs radicals are not capable of removing H atoms from hydrocarbon molecules at 77°K. As the energy of the absorbed photons is increased, the proportion of "hot" "CHs radicals capable of forming methane increases.

Light-induced reactions in polymethylmethacrylate

280~

According to equation (2), intensification of the splitting off of ether groups should increase the rate of formation of unsaturated groups in the macromoleeules equivalently, so t h a t the accumulation of double bonds can exceed their consumption in reactions with free radicals. I n fact, irradiation of PMMA containing l~eCl3 with unfiltered light leads to an increase in the concentration of double bonds (an absorption band at 1660 cm -1 appears in the I R spectra), a fact t h a t was not observed in the case of light with ~>260 nm. The attribution of the accumulating unsaturated groups to the macromolecules is confirmed b y the fact that mass spectrometric measurements do not show evolution of at room temperature. The evolution of the monomer at room temperature was not observed either in a study of the radiolysis of purified PMMA [9], or in a study of the photolysis of PMMA labelled with C14 [10]. The rate of depolymerization in the PMMA-FeC1 s system does, however, increase markedly as the temperature is raised (this m a y be seen, for example, from the increase in the intensity of the peak with m/e:41, Table 1). I t m a y be suggested that the decomposition of radicals I with rupture of the macro-chain leads finally to the formation of terminal radicals (P~erm)that participate in depolymerization [1, 2, 10]. The marked temperature dependence of the rate of evolution of MMA m a y be explained by the comparatively high activation energy for the decomposition of the terminal radicals P'term-*P'term~-MMA In summarizing the discussion of the results obtained, one may write the following scheme for the reactions occurring in the PMMA-FeC13 system under the action of light: hv FeC13---~ FeClz~-CI" Cl'WPMMA-.radicals I, II, III Radicals II, I I I ~ intermediate products -, radical I. Radical I ~ intermediate products -~ P~erm~P', where P - is a macromolecule with an end double bond Radical I ~ P ~ ÷'COOCH 8 "COOCHs-~COI-~'CHs I -~CO+CHsO" - - ~ H C O O C H s - { - r a d i c a l s I, II, III T*

PUm-~Purm-{-MMA (depolymerization) P-+R'-~P'" Radical I, R~erm+R'-~ stable products, where R'='CHs; CH30"(CH2OH); Cl'. The recombination of the macroradicals with the light radicals "CHs and CHaO" (the last stage in the Scheme) can provide an explanation of the fact t h a t the rate of consumption of the methyl groups is, according to the I R measurements, less than the rates of consumption of the other functional groups in PMMA

2804

Yu. A. iV/ag~r~.YEVeta/.

(Fig. 1, 2, absorption in the region 1452 cm-1). It has recently been established that approximately 3 0 ~ of the light radicals participate in this recombination reaction [11]. A reduction in the rate of consumption of methyl radicals may also be favoured by their "adhering" to unsaturated groups being formed. It should be noted that the result of low-temperature experiments (77°K) also agree with the suggested occurrence of reaction (2). For example, when PMMA containing deuterated ester groups, that has undergone radiolysis as a preliminary, is subjected to photolysis, CD~ radicals are formed [12], a fact that may be considered as a confirmation of this since, according to [6], specimens having undergone radiolysis at 77°K contain radicals I amongst the total set of radicals. On the basis of the material that has been discussed, it should be noted that it is obiously necessary to discuss the results in other papers [6, 12, 13] by taking into account reaction (2), the more so because the formation of a number of radicals that have been observed by the E P R method may be readily explained by photo-transitions of the ester fragment: hv

"COOCH,--*CO2+'CH3; CO+ CI-I30"(C'H2OH)~ HCO It should be emphasized that "COOCH s radicals ought to be characterized in the E P R spectrum by a singlet line and by sensitivity to light. In fact, the presence of this E P R line in the absorption of radicals sensitive to light has been reported [6, 12, 13]. The small quantity of unidentified radicals with a triplet hyperfine structure [6] may represent the residues of "CH2OH radicals, which are known to be very effectively decomposed by the action of light in the region 290-390 mn with the formation of formyl (HCO) radicals [8, 14]. With regard to the formation of "CH~OH radicals in the photo-decomposition of radicals I, it may be postulated that they are formed either directly during the photolysis of the "COOCH s fragment or by the isomerization of CHsO" radicals in a manner similar to ethoxyl radicals. Isomerization of the latter takes place both by a thermal and also by a photochemical path r

Av,

CH3CH,O'~ and ~CH,CHOH, /Jvs

where hv1< hv2 [7]. The contributions made by reactions (1) and (2) towards the photo-decomposition of radicals I may be compared. It has been shown [12, 13] that, during the photolysis of PMMA specimens thought to contain radicals I [6], the radical products are chiefly formed through the rupture of the ester group. If it is assumed that radicals I are also formed in the direct photolysis of PMMA, then the absence of the removal of ~-methyl groups and the decomposition of only the ester groups in experiments with PMMA labelled with C14 [10] may also be considered as evidence of the dominating part played by reaction (2). Trana~Jf~d by G. F. MODLEN

Polymethacrylates containing sulphur

2805

REFERENCES 1. Yu. A. MIKHEYEV, G. B. PARHSKII, V. F. SHUBNYAKOV and D. Ya. TOPTYGIN, Khlmlya vysokikh energii 5: 77, 1971 2. V. 1~. GOL'DBERG, I. M. BEL'GOVSKH~ G. I. KORNIYENKO, I. A. KRASOTKINA, N. I. ZAITSEVA and D. Ya. TOPTYGIN, Dokl. AN SSSR 198: 872, 1971 3. Ya. M. BUS, N. D. SHCHERBA and A. N. TYNNII, Fiziko-khimicheskaya mekhanika materialov 6: 114, 1970 4. A. CERNU and R. MASSOT, Compilation of Mass Spectral Data, London-Paris, 1966 5. T. C. McNEHJ and D. NEILL, Europ. Polymer J. 6: 569, 1970 6. G. B. PARIISKII, Ye. Ya. DAVYDOV and D. Ya. TOPTYGIN, Izv. AN SSSR, seriya khimich., 2207, 1972 7. P. Yu. BUTYAGIN, Dokl. AN SSSR 165: 103, 1965 8. B. N. SHELIMOV, Dissertation, 1964 9. A. TODD, J. Polymer Sci. 42: 223, 1960 10. M. I. FROLOVA, L. V. NEVSKII and A. V. RYADOV, Vysokomol. soyed. 8: 877, 1961 (Translated in Polymer Sci. U.S.S.R. 3: 4, 703, 1961) 11. C. DAVID, D. FULl) and G. GEUSKENS, Makromolek. Chem. 189: 269, 1970 12. V. G. VINOGRADOVA, B. N. SHEIJIMOV and N. V. FOK, Khimiya vysokikh energii 2: 128, 1968 13. V. G. VINOGRADOV, B. N. SHELIMOV and N. V. FOK, Khimiya vysokikh energii 2: 136, 1968 14. B. N. SHELIMOV, N. V. FOK and V. V. VOYEVODSKII, Kinetika i kataliz 4: 539, 1963

POLYMETHACRYLATES CONTAINING SULPHUR: MECHANISM OF STABII,IZATION AGAINST THERM0-OXIDATIVE DEGRADATION* N. K. AVDONINA, A. A. BERLIN, G. N. SHVAREVAand G. B. SH~tAKOV (Received 28 February 1972) W i t h a methyl methacrylate-ethylthioethylmethaerylate copolymer as an example, the part played by sulphidie sulphur and the effect of the "foreign" link on the stabilization of sulphur-ecntaining polymethacrylates against thermo-oxidat i r e degradation were investigated. Investigation of the composition of the volatile products and the values of the copolymerization constants showed that stabilization is based on the decomposition of thioalkylacrylate units formed in the breakdown of hydroperoxides by sulphidic groups.

THE stabilization of polymethacrylates against thermo-oxidative decomposition by the introduction of thioalkylacrylates into the polymeric chain has been reported [1, 2]. * Vysokomol. soyed. A15: No. 11, 2477-2479, 1973.