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The mechanism of the light-shielding action of metal dialkyl dithiocarbamates in polydienes
1882
V.B.
IvA_~ov ~ a/.
6. M. B. LACHINOV, R. A. SIMOYAN, T. O. GEORGIEVA, V. P. ZUBOV a n d V. A. K A B & NOV, J. P o l y m e r Sci., P o l y m e r Chem. Ed. 17: 613, 1979 7. M. B. LACHINOV, V. Ire. DREBAL, V, A. KASAIKIN, V. A. SIMONYAN, N. I. SHINUL I ~ A , V. P. ZUBOV and V. A. KABANOV, Vysokomol. soyed. AI9: 741, 1977 (Translated in P o l y m e r Sci. U.S.S.R. 19: 4, 859, 1977) 8. S. T. BALKE a n d A. E. HAMIELEC, J. Appl. P o l y m e r Sei. 17: 905, 1973 9. B. W. BROOKS, Proc. Roy. Soe. 357: 183, 1977 10. J. DIONISIO, H. K. MAHABADI, K. F. DRISCOLL, E. ABUIN a n d E. A. LISSI, J . P o l y m e r Sci., P o l y m e r Chem. Ed. 17: 1891, 1979 11. M. DAOUD, J. P. COTTON, B. FARNOUX, G. JANNINK, G. SARMA, H. BENOIT~ R. DUPLESSIX, C. PICOT a n d P. G. DE GENNES, Macromolecules, 8: 804, 1975 I2. P. G. D E GENNES, J. Chem. Phys. 55: 572, 1971 13. P. G. DE GENNES, Macromoleeulos 9: 587, 1976 14. J. KLEIN, Macromoleeules 11: 582, 1978 15. A. R. KHOKHLOV, Vysokomol. soyed. A20: 2754, 1978 (Translated in Polymer S c i , U.S.S.R. 20: 12, 3087, 1978) 16. M. DAOUD and G. JANNINK, J. Phys. 87: 973, 1976 17. S. I. KUCHANOV a n d Ye. S. POVOLOTSKAYA, Dokl. A N SSS]~ 227: 1147, 1976 18. E.S. GARING, A. V. OLENIN, V. P. ZUBOV, S. I. KUCHANOV, Ye. S.POVOLOTSKAYA and V. KABANOV, J. Polymer. Sei., Polymer Chem. Ed. 16: 2199, 1978
Polymer Science U.S.S.R. Vol. 23, No. 8, pp. 1882-1888, 1981 Printed in Poland
0032-3950/81/081882-07507.5010, © 1982 Pergamon Press Ltd.
THE MECHANISM OF THE LIGHT-SHIELDING ACTION OF METAL DIALKYL DITHIOCARBAMATES IN POLYDIENES* V. B . Ivn~cov, I~ DE~¢ Su, Y~. L. LOZOVSKAYAand V. YA. SHLYA]?ZIqTOKtt. I n s t i t u t e of Chemical Physics, U.S.S.R. A c a d e m y of Sciences
(Received 7 May 1980) The efficiency of light-shielding action of m e t a l dialkyldithioearbamates in polyflienes is m a i n l y a result of t h e i r a b i l i t y to decompose hydroperoxides and also, in a certain measure, of their absorptive c a p a c i t y and photochemical stability. The rate constants a n d stoichiometric reaction coefficients were determined for a series of complexes with peroxide macroradicals and hydroperoxides in an isoprene-styrene block eopolymer.
I)IALKYLDITHIOCARBAMATESof transition metals are efficient light and thermal' s t a b i l i z e r s f o r p o l y m e r s [1, 2]. T h e y a r e w i d e l y u s e d a s s t a b i l i z e r s a n d v u l c a n i zation accelerators for materials based on polydienes. When the light-shielding: * Vysokomol. soyed. A23: No. 8, 1711-1716, 1081.
Metal dialkyl dithiocarbamates
i883
a c t i o n o f t h e s e e o m p m m d s h a s b e e n e x a m i n e d , it h a s g e n e r a l l y b e e n s u g g e s t e d t h a t t h e y s u p p r e s s t h e e x c i t e d c o n d i t i o n o f m i x t u r e s a n d c h r o m o p h o r i c groups, r e a c t w i t h free r a d i c a l s a n d h y d r o p e r o x i d e s a n d shield t h e p o l y m e r f r o m t h e ~etion of l i g h t [3]. H o w e v e r , t h e significance of e a c h of these m e c h a n i s m s a n d t h e i r c o n t r i b u t i o n t~o t h e general s t a b i l i z a t i o n effect h a s n o t y e t b e e n e s t a b l i s h e d . I t is also n e c e s s a r y to consider t h e f a c t t h a t some p e r o x i d e / m e t a l c o m p l e x e s m a y , in principle, a c t as p h o t o i n i t i a t o r s . I n t h i s work, we h a v e s t u d i e d t h e et~ciency o f Bi, Co. Cu, Fe, Ni a n d Z n d i a l k y l d i t h i o c a r b a m a t e s as l i g h t - s t a b i l i z e r s for p o l y d i e n e s in d i r e c t a n d sensitized o x i d a t i o n a n d also t h e k i n e t i c s of t h e c o n s u m p t i o n of t h e c o m p l e x e s in t h e p o l y m e r . W e s t u d i e d t h e w i d e s t a p p l i c a t i o n s of zinc a n d b i s m u t h d i e t h y l d i t h i o c a r b a m a t e s a n d also nickel d i b n t s l d i t h i o c a r b a m a t e . Besides this, we i n v e s t i g a t e d copper, iron al~ c o b a l t d i e t h v l d i t h i o c a r b a m a t e s ~ h i c h can b e f o r m e d in t h e p o t y n w r f r o m m i x t u r e s with th~ c o r r e s p o n d i n g m e t a l salts. The study was earried out with samples of isoprene-styrene thermoelastoplasts, grad, 1ST-30 with 3I ~ 100,o00, ,~btaiued by solution polymerizati(m with organolithium catalysts. The polymer was t)urified by 4-fold precipitation of the benzene solution with isopropan~,l. Benzene, grade Ch., i-~)propanol, grade Ch. D. A. and chlor()form, grade Ch. were used withcn ~t further purification. Benzil (CsHsCOCOC6I~s) was t.~vice recrystallized from ethan, I, 2,2,6,6-tetramett~yl-4-bcnzoyloxypiperidine-l-oxide from heptane. Nickel dibutyld[thiocarbamate, Ni[Bu:NCS,.~2, the diethyldithiocarbamates of copper, Cu[Et~NCS,],, bismuth, Bi[Et,NCS2]s, zinc, ZulE .:NCS~],, iron, Fe[Et2NCS,]z m , t cobalt, Co[Et~NCS2] were twic,, r('crystMlized from benzene or acetone. Films, 30-120 lm~ thM~, were formed by slow cvapora.tio~ of 5% polymer s~)lntiens in chloroform on a (',,Ih~pban(~ backing, followed by peeling ,ff ill water. These samt)les w(~r'~ (~xposed at 20j=2 '~ ~:~)t.be light from a CVD-120 lamp with ~>300 nm or to monochromatic light with ),=-365, 4t~5 or 436 mn from DRSH-1000 or CVD-120 lamps using filters. Th,, oxidation kinetics of the polymer were studied by folloxving the increase i~t optical density in the ranges of 3450 cm -J (OI-l:groups) or 1720 riD.-1 (CO groups) [4] mid also by the mar~.ometric method. T}te sensitivity of the ma~tometer assembly was 1 × 10 -s mole, based on O., absorption. Consm~q)ti m of stable nitroxyl radicals was followed by the .ESR method m~d that of metal eomt)lexes spectrophotometrically by chang- of long wave band absorpti(,~. F i g u r e 1 show,~ t h e k i n e t i c curves for a c c u m u l a t i o n of h y d r o x y l g r o u p s a n d t h e c o n s u m p t i o n of c o m p l e x e s w i t h light, e x p o s u r e of s a m p l e s c o n t a i n i n g t h e d i a l k y l d i t h i o e a , r b a m a t e s in c o n c e n t r a t i o n s of 0.10 mole/kg. S p e c t r o p h o t o m e t r y was n o t s u i t a b l e for t h e k i n e t i c s o f Zn [Et2NCS,~]2 c o n s u m p t i o n , as this c o m p l e x (lid n o t a b s o r b in the n e a r U V or visible regions. T h e c o m p l e x e s m a y be conv e n t i o n M l y d i v i d e d into 3 groups: t h o s e h a v i n g p r a c t i c a l l y no light-stabilizing p o w e r (Co [Et~N ~S~]~ a n d Fe[Et~NCS~]a), stabilizers (Zn[Et~NCS.,]~, Cu (Et,,NCS°]) and Bi [Et,2NCS~.]a) an(] h i g h l y effective ones (Ni [Bu~NCS,/]). All complex(~s s h o w e d a c o r r e l a t i o n l)etween light s t a b i l i z i n g p o w e r a n d t h e i r r a t e o f c o n s u m p tion, i.e. t h e worse t h e stabilizer, t h e q u i c k e r it b r e a k s d o w n w h e n t h e s a m p l e is e x p o s e d to light. H o w e v e r , t h a t this is l i m i t e d a n d r e l a t e s to t h e c o n s n m p t i o n of t h e c o m p l e x as a whole. As Fig. lb shows, t h e r a t e of b r e a k - d o w n of a c o m p l e x m ~ y b o t h increase (e.g. for Bi a n d Ni c o m p l e x e s ) a n d d e c r e a s e (for Cu [Et,~CS~],,),
1884
V.B.
Iv~ov
~ a/.
w i t h the extent of tranaformation. Therefore one can only speak of a correlation between the time in which the concentration falls significantly and the length ,of the induction period of the oxidation. I t is not possible to connect the stabilizing action of complexes in these conditions with their ability to suppress the excited condition of the chromophores o f the polymer. This is because the basic photochemically active intermediate in photo-oxidation of polydienes is a hydroperoxide [4] the life of which, in an e x c i t e d state, is very short. Besides, one must take into account t h a t with an solutions, a significant stabilizing effect m a y only be attained at very high concentrations (0.l-1 mole]kg). Final|y, it is known t h a t benzotriazole derivatives, which effectively suppress the excited state, very weakly influence the photo-oxidation of polydienes [5].
0"61 f
c.
Z
3
a
,.ol /ke
o
e
b
#
q
O
16
50 T/me ~ he
2q
70
90
FIG. l. Kinetic curves for photo-oxidation of the. polymer (a) and consumption of complex (b) with exposure of 170 ;,n thick films to light of 2>300 run from an SVI)-120 lamp: 1-5--in presence of the diethyldithiocarbamates of cobalt (1), iron (2), copper (3), bismuth (4) and zinc (5); 6--nickel dibutyldithioearbamate, 7--unstabilized polymer. Additive concentration, (~01 mole/kg.
l~Ietal dialkyl dithiocarbamates
1885
In order to explain the contribution of shielding to the general light stabilization effect, the photo-oxidation of thinner samples (/----30 /~m), containing the same complexes at a concentration of 0.01 mole/kg, was studied. It was shown t h a t here the efficiency of the complexes changed in the same order as for the thicker samples, although the length of the induction periods in the presence of Cu, Bi, Zn and Ni compounds were somewhat decreased compared with those for the thicker samples. Thus shielding b y complexes makes a definite contri-
dDaqso O.q]
~~~~2~I
20
60
100 T[rne, min
fqO
FI¢~. 2. Accumulation of hydroperoxide groups in benzil-sensitized (0. ] mole/kg) photo-oxidation of 170 gm thick films under the action of light of ~=405 nm in the presence of
~002 mole/kg of metal complexes. The figures on the curves correspond to the s~me stabilizers as in Fig. 1. bution to the general stabilizing effect b u t this does not explain the varying efficiency of different complexes. This is evident from the following experiment.. Samples ~ 8 0 pm thick and containing 0.003 mole]kg of Ni [Bu2NCS,]~ ol, Zu[Et2NCS~] 2 were exposed to light of ,~-----313 nm. The nickel complex absorbed light appreciably at this longer wave length b u t the zinc one did not. The inductiou periods for the oxidation of these samples were 230 and 98 hr respectively. i f one makes the optical density of the sample with Zn [Et3~NCS2]2 the same as that containing Ni [BueNCS~]~ b y adding 0.008 mole/kg of 2-hydroxY-4-octyloxybenzophenone then the induction period is increased to 170 hr. We know that 2-hydroxy-4-octyloxybenzophenone slightly influences the induction period of photo-oxidation of polydienes. It follows that a mixture of a UV absorber and a zinc complex possesses synergistic properties b u t the effect attained b y its application is appreciably less than that which the nickel complex provides. In order to determine whether a difference in efficiency of light shielding was connected with differences in inhibiting properties of complexes, their influence on the benzil-sensitized oxidation of a polymer was studied. I t was shown that in these conditions, the induction periods differed little (Fig. 2) and that Ou[Et2NCSa],is the most effective stabilizer. The coefficient of inhibition f can be
.1886
V.B.
IVANOV et al.
determined from the value (v) of the induction period
f =A'r'wl/c where wj is rate of initiation, A~ is the difference between values of the induction period in presence and in absence of a complex and c is the concentration of the complex. The initiation rate was determined from the rate of consumption of the stable nitroxyl radical, 2,2,6,6-tetramethyl-4-benzoyloxypiperidine-l-oxide. It was shown t h a t the inhibition coefficients had anomalously small values -- 0.16, 0.]9, 0.22, 0.22 and 0.23 for complexes of zinc, nickel, iron, bismuth and copper respectively. We note that the values of f for Cu [Et2NCS~J ~, determined with different initiation rates (from 0.22 to 1.7 × 10 -7 mole/kg.sec) and concentrations (0.1 to l × 10 -8 mole/kg)are practically the same (0.23_+0.03); f has a similar value when determined from the rate of consumption of Ou[Et~NCS~J2 and also from the induction period in the benzophenone sensitized process. These facts affirm that the basic process is a free radical reaction which is involved in the consumption of Cu [Et~NCS~]2 during sensitized polymer oxidation. Figure 3 presents kinetic curves of oxygen absorption during benzil sensitized oxidation of films, containing 0.01 mole/kg of N i ~ u ~ C S ~ ] 2 or Cu[Et~NCS.,]~. It can be seen that under these conditions, the complexes have a fairly weak influence on polymer oxidation. Since the nickel complex is hardly consumed when exposed to light in a vacuum (pressure ~<10 -6 Pa) but the copper complex is used up at almost the same rate as in air, it is thought that ~ i ~ u ~ C S 2 ] ~
10-
1
2
3
5
/00
300
500 Time ~rain
700
F~G. 3. Kinetic curves for oxygen absorption in benzi]- (0-1 mole/kg) sensitized oxidation of ~mstabilized films (1) and film~ containing ~01 mole]kg of Ni[BuINCS,], (2) or Cu[E~NC~=]= (3). E x p o s u r e ¢o light with ~ = 365 (/, 3) or 436 nm (1, 2).
Metal dialkyl dithiocarbamates
1887
reacts mainly with peroxides whereas Cu[Et~CS~]2 may react either with peroxides or allylic maeroradicals. Then, from the data of :Fig. 3; corresponding to the schematic inhibited oxidation of the polymer: h,
Sensitizer P---H,O7 PO~ wi
PO~+PH
~ POoH+P" kpr
P'+02---~ PO~
ko
PO~-In---~ products ki~ based on the expression:
(kp~[PH])/k,~=(wo,. [In])/w~ where wo, is the rate of polymer oxidation, [In] is the concentration of the complex, wi-:4.8 × 10-s mo]e/kg.sec for Ni[Bu21ffCS~]2, we may determine the ratio of the chain growth reaction constant to inhibitor breakdown: (kpr[PH])/(kIn'f)=0-30 mole/kg. The value kpr [PH]=3.2 sec -1 for the block copolymer IST-30 was calculated from the results of unstabilized polymer oxidation. Therefore for the nickel complex, f'/~n= 11 kg/mole.sec. Thus the results obtained show that the metal dithiocarbamates studied are weak inhibitors in polydienes, owing to the small difference in their coefficients and inhibition constants. Therefore the light shielding power of the complexes is only connected in small measure with their inhibiting ability and the~L only at relatively high concentrations (~10 -3 mole/kg). We note that unlike liquid hydrocarbons [6] and polypropylene [7], the complexes have a very weak inhibiting action (low values of f and fin) in polydienes. Thus for example, the values off=0.19 and f. krn/kpr [ P H I = 3"3 kg/mole in a polydiene for Ni [Bu~CS~] 2 is more than an order smaller than in polypropylene at 115° ( f = 5.2; f. kIn/kpr [PH] =-99 kg/mole [7]) although the values of/~n/kpr [PH] are practically the same and amouut to 17 and 19 kg/mole, respectively. This is evidently explained by the participation of reaction products of the complex with free radicals in the oxidative chain growth rc~ction which is, in its turn, connected with the high reactivity of polydienes. It is proposed from the data, that the difference in efficiency of the complexes may be explained by a difference in their light stability and their reactivity towards hydroperoxides. Actually, the rate of oxygen absorption of samples exposed to light of )~> 300 nm containing 0-01 mole/kg complex is notably larger in the presence of Cu[Et~NCS2]2 than lffi[BuNCS~]~in spite of the fact that the copper compound has a higher inhibiting power at the same concentration (Fig. 3). Therefore the copper complex is significantly less stable to light than the nickel one. Qualitative confirmation of this result can be seen in Fig. 1. Unlike the nickel complex, under the conditions used for testing the polymer
1888
V.B.
Iv~ov
e~ a/.
light stability, the copper complex breaks up rapidly as soon as exposure to light begins. Whilst studying the breakdown of the copper and nickel complexes under conditions of sensitized oxidation under high intensity illumination, we found t h a t the complex concentration contained to fall after the light was extinguished i.e. a photochemical after-effect was observed. Knowing the initiation rate (wi=5.4 × 10 -7 mole/kg.see) in experiments with Cu[Et~NCS2]~ (initial concentration 0.01 mol/kg) we can determine the concentration of peroxide being formed in the polymer b y exposure to light ([POOH]=0.003 mole/kg) and then according to ~he expression
Wr-~r "fr"[POOH] [In] (wr is the rate of complex break-down after cutting off the light) we can determine the effective constant for hydroperoxide decomposition f r ' k r = 0 " 0 3 kg/mole.see. The stoichiometrie coefficient for the hydroperoxide decomposition reaction m a y be determined from the formula f r - - [POOH/[In] For the copper complex, fr-----1. In the same way, we obtain kr----1 kg/mole.see a n d f r ~ 30 for Ni [Bu~CS2]~. However, in the latter case the after-effect could only be observed at small concentrations of the complex ( _ 10 -3 mole/kg) whilst the characteristic reaction time even in these conditions ( _ 5-10 min) is almost an order less than in experiments with Cu [Et~NCS~]2. Thus the high efficiency of the light-shielding action of the nickel complex in a polydiene is explained mainly b y the high efficiency of hydroperoxide decomposition. Translated by C. W. CAPP REFERENCES
1. B. RENBY andYa.RABEK, Fotodostruktsiya, foteokisleniye, fotostabiliza~siya polimerov, p. 437, iKir, 1978 2. I. FOIGT, S~abilizatsiya sintetich, polimerov protlv deystviyu sveba i tepla, p. 544, Khimia, 1972 3. V. Ya. SHLIAPINTOKH, Fotokhimich. prevr, i stabiliz, polimerov, p. 331, Khimiya, 1979 4. V. B. IVANOV, lYI. N. KUSHNETSOVA, L. G. ANGERT and V. Ya. SttI.TAPINTOKH, Vysokomol. soyed. A20: 465, 1978 (Translated in Polymer Sci. U.S.S.R. 20: 2, 526, 1978) 5. M. N. KUZNETSOVA, L. G. ANGERT, V. X. IVANOV and A. B. SHAPIRO, Kauchuk i rezina, No. 2,22, 1977 6. L. L. GERVITS, N. V. ZOLOTOVA and Ye. T. DENISOV, l~eftekhimia 15: 135, 1975 7. L. L. GERVITS, N. V. ZOLOTOVAand Ye. T. DENISOV, Vysokomol. soyed. AI7: 2112, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 9, 2437, 1975) 8. A. N. ZVEREV and V. G. VINOGRADOVA,Izv. ANTSSSR, ser. khim., No. 1, 23, 1979
V.B.
IvA_~ov ~ a/.
6. M. B. LACHINOV, R. A. SIMOYAN, T. O. GEORGIEVA, V. P. ZUBOV a n d V. A. K A B & NOV, J. P o l y m e r Sci., P o l y m e r Chem. Ed. 17: 613, 1979 7. M. B. LACHINOV, V. Ire. DREBAL, V, A. KASAIKIN, V. A. SIMONYAN, N. I. SHINUL I ~ A , V. P. ZUBOV and V. A. KABANOV, Vysokomol. soyed. AI9: 741, 1977 (Translated in P o l y m e r Sci. U.S.S.R. 19: 4, 859, 1977) 8. S. T. BALKE a n d A. E. HAMIELEC, J. Appl. P o l y m e r Sei. 17: 905, 1973 9. B. W. BROOKS, Proc. Roy. Soe. 357: 183, 1977 10. J. DIONISIO, H. K. MAHABADI, K. F. DRISCOLL, E. ABUIN a n d E. A. LISSI, J . P o l y m e r Sci., P o l y m e r Chem. Ed. 17: 1891, 1979 11. M. DAOUD, J. P. COTTON, B. FARNOUX, G. JANNINK, G. SARMA, H. BENOIT~ R. DUPLESSIX, C. PICOT a n d P. G. DE GENNES, Macromolecules, 8: 804, 1975 I2. P. G. D E GENNES, J. Chem. Phys. 55: 572, 1971 13. P. G. DE GENNES, Macromoleeulos 9: 587, 1976 14. J. KLEIN, Macromoleeules 11: 582, 1978 15. A. R. KHOKHLOV, Vysokomol. soyed. A20: 2754, 1978 (Translated in Polymer S c i , U.S.S.R. 20: 12, 3087, 1978) 16. M. DAOUD and G. JANNINK, J. Phys. 87: 973, 1976 17. S. I. KUCHANOV a n d Ye. S. POVOLOTSKAYA, Dokl. A N SSS]~ 227: 1147, 1976 18. E.S. GARING, A. V. OLENIN, V. P. ZUBOV, S. I. KUCHANOV, Ye. S.POVOLOTSKAYA and V. KABANOV, J. Polymer. Sei., Polymer Chem. Ed. 16: 2199, 1978
Polymer Science U.S.S.R. Vol. 23, No. 8, pp. 1882-1888, 1981 Printed in Poland
0032-3950/81/081882-07507.5010, © 1982 Pergamon Press Ltd.
THE MECHANISM OF THE LIGHT-SHIELDING ACTION OF METAL DIALKYL DITHIOCARBAMATES IN POLYDIENES* V. B . Ivn~cov, I~ DE~¢ Su, Y~. L. LOZOVSKAYAand V. YA. SHLYA]?ZIqTOKtt. I n s t i t u t e of Chemical Physics, U.S.S.R. A c a d e m y of Sciences
(Received 7 May 1980) The efficiency of light-shielding action of m e t a l dialkyldithioearbamates in polyflienes is m a i n l y a result of t h e i r a b i l i t y to decompose hydroperoxides and also, in a certain measure, of their absorptive c a p a c i t y and photochemical stability. The rate constants a n d stoichiometric reaction coefficients were determined for a series of complexes with peroxide macroradicals and hydroperoxides in an isoprene-styrene block eopolymer.
I)IALKYLDITHIOCARBAMATESof transition metals are efficient light and thermal' s t a b i l i z e r s f o r p o l y m e r s [1, 2]. T h e y a r e w i d e l y u s e d a s s t a b i l i z e r s a n d v u l c a n i zation accelerators for materials based on polydienes. When the light-shielding: * Vysokomol. soyed. A23: No. 8, 1711-1716, 1081.
Metal dialkyl dithiocarbamates
i883
a c t i o n o f t h e s e e o m p m m d s h a s b e e n e x a m i n e d , it h a s g e n e r a l l y b e e n s u g g e s t e d t h a t t h e y s u p p r e s s t h e e x c i t e d c o n d i t i o n o f m i x t u r e s a n d c h r o m o p h o r i c groups, r e a c t w i t h free r a d i c a l s a n d h y d r o p e r o x i d e s a n d shield t h e p o l y m e r f r o m t h e ~etion of l i g h t [3]. H o w e v e r , t h e significance of e a c h of these m e c h a n i s m s a n d t h e i r c o n t r i b u t i o n t~o t h e general s t a b i l i z a t i o n effect h a s n o t y e t b e e n e s t a b l i s h e d . I t is also n e c e s s a r y to consider t h e f a c t t h a t some p e r o x i d e / m e t a l c o m p l e x e s m a y , in principle, a c t as p h o t o i n i t i a t o r s . I n t h i s work, we h a v e s t u d i e d t h e et~ciency o f Bi, Co. Cu, Fe, Ni a n d Z n d i a l k y l d i t h i o c a r b a m a t e s as l i g h t - s t a b i l i z e r s for p o l y d i e n e s in d i r e c t a n d sensitized o x i d a t i o n a n d also t h e k i n e t i c s of t h e c o n s u m p t i o n of t h e c o m p l e x e s in t h e p o l y m e r . W e s t u d i e d t h e w i d e s t a p p l i c a t i o n s of zinc a n d b i s m u t h d i e t h y l d i t h i o c a r b a m a t e s a n d also nickel d i b n t s l d i t h i o c a r b a m a t e . Besides this, we i n v e s t i g a t e d copper, iron al~ c o b a l t d i e t h v l d i t h i o c a r b a m a t e s ~ h i c h can b e f o r m e d in t h e p o t y n w r f r o m m i x t u r e s with th~ c o r r e s p o n d i n g m e t a l salts. The study was earried out with samples of isoprene-styrene thermoelastoplasts, grad, 1ST-30 with 3I ~ 100,o00, ,~btaiued by solution polymerizati(m with organolithium catalysts. The polymer was t)urified by 4-fold precipitation of the benzene solution with isopropan~,l. Benzene, grade Ch., i-~)propanol, grade Ch. D. A. and chlor()form, grade Ch. were used withcn ~t further purification. Benzil (CsHsCOCOC6I~s) was t.~vice recrystallized from ethan, I, 2,2,6,6-tetramett~yl-4-bcnzoyloxypiperidine-l-oxide from heptane. Nickel dibutyld[thiocarbamate, Ni[Bu:NCS,.~2, the diethyldithiocarbamates of copper, Cu[Et~NCS,],, bismuth, Bi[Et,NCS2]s, zinc, ZulE .:NCS~],, iron, Fe[Et2NCS,]z m , t cobalt, Co[Et~NCS2] were twic,, r('crystMlized from benzene or acetone. Films, 30-120 lm~ thM~, were formed by slow cvapora.tio~ of 5% polymer s~)lntiens in chloroform on a (',,Ih~pban(~ backing, followed by peeling ,ff ill water. These samt)les w(~r'~ (~xposed at 20j=2 '~ ~:~)t.be light from a CVD-120 lamp with ~>300 nm or to monochromatic light with ),=-365, 4t~5 or 436 mn from DRSH-1000 or CVD-120 lamps using filters. Th,, oxidation kinetics of the polymer were studied by folloxving the increase i~t optical density in the ranges of 3450 cm -J (OI-l:groups) or 1720 riD.-1 (CO groups) [4] mid also by the mar~.ometric method. T}te sensitivity of the ma~tometer assembly was 1 × 10 -s mole, based on O., absorption. Consm~q)ti m of stable nitroxyl radicals was followed by the .ESR method m~d that of metal eomt)lexes spectrophotometrically by chang- of long wave band absorpti(,~. F i g u r e 1 show,~ t h e k i n e t i c curves for a c c u m u l a t i o n of h y d r o x y l g r o u p s a n d t h e c o n s u m p t i o n of c o m p l e x e s w i t h light, e x p o s u r e of s a m p l e s c o n t a i n i n g t h e d i a l k y l d i t h i o e a , r b a m a t e s in c o n c e n t r a t i o n s of 0.10 mole/kg. S p e c t r o p h o t o m e t r y was n o t s u i t a b l e for t h e k i n e t i c s o f Zn [Et2NCS,~]2 c o n s u m p t i o n , as this c o m p l e x (lid n o t a b s o r b in the n e a r U V or visible regions. T h e c o m p l e x e s m a y be conv e n t i o n M l y d i v i d e d into 3 groups: t h o s e h a v i n g p r a c t i c a l l y no light-stabilizing p o w e r (Co [Et~N ~S~]~ a n d Fe[Et~NCS~]a), stabilizers (Zn[Et~NCS.,]~, Cu (Et,,NCS°]) and Bi [Et,2NCS~.]a) an(] h i g h l y effective ones (Ni [Bu~NCS,/]). All complex(~s s h o w e d a c o r r e l a t i o n l)etween light s t a b i l i z i n g p o w e r a n d t h e i r r a t e o f c o n s u m p tion, i.e. t h e worse t h e stabilizer, t h e q u i c k e r it b r e a k s d o w n w h e n t h e s a m p l e is e x p o s e d to light. H o w e v e r , t h a t this is l i m i t e d a n d r e l a t e s to t h e c o n s n m p t i o n of t h e c o m p l e x as a whole. As Fig. lb shows, t h e r a t e of b r e a k - d o w n of a c o m p l e x m ~ y b o t h increase (e.g. for Bi a n d Ni c o m p l e x e s ) a n d d e c r e a s e (for Cu [Et,~CS~],,),
1884
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w i t h the extent of tranaformation. Therefore one can only speak of a correlation between the time in which the concentration falls significantly and the length ,of the induction period of the oxidation. I t is not possible to connect the stabilizing action of complexes in these conditions with their ability to suppress the excited condition of the chromophores o f the polymer. This is because the basic photochemically active intermediate in photo-oxidation of polydienes is a hydroperoxide [4] the life of which, in an e x c i t e d state, is very short. Besides, one must take into account t h a t with an solutions, a significant stabilizing effect m a y only be attained at very high concentrations (0.l-1 mole]kg). Final|y, it is known t h a t benzotriazole derivatives, which effectively suppress the excited state, very weakly influence the photo-oxidation of polydienes [5].
0"61 f
c.
Z
3
a
,.ol /ke
o
e
b
#
q
O
16
50 T/me ~ he
2q
70
90
FIG. l. Kinetic curves for photo-oxidation of the. polymer (a) and consumption of complex (b) with exposure of 170 ;,n thick films to light of 2>300 run from an SVI)-120 lamp: 1-5--in presence of the diethyldithiocarbamates of cobalt (1), iron (2), copper (3), bismuth (4) and zinc (5); 6--nickel dibutyldithioearbamate, 7--unstabilized polymer. Additive concentration, (~01 mole/kg.
l~Ietal dialkyl dithiocarbamates
1885
In order to explain the contribution of shielding to the general light stabilization effect, the photo-oxidation of thinner samples (/----30 /~m), containing the same complexes at a concentration of 0.01 mole/kg, was studied. It was shown t h a t here the efficiency of the complexes changed in the same order as for the thicker samples, although the length of the induction periods in the presence of Cu, Bi, Zn and Ni compounds were somewhat decreased compared with those for the thicker samples. Thus shielding b y complexes makes a definite contri-
dDaqso O.q]
~~~~2~I
20
60
100 T[rne, min
fqO
FI¢~. 2. Accumulation of hydroperoxide groups in benzil-sensitized (0. ] mole/kg) photo-oxidation of 170 gm thick films under the action of light of ~=405 nm in the presence of
~002 mole/kg of metal complexes. The figures on the curves correspond to the s~me stabilizers as in Fig. 1. bution to the general stabilizing effect b u t this does not explain the varying efficiency of different complexes. This is evident from the following experiment.. Samples ~ 8 0 pm thick and containing 0.003 mole]kg of Ni [Bu2NCS,]~ ol, Zu[Et2NCS~] 2 were exposed to light of ,~-----313 nm. The nickel complex absorbed light appreciably at this longer wave length b u t the zinc one did not. The inductiou periods for the oxidation of these samples were 230 and 98 hr respectively. i f one makes the optical density of the sample with Zn [Et3~NCS2]2 the same as that containing Ni [BueNCS~]~ b y adding 0.008 mole/kg of 2-hydroxY-4-octyloxybenzophenone then the induction period is increased to 170 hr. We know that 2-hydroxy-4-octyloxybenzophenone slightly influences the induction period of photo-oxidation of polydienes. It follows that a mixture of a UV absorber and a zinc complex possesses synergistic properties b u t the effect attained b y its application is appreciably less than that which the nickel complex provides. In order to determine whether a difference in efficiency of light shielding was connected with differences in inhibiting properties of complexes, their influence on the benzil-sensitized oxidation of a polymer was studied. I t was shown that in these conditions, the induction periods differed little (Fig. 2) and that Ou[Et2NCSa],is the most effective stabilizer. The coefficient of inhibition f can be
.1886
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IVANOV et al.
determined from the value (v) of the induction period
f =A'r'wl/c where wj is rate of initiation, A~ is the difference between values of the induction period in presence and in absence of a complex and c is the concentration of the complex. The initiation rate was determined from the rate of consumption of the stable nitroxyl radical, 2,2,6,6-tetramethyl-4-benzoyloxypiperidine-l-oxide. It was shown t h a t the inhibition coefficients had anomalously small values -- 0.16, 0.]9, 0.22, 0.22 and 0.23 for complexes of zinc, nickel, iron, bismuth and copper respectively. We note that the values of f for Cu [Et2NCS~J ~, determined with different initiation rates (from 0.22 to 1.7 × 10 -7 mole/kg.sec) and concentrations (0.1 to l × 10 -8 mole/kg)are practically the same (0.23_+0.03); f has a similar value when determined from the rate of consumption of Ou[Et~NCS~J2 and also from the induction period in the benzophenone sensitized process. These facts affirm that the basic process is a free radical reaction which is involved in the consumption of Cu [Et~NCS~]2 during sensitized polymer oxidation. Figure 3 presents kinetic curves of oxygen absorption during benzil sensitized oxidation of films, containing 0.01 mole/kg of N i ~ u ~ C S ~ ] 2 or Cu[Et~NCS.,]~. It can be seen that under these conditions, the complexes have a fairly weak influence on polymer oxidation. Since the nickel complex is hardly consumed when exposed to light in a vacuum (pressure ~<10 -6 Pa) but the copper complex is used up at almost the same rate as in air, it is thought that ~ i ~ u ~ C S 2 ] ~
10-
1
2
3
5
/00
300
500 Time ~rain
700
F~G. 3. Kinetic curves for oxygen absorption in benzi]- (0-1 mole/kg) sensitized oxidation of ~mstabilized films (1) and film~ containing ~01 mole]kg of Ni[BuINCS,], (2) or Cu[E~NC~=]= (3). E x p o s u r e ¢o light with ~ = 365 (/, 3) or 436 nm (1, 2).
Metal dialkyl dithiocarbamates
1887
reacts mainly with peroxides whereas Cu[Et~CS~]2 may react either with peroxides or allylic maeroradicals. Then, from the data of :Fig. 3; corresponding to the schematic inhibited oxidation of the polymer: h,
Sensitizer P---H,O7 PO~ wi
PO~+PH
~ POoH+P" kpr
P'+02---~ PO~
ko
PO~-In---~ products ki~ based on the expression:
(kp~[PH])/k,~=(wo,. [In])/w~ where wo, is the rate of polymer oxidation, [In] is the concentration of the complex, wi-:4.8 × 10-s mo]e/kg.sec for Ni[Bu21ffCS~]2, we may determine the ratio of the chain growth reaction constant to inhibitor breakdown: (kpr[PH])/(kIn'f)=0-30 mole/kg. The value kpr [PH]=3.2 sec -1 for the block copolymer IST-30 was calculated from the results of unstabilized polymer oxidation. Therefore for the nickel complex, f'/~n= 11 kg/mole.sec. Thus the results obtained show that the metal dithiocarbamates studied are weak inhibitors in polydienes, owing to the small difference in their coefficients and inhibition constants. Therefore the light shielding power of the complexes is only connected in small measure with their inhibiting ability and the~L only at relatively high concentrations (~10 -3 mole/kg). We note that unlike liquid hydrocarbons [6] and polypropylene [7], the complexes have a very weak inhibiting action (low values of f and fin) in polydienes. Thus for example, the values off=0.19 and f. krn/kpr [ P H I = 3"3 kg/mole in a polydiene for Ni [Bu~CS~] 2 is more than an order smaller than in polypropylene at 115° ( f = 5.2; f. kIn/kpr [PH] =-99 kg/mole [7]) although the values of/~n/kpr [PH] are practically the same and amouut to 17 and 19 kg/mole, respectively. This is evidently explained by the participation of reaction products of the complex with free radicals in the oxidative chain growth rc~ction which is, in its turn, connected with the high reactivity of polydienes. It is proposed from the data, that the difference in efficiency of the complexes may be explained by a difference in their light stability and their reactivity towards hydroperoxides. Actually, the rate of oxygen absorption of samples exposed to light of )~> 300 nm containing 0-01 mole/kg complex is notably larger in the presence of Cu[Et~NCS2]2 than lffi[BuNCS~]~in spite of the fact that the copper compound has a higher inhibiting power at the same concentration (Fig. 3). Therefore the copper complex is significantly less stable to light than the nickel one. Qualitative confirmation of this result can be seen in Fig. 1. Unlike the nickel complex, under the conditions used for testing the polymer
1888
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light stability, the copper complex breaks up rapidly as soon as exposure to light begins. Whilst studying the breakdown of the copper and nickel complexes under conditions of sensitized oxidation under high intensity illumination, we found t h a t the complex concentration contained to fall after the light was extinguished i.e. a photochemical after-effect was observed. Knowing the initiation rate (wi=5.4 × 10 -7 mole/kg.see) in experiments with Cu[Et~NCS2]~ (initial concentration 0.01 mol/kg) we can determine the concentration of peroxide being formed in the polymer b y exposure to light ([POOH]=0.003 mole/kg) and then according to ~he expression
Wr-~r "fr"[POOH] [In] (wr is the rate of complex break-down after cutting off the light) we can determine the effective constant for hydroperoxide decomposition f r ' k r = 0 " 0 3 kg/mole.see. The stoichiometrie coefficient for the hydroperoxide decomposition reaction m a y be determined from the formula f r - - [POOH/[In] For the copper complex, fr-----1. In the same way, we obtain kr----1 kg/mole.see a n d f r ~ 30 for Ni [Bu~CS2]~. However, in the latter case the after-effect could only be observed at small concentrations of the complex ( _ 10 -3 mole/kg) whilst the characteristic reaction time even in these conditions ( _ 5-10 min) is almost an order less than in experiments with Cu [Et~NCS~]2. Thus the high efficiency of the light-shielding action of the nickel complex in a polydiene is explained mainly b y the high efficiency of hydroperoxide decomposition. Translated by C. W. CAPP REFERENCES
1. B. RENBY andYa.RABEK, Fotodostruktsiya, foteokisleniye, fotostabiliza~siya polimerov, p. 437, iKir, 1978 2. I. FOIGT, S~abilizatsiya sintetich, polimerov protlv deystviyu sveba i tepla, p. 544, Khimia, 1972 3. V. Ya. SHLIAPINTOKH, Fotokhimich. prevr, i stabiliz, polimerov, p. 331, Khimiya, 1979 4. V. B. IVANOV, lYI. N. KUSHNETSOVA, L. G. ANGERT and V. Ya. SttI.TAPINTOKH, Vysokomol. soyed. A20: 465, 1978 (Translated in Polymer Sci. U.S.S.R. 20: 2, 526, 1978) 5. M. N. KUZNETSOVA, L. G. ANGERT, V. X. IVANOV and A. B. SHAPIRO, Kauchuk i rezina, No. 2,22, 1977 6. L. L. GERVITS, N. V. ZOLOTOVA and Ye. T. DENISOV, l~eftekhimia 15: 135, 1975 7. L. L. GERVITS, N. V. ZOLOTOVAand Ye. T. DENISOV, Vysokomol. soyed. AI7: 2112, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 9, 2437, 1975) 8. A. N. ZVEREV and V. G. VINOGRADOVA,Izv. ANTSSSR, ser. khim., No. 1, 23, 1979