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Photo-oxidative degradation of polyetherurethanes
1386
V . A . KOSOBUTSKH e$ at/.
The data obtained thus show, as a whole, that the polycyclotetramerization of tetraeyanobenzene occurs in bulk by a step-wise mechanism. Soluble ribbon-like oligomers are first formed, the number of azaporphin rings being from 3 to 7-8; these gradually transform into insoluble crosslinked three dimensional structures identical to those obtained by the additional heating of the soluble oligomers. The synthesis of soluble oligomers of PPhC and their transformation into insoluble flat-network or graphite-like structures considerably extent the possibility of investigating and using polymeric porphyrazines practically. Translated by G. F. MODLEI~ REFERENCES 1. A. A. BERLIN and A. J. SHERLE, Inorgan. Macromolec. Rcv. 1: 235, 1971 2. E. A. LAWTON and D. D. MoRITCHE, J. Organ. Chem. 24: 26, 1959 3. Ye. Yu. BEKHT.I, D. D. NOVIKOV and S. G. ENTELIS, Vysokomol. soyed. A: 2754, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 12, 3717, 1967) 4. N. I. SHAPIRO, I. P. SUZDALEV, V. I. GOLD'ANSKII, A. L SHERLE and A. A. BERLIN, Teoret;. eksperim, khim. 11: 330, 1975 5. Ye. A. LUK'YANETS, S. A. MIKHALENKO and Ye. A. K0VSHEV, Zh. obshch, khim. 41: 934, 1971 6. L. G. CHERKASHINA, Ye. L. FRANKEVICH, N. V. YEREMINA, Ye. I. BALABANOV and A. A, BERLIN, Vysokomol. soyed. 7: 1264, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 8, 1478, 1965)
Polymer Scien(~U.S.S.R. Vol. 22, No. 6, pp. 1386-1893.1980 Printed in Poland
0032-3050180/1386-8507.50/0
1081 Pergamon Preaq Ltd.
P H O T O - O X I D A T I V E D E G R A D A T I O N OF P O L Y E T H E R U R E T H A N E S * V. A. KOSOBUTSKII, M. N. KURGANOVA, O. G. TARAKANOV a n d V. K . BELYAKOV All-Union Scientific Research Institute for Synthetic Resins
(Received 3 April 1979) The photo-oxidative degradation of polyotherurothanes based on polyoxypropylone glycol with a molecular weight of 1000 a n d diisocyanatos with various chemical structures has been studied. I t has boon established t h a t aromatic urethane fragments stabilize the oligoether part of the maeromoleeules against photo-oxidation. The stabilizing effect of the urethano fragments is connected with the inhibition of oxidation through the formation of intermediate complexes with peroxide compounds and is determined by the electron-donor properties of the urethanes. A correlation has been established between the stabilizing effect and the urethane's ionization potential. * Vysokomol. soyed. A22: No. 6, 1264-1269, 1980.
Photo-oxidative (leg~adatiou ,)f polyetherurethanes
1387'
BECAUSE o f t h e d e v e l o p m e n t o f o x i d a t i v e d e g r a d a t i o n , c o m p o n e n t s m a d e o f p o l y e t h e r u r e t h a n e s ( P E U ) p a r t l y change colour u n d e r the action of . ~ n l i g h t and t h e i r p h y s i c o - m e c h a n i c a l p r o p e r t i e s a r e i m p a i r e d . T h e r e s i s t a n c e of p ( ) l y m e r s t o p h o t o - o x i d a t i o n is d e t e r m i n e d , in th(. first i n s t a n c e , b y t h(~ chemic:t! s t r u c t u r e of the polymeric chain. T h e p r e s e n t w o r k is d e v o t e d to a n i n v e s t i g a t i o n of" t h e effect of th,, ~.hemical s t r u c t u r e o f ~ r e t h a n e f c a g m e n ~ s o n t h e p h o t : o - o x i ( t a t i v e degradatiot~ ()f" PELT. O n t h e b a s i s o f i n f o r m a t i o n in t h e l i t e r a t u r e a b o u t t h e use o f low-molc*:ttlar urct h a n c s , u r e a s a n d t h e i r d e r i v a t i v c s as h e a t a n d l i g h t s t a b i l i z e r s [I--~;!, i t m a y , m o r e o v e r , b e exI)ectcd t h a t t h e u r e t h a n ( ; f r a g m e n t s o f t h e maeron~e*!(,('ule will stabiliz(; a g a i n s t t h e p h o t o - o x i d ~ t t i o n o f t h e o l i g o c t h e r p a r t . I f such s t a b i l i z i n g ~t(:tion e x i s t s , it m u s t b e t a k e n irate a c c o u n t in a t t e m p t i n g to find t h e o t ) t i m u m m e t h o ( t s o f s t a b i l i z i n g P E U . T h e p o s s i b i l i t y t h a t t h e r e is ~ s t a b i l i z a t i ( , u effect e x e r t e d b y t h e u r e t h a n e f r a g m e n t s a n d t h e n a t u r e o f t h i s effect are ~lso d i s c u s s e d i n t h e p r c s e n t p~ti)er. P E U were synthesized from poly()xypropylene glycol with M = I000 a~ld diisocyanat~,s with various chemical structures (ratio OH : N C O = l :2). tn order to exclud(, ~lny effect. of free isocyanate groupso, the P E U were treated with absolute metha~lol. The following diisocyanates were used: hexamethylene- (HMD), diphonylsulphone- (DPSD), diphenylmethane- (MD), p-phenylene- (PD), 1,5-naphthalene- (ND) and diphenyloxide- (DPOD). Before the synthesis, the polyoxypropylcne glycol was dried, the solid diisocyan~ttes were sub. limed ~md the liquid diisocyanates were distilled. The concentration of the bnsic, substance in the diisocyanates used was not less t]mn 99-5°/o. The diurethanes were synthesized from absolute methanol and the c~,ri~,spondin;~ diisocyanate. The ureas were synthesized by the addition of the diisocyan~te t~ exccs.~ diethylamine. The compounds obtained were identified from chemical analysis dat~. A PRK-2 lanlp was used as the illumination source. In the kinetic inve.~tig~tions, the lamp was placed at a distance of 20 cm fr~)m the specimen. The cell with the sp~'eirnens was ])laced iHa water thermostat to eliminate thermal heatiug. A "Tsvet-102" gas chromatograph was used to analyse the gaseous prodtl,-1~ of degr~,darien. The sensitivit,y of the instrument in determining oxides of carbon was Io -3 vol. o, aild in determining acetone and acetaldehyde 2 ~ 10 ~ vol. ° o. i n order to incr(.as~ the accuracy of analysis and to carry out the expori~r~ent automatically, a feed system x ~ constru(:ted to the (~hromatogr~iph which xv~s cormected directly to the cell being irradiated. ~:hen this system ~w~s tlsed, the error in the a m o u n t of the sample introduced did ~lot (.x(-eed 2o0. In order to exclude the influence of diffusion effects on the kinetics of ph~t~-,)xidati~l, of PEU. the specixnens used had a t hieluless of 0.003-0-005 Inm. Th(. UV spectra were recor(lcd with a SF-16 spectrophotomcter. The el(;ctroni(' structure of the urethal~es and ureas wer(~ e~leulated using-' th~ Pariser Parr P~q)le n~etho(t. The details of the calculations basically correspond to those in If0]: the geo~nclrical properties of the molecules were optimiz,,d during the calcuh~ti,)ns t)y ~ proc(~uro proposed by Dewar [8]. The initial geometrical characteristics w~re selected as fi)llovcs: all the rings were taken to be regular polygons with sides 0.I397 nm long; all the valency angles were 120 degrees and the longShs of the N - - C , C--O and C = O bonds wore, taken to be equal to 0.142, 0.136 a n d 0-142 nm respectively. The calculations were carried out by means of a computer programme [9]. The ioaizatAon potentials wore assessed using ~¢.oopmans theorem.
V. A. KOSOBUTSKII et at.
1388
Investigation of the gaseous product of the photo-oxidative degradation of P E U with various chemical structures showed that the qualitative composition of the gaseous phase was the same for all tho polymers investigated, which is evidence of the commonality of the processes occuring. The principal degradation products are carbon monoxide and carbon dioxide, acetone and acetaldehyde, carbon monoxide being the predominant amongst these. Figure 1 shows the kinetics of evolution ()f carbon monoxide during the action of UV radiation on polyoxypropyleue glycol and the P E U based on diisocyanatcs with various chemical structures. Under thcse conditions, oxidation processes initiated by light and photo-decomposition processes occur simultaneously; the latter are known from the literature [10] also to lead to the formation of oxides of carbon. CO,g, 105 -
n
5
2
18
7
4, 2
t
C
12
2 3
6 1
6
~
i
I
12
,
~
I
18
3
6
T/me, ho
3
6
FIe. 1. Evolution of carbon monoxide during the irradiation of the following with unfiltered light from a PRK-2 lamp: 1--polyoxypropyleneglycol; 2--polyetherurethane based on diphenyhnethanodiisoeyanato; 3--polyetherurethane based on diphenyloxidediisocyanate: a--in vacuum; b--in oxygen; c--as (b) ,but calculated curves for gas evolution with a correction for photolysis. I n order to separate out that part of the products which relate to oxidation processes, the kinetics of gas evolution were investigated with irradiation of polyoxypropylene glycol and aromatic P E U in vacuum. No oxides of caTbon were found during the irradiation of polyoxypropylene glycol. The photo-degradation of P E U based on aromatic diisocyanates is accommpanied b y the evolution of oxides of carbon (Fig. la). The kinetics of gas-evolution is described by a straight line proportional to the time of irradiation. In the absence of oxygen, oxides of carbon are thus evolved only through the breakdown of urethane groups linked to the aromatic nucleus. I t m a y be seen from Fig. lb, that, for irradiation in oxygen, the rates of evolution of oxides of carbon become comparable for polyoxypropylene glycol and P E U . I t m a y therefore be concluded that, during the photo-oxidation of P E U based on aromatic dii~ocyanates, the gaseous products are formed both through the photo-degradation of the urethane groups and also through the oxidation of the
Photo-oxidative degradation of polyetherurethanes
1389
oligoether section. Thus the amount of the gaseous products, in particular carbon monoxide, formed during the photo-oxidation of P E U should be equal to the sum of the gaseous products evolved during thc photolysis of" P E U and the photo-oxidation of polyoxypropylene glycol. It mav be seen fl'om an analysis of Fig. lc, that 1his quantity is considerably smaller. The curves for the evohttion of carbon monoxide during the photo-oxidation of PEU based on MI) and I)POD, a corre~'t ion being made for photolysis, lie; below the curve tbr ,_,as (~volution during lhe photo-oxidation of polyoxypropylcne glycol. This fact iltdicates t h a t tim aromatic: urethanc fragments stabilize the oligoethcr part against photo-oxidation. The stabilizing cffcct of these fragments appears most clearly for irradiation by light with a wavelength greater than 300 nm, that is, in the region where the, ~romatic PEU, apart from the PEU based on NI), (1o not have any absorl)tion of their own. A clearer pattcrn can be found if the products investigated arc those which arc evolved during the decomposition of only the oligocther part of the macromolccule, since oxidcs of carbon are formed during the degradation of both the urethane and also the oligoether part. Acetone and acetaldehyde are found to be products of this type. They are evolved during the photo-oxidation of the oligocther and no such products are found Cluring the photo-oxidation of the urethanc based on phenol and phenylisocyanate.
9" ~°5 cl
>I" z'a
F
" 2'3
~4
o.e
2
6
I0 2 Time, hr
8
10
Fro. 2. Evolution of a--acetaldehyde and b--am.~tono during tim irradiation of: l--polyoxypropyloneglycol; 2-7--polyethcrurethanes based on the follo~4ng diisoycanatos; 2-hexarnethylene-; 3 -- diphenylsulphone-; 4 -- diphcnylmethane-; 5 -- p-phenyleno; 6 -- naphtha leno-; 7-- diphonyloxidc-. The kinetic curves for acetone and acetaldehyde are shown in Rig. 2. It may be seen t h a t the induction period for the evolution of gaseous products is increased for P E U based on aromatic diisocyanatcs. Analysis of these kinetic curves shows P E U stability in relation to diisocyanate structure decreases in the order DPOD > N P > P D > M D > DPSD ~_HMD. The stabilizing capacity of the urethane fragments will also increase in the same way. The stabilizing effect of aromatic urethane fragments may be caused either by filtering of the UV radiation, since they absorb in the UV region of the spectrum,
1390
V.A. KOSOBUTSKIIet aL
or because of their anti-oxidant capacity caused by the presence of a mobile hydrogen atom. I n order to clarify the part played by the filtering effect, the evolution of gaseous degradation products from P E U was studied by irradiating them in vacuum with light having a wavelength greater than 300 nm, since the filtration effect should appear quite clearly under vacuum conditions. I t m a y be seen from Fig. 3 t h a t a small amount of acetone is evolved during irradiation in vacuum, the curves for gas-evolution being the same for all these polymers apart from POPG. The stabilizing effect of the aromatic fragments is, consequently, connected not with the filtration of UV light but with their antioxidant capacity. This is also confirmed by the fact t h a t the values of activation energy for the evolution of acetone (15.Skcal/mo]e) and acetaldehyde (29.7 kcal]mole) during the photo-oxidation of PEU, for example from t h a t based on MD, are greater t h a n those for the photo-oxidation of POPG (9.6 and 11.4 kcal/ /mole respectively}.
02,% 0.08 1
I
A, ~, I0 5
2 x
0.02
-
>~ Z
"~
x l_
"~~~
0.01' ~!
I
I
I
q
0.04
/, Z ,13 I
I
1
T[me ~hi, FIG. 3
1
8
I
I
I
[
t2
60
T/me, rnin
/20
FIG. 4
Fro. 3. Evolution of acetone (A) during the irradiation in vacuum of the fo|lowing with light a wavelength greater than 300 nm: 1--polyoxypropy|eneglyco|; 2--polyetherurethane based on diphenylmethanedfisocyanate; 3--polyetherurethane based on naphthalenedfisocyanate. lrzG. 4. Accumulation of peroxides during the oxidation of polyoxypropyleneglycol in the presence of low-molecular urethancs and ureas: 1--w~thout any addition or with the addition of 0.0016 mole/kg of hexamcthylenediurethane; 2--with the addition of dipheny]methanediurethane, diphenylmethanediurea or N-diphenyldiurethane ethylene glycol (concentration of the addition, 0.0016 rnole/kg). A number of possible mechanisms for the action of inhibitors containing a mobile hydrogen atom has been discussed in the literature. The inhibitor m a y show the effect by reducing a hydxoperoxide [11]. Moreover, the first stage in inhibition m a y be the formation of a~a intermediate complex between the inhibitor and peroxide compounds, with subsequent transformation to stable products [12-14]. B y measuring the accumulation of peroxides during the oxidation of POPG in
Photo-oxidative degradation of polyetherurethan(~
1391
the dark in the presence of low-molecular urethanes and ureas, it was established Ihat the N-diphenylurethane of ethylene glycol, in exactly the .~ame way as other aromatic urethanes and ureas, increases the induction period of oxidation (Fig. 4). Since this urethane does not have a mobile hydrogen atom. it can be assumed that, out of all the possible inhibition reactions, the tbrmation of an interinediate complex between peroxide compounds and urethane group~ is the t)rcdominant. There is also evidence in favour of this assumption in that, as has been shown hy an investigation of the UV spectra of solutions of urethanes and ureas with dipheuylamine (an electron donor) and p-chloranil (an electron acccptor), aromatic urethanes and ureas have electrorL-donor properties. Peroxide compou:l(Is ave also electrophilic reagents [15]. The electron-donor properties of compounds are determined by their ionization potentials. A correlation is theretbre to be expected between the antioxi(lant capacity of urethanes and ureas and their ionization potentials. This hypothesis is confirmed by comparing the results of quantum-chemical calculations of the electronic structure of urethanes with the experimental data (see Table). RESL'LTS OF QUA~;TUM-CHEMICAL NIC STRUCTURE
CALCIYLATIONS OF THE ELECTRO-
OF LrRETHANE
A~I)
Cha.rg~
Coinpound
UREA.~ ()it
the nitrog(~n iI~oln
Diphenyloxidediurethane Naphthalenediurethane p-Phcnylcnediurethanc N-diphenyldiuretha~e ethylene glycol Diphenylmethanediurea Diphenylmethanediurethano Benzophcnonediurethane Hexamethylencdiureth~me Hexamethylenediurca
l,mization pottmtial, oV
-=0.2746 -:-0-2838 -:-0.2810 -!0.3416
~.4149 8-4888 S.7901 9-1368
~-0.2693 -?0.2755 i-0.2839 • 0-2040 ~0.1949
:~.2110 9.3008 9.5382 10.314(i 1o.0373
Analysis of the data in the Table and in Figs. 2 and 4 indicates that the inhibiting effect of both low-molecular urethanes and also the urethane fragments is, in fact, determined by their electron-donor propertics. The induction periods in ttle oxidation of polyoxypropylene glycol with additions of diphenylmethane(liurethane and the N-dii)henylurethane of ethylene glycol, which arc compounds with approximately the same ionization potential, are the same. The inhibiting effect of the urethane fragments in P E U increases in order of the urethanes, decreasing ionization potential. It is impossible to calculate the electronic structure of diphenylsulphonediurethane b y the method of calculation used. This is therefore discussed with
1392
V.A. KOSOBVTS]LIIet
el.
benzophenonediurethane as an example which also contains an electron-accepter bridging group. I t may be seen from the Table t h a t the introduction of an electronaccepter group leads to an increase in the ionization potential. When the SO, group is introduced, the iozrization potential should increase still more since the SO,, group has stronger electron-accepter propertics. Therefore the diphenylsulphone urcthane fragment either should not exhibit any inhibiting effect or it should be very weak. This is confirmed by the dat a in Fig. 2. I t should be noted t h a t no correlation is observed between the mobility of the hydrogen atom, which could be assessed from the positive charge on the nitrogen atom, and the antioxidant properties of the urethane fragments. We can a t t e m p t to construct a correlation relationship between the ionization potential of the urethanes and their antioxidant capacity on the basis of the results presented above. The induction period for gas evolution or the a m o u n t o f products evolved in a definite time interval m a y be selected to assess the antioxidant capacity. A relationship of this t ype is shown in Fig. 5. In either case, the relationship is linear. The correlatian obtained can serve as the basis ibr qualitatively predicting the antioxidant capacities of urethanes.
B,g~los
"l;h.
0"3
0"I
x" 6
~
S
~
~
I
~
I -8
-8
3
I -I0
Zonizafionpofenf[al, eV
Fro. 5. Dependence of 1--the induction period and 2--the amount of acetaldehyde evolved after 6 hr (Br~) on the ionization potential of the corresponding urethanes for the U-V irradiation of polyetherurethanes with various chemical structures. Ap ar t from urethane groups, commercial P E U m ay also contain urea groups formed during the synthesis of P E U in the presence of water and the chain extenders which are amines. W i t h o u t touching on the relative stability of oligoetherurcas, * we a t t e m p t e d to clarify the part played by urea groups in the photo-oxidation of the oligoethcr par t of the macromoleculcs by using the results .obtained above as a basis. F,xperiments involving the oxidation of polyoxypropylene glycol in the dark in the presence of model a r e a s (Fig. 4) show t h a t the aromatic ureas increase the induction period for oxidation of this compound to the same e x t e n t as do the analogous aromatic urethanes. I t m a y be seen from an analysis of the data from * We had no possibility to synthesize an oligoothorurea because no oligoether with -end amine groups was available.
Photo-oxi, lativo (h,grad~,tion of polyothcnlrathanes
1393
( l u a n t u m - c h e m i c a l calculations (sec Table) t h a t the ionization p o t e n t i a l s of t h e a r o m a t i c ureas ark c o m p a r a b l e to those of the u r e t h a n c s w i t h similar s t r u c t u r e s . B y m a k i n g a n a n a l o g y w i t h the effect of u r e t h a n e groups, it m a y t h c r e f b r e be c()n(,h~ded t h a t thc a r o m a t i c ureas, e n t e r i n g !rite t h e c o m p o s i t i o n of lhe I ) E U macromoleculcs, will also stabilize t h e oligoethcr })art against p h o t o o x i d a t i v c (lcgradation. I t has thus b e e n s h o w n a b o v e t h a t the a r o m ~ t i c m'cth~ne f r a g m e n t s st~d)ilize t h e oligocther p a r t of the m a c r o m o l e c u l c a g a i n s t l)hoto-oxidation. B u t a t th(.' ~ame t i m e ! h e y themselx'cs u n d e r g o d e g r a d a t i o n ullder the effects of UV radiation and this leads to a re(iuction in their c o n c e n t r a t i o n a n d to a decrease in the st~bilizi~lg effect. T h e overall effect is determirlc(| 1)v the r a t i o b c t w e e n the r a t c s of the c o m p e t i n g rc~ctions. I f thc d e c o m p o s i t i ( m of t h e u r e t h a u c f l ' a g m c n t s is el!re!mired, their stabilizing effect can bc retained. To do this, it is necessary to i~troducc light-stabilizcrs into t h e P E U ; these either a b s o r b the c h e m i c a l l y a c t i v e r a d i a t i o n or e x t i n g u i s h the excited s t a t c s of t h e a r o m a t i c u r e t h a n e fragm e n i s. Translated by O. F..'~'[ODLEN"
REFERENCES
1. MATSUI TOSIYOSI, ONe KHIROSI and SUGIYAMA IKUO, Japanese Pat. 41109, 1970 2. KISANISI YASUKttISA, TSUTA TADAKADZU and NAKANO TAKUO, Japanese Pat. 998, 1970 3. ISIDZUKA IOSIO, TANEDA YASUO and NAKANO TAKUO, Japanese Pat. 23099, 1967 4. J. W. CAItILL, Pat. U.S.A. 3464851, 1969 5. G. O. SEDWICK, Pat. U.S.A. 3577383, 1971 6. R. J. KNOPF, L. M. BOWNE and R. D. HISER, Pat. U.S.A. 3663506, 1972 7. G. I. KAGAN, V. A. KOSOBUTSKII, V. K. BELYAKOV and O. G. TARAKANOV, Khimiya geterotsiklich, soyed., No. 7, 794, 1972 8. N. MATAGA and K. NISHL)IOTO, J. Phys. Chem. 13: 140, 1957 9. V. A. KOSOBUTSKII, Candidate's dissertation, Roster State University, 1974 10. L. V. NEVSKH, O. G. TARAKANOV amd V. K. BELYAKOV, Plast, massy. No. 7, 44, 1966 11. N. M. EMA~NUEL', Uspeldfi khimii organicheskikh soyedinenii i autookisleniye (Advances in the Chemistry of Organic Compounds and Auto-oxidation) p. 329, "Khimiya", 1969 12. G. S. HAMMOND, C. E. BOOZER, C. E. HAMILTON and J. N. SEN, J. Anmr. Chem. Soc. 77: 3242, 1955 13. J. R. THOMAS, J. Amer. Chem. Soc. 85: 591, 1963 14. C. E. BOOZER, J. Amer. Chem. Soc. 76: 3861, 1954 15. P. LYKES, Mekhanizmy reaktsii v organieheskoi khimii (Reaction Mechanisms in Organic Chemistry). p. 48, "Khimiya", 1973 (ICussian translation)
V . A . KOSOBUTSKH e$ at/.
The data obtained thus show, as a whole, that the polycyclotetramerization of tetraeyanobenzene occurs in bulk by a step-wise mechanism. Soluble ribbon-like oligomers are first formed, the number of azaporphin rings being from 3 to 7-8; these gradually transform into insoluble crosslinked three dimensional structures identical to those obtained by the additional heating of the soluble oligomers. The synthesis of soluble oligomers of PPhC and their transformation into insoluble flat-network or graphite-like structures considerably extent the possibility of investigating and using polymeric porphyrazines practically. Translated by G. F. MODLEI~ REFERENCES 1. A. A. BERLIN and A. J. SHERLE, Inorgan. Macromolec. Rcv. 1: 235, 1971 2. E. A. LAWTON and D. D. MoRITCHE, J. Organ. Chem. 24: 26, 1959 3. Ye. Yu. BEKHT.I, D. D. NOVIKOV and S. G. ENTELIS, Vysokomol. soyed. A: 2754, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 12, 3717, 1967) 4. N. I. SHAPIRO, I. P. SUZDALEV, V. I. GOLD'ANSKII, A. L SHERLE and A. A. BERLIN, Teoret;. eksperim, khim. 11: 330, 1975 5. Ye. A. LUK'YANETS, S. A. MIKHALENKO and Ye. A. K0VSHEV, Zh. obshch, khim. 41: 934, 1971 6. L. G. CHERKASHINA, Ye. L. FRANKEVICH, N. V. YEREMINA, Ye. I. BALABANOV and A. A, BERLIN, Vysokomol. soyed. 7: 1264, 1965 (Translated in Polymer Sci. U.S.S.R. 7: 8, 1478, 1965)
Polymer Scien(~U.S.S.R. Vol. 22, No. 6, pp. 1386-1893.1980 Printed in Poland
0032-3050180/1386-8507.50/0
1081 Pergamon Preaq Ltd.
P H O T O - O X I D A T I V E D E G R A D A T I O N OF P O L Y E T H E R U R E T H A N E S * V. A. KOSOBUTSKII, M. N. KURGANOVA, O. G. TARAKANOV a n d V. K . BELYAKOV All-Union Scientific Research Institute for Synthetic Resins
(Received 3 April 1979) The photo-oxidative degradation of polyotherurothanes based on polyoxypropylone glycol with a molecular weight of 1000 a n d diisocyanatos with various chemical structures has been studied. I t has boon established t h a t aromatic urethane fragments stabilize the oligoether part of the maeromoleeules against photo-oxidation. The stabilizing effect of the urethano fragments is connected with the inhibition of oxidation through the formation of intermediate complexes with peroxide compounds and is determined by the electron-donor properties of the urethanes. A correlation has been established between the stabilizing effect and the urethane's ionization potential. * Vysokomol. soyed. A22: No. 6, 1264-1269, 1980.
Photo-oxidative (leg~adatiou ,)f polyetherurethanes
1387'
BECAUSE o f t h e d e v e l o p m e n t o f o x i d a t i v e d e g r a d a t i o n , c o m p o n e n t s m a d e o f p o l y e t h e r u r e t h a n e s ( P E U ) p a r t l y change colour u n d e r the action of . ~ n l i g h t and t h e i r p h y s i c o - m e c h a n i c a l p r o p e r t i e s a r e i m p a i r e d . T h e r e s i s t a n c e of p ( ) l y m e r s t o p h o t o - o x i d a t i o n is d e t e r m i n e d , in th(. first i n s t a n c e , b y t h(~ chemic:t! s t r u c t u r e of the polymeric chain. T h e p r e s e n t w o r k is d e v o t e d to a n i n v e s t i g a t i o n of" t h e effect of th,, ~.hemical s t r u c t u r e o f ~ r e t h a n e f c a g m e n ~ s o n t h e p h o t : o - o x i ( t a t i v e degradatiot~ ()f" PELT. O n t h e b a s i s o f i n f o r m a t i o n in t h e l i t e r a t u r e a b o u t t h e use o f low-molc*:ttlar urct h a n c s , u r e a s a n d t h e i r d e r i v a t i v c s as h e a t a n d l i g h t s t a b i l i z e r s [I--~;!, i t m a y , m o r e o v e r , b e exI)ectcd t h a t t h e u r e t h a n ( ; f r a g m e n t s o f t h e maeron~e*!(,('ule will stabiliz(; a g a i n s t t h e p h o t o - o x i d ~ t t i o n o f t h e o l i g o c t h e r p a r t . I f such s t a b i l i z i n g ~t(:tion e x i s t s , it m u s t b e t a k e n irate a c c o u n t in a t t e m p t i n g to find t h e o t ) t i m u m m e t h o ( t s o f s t a b i l i z i n g P E U . T h e p o s s i b i l i t y t h a t t h e r e is ~ s t a b i l i z a t i ( , u effect e x e r t e d b y t h e u r e t h a n e f r a g m e n t s a n d t h e n a t u r e o f t h i s effect are ~lso d i s c u s s e d i n t h e p r c s e n t p~ti)er. P E U were synthesized from poly()xypropylene glycol with M = I000 a~ld diisocyanat~,s with various chemical structures (ratio OH : N C O = l :2). tn order to exclud(, ~lny effect. of free isocyanate groupso, the P E U were treated with absolute metha~lol. The following diisocyanates were used: hexamethylene- (HMD), diphonylsulphone- (DPSD), diphenylmethane- (MD), p-phenylene- (PD), 1,5-naphthalene- (ND) and diphenyloxide- (DPOD). Before the synthesis, the polyoxypropylcne glycol was dried, the solid diisocyan~ttes were sub. limed ~md the liquid diisocyanates were distilled. The concentration of the bnsic, substance in the diisocyanates used was not less t]mn 99-5°/o. The diurethanes were synthesized from absolute methanol and the c~,ri~,spondin;~ diisocyanate. The ureas were synthesized by the addition of the diisocyan~te t~ exccs.~ diethylamine. The compounds obtained were identified from chemical analysis dat~. A PRK-2 lanlp was used as the illumination source. In the kinetic inve.~tig~tions, the lamp was placed at a distance of 20 cm fr~)m the specimen. The cell with the sp~'eirnens was ])laced iHa water thermostat to eliminate thermal heatiug. A "Tsvet-102" gas chromatograph was used to analyse the gaseous prodtl,-1~ of degr~,darien. The sensitivit,y of the instrument in determining oxides of carbon was Io -3 vol. o, aild in determining acetone and acetaldehyde 2 ~ 10 ~ vol. ° o. i n order to incr(.as~ the accuracy of analysis and to carry out the expori~r~ent automatically, a feed system x ~ constru(:ted to the (~hromatogr~iph which xv~s cormected directly to the cell being irradiated. ~:hen this system ~w~s tlsed, the error in the a m o u n t of the sample introduced did ~lot (.x(-eed 2o0. In order to exclude the influence of diffusion effects on the kinetics of ph~t~-,)xidati~l, of PEU. the specixnens used had a t hieluless of 0.003-0-005 Inm. Th(. UV spectra were recor(lcd with a SF-16 spectrophotomcter. The el(;ctroni(' structure of the urethal~es and ureas wer(~ e~leulated using-' th~ Pariser Parr P~q)le n~etho(t. The details of the calculations basically correspond to those in If0]: the geo~nclrical properties of the molecules were optimiz,,d during the calcuh~ti,)ns t)y ~ proc(~uro proposed by Dewar [8]. The initial geometrical characteristics w~re selected as fi)llovcs: all the rings were taken to be regular polygons with sides 0.I397 nm long; all the valency angles were 120 degrees and the longShs of the N - - C , C--O and C = O bonds wore, taken to be equal to 0.142, 0.136 a n d 0-142 nm respectively. The calculations were carried out by means of a computer programme [9]. The ioaizatAon potentials wore assessed using ~¢.oopmans theorem.
V. A. KOSOBUTSKII et at.
1388
Investigation of the gaseous product of the photo-oxidative degradation of P E U with various chemical structures showed that the qualitative composition of the gaseous phase was the same for all tho polymers investigated, which is evidence of the commonality of the processes occuring. The principal degradation products are carbon monoxide and carbon dioxide, acetone and acetaldehyde, carbon monoxide being the predominant amongst these. Figure 1 shows the kinetics of evolution ()f carbon monoxide during the action of UV radiation on polyoxypropyleue glycol and the P E U based on diisocyanatcs with various chemical structures. Under thcse conditions, oxidation processes initiated by light and photo-decomposition processes occur simultaneously; the latter are known from the literature [10] also to lead to the formation of oxides of carbon. CO,g, 105 -
n
5
2
18
7
4, 2
t
C
12
2 3
6 1
6
~
i
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12
,
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6
T/me, ho
3
6
FIe. 1. Evolution of carbon monoxide during the irradiation of the following with unfiltered light from a PRK-2 lamp: 1--polyoxypropyleneglycol; 2--polyetherurethane based on diphenyhnethanodiisoeyanato; 3--polyetherurethane based on diphenyloxidediisocyanate: a--in vacuum; b--in oxygen; c--as (b) ,but calculated curves for gas evolution with a correction for photolysis. I n order to separate out that part of the products which relate to oxidation processes, the kinetics of gas evolution were investigated with irradiation of polyoxypropylene glycol and aromatic P E U in vacuum. No oxides of caTbon were found during the irradiation of polyoxypropylene glycol. The photo-degradation of P E U based on aromatic diisocyanates is accommpanied b y the evolution of oxides of carbon (Fig. la). The kinetics of gas-evolution is described by a straight line proportional to the time of irradiation. In the absence of oxygen, oxides of carbon are thus evolved only through the breakdown of urethane groups linked to the aromatic nucleus. I t m a y be seen from Fig. lb, that, for irradiation in oxygen, the rates of evolution of oxides of carbon become comparable for polyoxypropylene glycol and P E U . I t m a y therefore be concluded that, during the photo-oxidation of P E U based on aromatic dii~ocyanates, the gaseous products are formed both through the photo-degradation of the urethane groups and also through the oxidation of the
Photo-oxidative degradation of polyetherurethanes
1389
oligoether section. Thus the amount of the gaseous products, in particular carbon monoxide, formed during the photo-oxidation of P E U should be equal to the sum of the gaseous products evolved during thc photolysis of" P E U and the photo-oxidation of polyoxypropylene glycol. It mav be seen fl'om an analysis of Fig. lc, that 1his quantity is considerably smaller. The curves for the evohttion of carbon monoxide during the photo-oxidation of PEU based on MI) and I)POD, a corre~'t ion being made for photolysis, lie; below the curve tbr ,_,as (~volution during lhe photo-oxidation of polyoxypropylcne glycol. This fact iltdicates t h a t tim aromatic: urethanc fragments stabilize the oligoethcr part against photo-oxidation. The stabilizing cffcct of these fragments appears most clearly for irradiation by light with a wavelength greater than 300 nm, that is, in the region where the, ~romatic PEU, apart from the PEU based on NI), (1o not have any absorl)tion of their own. A clearer pattcrn can be found if the products investigated arc those which arc evolved during the decomposition of only the oligocther part of the macromolccule, since oxidcs of carbon are formed during the degradation of both the urethane and also the oligoether part. Acetone and acetaldehyde are found to be products of this type. They are evolved during the photo-oxidation of the oligocther and no such products are found Cluring the photo-oxidation of the urethanc based on phenol and phenylisocyanate.
9" ~°5 cl
>I" z'a
F
" 2'3
~4
o.e
2
6
I0 2 Time, hr
8
10
Fro. 2. Evolution of a--acetaldehyde and b--am.~tono during tim irradiation of: l--polyoxypropyloneglycol; 2-7--polyethcrurethanes based on the follo~4ng diisoycanatos; 2-hexarnethylene-; 3 -- diphenylsulphone-; 4 -- diphcnylmethane-; 5 -- p-phenyleno; 6 -- naphtha leno-; 7-- diphonyloxidc-. The kinetic curves for acetone and acetaldehyde are shown in Rig. 2. It may be seen t h a t the induction period for the evolution of gaseous products is increased for P E U based on aromatic diisocyanatcs. Analysis of these kinetic curves shows P E U stability in relation to diisocyanate structure decreases in the order DPOD > N P > P D > M D > DPSD ~_HMD. The stabilizing capacity of the urethane fragments will also increase in the same way. The stabilizing effect of aromatic urethane fragments may be caused either by filtering of the UV radiation, since they absorb in the UV region of the spectrum,
1390
V.A. KOSOBUTSKIIet aL
or because of their anti-oxidant capacity caused by the presence of a mobile hydrogen atom. I n order to clarify the part played by the filtering effect, the evolution of gaseous degradation products from P E U was studied by irradiating them in vacuum with light having a wavelength greater than 300 nm, since the filtration effect should appear quite clearly under vacuum conditions. I t m a y be seen from Fig. 3 t h a t a small amount of acetone is evolved during irradiation in vacuum, the curves for gas-evolution being the same for all these polymers apart from POPG. The stabilizing effect of the aromatic fragments is, consequently, connected not with the filtration of UV light but with their antioxidant capacity. This is also confirmed by the fact t h a t the values of activation energy for the evolution of acetone (15.Skcal/mo]e) and acetaldehyde (29.7 kcal]mole) during the photo-oxidation of PEU, for example from t h a t based on MD, are greater t h a n those for the photo-oxidation of POPG (9.6 and 11.4 kcal/ /mole respectively}.
02,% 0.08 1
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/20
FIG. 4
Fro. 3. Evolution of acetone (A) during the irradiation in vacuum of the fo|lowing with light a wavelength greater than 300 nm: 1--polyoxypropy|eneglyco|; 2--polyetherurethane based on diphenylmethanedfisocyanate; 3--polyetherurethane based on naphthalenedfisocyanate. lrzG. 4. Accumulation of peroxides during the oxidation of polyoxypropyleneglycol in the presence of low-molecular urethancs and ureas: 1--w~thout any addition or with the addition of 0.0016 mole/kg of hexamcthylenediurethane; 2--with the addition of dipheny]methanediurethane, diphenylmethanediurea or N-diphenyldiurethane ethylene glycol (concentration of the addition, 0.0016 rnole/kg). A number of possible mechanisms for the action of inhibitors containing a mobile hydrogen atom has been discussed in the literature. The inhibitor m a y show the effect by reducing a hydxoperoxide [11]. Moreover, the first stage in inhibition m a y be the formation of a~a intermediate complex between the inhibitor and peroxide compounds, with subsequent transformation to stable products [12-14]. B y measuring the accumulation of peroxides during the oxidation of POPG in
Photo-oxidative degradation of polyetherurethan(~
1391
the dark in the presence of low-molecular urethanes and ureas, it was established Ihat the N-diphenylurethane of ethylene glycol, in exactly the .~ame way as other aromatic urethanes and ureas, increases the induction period of oxidation (Fig. 4). Since this urethane does not have a mobile hydrogen atom. it can be assumed that, out of all the possible inhibition reactions, the tbrmation of an interinediate complex between peroxide compounds and urethane group~ is the t)rcdominant. There is also evidence in favour of this assumption in that, as has been shown hy an investigation of the UV spectra of solutions of urethanes and ureas with dipheuylamine (an electron donor) and p-chloranil (an electron acccptor), aromatic urethanes and ureas have electrorL-donor properties. Peroxide compou:l(Is ave also electrophilic reagents [15]. The electron-donor properties of compounds are determined by their ionization potentials. A correlation is theretbre to be expected between the antioxi(lant capacity of urethanes and ureas and their ionization potentials. This hypothesis is confirmed by comparing the results of quantum-chemical calculations of the electronic structure of urethanes with the experimental data (see Table). RESL'LTS OF QUA~;TUM-CHEMICAL NIC STRUCTURE
CALCIYLATIONS OF THE ELECTRO-
OF LrRETHANE
A~I)
Cha.rg~
Coinpound
UREA.~ ()it
the nitrog(~n iI~oln
Diphenyloxidediurethane Naphthalenediurethane p-Phcnylcnediurethanc N-diphenyldiuretha~e ethylene glycol Diphenylmethanediurea Diphenylmethanediurethano Benzophcnonediurethane Hexamethylencdiureth~me Hexamethylenediurca
l,mization pottmtial, oV
-=0.2746 -:-0-2838 -:-0.2810 -!0.3416
~.4149 8-4888 S.7901 9-1368
~-0.2693 -?0.2755 i-0.2839 • 0-2040 ~0.1949
:~.2110 9.3008 9.5382 10.314(i 1o.0373
Analysis of the data in the Table and in Figs. 2 and 4 indicates that the inhibiting effect of both low-molecular urethanes and also the urethane fragments is, in fact, determined by their electron-donor propertics. The induction periods in ttle oxidation of polyoxypropylene glycol with additions of diphenylmethane(liurethane and the N-dii)henylurethane of ethylene glycol, which arc compounds with approximately the same ionization potential, are the same. The inhibiting effect of the urethane fragments in P E U increases in order of the urethanes, decreasing ionization potential. It is impossible to calculate the electronic structure of diphenylsulphonediurethane b y the method of calculation used. This is therefore discussed with
1392
V.A. KOSOBVTS]LIIet
el.
benzophenonediurethane as an example which also contains an electron-accepter bridging group. I t may be seen from the Table t h a t the introduction of an electronaccepter group leads to an increase in the ionization potential. When the SO, group is introduced, the iozrization potential should increase still more since the SO,, group has stronger electron-accepter propertics. Therefore the diphenylsulphone urcthane fragment either should not exhibit any inhibiting effect or it should be very weak. This is confirmed by the dat a in Fig. 2. I t should be noted t h a t no correlation is observed between the mobility of the hydrogen atom, which could be assessed from the positive charge on the nitrogen atom, and the antioxidant properties of the urethane fragments. We can a t t e m p t to construct a correlation relationship between the ionization potential of the urethanes and their antioxidant capacity on the basis of the results presented above. The induction period for gas evolution or the a m o u n t o f products evolved in a definite time interval m a y be selected to assess the antioxidant capacity. A relationship of this t ype is shown in Fig. 5. In either case, the relationship is linear. The correlatian obtained can serve as the basis ibr qualitatively predicting the antioxidant capacities of urethanes.
B,g~los
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-8
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Zonizafionpofenf[al, eV
Fro. 5. Dependence of 1--the induction period and 2--the amount of acetaldehyde evolved after 6 hr (Br~) on the ionization potential of the corresponding urethanes for the U-V irradiation of polyetherurethanes with various chemical structures. Ap ar t from urethane groups, commercial P E U m ay also contain urea groups formed during the synthesis of P E U in the presence of water and the chain extenders which are amines. W i t h o u t touching on the relative stability of oligoetherurcas, * we a t t e m p t e d to clarify the part played by urea groups in the photo-oxidation of the oligoethcr par t of the macromoleculcs by using the results .obtained above as a basis. F,xperiments involving the oxidation of polyoxypropylene glycol in the dark in the presence of model a r e a s (Fig. 4) show t h a t the aromatic ureas increase the induction period for oxidation of this compound to the same e x t e n t as do the analogous aromatic urethanes. I t m a y be seen from an analysis of the data from * We had no possibility to synthesize an oligoothorurea because no oligoether with -end amine groups was available.
Photo-oxi, lativo (h,grad~,tion of polyothcnlrathanes
1393
( l u a n t u m - c h e m i c a l calculations (sec Table) t h a t the ionization p o t e n t i a l s of t h e a r o m a t i c ureas ark c o m p a r a b l e to those of the u r e t h a n c s w i t h similar s t r u c t u r e s . B y m a k i n g a n a n a l o g y w i t h the effect of u r e t h a n e groups, it m a y t h c r e f b r e be c()n(,h~ded t h a t thc a r o m a t i c ureas, e n t e r i n g !rite t h e c o m p o s i t i o n of lhe I ) E U macromoleculcs, will also stabilize t h e oligoethcr })art against p h o t o o x i d a t i v c (lcgradation. I t has thus b e e n s h o w n a b o v e t h a t the a r o m ~ t i c m'cth~ne f r a g m e n t s st~d)ilize t h e oligocther p a r t of the m a c r o m o l e c u l c a g a i n s t l)hoto-oxidation. B u t a t th(.' ~ame t i m e ! h e y themselx'cs u n d e r g o d e g r a d a t i o n ullder the effects of UV radiation and this leads to a re(iuction in their c o n c e n t r a t i o n a n d to a decrease in the st~bilizi~lg effect. T h e overall effect is determirlc(| 1)v the r a t i o b c t w e e n the r a t c s of the c o m p e t i n g rc~ctions. I f thc d e c o m p o s i t i ( m of t h e u r e t h a u c f l ' a g m c n t s is el!re!mired, their stabilizing effect can bc retained. To do this, it is necessary to i~troducc light-stabilizcrs into t h e P E U ; these either a b s o r b the c h e m i c a l l y a c t i v e r a d i a t i o n or e x t i n g u i s h the excited s t a t c s of t h e a r o m a t i c u r e t h a n e fragm e n i s. Translated by O. F..'~'[ODLEN"
REFERENCES
1. MATSUI TOSIYOSI, ONe KHIROSI and SUGIYAMA IKUO, Japanese Pat. 41109, 1970 2. KISANISI YASUKttISA, TSUTA TADAKADZU and NAKANO TAKUO, Japanese Pat. 998, 1970 3. ISIDZUKA IOSIO, TANEDA YASUO and NAKANO TAKUO, Japanese Pat. 23099, 1967 4. J. W. CAItILL, Pat. U.S.A. 3464851, 1969 5. G. O. SEDWICK, Pat. U.S.A. 3577383, 1971 6. R. J. KNOPF, L. M. BOWNE and R. D. HISER, Pat. U.S.A. 3663506, 1972 7. G. I. KAGAN, V. A. KOSOBUTSKII, V. K. BELYAKOV and O. G. TARAKANOV, Khimiya geterotsiklich, soyed., No. 7, 794, 1972 8. N. MATAGA and K. NISHL)IOTO, J. Phys. Chem. 13: 140, 1957 9. V. A. KOSOBUTSKII, Candidate's dissertation, Roster State University, 1974 10. L. V. NEVSKH, O. G. TARAKANOV amd V. K. BELYAKOV, Plast, massy. No. 7, 44, 1966 11. N. M. EMA~NUEL', Uspeldfi khimii organicheskikh soyedinenii i autookisleniye (Advances in the Chemistry of Organic Compounds and Auto-oxidation) p. 329, "Khimiya", 1969 12. G. S. HAMMOND, C. E. BOOZER, C. E. HAMILTON and J. N. SEN, J. Anmr. Chem. Soc. 77: 3242, 1955 13. J. R. THOMAS, J. Amer. Chem. Soc. 85: 591, 1963 14. C. E. BOOZER, J. Amer. Chem. Soc. 76: 3861, 1954 15. P. LYKES, Mekhanizmy reaktsii v organieheskoi khimii (Reaction Mechanisms in Organic Chemistry). p. 48, "Khimiya", 1973 (ICussian translation)