Photochemical changes in polyvinyl-p-azidobenzoate in the presence of compounds containing double bonds

PHOTOCHEMICAL CHANGES IN P O L Y V I N Y L - p - A Z I D O B E N Z O A T E IN T H E PRESENCE OF COMPOUNDS CONTAINING D O U B L E BONDS* A. G. FILIMOSItKIN, R. ~-~. NEVEDOMSKAYA, I. P. ZHEREBTSOV and R. M. LIVSHITS Petroleum Chemistry Institute, S.D.U.S.S.R. Academy of Sciences Tomsk State University (Received 2 September 1974)

It was found in studies of polyvinyl-p-azidobenzoate copolymers that the pho~lysis of azido groups in the presence of compounds containing polarized double bonds is accompanied by stabilization of nitrones through addition to the double bond with the formation of aziridine rings. The main reaction of 1,1-ring addition is accompanied by a number of secondary reactions. It is shown that the direction of the reactions is determined by the initial molar ratio of double bonds and azido groups in the copolymcr composition. THE development of radio-electronics and of semi-conducting techniques has resulted in greatly increased interest in photosensitive polymers (PSP) where photoresistant materials can be prepared as a result of photochemical changes in PSP. The preparation of these materials, which is based on changes in the s o l ubility of PSP under irradiation, has been achieved as a result of the following two processes: a) crosslinking of the macromolecules; b) polymer analogue changes in macromolecular chains during interaction of photolysis products of photosensitive groups with low molecular photoinsensitive compounds present in the films. Using process a) only negative images are obtainable, whereas with process b) it should be possible to obtain either a negative or a positive, depending on which solvent is selected. Among the widely used PSP are high molecular compounds containing azido groups, particularly polyvinyl-p-azidobenzoate (PVAB). The photolysis of PVAB is accompanied by the formation of insoluble products on account of erosslinking due to the recombination of nitrenes, which are products of photolysis of azido groups [1] ~3

- ~v - ~ ~,~N. - - - , ~ , N = N ~ , ~

* Vysokomol. soyed. A17: No, I0, 2260-2265, 1975. 2605

(1)

2606

fix. G. FILIMOSHKIN ei~ al.

Thus negative photoresistors are obtainable from PSP with azido side groups. I f nitrene accepters are introduced into films of these polymers, particularly if the accepters are compounds containing unsaturated bonds, it is to be expected t h a t reaction (1) will be suppressed, and that polymer analogue transformations will take place with the formation of polymers having aziridine rings, in accordance with the scheme CH

(2) CH--R

Given dissimilar solubilify of the initial and the obtained polymers there is the possibility of preparing either negative or positive photoresistors from P S P containing azido group. Moreover, with appropriate selection of the right unsaturated compound, reaction (2) may serve as a means of modifying the polymers, giving the latter a number of good properties. The results reported below were obtained by analysis of photochemical changes in PVAB in the presence of compounds containing double b o n d s . The specimens investigated were copolymers of vinyl acetate and VAB with v ar y i n g contents of vinyl azidobenzoate units. The initial P V A B were prepared by esteriilcation o f polyvinyl alcohol with p-azidobenzoie acid chloride and acetic anhydride [2] (Table 1). As initial compounds containing double bonds we selected compounds varying as to the polarity of the double bonds: 9-(B-hydroxy)ethylcarbazole (AEC) and maleie anhydride (~A). T~BLE 1. C ~ C T E ~ m T I C S PVAB copolymer* PVAB-92 PVAB-42 PVAB-15

OF ~HE I ~ I T I ~ POLYMERS

Nitrogen, wt. ~o

2tmax, n m

[v]], dl/g$

20"30 13.60 7.18

274 274 290

0"88 0.71 0.66

* The figures after the copolymercode name relate to the molar percentage of vinyl azidobenzoate units determined from the electronic spectra. t The absorption of a film coated on quartz glass. * In chloroform solution at 25°.

AE C prepared by esterification of 9m(B-hydroxy)ethylcarbazole with acrylic acid [3] after three recrystallizations from methanol had: m.p. 78-79°; N, %: found--5.33; calcul a t e d - 5 . 2 8 . MA was purified by distillation, m.p. 53 °. The choice of compounds was governed by the fact t h a t the latter underwent no changes when exposed, to monochromatic light with 2----254 nm, and, in addition to being nonvolatile, have a common solvent, and together with P V A B give translucent films of v a r y i n g composition. Moreover, A E C has a number of characteristic bands in the I R spectra which facilitate observation of degrees of conversion. Investigation of the reaction meebA.ni~m involved synthesis of a model compound

Photochemical changes in polyvinyl-p-azidobenzoate

2607

containing the aziridine ring

CI-I3

I

CH--C--O--CH2--CH2--N?

°[I

~

/'

(a)

CH, The compound was synthesized via a stage of preparation of A2-1,2,3-triazoline with subsequent thermolysis. The A2-triazoline derivative was prepared b y the method described in ref. [4]. AEC was dissolved in p-azidoethylbenzoate, and was left standing a t 20 ° for 5 days. The obtained product was then washed r e p e a t e d l y with ether. This resulted in Aa-l,2,3-triazoline, a white substance with m.p. 116-117 ° (decomp.). Thermolysis of the intermediate triazoline was carried out in solution (a 1 : 1 mixture of CX-ICla a n d toluene) a t 85 ° for 7 hr [5]. The bright brown product was rccrystallized from methanol, a n d had m.p. 120-122% The I R spectrum shown in Fig. l a bears out the structure of the l a t t e r compound. p-Azidocthylbenzoate (AEB) was prepared as follows: 1 mole of p-azidobenzoic acid chloride was m i x e d with 1 mole of abs. ethyl alcohol, a n d cooled to 0 °, follewod b y t h e dropwise addition, while stirring, of a mixture of 0"5 mole of abs. ethanol mad 1 mole of pyridine, after which the mixture was left standing for 0.5 hr, was then r a p i d l y filtered, and the filtrate was e v a p o r a t e d under vacuum. The resulting oily liquid was recrystallized from alcohol. The bright yellow A E B needles h a d m.p. 17.5 °, d 2~ 1.1800 g/era3; n ~3 1.5592; Amax=274 nm, emax= 1-90 × l0 t 1./mole.era (in CttCla). The I R spectrum of the liquid film is identical to t h a t of the PVAB-92 film. The kinetics of photochemical changes were investigated with the samples exposed to short wave UV light with A=-254 nm, using a BUV-30P low pressure mercury l a m p a t a distance of 20 cm. As the azides do not absorb in the vicinity of A>300 nm, there was no need for a filter for the long wave UV light which makes up 10-15% of the total radiation. The intensity of the light on the samples, determined with the aid of a F e r r y oxolate actinometer, was 10 -s einstein/era ~. sec. The measurements were carried out on the basis of 15-20~/o conversion. Samples were p r e p a r e d in film form on NaC1 backings, and the films were coated on from 2 - 4 % solutions in cyclohexanonc using a centrifuge. The film thicknesses were verified b y optical density measurements, and the irradiation of the polymers was carried out directly in the spectrophotometer cells. The rates of azide group decomposition, and of double opening were estimated from changes in the appropriate bands in the I R spectra. An IKS-22 spectrophotometer was used for recording the spectra, Preliminary tests showed t h a t the II~ spectrum of a polyvinylacetate film coated on under s t a n d a r d conditions and containing AEC (or MA) remained unaltered during irradiation under the conditions adopted. T h e i r r a d i a t i o n o f P V A B is a c c o m p a n i e d b y r e g u l a r r e d u c t i o n i n t h e i n t e n s i t y o f t h e b a n d a t 2120 c m -1 (r Ns) a n d b y t h e a p p e a r a n c e o f f r e s h b a n d s r e l a t i n g t o a z o b o n d s (1660, 910 c m - 1 ) o r t o ~ q - - H (3400, 1540 c m - 1 ) [ 1]. I n t h e p r e s e n c e o f A E C t h e r e is a l s o a r e d u c t i o n i n t h e i n t e n s i t y o f t h e 2120 era -1 b a n d a c c o m p a n i e d b y r e d u c e d a b s o r p t i o n a t 805 c m -1 ( F i g . 1) r e l a t i n g t o g c - ~ o f t h e d o u b l e b o n d o f A E C . T h e a s s i g n m e n t o f t h e l a t t e r b a n d is b o r n e o u t b y i t s d i s a p p e a r a n c e d u r i n g t h e r a d i c a l p o l y m e r i z a t i o n o f A E C [3]. T h e i n t e n s i t y o f t h e 805,

2608

A . G . FILIMOSHKI~ 6t al.

30

70

I

30

lg t

~;

_l

lZ

E

15

I

!

I3

I

I

1!

I

,I

9

I

f

7 j). /~-2 Cm- I

FIG. 1. I R spectrum of the PVAB-92 film ( N s : A E C = I . 0 : 1.5) before (a) a n d after irradiation for 30 min under a P R K - 5 l a m p a t a distance of 20 em, the unreacted .kEG removed with ether (b); I R spectrum of the model compound (formula (3)) (c).

cm -1 band is proportional to the AEC content of the film, which provides a basis f-or estimating the number of double bonds in the film. Under the conditions of the experiments a reduction in the number of double bonds during irradiation is due to the film containing azido groups, which generate nitrenes (see reaction (2)), as the spectrum of AEC in polyvinylacetate films remains unchanged when exposed to light with ~ 2 5 4 nm. Moreover, a band at 1150 cm -1 appears in the I R spectrum of the irradiated PVAB-92 containing AEC, and there is increased absorption at 1220 and 1360 cm -1 (Fig. 1). It appears from ref. [6] that these changes are attributable to v ~ C - - N < and P h - - N in aziridine rings. To verify the formation of aziridine rings during the irradiation of PVAB in presence of AEC we synthesized a model compound (3), the I R spectrum of which (Fig. lc) is similar to that seen in Fig. lb. In view of these findings one can assume that the irradiation of PVAB-92 in presence of AEC leads to decay of nitrenes due to attachment to the double bond of a low molecular compound with the formation of aziridines. I t is characteristic that the I R spectra of PVAB-92 irradiated in presence of AEC have no bands relating to azo bonds, which are typical for products of photochemical transformations of PVAB in the absence of low molecular compounds. Further confirmation of the absence of intermolecular bonds is seen in the preservation of the solubility of the film. On comparing the spectra of the irradiated PVAB copolymers with and without MA it appears that bands relating to aziridine rings are absent. In addition, loss of solubility is observed for the irradiated PVAB film, both in the presence,

Photochemical changes in polyvinyl-p-azidobonzoato

2609

a n d t h e absence, of MA. D e t e r m i n a t i o n o f the a m o u n t o f c a r b o x y l groups in t h e p r o d u c t of t h e p h o t o - r e a c t i o n a f t e r the e x t r a c t i o n of MA points to t h e absence of t h e l a t t e r in the m a c r o m o l e c u l a r chains. I n view of these findings we conclude t h a t n i t r e n e s do n o t r e a c t w i t h the double b o n d of MA. T h e a m o u n t of nitrenes reacting with the double b o n d of AEC will depend, o t h e r conditions being identical, on the c o n t e n t of Ns groups in the P V A B , a n d on t h e molar ratio o f N s : AEC in the film. T o clarify this problem a n d find t h e o p t i m u m conditions for t h e reaction, parallel studies were m a d e o f t h e kinetics o f p h o t o l y s i s of azido groups based on the 2120 cm -1 band, a n d t h e depletion o f double bonds, according to t h e b a n d a t 805 cm -1. A t the same t i m e t h e ratio o f the n u m b e r of moles of r e a c t e d AEC a to the n u m b e r of moles of d e c o m p o s e d azido g r o u p s b ( q = a / b ) q u a n t i t a t i v e l y characterizes the p r o p o r t i o n of the reaclog~ 5-

N

I-

,,,,I

....x~x

2

Z

.o 4'

2r-

3

i

i

i

t

i

I

i

1"68

3"30

5"g

6"65

8"35

250

300

TZme , m[n

FIG. 2

k~l 3*0

t 380 ,,t, nm

Fro. 3

FIG. 2. Aznount ofazido groups decomposed versus exposure time for PVAB - 92 (•); PVAB 92: M A = I : 2 (1'); PVAB - 92 : AEC=I-0 : 0.5 (2); 1 : 1 (3); 1.0 : 1.5 (4). FIG 3. U V spectra of PVAB - 92 (1) and AEC (2) in CI-ICla (l= 1 era, concentration of vinylazidobenzoate units c × 10~=3-88 (1) and AEC--4 mole/1. (2)). tion of 1,1-cycloaddition o f AEC, a n d if we assume t h a t the q u a n t u m yield in the case of photolysis of Na groups is close to u n i t y , as was p r e v i o u s l y shown in ref. [1], t h e n q is n u m e r i c a l l y equal to the q u a n t u m yield according to double bonds. I t should be n o t e d t h a t the r a t e of decomposition of azido groups is r e d u c e d o n i n t r o d u c i n g AEC into t h e film (Fig. 2) as AEC absorbs in the v i c i n i t y o f 254 n m (Fig. 3), b u t does n o t itself polymerize u n d e r the conditions of the e x p e r i m e n t .

2610

A.G.

TABLE 2. EFFECT

OF CONTENT RATIO

Time of exposure, min

FILIMOSHKI~r et al.

OF AZIDO GROUPS IN PVAB

OF AZIDO

GROUPS

AND

AEC

ON q, AND q IN RELATION IN THE

T O THw.

FILM

Molar ratio of azido groups to AEG content in film I I 1.o: 0.5 I 1.0:0.75 I 1:1 I 1.0:1-5 1:2

0"16 0"50

PVAB-92 0.84 0.87 0.84 0-85 0.87 0.83 0.87* 0.85

0"64 0"62 0'61 0"61 0"65 0"58 0"62*

1"00 1"67

2"67 4"67 9"67

~0.62

1"00 0"92 0"98 0"91 0"99 0"93 o.98t 0.97

1"02 1"14 1"00 1"08 1"12 0"97 1.04t 1-05

1"31 1.29~ 1-27

2.26

2"09

1.37

2.s

2.6

1.37 1.32 1-20

1"28 1"27

PVAB-42*

1

91.3

I

I

91.6

r

-

I

PVAB-15* * The film is insoluble,

-

I

1-2

t Film dissolves in cyclohexanone.

T h e d a t a in T a b l e 2 show t h a t values of q in t h e studied systems are p r a c t i cally i n d e p e n d e n t o f conversion, a n d t h a t when P V A B w i t h a high c o n t e n t o f N3 groups (PVAB-92) is used as t h e initial p o l y m e r , t h e average value o f q (~) rises u n i f o r m l y w i t h increasing c o n t e n t of double bonds in the film, ~ b e i n g < 1 in t h e case o f N8 : A E C ~ I . 0 : 0.5 a n d 1.0 : 0.75, a n d close to u n i t y a t Ns:AEC~< ~<1. I n t h e case o f initial N3/AEC ratios ~< 1 the i r r a d i a t e d films are soluble, b u t w i t h N s / A E C > I insoluble films are obtained. I n e x p e r i m e n t s with P V A B - 4 2 a n d PVAB-15, w h e n nitrenes f o r m e d during photolysis are s u r r o u n d e d b y a c e t a t e groups, t h e relationship b e t w e e n ~ a n d the n u m b e r of double bonds in t h e films is o f a n e x t r e m a l t y p e . T h e e x p e r i m e n t a l findings are u n d e r s t a n d a b l e if one bears in m i n d t h a t along w i t h t h e m a i n reaction of 1,1-cycloaddition of A E C t o n i t r e n e s s e c o n d a r y processes also t a k e place during the irradiation of P V A B in presence o f AEC, a n d m a y be described as follows: CHj~CHR ~N,

hv_~ ~

• ] N.-- ~ - ~

/I ~

CH,

CHR N~--N~ (4)

~0COCHs CH, = C H R ) -~ ~ N H + M~ OCO~H~ C ~R j,D decay with intermopolymerlecular bond formation ization

Photoohemical changes in polyvinyl-p-azidobenzoato

2611

As was shown in ref. [1], the decay of nitrenes in PVAB-92 takes place mainly in accordance with reaction B in scheme (4), though reaction C occurs as well. During the irradiation of PVAB-92 in presence of AEC with relatively low concentrations of the latter, some of the nitrenes react according to reaction B. On increasing the amount of double bonds the proportion of reaction A is increased. Given an initial N3/AEC ratio of 1.0 to 1-5 the reaction t h a t takes place is almost exclusively reaction A, i.e. polymer analogue transformation of azido groups to aziridine rings. With PVAB-42 and PVAB-15 as the starting products the surrounding acetate groups will result in a greatly increased probability of monoradicals being formed in accordance with reaction C, and direct evidence was obtained in ref. [7] to show t h a t these monoradieals do appear in the system while nitrene decay is proceeding. The presence of monoradicals in the system m a y initiate the polymerization of AEC, and it is this which gives rise to a larger n u m b e r of reacted double bonds relative to the amount of nitrenes formed which do n o t initiate polymerization. An increase in the amount of AEC in the film increases both the probability of polymerization (reaction D) and the probability of 1,1-cycloaddition (reaction A), and polymerization is bound to increase ~. On the other hand a reduction in ~ in the case of a large amount of AEC in PVAB-42 and PVAB-15 films means t h a t reaction A is predominating, and t h a t reaction D is being suppressed, since there is marked inhibition of the formation of monoradicals (polymerization initiators), the latter reaction having high activation energies [8]. This means, therefore, t h a t whether Or not polymer analogue photochemical transformation of azido groups to aziridine rings takes place will depend on the number of double bonds, which in turn depends on the surrounding groups. Translated by R. J. A. HEND:aY REFERENCES

1. A. G. FILIMOSttKIN, Yu. A. YERSHOV and R. M. LIVSHITS, Vysokomol. soyed. A16: 1078, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 5, 1245, 1974) 2. S. H. MERRIL and C. C. UNRUH, J. Appl. Polymer Sci. 7: 273, 1963 3. V. P. LOPATINSKII and I. P. ZHEREBTSOV, Izv. Tomsk Polyteeh. Inst. 175: 10, 1971 4. R. HUISGEN, G. SZEIMIES and L. MOBIUS, Chem. Ber. 99: 491, 1966 5. G. SZEIMIES and R. HUISGEN, Chem. Ber. 99: 475, 1966 6. Y. GILLIAMS and G. SMETS, Makromolek Chem. 117: 1, 1968 7. Yu. A. YERSHOV, V. Ye. SEROKHVOSTOVA, V. B. MILLER, A. G. FILIMOSHK1N and R. M. LIVSHITS, Izv. S. D. U.S.S.R. Academy of Sciences, seriya khlm., 105, 1974 8. A. REISER, F. WILLETS, G. TERRY, V. WILLIAMS and B. MARLEY, Trans. Faraday Soc. 64: 3265, 1968