A C T A U N I V E R S I T A T I S L O D Z I E N S I S F O LIA C H I MI CA 10, 1993 W oj c i e c h 3. Kin art* , C e z ar y M. K in a r t * * THE E L E C T R O C H E M I C A L S T U D I E S O N W A T E R - S O L U B L E P O L Y M E R I Z A T I O N I N I T I A T O R S B A S E D ON THE 9 H - T H I 0 X A N T H - 9 - 0 N E S T R U C T U R E This paper r e p or ts p o l a r o g r a p h i c s t u d i e s on a g ro up of five w a t e r - s o l u b l e i n i t i at o rs of p o l y m e r i z a t i o n , the d e r i v a tives of 9 H - t h i o x a n t h - 9 - o n e . Their r e d u c t i o n on the m e r cu ry c a t h o d e is a t w o - e l e c t r o n pro cess , and it leads to the c o r r e s p o n d i n g d e r i v a t i v e s of 9 H - t h i o x a n th en - 9- ol .
The r e s p e c t i v e p h o t o i n i t i a t o r s s u b s t i t u t e d by the methyl g ro up in p o s i t i o n 1 e x h ib it only sin gle w av es w h e r e a s the o t h e rs e x h i b i t two waves. The a p p e a r a n c e of the sec on d wave is a t t r i b u t e d to f o r m a ti o n of a h yd r a t e by the s t u di ed 9H- - t h i o x a n t h - 9 - o n e s . The an a ly si s of c ha n g e s in the l im itin g d i f u s s i o n c u r r e n t for a q u eo us s o l u t i o n s of those c o m p o u n d s as a fu n ct io n of pH e n a bl e us to e x p l a i n the e f f ec t of m e t h y l a t i o n and ionic s u b s t i t u e n t on the a c i d - b a s e e q u i l i b r i a o c c u r i n g there.
I N T R OD U CT IO N
R ec e ntl y, s ev eral types of k e t o n e s have a cq u i r e d some i m p o r t a n c e as p h o t o i n i t i a t o r s for the u n s a t u r a t e d h y d r o c a r b o n p o l y m e r i z a t i o n [ l ] . The a p p l i c a t i o n of some i on ized d e r e v a t i v e s of b e n z o p h e n o n e or benzil, a b s o r b i n g the UV light b e t w e e n 200 an 3 50 nm, has been r e p o r t e d for g r a f t i n g w a t e r - s o l u b l e v i n yl ic m o n o m e r s on to c e l l u l o s e or wool [2]. How ever , in spi te of their r e a c ti v it y, these p h o t o i n i t i a t o r s are act iv e only then when they
*
D e p a r t m e n t of O r g a n i c Ch e mi st r y, U n i v e r s i t y of Lodź, N a r u t o w i c z a 68, 90- 136 Łódź, Poland.
are i r r a d i a t e d at 365 nm or below in the UV sp e ct ra l range, t h e r e f o r e their use in ma ny te c hn ic a l appli ca ti on s which r eq u ir e s the i l l u m i n a t i o n in the blue and of the v i s i b l e s p e c t r u m is i m p os s ib l e. In o rd er to o v e r c o m e s o m e h o w these d ra w b a c k s , several r e s e a r c h g r o u ps had s y n t h e s i z e d some 9H-thioxanth-9-one d e r i v a t i v e s w h i c h us ed in c o n j u c t i o n with a c t i va t or s, were r e p o r t e d to be p a r t i c u l a r l y s u i t a b l e for c u r in g some c o a t i n g f o r m u l a t i o n s b e t ween 350 and 450 nm. These c o m p o u n d s c ar ry a s i n g le c a t i o n i c s o l u b i l i z i n g group as it is s ho wn b e l o w for five d e r i v a t i v e s of 9 H - t h i o x a n t h - 9 - o n e s t u d i e d by us. R^, R 2 , R-j, R^ = A or 8 or C A = 0 - C H 2 - C H - C H 2 N + ( C H 3 )3 Cl'; B = C H 3 ; C = H GH T a b l e 1 F o r m u l a s of the wat er - -soluble 9 h - t h i o x a n t h - 9 - o n e d e r i v a t i v e s C o m p o u n d R 1 r2 R 3 R 4 1 C A B B 2 B A C B 3 C A C C 4 B C C A 5 C C C A
Used in this work s y m b o l s (1) - (5) in d ic at e r e s p e c t i v e l y : (1) - 2 - h y d r o x y - 3 - ( 3 , 4 - d i m e t h y l - 9 - o x o - 9 H - t h i o x a n t h e n - 2 - y l o x y ) - N . N , N - t r i m e t h y l a m m o n i u m - l - p r o p a n i u m chl orid e,
(2) - 2 - h y d r o x y - 3 - ( l , 4 - d i m e t h y l ^ 9 - o x o - 9 H - t h i o x a n t h e n - 2 - y l o x y ) - N , N , N - t r i m e t h y l a m m o n i u m - l - p r o p a n i u m chl orid e, (3) - 2 - h y d r o x y - 3 - ( 9 - o x o - 9 H - t h i o x a n t h e n - 2 - y l o x y ) - N , N , N - t r i m e t h y - l a m m o n i u m - l - p r o p a n i u m chl orid e, (4) - 2 - h y d r o x y - 3 - ( 1 - m e t h y l - 9 - o x o - 9 H - t h i o x a n t h e n - 4 - y l o x y )-N ,N,N - - t r i m e t h y l a m m o n i u m - l - p r o p a n i u m chl orid e, (5) - 2 - h y d r o x y - 3 - ( 9 - o x o - 9 H - t h i o x a n t h e n - 4 - y l o x y )- N , N , N-trimethy- l a m m o n i u m - l - p r o p a n i u m chloride.
L o u f i n o t at al. [3] have r e p o r t e d that i n t r o d u c t i o n of the ionic s u b s t i t u e n t into the 9 H - t h i o x a n t h - 9 - o n e s k e l e t o n leads to very high rates of p o l y m e r i z a t i o n . F u r t h e r s u b s t i t u t i o n by the met hyl g r o up s into a v a i l a b l e p o s i t i o n s of the b e n ze ne ring c a r r y i n g a l r e a d y the ionic s u b s t i t u e n t r e s ul ts in p ro f o u n d c ha n g e s in the re a ct iv i ty . The p e r f o r m a n c e of c o m p o u n d s (2) and (4) w h i c h car ry a m et hyl in p o s i t i o n 1, and w hi ch are apt to u nd e r g o the k e t o - e n o l t a u t o m e r i s m in their e x c i t e d states, does not seem to be d i m i n i s h e d by such an i n t e r m o l e c u l a r d e a c t i v a t i n g p r o c e s s .
L o u g n o t at al. [3] have also c o m p a r e d the r at es of p o l y m e r i z a t i o n (R p ) a c r y l a m i d e in a q u e o u s s o l u t io n s, in the p r e s e n c e of t h i o x a n t h o n e s (1), (3), (4), (5).
It seems to be quite i n t e r e s t i n g that Rpdisplays its h i g he st value for c o m p o u n d s (4) and (5), is much lower for c o m p o u n d (1), and is lowest for c o m p o u n d (3). A d d i t i o n a l l y , 2 - h y d r o x y - 3 - ( 1,3, 4 - t r i i n e t h y l - 9 - o x o - 9 H - t h i o x a n t h e n - 2 - y l o x y ) - N , N , N - t r i m e t h y l a m m o n i - u m - l - p r o p a n i u m c h l o r i d e w hi ch is s i m il ar to compound (2) s t u di ed by us, d e m o n s t r a t e s the h i g h e s t a ct i v i t y as p h o t o i n i t i a t o r , in c o m p a r i s o n with the oth er c o m p o u n d s mentioned. U n f o r t u n a t e l y , the ef fect of s u b s t i t u t i o n by the met hy l g r o up s and the ion ic sub- s t i t u t e n t was not e x p l a i n e d in full. A d d i t i o n a l l y , all the five s tu d ie d p h o t o i n i t i a t o r s C 1)-(5) e x n i b i t the loss of a c t i v i t y in ac idic and n e u tr al media. No d e f i n i t e rea so n was given as to a c c o u n t for this effect. The refore, the aim of the p r e s e n t study is to e x p la in the acid - base e q u i l i b r i a o c c u r i n g in a q u eo us s o l u t i o n s of 9 H - t h i o x a n t h - 9 - o n e s s t u d i e d w i t h i n the p os s i b l y full range of pH. We have h op ed that s c r u t i n y of such the e q u i lib ria w ou ld e n a b le us to e x p la in the above m e n t i o n e d s t r u c t u r a l e f f e c t s .
E X P E R I M E N T A L
All s t u d i e d c o m p o u n d s ( 1 ) — (5) were m a n u f a c t u r e d by I n t e r n a tio nal B i o - S y n t h e t i c s in the U ni ted Kingdom.
P o l a r o g r a p h y was c a r r i e d out for 0.001 M a q u ou s s o l u t i o n s of five the title p h o t o i n i a t o r s . A sam pl e for the m e a s u r e m e n t was a l w a ys p r e p a r e d by d i l u t i o n of 1 c m 3 of 0.025 M s o l u t i o n of 9H- - t h i o x a n t h - 9 - o n e s t u d i e d by a d d i t i o n of the B ri tten b u f f er up to the vol um e of 25 cm^ (pH was var ie d from 2 to 10). A p o l a r o g r a p h P PW-1 was m ad e by E lp an in Poland, and the t h r e e - e l e c t r o d e s y s t e m c o n s i s t i n g of: a s a t u r a t e d c al o m e l el e ct ro d e, a d ro p pi n g m e r c u r y e l e c t r o d e and a c o m p e n s a t i o n e l e c t r o d e formed by the "bo ttom " m e r c u r y were used 0.1 M KC1 was used as a s u p p o r t i n g e l e c t r o l y t e . IR s p e ct ra were r ec o r d e d u si ng an IR p h o t o s p e c t r o - m e t e r of the S p e c o r d 71 IR type.
RE S U L T S AND D I S C U S S I O N
We have p r e v i o u s l y found [4] that the c o u l o m e t r i c r e d u c ti o n of c o m p o u n d (1) in its a q u e o u s s ol u t i o n at the m e r c u r y e l e c t r o d e was a t w o - e l e c t r o n p r o c e s s o c c u r i n g a c c o r d i n g to the m e c h a n i s m g i v e n below:
Fig. 1. C h a n g e s in the l im i t i n g d i f f u s i o n c ur rent , i ^ , and the h a l f - p e a k r e d u c t i o n po t en ti a l, E ^ C a t t r i b u t e d to the k e t o n e r e d uc t io n ), and the c o r r e s p o n d i n g values, i 2 , E 2 ( a t t r i b u t e d to the h y d r a t e r e d u c t i o n ) d ra wn as a f un c t i o n of pH for 1 • l O -3 M a qu e o u s s o l u t i o n s of c o m p o u n d (1) b u f f e r e d by the B r i t t e n - R o b i n - son buf fers , o b t a i n e d from p o l a r o g r a p h i c r e d u c t i o n of (1) in the
t h r e e - e l e c t r o d e sys te m
i rf - i
Also the a n a l y s i s of the slo pe of log - --t— d r a w n as a f u n c ti on of the e l e c t r o d e p o t e n ti a l for all the p o l a r o g r a p h i c wav es r e c o r d e d for the five 9 H - t h i o x a n t h - 9 - o n e s s t u d i e d i n d i c a t e s a t w o - e l e c t r o n m e c h a n i s m tor this re d uc ti o n. Th e re fo r e, we have a s s u me d that the r e d u c t i o n of all the 9 H - t h i o x a n t h - 9 - o n e s s t u d i e d by us leads to the c o r r e s p o n d i n g D e r i v a t i v e s of 9 H - t h i o x a n t h e n - -9-ol. A d d i t i o n al ly , the h a l f - p e a k r e d u c t i o n p o t e n t i a l s for all the c o m p o u n d s s t u d i e d d ra wn as a f un c t i o n of pH sho ws a linear g r o w th with the in c re as e of pH (see Fig. 1-5). H o w e v e r the d i a g r ams of c h a n g e s in the l im i t i n g d i f f u s i o n c u r r e n t - i^ d ra wn as a fu n ct io n of pH are e nt i r e l y d i f f e r e n t for all the p h o t o i n i t i a - tors studied. Tho se substitited in p o s i t i o n 1 (i.e. c o m p o u n d s (2)
Fig 2. C h a n g e s in the l i m i t i n g d i f f u s i o n cur rent , i, and half- -peak r e d u c t i o n p o t e n ti a l, E ^ / 2 ’ d ra wn as a f u n c t i o n of pH for 1 ■ 1 0 ” "5 M a qu e o u s s o l u t i o n of c om p o u n d (2) b u f f e r e d by the B r i t t e n - R o b i n s o n buf fers , o b t a i n e d from p o l a r o g r a p h i c r e d u c t i o n
of (2) in the t h r e e - e l e c t r o d e s ys tem
and (4) - see Tab. 1) e x h i b i t a s i n g l e - w a v e for ea ch value of pH. The m e a s u r e d v a l ue s of i^ for c o m p o u n d s (2) and (4) d e c r e a s e g r a d u a l l y w it h the i nc r e a s e of pH. Only for a c i d ic s ol u tio ns, (see Fig. 2 and 4). This e f f e c t is p r e s u m a b l y due to p r o t o n a t i o n
of the ketone. The r e m a i n i n g three p h o t o i n i t i a t o r s reveal a s e r ie s of t w o - e l e c t r o n w av es clo se to each other. The a p p e a r a n c e of such two s e p a r a t e w av es is a t r i b u t e d to a c o e x i s t e n c e of two d i f f e r e n t forms of the s t u d i e d salt in the a q u eo us s o l u t io n s (i.e. k e t o n e and its h yd rate ) a c c o r d i n g to the e q u i l i b r i u m given below, where: R 1 = R 2 H or C ^ Cl © +h2o(h+ ) -h2o Cle
The f o r m a t i o n of the a n a l o go u s h yd r a t e has p r e v i o u s l y been d e s c r i b e d for l - m e t h y l - 4 - p i p e r i d o n e h y d r o c h l o r i d e [5]. The first wave for w hi ch the l i m i t i n g d i f f u s i o n c u r r e n t was d e n o t e d as i was a t r i b u t e d to the r e d u c t i o n of ket one, w h e r e a s the
one with the l im i t i n g d i f f u s i o n c u r r e n t i
d se cond to the r e d u c t i o n of its hydrate. A dd i t i o n a l l y , the IR s p e c t r a of s am p l e s of c o m p o u n d s (1), (3) and (5) r e c r y s t a l l i z e d from s t r o n g l y a ci dic a qu e o u s s o l u t i o n s ha ve shown a w e a k e n i n g the a b s o r p t i o n band at ca. 1670 c m " 1 , w h i c h is c h a r a c t e r i s t i c of the C = 0 d o u b le bond s t r e t c h i n g fre quen cy, and a c o n s i d e r a b l e g r o w t h of the a b s o r p t i o n band at 3400 c m ’ 1 c h a r a c t e r i s t i c of the OH s t r e t c h i n g frequency. C o n t r a r i w i s e , c o m p o u n d s (2) and (4) have not s ho wn the a n a l o g o u s beh avio ur.
Co m p o un d (1) r e v ea ls two close p o l a r o g r a p h i c waves. 1^ for the fir st wave a t t r i b u t e d to k e t o ne n e a rl y does not d e p e n d on
pH (see Fig. 1), w h e r e a s i . of the sec on d wave d e c r e a s e s q u i ck ly
2
w it h the i n c r e a s e of pH. It r e a c h e s a c o n s t a n t value for pH equal to c le a r l y ca 7.
Fig. 3. C h a n g e s in the l i m i t i n g d i f f u s i o n c ur rent , i and h a l f - p e a k r e d u c t i o n p o t e n ti a l, ( a t t r i b u t e d to the k e t o n e r e d u c t io n ) and the c o r r e s p o n d i n g values, i 2 > E 2 ( a t t r i b u t e d to the h y d r a t e r e d u c t i o n ) d r a wn as a f u n c t i o n of pH for 1 • 1 0 ” '5 M a q u e o u s s o l u t i o n of c o m p o u n d (3), b u f f e r e d by the B r i t t e n - R o b i n - son b uf fers , o b t a i n e d from p o l a r o g r a p h i c r e d u c t i o n of (3) in the
Fig. 4. C h a n g e s in the l im i t i n g d i f f u s i o n c ur rent , i, and half- -peak r e d u c t i o n p o t e n ti a l, E ^ / 2 ’ d ra wn as a f un c t i o n of pH for 1 ' 10 ^ M a q u eo us s o l u t i o n s of c o m p o u n d (4), b u f f e r e d by the B r i t t e n - R o b i n s o n buf fers , o bt a i n e d from p o l a r o g r a p h i c r e d u c t i o n
of (4) in the t h r e e - e l e c t r o d e s y s t e m
This v al ue of pH w ou ld c o r r e s p o n d to a m i n i m u m h y d r a t e c o n c e n t r a t i o n in the s ol utio n, and as it is k n o w n from l i t e r a t u r e [3], at this pH c o m p o u n d (1) r e v ea ls the h i g h e s t a c t i v i t y as the p h o t o i n i t i a t o r . Also p o l a r o g r a p h y of c o m p o u n d (3) r e v e a l s a set of two w a v e s (Fig. 3). N e i th er i. nor i, d i s p l a y (any)
con-°1 2
s i d e r a b l e c h a n g e s for pH b el ow 7. H ow ever , for pH a bo ve 7 only a s in gle w av e c ou ld be obs erve d. W i t h i n this r e g i o n of pH, the
Fig. 5. C h a n g e s in the l im i t i n g d i f f u s i o n c ur rent , i^, and h al f- -peak r e d u c t i o n p o t e n ti a l, ( a t t r i b u t e d to the k e t o n e r e d u c tion), and the c o r r e s p o n d i n g values, i 2 E 2 ( a t t r i b u t e d to the h y d r a t e r e d u c t i o n ) drawn as a f un c t i o n of pH for 1 • 10 3 M aqueous s o l u t i on s c o m p o u n d s (5), b u f f e r e d by the B r i t t e n - R o b i n s o n b uf fers , o b t a i n e d from p o l a r o g r a p h i c r e d u c t i o n of (5) in the three-electro
l i m i t i n g d i f f u s i o n c u r r e n t b ei ng now a sum of i , + i . , gradually
2
i n c r e a s e s with the i nc r e a s e of pH. P re s u m a b l y , also the 0H~ ions w ou ld c a t a l y s e here f o r m a t i o n of the h yd rate s. F in a ll y , the most u n i q u e set of p o l a r o g r a p h i c w a v es was o b s e r v e d for c o m p o u n d (5) (Fig. 5). For pH above 5, the sec on d wave d is a p p e a r s . This i n d i c at es that c o n c e n t r a t i o n of the h y d r a t e w ou ld be very low here, t h e r e f o r e the a c t i v i t y of the p h o t o i n i t i a t o r s t u d i e d is hig he r as c o m p a r e d with that in aci di c s ol u tio ns.
R EF E R E N C E S [1] G. A m i r z a d e h , W. S c h a n a b e l , M a k r o m o l . Chem., 1 8 2 , 2821 (1981). R . S . D a v i d s o n , 3. W . G o o d i n , Eur. Polym. 3., _18, 587 (1982). M. R. S a n d n e r , C. L. O s b o r n , D. 3. T r e c k e r , J. Polym. Sci., Polym. Chem. Ed., 1 0 8 , 3173 (1972).
3. P. F o u a s s i e r, 3. Chim. Phys., 80, 339 (1983). 5 . F . Y a t e s , G . B . S c h u s t e r , 3. Org. Chem., 4 9 , 3349 (1984). [2] R . A . B o t t o m , 3. T . G u t h r i e , Polym. P h o t o ch e m. , 6, 111 (1985). 3. T . G u t h r i e , P . S . 8 a k e r , R . A., B o t t o m , P. N. G r e e n , S pe c tro sc. Chem., _5, 4 (1985). [3] D . 3. L o u g n o t , C . T u r c k , 3. P . F o u a s s i e r, M a c r o m o l ., 22, 108 (1989). [4] W . 3. K i n a r t , et al., 3. E le c tr o a n a l . Chem., 2 9 4 , 293 (1990) . [5] R . E . L y 1 e , R . E . A d e 1, G . G . L y 1 e , 3. Org. Chem., 24, 342 (1959).
W o j c i e c h J. Kinart, C e z a ry M. Kin ar t
E L E K T R O C H E M I C Z N E B A D A N I A R O Z P U S Z C Z A L N Y C H W WOD ZIE I N I C J A T O R Ó W P O L I M E R Y Z A C J I
O P A R T Y C H NA U KŁ A D Z I E 9 H - T I 0 K S A N T E N - 9 - 0 N U
P ra ca ta d o t y c z y p o l a r o g r a f i c z n y c h badań sz e r e g u r o z p u s z c z a l nych w w o d z i e i n i c j a t o r ó w p o l i m e r y z a c j i . S t w i e r d z o n o , że ich e l e k t r o c h e m i c z n a r ed u k c j a na k a t o d z i e r t ę c i ow e j jest p r o c e s e m d w u e l e k t r o n o w y m p r o w a d z ą c y m do o d p o w i a d a j ą c y c h im p o c h o d n y c h t i o k s a n t e n - 9 - o l u . Te ze z b a d a n y c h f o t o i n i c j a t o r ó w , k tó re były p o d s t a w i o n e grupą m e t y l o w ą w p o z yc ji 1 w y k a z y w a ł y t y l k o p o j e d y n cze fale p o l a r o g r a f i c z n e , p o d c z a s gdy p o z o s t a ł e - podwójne' fale p o l a r o g r a f i c z n e . Z j a w i s k o p o j a w i a n i a się d r u gi ej fali na p o l ar o- g r a m a c h b a d a n y c h z w i ą z k ó w z o s ta ło w y t ł u m a c z o n e t w o r z e n i e m się h y d r a t ó w b a d a n y c h t i o k s a n t e n o n ó w . A n a li za zmian g r a n i c z n e g o p r ą du d y f u z y j n e g o dla ich r o z t w o r ó w w o d n y c h w y k r e ś l o n a w funkcji pH p o z w o l i ł a w y j a ś n i ć efekt p o d s t a w i e n i a do u k ł ad u m a c i e r z y s t e g o gr up m e t y l o w y c h i b o c z n e g o ł ań cuch a na z a c h o d z ą c e tamże r ó w n o w a gi k w a s o w o - z a s a d o w e .
