Hydrogels for Medical and Related Applications

8 Analysis and Purification of 2-Hydroxyethyl Methacrylate by Means of Thin-Layer Chromatography U. A. TH. BRINKMAN, T. A. M. VAN SCHAIK, and G. DEVRIES Department of Analytical Chemistry, Free Reformed University, Amsterdam, The Netherlands A. C. DEVISSER

Downloaded by COLUMBIA UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0031.ch008

Department of Materials Science, Free Reformed University, Amsterdam, The Netherlands T h r e e - d i m e n s i o n a l networks o f h y d r o p h i l i c methac r y l a t e polymers, have been r e c o g n i z e d ( 1 , 2 ) as usef u l m a t e r i a l s f o r b i o m e d i c a l and s u r g i c a l a p p l i c a t i o n s Implants and o t h e r prothèses a r e b e i n g p r e p a r e d from such h y d r o g e l s o f t e n r e i n f o r c e d w i t h a f a b r i c s t r u c t u r e . The r e s e a r c h program o f o u r departments aims a t the development o f a h y d r o g e l t h a t l j combined w i t h a f i l l e r can s e r v e t o s u b s t i t u t e h a r d t i s s u e (bone), and 2 ) promotes c a l c i f i c a t i o n and/or bone growth i n i t s m a t r i x , so t h a t t h e i n i t i a l l y s o f t m a t e r i a l i s f i l l e d up by t h e body i t s e l f w i t h h a r d t i s s u e . As h y d r o g e l , p o l y ( h y d r o x y e t h y l m e t h a c r y l a t e ) (PHEMA), has been s e l e c t e d which can be p r e p a r e d from t h e commerc i a l l y a v a i l a b l e monomer, 2 - h y d r o x y e t h y l m e t h a c r y l a t e (HEMA) by v a r i o u s methods ( 3 - 5 ) . PHEMA, which has been w i d e l y s t u d i e d and d i s c u s sed f o r m e d i c a l a p p l i c a t i o n s , has l a r g e l y been accept s (1*6) b i o c o m p a t i b l e , s a f e and s t a b l e m a t e r i a l f o r m e d i c a l u s e . I n o r d e r t o p r e p a r e a g e l t h a t meets such r e q u i r e m e n t s , t h e i n i t i a l monomer s h o u l d be o f h i g h p u r i t y . The c o m m e r c i a l l y a v a i l a b l e HEMA c o n t a i n s r e l a t i v e l y l a r g e p r o p o r t i o n s o f m e t h a c r y l i c a c i d (MAA) and t h e d i e s t e r e t h y l e n e d i m e t h a c r y l a t e (EDMA); t h e l a t t e r can be formed from HEMA by t r a n s e s t e r i f i c a t i o n . Other contaminants may a l s o be p r e s e n t , as i s d i s c u s s e d below. The removal o f EDMA from HEMA, as w e l l as MAA and i t s o t h e r e s t e r s , i s p a r t i c u l a r l y d e s i r a b l e , because t h e p r o p o r t i o n o f d i e s t e r i n t h e p o l y m e r i z i n g mixture a f f e c t s the nature o f the network formed (1)· S i n c e t h e r a t e o f both t h e p o l y m e r i z a t i o n and t h e t r a n s e s t e r i f i c a t i o n r e a c t i o n s t r o n g l y i n c r e a s e s w i t h i n c r e a s i n g temperature, t e c h n i q u e s such as d i s t i l l a t i o n and gas chromatography do n o t appear t o be e n t i r e l y f a v o r a b l e f o r t h e u l t i m a t e p u r i f i c a t i o n and a n a l y s i s o f HEMA samples. f

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In Hydrogels for Medical and Related Applications; Andrade, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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T h e r e f o r e , i n t h e p r e s e n t s t u d y , t h i n - l a y e r chromato graphy ( t i c ) has been s e l e c t e d f o r both q u a l i t a t i v e a n a l y s i s and s m a l l - s c a l e p r e p a r a t i v e work.


M a t e r i a l s and Methods 2 - H y d r o x y e t h y l m e t h a c r y l a t e was p u r c h a s e d from BDH ( P o o l e , England) and Merck (Darmstadt, W. Germany); t h e s e p r o d u c t s have a y e l l o w c o l o r . C o l o r l e s s h i g h l y pure (> 99%) HEMA was o b t a i n e d as a g i f t from Hydro Med S c i e n c e s (New Brunswick, N.J., U.S.A.). E t h y l e n e d i m e t h a c r y l a t e was o b t a i n e d from BDH ( P o o l e , England) and Koch and L i g h t (Colnbrook, Eng l a n d ) . M e t h a c r y l i c a c i d , e t h y l e n e g l y c o l , 2-hydroxyp r o p y l m e t h a c r y l a t e , m e t h y l m e t h a c r y l a t e , hydroquinone and £-methoxyphenol, were p u r c h a s e d from Merck. D i e t h y l e n e g l y c o l m e t h a c r y l a t e was s y n t h e s i s e d (8) by m i x i n g a p p r o p r i a t e amounts o f d i e t h y l e n e g l y c o l (855 g) and methyl m e t h a c r y l a t e (500 g) a t a temperature o f 60°C, a d d i n g a 4 Ν s o l u t i o n o f sodium methanolate i n methanol (10 g) and h e a t i n g t h e r e a c t i o n m i x t u r e f o r a p e r i o d o f time o f 30 min. S u b s e q u e n t l y , t h e m i x t u r e was poured i n t o water (400 g ) , washed w i t h n-hexane and e x t r a c t e d t w i c e w i t h d i e t h y l e t h e r . A f t e r r e p e a t e d washings w i t h w a t e r , t h e e t h e r e x t r a c t was d r i e d and d i e t h y l e n e g l y c o l m e t h a c r y l a t e was d i s t i l l e d a t r e d u ced p r e s s u r e . A l l f u r t h e r c h e m i c a l s used i n t h i s study were r e a g e n t - g r a d e p r o d u c t s , and were used w i t h o u t f u r t h e r purification. T h i n - l a y e r chromatography was c a r r i e d o u t i n Hellendahl s t a i n i n g j a r s , using precoated s i l i c a g e l p l a t e s ( K i e s e l g e l 60 F 9 5 4 ' o ^^ * a p p r o p r i a t e s i z e (4 χ 8 cm ) . Spots were a p p l i e d w i t h a p o i n t e d paper wick and chromatography i s c a r r i e d o u t i n a n o n - s a t u r a t e d atmosphere. A f t e r development o v e r a l e n g t h o f r u n o f a p p r o x i m a t e l y 7 cm, d e t e c t i o n i s done u s i n g t h e me thods r e p o r t e d below. R e p r o d u c i b i l i t y o f t h e Rp v a l u e s i n t h e s o l v e n t systems used i n t h e p r e s e n t study was satisfactory. P r e p a r a t i v e - s c a l e t h i n - l a y e r chromatography was c a r r i e d o u t on 20 χ 20 cm2 2-mm t h i c k p r e c o a t e d s i l i c a g e l p l a t e s ( K i e s e l g e l 60 F 2 5 4 , Merck). 100-150 mg HEMA, as a 20% (v/v) s o l u t i o n i n e t h a n o l , were a p p l i e d w i t h a Camag chromatocharger equipped w i t h a d i s p o s a b l e p l a s t i c s y r i n g e . Development o v e r a d i s t a n c e o f 15-17 cm, w i t h o u t p r i o r a c c l i m a t i s a t i o n , i s done i n a normal r e c t a n g u l a r tank. M e r c

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BRiNKMAN E T A L .

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R e f r a c t i v e i n d i c e s were measured on a t h e r m o s t a t t e d (20 + 0.1°C) Abbe r e f r a c t o m e t e r . I R - s p e c t r a were run on a Shimadzu IR 400 s p e c t r o m e t e r , u s i n g c e l l s w i t h NaCl windows. R e s u l t s and D i s c u s s i o n Reference Substances. As has been s t a t e d i n t h i s i n t r o d u c t i o n , HEMA g e n e r a l l y c o n t a i n s s e v e r a l i m p u r i t i e s * * 9 · EDMA, m e t h a c r y l i c a c i d and e t h y l e n e g l y c o l . S i n c e m e t h a c r y l i c a c i d and i t s d e r i v a t i v e s tend t o p o l y m e r i z e even a t room temperature, an i n h i b i t o r such as h y d r o q u i n o n e , £-methoxyphenol, p h e n o t h i a z i n e o r o c t y l p y r o c a t e c h o l i s u s u a l l y added t o these p r o d u c t s (9). A c c o r d i n g t o t h e s p e c i f i c a t i o n s , t h e HEMA samples o b t a i n e d from BDH and Merck c o n t a i n 200 ppm p-methoxyphenol, and 100 ppm hydroquinone p l u s 100 ppm £-methoxyphenol, r e s p e c t i v e l y ; HEMA from Hydro Med S c i e n c e s , I n c . , c o n t a i n s 36 ppm £-methoxyphenol. Hydroquinone i s a l s o p r e s e n t i n EDMA and m e t h a c r y l i c a c i d samples. M e t h y l m e t h a c r y l a t e may a l s o be p r e s e n t as an i m p u r i t y i n c o m m e r c i a l l y a v a i l a b l e HEMA, s i n c e i t i s one o f t h e s t a r t i n g m a t e r i a l s i n i t s s y n t h e s i s . However, d u r i n g t i c , i t e v a p o r a t e s o f f from t h e c h r o matoplate (b.p., 100°C) and escapes d e t e c t i o n i n q u a l i t a t i v e a n a l y s i s . In p r e p a r a t i v e - s c a l e work, such e v a p o r a t i o n d u r i n g development ensures i t s removal. By d e v e l o p i n g a sample o f pure methyl m e t h a c r y l a t e and s p r a y i n g w i t h water (JL0) immediately a f t e r t h e r u n , we have shown i t s Rp v a l u e t o be c o n s i d e r a b l y h i g h e r than t h a t o f HEMA i n b o t h s o l v e n t systems recommended below. As r e g a r d s t h e presence o f water, one i s r e f e r r e d t o F i g . 3 and t h e accompanying t e x t . As a consequence o f t h e above, i n a d d i t i o n t o HEMA, 5 compounds were i n c l u d e d i n o u r s t u d y , v i z . EDMA, m e t h a c r y l i c a c i d , e t h y l e n e g l y c o l , hydroquinone and £-methoxyphenol ( c f . T a b l e I ) . The p u r i t y o f t h e samples o b t a i n e d as r e f e r e n c e s u b s t a n c e s was t e s t e d , u s i n g two o r more o f t h e s o l v e n t systems d i s c u s s e d below. The samples were found t o be c h r o m a t o g r a p h i c a l l y pure^ a p a r t from t h e presence o f an i n h i b i t o r , w i t h t h e e x c e p t i o n o f EDMA. Here, t i c w i t h n-hexane d i e t h y l e t h e r (1:1) as mobile phase r e v e a l e d t h e p r e s e n c e o f some e i g h t a d d i t i o n a l s p o t s ( F i g s , l a and l b ) . These contaminants were removed by d i s t i l l a t i o n a t reduced p r e s s u r e (b.p. o f EDMA a t 4 mm Hg, 90°C) o r , p r e f e r a b l y , by p r e p a r a t i v e - s c a l e t i c ( F i g s , l c and d ) .


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0.60.6

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15

100

I

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0.2

0.2

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(yg/μΐ)

0.2

0.2

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20 F o r HEMA, d has a v a l u e o f 1.070-1.074 U 4 ) . T h e r e f o r e , l y g contaminant/μΐ o f HEMA r o u g h l y c o r r e s p o n d s w i t h 1000 ppm o r 0.1 wt.%.

15

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Ethylene g l y c o l

£-Methoxyphenol

30

Methacrylic acid

15

30

Ethylene dimethacrylate (EDMA)

Hydroquinone

30

UV

Detection l i m i t

2-Hydroxyethyl methacrylate (HEMA)

Name

T a b l e I . D e t e c t i o n l i m i t s o f HEMA and some o f i t s contaminants,


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With t h e former t e c h n i q u e , a d d i t i o n o f hydroquinone o r o f a m i x t u r e o f KC1, B 1 C I 3 and B 1 I 3 (1_1) i s n e c e s sary i n order t o prevent polymerization. Detection. From among a l a r g e number o f non s p e c i f i c methods o f i d e n t i f i c a t i o n , t h r e e d e t e c t i o n methods were s e l e c t e d f o r f u r t h e r r e s e a r c h . Under U.V.l i g h t (254 nm) a l l compounds b u t e t h y l e n e g l y c o l a r e v i s i b l e as dark s p o t s on a f l u o r e s c e n t background. Treatment w i t h i o d i n e vapor (red-brown s p o t s on a y e l l o w - w h i t e background) i s a s u i t a b l e means t o d e t e c t the g l y c o l t o o . U n f o r t u n a t e l y , t h e s e n s i t i v i t y o f t h e combined U . V . / I p r o c e d u r e i s r a t h e r low (cf:. T a b l e I ) . T h e r e f o r e as an a l t e r n a t i v e , d e t e c t i o n has been done by s p r a y i n g w i t h a 0.2% s o l u t i o n o f KMnO4 i n acetone. White s p o t s a r e formed on a brownish background f o r a l l compounds e x c e p t £-methoxyphenol, which shows up as a p a l e orange s p o t . S i n c e KKn04 r e a c t s w i t h i m p u r i t i e s i n t h e acetone t o form Μηθ2, which mars t h e detec t i o n , t h e r e a g e n t s o l u t i o n s h o u l d be f r e s h l y p r e p a r e d . Keeping i n mind t h a t c h e c k i n g t h e p u r i t y o f HEMA i s done by t i c o f u n d i l u t e d samples, one may c o n c l u d e from t h e d a t a i n T a b l e I t h a t s p r a y i n g w i t h KMn04 a l l o w s t h e d e t e c t i o n o f i m p u r i t i e s down t o a t l e a s t 0.1%. Since the i n h i b i t o r s are present i n the v a r i o u s samples o f HEMA, EDMA and m e t h a c r y l i c a c i d i n concen t r a t i o n s o f up t o 0.0 2% o n l y , they w i l l escape d e t e c t i o n w i t h t h e above p r o c e d u r e s . T h e r e f o r e , two a l t e r n a t i v e methods o f i d e n t i f i c a t i o n (12,13) have been t e s t e d : 1) s p r a y i n g w i t h a f r e s h l y p r e p a r e d 0.5% s o l u t i o n o f t h e d i a z o r e a g e n t E c h t b l a u s a l z Β i n water, and e i t h e r s p r a y i n g w i t h 0.1 Ν NaOH o r , p r e f e r a b l y , exposure t o vapor o f NH3 ; 2) s p r a y i n g w i t h a m i x t u r e o f 0.09 g s u l p h a n i l i c a c i d d i s s o l v e d i n 10 ml 1.1 Ν HC1 and 10 ml o f an aqueous 4.5% NaN02 s o l u t i o n , k e p t a t 0°C and d i l u t e d w i t h an e q u a l volume o f a 10% Na2C03 s o l u t i o n immediately b e f o r e use. With both r e a g e n t s , brownish s p o t s show up on a n e a r l y white back ground. As can be seen from t h e d a t a i n T a b l e I , t h e use o f even these r e a c t i o n s b a r e l y s u f f i c e s f o r t h e i d e n t i f i c a t i o n o f the i n h i b i t o r s i n methacrylic a c i d and i t s d e r i v a t i v e s . F o r t u n a t e l y , improved r e s u l t s a r e o b t a i n e d i f s u l p h a n i l i c a c i d i s used t o d e t e c t t h e p h e n o l i c compounds by a drop t e s t p r o c e d u r e , i n which 1 drop o f t h e d i a z o t i s e d r e a g e n t i s mixed w i t h 1 drop o f sample ( d i s s o l v e d i n a minimum amount o f e t h a n o l , i f n e c e s s a r y ) and 1 drop o f a 10% Na2C03 s o l u t i o n . The d e t e c t i o n l i m i t now i s a p p r o x i m a t e l y 0.01% f o r h y d r o quinone ( g r e e n - t o - y e l l o w s p o t s ) , and 0.001% f o r η methoxyphenol ( r e d - t o - g r e e n s p o t s ) . 2



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S o l v e n t Systems. A f t e r s e v e r a l t r i a l runs o f the f o u r main components w i t h s i n g l e - s o l v e n t systems r a n g i n g from n o n - p o l a r (n-hexane, cyclohexane) t o p o l a r s o l v e n t s ( d i e t h y l e t h e r , m e t h y l i s o b u t y l ketone (MIBK), a c e t o n e ) , m i x t u r e s o f n-hexane w i t h d i e t h y l e t h e r o r ( a n ) o t h e r c o m p o n e n t ( s T were s e l e c t e d f o r f u r t h e r r e s e a r c h . S a t i s f a c t o r y r e s u l t s were o b t a i n e d w i t h n-hexane - d i e t h y l e t h e r (1:1, v/v) and n-hexaneMIBK - n - o c t a n o l (9:2:1, v/v) ( F i g . 2a and d ) . As a drawback, however, i t was o b s e r v e d t h a t the s p o t due t o m e t h a c r y l i c a c i d tends t o t a i l , e s p e c i a l l y i n the l a t t e r s o l v e n t system. Such t a i l i n g , which i s p a r t i c u l a r l y harmful i n p r e p a r a t i v e - s c a l e t h i n - l a y e r or column chromatography, can e f f e c t i v e l y be reduced by a d d i n g an a c i d i c component t o the system. Two a l t e r n a t i v e s ' have been i n v e s t i g a t e d . 1) The s o l v e n t system i s s a t u r a t e d w i t h an e q u a l volume o f 25% (v/v) H N O 3 . When a h i g h e r p e r c e n t a g e o f a c i d i s used, decomposit i o n o f one o r more o f the compounds under i n v e s t i g a t i o n o c c u r s d u r i n g development, as e v i d e n c e d by the o c c u r r e n c e o f a brown s t r e a k i n g zone on the chromatogram. 2) Development i s done on a s i l i c a g e l p l a t e soaked f o r 15 min w i t h 1% H 2 S O 4 , and s u b s e q u e n t l y d r i e d o v e r n i g h t a t 6 0 ° C . As i s m a n i f e s t from the d a t a i n F i g . 2, w i t h each o f these t e c h n i q u e s , the r e s u l t s i n d e e d improved. F o r q u a l i t a t i v e a n a l y s i s , development w i t h a HNOj s a t u r a t e d mobile phase s h o u l d be p r e f e r r e d t o t i c on an a c i d - i m p r e g n a t e d s u p p o r t , which i s a more time consuming p r o c e d u r e . From F i g . 2 i t i s e v i d e n t t h a t n-hexane - MIBK - n - o c t a n o l (9:2:1, s a t d . w i t h 25% H N O 3 ) i s a v e r y p o w e r f u l s o l v e n t system f o r the r e s o l u t i o n o f the f o u r main components. A d d i t i o n o f n i t r i c a c i d t o n-hexane - d i e t h y l e t h e r (1:1) a l s o improves the s e p a r a t i o n o f HEMA from m e t h a c r y l i c a c i d ; however, the s p o t s due t o m e t h a c r y l i c a c i d and EDMA now show an a p p r e c i a b l e o v e r l a p . T h e r e f o r e , as a t e n t a t i v e conc l u s i o n , we recommend the use o f both n-hexane d i e t h y l e t h e r (1:1) and n-hexane - MIBK - n - o c t a n o l (9:2:1, s a t d . w i t h 25% H N O 3 ) f o r q u a l i t a t i v e a n a l y s i s . F o r p r e p a r a t i v e - s c a l e t i c , chromatography on H 2 S O 4 -impregnated s i l i c a g e l has been p r e f e r r e d t o development w i t h a HNC^-saturated mobile phase, because s u l p h u r i c a c i d , but not n i t r i c a c i d , has a n e g l i g i b l y s m a l l s o l u b i l i t y i n the s o l v e n t used t o e l u t e HEMA from the s u p p o r t m a t e r i a l ( c f . b e l o w ) . n-hexane d i e t h y l e t h e r (1:1) has been s e l e c t e d as m o b i l e phase r a t h e r than the n-hexane - MIBK - n - o c t a n o l m i x t u r e , because o f the r e l a t i v e l y l a r g e time o f development o f the l a t t e r s o l v e n t system ( a p p r o x i m a t e l y 4 v s . 2.5 h


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Figure 1. Thin-layer chromatograms of EDMA samples, (a) EDMA from BDH; (b) EDMA from Koch and Light; (c) redis tilled EDMA (BDH); (d) EDMA (BDH) purified by preparative-scale tic. System: silica gel/n-hexane-diethyl ether (1:1).

3

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Figure 2. Tic of HEMA, EDMA, methacrylic acid (MA), and ethylene glycol (EG) in the systems: (a) silica gel/n-hexane-diethyl ether (1:1); (b) silica gel/n-hexane-diethyl ether (1:1, satd. with 25% HN0 ); (c) H S0^-impregnated silica gel/n-hex ane-diethyl ether (1:1); (d) silica gel/n-hexaneMIBK-n-octanol (9:2:1); (e) silica gel/n-hexaneMIBK-n-octanol (9:2:1, satd. with 25%> HN0 ). , acid (1), n-octanol (2) and MIBK (3) front. 3

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3



112


f o r a 16-cm r u n ) . Due t o t h e low v o l a t i l i t y o f n - o c t a n o l , i t s complete removal from t h e t h i n l a y e r i s n o t e a s i l y e f f e c t e d , and p a r t o f i t w i l l be e l u t e d t o g e t h e r w i t h HEMA. Next, a t t e n t i o n was p a i d t o t h e s e l e c t i o n o f a s o l v e n t s u i t e d t o e l u t e t h e s e p a r a t e d component(s) from t h e s i l i c a g e l . A t f i r s t s i g h t , t h i s c h o i c e does not appear t o be v e r y c r i t i c a l , because HEMA d i s s o l v e s f r e e l y i n s o l v e n t s such as water, acetone, d i e t h y l e t h e r , t e t r a c h l o r o m e t h a n e and, l e s s s o , n-hexane. However, when u s i n g any o f t h e f i r s t t h r e e h i g h l y p o l a r s o l v e n t s , t h e a c i d used t o impregnate t h e s i l i c a g e l i s e x t r a c t e d a l o n g w i t h HEMA. Moreover, when d e v e l o p ment i s c a r r i e d o u t on s i l i c a g e l plates, zinc o r i g i n a t i n g from t h e manganese-doped z i n c s i l i c a t e p r e s e n t as f l u o r e s c i n g i n d i c a t o r - as i t s s u l p h a t e s a l t , i s a l s o e x t r a c t e d . The presence o f both t h e a c i d and i t s z i n c s a l t i n p u r i f i e d HEMA have been demons t r a t e d by t i c on p l a i n s i l i c a g e l , w i t h n-hexane d i e t h y l e t h e r (1:1) as d e v e l o p i n g s o l v e n t . Both compounds remain a t t h e o r i g i n and a r e i d e n t i f i e d by s p r a y i n g w i t h a s u i t a b l e a c i d - b a s e i n d i c a t o r , e.g. thymol b l u e (pH range, 1.2-2.8), and 4 - ( 2 - p y r i d y l a z o ) r e s o r c i n o l , r e s p e c t i v e l y . As a consequence, t h e r e l a t i v e l y n o n - p o l a r t e t r a c h l o r o m e t h a n e i s p r e f e r r e d as eluent. When e x a m i n i n g t h e t i c d a t a i n o r d e r t o e v a l u a t e systems s u i t a b l e f o r column chromatography, one s h o u l d c o n s i d e r t h a t development w i t h a HNO^-saturated mobile phase causes t h e f o r m a t i o n o f an a c i d f r o n t a t Rp v a l u e s o n l y s l i g h t l y h i g h e r than those o f HEMA ( F i g . 2b and e ) . C o n s e q u e n t l y , i n chromatography on columns e q u i l i b r a t e d w i t h HNC^-saturated s o l v e n t systems, t h e s e p a r a t i o n o f HEMA from contaminants h a v i n g h i g h e r Rp v a l u e s , presumably w i l l be r a t h e r m a r g i n a l . T h e r e f o r e , p r e f e r e n c e s h o u l d be g i v e n t o chromatography on H 2 S O 4 impregnated s i l i c a g e l ; f o r t h e same reasons as mentioned above, n-hexane - d i e t h y l e t h e r (1:1) i s a g a i n recommended as mobile phase. Application. T h i n - l a y e r chromatograms o f a l l r e f e r e n c e substances and HEMA samples a r e p i c t u r e d i n F i g . 3. F u r t h e r i n f o r m a t i o n i s r e c o r d e d i n T a b l e I I . The chromatograms demonstrate t h a t - a p a r t from the i n h i b i t o r s - e t h y l e n e g l y c o l and/or o t h e r p o l a r m a t e r i a l , EDMA and m e t h a c r y l i c a c i d (BDH sample o n l y ) are p r e s e n t i n t h e HEMA samples from Merck and BDH. D e t e c t a b l e amounts o f m e t h a c r y l i c a c i d and EDMA a r e absent from HEMA o b t a i n e d from Hydro Med S c i e n c e s .



60 40

pM

MA

£-Methoxyphenol

Methacrylic

DGMA

EG/P

HEMA

Diethyleneglycol methacrylate

Ethylene g l y c o l / polar material

Cannot be d e t e c t e d : c f . t e x t .

Drop t e s t

Hy

HEMA

Hydroquinone

acid

70

EDMA

EDMA

00

10

25

25

Pure compounds

Abréviations (cf. F i g . 3)

Compound

hRp o f / i n :

gel/n-hexane - d i e t h y l e t h e r ( 1 : 1 )

00

+

+

+

10

00

10

25

-

25

60

—

HEMA (Hydro)

60

70

HEMA (Merck)

00

10

25

*

40

60

70

HEMA (BDH)

T a b l e I I . Rp d a t a f o r HEMA and i t s p r i n c i p a l c o n t a m i n a n t s i n t h e system


+

-

—

-

25

40

-

Methacrylic acid

silica


114


S t i l l , even t h i s h i g h l y pure p r o d u c t c o n t a i n s a t l e a s t two i m p u r i t i e s . The s p o t w i t h Rp 0.0 a g a i n may be a t t r i b u t e d t o e t h y l e n e g l y c o l and/or o t h e r p o l a r m a t e r i a l . As f o r t h e second s p o t , a compound d i s p l a y i n g Rp a p p r o x i m a t e l y 0.10 i n both chromatographic s o l v e n t systems, has a l s o been o b s e r v e d i n t h e HEMA samples o b t a i n e d from Merck and BDH. M i l l i g r a m amounts have been c o l l e c t e d by p r e p a r a t i v e - s c a l e t i c and c o lumn chromatography. On t h e b a s i s o f t h e mass spec trum (m/e v a l u e o f 144; p r e s e n c e o f 3 oxygen atoms) the unknown compound was i n i t i a l l y thought t o be h y d r o x y p r o p y l m e t h a c r y l a t e . However, comparison o f t h e t i c b e h a v i o r o f t h e unknown compound and a sample o f pure 2 - h y d r o x y p r o p y l m e t h a c r y l a t e demonstrated o u r h y p o t h e s i s t o be i n c o r r e c t . R e c e n t l y , from i n f o r m a t i o n o b t a i n e d from Hydro Med S c i e n c e s , i t has become e v i dent t h a t HEMA samples may c o n t a i n s e v e r a l t e n t h s o f a p e r c e n t o f d i e t h y l e n e g l y c o l m e t h a c r y l a t e . The l a t t e r p r o d u c t has been s y n t h e s i s e d i n o u r l a b o r a t o r y ( c f . above) and has been shown t o be i d e n t i c a l w i t h the unknown compound by both t i c and mass s p e c t r o metry . I n a d d i t i o n , we note t h a t a sharp s e p a r a t i o n o f HEMA and d i e t h y l e n e g l y c o l m e t h a c r y l a t e i s o b t a i n e d by c a r r y i n g o u t development w i t h pure d i e t h y l e t h e r i n s t e a d o f n-hexane - d i e t h y l e t h e r (1:1). The Rp v a l u e s now a r e 0.80 and 0.55 r e s p e c t i v e l y . As r e g a r d s t h e i n h i b i t o r s , w i t h n-hexane d i e t h y l e t h e r (1:1), hydroquinone d i s p l a y s an Rp v a l u e a p p r o x i m a t e l y e q u a l t o t h a t o f HEMA and cannot be detected with e i t h e r Echtblausalz Β o r d i a z o t i s e d s u l p h a n i l i c a c i d . When u s i n g n-hexane - MIBK - η o c t a n o l (9:2:1, s a t d . w i t h 25% H N O 3 ) as mobile phase, hydroquinone has a s l i g h t l y f a s t e r m i g r a t i o n r a t e than has HEMA, i t s s p o t o v e r l a p p i n g t h a t o f m e t h a c r y l i c a c i d . However, d e t e c t i o n o f s m a l l amounts o f h y d r o quinone even now i s n o t s u c c e s s f u l . I n s e p a r a t e expe r i m e n t s , we have demonstrated t h a t t h i s i s due t o s e v e r e s t r e a k i n g o f t h e hydroquinone s p o t , e f f e c t e d by t h e p r e s e n c e o f t h e l a r g e e x c e s s o f HEMA. As a consequence, hydroquinone i s d e t e c t e d i n i s o l a t e d i n s t a n c e s ( m e t h a c r y l i c a c i d ! ) o n l y . No such d i f f i c u l t i e s a r e e n c o u n t e r e d w i t h p-methoxyphenol; w i t h t h i s i n h i b i t o r , i d e n t i f i c a t i o n by t i c i s s u c c e s s ful. *As r e g a r d s t h e d i s c r e p a n c y between t h e mass o f d i e t h y l e n e g l y c o l m e t h a c r y l a t e (174) and t h e v a l u e quo t e d above (144), i t i s a well-known f a c t t h a t i n mass spectrometry p o l y g l y c o l s e a s i l y lose a p a r t o f t h e i r m o l e c u l e h a v i n g a mass o f 30; i . e . , t h e m/e v a l u e o f 144 s h o u l d be a t t r i b u t e d t o Im - C H o ] . 2

+



8.

BRiNKMAN E T A L .


115

S t i l l , w i t h both compounds, i n view o f t h e r a t h e r u n f a v o r a b l e d e t e c t i o n l i m i t s quoted i n T a b l e I , i t i s recommended t o a p p l y s e v e r a l m i c r o l i t e r s o f sample s o l u t i o n t o t h e c h r o m a t o p i a t e , u s i n g development o v e r a d i s t a n c e o f 10-15 cm. A l t e r n a t i v e l y , p r e c o n c e n t r a t i o n o f t h e i n h i b i t o r s by means o f t r e a t m e n t o f t h e sample s o l u t i o n w i t h Amberlyst A-27 r e s i n ( c f . below) may be used. I n c o n c l u s i o n , as i s e v i d e n t from t h e d a t a i n T a b l e I I , t h e drop t e s t p r o c e d u r e w i t h s u l p h a n i l i c acid i s e x c e l l e n t l y s u i t e d t o detect the presenc e , though n o t t h e t y p e , o f an i n h i b i t o r . Comparison o f t h e o v e r a l l p i c t u r e o f t h e s e r i e s o f chromatograms o b t a i n e d w i t h n-hexane - d i e t h y l e t h e r (1:1), and t h e s e r i e s o b t a i n e d w i t h n-hexane MIBK - n - o c t a n o l (9:2:1, s a t d . w i t h 25% H N O 3 ) demons t r a t e s t h a t a s m a l l e r number o f s p o t s i s o b s e r v e d when d e v e l o p i n g w i t h t h e l a t t e r s o l v e n t system. T h i s i s m a i n l y due t o t h e f a c t t h a t n - o c t a n o l has low v o l a t i l i t y and cannot e a s i l y be removed from t h e c h r o matogram p r i o r t o i d e n t i f i c a t i o n . As a consequence, upon s p r a y i n g w i t h a KMn04 s o l u t i o n , t h e c o l o r o f t h e background t u r n s brown r a t h e r r a p i d l y , thus o b s c u r i n g s e v e r a l o f t h e s m a l l e r s p o t s . I n summary, n-hexane d i e t h y l e t h e r (1:1) s h o u l d be p r e f e r r e d f o r q u a l i t a t i v e a n a l y s i s , both on account o f i t s f a s t e r m i g r a t i o n r a t e , and t h e b e t t e r d e t e c t i o n a c h i e v e d f o r a c t u a l l y a l l components p r e s e n t i n t h e HEMA and EDMA samples. P r e p a r a t i v e - s c a l e t i c on ^ S C ^ - i m p r e g n a t e d s i l i c a g e l has s u c c e s s f u l l y been c a r r i e d o u t w i t h HEMA samp l e s from Hydro Med S c i e n c e s and BDH. ( F o r a l l compounds s t u d i e d , t h e Rp v a l u e s i n t h i s system a r e e q u a l t o , o r s l i g h t l y h i g h e r than t h o s e i n t h e system s i l i c a gel/n-hexane - d i e t h y l e t h e r , s a t d . w i t h 25% HNCK; c f . F i g . 2 ) . A f t e r development w i t h n-hexane - d i e t h y l e t h e r (1:1) and e l u t i o n w i t h t e t r a c h l o r o m e t h a n e , t h e samples t u r n o u t t o be c h r o m a t o g r a p h i c a l l y pure. I f hydroquinone has been added t o t h e HEMA sample (Merck\ t h i s i n h i b i t o r w i l l s t i l l be p r e s e n t a f t e r chromatog r a p h i c p u r i f i c a t i o n (£f. above). G e n e r a l l y , t h e p r e s e n c e o f an i n h i b i t o r i s n o t h a r m f u l s i n c e i t can be removed q u a n t i t a t i v e l y from t h e PHEMA g e l by p r o l o n g e d washing w i t h water. However, s h o u l d q u a n t i t a t i v e removal o f hydroquinone a t an e a r l y stage be i m p e r a t i v e , then two c o n s e c u t i v e t r e a t m e n t s o f t h e sample t o be p u r i f i e d w i t h Amberlyst A-27 r e s i n (Rohm and Haas, P h i l a d e l p h i a , Pa. 19105, U.S.A.) s u f f i c e t o remove up t o 0.2% o f hydroquinone, and a l s o o t h e r p o l a r compounds; t h e s e can s u b s e q u e n t l y be e l u t e d w i t h methanol. The c o n c e n t r a t e d e x t r a c t so o b t a i n e d i s e x c e l l e n t l y s u i t e d t o d e t e c t t h e presence o f i n h i b i -


116


Table I I I . η

v a l u e s f o r HEMA and some o f i t s


contaminants Compound

η

HEMA

1.4525

I'

Diethyleneglycol methacrylate

1.4568

8

n

ri.4549 U.4549-1.4553 1.4313

EDMA Ethylene

glycol

Methacrylic Methyl

Ref.

acid

methacrylate

I,

2, 4 1

1.4314

1> 1

1.4142

5

t o r ( s ) , as was r e f e r r e d t o above. A f i n a l word about two f u r t h e r c r i t e r i a which a r e used t o a s s e s s t h e p u r i t y o f HEMA and EDMA, v i z . t h e r e f r a c t i v e i n d e x and t h e i n f r a r e d spectrum. A c c o r d i n g t o o u r e x p e r i e n c e s , n $ v a l u e s , which a r e r a t h e r gene r a l l y quoted i n t h e l i t e r a t u r e , a r e n o t v e r y r e l i a b l e c r i t e r i o n , since the i m p u r i t i e s normally present i n HEMA e x h i b i t n2Q v a l u e s which a r e both h i g h e r (EDMA and d i e t h y l e n e g l y c o l m e t h a c r y l a t e ) and lower (metha c r y l i c a c i d and e t h y l e n e g l y c o l ) than a r e those o f HEMA i t s e l f . As an i l l u s t r a t i o n , a s e r i e s o f d a t a i s recorded i n Table I I I . Besides, the r e f r a c t i v e index o f HEMA s t r o n g l y d e c r e a s e s w i t h i n c r e a s i n g water con t e n t , as i s demonstrated i n F i g . 4. I n f r a r e d s p e c t r o scopy has been recommended t o check t h e p u r i t y o f EDMA. Our r e s u l t s i n d i c a t e t h a t compounds such as HEMA and e t h y l e n e g l y c o l can be d e t e c t e d down t o app r o x i m a t e l y 1 wt.% due t o t h e s t r o n g a b s o r p t i o n o f ^ t h e i r h y d r o x y l groups (broad band a t 2800-3600 cm ) . However, one s h o u l d keep i n mind t h a t t h e i n f r a r e d spectrum does n o t e a s i l y a l l o w a c o n c l u s i o n r e g a r d i n g the n a t u r e o f t h e c o n t a m i n a n t ( s ) - e.g. HEMA, e t h y l e n e g l y c o l o r water. B e s i d e s , a compound such as metha c r y l i c a c i d goes u n d e t e c t e d a t t h e 1% l e v e l . 2


8.

BRiNKMAN E T AL.

117



HEMA

DGMA EG/P

Figure 3. Tic of three HEMA samples on silica gel, using n-hexane-diethyl ether (1:1) (a-c), and n-hexane-M IBK-n-octanol (9:2:1, satd. with 25% HNO ) (d-f) as mobile phases. HEMA samples obtained from BDH (a,d), Merck (b,e), and Hydro Med Sciences (c,f). For abbreviations, see Table II. , acid (1), n-octanol (2), and MIBK (3) front. s

1.45

1.40

1.35

100

80

60

40

20

0 % HEMA

0

20

40

60

80

1 0 0 % water

Figure 4. Dependence of n of HEMA

20

D

on water content


118


Abstract Poly(hydroxyethyl methacrylate) is generally accepted as a biocompatible, safe and stable hydrogel for medical use. The present paper describes the use of tlc for the analysis and small-scale preparation of the initial monomer, 2-hydroxyethyl methacrylate. Tlc on silica gel, with n-hexane - diethyl ether (1:1, v/v) and/or n-hexane - MIBK - n-octanol (9:2:1, v/v, satd. with 25% HNO) as mobile phase is recommen ded for qualitative analysis. Preparative-scale work is preferably carried out on HSO-impregnated silica gel, development being done with n-hexane - diethyl ether (1:1). Inhibitors are detected by means of tlc and of a drop test procedure with diazotised sulpha nilic acid. The nature of the contaminants present in several commercially available HEMA samples is dis cussed, n D values are reported for the system HEMA- water. 3


2

4

20

Literature Cited 1. Wichterle, O., and Lim, D., Nature (1960) 185, 117. 2. Wichterle, O., and Lim, D., U.S. Patent, (1961) 2, 976, 576. 3. Kopecek, J., Jokl, J., and Lim, D., J. Polymer Sci., [C] (1968) 16, 3877. 4. Refojo, M.F., J. Appl. Polymer Sci., (1965) 9, 3161. 5. Goedberg, A.I., Fertig, J., and Stanley, Η., U.S. Patent, (1962) 3, 059, 024. 6. Refojo, M.F., and Yasuda, H., J. Appl. Polymer Sci., (1965) 9, 2425. 7. Hasa, J., and Janácek, J., J. Polymer Sci., [C] (1967) 16, 317. 8. Kopecek, J., Thesis, Inst. Macromol. Chem., Prague, (1965) p. 35. 9. Mark, M.F., Gaylord, N.G., Bikales, N.M., (eds), "Encyclopedia of Polymer Science and Technology", Vol. 15, 1971, 273. 10. Zweig, G., and Sherma, J. (eds), "Handbook of Chromatography", CRC Press, Interscience, New York, 1971. 11. Crawford, J.W.C., U.S. Patent, (1939) 2, 143, 941. 12. Jatzkewitz, Η., and Lenz, U., Hoppe-Seylers Z. Physiol. Chem., (1956) 305, 53. 13. Jatzkewitz, Η., Hoppe-Seylers Z. Physiol. Chem., (1953) 292, 99. 14. Merck, Ε., Chemicalien, Reagenzien (1974), Darmstadt.


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