Polymeric Ligands for Ionic and Molecular Separations - ACS

Chapter 14

Polymeric Ligands for Ionic and Molecular Separations S. D . Alexandratos, D . W. Crick, D . R. Quillen, and C . E. Grady

Environmental Remediation Downloaded from pubs.acs.org by COLUMBIA UNIV on 12/10/18. For personal use only.

Department of Chemistry, University of Tennessee, Knoxville, T N 37996

An understanding of ligand-substrate interactions is important to the design of polymer -supported reagents for the selective complexation of a single species in a multi-component solution. Such polymers may be applied to the environmental separation of ions and molecules as well as in chemical sensors and as novel catalysts. A series of polymers has been synthesized where substrate selectivity arises through the polymer's bifunctionality. The dual mechanism bifunctional polymers are described and applied as both polymer beads and membranes for the selective complexation of metal ions and molecules.

The development of methodologies f o r the c o v a l e n t bonding of l i g a n d s t o p o l y m e r i c supports i s important f o r the p r e p a r a t i o n of n o v e l reagents t o be used i n i o n i c and molecular s e p a r a t i o n s which are important t o the environment and f o r the p r e p a r a t i o n o f chemical sensors which can be used i n the m o n i t o r i n g of e n v i r o n mentally s e n s i t i v e areas. Our r e s e a r c h focuses on i d e n t i f y i n g the mechanisms by which i m m o b i l i z e d l i g a n d s 0097-6156/92/0509-0194S06.00/0 © 1992 American Chemical Society

14. ALEXANDRATOS ET AL.

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can i n t e r a c t s e l e c t i v e l y w i t h t a r g e t e d s u b s t r a t e s and then s y n t h e s i z i n g the corresponding polymer-supported reagents. The s e l e c t i v e complexation of i o n s and molecules by d i f f e r e n t reagents i s best understood w i t h i n the c o n t e x t of host-guest chemistry (1). Crown e t h e r s d i s c r i m i n a t e among metal i o n s based on t h e i r i o n i c diameter ( 2 ) , cryptands are s t i l l more s e l e c t i v e (3) due t o a h i g h e r degree of pre-organization (4), and cyclodextrins are shape selective for molecular s u b s t r a t e s ( 5 ) . Template p o l y m e r i z a t i o n i s one method of p r e - o r g a n i z i n g the l i g a n d s on a polymer f o r g r e a t e r substrate s e l e c t i v i t y (6). Polymer-supported crown e t h e r s (7) and cryptands (8) attempt t o combine the s e l e c t i v i t i e s i n h e r e n t t o the l i g a n d w i t h the advantages of u s i n g polymeric reagents, i n c l u d i n g ease of recovery and a d a p t a b i l i t y t o continuous processes (9). R e a c t i v e polymers (10,11) form a unique category of reagents which i n f l u e n c e the s e l e c t i v i t i e s of i o n exchange r e s i n s by superimposing an a d d i t i o n a l r e a c t i o n (such as c h e l a t i o n ) on the exchange mechanism. The dominant f r e e energy of the superimposed r e a c t i o n then d i r e c t s the s e l e c t i v i t y towards a t a r g e t e d i o n . Metal Ion Complexation Reactions by Polymer-Supported Reagents Our approach towards the p r e p a r a t i o n of h i g h l y s e l e c t i v e reagents i s t o s y n t h e s i z e b i f u n c t i o n a l c r o s s l i n k e d polymers and study c o o p e r a t i n g l i g a n d p a i r s w i t h g r e a t e r s e l e c t i v i t i e s than the corresponding monofunctional reagents. These dual mechanism bifunctional polymers (DMBPs) have been d i v i d e d i n t o t h r e e c l a s s e s , each of which d i s p l a y s a d i f f e r e n t r e c o g n i t i o n mechanism f o r metal i o n s (12). The C l a s s I DMBPs superimpose a r e d u c t i o n r e a c t i o n on the i o n exchange mechanism (13). The polymer w i t h

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primary and secondary phosphinic a c i d l i g a n d s ( F i g u r e 1) has been s t u d i e d e x t e n s i v e l y . The primary phosphinic a c i d groups w i l l reduce metal i o n s , w i t h h i g h e s t s e l e c t i v i t y f o r the mercuric i o n (equation 1 ) . The r e d u c t i o n p o t e n t i a l of Cu(II) i s a t t h e lower l i m i t o f t h e polymer*s a b i l i t y t o reduce i o n s t o t h e f r e e metal. The Cu(II) r e d u c t i o n approaches completion over a p e r i o d o f 240 hours w h i l e t h e Hg(II) r e d u c t i o n occurs over a few hours a t room temperature. I n c r e a s i n g t h e temperature leads t o a l a r g e i n c r e a s e i n t h e r a t e o f reduction. Optimization of the k i n e t i c s w i l l also r e q u i r e a study o f t h e e f f e c t o f t h e polymer's macroporosity. The C l a s s I I DMBPs a r e able t o i o n exchange w i t h one l i g a n d and c o o r d i n a t e w i t h a d i f f e r e n t l i g a n d . The phosphonate monoester/diester polymer ( F i g u r e 1) i s a r e p r e s e n t a t i v e example and w i l l be d i s c u s s e d i n more d e t a i l below (14). The phosphonic acid/dimethylamine polymer i s another example o f t h i s c l a s s . The C l a s s I I I DMBPs ( F i g u r e 1) combine i o n exchange w i t h a p r e c i p i t a t i o n r e a c t i o n , t h e l a t t e r a r i s i n g from a quaternary amine l i g a n d whose a s s o c i a t e d anion can be v a r i e d a c c o r d i n g t o which c a t i o n i s t o be p r e c i p i t a t e d (15). Proper c h o i c e o f t h e anion and a l k y l groups on t h e amine leads e x c l u s i v e l y t o i n t r a p a r t i c l e p r e c i p i t a t i o n (equation 2 ) . Thus, f o r example, a b i f u n c t i o n a l polymer w i t h tributylammonium i o n l i g a n d s coupled with the thiocyanate a n i o n , when contacted w i t h a 0.01N AgNO^ s o l u t i o n , leads t o p r e c i p i t a t i o n o f t h e AgSCN w i t h i n t h e polymer m a t r i x (70% s o r p t i o n a f t e r a 17 h s t i r ) . Exchanging t h e polymer w i t h K I 0 y i e l d s a reagent s p e c i f i c f o r P b ( I I ) as i o d a t e becomes t h e a s s o c i a t e d anion and P b ( 1 0 ^ ) precipitates. 3

2

Supported Ligand Synergistic Interaction. The complexa t i o n o f metal i o n s by a p a i r o f s o l u b l e complexing agents w i t h one o p e r a t i n g by i o n exchange and t h e other

ALEXANDRATOS ET AL.


F i g u r e 1. Chemical s t r u c t u r e s of the C l a s s I , I I , and I I I d u a l mechanism b i f u n c t i o n a l polymers.

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by c o o r d i n a t i o n and i n such a way t h a t both groups complex more metal i o n than e i t h e r c o u l d alone, i s well-known i n s o l v e n t e x t r a c t i o n chemistry (16). We f i r s t s y n t h e s i z e d the phosphonate monoester/ d i e s t e r polymer as w e l l as the corresponding monofunctional polymers i n order t o d e f i n e a polymeric analogue t o this synergistic enhancement. In studying the complexation of F e ( I I I ) , H g ( I I ) , A g ( I ) , M n ( I I ) , and Z n ( I I ) from a c i d i c s o l u t i o n s (4.0 t o 0.2N HNO^) at a constant 4N n i t r a t e background by the a d d i t i o n of v a r y i n g l e v e l s of sodium n i t r a t e , i t i s found t h a t the behavior of the b i f u n c t i o n a l polymer i s s i m i l a r t o t h a t found f o r the monofunctional r e s i n s f o r a l l i o n s except silver. Thus, from a background s o l u t i o n of 4N HNO^, the d i s t r i b u t i o n c o e f f i c i e n t s f o r Ag(I) are 439, 463, and 2924 f o r the monoester, d i e s t e r , and b i f u n c t i o n a l r e s i n s , r e s p e c t i v e l y . On the other hand, the values f o r Hg(II) are 23, 3, and 15, i n the order g i v e n above. The reason for a supported ligand synergistic i n t e r a c t i o n w i t h Ag(I) has y e t t o be determined but i s probably r a t i o n a l i z e d by a similar polarizability between the metal i o n and the phosphoryl oxygen ligands. A s t r o n g c o o r d i n a t i v e i n t e r a c t i o n i s one mechanism by which r e a c t i v i t y c o n t r o l s r e c o g n i t i o n . A d d i t i o n a l s t r o n g i o n / l i g a n d i n t e r a c t i o n mechanisms which can be used t o c o n t r o l r e c o g n i t i o n are r e d u c t i o n and p r e c i p i tation. S t e r i c hindrance can a l s o be used as another method of c o n t r o l l i n g r e c o g n i t i o n by l i m i t i n g access t o the c o o r d i n a t i n g s i t e s . T h i s seems t o be t r u e w i t h the p u r e l y c o o r d i n a t i n g d i e s t e r r e s i n s which d i s p l a y a s e l e c t i v i t y depending upon the b u l k i n e s s of the e s t e r moieties.

14. ALEXANDRATOS ET AL. Polyfunetional


Membranes v i a Interpenetrating Polymer

Networks: Application to Metal Ion Separations P o l y p r o p y l e n e membranes which have been m o d i f i e d by s o r b i n g o r g a n i c s o l u t i o n s o f complexing agents i n t o the porous s t r u c t u r e , have been u t i l i z e d f o r h i g h l y s e l e c t i v e metal i o n t r a n s p o r t from aqueous s o l u t i o n s (17) . These supported l i q u i d membranes (SLMs) a r e important t o t h e s e p a r a t i o n of a wide v a r i e t y o f metal i o n s due t o t h e i r v e r s a t i l i t y : t h e s e l e c t i v i t y o f t h e membrane i s dependent on t h e complexing agent sorbed w i t h i n t h e p o l y p r o p y l e n e and t h i s can o b v i o u s l y be changed depending upon t h e t a r g e t e d i o n . A c r i t i c a l problem w i t h the SLMs i s t h a t o f s t a b i l i t y : metal i o n t r a n s p o r t w i l l e v e n t u a l l y cease as t h e complexing agent and/or i t s a t t e n d a n t s o l v e n t d i s s o l v e out i n t o the aqueous phase. Our r e s e a r c h w i t h the SLMs has c e n t e r e d on t h e s y n t h e s i s o f h i g h l y s t a b l e membranes f o r l o n g term metal i o n t r a n s p o r t . S i n c e i t i s advantageous t o m a i n t a i n p o l y p r o p y l e n e as t h e b a s i c support s t r u c t u r e due t o i t s commercial a v a i l a b i l i t y , a v e r s a t i l e memb rane m o d i f i c a t i o n technique has been found t o be t h a t involving i n t e r p e n e t r a t i n g polymer networks [IPNs] (18) . I n t h i s t e c h n i q u e , a p o l y m e r i z a b l e metal i o n complexing agent i n a hydrophobic s o l v e n t i s sorbed by the p o l y p r o p y l e n e and p o l y m e r i z e d w i t h a f r e e r a d i c a l i n i t i a t o r i n t h e membrane e i t h e r w i t h o r w i t h o u t a d d i t i o n a l comonomers ( 1 9 ) . S t a b i l i t y s t u d i e s a r e c a r r i e d out by d e t e r m i n i n g t h e membrane's a b i l i t y t o t r a n s p o r t C u ( I I ) from pH 4.2 s u l f a t e / b i s u l f a t e b u f f e r s o l u t i o n s over a number o f weeks (20). The concept i s i l l u s t r a t e d i n F i g u r e 2. D i ( u n d e c e n y l ) p h o s p h o r i c a c i d , [CH =CH(CH )^0] P(Ο)OH, (DUP) was found t o have the necessary p r o p e r t i e s needed f o r a complexing agent t o be p o l y m e r i z e d w i t h i n a memb rane: t h e i o n exchange group i s s e p a r a t e d by a spacer 2

2

2

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IPN F i g u r e 2. H i g h l y s t a b l e polypropylene membranes through i n t e r p e n e t r a t i n g polymer network s y n t h e s i s .

14. ALEXANDRATOS ET A L


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group from t h e v i n y l moiety a l l o w i n g i t t o remain mobile a f t e r p o l y m e r i z a t i o n ; and t h e - C H - adjacent t o the v i n y l group leads t o c h a i n t r a n s f e r d u r i n g t h e f r e e r a d i c a l p o l y m e r i z a t i o n ( i n t h e manner expected o f a l l y l groups) which l i m i t s t h e degree o f p o l y m e r i z a t i o n thus maintaining ligand m o b i l i t y . One-Component I P N s . A SLM o f 0.4M d i ( 2 - e t h y l h e x y l ) phosphoric a c i d (DEHP) i n dodecane was s t u d i e d as t h e c o n t r o l a g a i n s t which t h e performance o f DUP membranes c o u l d be measured. I t was found t o have a p e r m e a b i l i t y c o e f f i c i e n t o f 4x10"^ cm/sec and a l i f e t i m e o f 45 days ( e x t r a p o l a t e d from i t s performance over 14 days). Since e a r l i e r s t u d i e s had e s t a b l i s h e d t h a t a minimum c o n c e n t r a t i o n of 1M DUP i s r e q u i r e d i n order t o prevent disentanglement of t h e polymerized product from t h e i n i t i a l polymer network (21), SLMs a t t h a t DUP concentr a t i o n were prepared. I n t e r e s t i n g l y , t h e 1M DUP IPN membrane had a s h o r t e r l i f e t i m e than the DEHP membrane, -4 though i t s i n i t i a l p e r m e a b i l i t y , 2.5x10 cm/sec, i n d i c a t e d a t r a n s p o r t r a t e comparable t o t h e more mobile DEHP. E i t h e r IPN formation w i t h 1M DUP d i d n o t produce s u f f i c i e n t entanglement w i t h t h e polypropylene network, o r t h e c o n c e n t r a t i o n o f phosphorus a c i d m o i e t i e s was h i g h enough t o permit h y d r a t i o n o f t h e membrane pores. Two-Component I P N s . I n order t o i n c r e a s e t h e degree o f p o l y m e r i z a t i o n w h i l e d e c r e a s i n g t h e DUP c o n c e n t r a t i o n , IPN membranes were prepared w i t h 0.25M DUP and 1.5M a c r y l o n i t r i l e (AN). DUP/AN IPNs were f i r s t formed i n p o l y s t y r e n e beads and found t o be s t a b l e as long as t h e double bond c o n c e n t r a t i o n o f t h e o v e r a l l mixture was 2M. Membranes, however, made w i t h t h i s DUP/AN r a t i o , as w e l l as w i t h o t h e r s having somewhat lower AN l e v e l s and c o r r e s p o n d i n g l y higher DUP l e v e l s , d i d n o t permit any Cu(II) t r a n s p o r t , probably due t o t h e DUP*s l a c k o f m o b i l i t y i n t h e predominately AN copolymer. 2

202


Three-Component IPNs. H i g h l y s t a b l e IPN membranes were prepared from p o l y p r o p y l e n e when c o n c l u s i o n s r e p o r t e d e a r l i e r w i t h the i o n exchange/coordination b i f u n c t i o n a l polymers were extended t o the membranes. S i n c e n e u t r a l compounds w i t h a phosphoryl oxygen moiety are important metal i o n c o o r d i n a t o r s , t r i ( u n d e c e n y l ) p h o s p h i n e o x i d e (TUP) was s y n t h e s i z e d and copolymerized w i t h DUP. The l e s s h y d r o p h i l i c TUP c o u l d thus be p o l y m e r i z e d w i t h DUP i n order t o a s s i s t i n the metal i o n t r a n s p o r t . I t c o u l d a l s o be expected t o lower the degree o f p o l y m e r i z a t i o n when AN i s a comonomer thus a l l o w i n g f o r h i g h l i g a n d m o b i l i t y . A DUP/TUP/AN c o n c e n t r a t i o n o f 0.40M, 0.28M, 0.85M, r e s p e c t i v e l y , was found t o o f f e r t h e b e s t compromise between s t a b i l i t y and metal i o n t r a n s p o r t a f t e r p o l y m e r i z a t i o n w i t h i n the p o l y p r o p y l e n e membrane: -4 a p e r m e a b i l i t y c o e f f i c i e n t o f 1.25x10 cm/sec remained c o n s t a n t over 36 days so t h a t , under the c o n d i t i o n s o f the p r e s e n t experiment, t h e membrane was found t o be s t a b l e over an i n d e f i n i t e l y l o n g p e r i o d o f time. Molecular Complexation Reactions by Polymer-Supported Reagents The complexation o f n e u t r a l molecules by l i g a n d s c o v a l e n t l y bound t o polymer supports r e q u i r e s the study of a s e t o f parameters d i f f e r e n t from those i n v o l v i n g the complexation r e a c t i o n s o f metal i o n s . While t h e l a t t e r emphasizes i o n exchange mechanisms, t h e former r e l i e s on r e a c t i o n s such as acid/base i n t e r a c t i o n s f o r selectivity. The r o l e o f t h e polymer support i n m o l e c u l a r complexation r e a c t i o n s has been the focus o f t h i s p a r t o f our r e s e a r c h : s t u d i e s are b e i n g undertaken t o determine whether the polymer a c t s o n l y as an i n e r t m a t r i x on which t o bond a p p r o p r i a t e l i g a n d s , o r whether i t can a l s o i n f l u e n c e the l i g a n d - m o l e c u l e i n t e r a c t i o n . P o l y s tyry1-bound dimethylamine l i g a n d s can be expected t o complex both i n o r g a n i c and o r g a n i c a c i d s .



203

Benzoic a c i d s , i n p a r t i c u l a r , can be used t o e v a l u a t e the s e l e c t i v i t y of the l i g a n d t o v a r i a t i o n s i n the e l e c t r o n d e n s i t y a t the a c t i v e s i t e by d e t e r m i n i n g the amount complexed from s o l u t i o n as e l e c t r o n withdrawing/ donating s u b s t i t u e n t s on the aromatic r i n g i n c r e a s e or decrease the a c i d i t y of the f u n c t i o n a l group. The i n t e r a c t i o n may be summarized by e q u a t i o n 3.

The s e l e c t i v i t y of the l i g a n d can be q u a n t i f i e d a c r o s s a s e r i e s of s u b s t r a t e s by d e t e r m i n i n g the b i n d i n g constant of each s u b s t r a t e v i a i t s a d s o r p t i o n isotherm and corresponding y - r e c i p r o c a l p l o t (equation 4). c/r = (1/S )c + ( l / K S ) t

t

(4)

I n t h i s e q u a t i o n , c i s the m i l l i e q u i v a l e n t s (mequiv) of s u b s t r a t e i n s o l u t i o n a t e q u i l i b r i u m per mL s o l u t i o n , r i s the mequiv of s u b s t r a t e bound per gram of polymer, S i s the s a t u r a t i o n c a p a c i t y of the polymer i n mequiv per gram, and Κ i s the b i n d i n g constant (22). I n the i n i t i a l s t u d i e s c a r r i e d out t o date, p o l y styrene-supported dimethylamine polymers c r o s s l i n k e d w i t h 2 and 15% d i v i n y l b e n z e n e (DVB) were complexed w i t h m-chlorobenzoic a c i d and the a d s o r p t i o n isotherms defined. The corresponding b i n d i n g c o n s t a n t s were c a l c u l a t e d t o be 126.19 and 67.99 M , respectively. I t i s thus e v i d e n t t h a t the polymer support a f f e c t s the s t r e n g t h of the b i n d i n g i n t e r a c t i o n between the l i g a n d and s u b s t r a t e . M e c h a n i s t i c s t u d i e s t o understand the b a s i s f o r t h i s i n f l u e n c e and i t s use i n the d e s i g n of polymers f o r s e l e c t i v e molecular complexation r e a c t i o n s are i n p r o g r e s s . t

-1

204


Acknowledgment. We g r a t e f u l l y acknowledge t h e support of t h e Department o f Energy, O f f i c e o f B a s i c Energy S c i e n c e s , through grant DE-FG05-86ER13591.

Literature Cited 1. Cram, D . J . ; Cram, J.M. Acc. Chem. Res. 1978, 11, 8. 2. Pedersen, C.J. J. Am. Chem. Soc. 1967, 89, 7017. 3. Lehn, J.M. Pure Appl. Chem. 1980, 52, 2441. 4. Timko, J.M.; Helgeson, R.C.; Newcomb, M.; Gokel, G.W.; Cram, D.J. J . Am. Chem. Soc. 1974, 96, 7097. 5. Trainor, G.L.; Breslow, R. J . Am. Chem. Soc. 1981, 103, 154. 6. Wulff, G.; Grobe-Einsler, R.; Vesper, W.; Sarhan,A. Makromol. Chem. 1977, 178, 2817. 7. Smid, J . Pure Appl. Chem. 1982, 54, 2129. 8. Dietrich, B.; Lehn, J . M . ; Sauvage, J.P. Tetrahedron Lett. 1969, 34, 2885. 9. Polymer-Supported Reactions in Organic Synthesis; Hodge,Ρ.; Sherrington,D.C., Eds., Wiley: Chichester, 1980. 10. Helfferich, F. J . Phys. Chem. 1965, 69, 1178. 11. Janauer, G.Ε.; Ramseyer, G.Ο.; Lin, J.W. Anal.Chim. Acta 1974, 73, 311. 12. Alexandratos,S.D. Sep. Purif. Methods 1988, 17, 67. 13. Alexandratos,S.D.; Wilson,D.L. Macromolecules 1986, 19, 280. 14. Alexandratos,S.D.; Crick, D.W.; Quillen, D.R. Ind. Eng. Chem. Res. 1991, 30, 772. 15. Alexandratos,S.D.; Bates, M.E. Macromolecules 1988, 21, 2905. 16. Choppin, G.R. Sep. Sci. Technol. 1981, 16, 1113. 17. Baker, R.W.; Tuttley, M.E.; Kelly, D . J . ; Lonsdale, H.K. J . Membr. Sci. 1977, 2, 213. 18. Sperling, L.H. Interpenetrating Polymer Networks and Related Materials; Plenum: New York, NY, 1981.



19. Alexandratos, S.D.; Strand, M . A . ; Callahan, C.M. In Advances i n Interpenetrating Polymer Networks; Klempner, D . ; F r i s c h , K . C . , Eds.; Technomic Publishing: Lancaster, PA, 1990, V o l . 2; pp. 73-99. 20. Danesi, P . R . ; C h i a r i z i a , R . ; Castagnola, A.J. J. Membr. S c i . 1983, 14, 161. 21. Alexandratos, S.D.; Strand, M.A. Macromolecules 1986, 19, 273. 22. Connors,K.A. Binding Constants; Wiley: New York,NY, 1987. RECEIVED January 30, 1992

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