Interpenetrating Polymer Networks - American Chemical Society

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8 Bifunctional Interpenetrating Polymer Networks Spiro D. Alexandratos, Corinne Ε. Grady, Darrell W. Crick, and Robert Beauvais Department of Chemistry, University of Tennessee, Knoxville, TN 37996

Interpenetrating polymer networks (IPNs) with two different func tional groups on one of the two cross-linked networks have been synthesized. Bifunctional IPNs with imidazole and carboxylic acid groups are described and their binding affinity for copper and cobalt ions is quantified via Langmuir adsorption isotherm plots. The microenvironment within the imidazole-acid IPNs varies as the ratio of ligands changes along the series 1:0, 0.5:0.5, and 0:1. Binding con stants for the foregoing IPNs are 3130, 1556, and 77 Ν for Cu(II) and 294, 189, and 26 Ν for Co(II), respectively. Conditions under which different ligands can cooperate synergistically in the complexation of substrates are being defined to enable design of highly selective chemical sensors and complexing agents for environmental separa tions. -1

-1

THE MODIFICATION OF POLYMERS BY COVALE NT BONDING OF LIGANDS on a

support matrix has long been recognized as an important technique for the preparation of polymer-supported reagents (I). Such reagents are used for the complexation of ions and molecules from solutions (2) as well as in the synthesis of organic molecules such as proteins (3) and polysaccharides (4). A number of procedures that are specific for targeted substrates have evolved for the preparation of polymeric reagents. Template polymerization creates cavities within a polymer matrix to produce selectivity based on size of the substrate (5). This shape-selectivity arises from polymerizing monomers around a template; the resulting cavities retain the dimensions of the tem plate after the template is removed from the matrix. This approach has been 0065-2393/94/0239-0197$06.00/0 © 1994 American Chemical Society

Klempner et al.; Interpenetrating Polymer Networks Advances in Chemistry; American Chemical Society: Washington, DC, 1994.


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INTERPENETRATING POLYMER NETWORKS

successful for molecular recognition (6) and shows promise in metal ion selectivity studies as well (7). Ligands with high levels of preorganization, such as crown ethers, have also been bonded to polymer matrices and retain much of their inherent selectivity (8). Hard-soft acid-base theory (9) can provide the framework for choosing ligands that can be expected to display a certain ionic selectivity (JO). In most cases, a single type of ligand is bonded to the polymer and its selectivity series then is determined. A n alternate approach is to bond two types of ligands on a polymer and then define the conditions under which the ligands cooperate in complexing greater levels of metal ions than either individual could complex (11). In one example, ion exchange-coordination resins were syn thesized with phosphonate monoester and diester ligands covalently bound on cross-linked polystyrene beads (12). This resin complexed far more Ag(I) than expected from results with the corresponding monofunctional resin (13). The synthesis of bifunctional polymers to study the synergistic interaction of supported ligands required a new preparative technique due to the limited number of reactions that could immobilize well-defined pairs of ligands on polymers. The development of bifunctional interpenetrating polymer net works (IPNs) proved ideal for the synthesis of reagents with two types of ligands bound within a single matrix in bead, rather than granular, form. Polymers in bead form are important in rate studies due to their controled particle size and shape; they are preferred in separations applications due to their adaptability to continuous processes and long-term stability. IPNs have been defined as "a combination of two polymers both in network form, at least one of which is synthesized in the presence of the other" (14). The modification of the engineering properties displayed by a given polymer (tensile strength, flexibility, etc.) has been the primary objec tive of much research ( 15). The sequential method of I P N synthesis involves starting with a preformed cross-linked polymer, contacting it with a monomer solution, and forming the second network within the first. The final proper ties displayed by the sequential IPNs are dominated by the properties of the first network (16). The I P N morphology seen by electron microscopy is set by the cross-link density in the first network; changes in the degree of cross-linking in the second network have little effect (17). In a comprehen sive study on the morphology of sequential IPNs, a number of such IPNs were synthesized, including two IPNs with equivalent levels of polystyrene and poly(ethyl acrylate): in one, the polystyrene was the initial matrix and the ethyl acrylate polymerized within it; in the other, the polyacrylate formed the initial matrix and styrene polymerized within it. In both cases, the initial matrix formed a continuous cellular structure with the second network found mainly within the cells (18). It is important to note that both networks were lightly cross-finked with 0.5 v o l % tetraethylene glycol dimethacrylate; higher cross-fink levels are expected to diminish the size of the cells. Dynamic mechanical spectroscopy on the same IPNs shows that the polystyrene-poly-



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acrylate I P N is stiffer than the polyacrylate-polystyrene I P N , which confirms that the first network dominates the final I P N properties (19). The final properties also can depend on small changes in the synthesis: sequential IPNs of polyurethane~poly(methyl methacrylate), where the methacrylate network is initiated with azobis(4'-cyanovaleric acid), are more ductile than IPNs made with azobis(isobutyronitrile) (20). IPN-modified polystyrene has been the subject of previous studies, including research by Millar, who formed a second polystyrene network within the original matrix (21), Sperling and Friedman, who modified the network with ethyl acrylate (22), and Kolarz, who used methyl methacrylate as the second network (23). IPNs also have been prepared as ion exchange resins. Polystyrene was functionalized with quaternary ammonium ligands within a polyethylene network (24), polystyrylsulfonic acid and polystyryltrimethylammonium ion ligands formed a network capable of both cation and anion exchange (25), and polystyryldimethylamine within polyacrylic acid formed a weak base-weak acid I P N (26). The sequential synthesis of IPNs was adapted to the preparation of new dual-mechanism bifunctional polymers (27) that were developed as sub strate-selective reagents. In the IPNs synthesized to date, polystyrene beads form the first matrix into which a second network with ion exchange or coordinating ligands or both is immobilized. Ideally, the selectivity of the bifunctional network will be greater than the selectivity of the corresponding monofunctional IPNs. The objective of the current research is to define the manner in which bifunctional IPNs can be prepared and to understand whether their ability to complex substrates varies as a function of the ratio of ligands. Metal ions are used to probe the I P N environment. Binding constants, evaluated through Langmuir adsorption isotherms (28), are used to quantify changes i n the microenvironment.

Experimental Details The IPNs are synthesized from polystyrene beads that are cross-linked with 2% divinylbenzene (DVB). The 250-425-μηι diameter polystyrene xerogel divinyl benzene beads were prepared by suspension polymerization wherein the organic phase consisted of 172 g of styrene, 6.50 g of technical grade D V B (55.4% meta and para isomers), 1.80 g of benzoyl peroxide, and 120 g of toluene as porogen. Synthesis of the polystyrene-poly(N-vinylimidazole-co-acrylic acid) I P N is typical of the IPNs in this study. Monomer solution was prepared by dissolving 10.33 g of DVB, 16.99 g of N-vinylimidazole, 18.08 g of ethyl acrylate, and 1.55 g of azobis(isobutyronitrile) in enough toluene to give 85 mL of solution. The total concentration is 4.25 M in complexing monomers (2.125 M each in imidazole and acrylate; the N-vinylimidazole IPN is 4.25 M in imidazole alone). The solution was sparged with nitrogen for 5 min, then contacted overnight with 10 g of polystyrene. The beads swelled to 4.5 times their original volume. The second network was polymerized by adding the swollen beads to 134 g of an aqueous


200


solution of 1.5 wt% polyvinyl alcohol) (based on the weight of monomers within the beads) and 70 g of C a C l dihydrate. The mixture was stirred at 80 ° C for 12 h, after which time the beads were washed with hot water and Soxhlet extracted with methanol for 17 h. The IPNs were then eluted with 1 L of H 0 , 1 Ν NaOH, H 0 , 1 N H C 1 , and H 0 . The ester groups were hydrolyzed to the acid by refluxing the beads in 250-mL of 2 Ν K O H in 90% aqueous dioxane for 72 h, then eluting with 2 L of H 0 , 1 L of 1 N H C 1 , and 1 L of H 0 . The imidazole IPN has a base capacity of 4.05 mequiv/g of dry resin, the carboxylate I P N has an acid capacity of 5.32 mequiv/g of dry resin, and the bifunctional IPN has acid-base capacities of 2.50/2.65 mequiv/g of dry resin. The analyses were made by acid or base titrations and confirmed by elemental analyses. Metal ion solutions were prepared by dissolving varying amounts of metal nitrate and 0.06 equiv of acetic acid in 80 mL of H 0 , adjusting the p H to 5.00 with 6 Ν NaOH, and diluting the solution to 100 mL with water. Binding isotherms were determined by contacting 1 mequiv of each IPN (0.2469-, 0.1880, and 0.1942 g of dry weight of the imidazole, acid, and bifunc tional IPNs) with 10 mL of metal-containing buffer solution. The vials were shaken for 7 days on a wrist-action shaker to ensure that equilibrium had been reached. The concentration of metal remaining in solution was quantified with an atomic absorption spectrophotometer (Perkin-Elmer 3100). The wavelengths used for the analysis were 324.7 nm for Cu(II) and 240.7 nm for Co(II). 2

2

2

2


2

2

2

Results Initial studies with bifunctional IPNs focused on the carboxylic acid ligand for ion exchange and the imidazole ligand for metal ion coordination. Acrylic acid was first tried as the monomer that would provide the ion exchange sites; ethyl acrylate was later found to be a better choice despite the fact that it necessitated a subsequent hydrolysis after I P N formation. The coordinating monomer was iV-vinylimidazole. The acid-base capacities of the imidazole, acid, and bifunctional IPNs were 0/4.05-, 5.32/0-, and 2.50/2.65-mequiv/g of dry resin, respectively. In each case, the calculated capacities based on monomer uptake were in agreement with experimental values, which indicates almost complete incor poration of monomer within the original polystyrene matrix. Fourier trans form infrared (FTIR) analyses are consistent with I P N formation (29) as is a solid-state C N M R study (Crick, D . W.; Alexandratos, S. D . , in press, Macromolecules). The binding constants for the imidazole, acid, and bifunctional IPNs were determined for Cu(II) and Co(II) in p H 5 acetate buffer solutions. The Cu(II) and Co(II) isotherms for the bifunctional I P N (Figure 1) are represen tative. Though the Cu(II) isotherm has not reached a well-defined plateau, no further data points were gathered because the inherent nonideality of the solutions at the required concentrations preclude their use in a Langmuir plot. The binding constants are calculated from an iterative solution of the 1 3


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isotherm equation r=(S c)/(Kt

1

where r is the milliequivalent eomplexed M per gram of polymer, c is the corresponding milliequivalent M in 1 m L of solution, S is the saturation capacity (milliequivalents per gram), and Κ is the binding constant (in units of reciprocal equivalents, i.e., normality). The binding constants for the IPNs are given in Table I. Note that the Cu(II) saturation capacity of the acid I P N (5.25 mequiv/g) is very close to the total capacity measured by base titration (5.32 mequiv/g), which indicates that I P N formation does not hinder ligand-substrate complexation. n

n


(1)

+c)

0 0.00 1

+

+

t

· 0.04

1

0.02

« 0.06

e

1

0.08

Figure 1. Adsorption isotherm plots for Cu(ll) and Co(ll) binding with the bifunctional poly(N-vinylimidazole-co-acrylic acid) polystyrene-based IPN. Table I. Binding Constants ( Κ ) and Saturation Capacities (S ) for the IV-Vinylimidazole (Vim), Carboxylic Acid, and Bifunctional (Vim-Acid) IPNs for Cu(ll) and Co(ll) Complexation t

IPN Cu(H) Vim Cu(II) Acid Cu(II) Vim-acid Co(II) Vim Co(II) Acid Co(II) Vim-acid

K(M" ) J

3130 77 1556 294 26 189

S (mequiv / g) t

2.37 5.25 3.05 1.92 3.06 1.56


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Discussion The substrate-complexing properties of ligands immobilized within a polymer network can be affected by the microenvironment formed by the polymer itself around the ligands (30). In the synthesis of IPNs and the study of their substrate recognition properties, it is important to define (1) whether differ ent ligand ratios in the bifunctional IPNs are sensitive to variations in the structure of substrates and (2) what role the polymeric microenvironmental plays in the complexation reaction. The role of polymeric microenvironments is addressed by the synthesis of a series of sequential IPNs from different initial supports [polystyrene, poly(methyl methacrylate), etc.] that will be reported in due course. The results from the foregoing study show that bifunctional IPNs offer a different binding environment to metal ions than the monofunctional ana logues. The use of copper and cobalt as the probe ions revealed that the bifunctional network is capable of distinguishing between different ions, depending on the inherent ligand ion affinities determined by monofunc tional counterparts. Under the present conditions, the ligands interact with the metal ions in a noncooperative manner: the relationship between ligand ratio and binding constants is linear. The imidazole ligand has a binding constant that is 10 times greater for Cu(II) than for Co(II), and this selectivity is retained in the bifunctional I P N , though at a lower absolute value, which thus points to different binding environments for the two IPNs. Current research is focused on definition of the conditions under which there is synergistic enhancement in the metal ion complexation reactions. Additional studies center on the microenvironmental effect, as previously defined, and on the influence of I P N morphology on substrate binding constants.

Acknowledgment We gratefully acknowledge the support of the Department of Energy, Office of Basic Energy Sciences, through grant DE-FG05-86ER13591.

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RECEIVED for review November 2 6 , 1 9 9 1 . ACCEPTED revised manuscript September 10, 1992.


Interpenetrating Polymer Networks - American Chemical Society

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