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The effect of PEG groups on swelling properties of PEG-grafted-polystyrene resins in various solvents
Reactive & Functional Polymers 44 (2000) 41–46 www.elsevier.com / locate / react
The effect of PEG groups on swelling properties of PEGgrafted-polystyrene resins in various solvents a b, Byeong-Deog Park , Yoon-Sik Lee * a b
Aekyung Central Research Center, 72 -6 Yong Jeon-Dong, Taejeon 300 -200, South Korea School of Chemical Engineering, Seoul National University, Seoul 151 -742, South Korea
Received 9 November 1998; received in revised form 21 June 1999; accepted 25 June 1999
Abstract Polyethylene glycol grafted polystyrene (PS-g-PEG) resins with different PEG grafting levels were prepared and their swelling properties were examined. The swelling properties together with the surface morphology were discontinuous at around 70 wt.% of the PEG content. 2000 Elsevier Science B.V. All rights reserved. Keywords: PS-g-PEG; Grafting level; Surface morphology
1. Introduction Polyethylene glycol grafted polystyrene (PSg-PEG) resins have drawn much attention in many research areas such as pharmaceutical chemistry, organic chemistry, biochemistry, and biochemical engineering [1–5]. One of the most interesting applications of PS-g-PEG resin can be found in the building of combinatorial libraries of peptides, nucleotides and small organic molecules, which have become a very powerful tool for the development of therapeutic agents [6–8]. PS-g-PEG resins show good swelling properties in various kinds of solvent, such as alcohols, dichloromethane, DMF, THF, acetonitrile, and water. These properties are due to the amphiphilic character of the resins. The *Corresponding author. E-mail address: [email protected] (Y.-S. Lee)
polystyrene backbone of the PS-g-PEG resins is hydrophobic and exerts strong mechanical and chemical stability. It possesses good swelling properties in organic solvents such as dichloromethane, DMF. The grafted PEG moieties which show good compatibility with biological systems are hydrophilic in nature and swell well in hydrophilic solvent such as alcohols and water [9–11]. Therefore, the amount of grafted PEG to the polystyrene backbone in PS-g-PEG resin is an important factor affecting many physical properties as well as chemical properties. As a polymer support, the swelling property is very important when used in organic syntheses. The reactions are heterogeneous in nature, which means that if a reagent should react with any active site at the interior of the polymer matrix, it has to diffuse from the bulk phase to the resin matrix. High swelling properties of the
1381-5148 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S1381-5148( 99 )00075-9
42
B.-D. Park, Y.-S. Lee / Reactive & Functional Polymers 44 (2000) 41 – 46
polymer chain may induce the reagent to diffuse quickly inside the resin and the reaction proceeds smoothly in a more homogeneous phase. Many synthetic chemists and biologists have already noticed the importance of the swelling properties of polymer supports [12– 14]. But, there has been no systematic study dealing with the relationship between the swelling properties of PS-g-PEG resins and the amount of grafted PEG. We would like to report some interesting observations in the swelling properties and the surface morphology of the PS-g-PEG resins as the grafting levels of PEG were increased from 2-(1-methyl)hydroxyethyl polystyrene (PS-PO) resin [15,16].
with nitrogen gas for 30 min at 1008C to remove moisture. Ethylene oxide (50 ml, 1.00 mol) was then added to the reactor and the reaction mixture was maintained at 1108C while stirring. After 9 h, the resin was washed with dioxane / water (1:1, 3 3), 0.1 N aq. HCl ( 3 3), water ( 3 3), ethanol ( 3 5), and dichloromethane ( 3 3) and dried in vacuo. The grafting levels were determined by weight increase.
2.3. Measurement of swelling properties
2. Experimental
The swelling volumes of various PS-g-PEG resins were measured in a fritted column (ID 0.9 cm, length 40 cm). The resins were swollen in a solvent for 15 min and washed with a 10-fold volume of each solvent. After filtering the resins, the volumes of the resins were measured.
2.1. 2 -(1 -Methyl)hydroxyethyl polystyrene ( PSPO)
3. Results and discussion
After thorough washing [17], polystyrene-1% divinylbenzene copolymer beads (Bio-Rad Co., BioBead S-X1, 200–400 mesh, 5 g, 47.5 mmol) were swollen in dichloromethane (80 ml) in a three-necked round bottomed flask, and SnCl 4 (11.1 ml, 95 mmol) was added to the resin mixture while stirring in an ice bath. Propylene oxide (6.5 ml, 95 mmol) in 20 ml of dichloromethane was then added dropwise for 10 min while stirring. After 5 h of reaction at 08C, 100 ml of methanol were added to quench the remaining catalyst, and then the resin was washed with ethanol ( 3 3), 0.1 N aq. HCl ( 3 3), water ( 3 3), methanol ( 3 2) and dried in vacuo, yielding 5.74 g (substitution level: 2.23 mmol / g).
2.2. Polyethylene glycol grafted polystyrene ( PS-g-PEG) The PS-PO resin (5 g) and KOH (0.63 g, 1.1 equiv.) were added in dioxane (150 ml) in a pressure reactor (Parr Model 4561) and purged
As a precursor of PS-g-PEG resin, PS-PO resin was prepared from polystyrene resin (Bio Bead S-X1) by Friedel-Crafts alkylation with propylene oxide in the presence of SnCl 4 catalyst [15,16]. The initial substitution level of the hydroxy group was 2.23 mmol / g, which means that every one out of four phenyl rings was grafted. The PS-PO resin was swelled in dioxane with 1.1 equiv. of KOH, and reacted with various amounts of ethylene oxide (EO) yielding six kinds of PS-g-PEG resins. The results are summarized in Table 1. As the content of PEG is increased, the final substitution levels of hydroxy group in PS-gPEG resins should decrease accordingly. Therefore, to obtain the optimal substitution levels, the amount of EO added to the PS-PO resin should be controlled, and the physical properties of the resulting PS-g-PEG resins checked. Thus, the swelling properties of each of the resins in various solvents were measured and the results are depicted in Fig. 1. As shown in Fig. 1, the swelling volumes of
B.-D. Park, Y.-S. Lee / Reactive & Functional Polymers 44 (2000) 41 – 46
43
Table 1 Various PS-g-PEG resins for the swelling property tests Resin
Initial sub’n level a (mmol / g)
Added PEG g / g of PS-PO b
Avg. Mw of PEG c
Fraction of PEG (%)c
EO p c
Final sub’n level c (mmol / g)
1 2 3 4 5 6
2.23 2.23 2.23 2.23 2.23 2.23
0.00 0.38 1.47 1.87 2.52 4.05
0 170 660 840 1130 1840
0 28 60 65 72 81
0.0 3.9 15.0 19.0 25.7 41.8
2.23 1.62 0.90 0.78 0.63 0.44
a
Initial substitution level was calculated from N content of the modified PS-PO resin with 10 equiv. of phenyl isocyanate. The values were determined by measuring the increased weight after drying at 658C in vacuo. c These values were calculated from a,b values. b
Fig. 1. Swelling properties of various PS-g-PEG resins for various solvents (water (d); ethanol (s); acetonitrile ( 3 ); DMF (h); THF (j); dichloromethane (m)).
PS-g-PEG resins in water were increased constantly as the content of PEG was increased. This swelling property in water is reasonable because PEG is hydrophilic and compatible with water. But the swelling behavior in a solvent which is miscible with water (such as THF, DMF, acetonitrile, and ethanol) is totally different from that in water. For example, the swelling volumes in dichloromethane, THF, and DMF were decreased continuously as the content of PEG was increased up to 72 wt.% (2.52 g PEG / 1 g PS). Polystyrene resin itself is not so swellable in a polar solvent such as acetonitrile and ethanol. But the swelling properties in those polar solvents were increased up to 60 wt.%
(1.47 g PEG / 1 g PS) of PEG content and decreased at 72 wt.% (2.52 g PEG / 1 g PS) of PEG content. The incorporation of hydrophilic PEG moieties makes the resin swell better in acetonitrile and ethanol, but not in the range from 60% (1.47 g PEG / 1 g PS) to 72 wt.% (2.52 g PEG / 1 g PS) of PEG content. When the PEG content is low, as in case of resin 2 (28 wt.% of PEG, 0.38 g PEG / 1 g PS), the swelling volumes show big differences as the solvent varies. But, the differences became smaller as the PEG content is increased, reaching to a minimal point at 72 wt.% (2.52 g PEG / 1 g PS) of PEG content (resin 5). Further addition of PEG to the resin made swelling volumes in-
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B.-D. Park, Y.-S. Lee / Reactive & Functional Polymers 44 (2000) 41 – 46
crease in all the solvents. These discontinuous swelling properties at around 72 wt.% (2.52 g PEG / 1 g PS) of PEG grafting are very curious phenomena. We have tried to correlate the swelling properties of the resins with solvent polarity parameters. The dielectric constants of several water miscible solvents are in the following order; THF , EtOH , DMF , MeCN (the dielectric constants are as follows: THF, 7.58; DMF, 37.0; MeCN, 37.5; EtOH, 24.55. The empirical parameters of solvent polarity constants are as follows. ET (30) values: THF, 37.4; DMF, 43.8; MeCN, 46.0; EtOH, 51.9. Z values: THF, 58.8; DMF, 68.4; MeCN, 71.3; EtOH, 79.6. Data are from Ref. [18]). Up to the 72 wt.% (2.52 g PEG / 1 g PS) PEG grafting level, the order of swelling volumes are as follows THF . DMF . MeCN . EtOH. It is evident that there is no direct relationship between the dielectric constants and the swelling properties. On the other hand, the swelling properties hold some relationship with ET (30) values or Z values [18]. The order of ET (30) or Z values are THF , DMF , MeCN , EtOH, which are exactly the reverse order of the swelling volumes in those solvents, and the differences become smaller as the PEG content increases. But, when the PEG grafting level was further increased, all the swelling volumes were governed by the excess PEG chains and revealed similar swelling properties in various solvent systems. Thus, when the PEG contents are not so high (resin 2, resin 3), the swelling behavior of the PS-g-PEG resins is governed by the property of the polystyrene backbone, solvation of which is dependent on the solvent polarity parameters related to charge-transfer interactions or p–p interactions between the phenyl rings within the polymer matrixes [18]. As the PEG content increases, those interactions are interrupted or replaced by the PEG chains reaching to a saturation point at 72 wt.% (2.52 g PEG / 1 g PS) of PEG grafting level, where the ratio of EO unit / benzene ring is ca. 6.0. We performed DSC experiments with the resins, which revealed that T g of PS (1% DVB
cross-linked) resin is around 1158C and that of PS-PO is around 1228C (rather broad heat absorption occurred from 98.5 to 1398C). As the amount of grafted PEG level was increased, a new T g peak appeared around 45–508C and the T g peak of PS backbone was decreased accordingly. At 60 wt.% (1.47 g PEG / 1 g PS) PEG level, the T g around 95–1308C nearly disappeared. Above 72 wt.% (2.52 g PEG / 1 g PS) PEG, we could not detect any T g peak of the PS backbone. These results clearly show that there is a phase separation at the grafting points between the PS backbone and the growing PEG chains. But, even though a phase separation may exist within the polymer matrix, it cannot explain properly the curious swelling properties. When the temperature was varied from room temperature to 708C, the swelling properties in DMF and in water remained almost unchanged. We took SEM pictures of the PS-g-PEG resins and noticed that above 72 wt.% (2.52 g PEG / 1 g PS) of PEG grafting, the size of the resins was doubled and the morphologies were changed from very smooth surfaces to orange skin-like rough surfaces accordingly (Fig. 2). The SEM pictures revealed that before the PEG grafting level reached to a saturation point, all the PEG chains were buried within the PS matrix.
4. Conclusions From the above results, the discontinuity of the swelling property of the PS-g-PEG resins can be explained as follows. Up to 72 wt.% (2.52 g PEG / 1 g PS) PEG grafting levels, all the grafted PEG chains are buried inside the resins interrupting p–p interactions between the phenyl rings within the polymer matrix. By simple calculations, six ethylene oxide units of PEG interact with one phenyl ring at the saturation point. Above that point, excess parts of the PEG chains are not bound to such interactions and are free to be solvated. Such interactions may be an important factor to be considered when the resins are to be used as
B.-D. Park, Y.-S. Lee / Reactive & Functional Polymers 44 (2000) 41 – 46
45
Fig. 2. Scanning electron micrographs of resins: (A) PS resin ( 3 1000); (B) PS-g-PEG resin (72 wt.% of PEG) ( 3 500).
polymer supports in combinatorial organic synthesis [19–24] or as a carrier of ligands in an in vitro bio-assay system [25,26]. To conclude, special interactions between the PEG chains and the phenyl rings within the polymer matrix are responsible for the discontinuous swelling behavior as the PEG content is increased in PS-g-PEG resins. Therefore, when preparing PS-g-PEG resins, controlling the PEG grafting level is necessary and important to attain optimal physical and chemical properties. Acknowledgements We thank KOSEF (96-0300-13-01-3) for their financial support to our research. References [1] E. Bayer, Angew. Chem. Int. Ed. Engl. 30 (1991) 113–216. [2] G. Barany, F. Albericio, S. Biancalana, S.L. Bontems, J.L. Chang, R. Eritja, M. Ferrel, C.G. Fields, in: J.A. Smith, J.E. Rivier (Eds.), Peptides: Chemistry and Biology, Proceedings of the 12th American Peptide Symposium, ESCOM, Leiden, 1992, pp. 603–604.
[3] L. Zhang, W. Rapp, E. Bayer, in: E. Giralt, D. Andreu (Eds.), Peptide 1990: Proceedings of 21st European Peptide Symposium, ESCOM, Leiden, 1991, pp. 196–197. [4] W. Rapp, L. Zhang, R. Haebich, E. Bayer, in: G. Jung, E. Bayer (Eds.), Peptide: Proceedings of the 20th European Peptide Symposium, W. de Gruyter, Berlin, 1989, pp. 199– 201. [5] E. Bayer, W. Rapp, US Patent 4,908,405 (03 / 13 / 90). [6] L.A. Thompson, J.A. Ellman, Chem. Rev. 96 (1996) 555– 600. [7] R. Cortese, Combinatorial Libraries: Synthesis, Screening, and Application Potential, W. de Gruyter, New York, 1995. [8] M.A. Gallo, R.W. Barrett, W.J. Dower, S.P.A. Fodor, E.M. Gorden, J. Med. Chem. 37 (1994) 1233–1251. [9] Y. Inada, K. Takahashi, T. Yoshimoto, A. Ajima, A. Matsushima, Y. Saito, TIBTECH (1986) 190–195. [10] V.N.R. Pillai, M. Mutter, Naturwissenschaften 68 (1981) 558–566. [11] K. Kempe, G. Barany, J. Am. Chem. Soc. 118 (1996) 7083. [12] W.S. Hancock, D.J. Prescott, P.R. Vagelos, G.R. Marshall, J. Org. Chem. 38 (1973) 774–781. [13] V.K. Sarin, S.B.G. Kent, R.B. Merrifield, J. Am. Chem. Soc. 102 (1980) 5463–5470. [14] G.B. Fields, C.G. Fields, J. Am. Chem. Soc. 113 (1991) 4202–4207. [15] Y.S. Lee, B.D. Park, H.I. Lee, US Patent 5,466,758 (11 / 14 / 95). [16] B.D. Park, H.I. Lee, S.J. Ryoo, Y.S. Lee, Tetrahedron Lett. 38 (1997) 591–594. [17] A.R. Mitchel, S.B.H. Kent, M. Engerhard, R.B. Merrrifield, J. Org. Chem. 43 (1978) 2845. [18] C. Reichardt, in: 2nd ed, Solvents and Solvent Effects in Organic Chemistry, VCH, 1988, pp. 361–371.
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[19] S. Zalipsky, J.L. Chang, F. Albericio, G. Barany, React. Polym. 27 (1994) 243–258. [20] H. Hellerman, H.W. Lucas, J. Maul, V.N.R. Pillai, M. Mutter, Makromol. Chem. 84 (1983) 2603–2617. [21] E. Bayer, W. Rapp, Biotechnical and biomedical applications, in: J.M. Harris (Ed.), Polyethylene Glycol Chemistry, Plenum Press, New York, 1992, pp. 325–345. [22] M. Meldal, Tetrahedron Lett. 33 (1992) 3077–3080. [23] M. Renil, R. Nagaraj, V.N.R. Pillai, Tetrahedron 50 (1994) 6681–6688.
[24] M. Kempe, G. Barany, J. Am. Chem. Soc. 118 (1996) 7083–7093. [25] J.S. Shin, B.G. Kim, D.H. Kim, Y.S. Lee, Anal. Biochem. 236 (1996) 9–13. [26] Y.S. Lee, D.H. Kim, in: R. Ramage, R. Epton (Eds.), Peptide 1996, Mayflower Scientific, Birmingham, 1997, pp. 571– 572.
The effect of PEG groups on swelling properties of PEGgrafted-polystyrene resins in various solvents a b, Byeong-Deog Park , Yoon-Sik Lee * a b
Aekyung Central Research Center, 72 -6 Yong Jeon-Dong, Taejeon 300 -200, South Korea School of Chemical Engineering, Seoul National University, Seoul 151 -742, South Korea
Received 9 November 1998; received in revised form 21 June 1999; accepted 25 June 1999
Abstract Polyethylene glycol grafted polystyrene (PS-g-PEG) resins with different PEG grafting levels were prepared and their swelling properties were examined. The swelling properties together with the surface morphology were discontinuous at around 70 wt.% of the PEG content. 2000 Elsevier Science B.V. All rights reserved. Keywords: PS-g-PEG; Grafting level; Surface morphology
1. Introduction Polyethylene glycol grafted polystyrene (PSg-PEG) resins have drawn much attention in many research areas such as pharmaceutical chemistry, organic chemistry, biochemistry, and biochemical engineering [1–5]. One of the most interesting applications of PS-g-PEG resin can be found in the building of combinatorial libraries of peptides, nucleotides and small organic molecules, which have become a very powerful tool for the development of therapeutic agents [6–8]. PS-g-PEG resins show good swelling properties in various kinds of solvent, such as alcohols, dichloromethane, DMF, THF, acetonitrile, and water. These properties are due to the amphiphilic character of the resins. The *Corresponding author. E-mail address: [email protected] (Y.-S. Lee)
polystyrene backbone of the PS-g-PEG resins is hydrophobic and exerts strong mechanical and chemical stability. It possesses good swelling properties in organic solvents such as dichloromethane, DMF. The grafted PEG moieties which show good compatibility with biological systems are hydrophilic in nature and swell well in hydrophilic solvent such as alcohols and water [9–11]. Therefore, the amount of grafted PEG to the polystyrene backbone in PS-g-PEG resin is an important factor affecting many physical properties as well as chemical properties. As a polymer support, the swelling property is very important when used in organic syntheses. The reactions are heterogeneous in nature, which means that if a reagent should react with any active site at the interior of the polymer matrix, it has to diffuse from the bulk phase to the resin matrix. High swelling properties of the
1381-5148 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S1381-5148( 99 )00075-9
42
B.-D. Park, Y.-S. Lee / Reactive & Functional Polymers 44 (2000) 41 – 46
polymer chain may induce the reagent to diffuse quickly inside the resin and the reaction proceeds smoothly in a more homogeneous phase. Many synthetic chemists and biologists have already noticed the importance of the swelling properties of polymer supports [12– 14]. But, there has been no systematic study dealing with the relationship between the swelling properties of PS-g-PEG resins and the amount of grafted PEG. We would like to report some interesting observations in the swelling properties and the surface morphology of the PS-g-PEG resins as the grafting levels of PEG were increased from 2-(1-methyl)hydroxyethyl polystyrene (PS-PO) resin [15,16].
with nitrogen gas for 30 min at 1008C to remove moisture. Ethylene oxide (50 ml, 1.00 mol) was then added to the reactor and the reaction mixture was maintained at 1108C while stirring. After 9 h, the resin was washed with dioxane / water (1:1, 3 3), 0.1 N aq. HCl ( 3 3), water ( 3 3), ethanol ( 3 5), and dichloromethane ( 3 3) and dried in vacuo. The grafting levels were determined by weight increase.
2.3. Measurement of swelling properties
2. Experimental
The swelling volumes of various PS-g-PEG resins were measured in a fritted column (ID 0.9 cm, length 40 cm). The resins were swollen in a solvent for 15 min and washed with a 10-fold volume of each solvent. After filtering the resins, the volumes of the resins were measured.
2.1. 2 -(1 -Methyl)hydroxyethyl polystyrene ( PSPO)
3. Results and discussion
After thorough washing [17], polystyrene-1% divinylbenzene copolymer beads (Bio-Rad Co., BioBead S-X1, 200–400 mesh, 5 g, 47.5 mmol) were swollen in dichloromethane (80 ml) in a three-necked round bottomed flask, and SnCl 4 (11.1 ml, 95 mmol) was added to the resin mixture while stirring in an ice bath. Propylene oxide (6.5 ml, 95 mmol) in 20 ml of dichloromethane was then added dropwise for 10 min while stirring. After 5 h of reaction at 08C, 100 ml of methanol were added to quench the remaining catalyst, and then the resin was washed with ethanol ( 3 3), 0.1 N aq. HCl ( 3 3), water ( 3 3), methanol ( 3 2) and dried in vacuo, yielding 5.74 g (substitution level: 2.23 mmol / g).
2.2. Polyethylene glycol grafted polystyrene ( PS-g-PEG) The PS-PO resin (5 g) and KOH (0.63 g, 1.1 equiv.) were added in dioxane (150 ml) in a pressure reactor (Parr Model 4561) and purged
As a precursor of PS-g-PEG resin, PS-PO resin was prepared from polystyrene resin (Bio Bead S-X1) by Friedel-Crafts alkylation with propylene oxide in the presence of SnCl 4 catalyst [15,16]. The initial substitution level of the hydroxy group was 2.23 mmol / g, which means that every one out of four phenyl rings was grafted. The PS-PO resin was swelled in dioxane with 1.1 equiv. of KOH, and reacted with various amounts of ethylene oxide (EO) yielding six kinds of PS-g-PEG resins. The results are summarized in Table 1. As the content of PEG is increased, the final substitution levels of hydroxy group in PS-gPEG resins should decrease accordingly. Therefore, to obtain the optimal substitution levels, the amount of EO added to the PS-PO resin should be controlled, and the physical properties of the resulting PS-g-PEG resins checked. Thus, the swelling properties of each of the resins in various solvents were measured and the results are depicted in Fig. 1. As shown in Fig. 1, the swelling volumes of
B.-D. Park, Y.-S. Lee / Reactive & Functional Polymers 44 (2000) 41 – 46
43
Table 1 Various PS-g-PEG resins for the swelling property tests Resin
Initial sub’n level a (mmol / g)
Added PEG g / g of PS-PO b
Avg. Mw of PEG c
Fraction of PEG (%)c
EO p c
Final sub’n level c (mmol / g)
1 2 3 4 5 6
2.23 2.23 2.23 2.23 2.23 2.23
0.00 0.38 1.47 1.87 2.52 4.05
0 170 660 840 1130 1840
0 28 60 65 72 81
0.0 3.9 15.0 19.0 25.7 41.8
2.23 1.62 0.90 0.78 0.63 0.44
a
Initial substitution level was calculated from N content of the modified PS-PO resin with 10 equiv. of phenyl isocyanate. The values were determined by measuring the increased weight after drying at 658C in vacuo. c These values were calculated from a,b values. b
Fig. 1. Swelling properties of various PS-g-PEG resins for various solvents (water (d); ethanol (s); acetonitrile ( 3 ); DMF (h); THF (j); dichloromethane (m)).
PS-g-PEG resins in water were increased constantly as the content of PEG was increased. This swelling property in water is reasonable because PEG is hydrophilic and compatible with water. But the swelling behavior in a solvent which is miscible with water (such as THF, DMF, acetonitrile, and ethanol) is totally different from that in water. For example, the swelling volumes in dichloromethane, THF, and DMF were decreased continuously as the content of PEG was increased up to 72 wt.% (2.52 g PEG / 1 g PS). Polystyrene resin itself is not so swellable in a polar solvent such as acetonitrile and ethanol. But the swelling properties in those polar solvents were increased up to 60 wt.%
(1.47 g PEG / 1 g PS) of PEG content and decreased at 72 wt.% (2.52 g PEG / 1 g PS) of PEG content. The incorporation of hydrophilic PEG moieties makes the resin swell better in acetonitrile and ethanol, but not in the range from 60% (1.47 g PEG / 1 g PS) to 72 wt.% (2.52 g PEG / 1 g PS) of PEG content. When the PEG content is low, as in case of resin 2 (28 wt.% of PEG, 0.38 g PEG / 1 g PS), the swelling volumes show big differences as the solvent varies. But, the differences became smaller as the PEG content is increased, reaching to a minimal point at 72 wt.% (2.52 g PEG / 1 g PS) of PEG content (resin 5). Further addition of PEG to the resin made swelling volumes in-
44
B.-D. Park, Y.-S. Lee / Reactive & Functional Polymers 44 (2000) 41 – 46
crease in all the solvents. These discontinuous swelling properties at around 72 wt.% (2.52 g PEG / 1 g PS) of PEG grafting are very curious phenomena. We have tried to correlate the swelling properties of the resins with solvent polarity parameters. The dielectric constants of several water miscible solvents are in the following order; THF , EtOH , DMF , MeCN (the dielectric constants are as follows: THF, 7.58; DMF, 37.0; MeCN, 37.5; EtOH, 24.55. The empirical parameters of solvent polarity constants are as follows. ET (30) values: THF, 37.4; DMF, 43.8; MeCN, 46.0; EtOH, 51.9. Z values: THF, 58.8; DMF, 68.4; MeCN, 71.3; EtOH, 79.6. Data are from Ref. [18]). Up to the 72 wt.% (2.52 g PEG / 1 g PS) PEG grafting level, the order of swelling volumes are as follows THF . DMF . MeCN . EtOH. It is evident that there is no direct relationship between the dielectric constants and the swelling properties. On the other hand, the swelling properties hold some relationship with ET (30) values or Z values [18]. The order of ET (30) or Z values are THF , DMF , MeCN , EtOH, which are exactly the reverse order of the swelling volumes in those solvents, and the differences become smaller as the PEG content increases. But, when the PEG grafting level was further increased, all the swelling volumes were governed by the excess PEG chains and revealed similar swelling properties in various solvent systems. Thus, when the PEG contents are not so high (resin 2, resin 3), the swelling behavior of the PS-g-PEG resins is governed by the property of the polystyrene backbone, solvation of which is dependent on the solvent polarity parameters related to charge-transfer interactions or p–p interactions between the phenyl rings within the polymer matrixes [18]. As the PEG content increases, those interactions are interrupted or replaced by the PEG chains reaching to a saturation point at 72 wt.% (2.52 g PEG / 1 g PS) of PEG grafting level, where the ratio of EO unit / benzene ring is ca. 6.0. We performed DSC experiments with the resins, which revealed that T g of PS (1% DVB
cross-linked) resin is around 1158C and that of PS-PO is around 1228C (rather broad heat absorption occurred from 98.5 to 1398C). As the amount of grafted PEG level was increased, a new T g peak appeared around 45–508C and the T g peak of PS backbone was decreased accordingly. At 60 wt.% (1.47 g PEG / 1 g PS) PEG level, the T g around 95–1308C nearly disappeared. Above 72 wt.% (2.52 g PEG / 1 g PS) PEG, we could not detect any T g peak of the PS backbone. These results clearly show that there is a phase separation at the grafting points between the PS backbone and the growing PEG chains. But, even though a phase separation may exist within the polymer matrix, it cannot explain properly the curious swelling properties. When the temperature was varied from room temperature to 708C, the swelling properties in DMF and in water remained almost unchanged. We took SEM pictures of the PS-g-PEG resins and noticed that above 72 wt.% (2.52 g PEG / 1 g PS) of PEG grafting, the size of the resins was doubled and the morphologies were changed from very smooth surfaces to orange skin-like rough surfaces accordingly (Fig. 2). The SEM pictures revealed that before the PEG grafting level reached to a saturation point, all the PEG chains were buried within the PS matrix.
4. Conclusions From the above results, the discontinuity of the swelling property of the PS-g-PEG resins can be explained as follows. Up to 72 wt.% (2.52 g PEG / 1 g PS) PEG grafting levels, all the grafted PEG chains are buried inside the resins interrupting p–p interactions between the phenyl rings within the polymer matrix. By simple calculations, six ethylene oxide units of PEG interact with one phenyl ring at the saturation point. Above that point, excess parts of the PEG chains are not bound to such interactions and are free to be solvated. Such interactions may be an important factor to be considered when the resins are to be used as
B.-D. Park, Y.-S. Lee / Reactive & Functional Polymers 44 (2000) 41 – 46
45
Fig. 2. Scanning electron micrographs of resins: (A) PS resin ( 3 1000); (B) PS-g-PEG resin (72 wt.% of PEG) ( 3 500).
polymer supports in combinatorial organic synthesis [19–24] or as a carrier of ligands in an in vitro bio-assay system [25,26]. To conclude, special interactions between the PEG chains and the phenyl rings within the polymer matrix are responsible for the discontinuous swelling behavior as the PEG content is increased in PS-g-PEG resins. Therefore, when preparing PS-g-PEG resins, controlling the PEG grafting level is necessary and important to attain optimal physical and chemical properties. Acknowledgements We thank KOSEF (96-0300-13-01-3) for their financial support to our research. References [1] E. Bayer, Angew. Chem. Int. Ed. Engl. 30 (1991) 113–216. [2] G. Barany, F. Albericio, S. Biancalana, S.L. Bontems, J.L. Chang, R. Eritja, M. Ferrel, C.G. Fields, in: J.A. Smith, J.E. Rivier (Eds.), Peptides: Chemistry and Biology, Proceedings of the 12th American Peptide Symposium, ESCOM, Leiden, 1992, pp. 603–604.
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