Polymer Journal, Vol.34, No. 6, pp 437—442 (2002) Polymer Effect on Molecular Recognition. Enhancement of Molecular-Shape Selectivity for Polycyclic Aromatic Hydrocarbons by Poly(acrylonitrile) Makoto TAKAFUJI, Wei DONG, Yoshihiro GOTO, Toshihiko SAKURAI, ∗ † Shoji NAGAOKA, and Hirotaka IHARA Department of Applied Chemistry & Biochemistry, Faculty of Engineering, Kumamoto University, Kumamoto 860–8555, Japan ∗Kumamoto Industrial Research Institute, Kumamoto 862–0901, Japan (Received December 10, 2001; Accepted April 25, 2002) ABSTRACT: Poly(acrylonitrile) immobilized onto porous silica (Sil-ANn) was prepared to evaluate the effect of polymerization degree of poly(acrylonitrile) on selective interaction with polycyclic aromatic hydrocarbons. The HPLC using the packed column (Sil-ANn) and an aqueous solution as a mobile phase showed higher selectivity for structural isomers of polycyclic aromatic hydrocarbons compared with simply cyanopropylated silica (Sil-CN) and alkylated silica (Sil-C4). It was considered that silica-supported poly(acrylonitrile) recognized molecular aromaticity of π-electron con- taining compounds rather than molecular hydrophobicity. Furthermore, similar results were obtained in the selectivity towards geometrical isomers such as trans- and cis-stilbenes or triphenylene and o-terphenyl and structural isomers such as o-, m-, p-terphenyls. Also the separation factors increased with an increase in polymerization degree of ANn. This paper discusses that polymeric structures enhance the selectivity. KEY WORDS Polymer Grafting Silica / High Performance Liquid Chromatography (HPLC) / Multiple π–π Interactions / CN–π–to-benzene–π Interaction / Molecular Aromaticity / Geometrical Isomers / Structural Isomers / Specific behaviors of many polymers are based on sult, we have found that the polymer-immobilized sil- their configurations and conformations. The chemi- ica showed much better selectivity for structural iso- cal structures of monomer units and their arrangements mers of polycyclic aromatic hydrocarbons than the cor- influence on the conformational structures of polymer responding monomer-immobilized silica. In this paper, chain and then closely relate to their functions whose we wish to report the effects of polymerization degree are carried out by multiple interactions through accu- of polymers on separation behavior of polycyclic aro- mulation of their functional groups. In biological sys- matic hydrocarbons by using poly(acrylonitrile). tems, there are many examples of protein polymers, the structures of which determine the biological functions. EXPERIMENTAL The molecular assemblies such as bio-membranes also show specific behaviors in bio-systems. Materials On the other hand, synthetic polymers are used as Acrylonitrile (AN; Nacalai tesque, Japan) was dis- many industrial materials such as films, fibers and ad- tilled and the fraction boiling at 72–74◦C was used. 3- hesions. It is well-known that crystallinity and poly- Mercaptopropyltrimethoxysilane (MPS) as telogen was merization degree influence the properties of polymers. purchased from Chisso Co., Japan. Azobisisobutyroni- Many researchers have reported physical and chemi- trile (AIBN) as an initiator was purified by recrystal- cal properties of polymers with functional groups in lization from methanol. A typical telomerization pro- their main and side chains because it is important to cedure of poly(acrylonitrile), ANn, where n is the av- mimic the functions of biopolymers using simply syn- erage of polymerization degree, was as follows: AN thetic polymers. This will expand possible applications (35 mL, 532 mmol) and MPS (10 mL, 53 mmol) were of polymers. dissolved in ethanol (200 mL). AIBN (0.14 g, 0.5 wt%) We have investigated the hybridization between was added to the solution at 60◦C. The mixture was 1–10 1 ◦ silica gels and polymers such as polystyrene, stirred for 72 h at 60 C under N2 gas atmosphere. After poly(acrylonitrile),6 and poly(methyl acrylate)3 which the reaction, the white precipitates obtained were gath- were applied to separation. These polymers possess π- ered by concentration and filtration, washed succes- electrons to be used as a π–π interaction source and sively with ethanol and ether, and then dried in vacuo. thus have been applied as packing materials for liquid The polymerization degree (n)ofANn was determined π 1 chromatography with -electron recognition. As a re- by H NMR spectroscopy (in d6-dimethylsulfoxide, δ = δ = †To whom correspondence should be addressed. 2.05 ppm (CH2CH–CN), 3.15 ppm (CH2CH–CN), 437 M. TAKAFUJI et al. and δ = 3.55 ppm (SiOCH3)). group (ANn) was prepared by telomerization of acry- ANn was readily introduced onto porous silica par- lonitrile with 3-mercaptopropyltrimethoxysilane. The ticle by mixing in dimethylformamide at 80◦C. YMC average of polymerization degree was easily controlled 2 120-S5 silica (diameter 5 µm, pore size 120 Å, 295 m by adjustment of the molar ratio. As a result, ANn with g−1) was used as porous silica. The resulting particles n = 3 and 21 were prepared for our purpose. were washed successively with dimethylformamide The ANn was immobilized onto porous silica. This and ether. The amount of ANn introduced onto silica was done through the reaction between the terminal was determined by elemental analysis. trimethoxysilyl groups of ANn and silanol groups from Wakopak (Wakosil 5CN, 4.6 mm i.d. × 250 mm) silica. The resulting silica successive washing with was used as a CN-containing silica (Sil-CN). Mightysil dimethylformamide showed no change in weight. IR (RP-4 GP, 4.6 mm i.d. × 250 mm) was used as a simply- spectroscopy showed the specific absorption due to −1 hydrophobized silica (Sil-C4). CN groups (νC−N, 2273 cm ). The elemental analysis All aromatic hydrocarbons as a sample were com- showed that 11.5 wt% (n = 3, Sil-AN3) and 13.8 wt% mercially obtained and used without further purifica- (n = 21, Sil-AN21)ofANn introduced onto the silica. tion. Benzene, anthracene, pyrene, triphenylene and terphenyl isomers were purchased from Nacalai tesque Selectivity for Structural Isomers of Polycyclic Aro- (Kyoto, Japan). Naphthacene was purchased from matic Hydrocarbons Tokyo Kasei Kogyo Co., Ltd. (Tokyo, Japan). Naph- The interaction between ANn and polyaromatic hy- thalene, dinitrobenzene isomers and dinitrotoluene iso- drocarbons was evaluated by using retention factors mers were purchased from Kanto Kagaku (Tokyo, in column liquid chromatography. Typical chro- Japan). Chrysene and stilbene isomers were pur- matograms are shown in Figure 1. The resultant reten- chased from Sigma-Aldrich Co. (St. Louis, MO). tion factor (k) and separation factor (α) were summa- Benz[a]anthracene was purchased from Kishida Chem- rized in Table I, II, and III. As shown in Table I, when ical Co., Ltd. (Osaka, Japan). Measurements The polymer-grafted silica (Sil-ANn) was packed into a stainless steel column (4.6 mm i.d. × 250 mm) using a hexanol-chloroform mixture. The liquid chro- matographic properties were examined using methanol- water mixtures as mobile phases. The chromatograph included a JASCO PU-980 intelligent HPLC pump, and a JASCO MD-910 multiwavelength detector. JASCO- BORWIN (Ver. 1.5) software was used for system con- trol and data analysis. 10 micro-liters of the sample dis- solved in methanol was injected through a HAMILTON 80365 injector. The mobile phases were methanol- water (6 : 4) for Sil-CN and Sil-ANn columns, and methanol-water (7 : 3) for Sil-C4 column. Chromatog- raphy was carried out at flow-rate of 1.0 mL min−1. The retention factor (k) was determined by (te- t0)/t0, where te and t0 are retention time of samples and methanol, respectively. The separation factor (α) was defined by the ratio of retention factor. Water-1-octanol partition coefficient (log P) was de- termined by retention factor with octadecylated silica, ODS (Inertsil ODS, 4.6 mm i. d. × 300 mm, GL Science Co., Ltd.) : log P = 3.2622 + 4.208 log k.11 RESULTS AND DISCUSSION Figure 1. Typical chromatograms with Sil-C4, Sil-CN, Sil- AN3, and Sil-AN21 columns. Mobile phase: methanol-water (6 : 4) for Sil-CN, Sil-AN3, and Sil-AN21 and methanol-water (7 : 3) for Preparation of Silica-Supported Poly(acrylonitrile) ◦ Sil-C4, Temperature: 35 C, UV detection wavelength: 250 nm, The Poly(acrylonitrile) with a terminal trimethoxysilyl small arrows indicate t0. 438 Polym. J., Vol.34, No. 6, 2002 Polymer Effect on Molecular Recognition Table I. Retention factors (k) and separation factors (α) for polycyclic aromatic hydrocarbons with Sil-C4, Sil-CN, Sil-AN3, and −1 ◦ Sil-AN21. Mobile phase: methanol/water = 6/4 (Sil-CN, Sil-AN3, and Sil-AN21), 7/3 (Sil-C4), Flow rate: 1.0 mL min , Temperature: 35 C, UV detection wavelength: 250 nm N kk k k Table II. Retention factors (k) and separation factors (α) for polycyclic aromatic hydrocarbons with Sil-C4, Sil-CN, Sil-AN3, and −1 ◦ Sil-AN21. Mobile phase: methanol/water = 6/4 (Sil-CN, Sil-AN3, and Sil-AN21), 7/3 (Sil-C4), Flow rate: 1.0 mL min , Temperature: 35 C, UV detection wavelength: 270 nm N k k k k Table III. Retention factors (k) and separation factors (α) for polycyclic aromatic hydrocarbons with Sil-CN, Sil-AN3, and Sil-AN21. Mobile phase: methanol/water = 6/4, Flow rate: 1.0 mL min−1, Temperature: 35◦C, UV detection wavelength: 250 nm k k k the samples were benzene, naphthalene, anthracene, thacene and benzene were 16.6 and 27.3 in Sil-AN3 and pyrene and naphthacene, k increased with an increase Sil-AN21 but 9.4 and 5.8 in Sil-CN and Sil-C4. Here, in the number of aromatic rings and no difference was it should be emphasized that Sil-C4 shows the small- confirmed in the retention order. However, it is clear the est value. This is attributed that only Sil-C4 has no selectivities were much higher in Sil-ANn than those π-electron sources for π–π interaction with the sam- in Sil-CN and Sil-C4.