Synthesis and Characterization of Poly(N-vinylimidazole-co-acrylonitrile) and Determination of Monomer Reactivity Ratios

1088 Full Paper Summary: Radical-initiated solution copolymerization of N-vinylimidazole (VIM) and acrylonitrile (AN) was carried out with 2,2 0 -azobisisobutyronitrile (AIBN) as an initiator in benzene at 70 8C in nitrogen atmosphere. The structure and composition of synthesized copolymers for a wide range of monomer feeds were determined by FTIR, 1 H and 13 C NMR spectroscopy with the aid of recorded analytical absorption bands for VIM (667 cm 1, C N of imidazole ring) and AN (2 242 cm 1, CN group), as well as by using the areas of proton and carbon atom signals from corresponding functional groups of monomer units. Monomer reactivity ratios for VIM (M 1 )-AN (M 2 ) pair were determined by nonlinear regression (NLR), Kelen Tüdös (KT) and Fineman Ross (FR) methods. They were found to be r 1 ¼ 0.24 and r 2 ¼ 0.15 for the NLR method, r 1 ¼ 0.22 and r 2 ¼ 0.094 for the KT method, and r 1 ¼ 0.24 and r 2 ¼ 0.12 for the FR method, respectively. The relatively high activity observed of VIM growing macroradical and the results of FTIR and 1 H NMR structural analysis of copolymers suggest the formation of complexed linkages between monomers and growing radicals in chain propagation reactions. Similar complexation between monomer comonomer units in the structure of formed macromolecules showed an increase in isotactic triad fractions in the copolymer. Complex formation between the imidazole ring and nitrile group in both the monomer mixture and chain growing reactions. Synthesis and Characterization of Poly(N-vinylimidazole-co-acrylonitrile) and Determination of Monomer Reactivity Ratios Nursel Pekel,* 1 Zakir M. O. Rzaev, 2 Olgun Güven 1 1 Department of Chemistry, Hacettepe University, Beytepe, 06532 Ankara, Turkey Fax: (þ90) 312 297 7965; E-mail: nursel@hacettepe.edu.tr 2 Department of Chemical Engineering, Hacettepe University, Beytepe, 06532 Ankara, Turkey Received: October 12, 2003; Revised: March 26, 2004; Accepted: March 26, 2004; DOI: 10.1002/macp.200300130 Keywords: acrylonitrile; complex formation; NMR; radical polymerization; N-vinylimidazole Introduction The copolymerization reactions involving N-vinylimidazole (VIM) and its derivatives have stimulated great interest due to the wide possibilities of the preparation of new materials, including the synthetic analogues of biopolymers with imidazole fragments in the macromolecules. These materials showed unique properties such as ion exchange and complexing behaviors, catalytic, biological and physiological activities, and heat resistance. It is known that the homo- and copolymers and block copolymers of N- vinylimidazole (VIM) are used as a carrier agent for protein separations [1,2] and for an active moiety of electrolytic enzymes [3 5] and for the preparation of strong anion exchange membranes [6] and new steric stabilizers for polyaniline colloids. [7] The catalytic activity of the imidazole monomers and polymers and imidazole-containing proteins have been studied in detail. [8,9] Poly(VIM) ligands form multidentate complexation systems with Ag þ and Cu 2þ ions with two and four coordination numbers, respectively. [10] The copolymerization of VIM with different donor acceptor monomers, such as styrene, [11] 4-amino-styrene, [7] vinyl acetate, [11] N-vinylpyrrolidone, [11] methyl acrylate, [12] methyl methacrylate, [11] maleic anhydride, [13,14] dialkyl fumarates, and maleates, [15,16] and the determination of the monomer reactivity ratios have been also reported. It was shown that these imidazole-containing copolymers obtained from a wide range of monomer feeds had different compositions, which vary from random to alternating. On the other hand, poly(acrylonitrile) (PAN) as a cyanocontaining polymer precursor was modified to improve its properties by the incorporation of suitable acidic comonomers such as maleic and itaconic acids during polymerization, which increases its hydrophilicity and catalyzes the Macromol. Chem. Phys. 2004, 205, 1088 1095 DOI: 10.1002/macp.200300130 ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Synthesis and Characterization of Poly(N-vinylimidazole-co-acrylonitrile) and Determination of Monomer Reactivity Ratios 1089 cyclization of nitrile groups during the heat treatment of PAN. [17 22] DMF is the solvent usually recommended for the copolymerization of AN. [22] This highly hygroscopic solvent and the presence of water are known to affect the monomer reactivity ratio; therefore, special care must be taken to remove water. [23] The effect of reaction medium on radical homo- and copolymerization of functional monomers has been reported by Plochocka, [24] who considered the electrostatic and polar polar interactions and H-bonding factors as responsible for functional monomer and radical reactivities in copolymerization reactions. Recently, the synthesis of poly(vim-co-an) and its amidoxime derivative and new chelating hydrogels has been reported by Pekel at al. [25] These copolymers were prepared by 60 Co-g-irradiation of VIM/AN monomer mixture and then amidoximation of cyano group of the formed copolymers in aqueous solution. In our other work, [26] we studied the free-radical copolymerization of N-vinylimidazole with ethyl methacrylate initiated by AIBN in benzene at 70 8C. It was found that this copolymerization follows the simple terminal model of copolymerization, and the monomer reactivity ratios have been evaluated by different methods using elemental analysis and FTIR data of copolymer compositions obtained at a wide range of monomer feed ratios. Poly(VIM-co-ethyl methacrylate)s with different compositions and high values of glass-transition temperature and thermal stability were synthesized. Monomer reactivity ratios determined by different methods, including FTIR spectroscopy, indicated that the random copolymerization is realized in the studied monomer system, and obtained copolymers predominantly are broadened with acrylate monomer units. [26] The present work is aimed at (1) the synthesis of poly(vim-co-an)s by radical-initiated solution copolymerization, (2) the determination of monomer reactivity ratios, (3) the characterization of synthesized copolymers, and (4) the evaluation of the effect of complex formation in the alternating chain growth reactions. Experimental Part Materials VIM (Aldrich) and AN (Merck) were distillated in vacuo before use: VIM: b.p. 78.5 8C/13 mmhg d 4 20 : 1.0388, n D 20 : 1.5290; AN: d 4 20 : 0.8100, n D 20 : 1.3910. The 2,2 0 -azobisisobutyronitrile (AIBN) radical initiator (Merck) was recrystallized just before use from methanol solution. The procedure was repeated twice. The melting point was 102 8C. The solvents (benzene, methanol, n-heptane and diethyl ether) used as copolymerization medium and for purification of copolymers by precipitation and washing, were all analytical grade reagents. They were purified using standard methods and were distilled before use. Copolymerization Procedure The homogeneous solution copolymerization reactions of VIM-AN monomer pair were carried out in benzene in glass tubes (18 3 ml) at 70 8C with AIBN as the initiator. The appropriate amount of comonomers in various molar ratios, [VIM]/[AN] ¼ 4.0 0.25, at a constant total concentration were mixed with the solvent and initiator and placed into the tubes. The reaction mixture was cooled by liquid nitrogen and flushed with nitrogen gas for 2 min, then thawed. This procedure was repeated twice, and then the tubes were sealed and placed in a thermostatically controlled silicon oil bath at 70 0.1 8C. The time of copolymerization was established experimentally to keep the conversion of monomers less than 10% (about 1.0 1.5 h). Poly(VIM-co-AN)s were isolated from the reaction mixture by precipitation with n-heptane and purified by two reprecipitation procedures from benzene to diethyl ether, and by washing with several portions of n-heptane and diethyl ether. The copolymers were then isolated by centrifugation and dried under vacuum at 50 8C to constant weight. The copolymer compositions were determined by FTIR, 1 H and 13 C NMR (400 MHz) spectroscopy using absorption bands and integral area for the imidazole rings (for VIM units) and CN groups (for AN units) as analytical bands and signals, respectively, for quantitative analysis. The homopolymers of VIM and AN were synthesized by common radical solution method in benzene using AIBN as an initiator at 70 8C. Both polymers were purified by the similar techniques as copolymer has been done. Obtained homopolymers have the following characteristics: polyðvimþ[z] 0.42 dl g 1 in DMF at 25 0.1 8C, M v 65 000 g mol 1 and polyðanþ[z] 1.87 dl g 1 in DMF at 25 0.1 8C, M v ¼ 142 000 g mol 1. The copolymers synthesized from equimolar ratio of monomer feed had the following average characteristics: [Z] 0.55 dl g 1 in DMF at 25 0.1 8C, m 1 /m 2 ¼ 1.11. Calcd. C 65.30, H 6.10, N 28.57; Found C 64.57, H 6.05, N 29.38. IR spectra (KBr): VIM unit: 3 110 (C H (ring) stretching), 2 950 (C H and CH 2 (main chain) stretching), 1 650 (C C (ring) stretching), 1 500 (C C and C N (ring) stretching), 1 290, 1 228 and 1 085 (C H (ring) in-plane bending, C N (ring) stretching), 915 (ring stretching and in-plane bending) and 667 cm 1 (C N (ring) stretching); AN unit: 2 943 2 870 (CH 2 stretching), 2 242 (C N stretching), 1 420 cm 1 (CH 2 band of backbone). 1 H NMR (DMSO-d 6,278C): d ¼ 1.56 2.09 (4H, backbone CH 2 from VIM and AN linkages), 3.41 (1H, backbone CH from AN unit), 3.93 4.52 (1H, backbone CH from VIM unit), 7.37 6.78 (3H, CH groups in free imidazole ring) and 7.35 7.85 (3H, CH groups in complexed imidazole ring).

1090 N. Pekel, Z. M. O. Rzaev, O. Güven Under similar conditions, the homopolymers of VIM and AN were synthesized and characterized. Poly(VIM): 1 H NMR spectra (H 2 O-d 2,278C): d ¼ 1.99 2.04 (2H, doublet from CH 2 backbone), 2.48 3.62 (1H, triplet from CH backbone for the syndiotactic triads, s), 3.09 (1H, singlet from CH backbone for the heterotactic triads, h), 3.62 (1H, singlet from CH backbone for the isotactic triads, i), 7.36 6.47 (3H, CH groups of imidazole ring). Poly(AN): 1 H NMR spectra (DMSO-d 6,278C): d ¼ 2.05 2.16 (2H, CH 2 backbone), 3.13 3.17 (1H, CH backbone). Measurements FTIR spectra of the copolymers (KBr pellet) were recorded with FTIR Nicolet 520 spectrometer in the 4 000 400 cm 1 range, where 10 scans were taken at 4 cm 1 resolution. 1 H NMR spectra were recorded on a JEOL 6X-400, 400 MHz high performance digital FT-NMR spectrometer with DMSO-d 6 and H 2 O-d 2 as solvents at 27 8C. The compositions of the copolymers synthesized using various monomer feed ratios were determined by known FTIR [26,27] and NMR [28] methods and were achieved by comparing the absorption bands and the integrals of the methyne and nitrile groups regions in the spectra of VIM and AN units, respectively. Molar fractions of the comonomer units (m 1 and m 2 ) in poly(vim-co-an)s using 1 H NMR analysis data were calculated according to Equation (1) and (2). Am 1 ðch of imidazole ringþ=a total ¼ n 1 m 1 =ða 1 m 1 þ b 2 m 2 Þ ð1þ Am 2 ðch of AN unitþ=a total ¼ n 2 m 2 =ða 1 m 1 þ b 2 m 2 Þ ð2þ where Am 1 and Am 2 are the normalized areas per carbon atoms from the corresponding functional groups of the monomer unit regions in 1 H NMR spectra; A total is the total area of carbon atoms in the copolymer; n 1 and n 2 are the integers of proton(s) in the functional group of the monomers; a and b are the integers of protons in the monomer units (m 1 and m 2 ); in the case of (m 1 þ m 2 ) ¼ 1, monomer unit ratios can be calculated by ratioing of Equation (1) and (2) using the following simplified form [Equation (3)]. m 1 =m 2 ¼ f ¼ n 2 Am 1 ðch of ring or chain CHÞ=n 1 Am 2 ðch of AN unitþ ð3þ The CHNS-932 Model LECO Elemental Analyzer was used for the determination of C, H and N contents in the copolymers synthesized. Intrinsic viscosities of the homo- and copolymers were determined in DMF at 25 0.1 8C in the concentration range of 0.1 1.0 g dl 1 using an Ubbelohde viscometer. Results and Discussion Some Observations on the Peculiarities of Copolymerization VIM is a polar, electron-donor monomer with (p 0! p! p) conjugated system characterized by a negative polarity of its vinyl double bond (e 1 ¼ 0.61). [26] On the other hand, AN is also polar monomer, but an electron-acceptor monomer with a positive charge on its vinyl b-carbon atom (e 2 ¼þ1.23). [16] So in the solution copolymerization of this donor acceptor monomer system, both monomers can show higher activity and some tendency to alternating the monomer units in copolymer chains. The poly(vim-co-an)s synthesized were characterized by FTIR spectroscopy to determine the VIM and AN unit contents. The absorption value ratios between characteristic analytical bands of 667 cm 1 (for imidazole ring band in VIM unit), 2 242 cm 1 (for nitrile stretching band in AN unit) and the least changing absorption band of 1 360 cm 1 as a standard band (A ¼ log (I o /I), DA i ¼ A i /A 1360 ) were used to calculate the copolymer compositions. Molar fractions (in mol-%) of comonomer units (m 1 and m 2 ) in copolymers using FTIR analysis data are calculated according to Equation (4) and (5). m 1 ¼ m 2 ¼ DA 667 =M 1 DA 667 =M 1 þ DA 2242 =M 2 100 DA 2242 =M 2 DA 667 =M 1 þ DA 2242 =M 2 100 ð4þ ð5þ where m 1 /m 2 ¼ [DA 667 /M 1 ]/[DA 2242 /M 2 ]; M 1 and M 2 are molecular weights of VIM and AN monomer units, respectively. Results of FTIR analyses of copolymers by using various initial monomer ratios are illustrated in Figure 1. On the basis of these data, the values of absorption bands for the comonomer units are calculated which are used for the determination of copolymer compositions according to Equation (4) and (5). The results obtained are presented in Table 1. As evidenced from these data, the change in the concentrations of VIM (with electron-donor double bond) and AN (with electron-acceptor double bond) in the monomer feed from 40 to 60 mol-% leads to the formation of copolymers with almost identical molar ratios of m 1 /m 2 monomer units. The studied pair of monomers shows a tendency of alternating copolymerization. This can be explained by the effect of complex formation between donor acceptor double bonds and between imidazole ring and nitrile group in both the monomer mixture and chain growing reactions, respectively, as shown in Scheme 1. Intra- or intermolecular complex formation or both can proceed between the acceptor p system of an imidazole ring and the donor cyano group in the studied monomer and polymer systems. The formation of proposed complex between monomers and macroradicals is confirmed by the FTIR analysis of monomer and homopolymer mixtures (Figure 2). FTIR analysis of free monomers and their mixture (Spectra (A)), as well as homopolymers of VIM and AN, and their mixtures (Spectra (B)) indicated that the characteristic band of C N (ring) group is shifted from

Synthesis and Characterization of Poly(N-vinylimidazole-co-acrylonitrile) and Determination of Monomer Reactivity Ratios 1091 Figure 1. FTIR spectra of homo- and copolymers: (a) poly(an), (g) poly(vim) and poly(vim-co-an)s with different contents of imidazole unit: (b) 20, (c) 40, (d) 50, (e) 60, and (f) 80 mol-%. 1 500 to 1 512 cm 1 (for the monomer mixture) and from 1 495 to 1 503 cm 1 (for the polymer mixture), respectively. The relatively weak shift in the C N band in the polymer mixture can be explained by the disappearance of the conjugation effect between double bond and triple bond corresponding functional group during chain propagation reaction. Similar weak band shifts are observed in the spectra of VIM-AN copolymers (Figure 1), whereas the characteristic band of imidazole (C N ring stretching) apparently shifted from 656 to 680 cm 1. It is known fact that poly(vim) as a polybase polymer forms interpolymer complex with poly(acrylic acid) through a donor acceptor interaction between the imidazole rings and carboxylate anions. [29,30] The structure and composition of synthesized copolymer is also confirmed by 1 H NMR analysis (Figure 3). The comparative analysis of 1 H NMR spectra of the homopolymers of VIM and AN and poly(vim-co-an), which are synthesized under similar conditions, indicates the significant changes in the chemical shifts of imidazole ring protons and backbone CH protons of VIM and AN linkages, when these groups form the backbone of the copolymer macromolecules. The spectra of poly(vim) (Figure 3a) contain the characteristic proton signals from imidazole ring (multiplet with 6.47 7.36 ppm), backbone CH 2 (doublet with 1.99 2.04 ppm) group and splitting chain- CH group [isotactic (i), heterotactic (h) and syndiotactic (s) triads]. It is well-known that backbone CH groups of poly(vim) are sensitive to macromolecular chain configuration and allow the determination of polymer tacticity and ratios of different triads. [31,32] As evidenced from character and position of these chemical shifts, poly(vim) Table 1. Radical copolymerization of VIM (M 1 ) with AN (M 2 ). Reaction conditions: solvent: benzene; 70 0.1 8C; initiator, [AIBN] ¼ 8.0 10 3 mol L 1 ; [M] total ¼ 4.0 mol L 1 ; conversion 10% (around 5.6 9.6%). Monomer feed FTIR analysis Copolymer composition Parameter of FR-eq. Parameters of KT-eq. [M 1 ] [M 2 ] Am 1 (667) Am 2 (2 242) m 1 m 2 F 2 /f F(f 1)/f F 2 /f þ a a) x Z mol-% mol-% mol-% mol-% 20 80 10.33 7.75 42.92 57.02 0.08 0.08 0.90 0.09 0.09 40 60 11.67 6.48 44.42 55.58 0.56 0.07 1.38 0.41 0.05 50 50 13.11 5.65 52.65 47.35 0.90 0.10 1.72 0.52 0.06 60 40 13.56 5.42 54.34 45.66 1.89 0.24 2.71 0.70 0.09 80 20 15.56 4.61 65.61 34.39 8.38 1.91 9.20 0.91 0.21 a) a (arbitrary parameter) ¼ H(F 2 /a) min.(f 2 /a) max ¼ 0.82.

1092 N. Pekel, Z. M. O. Rzaev, O. Güven Scheme 1. shows predominantly atactic configuration (h > s > i) as poly(vim) synthesized by radical polymerization of VIM with AIBN in methanol at 50 8C by Barboiu et al. [31] However, it was observed that CH proton signals of poly[vim-co-an (45.7 mol-%)] (d c ) significantly shifted to lower field [Dd ¼ d c d p ¼ 0.60, 0.84 and 1.04 for (i), (h) and (s) triads, respectively] with increasing intensity of proton signal from isotactic triad (i >> h i) (Figure 3b) in comparison with those for poly(vim) (d p ). Simultaneously, the essential shift in some fraction of the imidazole ring protons (44%) (Dd ¼ 0.5) is taking place. It can be proposed that this unusual fact related to the formation of two different types of imidazole fragments such as free (6.78 7.37 ppm with integral area of 1 745) and cyanocomplexed rings (7.35 7.85 ppm with integral area of 1 287). On the other hand, backbone methyne group in conjugated >CH C N fragment of AN linkage is also shifted from 3.15 (for homopolymer) to 3.41 ppm (for copolymer) (Figure 3c). All these phenomena observed can be explained by strong inter- and intramolecular incorporations between imidazole ring and cyano group which play an important role in the formation of copolymer structure with possible controlled configuration. Thus, the formation of complexed linkages between monomers and growing radicals in chain propagation reactions, as well as between monomer comonomer units in the structure of formed macromolecules is providing an increase in the isotactic triad fractions in copolymer. In synthesized poly(vim-co-an), the possible configurational repeating units can be presented as shown in Scheme 2. From the values of the integral areas, Am 1 (0.56 for 3H of imidazole ring or 0.178 for 1H CH backbone of VIM unit) and Am 2 (0.146 for 1H CH backbone of AN unit), and the use of Equation (3), the composition of copolymer can be calculated: m 1 ¼ 56.07 and m 2 ¼ 43.93 mol-%. These values obtained by 1 H NMR method agree well with the monomer unit composition of this copolymer calculated from FTIR spectroscopy data (m 1 ¼ 54.34 and m 2 ¼ 45.66 mol-%). Determination of Monomer Reactivity Ratios Copolymerization reactions of VIM with AN were carried out under low conversion conditions (10%) to determine the monomer reactivity ratios (r 1 and r 2 ) in the steady-state by using known terminal model of the Fineman-Ross [33] and Kelen Tüdös (KT) equations [Equation (6) and (7)] given below: [34] Fðf 1Þ=f ¼ r 1 ðf 2 =f Þ r 2 Z ¼ðr 1 þ r 2 =aþx r 2 =a ð6þ ð7þ where Z ¼ [F(f 1)/f]/(F 2 /f þ a); x ¼ (F 2 /f )/(F 2 /f þ a); a (arbitrary constant) ¼ H(F 2 /f) min (F 2 /f) max ; F ¼ [VIM]/ [AN]; and f ¼ m 1 /m 2. For comparison, the nonlinear regression (NLR) procedure using a microcomputer program [35] has also been applied to calculate copolymerization constants. Figure 2. Fragments of the FTIR spectrum of monomer feed (A) [(1) VIM, (2) VIM-AN mixture with 50 mol-% of VIM content, (3) AN] and homopolymer mixtures (B) [(1) poly(vim), (2) poly (VIM)/poly(AN) mixture with 50 mol-% of poly(vim) content, (3) poly(an)].

Synthesis and Characterization of Poly(N-vinylimidazole-co-acrylonitrile) and Determination of Monomer Reactivity Ratios 1093 Figure 3. 1 H NMR (400 MHz) spectra of (a) poly(vim), (b) poly(vim-co-an) with 60 mol-% of poly(vim) content in DMSO-d 6 at 27 8C and (c) poly(an). Monomer reactivity ratios (r 1 and r 2 ) were obtained using experimental data, presented in Table 1, from FR plots of (F 2 /f) versus F(f 1)/f and KT plots of x versus Z and from NLR analysis. As evidenced from these values, which are summarized in Table 2, some alternating copolymerization reactions are realized in monomer systems with visibly high degree of formation of monomer unit diads in the VIM-AN system. Reactivity ratios determined by different methods have similar values indicating reasonable agreement between the used methods. As an additional confirmation of the alternating tendency of the monomers in the both studied systems, the monomer sequence lengths (m 1 and m 2 ) are calculated from the well-known equations [Equation (8) and (9)]. [36] m 1 ¼ 1 þ r 1 ðm 1 =m 2 Þ m 2 ¼ 1 þ r 2 ðm 2 =m 1 Þ ð8þ ð9þ The values of m 1 and m 2 are presented in Table 3. As seen from these values for different monomer-copolymer compositions, the value of m 1 (VIM unit sequence length) insignificantly changes (from 1.16 to 1.42) in the copolymers with increasing VIM feed concentration. On the other hand, the mean unit sequence lengths for AN units (m 2 ) have Table 2. Constants of copolymerization (r 1 and r 2 ) for VIM (M 1 ) AN (M 2 ) monomer pair obtained by using FTIR analysis data. Methods of determination and calculation r 1 r 2 r 1 r 2 Scheme 2. Fineman Ross 0.24 0.12 0.029 Kelen Tüdös 0.22 0.094 0.021 Nonlinear regression 0.24 0.15 0.036

1094 N. Pekel, Z. M. O. Rzaev, O. Güven Table 3. Mean sequence lengths (m 1 and m 2 ) and probability (P ij ) for different homo- and heterodiad linkages as the functions of monomer-copolymer composition. Monomer feed ratio Mean sequence length Homodiad (P ii ) a) Heterodiad (P ij ) a) F m 1 m 2 P 11 P 22 P 12 P 21 0.25 1.16 1.12 0.05 0.02 0.95 0.98 0.67 1.18 1.11 0.13 0.06 0.87 0.94 1.00 1.24 1.08 0.18 0.09 0.82 0.91 1.50 1.26 1.08 0.25 0.12 0.75 0.88 4.00 1.42 1.05 0.47 0.27 0.53 0.73 a) P ij is the probability for the ij diad linkages in the copolymer chain which is calculated from constants of copolymerization [r 1 ¼ 0.22 (VIM) and r 2 ¼ 0.094 (AN)] using the following known equations: [37] P ij ¼ 1/(1 þ r i F) and P ii ¼ 1 P ij. relatively low values (from 1.05 to 1.12). These facts correlate well with almost two-fold values of r 1 compared with the reactivity of AN and confirm some alternating character of the monomer unit distributions in the copolymers (r 1 < 1, r 2 1 and r 1 r 2! 0). The relative reactivity of VIM and AN as a 1/r 2 (for styrene-standard comonomer) parameter can also calculated from known values of monomer reactivity ratios for VIM-styrene (r 1 ¼ 0.071 and r 2 ¼ 0.029) and AN-styrene (r 1 ¼ 0.03 and r 2 ¼ 0.37) pairs. [16] These calculated values are 34.5 for VIM and 2.7 for AN, which indicated the high activity of VIM in radical copolymerization and supported our experimental results. Generally, these results suggest that the chain growth reactions proceed predominantly by the addition of AN comonomer to the VIM. macroradical through intermediate formation of charge transfer complex between imidazole (donor) and AN (acceptor) double bonds and intermolecular complexes between imidazole and nitrile functional groups of the monomers and corresponding growing macroradicals according to the above mentioned scheme. The predominant formation of complexed heterodiad linkages in copolymer chains is also confirmed by calculated values of different probabilities (P ij ) for m 1 m 2 (around 0.53 0.95) and m 2 m 1 (around 0.73 0.98) alternating linkages (Table 3). P ij is the probability for ij diad linkages in the copolymer chain which is calculated from constants of copolymerization [r 1 ¼ 0.22 (VIM) and r 2 ¼ 0.094 (AN)] using the known Equation (10). [37] P ij ¼ 1=ð1 þ r i FÞ and p ii ¼ 1 P ij ð10þ (2) the study of the structure and compositions of copolymers by FTIR, 1 H and 13 C NMR spectroscopy; (3) the determination of monomer reactivity ratios by FR, KT and NLR methods; (4) the evaluation of the effect of complex formation between an imidazole ring and a cyano group on the chain growth reactions and copolymer composition of the synthesized copolymer systems. It was shown that the copolymer composition is dependent on the monomer feed ratio and changed from random ([VIM] >> [AN] or [AN] >> [VIM]) to alternating structures (m 1 /m 2 ¼ 0.80 1.19). The values obtained for the reactivity ratios (r 1 < 1, r 2 1 and r 1 r 2! 0), the monomer sequence lengths and the probability for the various linkages (Table 3), all indicated some alternating character of the monomer unit distributions in the copolymers and reasonably correlate with complex-radical mechanism of the alternating copolymerization. The proposed realization of donor acceptor interactions between p- electron system of imidazole rings (VIM monomer unit and growing macroradical) and nitrile groups (AN) and its effect on the chain growth is confirmed by FTIR and NMR spectroscopy. Acknowledgements: This study was carried out in accordance with the Polymer Science and Technology Program of Chemistry and Chemical Engineering Departments, Hacettepe University. Support by TÜBİTAK (Turkish National Scientific and Technical Research Council) through Project MISAG 146 is acknowledged. Conclusion This paper presents: (1) the synthesis of poly(vim-co-an)s with different compositions by radical copolymerization of respective monomers with AIBN initiator in benzene at 70 8C under a nitrogen atmosphere; [1] R. Lemque, C. Vidal-Madjar, M. Racine, J. Piquion, B. Sebille, J. Chromatogr. 1991, 553, 165. [2] P. Cysewski, A. Jaulmes, R. Lemque, B. Sebille, C. Vidal- Madjar, G. Jilde, J. Chromatogr. 1991, 548, 61. [3] C. G. Overberger, T. J. Pacansky, J. Polym. Sci. Polym. Symp. 1974, 45, 39.

Synthesis and Characterization of Poly(N-vinylimidazole-co-acrylonitrile) and Determination of Monomer Reactivity Ratios 1095 [4] C. G. Rounds, W. D. Rounds, F. E. Regnier, J. Chromatogr. 1987, 397, 25. [5] J. A. Alpert, F. E. Regnier, J. Chromatogr. 1979, 185, 375. [6] G. Jilde, B. Sebille, C. Vidal-Madjar, R. Lemque, K. K. Unger, Chromatographia 1993, 37, 603. [7] R. F. C. Bay, S. P. Armes, C. J. Pickett, K. S. Ryder, Polymer 1991, 32, 2456. [8] J. F. Kirsch, W. P. Jencks, J. Am. Chem. Soc. 1964, 86, 833. [9] D. H. Gold, H. P. Gregor, J. Phys. Chem. 1960, 64, 1461. [10] T. Miyajima, H. Nishimura, H. Kodama, S. Ishigura, React. Funct. Polym. 1998, 38, 183. [11] K. L. Petlak, J. Polym. Sci. Polym. Lett. Ed. 1978, 16, 393. [12] S. Machida, Y. Shimizu, T. Makita, Japan Tappi J. 1967, 21, 352. [13] M. Araki, K. Kato, T. Koyanagi, S. Machida, J. Macromol. Sci. Chem. 1977, 11, 1039. [14] C. Göksel, B. Hacoǧlu, U. Akbulut, J. Polym. Sci. Part A: Polym. Chem. 1997, 35, 3578. [15] G. Saini, A. Leoni, C. Franco, Makromol. Chem. 1971, 144, 235. [16] [16a] Polymer Handbook, 3rd edition, J. Brandrup, E. H. Immergut, Eds., Wiley, New York 1989, p. II/271; [16b] Polymer Handbook, 3rd edition, J. Brandrup, E. H. Immergut, Eds., Wiley, New York 1989, p. II/184. [17] J. S. Tsi, C. H. Lin, J. Mater. Sci. Lett. 1990, 9, 869. [18] A. K. Gupta, D. K. Paliwal, P. Baipai, J. Macromol. Sci. Rev. Macromol. Chem. 1991, C31,1. [19] C. Zhang, R. D. Gilbert, R. E. Fornes, J. Appl. Polym. Sci. 1995, 58, 2067. [20] P. Li, H. Shan, J. Appl. Polym. Sci. 1995, 56, 877. [21] P. Baipai, T. V. Sreekumar, K. Sen, Polymer 2001, 79, 1707. [22] R. Devasia, C. P. R. Nair, K. N. Ninan, Eur. Polym. J. 2002, 38, 2003. [23] J. M. G. Cowie, I. J. McEwen, D. J. Yule, Eur. Polym. J. 2000, 36, 1795. [24] K. Plochocka, J. Macromol. Sci.-Rev. Macromol. Chem. Phys. 1981, C20(1), 67. [25] N. Pekel, N. Şahiner, O. Güven, Radiat. Phys. Chem. 2000, 59, 485. [26] N. Pekel, N. Şahiner, O. Güven, Z. M. O. Rzaev, Eur. Polym. J. 2001, 37, 2443. [27] M. Baştürkmen, Z. M. O. Rzaev, G. Akoval, D. Ksakürek, J. Polym. Sci. Part A: Polym. Chem. 1995, 33,7. [28] S. Dinçer, V. Köseli, H. Kesim, Z. M. O. Rzaev, E. Pişkin, Eur. Polym. J. 2002, 38, 2143. [29] I. Iliopoulos, R. Audebert, J. Polym. Sci. Part B: Polym. Phys. 1988, 26, 2093. [30] N. L. Mazyar, V. V. Annenkov, V. A. Kruglova, D. S. D. Toryashinova, E. N. Danilovtseva, Polym. Sci. U.S.S.R. 1999, A41, 246. [31] V. Barboiu, E. Streba, M. N. Holerca, C. Luca, J. Macromol. Sci. Chem. 1995, A32, 1385. [32] B. B. Dambatta, J. R. Ebdon, T. N. Hukerby, Eur. Polym. J. 1984, 20, 645. [33] M. Fineman, S. D. Ross, J. Polym. Sci. 1950, 5, 259. [34] T. Kelen, F. Tüdös, J. Macromol. Sci. Chem. 1975, A9,1. [35] M. Dube, R. Amin Sanayei, A. Penlidis, K. F. O Driscoll, P. M. Reilly, J. Polym. Sci. Part A: Polym. Chem. 1991, 29, 703. [36] C. W. Pyun, J. Polym. Sci. Part A: Polym. Phys. 1970, 8, 1111. [37] E. G. Chatzi, O. Kammona, A. Kentepozidou, C. Kiparissides, Macromol. Chem. Phys. 1997, 198, 2409.