Chemical initiation mechanism of maleic anhydride grafted onto styrene butadiene styrene block copolymer

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1 European Polymer Journal 39 (2003) Short communication Chemical initiation mechanism of maleic anhydride grafted onto styrene butadiene styrene block copolymer Zhang Aimin *, Li Chao The State Key Laboratory of Polymer Material Science and Engineering, Sichuan University, Chengdu , China Received 13May 2002; received in revised form 13November 2002; accepted 1 November 2002 Abstract The mechanism of grafting styrene butadiene styrene (SBS) tri-block copolymer with maleic anhydride (MAH) initiated by benzoperoxide (BP) or 2,2 0 -azo-bis-isobutyronitrile (AIBN) was studied by FTIR and 1 H NMR spectroscopies. The variation of C@C (double bond) content in SBS-g-MAH was used to verify the different graft mechanisms of BP and AIBN, indicating that the chemical initiation mechanisms of MAH grafted onto SBS of AIBN is different from that of BP. The graft reaction occurs by addition on C@C for AIBN, while by removal of an allylic hydrogen atom from SBS and by addition on C@C at the same time for BP. The graft efficiency of AIBN is higher than that of BP in this system. Ó 2003Elsevier Science Ltd. All rights reserved. Keywords: SBS; MAH; Mechanism 1. Introduction The introduction of polar constituents onto hydrophobic polymers is known to result in a considerable improvement in physical chemical properties. Several studies have appeared dealing with grafting of vinyl monomers onto polymers such as styrene butadiene block copolymers [1], styrene isoprene copolymers [2], styrene (ethylene-co-butene) styrene tri-block copolymers [3], poly-cis-butadiene rubber (PcBR) [], and acrylonitrile butadiene styrene (ABS) terpolymer [5,6]. In all cases the effects of time, temperature and concentration have been studied so that a good understanding of the grafting system is available, but a problem such as the site of these graft reactions remains unsure. Wilkie and co-workers [1,6] indicated that, both BP and AIBN function by removal of an allylic hydrogen atom * Corresponding author. Tel.: ; fax: address: amzhang@mail.sc.cninfo.net (Zhang Aimin). or by addition onto C@C; Mrrov and Velichksva [2] showed that MAH was grafted onto SIS only by removal of an allylic hydrogen atom; Madhusudan Rao and Raghunath Rao [5] showed that MAH grafted onto ABS by adding on the double bond initiated by BP; Kang and co-workers [] showed that BP functioned by removal of an allylic hydrogen atom while AIBN functioned by addition on the double bond. In this paper we studied the reaction of MAH with SBS in the presence of BP or AIBN as initiator by means of FTIR and 1 H NMR spectroscopies. Evidence will be presented to establish the site of initiation of graft copolymerization. 2. Experimental 2.1. Materials SBS was supplied by YueYang Petrochemical Company as YH-791 and contains about 70% butadiene. Solvents (trichloroethylene, butanone), MAH and BP are all analytically pure and were used as received; /03/$ - see front matter Ó 2003Elsevier Science Ltd. All rights reserved. doi: /s (02) 转载

2 1292 Zhang Aimin, Li Chao / European Polymer Journal 39 (2003) AIBN was recrystallized from ethanol and dried in a desiccator Grafting procedure Grafting was carried out in a mixed solvent of trichloroethylene and butanone (25:75 ml) at 0 C in a flask equipped with stirrer, thermometer, condenser and nitrogen inlet. SBS 3.5 g and MAH.9 g were dissolved in 100 ml mixed solvent, and a portion of initiator BP or AIBN was added. The reaction was terminated after 1 h. Then, the volatile component was removed by reduced pressure distillation, the nonvolatile sample was recovered and air-dried at room temperature. The residual MAH was extracted by methyl alcohol, and the grafted SBS was obtained after drying Characterization FTIR spectra were obtained from polymer films on a Nicolet-560 spectrometer, the scan time was 20 and the resolution was cm 1. The films used for spectroscopic studies were cast on a KBr cuvette from chloroform solutions and dried before testing. Quantitative analyses were made by the peak area ratio of the carbonylstretching region ( cm 1 ), the olefinic C H out-of-plane bending vibration of the trans-c@c of butadiene units region (97 95 cm 1 ), the olefinic C H out-of-plane bending vibration of the cis-c@c of butadiene units region (76 cm 1 ) and the vibration stretching of region ( cm 1 ). The ring breathing stretching region ( cm 1 ) of styrene units was used as the internal standard peak. The peak area of the cis-c@c was calculated by a peak resolution program because the peak c ¼CAH (76 cm 1 ) was overlapped with the peak characteristic of PS d (759 cm 1 ). 1 H NMR spectra were recorded on a Varian Unity Inova-00 MHZ spectrometer at 00 MHZ, CDCl 3 was used as solvent and tetramethylsilane (TMS) as internal standard. The acquisition time was 3.5 s with a relaxation delay of 1 s, the rotation speed of sample cell was 20 rpm, the scan time was 150, and the signal enhancement was Results and discussion 3.1. Indication of grafting From the FTIR analysis result we know that weak asymmetric and symmetric carbonyl vibration at 157, 170 cm 1 and the C C vibration at the cm 1 region together indicate that MAH has been grafted onto SBS [,5]. This can also be proved by Fig. 2. The absorbance peak at 1737 and 1712 cm 1 was assigned to carboxyl (hydrolyzed MAH) [7] and the product of photo or thermal degradation Graft efficiency of different initiators Fig. 1 shows the relationship between the graft ratio of MAH onto SBS and the concentration of initiators. The graph indicates that the grafting yields are increased with an increase of initiator concentration. The same results were observed in grafting reactions onto ABS [5], SBS [1] and cis-pb []. It also shows that AIBN has a higher grafting efficiency than BP in this system Site determination of grafting onto SBS Qualitative FTIR analysis In Fig. 2, the carbonyl bond at 1776 cm 1 of the sample initiated by AIBN, which is the characteristic C= /A AIBN BP Concentration of initiator, wt% Fig. 1. Comparison of initiation efficiency between BP and AIBN m C@ (170 and 173 cm 1 ), A 0 : interior label peak. (a) AIBN (b) BP σ (cm -1 ) Fig. 2. FTIR spectra of SBS-g-MAH of different initiators: (a) AIBN (2 wt.%); (b) BP (2 wt.%).

3 Zhang Aimin, Li Chao / European Polymer Journal 39 (2003) peak of the anhydride, shifts to somewhat lower numbers wave and becomes broader compared with the sample initiated by BP. The carbonyl group at 1735 cm 1 in the sample initiated by AIBN is stronger than that of BP, and the 2220 cm 1 peak characteristic of the cyanide group of the initiator was not observed in the spectrum of the sample initiated by AIBN. Mrrov and Velichkova [2] believed that as no peak characteristic of the cyanide group around 2220 cm 1 was found, the graft reaction should occur by removal of an allylic hydrogen atom from the butadiene block of SBS by AIBN according to the reaction (A), but it cannot explain the difference around 170 cm 1 in the FTIR spectrum. From the earlier observations [2,] and the observation of the FTIR spectrum of different samples, we propose that the graft reaction may occur by addition on the C@C bond initiated by AIBN according to the reaction (B). This is because cyanide was hydrolyzed by methanol when post treating in an acid environment, which results in more carboxyl groups on the grafting product. Inter-molecular hydrogen bonds may form when the hydrogen proton of a carboxyl group acts as the electron accepter while the oxygen atom of the anhydride acts as the donor, which causes the carbonyl bond at 170 cm 1 to broaden and shift to a lower number wave []. (A) Abstraction of a-hydrogen (allylic hydrogen) from the PB main chain by the initiator radical: fa (C=C)/A 0 (a) fa (C=C)/A 0 (b) cis total trans Concentration of BP, wt % 0 2 Concentration of AIBN, wt % cis total trans Fig. 3. Effect of graft ratio on the structure of SBS: cis: c cis@cah (76 cm 1 ), trans: c trans@cah (966 cm 1 ), total: (3006 cm 1 ), A 0 : interior label peak. (a) initiated by BP; (b) initiated AIBN f A (trans-c=c) / A 0 (trans-c=c) / A 0 + R C H 1 MAH RH (B) Addition of the radical to the double bond of the chain: C H R R MAH 7 2 R 2

4 129 Zhang Aimin, Li Chao / European Polymer Journal 39 (2003) (a) cis-1 trans trans-2,3 cis-2,3 PS CL δ (ppm) (b) δ (ppm) Fig.. 1 H NMR spectra of SBS-g-MAH initiated by (a) AIBN (2 wt.%); (b) BP (2 wt.%). 3.. Quantitative FTIR analysis It can be seen that the content of C@C in PB block of SBS-g-MAH varies as the initiator concentration increases (Fig. 3a and b). In Fig. 3a, as the concentration of BP increases, the content of butadiene C@C first decreases and then increases. In Fig. 3b, the content of butadiene C@C decreases with the increase of AIBN concentration. For BP, the graft reaction should occur by replacement of an allylic hydrogen atom and addition on the C@C of the butadiene portion of SBS at the same time. BP is more effective to an allylic hydrogen atom than to butadiene C@C for the graft reaction. When the concentration of BP increases, more and more allylic carbon atoms are occupied by MAH, so the butadiene C@C are protected because of steric hindrance, as a result the content of butadiene C@C then increases. This result agrees partly with previous studies [1, 6]. For AIBN, because the graft reaction should occur only by addition on the butadiene C@C, the content of butadiene C@C decreases as the concentration of AIBN increases. This is in agreement with Kang and co-workers [] while different from Mrrov and Velichksva [2] H NMR analysis The 1 H NMR spectra of SBS-g-MAH initiated by AIBN and BP are shown in Fig. a and b. The 1 H NMR assignments of SBS-g-MAH can be see from Fig. a and b together with the radical reactions formula. In Fig. a, besides the expected signals for the block copolymer, small peaks at 3.67 and 3.7 ppm appear in SBS-g-MAH initiated by AIBN, which could be assigned respectively to the methyne proton and the twomethylene protons of the succinic anhydride ring [2]. But in Fig. b, the chemical shifts of the succinic anhydride ring initiated by BP appeared at 3.796, 3.67, 3.7 ppm, among which the peak at 3.67 ppm is strong and wide. This is because the intensities and chemical shifts of the resonance are those expected of succinic anhy-

5 Zhang Aimin, Li Chao / European Polymer Journal 39 (2003) dride rings attached at various positions along the polybutadiene chain. When the succinic anhydride added onto the allylic carbon atom, the hydrogen proton on the anhydride ring would be in the deshielding region of the carbon carbon double bond, which would appear at lower field with higher chemical shifts (3.67, ppm). When the succinic ring added onto C@C, the hydrogen proton on the anhydride ring would appear at lower chemical shifts (3.7, 3.67 ppm) [9]. bservations of FTIR and 1 H NMR together with the proposed mechanism made us conclude that BP may function by removal of an allylic hydrogen atom and by addition on C@C while AIBN by addition on C@C only.. Conclusion The graft reaction occurs by addition on C@C when initiated by the AIBN radical while the BP radical functions by removal of an allylic hydrogen atom from SBS and by addition on C@C. Initiation of an allylic site must occur much more often as does initiation of C@C for BP. AIBN has higher grafting efficiency for grafting MAH onto SBS than BP. Acknowledgements This work was supported by the National Natural Science Foundation of China (299000) and 973 (G ). References [1] Jiang DD, Wilkie CA. Chemical initiation of graft copolymerization of methyl methacrylate onto styrene butadiene block copolymer. J Polym Sci, Part A: Polym Chem 1997;35: [2] Mrrov Z, Velichkova R. Modification of styrene isoprene block copolymer-3. Addition of maleic anhydride-mechanism. Eur Polym J 1993;29(): [3] Passaglia E, Ghetti S, Picchioni F, Ruggeri G. Grafting of diethyl maleate and maleic anhydride onto styrene b(ethylene co-1 butene) b styrene tribolck copolymer (SEBS). Polymer 2000;1: [] Jing Sheng, Xiao long lu, Kang De Yao. Investigaton of graft polymerization of maleic anhydride onto polybutadiene rubber. J Macromol Sci-Chem 1990;27(2): [5] Madhusudhan Rao B, Raghunath Rao P. Grafting of maleic anhydride onto acrylonitrile butadiene styrene terpolmer: synthesis and characterization. Polym Plast Technol Eng 1999;3(5): [6] Chandrasiri JA, Wilkie CA. Chemically initiated graft copolymerization of acrylic acid onto acrylonitrile butadiene styrene (ABS) terpolymer and its constituent polymers. J Polym Sci, Part A: Polym Chem 1996;3: [7] Gu H, Chai C. Solid phase graft copolymerization on PP powder surface. Cai Liao Gong Cheng 1999;1:10 3. [] Xue Q. Gao Fen Zi Jie Gou Yan Jiu Zhong De Guang Pu Fang Fa. Advanced Education Press; p [9] Zhao TZ. He Ci Gong Zhen Qing Pu. Bei Jing University Press; 193. p