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

European Polymer Journal 39 (2003) 1291–1295 www.elsevier.com/locate/europolj

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 610065, China Received 13 May 2002; received in revised form 13 November 2002; accepted 14 November 2002

Abstract The mechanism of grafting styrene–butadiene–styrene (SBS) tri-block copolymer with maleic anhydride (MAH) initiated by benzoperoxide (BPO) or 2,20 -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 BPO and AIBN, indicating that the chemical initiation mechanisms of MAH grafted onto SBS of AIBN is different from that of BPO. 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 BPO. The graft efficiency of AIBN is higher than that of BPO in this system. Ó 2003 Elsevier 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) [4], 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 BPO and AIBN function by removal of an allylic hydrogen atom

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 BPO; Kang and co-workers [4] showed that BPO 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 BPO 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

* Corresponding author. Tel.: +86-28-85405868; fax: +86-2885402465. E-mail address: [email protected] (Zhang Aimin).

SBS was supplied by YueYang Petrochemical Company as YH-791 and contains about 70% butadiene. Solvents (trichloroethylene, butanone), MAH and BPO are all analytically pure and were used as received;

0014-3057/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0014-3057(02)00371-3

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Zhang Aimin, Li Chao / European Polymer Journal 39 (2003) 1291–1295

AIBN was recrystallized from ethanol and dried in a desiccator.

assigned to carboxyl (hydrolyzed MAH) [7] and the product of photo or thermal degradation.

2.2. Grafting procedure

3.2. Graft efficiency of different initiators

Grafting was carried out in a mixed solvent of trichloroethylene and butanone (25:75 ml) at 80 °C in a flask equipped with stirrer, thermometer, condenser and nitrogen inlet. SBS 3.85 g and MAH 4.89 g were dissolved in 100 ml mixed solvent, and a portion of initiator BPO 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.

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 [4]. It also shows that AIBN has a higher grafting efficiency than BPO in this system.

16

/A 0

FTIR spectra were obtained from polymer films on a Nicolet-560 spectrometer, the scan time was 20 and the resolution was 4 cm1 . 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 (1695–1850 cm1 ), the olefinic C–H out-of-plane bending vibration of the trans-C@C of butadiene units region (987–945 cm1 ), the olefinic C–H out-of-plane bending vibration of the cis-C@C of butadiene units region (746 cm1 ) and the vibration stretching of the @CAH region (3016–2991 cm1 ). The ring breathing stretching region (1506–1483 cm1 ) 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 c¼CAH (746 cm1 ) was overlapped with the peak characteristic of PS dCH (759 cm1 ). 1 H NMR spectra were recorded on a Varian Unity Inova-400 MHZ spectrometer at 400 MHZ, CDCl3 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 16.

3.3.1. Qualitative FTIR analysis In Fig. 2, the carbonyl bond at 1776 cm1 of the sample initiated by AIBN, which is the characteristic

12

C=O

2.3. Characterization

3.3. Site determination of grafting onto SBS

8 AIBN BPO

4

0 0

1

2

3

4

5

Concentration of initiator, wt% Fig. 1. Comparison of initiation efficiency between BPO and AIBN mC@O (1780 and 1738 cm1 ), A0 : interior label peak.

1735

1776 1780

(a) AIBN

1737

3. Results and discussion 3.1. Indication of grafting (b) BPO

From the FTIR analysis result we know that weak asymmetric and symmetric carbonyl vibration at 1857, 1780 cm1 and the C–O–C vibration at the 1300–1100 cm1 region together indicate that MAH has been grafted onto SBS [4,5]. This can also be proved by Fig. 2. The absorbance peak at 1737 and 1712 cm1 was

2400

2200

2000

1800

1600

σ (cm-1) Fig. 2. FTIR spectra of SBS-g-MAH of different initiators: (a) AIBN (2 wt.%); (b) BPO (2 wt.%).

Zhang Aimin, Li Chao / European Polymer Journal 39 (2003) 1291–1295

peak of the anhydride, shifts to somewhat lower numbers wave and becomes broader compared with the sample initiated by BPO. The carbonyl group at 1735 cm1 in the sample initiated by AIBN is stronger than that of BPO, and the 2220 cm1 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 cm1 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 1780 cm1 in the FTIR spectrum. From the earlier observations [2,4] 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 1780 cm1 to broaden and shift to a lower number wave [8].

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f A (C=C) / A0

6.0 cis total trans

0.9

5.5

5.0 0.3

0

(a)

2

4

Concentration of BPO, wt % 7

f A (C=C) / A0

1.2 cis total trans

0.9

6

0.3 4 0

(b)

2

f A (trans-C=C) / A 0

0.6 5

(A) Abstraction of a-hydrogen (allylic hydrogen) from the PB main chain by the initiator radical:

(trans-C=C) / A0

0.6

4

Concentration of AIBN, wt %

Fig. 3. Effect of graft ratio on the structure of SBS: cis: ccis@CAH (746 cm1 ), trans: ctrans@CAH (966 cm1 ), total: m@CAH (3006 cm1 ), A0 : interior label peak. (a) initiated by BPO; (b) initiated AIBN.

CH 2

CH 3

CH2 1

+

CH2 4

+

C H

CH2

CH

MAH

CH

CH

R

CH

CH2

CH 5 O

6 O

O

(B) Addition of the radical to the double bond of the chain: CH CH2

+

CH2 O

O

R

O

MAH 8

CH

C H

CH

R 7

CH2

CH

CH CH2

CH2

R

CH2

RH

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Zhang Aimin, Li Chao / European Polymer Journal 39 (2003) 1291–1295

(a) cis-1 11

trans-1

12

CH2CH CH 9

CH2

trans-2,3 cis-2,3

10

PS CHCL3

8.5

12

8

10

9

11

7

6.5

4.5

2.5

δ (ppm)

0.0

(b)

3.796

3.687 3.474

6 7 5 8

8.5

6.5

4.5

δ (ppm)

2.5

0.0

Fig. 4. 1 H NMR spectra of SBS-g-MAH initiated by (a) AIBN (2 wt.%); (b) BPO (2 wt.%).

3.4. 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 BPO 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 BPO, 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. BPO is more effective to an allylic hydrogen atom than to butadiene C@C for the graft reaction. When the concentration of BPO 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 [1,4–6]. 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 [4] while different from Mrrov and Velichksva [2]. 3.4.1. 1 H NMR analysis The 1 H NMR spectra of SBS-g-MAH initiated by AIBN and BPO are shown in Fig. 4a and b. The 1 H NMR assignments of SBS-g-MAH can be see from Fig. 4a and b together with the radical reactions formula. In Fig. 4a, besides the expected signals for the block copolymer, small peaks at 3.674 and 3.478 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. 4b, the chemical shifts of the succinic anhydride ring initiated by BPO appeared at 3.796, 3.687, 3.474 ppm, among which the peak at 3.687 ppm is strong and wide. This is because the intensities and chemical shifts of the resonance are those expected of succinic anhy-

Zhang Aimin, Li Chao / European Polymer Journal 39 (2003) 1291–1295

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 (3.687, 3.796 ppm). When the succinic ring added onto C@C, the hydrogen proton on the anhydride ring would appear at lower chemical shifts (3.474, 3.687 ppm) [9]. Observations of FTIR and 1 H NMR together with the proposed mechanism made us conclude that BPO may function by removal of an allylic hydrogen atom and by addition on C@C while AIBN by addition on C@C only. 4. Conclusion The graft reaction occurs by addition on C@C when initiated by the AIBN radical while the BPO 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 BPO. AIBN has higher grafting efficiency for grafting MAH onto SBS than BPO. Acknowledgements This work was supported by the National Natural Science Foundation of China (29904004) and 973 (G1999064809).

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