Synthesis, Characterization, and Hydrolysis of PVAc-PS- PVAc via Charge Transfer Polymerization

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1 Synthesis, Characterization, and Hydrolysis of PVAc-PS- PVAc via Charge Transfer Polymerization ZAIJUN LU, XIAOYU HUANG, JUNLIAN HUANG Department of Macromolecular Science, Fudan University, The Open Laboratory of Molecular Engineering of Polymer, Education Ministry of China, Shanghai , China Received 22 October 1998; accepted 18 January 1999 ABSTRACT: An ABA triblock copolymer of polyvinyl acetate-b-polystyrene-b-polyvinyl acetate (PVAc-PS-PVAc) was successfully synthesized with a binary system composed of polystyrene with N,N-dimethylaniline end groups (PS da ) and benzophenone to initiate the polymerization of vinyl acetate under UV irradiation. The PS da was obtained by capping the living polystyrene macrodianion with p-(dimethylamino) benzaldehyde in excess. The PVA-PS-PVA could then be obtained by hydrolysis of PVAc-PS-PVAc in the sodium ethoxide benzene solution. The intermediates and desirable copolymers were characterized by GPC, IR, and 1 H-NMR in detail John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: , 1999 Keywords: triblock copolymer; polyvinyl acetate; polystyrene; hydrolysis; polyvinyl alcohol; capping reaction; photo-induced charge transfer polymerization INTRODUCTION Interest in block copolymers with poly(vinyl acetate) (PVAc) and polyvinyl alcohol (PVA) blocks is increasing, from both theoretical and experimental points of view. Some of these copolymers, such as PMMA-b-PVAc, can be used as compatibilizer for blend pairs with a lower critical solution temperature (LCST), such as PMMA/PVAc, 1 in which the blend pairs are homogeneous and miscible at low temperature and immiscible at high temperature. Some copolymers with hydrophobic and hydrophilic blocks, as well as crystalline and noncrystalline blocks, such as PS-b-PVA and PMMA-b-PVA, not only show the intriguing phenomenon of self-assembling as well as a unique morphology and phase structure, but also have found important applications in coatings, adhesives, thin film, stabilizers of dispersion polymerization, 2 pharmaceuticals, photographic technologies, oil recovery, an others. Correspondence to: Junlian Huang Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 37, (1999) 1999 John Wiley & Sons, Inc. CCC X/99/ In the preparation of block copolymers, anionic living polymerization is generally the most useful procedure. However, it is only suitable for monomers with low polarity such as styrene, butadiene, isoprene, and others, and is inapplicable for monomers such as vinyl acetate and vinyl alkyl ketone. Recently we reported a useful method for making block copolymers via combination of anionic with photo-induced charge transfer polymerization (CTP), in which p-aminophenol was used as a potential bifunctional initiator. A series of block copolymers, such as PEO-b-PS, 3 PEO-b-PVAc, 4 were prepared. In this report, a new route is suggested to prepare the triblock copolymer of PVAc-PS-PVAc and PVA-PS-PVA. It has universal significance for the preparation of other block copolymers with novel structures. EXPERIMENTAL Materials Styrene (St) was dried with calcium hydride, then distilled under reduced pressure. Tetrahydrofu- 2595

2 2596 LU, HUANG, AND HUANG ran (THF) was refluxed over solid KOH for 10 h, then dried with calcium hydride and distilled. Naphthalene (Shanghai Yuan Hang Chemical Factory), p-(dimethylamino) benzaldehyde (DMAB) (Shanghai Third Regent Factory) and benzophenone (BP) (Beijing Chemical Factory) were recrystallized twice from ethanol. Vinyl acetate (VAc) and benzene were distilled before used. Anionic Polymerization of Styrene Using Lithium Naphthalenide as Initiator Lithium naphthalenide was synthesized by the reaction of naphthalene with metallic lithium in anhydrous THF according to the procedure described. 5 The concentration was 1.4 mol/l. Anionic polymerization of styrene was performed with lithium naphthalenide in THF at 78 C for 0.5 h by the standard method, and an aliquot of the polymer solution was taken out for gel permeation chromatography (GPC) before the addition of DMAB. Capping Reaction of Polystyrene Macrodianion with DMAB The above-mentioned living macrodianion solution (60 ml, mol/l) was added dropwise to g ( mol) of DMAB at 50 C in 20 min, the system was stirred for 0.5 h at room temperature, then stopped by methanol in excess. The desirable product (PS da ) was purified by repeated dissolving/precipitating with chloroform/methanol and dried at 60 C for 4 h. Synthesis of Polystyrene with Aromatic Tertiary Amine at 1 End (PS s ) Polystyrene with aromatic tertiary amine at 1 end (PS s ) was prepared by the reaction of DMAB with poly (styryl) lithium obtained from n-butyllithium-initiated polymerization of styrene according to the reported method. 6 Preparation of PVAc-PS-PVAc In a 100-mL ampoule, g ( mol) PS da, g ( mol) benzophenone, 5 ml VAc, and 5 ml benzene were introduced. The ampoule was degassed 3 times by freeze-pump-thaw cycles, sealed under N 2, and irradiated by a highpressure mercury lamp (DDZ-300, Shanghai YaMing Lamp Factory) at 25 C for 9 h. The aqueous cupric sulfate solution was used as a photofilter to obtain 365-nm monochromatic light. The desirable products were precipitated with methanol and purified by extracting 24 h with cyclohexane to remove the unreacted PS da and another 24 h with methanol to remove possible remaining homopolymer of VAc. (PVAc was not detected.) Hydrolysis of PVAc-PS-PVAc To 300 ml 5% sodium ethoxide benzene was added 16 g ( mol) PVAc-PS-PVAc. Hydrolysis was conducted at 70 C with stirring for 12 h, and no acetoxy or acetyl groups remained in the hydrolyzed product, as shown by IR and NMR. Measurements The molecular weight and molecular weight distribution of the polymers and copolymers were measured with a Shimadzu LC-3A gel permeation chromatograph equipped with a UV detector, using CHCl 3 as an eluent and monodistribution polystyrene as standards. UV spectra were recorded on a 756 MC UV-Vis spectrophotometer Scheme 1.

3 SYNTHESIS OF PVAC-PS-PVAC VIA CHARGE TRANSFER POLYMERIZATION 2597 Figure 1. UV spectra of PS (A), DMA(B), and PS da (C). Solvent CHCl 3 ; concentration 1.7, 6.3, and mol/l for (A), (B), and (C), respectively. Figure 2. UV spectra of TCNE (A), PS da (B), and a mixture of TCNE and PS da [1:1 (mol/mol)] (C). Solvent CHCl 3 ; concentration mol/l for all samples. (Shanghai Third Analytical Instrument Factory). FT-IR spectra were scanned by a Perkin- Elmer 983G spectrometer. 1 H-NMR spectra were scanned on a JEOL FX-90Q spectrometer using CDCl 3 as solvent and tetramethylsilane (TMS) as internal standard. Scheme 2.

4 2598 LU, HUANG, AND HUANG Table I. Block Copolymerization Data Run Initiator Conc. of Initiator ( 10 3 mol/l) Conversion of VAc a (%) Conversion of PS b (%) Block Copolymer M n c ( 10 4 ) M w /M n 1 DMA d e PS s f PS ba VAc vinyl acetate; PS polystyrene; DMA dimethylaniline; BP benzophenone. [BP]: mol L; VAc: 5 ml; Benzene: 5 ml; Polymer time: 9 h. a wt % [weight of reacted monomer (g)/weight of total added monomer (g)] 100. b Determined by GPC, comparing peak areas of unreacted PS b and block copolymer. c Obtained by GPC (calibration with polystyrene standards). d N,N-dimethyl aniline. e Polystyrene with DMA at one chain end; M n 7,900, M w /M n f Polystyrene with DMA at both chain ends; M n 7,200, M w /M n RESULTS AND DISCUSSION Synthesis and Characterization of PS da The PS da was prepared by the reaction of PS macrodianions with DMAB in excess shown in Scheme 1. When the living polystyrene macrodianion solution was added to DMAB, the brownred color immediately changed to orange-yellow, suggesting that the reaction had proceeded rapidly. The UV spectra of common PS, N,N-dimethylaniline (DMA), and PS da are shown in Figure 1. By comparison with the A and B spectra, it was confirmed that the absorption band at 260 nm in C was attributed to 3 * for the benzene ring; the 315-nm band was attributed to n3 * for aromatic ternary amino groups. The existence of DMA end groups in PS da could also be deduced from the UV spectrum of the complex formed by tetracyanoethylene (TCNE) with PS da. In Figure 2, A and B represent the UV spectra of TCNE and PS da ; their maximum absorbance peaks were at 268, 277.5, and 315 nm, respectively. When TCNE and PS da were mixed in the ratio 2:1 (mol/ mol), however, new bands were observed at ca.400 and 418 nm, as shown in Figure 2 (C). These were attributed to the formation of a complex between TCNE and DMA end groups of PS da through charge transfer. This is a sensitive method to check if the aromatic amine groups existed in a molecule. 7 Using the linear relationship between the concentration of DMA and its absorbance, the capping efficiency of DMAB on PS macrodianions and the molecular weight of PS da could be calculated. The capping efficiency was nearly 100%, and the molecular weight was 7,600, which agrees with the 7,200 obtained by GPC. Synthesis and Characterization of PVAc-PS-PVAc The photo-induced charge transfer polymerization of VAc was carried out with the initiation mechanism shown in Scheme 2. With 365-nm UV irradiation, excited BP formed an exciplex with the DMA end groups of PS da first, then the diphenyl methanol radicals and aromatic tertiary amine radicals of PS da were produced through proton transfer. 8 The former was stable at room temperature and served as a chain terminant, 9 and the latter initiated the polymerization of VAc. To determine if polymerization of VAc was propagated from 1 end or 2 ends of the PS da,a Figure 3. Gel permeation chromatograms of block copolymers A (35 C) and B (25 C).

5 SYNTHESIS OF PVAC-PS-PVAC VIA CHARGE TRANSFER POLYMERIZATION 2599 Figure 4. IR spectra of PVAc-PS-PVAc (A) and hydrolyzed product PVA-PS-PVA (B). group of experiments using DMA/BP and PS s /BP as the initiation system (in which both have only 1 DMA group) were carried out under the same conditions except that DMA and PS s concentrations were twice that of PS da. The results are summarized in Table I. We observed that the molecular weights of formed PVAc were rather close for the systems of DMA/BP and PS s /BP; the former was and the latter was However, when the PS da /BP was used, the molecular weight of PVAc segments was ca , which is 5 times the molecular weight of the DMA/BP and PS s /BP systems. This phenomenon illustrated that the initiating species were bis Figure 5. PVA (B). 1 H-NMR spectra of PVAc-PS-PVAc (A) and hydrolyzed product PVA-PS-

6 2600 LU, HUANG, AND HUANG Scheme 3. radicals for the system of PS da /BP and the propagation of VAc was conducted from the both ends of PS chain. However, it was found that if the polymerization of VAc was carried out at a higher temperature ( 35 C), GPC of the product revealed 2 peaks [Fig. 3 (A)], which means that the coupling and disproportionating terminations might coexist, although the latter was dominant. When polymerization of VAc was conducted at 25 C, the peak with the smaller retention volume [Fig. 3 (B)] disappeared. Therefore, in our experimental conditions the resultant block copolymers were expected to be mainly the ABA type. It was also found by us 10 and other researchers 11 that only 1 methyl group for 1 DMA took part in the reaction; the other methyl groups are inert and nonreactive under the given experimental conditions. The structure of the block copolymers was confirmed by IR and 1 H-NMR measurements. In Figure 4 (A), the strong absorption at 1735 cm 1 (C O ) for PVAc blocks and at 3026, 1601, 1435, and 701 cm 1 (benzene ring) for PS showed the existence of PS and PVAc segments. The same conclusion could also be drawn from the Figure 5 (A) spectra in which all resonance signals corresponding to the structure of PS and PVAc segments appeared, such as those at (benzene ring), 1.84 ( CH connected to the benzene ring), and 1.42 ppm ( CH 2 ) for the PS block, and for the PVAc block, at 1.44 ( CH 2 ), 4.88 ( CH connected to CH 3 COO) and 2.07 ppm (CH 3 CO). Hydrolysis of PVAc-PS-PVAc The hydrolysis of PVAc-PS-PVAc (Scheme 3) was carried out successfully and thoroughly. All peaks associated with acetyl groups at 1735 cm 1 (IR) and at 2.07 ppm (NMR) disappeared after hydrolysis, and some new peaks attributed to hydroxyl group, such as at 3395 cm 1 [Fig. 4 (B)] and at 4.64 ppm [Fig. 5 (B)] were observed. We also observed no chain cleavage under the given conditions. We appreciate the financial support from the Natural Science Foundation of China and Doctor Training Foundation of Education Ministry of China. REFERENCES AND NOTES 1. Song, M.; Liang, H. S.; Jiang, B. Z. Polym Bull 1990, 23, Terada, K.; Miyazaki, H.; Yoshihara, M.; Sato, T.; Maruyama, H.; Okaya, T. Kobunshi Ronbunshu 1993, 50, Huang, J.; Huang, X.; Zhang, S. Macromolecules 1995, 28, Huang, J.; Huang, X.; Hu, W. Macromol Chem Phys 1997, 198, Morishima, Y.; Hashimoto, T.; Itoh, Y.; Kamachi, M.; Nozakura, S. J Polym Sci Polym Chem Ed 1982, 20, Quirk, R. P.; Alsamarrale, M. Ind Eng Chem Prod Res Dev 1986, 25, Du, F.; Zheng, P.; Li, F. Acta Polym Sin 1992, 3, Ghosh P. G.; Ghosh, R. Eur Polym J 1981, 17, Kubota H.; Ogiwara, Y. J Appl Polym Sci 1982, 27, Lu, Z.; Huang, X.; Huang, J. Macromol Rapid Commun 1998, 19, Ledwith, A. J Oil Col Chem Assoc 1976, 59, 157.