A novel synthesis of acrylic acid containing polymers

Macromol. Rapid Commun. 18,385-391 (I 997) 385 A novel synthesis of acrylic acid containing polymers Maoliang Xianga), Ming Jiang *a), Xiangming Kongb), Yiqing Yangb), Wenkui Lub) a) Institute of Macromolecular Science and Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, China b, Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China (Received: December 18, 1996) SUMMARY A novel synthesis of linear acrylic acid containing polymers, poly(styrene-co-acrylic acid) and poly(acry1ic acid), was accomplished through hydrolysis of the respective parent polymers, i. e. poly(styrene-co-methyl acrylate) and poly(methy1 acrylate), with trimethylsilyl iodide under mild conditions. Combination of 'H NMR, 13C NMR, FTIR, DSC and chemical titration confirms that the conversion from methoxycarbonyl to carboxyl is almost complete. This method is further successfully applied to synthesize poly- (ethyl methacrylate-co-acrylic acid) through selective hydrolysis of the methyl acrylate units in poly(ethy1 methacrylate-co-methyl acrylate). Introduction Acrylic acid containing homopolymer and copolymers are of interest on both practical and theoretical grounds'). However, preparation of these copolymers is not an easy job. Free-radical polymerization of the corresponding monomers usually gives product with broad molecular weight and/or composition distribution2), and the system becomes heterogeneous as the fraction of acrylic acid in the feed increases. Although template polymerization can produce (meth)acrylic acid homopolymer, it is hard to separate it from the formed complex3). Such technique was also used in copolymerization of methacrylic acid and styrene, however, it usually leads to a copolymer with long average styrene and methacrylic acid sequences4). With respect to block copolymers, sequential anionic polymerization is generally not amenable to polar and/or ionic monomers5). It was reported that poly- ((meth)acrylic acid) and low molecular weight poly(styrene-b-methacrylic acid) could be obtained through hydrolysis of the corresponding ester polymers under either acidic or basic aqueous conditions6). However, attempts to prepare rather high molecular weight (meth)acrylic acid containing copolymers, either random or block, were unsuccessful under such conditions7), probably due to the existence of unreactive hydrophobic monomer units. Therefore, it is still necessary to develop a general method to prepare random or block copolymers containing (meth)acrylic acid. In case of small molecules, it is noted that trimethylsilyl iodide (TMSI) is effective to cleave ether or ester groups under mild conditions to yield the corresponding hydroxyl or carboxyl groups in high Recently, this reaction has been successfully transferred to, and developed for, synthesizing hydroxyl containing polymers in our laboratory'*). Then, our next aim is to extend this method to synthesis of carboxyl containing polymers. In the case of small molecules, Jung et al9) reported 0 1997, Hiithig & Wepf Verlag, Zug CCC 1022-1336/97/$02.50

386 M. Xiang, M. Jiang, X. Kong, Y. Yang, w. Lu the successful hydrolysis of the esters of aromatic acids and most aliphatic acids without a-substituent. In addition, methyl 1-adamantylate, which is the only ester with a-substituent mentioned, can be hydrolyzed to get isolated 1 -adamantanecarboxylic acid in high yield (90%) as well. As to polymeric systems, it was reported that t-buty17' or benzyl") ester containing polymers can be readily hydrolyzed by TMSI to produce the corresponding carboxyl containing polymers. Since those group-protected monomers are expensive and not easy to synthesize, studies on hydrolysis of other readily available alkyl esters, which have never been reported in the literature, would be interesting and worthwhile. In this communication, we report the successful synthesis of acrylic acid containing polymers via hydrolysis of the corresponding acrylate containing polymers. Experimental part Materials Styrene, methyl acrylate, methyl methacrylate, and ethyl methacrylate were distilled at reduced pressure in the presence of calcium hydride just prior to use. TMSI was synthesized as described previouslyg9 lo). Chloroform was washed thrice by deionized water, pre-dried with anhydrous calcium chloride, refluxed with P2O5 and distilled just before use. Other chemicals were used as received. Syntheses Preparation of the parent polymers: Poly(styrene-co-methyl acrylate) (SMA) and poly(styrene-co-methyl methacrylate) (SMMA) were prepared in bulk through radical copolymerization of the respective monomers with azoisobutyronitrile (AIBN) as the initiator. The composition of these copolymers was determined by 'H NMR spectroscopy from the ratio of the integral of methoxy peak (6 = 2.95-3.55 ppm, -COOm3) to that of the aromatic protons (S = 6.5-7.4 ppm). Poly(ethy1 methacrylate-co-methyl acrylate) (EMA-MA) and poly(methy1 acrylate) (PMA) were synthesized in benzene with AIBN as the initiator. The composition of EMA-MA was evaluated by 'H NMR spectroscopy from the ratio of the integral of methoxy peak (6 = 3.6 ppm) to that of ethoxy peak (6 = 4.0 ppm, -COOCCCH3). Hydrolysis of SMA, SMMA and EMA-MA: A 250 ml three-necked round bottom flask equipped with a condenser and a nitrogen inlet was charged with 1.0 g copolymer and 100 ml freshly distilled chloroform. After the copolymer was completely dissolved, 3 equiv. TMSI (relative to the moles of methoxycarbonyl in these copolymers) was added under nitrogen atmosphere. After stirring the mixture at 50 C for 2 days, several milliliters of H20/THF solution (2/8 v/v) were added to the resulting homogeneous dark red mixture. The color disappeared after sufficient saturated aqueous Na2S203 was added. The polymer, obtained after removing chloroform with a rotatory evaporator, was dissolved in THE After the insoluble yellow particles of sulfur were separated by centrifugation, the filtrate was precipitated into deionized water. The dissolution-precipitation cycle was repeated twice more. The final polymer was dried in vacuo at 80 C for 24 h in the presence of P2O5. Hydrolysis of PMA: To a solution of 0.5 g PMA in 100 ml freshly distilled chloroform, 5 equiv. TMSI was added under nitrogen atmosphere. The color of the mixture immediately turned yellow, then gradually to red in the first 2 h. After the homogeneous mixture

A novel synthesis of acrylic acid containing polymers 387 was further stirred at 50 C for 2 days, it was combined with sufficient H20/THF (218 v/v) mixture, then with saturated aqueous Na2S203. The solvent chloroform was removed using a rotatory evaporator. The remaining solid material was dissolved in N,N-dimethylformide (DMF), leaving insoluble yellow particles, which were then separated by centrifugation. The polymer, recovered via adding the clear DMF solution into a large amount of acetone, was dissolved in diluted NaOH solution, followed by repeated dialysis against deionized water with stirring. After acidifying with hydrochloric acid and removal of water, the resulting polymer was thoroughly dried in vucuo at 80 C for 24 h in the presence of P205. Characterization 'H NMR and 13C NMR spectra of polymers were recorded on a Bruker AMX-400 NMR spectrometer with tetramethylsilane as the internal standard. Infrared spectra were acquired on a Magna 550 FTIR spectrophotometer. The IR samples were prepared via solvent casting from THF or deionized water. Molecular weights were measured via size exclusion chromatography (SEC) using a Waters Model 510 pump and ERMA ERC- 7512 (Tokyo) refractive index detector. The stationary phase consisted of lo2, lo3, lo4, lo5, lo6 A pstyrage1 columns (Polymer Standards Service); THF was used as the mobile phase at a flow rate of 1.0 mjjmin. Narrow molecular weight polystyrene standards (Polyscience Inc.) were used for calibration. Thermal analysis was assessed on a Shimadzu DSC-50 instrument under nitrogen atmosphere at a heating rate of 10 "Clmin. The temperature corresponding to half of the heat capacity change was taken as the glass transition temperature (Tg). Acid content of the hydrolyzed PMA was calculated from its titration in water by 0.1 N aqueous NaOH with phenolphthalein as indicator. Titrations of acid contents of other hydrolyzed copolymers were done in THF using 0.1 N aqueous NaOH titrant. Results and discussion Reaction of SMA and PMA with TMSI It was found that the hydrolyzed product (SMA-h) of SMA could not be dissolved in chloroform, which is a good solvent of SMA. This implies that SMA-h has quite different properties to SMA. Fig. 1 shows the 'H NMR spectra of SMA and SMA-h. A i Fig. 1. 'H NMR spectra of (a) SMA in CDC13 and (b) SMA-h n / \ 12 10 8 6 4 2 0 6 in ppm

388 M. Xiang, M. Jiang, X. Kong, Y. Yang, W. Lu The ester methyl of SMA gives rise to a rather broad peak at the range of 2.95-3.55 ppm (Fig. la), which could be attributed to the effect of adjacent phenyl ring"). In Fig. lb, this broad peak disappears and a new small peak appears at about 12 ppm. There exists a sharp peak at 3.35 pprn which is caused by the impurity of the solvent (perdeuterated dimethyl sulfoxide, DMSO-d6). The FTIR spectrum of SMA exhibits a strong band at 1734 cm-', while that of SMA-h shows a strong band at 1703 cm-', and a new broad band at 3500-2500 cm-' as well. All these data indicate that the cleavage of methyl ester takes place and carboxyl groups are formed. Further, the acid content of SMA-h determined by titration (Tab. 1) is in good agreement with the methyl acrylate content in the parent copolymer SMA. This result strongly supports the complete cleavage of methyl acrylate. In addition, the T, of SMA-h is 43 C higher than that of SMA, and the apparent molecular weight of SMA-h is only one-third of that of SMA. Both of the changes can be attributed to the existence of intramacromolecular association of carboxylic acid in SMA-h, since TMSI never causes a molecular weight change in such mild conditions7). Tab. 1. Compositions, number-average molecular weights (@,) and glass tansition temperatures (T,) of the (co)polymers Before hydrolysis After hydrolysis Sample x an x T~ ("c) Sample Y,wn x 10-~ T~ (T) in %a) in %b) SMA 21.7 57.9 86.0 SMA-h 20.8 18.9 128.8 SMMA 15.1 39.9 104.6 SMMA-h - 39.3 104.9 EMA-MA 15.3 55.6 62.2 EMA-MA-h 14.1 17.1 96.4 PMA 100 178.8 17.0 PMA-h 94.0-103.5 a) Molar percentage of methoxycarbonyl in the (co)polymers evaluated from 'H NMR. b, Molar percentage of acrylic acid in the (co)polymers determined by titration. From the results mentioned above, it can be concluded that methyl acrylate containing copolymer could be quantitatively hydrolyzed to acrylic acid containing copolymer under mild conditions. Then, what is the hydrolysis behavior of methyl acrylate homopolymer? Fig. 2 shows the 'H NMR spectrum of the sodium salt of PMA hydrolyzed product (PMA-h-Na) in deuterated water (D,O). Obviously, the peak of ester methyl at 3.6 ppm greatly diminishes and becomes very small relative to that of backbone protons. Moreover, I3C NMR results show that- the peaks of methoxycarbonyl at 174.9 ppm and methoxyl at 51.7 pprn disappear, while a new peak at 185.6 ppm appears in the spectrum of PMA-h-Na. This new peak could clearly be attributed to the carbon atom in sodium carboxylate. FTIR spectrum of PMA-h-Na film exhibits a strong new band at 1567 cm-', which is consistent with that of carbonyl in sodium carboxylate. Meanwhile, there exists a weak shoulder-like band at 1715 cm-' but no apparent peak at 1733 cm-', the characteristic band of ester carbonyl. Based on these data, it can be concluded that the hydrolysis of PMA by TMSI is believed to be almost complete.

A novel synthesis of acrylic acid containing polymers 3 89 Fig. 2. 'H Nh4R spectrum of PMA-h-Na in D20; *solvent peak 5 ti in ppm 4 3 2 1 In addition, the acid content of PMA-h determined by titration (Tab. 1) is somewhat less than, but quite close to, that expected. The Tg of PMA-h is completely different to that of PMA, and is about the same as that of conventional poly(acry1ic acid)13). Selective hydrolysis of EMA-MA The hydrolysis of methyl methacrylate containing polymer, SMMA, was performed. It was interesting to see that the reaction of SMMA with TMSI in chloroform for 2 days was totally different from that of SMA at the same conditions. The hydrolyzed product of SMMA was spectroscopically identical to the starting copolymer, and even the molecular weight did not change (see Tab. 1). It is consistent with the case of attempted hydrolysis of poly(styrene-b-methyl methacrylate) with TMSI in methylene chloride7). We took advantage of this remarkable difference in hydrolysis between polymers containing acrylate and methacrylate to synthesize the random copolymer of acrylic acid and alkyl methacrylate by selectively hydrolyzing the corresponding parent copolymer by TMSI. Fig. 3 shows the 'H NMR spectra of EMA-MA and its hydrolyzed product (EMA-MA-h). The ethoxy peak of ethyl methacrylate remains in the spectrum of EMA-MA-h, while the methoxy peak of methyl acrylate disappears. The ratio of the integral of ethoxy peak at 4.0 ppm to that of total protons in the backbone and a- methyl remains constant (0.049) before and after reaction. It can be seen in Tab. 1 that the molar content of acid in EMA-MA-h was almost the same as that of methyl acrylate in EMA-MA, and the Tg of EMA-MA-h was 34 C higher than that of EMA-MA. All these data clearly demonstrate that the methyl acrylate can be selectively hydrolyzed to form acrylic acid, while ethyl methacrylate remains unchanged. Encouraged by the above results, our further target is to prepare acrylic acid containing amphiphilic block copolymers through hydrolysis of methyl acrylate containing block copolymers, which could be synthesized via living polymeri~ationl~). Such kind of mono-dispersed amphiphilic block copolymer is very interesting and

390 M. Xiang, M. Jiang, X. Kong, Y. Ymg, W. LU L r n A 1 ~ ~ ' ~ 1 ' ~ ' ~ 1 " ~ ~ I ~ ' ' ' I ' ~ ~ ~ I ' ' ' ~ I ~ ' ~ ~ I 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 6 in ppm Fig. 3. 'H NMR spectra of (a) EMA-MA in CDCl, and (b) EMA-MA-h in DMSO-d6; *solvent peak useful, especially for studies on the association and assembly behavior of macromolecules. In summary, a novel synthesis of linear acrylic acid containing polymers via hydrolysis of methyl acrylate was developed. The advantages of this procedure are as follows: 1) Methyl acrylate is cheaper and more readily available than the t-butyl and benzyl ester, which have widely been used as the piecursor of carboxyl. 2) Methyl acrylate can be quantitatively hydrolyzed to form acrylic acid at mild conditions; so acrylic acid containing random copolymers, as well as homopolymer, can be easily obtained. Particularly, since in copolymers containing acrylate and methacrylate units, acrylate units can be selectively hydrolyzed, this procedure makes it possible to synthesize copolymers containing acrylic acid and methacrylate units, which are usually difficult to produce. 3) This route has good prospects in preparing acrylic acid containing amphiphilic block copolymers. Acknowledgements: This work is supported by the National Natural Science Foundation of China and also by the National Key Projects for Fundamental Research "Macromolecular Condensed State ", the State Science and Technology Commission of China. '1 a) G. Riess, G. Hurtrez, l? Bahdur, in Encyclopedia of Polymer Science and Engineering; H. Mark, N. Bikales, C. Overberger, G. Menges, Eds., Wiley-Interscience, New York 1985, Vol. 2, pp. 324-434; b) H. L. Greenwald, L. S. Luskin, in Handbook of Water Soluble Gums and Resins, R. L. Davidson, Ed., McGraw-Hill Inc., New York 1980, Chapter 17, pp. 1-19 2, S. Omi, M. Iso, R. Tajima, K. Kaneko, M. Takesue, J. Polym. Sci., Part A: Polym. Chem. Ed. 32,571 (1994)

A novel synthesis of acrylic acid containing polymers 391 3, V. Y. Baronovsky, I. V. Kotlyarsky, V. S. Etlis, V. A. Kabanov, Eul: Polym. J. 28, 1427 (1992) 4, H. L. Frisch, Q. H. Xu, Macromolecules 25,5145 (1992) 5, A. Noshay, J. E. McGrath, in Block Copolymers: Overview and Critical Survey, Academic Press, New York 1977 6, a) C. W. Brown, I. F. White, J. Appl. Polym. Sci. 16, 2671 (1972); b) R. D. Allen, I. Yilgor, J. E. McGrath, in Coulombic Interactions in Macromolecular Systems, A. Eisenberg, F. E. Bailey, Eds., ACS Symposium Series No. 302, Washington DC 1986, p. 79 ) D. E. Bugner, Chapter 20 in Chemical Reactions on Polymers, J. L. Benham and J. F. Kinstle, Eds., ACS, Washington DC 1988 8, M. E. Jung, M. A. Lyster, J. 0%. Chem. 42,3761 (1977) 9, M. E. Jung, M. A. Lyster, J. Am. Chem. SOC. 99,968 (1977) lo) M. L. Xiang, M. Jiang, L. X. Feng, Macromol. Rapid Commun. 16,477 (1995) M. G. Perrott, B. M. Novak, Macromolecules 29, 1817 (1996) 12) A. Natansohn, A. Eisenberg, Macromolecules 20, 323 (1987) 13) J. Brandrup and E. H. Immergut, Eds., Polymer Handbook, Wiley-Interscience, New York 1989, Part VI, p. 215 14) E. Ihara, M. Morimoto, H. Yasuda, Macromolecules 28,7886 (1995)