Mass Spectrometric Analysis for High Molecular Weight Synthetic

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Anal. Chem. 2007, 79, 4182-4187

Mass Spectrometric Analysis for High Molecular Weight Synthetic Polymers Using Ultrasonic Degradation and the Mechanism of Degradation Hideya Kawasaki, Yoshiki Takeda, and Ryuichi Arakawa*

Department of Applied Chemistry, Kansai University, Suita, Osaka 564-8680, Japan

We have investigated ultrasonic degradations of poly(ethylene oxide) (PEG) and poly(methyl methacrylate) (PMMA) in aqueous media by means of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The ultrasonic degradation of polymers was monitored as a function of ultrasonication duration to examine the structural details of ultrasonic degradation polymers. PEG solution ultrasonication produced five types of oligomers (M h ∼1000 Da) with different end groups, irrespective of the initial average molecular masses (M h ) 2, 6, 20, and 2000 kDa). Several degradation pathways with free radical reactions have been suggested to explain these degradation products: the ultrasonic degradation of PEG is initiated by breaking of the C-O bond in the PEG chain, generating polymeric radicals with two terminal groups, i.e., X•(∼CH2CH2•) and Y•(∼CH2CH2O•), followed by termination with extraction or release of a hydrogen atom. However, PMMA (M h ) 1630 Da) ultrasonication generated only one type of degradation oligomer, which has a hydrogen group at both ends, the same as that of the original oligomer. It has been suggested that the presence of the radical terminal groups X•(∼CH2•) and Y•(∼(CH3)CCOO(CH3)C•) is due to selective C-C bond breaking in the chain during the ultrasonic degradation of PMMA. The MALDI-TOFMS combined with the ultrasonic degradation technique (UD/MALDITOFMS) developed in this study could be extended to the analysis of synthetic polymer structures with high molecular weights. Ultrasound as a research tool has a relatively recent history.1,2 The most important recent aspects of sonochemistry include its applications to studying the synthesis and modification of both organic and inorganic materials.2-6 Sonochemical processes in * To whom correspondence should be addressed. Tel: +81-6-6368-0781. Fax: +81-6-6339-4026. E-mail: [email protected]. (1) Price, G. J. The use of ultrasound for the controlled degradation of polymer solutions. In Advances in sono-chemistry; Mason T. J., Ed.; Jai Press: Cambridge, 1990; Vol. 1. (2) Suslick, K. S.; Price, G. J. Annu. Rev. Mater. Sci. 1999, 29, 295. (3) McAskill, N. A.; Sangster, D. F. Aust. J. Chem. 1979, 32, 2611. (4) Anderson, C. D.; Sudol, E. D.; El-Aasser, M. S. Macromolecules 2002, 35, 574. (5) Bradley, M. A.; Prescott, S. W.; Schoonbrood, H. A. S.; Landfester, K.; Griester, F. Macromolecules 2005, 38, 6346. (6) Price, G. J. Ultrason. Sonochem. 1996, 3, S229.

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solutions can induce cavitations (the formation, growth, and collapse of bubbles).7-9 During cavitations, bubble collapse produces intense local heating, high pressures, and very short lifetimes. These transient, localized hot spots drive high-energy chemical reactions. The high temperatures also result in the homolysis of water within the bubbles, producing reactive hydroxyl radicals.3,10-12 The application of ultrasonication to the degradation of synthetic and biopolymers has drawn considerable attention1,13-19 because the molecular weight is an important characteristic of polymers. Another advantage of ultrasonic treatment is that no chemicals are used such that there is no need for further purification steps, and the process itself is very simple to perform. Moreover, under special conditions, it may be possible to obtain samples with a specific molecular weight distribution and even of a specific architecture. The molecular weight of polymers reportedly decreases drastically at first and then gradually approaches a limiting molecular weight.6,7 The exact mechanism of ultrasonic degradation remains uncertain, but it is suggested that cavitation bubbles are primarily responsible for the degradation of polymers. Additionally, a recent application of ultrasonic-induced polymer degradation is the production of block copolymers from a homopolymer in a different monomer or two different homopolymer mixtures.20,21 Thus, the ultrasonic degradation of polymers has been extensively studied to date. However, as yet, structural details of polymers induced by ultrasonic degradation have received little research attention, because in most cases the (7) Leighton, T. G. The Acoustic Bubble; Academic Press: London, 1994. (8) Suslick, K. S.; Hammerton, D. A.; Cline, R. E. J. Am. Chem. Soc. 1986, 108, 5641-5642. (9) Suslick, K. S. Sonochem. Sci. 1990, 247, 1439. (10) Tauber, A.; Mark, G.; Schuchman, H. P.; Sonntag, C. V. J. Chem. Soc., Perkin Trans. 2 1999, 1129. (11) Suslick, K. S., Ed. Ultrasound: Its Chemical, Physical, and Biological Effects; VCH: New York, 1988. (12) Schmid, G.; Rommel, O. J. Phys. Chem. 1939, 185, 97. (13) Jellonel, H. H. G. Degradation of vinyl polymers; Academic Press: New York, 1955. (14) Kuijpers, M. W. A.; Kemmere, M. F.; Keurentjes, J. T. F. Ultrasound-induced radical polymerization. In Encyclopedia of Polymer Science and Technology; John Wiley and Sons: New York, 2004. (15) Price, G. J.; Smith, P. F. Polymer 1993, 34, 4111. (16) Koda, S.; Mori, H.; Matsumoto, K.; Nomura, H. Polymer 1993, 34, 30. (17) Sivaligan, G.; Madras, G. Polym. Degrad. Stab. 2003, 80, 11. (18) Gro ¨nroos, A.; Pirkonen, P.; Ruppert, O. Ultrason. Sonochem. 2004, 11, 9. (19) Kuijpers, M. W. A.; Prickaerts, R. M. H.; Kemmere, M. F.; Keurentjes, J. T. F. Macromolecules 2005, 38, 1493. (20) Fujiwara, H. Polym. Bull. 2001, 47, 247. (21) Madras, G.; Karmore, V. Polym. Int. 2001, 50, 683. 10.1021/ac062304v CCC: $37.00

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ultrasonic degradation of polymers has been examined mainly with gel permeation chromatography (GPC) and viscometry to monitor changes in the molecular weight of polymers. The recent development of matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry (MALDI-TOFMS) for the analysis of polymers has dramatically increased the mass range; this modality can detect intact polymer molecules of high molecular masses, depending on the kinds of polymers.22-32 Ions in the mass spectra originate from ions of intact polymer chains, and therefore, MALDI-TOFMS demonstrates structural details of polymers: average molar masses, chemical formulas, the molecular structures of monomer repeat units, and the masses of chain end groups. Structural details for polymers can be difficult to obtain above 5-15 kDa, depending on the kinds of polymer and the molecular weight distribution. In most cases, therefore, thermal-decomposition GC/MS or MALDI-TOFMS combined with thermal-decomposition techniques have been widely applied to the analysis of polymers with high molecular weights.24,28-30 However, thermal decompositions of polymers often produced secondary products due to heat, oxygen, or both, thereby yielding complicated MS spectra. Thus, it would be advantageous to develop new methods, other than the traditional thermaldecomposition techniques, for the analysis of polymers with high molecular weights. The applications of MALDI to the photooxidation, enzymatic, and ozonolysis degradations of polymers have recently been reported.33-36 Herein, we have investigated the ultrasonic degradation processes of poly(ethylene oxide) (PEG) and poly(methyl methacrylate) (PMMA) in aqueous solutions by MALDI-TOFMS. The average molecular masses (M h ) of the polymers used were 2, 6, 20, and 2000 kDa for PEG and 1.6 kDa for PMMA. To the best of our knowledge, this is the first mass spectrometric investigation of the ultrasonic degradation of polymers. The application of MALDI-TOFMS to the ultrasonic degradation of polymers offers the opportunity to directly study the structural changes of polymers induced by ultrasonic degradations. In particular, we are interested in the end group determination of ultrasoundinduced polymers because of their critical importance to understanding the mechanism of ultrasonic degradation, ultrasoundinduced polymer synthesis, and end use application of the final (22) Bahr, U.; Deppe, A.; Karas, M.; Hillenkamp, F. Anal. Chem. 1992, 64, 2866. (23) Montaudo, M. S.; Puglisi, C.; Samperi, F. Macromolecules 1995, 28, 4562. (24) Puglisi, C.; Samperi, F.; Carroccio, S.; Montaudo, G. Macromolecules 1999, 32, 8821. (25) Quick, R. P.; Mathers, R. T.; Wesdemiotis, C.; Arnould, M. A. Macromolecules 2002, 35, 2912. (26) Nielen, M. W. Mass Spectrom. Rev. 1999, 18, 309. (27) Hanton, S. D. Chem. Rev. 2001, 101, 527. (28) Carroccio, S.; Puglisi, C.; Montaudo, G. Macromolecules 2005, 38, 6849. (29) Montaudo, G.; Samperi, F.; Montaudo, M. S. Prog. Polym. Sci. 2006, 31, 277. (30) Sakurada, N.; Fukuo, T.; Arakawa, R.; Ute, K.; Hatada, K. Rapid Commun. Mass Spectrom. 1998, 12, 1895. (31) Arakawa, R.; Shimomae, Y.; Morikawa, H.; Ohara, K.; Okuno, S. J. Mass Spectrom. 2004, 18, 961. (32) Okuno, S.; Arakawa, R.; Okamoto, K.; Matsui, Y.; Seki, S.; Kozawa, T.; Tagawa, S.; Wada, Y. Anal. Chem. 2005, 77, 5364. (33) Carrccio, S.; Puglisi, C.; Montaudo, G. Macromolecules 2004, 37, 6576. (34) Sato, H.; Kiyono, Y.; Ohtani, H.; Tsuge, S.; Aio, H.; Sio, K. J. Anal. Appl. Pyrolysis 2003, 68-69, 37. (35) Taguchi, Y.; Ishida, Y.; Ohtani, H.; Matsubara, H. Anal. Chem. 2004, 76, 697. (36) Zoller, D. L.; Johnston, M. V. Macromolecules 2000, 33, 1664.

Figure 1. MALDI-TOF mass spectra of PEG6000. Ultrasonication time for (a) 0, (b) 2, (c) 4, and (d) 8 h.

products. In addition, it is our goal to present a new approach to the mass spectrometric analysis of high molecular weight synthetic polymers using MALDI-TOFMS combined with an ultrasonic degradation technique (UD/MALDI-TOFMS). EXPERIMENTAL SECTION Materials. Tetrahydrofuran (THF), acetonitrile, sodium iodide, and PEG (average molecular mass, M h ) 2, 6, 20, and 2000 kDa) were obtained from Wako Pure Chemical (Osaka, Japan), and are hereafter denoted by PEG2000, PEG6000, and so on. PMMA (Mn ) 1630 Da, Mw/Mn ) 1.10) was obtained from Polymer Laboratories. R-Cyano-4-hydroxycinnamic acid (CHCA) was purchased from Sigma-Aldrich (Milwaukee, WI). Ultrasonic Degradation. Solutions of 1.8 mg/mL PEG or PMMA were made in water and an acetonitrile/water mixture (1/1, v/v), respectively, and 1.5 mL of each solution was ultrasonically degraded in a horn-type sonicator (UR-20P, Tomy Seiko) at 15 °C. For the degradation, a 1.5-mL polymer solution was placed in the jacket flask, and its temperature was controlled by the circulation of thermostated water and sonication for 2, 4, and 8 h. The frequency of the ultrasound was 28 kHz, and the output was set at 20 W. MALDI Analysis. MALDI mass spectra were acquired in positive reflectron mode using an Axima-CFR time-of-flight mass spectrometer (Shimadzu/Kratos, Manchester, UK) with a pulsed nitrogen laser (337 nm). PEG (1.8 mg/mL) was dissolved in water, CHCA (10 mg/mL), and NaI (1.5 mg/mL) in THF/water (1/1, v/v). Solutions containing the matrix, cationizing agent, and polymers (0.5 µL) were deposited on a stainless sample target by the overlayer method, as follows; first deposition, matrix solution; second, cationizing agent solution; third, polymer solution. The analyte ions were accelerated at 20 kV under delayed extraction conditions. RESULTS AND DISCUSSION Ultrasonic Degradation of PEG6000. We have investigated the degradation of PEG6000 with various durations of sonication by MALDI-TOFMS to detect structural changes in PEG induced by the sonication. Figure 1 shows MALDI mass spectra (m/z ) 950-8000) for different sonication times (0, 2, 4, and 8 h). Before Analytical Chemistry, Vol. 79, No. 11, June 1, 2007

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Figure 2. MALDI-TOF mass spectrum of PEG6000 after ultrasonication for 8 h. Table 1. Structural Assignments of the Peaks in the MALDI-MS Spectrum of Figure 2

sonication (a), well-resolved ions with a spacing of 44 Da are detectable around m/z ) 6000, indicating identification of the PEG backbone. At the sonication time of 2 h (b), the mass spectrum consists of mixtures of ions from undegraded chains around m/z ) 6000 and degraded chains around m/z ) 500-3000 with the 44 Da spacing. In addition to these ions from degraded chains, the intensity of ions around m/z ) 3000-5000 also increases at the sonication time of 4 h (c), which may originate from the recombination of degraded chains. At the sonication time of 8 h (d), the ions from the degradation appear around m/z ) 5001000, accompanied by a marked decrease in the intensity of ions from undegraded chains and ions around m/z ) 3000-5000. These MALDI mass spectra indicate that the molecular weight of PEG6000 decreases drastically initially with ultrasonic degrada4184

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tion and then gradually approaches a limiting molecular weight of 500-1000. This ultrasonic degradation process is qualitatively consistent with that described in previous reports using GPC and viscometry to monitor molecular weight changes in polymers.6,17 End Group Determination of the Ultrasonic Degradation Products of PEG6000. The end groups of the ultrasonic degradation products of PEG6000 are determined as follows. One considers the Na adduct ions observed at m/z Mobs in the MALDI mass spectra and thereby obtains a sum of the end group masses: (E1 + E2) ) Mobs - 44n - 23, where E1 and E2 are the mass of an end group and n is the number of repeat units. The PEG molecular ion observed at m/z )1185.4 gives n ) 26 and E1 + E2 ) 18, indicating a hydroxyl group at one end of the chain and a hydrogen at the other.

Scheme 1. Ultrasonic Degradation Processes Occurring in PEG6000

Scheme 2. Ultrasonic Degradation Processes Occurring in PMMA (Mn ) 1630)

The expanded spectrum (m/z ) 1050-1350) of PEG6000 at a sonication time of 8 h (Figure 2) shows five distinct ion degradation products designated by the symbols A, A′, B, B′, and C, which were assigned by the procedure above in Table 1. On the other hand, the spectrum of the undegraded PEG indicated only one series of ions with the Na adduct, and the end group is a hydroxyl and a hydrogen atom, corresponding to the A series. The ions of A′ correspond to the PEG oligomer with a hydroxyl group at one terminus of the chain and an aldehyde group at the other. The ions of B are associated with a hydroxyl group and an ethyl group, indicating the most abundant product, as shown in Table 1. Identification of the ions of B′ raises two possibilities: (i) a cyclic oligomer of PEG and (ii) an oligomer with a hydroxyl and a vinyl group. These two species cannot be discriminated because they have the same mass. The ions of C are associated with an ethyl and a hydrogen group. Ultrasonic Degradation Pathways of PEG Polymers. Considering the five types of ultrasonic degradation products (Table 1), we propose the degradation pathways presented in Scheme 1. The first route involves the scission of PEG by

ultrasonication (I), yielding two daughter polymeric radicals with different terminal groups, X•(∼CH2CH2O•) and Y•(∼CH2CH2•), due to breaking of C-O bonds in the PEG chain somewhat less stable than C-C bonds, which was reported to occur preferentially at the midpoint of the chain.30 Subsequently, the termination of the generated X• radicals by extraction and release of a hydrogen atom yields the products A (II) and A′ (III), respectively. In the same way, the Y• radicals are also terminated by extraction and release to yield the products B (IV) and B′ (V), respectively. Further degradation of product B yields the polymeric radical Z•(CH3CH2∼OCH2CH2•). The resultant Z• radical extracts a hydrogen atom, leading to product C (VI). Finally, it is most likely that these degradation products of X• and Y• were generated by the breaking of C-O bonds in the PEG chain, followed by termination with extraction and release of a hydrogen atom. We also conducted ultrasonic experiments on other PEGs with different initial molecular masses (2, 6, 20, and 2000 kDa) using a sonication time of 8 h. The MALDI mass spectra (m/z 11601210) exhibited the same types of degradation products, irrespective of initial molecular weights (Figure 3). However, the relative Analytical Chemistry, Vol. 79, No. 11, June 1, 2007

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Table 2. Intensity Relative to That of Peak A of PEG in the MALDI Spectra of Figure 3 relative intensity M

A

B

C

2× 6 × 103 2 × 104 2 × 106

1.0 1.0 1.0 1.0

1.7 1.4 2.7 2.8

0.3 0.5 2.7 3.7

103

Figure 3. MALDI-TOF mass spectra of PEG at different initial molecular weights after the ultrasonication for 8 h. (a) 2 × 103, (b) 6 × 103, (c) 2 × 104, and (d) 2 × 106. These spectra are processed with the averaging treatments of 5 times.

Figure 4. MALDI-TOF mass spectrum of PMMA (Mn ) 1630). Ultrasonication times for (a) 0 and (b) 8 h.

intensities of the products did depend on the initial molecular weights (Table 2). The intensity of C increases much more strongly with initial molecular weight than that of B, suggesting that more consecutive degradation steps take place, as the initial molecular weight increases. Ultrasonic Degradation Products of PMMA. We have also investigated the ultrasonic degradation of PMMA (M h ) 1630 Da). An acetonitrile solution of PMMA generated no sonication products, and thus, a mixture solution of acetonitrile/water (1:1, v/v) was used for ultrasonic degradation (Figure 4). Before sonication of PMMA (a), a well-resolved MALDI mass spectrum with a spacing of 100 Da was detectable, indicating the both end groups of PMMA are single hydrogen atoms. However, a center mass of the ion distribution was identified around at m/z ) 1200, different from the m/z ) 1400 obtained in the acetonitrile solution of PMMA. This lower shift is entirely due to the addition of water and a decrease in ionization efficiency of PMMA with higher molecular weight, because it is also more likely that the solubility of higher molecular weight oligomers of PMMA in the water/ acetonitrile solution is less than in the acetonitrile. At the sonication time of 8 h (b), the degraded oligomers of PMMA at 4186 Analytical Chemistry, Vol. 79, No. 11, June 1, 2007

approximately m/z ) 700 appear to be accompanied by a significant decrease in the original chains. Both end groups of the degradation products of PMMA were determined to be a hydrogen atom. This was different from the results of the PEG degradation. To explain this phenomenon, we must consider the possibility that PMMA initiates C-C bond breaking of the chain (I) as shown in Scheme 2, producing two polymeric radicals with the radical terminal groups X•(∼CH2•) and Y•(∼(CH3)CCOO(CH3)C•), followed by termination with extraction of a hydrogen atom (II and III). It should be noted that the ultrasonic degradation of the PMMA chain does not occur at the C-C bond of the R position, as shown in Scheme 2. It has been proposed that polymers in solution can undergo chemical transformations under the influence of ultrasound by at least three mechanisms: (1) reactions with •OH and H• radicals, which are originated from the implosive collapse of a bubble by high-frequency ultrasound, (2) mechanochemical effect from the shear forces generated around collapsing cavitation bubbles, and (3) pyrolysis in the hot interfacial region between the bubble and the surrounding liquid.37 It has been reported that the effect of addition of radical trapping reagent on the ultrasonication of PEG with nominal average molecular masses of 20 and 9 kDa, in which this ultrasonic degradation process of PEG, was suggested to be caused by OH radicals as well as mechanochemical effects.37 A similar degradation mechanism is likely for the ultrasonic degradation of PEG in the present study, although the pyrolysis in the hot interfacial region cannot be neglected. The pyrolysis products with five major series from PEG (M h ) 8000, 16 000, 20 000) were explained via a free radical decomposition scheme by GC/MS,38 and some of them have the same end groups (ethyl ether/aldehyde/vinyl ether) with the ultrasonic products of PEG in this study. In pyrolysis/thermal degradation of PMMA, the polymer is known to be depolymerized to monomeric units above a given pyrolysis temperature.39-41 On the other hand, it is not clear whether the ultrasonic chain scission is induced by the release of monomers from the end group (i.e., unzipping) or random-chain scission around the center of the chain. This could be checked by investigating the ultrasonic degradation of PMMAs with unique end groups in the future. We have also noted that it is hard to generate ultrasonic degradation products from H• extraction by the incipient radicals for both PEG and PMMA. For example, the intensities of A′ and B′ from H• release are smaller than those of A and B from H• (37) Rokita, B.; Czechowska-Biskup, R.; Ulanski, P.; Rosiak, J. M. e-Polymers 2005, 24, 1. (38) Lattimer. R. P. J. Anal. Appl. Pyrolysis 2000, 56, 61. (39) Manring, L. E. Macromolecules 1989, 22, 2673. (40) Eger, C.; Kaminsky, W. J. Anal. Appl. Pyrolysis 2001, 58-59, 781. (41) AÄ da´mova´, M.; Orinˇa´k, A.; Hala´s, L. J. Chromatogr., A 2005, 1087, 131.

extraction, as that shown in Table 1 and Figure 3. These favorable ultrasonic products from H• extraction remain as a matter to be discussed further. CONCLUSION In this study, we have investigated ultrasonic degradations of PEG and PMMA in aqueous media by MALDI-TOFMS to examine the structural details of ultrasonic degradation polymers. MALDIMS combined with the ultrasonication procedure demonstrated five types of degradation products of PEG with different end groups to be identifiable, irrespective of the initial molecular masses (M h ) 2, 6, 20, and 2000 kDa). Five degradation pathways involving free radical reactions are suggested: the first route is the ultrasonic scission of PEG by C-O bond breaking, yielding two daughter products with different terminal radical groups, X•(∼CH2CH2O•) and Y•(∼CH2CH2•). The subsequent route is termination of the ultrasonically generated X• and Y• by extraction or release of a hydrogen atom. Further degradation of the resultant product yields the polymeric radical of Z• (CH3CH2∼OCH2CH2•). The Z• radical extracts a hydrogen atom, leading to another product.

In contrast to the ultrasonication of PEG, PMMA generated only one type of degradation product with a hydrogen group at both ends, the same as that of the original oligomer. It has been suggested that the radical terminal groups X•(∼CH2•) and Y•(∼(CH3)CCOO(CH3)C•) are present due to the selective breaking of C-C bonds in the PMMA chain during the ultrasonic degradation of PMMA. ACKNOWLEDGMENT This work was partially supported by the CREST of Japan Science and Technology Corporation (JST) and for Scientific Research, by Grants-in Aid for Scientific Research (18710102) and (18310069) from the Monbukagaku-shou, Japan, and by the Research and Development Organization of IndustrysUniversity Cooperation from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

Received for review December 5, 2006. Accepted March 28, 2007. AC062304V

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