Observation of Low Molecular Weight Poly(methylsilsesquioxane)s by

Oct 10, 2000 - The m/z peak at 393.5 is the sodium adduct of the styrene trimer with tert-butyl .... IX starts the fourth series (553.7 f .... Accepte...
0 downloads 0 Views 222KB Size
Download PDF
Anal. Chem. 2000, 72, 5673-5678

Observation of Low Molecular Weight Poly(methylsilsesquioxane)s by Graphite Plate Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Hie-Joon Kim,* Jin-Kyu Lee, Sung-Jun Park, Hyun Wook Ro, Dae Young Yoo, and Do Y. Yoon

School of Chemistry and Molecular Engineering, Seoul National University, Seoul 151-747, Korea

Mass spectra of polystyrene and poly(methylsilsesquioxane)(PMSSQ) derived from methyltriethoxysilane(MTES) were obtained in the 100-1000 Da range by laser desorption/ionization time-of-flight mass spectrometry using a graphite plate without a matrix. Clean mass spectra were obtained without interference from carbon clusters or other low molecular weight compounds. Initial reaction products derived from condensation of partially hydrolyzed MTES were observed. Upon 30 min of heating at 30 °C, the ethoxy groups were fully hydrolyzed to hydroxy groups. Many PMSSQ species consistent with predictable polymerization reaction pathways involving intermolecular condensation and intramolecular dehydration were observed. Thus, laser desorption/ionization time-of-flight mass spectrometry using a graphite plate, without added matrix materials, is shown to provide valuable information on low molecular weight polymer not available by MALDI-TOF-MS. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was developed in 1988 by Karas and Hillenkamp1 and by Tanaka et al.2 with the goal of determining molecular weights of biopolymers and synthetic polymers above 10 000 Da. In 1996, Schriemer and Li reported observation of polystyrene with a molecular weight as high as 1.5 million Da by MALDI-TOF-MS.3 In fact, the term “matrix-assisted laser desorption” was used in 1985 to explain the enhancement of laser desorption signal from alanine due to the presence of UVabsorbing tryptophan.4 However, MALDI-TOF-MS of small molecules received little attention, because the matrix signals usually complicate interpretation of sample signals in the low-mass region. Low molecular weight synthetic polymers observed by MALDIMS include 4 kDa poly(propylene glycol),2 1100 Da5 and 3750 Da6 poly(methyl methacrylate), and 1450 Da poly(ethylene glycol).7 (1) Karas, M.; Hillenkamp, F. Anal. Chem. 1988, 60, 2299. (2) Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshida, T. Rapid Commun. Mass Spectrom. 1988, 2, 151. (3) Schriemer, D. C.; Li, L. Anal. Chem. 1996, 68, 2721. (4) Karas, M.; Bachmann, D.; Hillenkamp, F. Anal. Chem. 1985, 57, 2935. (5) Jackson, C.; Larsen, B.; McEwen, C. Anal. Chem. 1996, 68, 1303. (6) Chen, H.; Guo, B. Anal. Chem. 1997, 69, 4399. (7) Rashidzadeh, H.; Guo, B. Anal. Chem. 1998, 70, 131. 10.1021/ac0003899 CCC: $19.00 Published on Web 10/10/2000

© 2000 American Chemical Society

The matrix signals could presumably be avoided using an insoluble high molecular weight material to absorb and transfer energy. Graphite is a solid material close to an ideal blackbody. Therefore, the use of graphite as an energy mediator in laser desorption/ionization has a potential to generate low-mass sample signals without generating matrix signals and thus to alleviate the low-mass complications. In the past, graphite mixed with liquid matrixes, such as glycerol, was found to be useful in obtaining time-of-flight mass spectra of proteins,8 2500 Da polystyrene, and 3000 Da poly(ethylene glycol).9 However, the so-called graphite surface-assisted laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) has not been applied to the analysis of low molecular weight polymers below 1000 Da. Direct use of graphite without a matrix was attempted by Zumbuhl et al.10 They obtained low-mass signals of triterpenes using a dry graphite surface prepared on the sample tip by evaporating a 30% suspension of 2-µm graphite particles in methanol. Kim and Kang performed LDI-TOF-MS of 2000 Da polypropyleneglycol and 2500 Da polystyrene using a 2-mm-thick graphite plate without a matrix.11 Using a graphite plate to absorb and transfer UV energy, we obtained a detailed molecular weight distribution of poly(methylsilsesquioxane) in the 100-1000 Da range by LDI-TOF-MS. Silsesquioxanes12,13 with the RSiO3/2 repeat unit have a great potential as low-dielectric insulators in microelectronic devices.14,15 In this paper, we demonstrate that graphite plate laser desorption/ ionization time-of-flight mass spectrometry (GPLDI-TOF-MS) can be used to observe low molecular weight PMSSQs and to investigate the initial polymerization pathways. EXPERIMENTAL SECTION PMSSQs were prepared by a hydrolysis-condensation reaction. A sample of 4.99 g of methyltriethoxysilane (MTES, Lancaster (8) Sunner, J.; Dratz, E.; Chen, Y.-C. Anal. Chem. 1995, 67, 4335. (9) Dale, M. J.; Knochenmuss, R.; Zenobi, R. Anal. Chem. 1996, 68, 3321. (10) Zumbuhl, S.; Knochenmuss, R.; Wulfert, S.; Dubois, F.; Dale, M. J.; Zenobi, R. Anal. Chem. 1998, 70, 707. (11) Kim, J.; Kang, W. Bull. Korean Chem. Soc. 2000, 21(4), 401. (12) Baney, R. H.; Itoh, M.; Sakakibara, A.; Suzuki, T. Chem. Rev. 1995, 95, 1409. (13) Loy, D. A.; Shea, K. J. Chem. Rev. 1995, 95, 1431. (14) Nguyen, C. V.; Carter, K. R.; Hawker, C. J.; Hedrick, J. L.; Jaffe, R. L.; Miller, R. D.; Remenar, J. F.; Rhee, H.-W.; Rice, P. M.; Toney, M. F.; Trollsas, M.; Yoon, D. Y. Chem. Mater. 1999, 11, 3080. (15) Miller, R. D. Science (Washington, D.C.) 1999, 286, 421.

Analytical Chemistry, Vol. 72, No. 22, November 15, 2000 5673

Figure 1. GPLDI-TOF mass spectrum of low-mass polystyrene standard. One microliter of 1 mg/mL polymer sample was loaded on the graphite plate without a matrix. Sodium adducts of styrene trimer through nonamer are observed.

Synthesis Ltd., U.K.) was added to 8.01 mL of 4-methyl-2pentanone (Aldrich) in a one-neck round-bottom flask in a glovebox under nitrogen. A sample of 0.42 mL of 0.2 N HCl was mixed with 4.65 mL of distilled water in a dropping funnel, and the mixture was delivered to the flask at room temperature over a 5-min period. A 10-µL aliquot was withdrawn for immediate analysis (0 min). The reaction mixture was heated at 40 °C for 30 min and 24 h, filtered, washed with distilled water, and dried by rotary evaporation and vacuum drying. The graphite plate was custom-made, using commercial 7-µm graphite particles, identical in dimensions to the stainless steel plate provided by the manufacturer of the MALDI-TOF mass spectrometer used (PerSeptive Biosystems, Framingham, MA). Typically, 1 µL of a 1 mg/mL polymer sample dissolved in tetrahydrofuran was applied to the graphite plate. Mass spectrometric experiments were carried out according to the conventional MALDI-TOF-MS procedure (337 nm nitrogen laser, 20 kV accelerating voltage, 1.2 m flight tube, 200 signals averaged). The polystyrene standard (Polymer Standards Service) with a Mn of 470 Da was used for calibration. MALDI-TOF-MS was performed using dithranol as a matrix.

RESULTS AND DISCUSSION Polystyrene Standard. Figure 1 shows a GPLDI-TOF mass spectrum of a low molecular weight polystyrene standard. The styrene oligomers (trimer through nonamer) are separated by 104.3 mass units, corresponding to the [(C6H5)CHCH2] repeat unit. The m/z peak at 393.5 is the sodium adduct of the styrene trimer with tert-butyl and hydrogen at the ends (calculated mass, 393.51). 5674

Analytical Chemistry, Vol. 72, No. 22, November 15, 2000

Synthetic polymers are known to produce sodiated cations in MALDI-MS.9 A smaller peak of the potassium adduct is also shown at 409.5. Kahr and Wilkins obtained Fourier transform mass spectra of low molecular weight polymers, including polystyrene, by silver nitrate chemical ionization.16 However, to our knowledge this is the first observation of low molecular weight polymer standards below 1000 Da by LDI-TOF-MS. Knowledge of the molecular mass distribution of polymer standards is important in gel permeation chromatography calibration. Even though mass spectra may not quantitatively represent mass distribution of the polymer sample, they still can be used to obtain reasonably good molecular mass distribution as Kahr and Wilkins have shown for several low molecular weight polymers.16 The sodiated trimer was used for calibration in the subsequent interpretation of PMSSQs. Poly(methylsilsesquioxane)s. Molecular weight determination by LDI-MS has been limited in the past to PMSSQs with molecular weights greater than 1000 Da,17-19 because the signals below 1000 Da are complicated by the matrix signals resulting from the MALDI process. Molecular weight information below 1000 Da could help understand and control the initial polymerization pathways, which would determine the ultimate molecular weight distribution and the local microstructures. (16) Kahr, M. S.; Wilkins, C. L. J. Am. Soc. Mass Spectrom. 1999, 4, 224. (17) Wallace, W. E.; Guttman, C. M.; Antonucci, J. M. J. Am. Soc. Mass Spectrom. 1999, 224. (18) Wallace, W. E.; Guttman, C. M.; Antonucci, J. M. Polymer 2000, 41, 2219. (19) Eisenberg, P.; Erra-Balsells, R.; Ishikawa, Y.; Lucas, J. C.; Mauri, A. N.; Nonami, H.; Riccardi, C. C.; Williams, R. J. J. Macromolecules 2000, 33, 1940.

Figure 2. GPLDI-TOF mass spectrum of PMSSQ produced from MTES at room temperature immediately after mixing with the acid catalyst. One microliter of 1 mg/mL polymer sample was loaded on the graphite plate without a matrix.

The MALDI mass spectrum in the 200-800 Da range was complicated by the dithranol matrix peaks (result not shown) and, therefore, the low-mass distribution of PMSSQs could not be observed by the conventional MALDI-TOF-MS. In contrast, a clean GPLDI-TOF mass spectrum was obtained above 100 Da from the MTES hydrolysis product and PMSSQs (Figure 2). As with polystyrene, sodium adducts were primarily observed. Assuming that one of the smaller peaks in Figure 2 corresponds to 1% of the total amount of the polymer loaded, 1 µL of the PMSSQ sample loaded containing 1 mg of total polymer per milliliter contains 20 pmol of that particular PMSSQ. A good mass spectrum was also obtained after 20-fold dilution by increasing the laser power. Initial Hydrolysis and Condensation. It was of interest whether the initial reactions in polymerization of silsesquioxanes involve acid-catalyzed hydrolysis of all three ethoxy groups in MTES (I, 178.3 Da, +Na+ peak expected at m/z 201.3) to methylsilanetriol (II, 94.1 Da), which would undergo condensation to 1,3-dimethyl-1,1,3,3-tetrahydroxydisiloxane, or condensation would take place as soon as methyldiethoxysilanol (III, 150.3 Da) is produced upon hydrolysis of one ethoxy group. Since acid-catalyzed hydrolysis of the ethoxy groups in MTES was known to be fast, an aliquot of the reaction mixture was withdrawn and analyzed as soon as MTES and the HCl catalyst were mixed. Figure 2 shows the mass spectrum obtained from this 0-min sample. A small peak of residual MTES was observed at m/z 201.2 (+Na+) and 217.4 (+K+). Hydrolysis of one ethoxy group decreases the mass by 28.0 Da. A strong peak corresponding to methyldiethoxysilanol (III, +Na+ expected at 173.1) was observed at 173.5. The 189.6 peak is the potassium adduct. Methylsilanetriol (+Na+ expected at 117.1) was not observed,

suggesting that condensation begins before hydrolysis of all three ethoxy groups.

A condensation reaction of a methyldiethoxysilanol molecule with another methyldiethoxysilanol or methyltriethoxysilane will produce 1,3-dimethyl-1,1,3,3-tetraethoxysiloxane (IV, 282.5 Da). Figure 2 shows its sodium adduct at 305.7. This observation clearly shows that condensation takes place as soon as one ethoxy group is hydrolyzed. When this first silsesquioxane to be produced is hydrolyzed to a monohydroxy compound, it will readily condense with III to a trimer, which was observed at 409.8 (Figure 2). Note that addition of a CH3-SiO-OC2H5 unit increases the mass by 104 Da. Upon hydrolysis, the 409.8 peak loses 28 Da sequentially to yield the 381.6 and the 353.5 peaks as well as their potassium adducts (Figure 2). Analytical Chemistry, Vol. 72, No. 22, November 15, 2000

5675

Figure 3. Proposed initial reaction pathway in the hydrolysis and condensation of methyltriethoxysilane, forming ladder-structure poly(methylsilsesquioxane). Silicon atoms at the intersections and oxygen atoms between all silicon atoms are omitted. Methyl groups at the open ends are also omitted. The expected mass of the sodium adduct for the proposed structure is indicated.

Addition of the CH3-SiO-OC2H5 unit to the monohydroxy trimer yields a series of peaks separated by 104 Da (m/z 381.6 f 485.9 f 590.4 f 694.6 in Figure 2). Addition of the same unit to the dihydroxy trimer yields another series of peaks separated by 104 Da (m/z 353.5 f 457.9 f 561.9 f 665.5 f 770.7). The dihydroxy tetramer (m/z 457.9) undergoes cyclization to yield the first ladder structure, a cyclotetrasiloxane derivative (V; 416.7 Da, +Na+ peak shown at 440.3 in Figure 2). Phenylcyclotetrasiloxanetetrol, a cyclotetrasiloxane derivative, was prepared by Brown in 1965.20 However, cyclotetrasiloxane derivatives have not been observed mass spectrometrically due to their low molecular weight and reactivity. As a matter of fact, the 440.3 peak was observed only from the 0-min sample and disappeared upon heating the reaction mixture. Observation of compound V suggests that cyclotetrasiloxanes could be an important intermediate in the formation of ladder-structure or cagelike polysilsesquioxanes.19 Alternatively, the 104 Da unit could be added to the dihydroxy tetramer to yield a dihydroxy pentamer (m/z 561.9 in Figure 2), which could form a cyclotetrasiloxane derivative as suggested in Figure 3. Surprisingly, extended ladder structures were observed even though efforts were made to minimize delay between the mixing of the monomer with the acid catalyst and the mass spectrometric measurement. For example, the 574.2 peak corresponds to addition of a (CH3Si(O)3/2)2 repeating unit (134.3 Da) to compound V. Apparently the initial hydrolysis and condensation reactions are very fast. As shown in Figures 2 and 3, a majority of the ethoxy groups in the 0-min sample remained unhydrolyzed even after (20) Brown, J. F., Jr. J. Am. Soc. Mass Spectrom. 1965, 87, 4317.

5676

Analytical Chemistry, Vol. 72, No. 22, November 15, 2000

formation of a variety of condensation products. Possible structures of the initial reaction products are shown in the proposed pathway in Figure 3. The peak at 425.7 might be due to the sodium adduct of the prismlike hexa(methylsilsesquioxane)(402.9 Da) as well as the potassium adduct of the 409.8 peak. Similar cage compounds were observed by Unno et al.21,22 and Laine et al.23 30-min Sample. The mass spectrum in Figure 4 represents PMSSQs obtained after 30 min of heating at 40 °C. Most of the peaks in Figure 4 could be accounted for by five series of ladder structures starting with compounds VI-X. Note that after 30 min of heating most of the ethoxy groups have been hydrolyzed. The 327.7 peak is the sodium adduct of VI, derived from hydrolysis of four ethoxy groups from compound V, which resembles tetraphenylcyclotetrasiloxanetetrol prepared by Brown.20 The potassium adduct was also observed at 343.5. Successive addition of the 134.3 Da (CH3Si(O)3/2)2 repeating unit leads to the first series of the ladder structure identifiable from in Figure 4 (m/z 327.7 f 461.2 f 595.0 f 733.4 f 867.5). The strongest peak at 477.6 in Figure 4 is 16 higher than 461.2 and appears to be a potassium adduct. The starting material of the second series (m/z 403.0 f 536.4 f 670.7 f 804.1 f 933.9), VII, is obtained by condensation of II with VI and was observed at m/z 403.0. Condensation of one methylsilanetriol increases the molecular weight by 76.1 Da. Condensation of another methylsilanetriol to VII yields VIII, expected at m/z 479.9. Unfortunately, it overlapped with the potassium adduct at 477.6 and could not be directly observed. The peak at 477.6 was unsymmetrical and had a shoulder toward higher m/z. The ladder analogue of VIII, 134.3 lower in mass, was expected at m/z 345.6 and was only partially resolved from the potassium adduct (m/z 343.5)(Figure 4, inset). This linear tetramer of methylsilanetriol starts the third series (345.6 f 479.9 f 612.2 f 747.1). Condensation of methylsilanetriol to VIII yields IX, observed at m/z of 553.7. IX starts the fourth series (553.7 f 687.9 f 822.4). Condensation of four methylsilanetriols to VI yields X, observed at m/z 629.6. Its ladder analogues were also observed at 764.0 and 897.7. Other PMSSQs shown in Figure 4 are related to one of the above-mentioned compounds. For example, the 705.6 peak corresponds to the condensation of a methylsilanetriol to X. Successive addition of the 134.1 Da rung to the 705.6 peak leads to the peaks at 839.6 and 973.8. Successive condensation of a methylsilanetriol to the 705.6 peak leads to the 781.7 and 857.3 peaks. Also, successive condensation of a methylsilanetriol to the 839.6 peak leads to the 915.6 and 991.4 peaks. The 933.9 and 943.6 peaks are related to the 857.3 and 867.5 peaks, respectively, by 76 Da. Thus, most of the peaks in Figure 4 could be accounted for by successive additions of a 76 Da unit from condensation of a methylsilanetriol or of a 134 Da (CH3Si(O)3/2)2 unit. Note that many of the above compounds are highly branched and have several hydroxyl groups that could lead to subsequent intermolecular or intramolecular condensations. The 385.5 peak probably (21) Unno, M.; Alias, S. B.; Arai, M. ; Takada, K.; Tanaka, R.; Matsumoto, H. Appl. Organomet. Chem. 1999, 13, 303. (22) Unno, M.; Alias, S. B. ; Saito, H.; Matsumoto, H. Organometallics 1996, 15, 2413. (23) Laine, R. M.; Zhang, C.; Sellinger, A.; Viculis, L. Appl. Organomet. Chem. 1998, 12, 715.

Figure 4. GPLDI-TOF mass spectrum of PMSSQs prepared from MTES upon heating at 40 °C for 30 min. One microliter of 1 mg/mL polymer sample was loaded on the graphite plate without a matrix.

corresponds to a bicyclopentasiloxane, XI, similar to 1,5-bis(1,1,2trimethylpropyl)-3,3,7,7,10,10-hexaphenylbicyclo[3,3,3]-pentasiloxane obtained by Unno et al.21 The 519.3 peak would be its ladder analogue. 24-h Sample. As the reaction time increases, an overall increase in the molecular weight is expected, as more monomers or small PMSSQs are added via condensation. At the same time, intramolecular dehydration will introduce additional Si-O-Si bonds, reducing the number of free hydroxyl groups. Figure 5 shows a shift in the molecular weight distribution in the 300-

1000 Da range when the reaction time increased from 30 min to 24 h. After 24 h of heating, four silicon (observed at m/z 327.7 and 343.5 in Figure 4) and five silicon (observed at m/z 385.5, 403.0, and 419.4) compounds disappeared, and the heptamer observed at 536.0 became predominant. The heptamer results from condensation of II with the tetrahydroxy compound (m/z 461.7) in Figure 3. In addition to the general shift to high molecular weight, an increase in the amount of dehydration products was also evident, as expected. For example, the 612.2 peak is higher than its

Analytical Chemistry, Vol. 72, No. 22, November 15, 2000

5677

Figure 5. GPLDI-TOF mass spectrum of PMSSQs prepared from MTES upon heating at 40 °C for 24 h. One microliter of 1 mg/mL polymer sample was loaded on the graphite plate without a matrix.

dehydration product, 595.0, in Figure 4. However, in Figure 5 the 594.3 peak is higher than the 612.1 peak. The same is true with the 536.0/553.6 pair. Proton NMR results showed that the percentage of hydroxyl protons decreases as the reaction time increases (results not shown). The stepwise increase in 134 Da due to the (CH3Si(O)3/2)2 repeating unit leads to the following series evident in Figure 5: 461.5 f 594.3 f 729.0 f 862.6; 477.9 f 612.1 f 746.0 f 880.3; 519.6 f 653.5 f 787.6; 536.0 f 670.2 f 804.7 f 939.0; 553.6 f 687.2 f 822.3 f 956.5. CONCLUSIONS UV energy is efficiently absorbed by graphite and utilized to desorb and ionize low molecular weight polymers without added matrixes. Thus, a clean mass spectrum can be obtained from low molecular weight polymers in the 100-1000 Da range. Laser desorption/ionization time-of-flight mass spectrometry using a

5678

Analytical Chemistry, Vol. 72, No. 22, November 15, 2000

graphite plate can provide valuable low molecular weight information not available by MALDI-TOF-MS. The technique would be useful not only in investigating the initial polymerization reaction pathways, as demonstrated for preparing poly(methylsilsesquioxane) from methyltriethoxysilane, but also in detecting low molecular weight nonpolar biomolecules. ACKNOWLEDGMENT Financial support from the Brain Korea 21 program (Ministry of Education) and the Korean Collaborative Project for Excellence in Basic System IC Technology (98-B4-C0-00-01-00-02) are appreciated. S.-J.P. and D.Y.Y. were recipients of the Brain Korea 21 fellowship.

Received for review April 6, 2000. Accepted July 5, 2000. AC0003899