Characterization of Polyether and Polyester Polyurethane Soft Blocks Using MALDI Mass Spectrometry

Anal. Chem. 2000, 72, 2490-2498

John T. Mehl,† Renata Murgasova,‡ Xia Dong,§ and David M. Hercules* Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235 Hartmut Nefzger Bayer Corporation, New Martinsville, West Virgina 26155

Selective degradation reactions combined with MALDI molecular weight. Urethane linkages are formed through reaction analysis have been applied for molecular weight (MW) of diisocyanate with the polyol and with a short chain diol (chain determination of polyether and polyester polyurethane extender). The chain extender and diisocyanate form the hard (PUR) soft blocks. Selective degradation allows recovery segments. Because of thermodynamic incompatibility of hard and of the polyols, and direct observation of the soft block soft segments, microphase separation occurs upon cooling. The oligomer distribution is possible for the first time by polyol soft segments form an amorphous matrix in which the hard using MALDI. Ethanolamine is applied for polyether PUR crystalline segments become embedded. Hard segments act as degradation. MALDI analysis indicates that the recovered thermohard blockreversible cross-links and fillers. The soft polytetrahydrofuran (pTHF) MW distribution is nearly blocks have easy segmental rotations at the temperature of use identical to the unreacted pTHF material. Reduction in (in many cases ambient temperature), thus imparting desired the ethanolamine reaction time allows observation of elastomeric properties. Both the chemical composition and mo- oligomer ions containing the diisocyanate linkage, which lecular weight distribution (MWD) of the incorporated soft block provide identification of the diisocyanate. Ethanolamine influence the macroscopic properties of the resulting PUR. This is not used for polyester PUR’s degradation because the is also true for the hard blocks. Thus, variation of the PUR ester bonds will be cleaved. Therefore, phenylisocyanate formulation covers a wide range of properties and applications is applied for polyester PUR degradation. Polybutylene (tailor-made elastomers). adipate (pBA) oligomers were directly observed in the A variety of conventional instrumental techniques such as 1,2 MALDI spectra of the degraded pBA-PUR samples. nuclear magnetic resonance spectroscopy (NMR), infrared 3 4 Comparison of the degraded pBA-PUR oligomer distribu- spectroscopy (IR), and size exclusion chromatography (SEC) tion with the unreacted pBA material indicates that low- have been applied to characterize PURs. Information about PUR mass oligomers are less abundant in the degraded pBA- terminal groups, diol or diamine extenders, polyol soft blocks, PURs. Oligomer ions containing the diisocyanate linkage and diisocyanates can be obtained. IR and NMR techniques based are also observed in the spectrum, providing a means for on end-group analysis can provide accurate determination of identifying the diisocyanate used for PUR syntheses. Size- number average molecular weight (Mn). However, end-group- exclusion chromatography (SEC) was combined with based methods do not provide weight average molecular weight MALDI to provide accurate MW determination. Narrow (Mw); hence, valuable information about polydispersity (PD) is MW fractions of the degraded and unreacted polyols were not obtained. SEC provides both Mn and Mw; however, because collected and analyzed by MALDI. This method allows of a lack of narrow molecular weight elution standards for polyethers and polyesters, large errors in M and M result. What precise calibration of the SEC chromatogram. The SEC- n w is required for PUR characterization is a method which provides MALDI results provide significantly larger Mw and PD reliable determination of both M and M for polyol soft blocks. values than MALDI alone. Using SEC-MALDI, it was n w determined that the PD indexes of the pTHF and pBA † Present address: Division of Bioengineering and Environmental Health, samples are larger than the assumed values, which are Massachusetts Institute of Technology, 77 Massachusetts Ave., Room 56-738, based on the polyol synthesis reactions. The combination Cambridge, MA 02139. ‡ Permanent address: Polymer Institute, Slovak Academy of Sciences, of selective degradation with SEC-MALDI, using either Dubravska cesta 9, 842 36 Bratislava, SK. ethanolamine or phenylisocyanate, is a viable method for § Present address: Evans East, 104 East Windsor Center, Suite 101, East polyurethane polyol characterization. Windsor, NJ 08520. (1) Yeager, F. W.; Becker, J. W. Anal. Chem. 1977, 49, 722-724. (2) Cambell, D.; White J. R. Polymer Characterization: Physical Techniques; Polyurethane (PUR) elastomers are multiblock copolymers Chapman and Hall: London, 1989. (3) Corish, P. J. Anal. Chem. 1959, 31, 1298-1306. with an alternating sequence of hard- and soft-block locks. The (4) Haken, J. K.; Burford, R. P.; Vimalsiri, P. A. D. T. J. Chromatogr. 1985, soft blocks are usually polyether or polyester polyols of varying 349, 347.

2490 Analytical Chemistry, Vol. 72, No. 11, June 1, 2000 10.1021/ac991283k CCC: $19.00 © 2000 American Chemical Society Published on Web 05/04/2000 Mass spectrometry can be used for polymer characterization. response.27,28 To overcome these limitations, several researchers Time-of-flight secondary ion mass spectrometry (TOF-SIMS),5-9 have used SEC to separate the polymer into narrower MW 10 field-desorption (FD-MS), and fast-atom bombardment (FAB- fractions. MALDI analysis of the fractions provides Mn, which, in MS)11,12 have all been applied for polymer analysis. Several turn, is used to calibrate the SEC trace without the need for investigations of PURs using TOF-SIMS 13,14 have been reported, external elution standards. Montaudo25 and others 29 have shown including average molecular weight (MW) estimation of PURs.15 that this method can provide reliable molecular weight determi- Though these techniques are highly informative in terms of nation for wide polydispersity polymers. identification of PUR components, low desorption/ionization of MALDI has been applied for the analysis of polyethers and higher masses and fragmentation limit the utility of these methods polyesters30-32 and more recently in combination with SEC.18,30 for average MW determination. The purpose of the present research is to extend this work for New ionization techniques such as matrix-assisted laser de- the analysis of PUR soft blocks. Through the use of selective sorption/ionization (MALDI)16 and electrospray (ESI) allow larger chemical degradation reactions, polyether and polyester soft MW’s to be determined. Poly(ethylene glycol) (PEG) oligomer blocks can be liberated from the PUR. Upon liberation, the soft ions in excess of 5 000 000 MW have been measured by ESI.17 blocks are amenable to characterization by MALDI and SEC- However, problems with multicharged oligomer ions have limited MALDI. Ethanolamine and phenylisocyanate are selective chemi- most ESI polymer analysis to molecular weights below 2000 Da.18 cal reagents which are used to analyze polyether and polyester MALDI, in contrast, generally produces singly charged oligomer PURs. In addition to identification and MW determination of the ions, and MW’s up to 1 500 000 for polystyrene (PS) have been constituent polyol, the method described here also identifies the measured.19 MALDI has been applied for MWD determination of diisocyanate linkage. a wide range of polymer types including PEGs,16 poly(methyl methacrylate)s (PMMA),20 poly(dimethylsiloxane)s (PDMS),21,22 EXPERIMENTAL polybutadienes (PBD), 23 and poly(acrylic acid)s (PAA).24 How- Synthesis of PURs. All polymer materials were supplied by ever, MW determination is generally reliable only for narrow Bayer Corp. (New Martinsville, WV). These included a series of MWDs (PD < 1.2).25 MALDI MW determination of high PD model PURs based on 4,4′-diphenylmethane-diisocyanate (MDI) polymers typically results in lower values compared with SEC. and either polytetrahydrofuran (pTHF) or polybutylene adipate Reasons for this discrepancy originate from mass bias during the of various MWs. Premixed polyol and chain extender (butanediol) MALDI desorption/ionization process and sample preparation26 were heated to 60 °C and reacted with an equimolar amount of and instrumental effects.27,28 Small oligomers typically desorb/ diisocyanate (60 °C) using a high speed stirrer. After ap- ionize more easily than large oligomers.18 Instrumental bias can proximately 1 min of intense mixing, the reacting mixture was occur due to detector saturation and velocity-dependent detector cast onto a preheated tray and annealed for 18 h at 110 ×bcC. Finally the PUR was allowed to cool to room temperature. (5) Bletsos, I. V.; Hercules, D. M.; van Leyen, D.; Hagenhoff, B.; Niehuis, E.; End-Group Titration. Number average molecular weights - Benninghoven, A. Anal. Chem. 1991, 63, 1953 1960. (M ) of the polyols were determined using the phthalic anhydride (6) Hercules, D. M. J. Mol. Struct. 1993, 292,49-64. n 34 (7) Dong, X.; Procter, A.; Hercules, D. M. Macromolecules 1997, 30,63-70. method with pyridine as solvent. After being hydrolyzed, the (8) Hittle, L. R.; Hercules, D. M. Surf. Interface Anal. 1994, 31, 217-225. remaining excess anhydride was titrated using normalized KOH - (9) Xu, K.; Proctor, A.; Hercules, D. M. Mikrochim. Acta 1996, 122,1 15. solution. The effective anhydride concentration was determined, (10) Lattimer, R. P.; Schulten, H. Anal. Chem. 1989, 61, 1201A-1214A. (11) Montaudo, G.; Montaudo, M.; Scamporrina, G.; Vitalini, D. Macromolecules as well, and taken into account. The resulting OH numbers are 1992, 25(5), 5099-5107. converted into number average molecular weight using (12) Ballistreri, A.; Garozza, D.; Giuffrida, M,; Montaudo, G. Anal. Chem. 1987, 59, 2024-2027. (56.1 g/mol)(1000 mg/g)(2 mol KOH/mol polyol) (13) Bletsos, I. V.; Hercules, D. M.; van Layen, D.; Benninghoven, A.; Karakat- M ) n # sanis, C. G.; Rieck, J. N. Anal. Chem. 1989, 61, 2142-2149. (OH mg KOH/g polyol) (14) Bletsos, I. V.; Hercules, D. M.; van Layen, D.; Benninghoven, A.; Karakat- sanis, C. G.; Rieck, J. N. Macromolecules 1990, 23, 4157-4163. (15) Cohen, L. R.; Hercules, D. M.; Karakatsanis, C. G.; Rieck, J. N. Macromol- where 56.1 g/mol is the formula weight of KOH, and the - ecules 1995, 28, 5601 5608. conversion factor of 2 is used for bifunctional polyol samples. The (16) Bahr, U.; Deppe, A.; Karas, M.; Hillencamp, F.; Giessman, U. Anal. Chem. 1992, 64, 2866. acid number of the polyesters was lower than 0.6 mg KOH/g and (17) Nohmi, T.; Fenn, J. B. J. Am. Chem. Soc. 1992, 114, 3241-3246. was not taken into account for the above determination. - (18) Hunt, S. M.; Derrick, P. J.; Sheil, M. M. Eur. Mass Spectrom. 1998, 4,1 12. MALDI-MS Analysis. All MALDI-MS spectra were acquired (19) Schriemer, D. C.; Li, L. Anal. Chem. 1996, 68, 2721. (20) Larsen, B. S.; Simonsick, W. J.; McEwen, C. N. J. Am. Soc. Mass Spectrom. using a Voyager-DE STR, MALDI-TOF mass spectrometer from 1996, 7, 287-292. (21) Belu, A. M.; DeSimone, J. M.; Linton, R. W.; Lange, G. W.; Friedman, R. M. (29) Nielen, W. F.; Malucha, S. Rapid Commun. Mass Spectrom. 1997, 11, 1194- J. Am. Soc. Mass Spectrom. 1996 7, 11-24. 1204. (22) Williams, J. B.; Gusev, A. I.; Hercules, D. M. Macromolecules 1996, 29, (30) Blais, J. C.; Tessier, M.; Bolback, G.; Remaud, B.; Rozes, L.; Guittard, J.; 8144-8150. Brunot, A.; Mare´chal, E.; Tabet, J. C. Int. J. Mass Spectrom. Ion Processes (23) Yalcin, T.; Schriemer, D. C.; Li, L. J. Am. Soc. Mass Spectrom. 1997, 8, 1995, 144, 131-138. 1220-1229. (31) Guittard, J.; Tessier, M.; Blais, J. C.; Bolbach, G.; Rozes, L.; Mare´chal, E.; (24) Danis, P. O.; Karr, D. E.; Mayer, F.; Holle, A.; Watson, C. H. Org. Mass Tabet, J. C. J. Mass Spectrom. 1996, 31, 1409-1421. Spectrom. 1992, 27, 843-846. (32) Williams, J. B.; Gusev, A. I.; Hercules, D. M. Macromolecules 1997, 37811- (25) Montaudo, G.; Montaudo, M. S.; Puglisi, C.; Samperi, F. Rapid Commun. 3787. Mass Spectrom. 1995, 9, 453-460. (33) Montaudo, G.; Garozzo, D.; Montaudo, M. S.; Puglisi, C.; Samperi, F. (26) Schriemer, D. C.; Li, L. Anal. Chem. 1997, 69, 4169-4175. Macromolecules, 1995, 28, 7983-7989. (27) Jackson, C.; Larsen, B.; McEwen, C. Anal. Chem. 1996, 68, 1303-1308. (34) Sorensen, W. R.; Cambell, T. M. Preparative Methods of Polymer Chemistry, (28) Schriemer, D. C.; Li, L. Anal. Chem. 1997, 69, 4176-4183. 2nd ed.; Interscience: New York, 1968; p 134.

Analytical Chemistry, Vol. 72, No. 11, June 1, 2000 2491 Perkin-Elmer Biosystems, (Forest City, CA). The instrument is equipped with a N2 laser emitting at 337 nm. Spectra were acquired in the positive-ion mode using the reflectron. The acceleration voltage ranged from 20 to 25 kV. Typically, 48-128 single-shot mass spectra were summed to give a composite spectrum. Polymer samples were dissolved in CHCl3 at 10 mg/ mL. The dithranol (Aldrich Chemical Co., Milwaukee, WI) matrix solution was prepared by dissolving 30 mg in 1 mL of CHCl3 (HPLC grade, Fisher Scientific, Pittsburgh, PA); matrix and polymer solutions were mixed in a 10:1 ratio. The MALDI target was pre-spotted with 1-2 µL of NaI solution (1 mg/mL in methanol) and allowed to air-dry. One to two microliters of matrix/polymer solution was deposited onto the sample target and air-dried. Size Exclusion Chromatography (SEC). SEC was performed using a preparative PLgel, mixed-D, linear PS/DVB from Polymer Laboratories, (Amherst, MA) with a refractive index detector from Knauer (Berlin, Germany). One hundered microliters of a 10% (wt/vol) polymer solution in THF (HPLC grade, Fisher Scientific, Pittsburgh, PA) was injected. THF was used as the eluent at a flow rate of 2 mL/min. For SEC fractionation, 20-30 fractions (0.8-1.0 mL) were collected manually in test tubes and evaporated to dryness overnight. Fifty to five hundred microliters of dithranol solution (30 mg/mL in CHCl3) was added prior to MALDI-MS Figure 1. Spectrum of pTHF 1000 (a), and spectrum of pBA 3560 analysis. (b). Open circles indicate cyclic oligomers, c1 indicates a monocarboxyl-terminated ester, and c indicates a dicarboxyl-terminated ester. MALDI was used to obtain a spectrum for each fraction. Using 2 the Voyager software package, the Mn value was determined for each spectrum. The software uses the peak height in the Mn Chloroform is a suspected carcinogen. Tetrahydrofuran can form calculation. An SEC calibration curve was generated by plotting explosive peroxides. Work with these substances was conducted log Mn for each fraction vs elution volume. The SEC peak was in a chemical ventilation hood wearing chemical-resistant gloves integrated using the SEC software (PL LogiCal version 6.01, and safety goggles. Polymer Laboratories, Amherst, MA), and the elution volumes for M and M were determined. Finally, the M and M elution n w n w RESULTS AND DISCUSSION volumes were converted to M and M using the SEC-MALDI n w MALDI Analysis. Successful MALDI analysis is highly de- calibration curve. pendent upon matrix selection and sample preparation. Several Chemical Degradation. pTHF-PUR. Two and two tenths matrixes have been reported for use with polyethers and polyes- milliliters of ethanolamine (Aldrich Chemical Co., Milwaukee, WI) ters: IAA, HABA, DHB, 5-CSA, HCCA, 30,31 and dithranol.18 Each was added to 2.25 g of PUR in a three-neck flask. The reac- of these matrixes was tested with the present polymer samples, ° tion proceeded under N2 (g) atmosphere at 150 C for 3.5 h. After and DHB and dithranol were found to provide the highest signal the reaction was complete, ethanolamine was removed under intensity. Dithranol was more reproducible than DHB; therefore, vacuum. Purification was carried out by washing the residue three this matrix was used for the analysis of the samples examined in times with ether. From each wash a transparent ether phase this study. was separated from an insoluble yellow phase by filtration. The Figure 1 shows positive-ion MALDI spectra of polytetrahydro- product was recovered from the ether by evaporation under furan (pTHF) (Mn ) 1000) and polybutylene adipate (pBA)(Mn vacuum. ) 2000), two polyols which are commonly used to make PUR pBA-PUR. Four and four-tenths of phenylisocyanate (Aldrich elastomers. For both polymers, intense [M + Na]+ oligomer ion Chemical Co., Milwaukee, WI) was added to 4.0 g of PUR in a peaks are observed. The samples were prepared by first pre- three-neck flask. The reaction proceeded under N2 (g) atmosphere spotting the target with NaI solution. Without additional sodium at 150-185 °C for 3.5-12 h. After the reaction mixture was cooled salt, only very weak signal intensity for oligomer ions was to room temperature, 6.8 mg of dibutylamine (Aldrich Chemical observed. It has been found by others that addition of extra Co., Milwaukee, WI) in 20 mL of THF were added dropwise, not sodium salt can result in enhanced signal intensity of Na+-adduct allowing the temperature to exceed 25 °C. The solution was ions.16,35 It should be noted, however, that other groups have poured into a dish and allowed to evaporate overnight. Purification indicated that addition of salt is not always necessary, depending was carried out by washing the solid raw product with ether. The upon the particular matrix and polymer used.30,33 ether was removed from the desired solid product by filtration, In the spectrum of pTHF (Mn ) 1000), linear oligomer peaks and the product was air-dried. range from n ) 3(m/z ) 257) up to typically n ) 59 (m/z ) Ethanolamine, dibutylamine, and phenylisocyanate are toxic corrosives. Avoid inhalation of vapors and direct skin contact. (35) Pasch, H.; Gores, F. Polymer 1995, 36, 1999.

2492 Analytical Chemistry, Vol. 72, No. 11, June 1, 2000 4292). The spacing between the main peak series is 72 Da, Scheme 1. Ethanolamine Degradation Reaction characteristic of the THF repeat unit. The wide mass range of of PTHF-PUR the ion peaks is typical for broad dispersity polymers such as polyethers and polyesters. Oligomers below n ) 3 are not observed. Absence of oligomer ions below n ) 3 has been reported by others18,20,30 and is thought to be due to a decrease in complex formation with sodium for small oligomers (n < 4).36,37 In addition to the linear oligomer ion peak series, there is a series of weak peaks located 18 Da below the linear oligomer ion peaks- (0). These peaks correspond to cyclic oligomer ions. ESI-TOF analysis also revealed the presence of cyclic oligomer ions.44 Electrospray is a gentle ionization process, imparting very little internal energy to the ionized molecules. Therefore, the presence of cyclic oligomer ions in both MALDI and ESI-TOF spectra suggests that the cyclic oligomers originate from the sample.

In the spectrum of pBA (Mn ) 2000), the main peak series corresponds to linear dihydroxy-terminated oligomer ions ranging from n ) 3(m/z ) 713) up to typically n ) 39 (m/z ) 7922). The spacing between the main peak series is 200 Da, characteristic of the polybutylene adipate monomer. Also present in the spectrum, at low signal intensity, are oligomers with different terminal then be reacted with fresh diisocyanate to form new PUR. Scheme groups. Located at 28 and 72 Da below the dihydroxyl-terminated 1 shows the degradation reaction of pTHF-PUR using ethanolamine. Ethanolamine cleaves only the urethane bonds, producing oligomer ion peaks are the dicarboxyl- (c2) and monocarboxyl- hydroxy-terminated polyethers. The amino group of ethanolamine (c1) terminated oligomers, respectively. Additionally, cyclic oligomer ions (0) are present at 90 Da below the linear dihydroxyl is involved in the reaction because it is more nucleophilic than terminated oligomer ions. Williams et al.32 also reported the the hydroxyl group of ethanolamine. presence of dicarboxyl- and monocarboxyl-terminated and cyclic Ethanolamine cleaves ester bonds, so therefore was not used oligomer ions in MALDI and ESI spectra of polyesters. for the analysis of polyester-PURs because the soft block would Selective Degradation. Degradation reactions are useful for degrade. For this reason, phenylisocyanate was used as a selective the analysis of insoluble or complex polymer systems. Various chemical reagent for polyester PURs. The reaction of isocyanate 42 methods have been used. Partial transesterfication using tri- with urethane bonds is well-known in PUR chemistry. Scheme flouroacetic acid (TFA) can cleave ester and urethane bonds and 2 shows the equilibrium reaction of phenylisocyanate with pBA- has been used in combination with TOF-SIMS for the analysis of PUR. The presence of an equilibrium has consequences with polyester-PURs.15 In addition, TOF-SIMS analysis of TFA esterfied respect to the yield which can be achieved. Under conditions of OH terminal groups was used to determine the branching excess phenylisocyanate, the major product is the N-phenylure- number.38 Hydrolysis with sodium hydroxide enables the analysis thane-terminated polyester. Phenylisocyanate cleaves only the of PUR diols and polyols.3 Thermal degradation using pyrolysis urethane bond, leaving the polyol ester bonds intact. Minor yields the constituent diol or polyol.39 MALDI analysis of pyrolysis products are the MDI-terminated, and mixed N-phenylurethane/ residues enabled the identification of linear polyester oligomers.40 MDI-terminated polyester. Deactivation of reactive isocyanate In the present research, two chemical degradation methods which groups is achieved using dibutylamine. have not been previously reported in conjunction with MALDI Figure 2a shows a MALDI spectrum of an ethanolamine analysis will be applied to enable determination of the polyol degraded pTHF-PUR. The main peak series corresponds to + + identity and MWD. Ethanolamine will be applied for polyether- oligomer [M Na] ions of pTHF. Oligomer ions extend out to ) ) PUR analysis, and phenylisocyanate will be applied for polyester- n 55 (m/z 4008), which is nearly identical to the unreacted PUR analysis. pTHF starting material (see Figure 1a). Inspection of the expanded Ethanolamine has been reported for the recycling of PURs by spectrum (see inset) reveals that there are no intermediate peaks enabling recovery of polyether polyols.41 The recovered polyol can between the oligomer ion peaks. This indicates that the ethanolamine reaction is complete, cleaving all of the urethane bonds.

(36) Bu¨rger, H. M.; Mu¨ller, H.-M.; Seeback, D.; Bo¨rnsen, K. O.; Scha¨r, M.; No evidence is found for cyclic oligomers formed during the Widmer, H. M. Macromolecules 1993, 26, 4738-4790. degradation reaction. Since the repeat pattern shows only pTHF (37) Reinhold, M.; Meier, R. J.; de Koster, C. G. Rapid Commun. Mass Spectrom. oligomer ions, information about the type of diisocyanate used is 1998, 12, 1962-1966. (38) Kim, Y. L.; Hercules, D. M. Macromolecules 1994, 27, 7855-7871. not available from the spectrum. (39) Berenbaum, M. B. In Chemical Reactions of Polymers; Fettes, E. M., Ed.; However, by stopping the reaction prior to completion, it is Interscience: New York, 1969; pp 983-984. possible to obtain an additional reaction product which contains (40) Lattimer, R. P.; Polce, M. J.; Wesdemiotis, C. J. Anal. Appl. Pyrolysis 1998, 48,1-15. the MDI segment. Figure 2b shows a MALDI spectrum of an (41) Yamamura, H. U.S. Patent 721,595, 1995. incompletely degraded pTHF-PUR; the reaction was stopped after (42) Bunge, W. Angew. Chem. 1960, 72, 1002. 1 h. An additional peak series is observed. This series is due to (43) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; p 95. species containing the PUR diisocyanate linkage. The peaks are (44) Mehl, J. T. Unpublished results. labeled A and B in Figure 2b, and the structures of these ions

Analytical Chemistry, Vol. 72, No. 11, June 1, 2000 2493 are illustrated in Figure 3a. This demonstrates that, by controlling Scheme 2. Phenylisocyanate Degradation reaction conditions, valuable information about the PUR structure Reaction of PBA-PUR can be obtained. In this case, the type of polyether and type of diisocyanate used to synthesize the PUR can be identified. Figure 4 shows a MALDI spectrum of a phenylisocyanate- degraded pBA-PUR. The separation between peaks of the main series is 200 Da, consistent with the polybutylene adipate repeat unit. The ions consist of a pBA chain with N-phenylurethane terminal groups. In addition to this series, there are two other series present. These peaks correspond to the expected minor products of the phenylisocyanate degradation reaction. The additional peak series are labeled Y and Z, and the ion structures are shown in Figure 3b. These ions provide identification of the diisocyanate linkage. Attempts to drive the phenylisocyanate degradation reaction further toward completion were not successful. Even after 12 h at 190 °C, degradation products containing the MDI linkage were still present in the MALDI spectra. The phenylisocyanate reaction is reversible and establishes system equilibrium and so it is therefore unlikely that all the urethane bonds will remain uncleaved; some urethane bonds will be reformed. In some cases the peaks corresponding to the minor products are more intense than shown in Figure 4 due to batch- to-batch variability of the degradation reaction. The number average molecular weight (Mn), the weight average molecular weight (Mw), and the polydispersity index (PD) can be determined by using the masses of the oligomer ions and the corresponding signal intensity. Mw and Mw are calculated using

) be due to either decreased cationization of small oligomers with Mn ∑ NiMi/∑ Ni sodium, loss of the more volatile smaller oligomers to the vacuum ) 2 system, or detector saturation from matrix and lower MW Mw ∑ NiMi /∑ NiMi contamination peaks in the low mass spectral region. For the ) pTHF 2000 sample, smaller oligomers are less abundant in the PD Mw/Mn oligomer distribution, and therefore, mass bias does not influence

the Mn determination. where Ni is the measured peak intensities (peak height) of a Table 2 gives the Mn and Mw values determined by MALDI molecular ion with degree of polymerization i and Mi is the mass for the pBA and degraded pBA-PUR samples. Mn values deter- of the ith oligomer. mined by end-group titration are included for the pBA samples.

Table 1 gives Mn and Mw values determined by MALDI for For the lowest MW sample (pBA 1000), the MALDI value is the pTHF and degraded pTHF-PUR samples. End-group titration higher than the titration value. This reflects a mass bias against values are also listed for the pTHF samples. There is very good smaller oligomers during MALDI analysis. The same behavior is agreement between the MALDI Mn and Mw values for the pTHF observed for the lower MW pTHF samples, as mentioned above. and degraded pTHF-PUR samples. This indicates that the deg- For the pBA 2000 sample, the MALDI and titration values are radation reaction allows recovery of a representative pTHF reasonably close. However, for the pBA 4000 sample, the titration oligomer distribution. The polydispersity (PD) indexes determined Mn value is higher than the MALDI value. This is also the case by MALDI are reasonable for pTHFs, which are expected to have for the pBA 3650 sample, which is a more narrowly distributed PD values of approximately 1.5, based on the polymerization con- sample than the pBA 4000 sample. This indicates that for the ditions. Though the PD values are not known for the pTHF sam- larger MW samples, MALDI under-represents the larger oligo- ples, the results suggest that MALDI analysis may be sufficient mers. There is a transition which occurs with MW for Mn for MW determination of small polyethers. In general it has been determination by MALDI. For lower MW pBA samples, the lower shown that MALDI does not provide accurate MW and PD values mass oligomers are discriminated against; however, for higher for polymers with polydispersities greater than 1.2.18,25 However, MW pBA samples, the higher mass oligomers are discriminated the results shown here indicate that for low MW polyethers against. The pBA 2000 sample coincides with an intersection of

MALDI appears to provide reasonable values of Mn and Mw. the two methods, where the Mn values determined by end-group The Mn values determined by MALDI for the two smaller titration and MALDI are similar. pTHF samples (pTHF 650 and 1000) are higher than the end- Comparing the pBA and degraded pBA-PUR samples, there group titration values. This indicates that MALDI analysis is biased are obvious differences between the PD indexes. The PD indexes against smaller oligomers (n e 3). The monomer and dimer peaks for the degraded samples are consistently lower, indicating that were not observed in the spectra of the pTHF samples. This could the oligomer distributions are different. The Mn values for the

2494 Analytical Chemistry, Vol. 72, No. 11, June 1, 2000 Figure 2. Spectrum of pTHF(1000)-PUR degraded using ethanolamine; reaction time is 3.5 h (a). Spectrum of degraded pTHF(1000)-PUR; reaction time is 1 h (b). The ion structures for the A and B series are shown in Figure 3a. degraded PUR samples are greater than the corresponding mass biasing in MALDI spectra of polymers.26 Further investiga- unreacted pBA samples, indicating that smaller oligomers are less tion of these possible effects on the recovered pBA oligomer abundant in the degraded material. distributions are necessary to determine the accuracy of this The purification procedure is the most likely cause for the degradation method for MW and PD determination. However, the observed difference in the oligomer distribution of the degraded results shown here indicate that the phenylisocyanate reaction pBA-PUR samples. Presently, simple washing of the solid raw allows recovery of a polyester soft-block oligomer distribution from product with ether is used to remove the degradation reaction PURs. Therefore, phenylisocyanate degradation in combination byproducts. Small pBA oligomers are soluble in ether and might with MALDI analysis is a promising method for soft-block char- be selectively removed during the washing step. Additionally, the acterization. MALDI analysis could be influenced by the presence of degrada- SEC-MALDI. SEC was used to separate the polymers into tion reaction byproducts not completely removed during the narrower molecular weight fractions. MALDI analysis of the purification procedure. The appearance of the MALDI crystals are fractions provides determination of Mn and Mw for each fraction. different when comparing the pBA and degraded pBA samples. Figure 5 shows SEC chromatograms for unreacted pTHF (2000) It has been shown that sample preparation can introduce serious and ethanolamine degraded pTHF(2000)-PUR. The large peak is

Analytical Chemistry, Vol. 72, No. 11, June 1, 2000 2495 Figure 3. Structure of ions observed in spectra of ethanolamine-degraded pTHF-PUR (a). Ion peaks are marked in Figure 2. Structure of ions observed in spectra of phenylisocyanate-degraded pBA-PUR (b). Ion peaks are marked in Figure 4.

Figure 4. Spectrum of pBA(2000)-PUR degraded using phenylisocyanate. Ion structures for the X, Y, and Z series are shown in Figure 3b.

due to pTHF. A second peak, at longer elution time, is present in to the degradation reaction product N,N′′-ethylenedi-4,1-phen- the degraded pTHF-PUR sample. Most likely this peak belongs ylene)bis[N′-(2-hydroxyethyl)]-urea, which is partially soluble in

2496 Analytical Chemistry, Vol. 72, No. 11, June 1, 2000 Table 1. Molecular Weights for Polytetrahydrofuran (pTHF) and Degraded Polyether Polyurethanes Determined by MALDI and SEC-MALDI

titrationc MALDI SEC-MALDI

sample Mn Mn Mw PD Mn Mw PD pTHF 650a 660 1059 1638 1.55 1168 2194 1.88 pTHF(650)-PURb 1104 1599 1.45 1205 2169 1.80 pTHF 1000a 984 1234 1944 1.58 1433 2675 1.87 pTHF(1000)-PURb 1304 1906 1.46 1348 2831 2.10 pTHF 2000a 2051 1914 2739 1.43 1873 4765 2.54 pTHF(2000)-PURb 1981 2786 1.41 1779 4082 2.29

a Unreacted polytetrahydrofuran (pTHF). b Degraded polyether polyurethane; values in parentheses are the nominal weights of polyether used to make original polyurethane. c End-group titration performed only on unreacted pTHF.

Table 2. Molecular Weights for Polybutylene Adipate (pBA) and Degraded Polyester Polyurethanes Determined by MALDI and SEC-MALDI

titrationd MALDI SEC-MALDI Figure 5. Size-exclusion chromatograms of pTHF 2000 (a), and

sample Mn Mn Mw PD Mn Mw PD ethanolamine-degraded pTHF(2000)-PUR (b). Selected fractions are marked in (b), and the corresponding MALDI spectra are shown in a pBA 1000 1022 1757 2713 1.55 1295 3303 2.55 Figure 6. pBA(1000)-PURb 1817 2381 1.31 1943 3069 1.58 pBA 2000a 2011 1872 2897 1.56 1910 4748 2.48 pBA(2000)-PURb 2731 3447 1.26 2604 5196 2.00 pBA(2000)-PUR-Ec 2893 3567 1.23 3456 5253 1.52 pBA 3650a,e 3650 3048 3783 1.24 3078 6200 2.01 pBA 4000a 3650 2327 3541 1.53 2980 7322 2.46 pBA(4000)-PURb 2875 3877 1.35 3515 8513 2.42

a Unreacted polybutylene adipate. b Degraded polyester polyurethane; values in parentheses are the nominal weight of polyesters used to make original polyurethane. c Same as footnote b; however, polyurethane included butanediol chain extender. d End-group titration performed only on unreacted polybutylene adipate. e Narrow distributed polybutylene adipate, assumed PD of approximately 1.5. Obtained by extracting smaller oligomers from normally distributed pBA 2000.

ether. MALDI analysis of the sharp peak indicated the absence of pTHF oligomers. For lower MW pTHF-PUR soft blocks, the sharp peak partially overlaps with the polymer SEC peak, because small oligomers have longer elution times. Peak interference in SEC can cause difficulties for peak integration because the baseline and integration end points are more difficult to establish. MALDI spectra of selected SEC fractions of an ethanolamine- degraded pTHF-PUR sample are shown in Figure 6. The polydispersity of each fraction is narrower than that of the unfractionated sample. Fractionation of the sample allowed detection of larger oligomer ions, up to n ) 174 (m/z ) 12580), compared with n ) Figure 6. Spectra of selected SEC fractions from ethanolamine- degraded pTHF(2000)-PUR. The location of the fractions are shown 79 (m/z ) 5736) for the unfractionated sample. Accurate Mn in Figure 5. determination of each fraction is possible using MALDI, and this method was used to calibrate the SEC trace. Mn and Mw values for each polymer were then determined from the SEC-MALDI to use of a refractive index (RI) SEC detector, which is less calibration curve and the respective elution times calculated sensitive to lower mass oligomers. through integration of the chromatogram by the SEC software. The Mw and PD values determined by SEC-MALDI are higher

Table 1 lists the results of SEC-MALDI analysis for the pTHF than the MALDI Mw and PD values. This indicates that SEC- and degraded pTHF-PUR samples. There is reasonable agreement MALDI is more sensitive to higher mass oligomers than MALDI. between the Mn values determined by MALDI and those deter- It is well recognized that MALDI sensitivity decreases with mined by SEC-MALDI. However, both methods yield higher increasing MW and PD.25 In contrast, the sensitivity of an RI values than end-group titration, indicating that SEC-MALDI also detector increases with MW. Therefore, more accurate determi- discriminates against lower mass oligomers. This could be due nation of Mw and PD can be made using SEC-MALDI compared

Analytical Chemistry, Vol. 72, No. 11, June 1, 2000 2497 with using MALDI analysis alone. The SEC-MALDI determined Two selective chemical degradation reagents, which have not been PD indexes range from 1.8 to 2.5, which is higher than the previously reported in combination with MALDI analysis, were expected value of 1.5. The expected PD value is based on the utilized. Ethanolamine was applied for the recovery of pTHF soft reaction condition used to synthesize the pTHF polyethers. blocks. MALDI analysis of the degraded polymers indicates that However, the PD indexes for these samples have not been ethanolamine gives recovery of a representative oligomer distribu- previously determined, and it is only assumed that the PD indexes tion. Allowing only partial degradation enables identification of are 1.5. The SEC-MALDI results indicate that the PD indexes are the diisocyanate using ions containing the diisocyanate linkage higher than 1.5 for the pTHF samples examined. which appear in the MALDI spectrum. The polydispersity indexes SEC-MALDI analysis was also performed on the pBA and determined by MALDI analysis are in reasonable agreement with degraded pBA-PUR samples, and the results are given in Table the expected values based on the pTHF synthesis reaction. This 2. The M values of the unreacted pBA samples determined by n suggests that, for small MW polyethers, MALDI analysis may be SEC-MALDI agree reasonably well with the end-group titration sufficient for M and M determination. values, indicating that SEC is not suffering from significant n w Phenylisocyanate, which does not cleave ester bonds, was discrimination against smaller pBA oligomers. The PD values applied for the analysis of polyester PURs. MALDI analysis determined by SEC-MALDI are higher than the MALDI deter- showed that the pBA oligomer distribution is recovered, along mined values. The same behavior is observed for the pTHF samples shown in Table 1. Once again, MALDI is less sensitive with minor degradation products containing the MDI linkage, to the larger oligomers. The combination of SEC with MALDI providing identification of both the polyester and the diisocyanate. can overcome this problem, enabling more accurate determination Comparison of the degraded pBA-PUR oligomer distributions with the unreacted pBA material indicates that smaller oligomers are of Mw and Mn. The PD values for pBA samples are close to 2.5, except for the pBA 3650, because of its preparation by extraction less abundant in the degraded samples. of smaller oligomers from a 2000 MW pBA sample. The PD SEC-MALDI was used to obtain more accurate MWD deter- indexes for these samples are expected to be close to 2.0, based minations than obtained with MALDI alone. The polydispersity on reaction theory;43 however, the PD indexes have not been indexes determined using SEC-MALDI are higher than the previously determined for these samples. The SEC-MALDI results MALDI determined PD indexes. The results presented here indicate that the pBA samples are more disperse than theory indicate that selective degradation in combination with SEC- predicts. MALDI analysis is a viable means for polyether and polyester The pBA polyols recovered from the phenylisocyanate degra- polyurethane soft-block characterization. dation reaction have lower PD indexes than the unreacted pBA samples, except for the largest MW sample. This means that the ACKNOWLEDGMENT recovered pBA samples have different oligomer distributions than This research was supported by the National Science Founda- the corresponding unreacted pBA samples. The Mn values determined by SEC-MALDI for the degraded samples are larger tion, Grant CHE-9520336. The authors wish to thank Dr. Costas than the unreacted pBA samples, indicating that smaller oligomers G. Karakatsanis for critical review of the manuscript. are less abundant in the degraded samples.

Received for review November 10, 1999. Accepted March CONCLUSIONS Selective chemical degradation combined with MALDI analysis 24, 2000. was used for the characterization of polyether and polyester PURs. AC991283K

2498 Analytical Chemistry, Vol. 72, No. 11, June 1, 2000