Structural Characterization of Quinoxaline Homopolymers and

Chrys Wesdemiotis , Nilüfer Solak , Michael J. Polce , David E. Dabney , Kittisak Chaicharoen , Bryan C. Katzenmeyer. Mass Spectrometry Reviews 2010,n...
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Anal. Chem. 2001, 73, 1948-1958

Structural Characterization of Quinoxaline Homopolymers and Quinoxaline/Ether Sulfone Copolymers by Matrix-Assisted Laser Desorption Ionization Mass Spectrometry Michael J. Polce,† Daniel J. Klein,‡,§ Frank W. Harris,‡ David A. Modarelli,† and Chrys Wesdemiotis*,†

Departments of Chemistry and Polymer Science, The University of Akron, Akron, Ohio 44325

Polyphenylquinoxalines (PPQs) are prepared from selfpolymerizable quinoxaline monomers that carry fluorine, hydroxyaryl (ArOH), and phenyl substituents. In basic media, these monomers self-polymerize via a series of nucleophilic aromatic substitution reactions (SNAr), in which aromatic enolates (ArO- nucleophiles) attack the electrophilic carbons bearing F leaving groups to effect fluoride displacement. Polyphenylquinoxaline/polyethersulfone (PPQ/PES) copolymers are synthesized similarly by combining self-polymerizable quinoxaline monomers with a 1:1 molar mixture of 4,4′-dichlorodiphenyl sulfone and bisphenol A. The MALDI mass spectra of the polymers reveal that the major products up to ∼15 000 Da molecular mass are homo- or copolymeric macrocycles. Linear byproducts are also observed, arising from nucleophilic ring opening of already formed macrocycles. Oligomers containing at least one PPQ unit readily protonate upon MALDI, whereas PES homopolymers require alkali metal ion addition to become detectable. Molecular orbital calculations point out that the nucleophilic and electrophilic reactivities of the PPQ monomer and the PPQ growing chains generated during propagation are comparable, allowing for continued condensations via SNAr, until cyclization terminates this process. The calculations also predict a significantly lower electrophilic reactivity for carbons substituted by chlorine instead of fluorine, justifying the discrimination against incorporation of PES units observed for the copolymers. The computationally optimized structures of PPQ and PPQ/ PES macrocycles show a diverse array of cavity sizes and geometries which depend on the size of the macrocycle, the sequence of the repeat units, and the position of the substituents in the quinoxaline ring; quinoxaline pendants (phenyl groups) are found to favor helical arrangements in the prepared macrocycles. Polyphenylquinoxalines (PPQs) constitute an important class of thermoplastics, characterized by high glass transition temper* To whom correspondence should be addressed: (phone) (330) 972-7699; (fax) (330) 972-7370; (e-mail) [email protected]. † Department of Chemistry. ‡ Department of Polymer Science. § Current address: Advanced Materials and Processing Branch, NASA Langley Research Center, Hampton VA 23681-2199.

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atures, low dielectric constants, high chemical and thermooxidative stabilities, and excellent mechanical properties.1-7 As a result, these polymers are suitable to replace many popular polyimides as high-temperature films, adhesives, and matrix resins for advanced composites.8 PPQs have traditionally been prepared by the reaction of aromatic tetraamines with aromatic tetraketones, which combine to form quinoxaline rings during the polymerization process.1,3-6,9 The latter monomers require, however, expensive and multistep syntheses, thereby preventing the widespread industrial use of PPQs. An alternative synthetic route to PPQs involves the polymerization of monomers with preformed quinoxaline rings.3-7,9-11 Scheme 1 depicts such a monomer (1) that has been produced in high yield by Harris et al.,3,5,7,9 using precursors of lower cost than the above-mentioned tetraamines and tetraketones. The synthesis leads to anti and syn isomers of the monomer. Both contain a nucleophilic site (OH) and an electrophilic C-atom bound to a potential leaving group (F) and can, thus, undergo selfpolymerization via a series of nucleophilic aromatic substitution reactions to yield a PPQ polymer (Scheme 1).3,5,7,9 The reaction takes place under basic conditions and is mechanistically analogous to the known condensation polymerization of 4,4′-dichlorodiphenyl sulfone (2) with bisphenol A (3), which also proceeds via nucleophilic aromatic substitution (in basic media), yielding a polyethersulfone (PES).5,12 PESs belong to the class of membrane and high-temperature specialty materials.8,13 Self-polymerizable (1) Hergenrother, P. M.; Levine, H. H. J. Polym. Sci., Polym. Chem. Ed. 1967, 5, 1453. (2) Labadie, J. W.; Hedrick, J. L. SAMPE J. 1989, 25, 18. (3) Korleski, J. E. Ph.D. Dissertation, The University of Akron, 1991. (4) Hedrick, J.; Twieg, R.; Matray, T.; Carter, K. Macromolecules 1993, 26, 4833. (5) Klein, D. J. Ph.D. Dissertation, The University of Akron, 1998. (6) Klein, D. J.; Baek, J.-B.; Harris, F. W. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1999, 40 (2), 882. (7) Kim, B.-S.; Korleski, J. E.; Zhang, Y.; Klein, D. J.; Harris, F. W. Polymer 1999, 40, 4553. (8) Cowie, J. M. G. Polymers: Chemistry and Physics of Modern Materials, 2nd ed.; Blackie Academic & Professional: London, 1991. (9) Harris, F. W.; Korleski, J. E. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1989, 6, 870. (10) Hedrick, J. L.; Labadie, J. W. Macromolecules 1988, 21, 1883; Macrocolecules 1990, 23, 1561. (11) Connell, J.; Hergenrother, P. M. Polymer 1992, 33, 3379. (12) Johnson, R. N.; Farnham, A. G.; Clendinning, R. A.; Hale, W. F.; Merriam, C. N. J. Polym. Sci. A-1 1967, 5, 2375. (13) Kessing, R. E. Synthetic Polymeric Membranes, 2nd ed.; John Wiley & Sons: New York, 1985. 10.1021/ac001247h CCC: $20.00

© 2001 American Chemical Society Published on Web 03/21/2001

Scheme 1. PPQ Formed by Self-Polymerization of Quinoxaline Monomer 1 in Basic Solution5,7a

a The curved, double-headed arrow indicates that 1 is a mixture of syn/anti isomers, both of which can be incorporated into the polymer.

Chart 1. Self-Polymerizable Quinoxaline Monomers, Containing a Hydroxyl-Substituted Naphthyl (4) or Biphenyl (5) Groupa

a The curved, double-headed arrows indicate that 4 and 5 are syn/anti mixtures.

quinoxaline monomers (such as the one shown in Scheme 1) can be copolymerized with bisphenol A and 4,4′-dichlorodiphenyl sulfone to afford PPQ/PES copolymers,5 which display different solubility and processability as compared to PPQ homopolymers.4,5 The thermal, mechanical, adhesive, and rheological properties of polyphenylquinoxalines qualify them as high-performance thermoplastics, prompting interest in their so far unknown microstructures. Here, we address this topic by employing matrixassisted laser desorption/ionization mass spectrometry (MALDIMS)14 to characterize the composition, architecture, and end groups of the PPQs generated from the aryl-substituted, selfpolymerizable monomers 1, 4, and 5 (cf. Scheme 1 and Chart 1).5,6 PPQ/PES copolymers of 1 with varying amounts of an equimolar mixture of 4,4′-dichlorodiphenyl sulfone (2) and bisphenol A (3) are also investigated.5 MALDI is a desorption/ionization method,15,16 producing quasimolecular ions from each oligomer present in the sample under analysis; fragment ions are generally not formed. The mass-tocharge ratios (m/z) of the oligomer ions observed in the mass spectrum reveal the repeat unit(s), composition, and end groups of the oligomers, while the overall appearance of the spectrum (i.e., relative abundance vs m/z) may provide insight about the molecular weight distribution of the polymer.17-22 MALDI-MS has (14) Karas, M.; Hillenkamp, F. Anal. Chem. 1988, 60, 2299. (15) Polce, M. J.; Wesdemiotis, C. In Mass Spectrometry of Polymers; Montaudo, G., Lattimer, R. P., Eds.; CRC Press: Boca Raton, FL, Chapter 1, in press. (16) Cotter, R. J. Time-of-Flight Mass Spectrometry. Instrumentation and Applications in Biological Research; ACS Professional Reference Books, American Chemical Society: Washington, DC, 1997; p 131.

successfully been used to elucidate the structures of several types of polymers.23,24 The present study extends its application to novel and industrially promising PPQ homopolymers and PPQ/PES copolymers. EXPERIMENTAL SECTION Polymerizations. The PPQ homopolymers were formed by heating the monomers (1, 4, 5) in a 1:1 (v/v) mixture of N-methyl2-pyrrolidinone (NMP) and toluene in the presence of potassium carbonate (Scheme 1). Under such basic conditions, the monomer OH groups are converted to O-, which can initiate polymerization via successive nucleophilic aromatic substitution reactions. The polymer synthesis consisted of four basic steps: (i) the reaction mixture was first stirred and heated for 4 h under N2 flow to facilitate the azeotropic removal of the water formed during the conversion of OH to O-; (ii) toluene was then removed by distillation (∼1 h), and (iii) the remaining NMP solution was heated for g3 h to complete the polymerization; (iv) finally, the resulting product was diluted with NMP and this mixture was added to methanol/acetic acid to precipitate the polymer, which was reprecipitated twice from chloroform. The PPQ/PES copolymers were formed similarly by mixing 1 (PPQ unit) with 2 and 3 (PES unit) in the molar ratios 3:1:1, 1:1:1, and 1:3:3, corresponding to theoretical molar PPQ/PES contents of 75%/25%, 50%/50%, and 25%/75%, respectively. Monomer 1 was also polymerized under pseudo-high-dilution conditions,25 involving the dropwise addition of an NMP solution of 1 to a mixture of NMP, toluene, and K2CO3, followed by steps ii-iv. A detailed description of the polymerization procedures and the synthesis of the corresponding monomers has been reported elsewhere.5-7 MALDI-MS Analysis. The MALDI mass spectra were acquired in positive (+20.00 kV) reflectron mode using a Bruker REFLEX time-of-flight (TOF) mass spectrometer (Bruker Daltonics, Billerica, MA). The instrument is configured with an LSI model VSL-337ND pulsed 337-nm nitrogen laser (Laser Science, Inc., Franklin, MA), a SCOUT26 ion source with gridless pulsed ion extraction, and a 1-GHz sampling rate digitizer. The mass scale was calibrated externally, using the peptides angiotensin II, bombesin, and somatostatin 28 (Sigma Chemical Co., St. Louis, MO). For polymer analysis, dithranol (ICN Biomedicals Inc., Aurora, OH) and potassium trifluoroacetate (Aldrich Chemical Co., Milwaukee, WI) were used as the UV-absorbing matrix and cationizing salt, respectively. Polymer samples were dissolved in HPLC grade benzene or tetrahydrofuran (Aldrich Chemical Co.) (17) (a) Danis, P. O.; Karr, D. E. Org. Mass Spectrom. 1993, 28, 923. (b) Danis, P. O.; Karr, D. E.; Xiong, Y.; Owens, K. G. Rapid Commun. Mass Spectrom. 1996, 10, 862. (18) (a) Montaudo, G.; Montaudo, M. S.; Puglisi, C.; Samperi, F. Macromolecules 1995, 28, 4562. (b) Montaudo, M. S.; Puglisi, C.; Samperi, F.; Montaudo, G. Macromolecules 1998, 31, 8666. (19) Dey, M.; Castoro, J. A.; Wilkins, C. L. Anal. Chem. 1995, 67, 1575. (20) Schriemer, D. C.; Li, L. Anal. Chem. 1996, 68, 2721. (b) Schriemer, D. C.; Whittal, R. M.; Li, L. Macromolecules 1997, 30, 1955. (21) Williams, J. B.; Gusev, A. I.; Hercules, D. M. Macromolecules 1997, 30, 3781. (22) Lattimer, R. P.; Polce, M. J.; Wesdemiotis, C. J. Anal. Appl. Pyrolysis 1998, 48, 1. (23) Nielen, M. W. F. Mass Spectrom. Rev. 1999, 18, 309. (24) Montaudo, G., Lattimer, R. P., Eds. Mass Spectrometry of Polymers; CRC Press: Boca Raton, FL, in press. (25) Srinivasan, S.; Hedrick, J. L.; Chan, K. P.; Hawker, C. J.; Twieg. R. Macromol. Symp. 1997, 122, 101.

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at a concentration of 10 mg/mL in polypropylene microcentrifuge tubes. Separate solutions of matrix and cationizing salt were likewise prepared at concentrations of 20 and 10 mg/mL, respectively. Protonated ions were observed when the benzene solutions of polymer and matrix were mixed in a 1:2 ratio; on the other hand, potassiated and protonated ions were observed when tetrahydrofuran solutions of polymer, matrix, and salt were mixed in a 1:2:1 ratio. In either case, 0.75 µL of the final sample solutions were deposited onto a polished stainless steel SCOUT26 sample holder and, after solvent evaporation, placed into the mass spectrometer. All m/z values quoted are monoisotopic, referring to the species containing only the lowest-mass isotopes of each element. The laser intensity and instrument parameters were adjusted to maximize the sensitivity at a resolution of 3000-8000 (50% valley) over the detected m/z range. For this, the laser attenuation was set at 70-75%, the delay time for pulsed extraction at 300 ns, the IS/2 (intermediate extraction) field at 15.81 kV, the reflector potential at 22.50 kV, and the detector voltage at 1.50 kV; the values set for the digitizing parameters were 10 Hz repetition rate, 1 ns time base, 38 000 ns data collection delay time, and 60 000 data point spectrum size. Under these conditions, the mass range m/z 800-5500 is recorded. Setting the data collection delay time at 0 or 80 000 ns allowed us to record the m/z ranges 0-1500 and 3500-15 000, respectively. During ion collection in the m/z 800-5500 and 3500-15 000 regions, products of m/z 10 000 (Mw) and an average polydispersity of 3.5.5 As expected for such characteristics, our MALDI mass spectrum is dominated by low-mass oligomers,30,31 containing detectable signals up to m/z ∼13 000 (vide supra). Up to that size, the major macromolecules observed are cyclic and no linear polymer with unreacted F and OH end groups is detected. The presence of very high molecular weight chains (.13 000), terminated by F and OH substituents, cannot however be excluded with confidence, unless the product is fractionated by GPC prior to MS analysis.23,24 PPQ/PES Copolymers. The starting reagents in the copolymerization process are the quinoxaline phenolate [1 - H]-, the sulfone 2, and the bisphenolate [3 - 2H]2-. The corresponding Mulliken charge distributions (Table 2) reveal a markedly lower δC value for the electrophilic C-atom of 2 versus that of [1 - H]-; this is due to the lower electronegativity of Cl versus F. On the other hand, the negative charge (δO) at the nucleophilic O- center is higher for [3 - 2H]2- than [1 - H]-, because the former is not as well delocalized. From these values, the reaction between 2 and either [1 - H]- or [3 - 2H]2- should be slow, discriminating this way the buildup of the PES component in the copolymer. Indeed, the most abundant products observed in the copolymer mass spectra contain a smaller proportion of PES units than expected from the molar ratio in which the PES and PPQ monomers were mixed.36 Judging from the δC and δO data (Table 2), the most facile initial step to the copolymer is nucleophilic displacement by [3 - 2H]2- of F- in the phenolate of a PPQ monomer (preferentially 1a) or PPQ oligomer, yielding a condensate with two reactive (42) In the cyclic product, the aromatic ether C-atoms have electrophilic character. The C(O) atom at the quinoxaline ring (δC ) +0.03 in syn and -0.04 in anti geometries) is less electropositive than the C(O) atom at the phenyl ring (δC ) +0.11 in all geometries). Ring-opening SNAr reactions at these C-atoms, by nucleophiles still present in the reaction mixture, would release an Ar-O- anion, which is a poor leaving group.38 As a result, such reactions cannot compete with propagation or ring closure via F- displacement. When they occur, for example with the NMP-derived enolate (Chart 2), the nucleophile should preferably attach at the more electrophilic phenolic end, as shown in Chart 2.

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Figure 11. Optimized structures (semiempirical AM1 level) of dimeric intermediates in the formation of PPQ/PES copolymers. (a) Dianion arising from the condensation of 1a (electrophile) and 3 (nucleophile), i.e., [1a/3 - 2H]2-; (b) anion arising from the condensation of 2 (electrophile) and 3 (nucleophile), i.e., [2/3 - H]-; and (c) condensate of 1a (nucleophile) and 2 (electrophile), i.e., 1a/2. See Chart 3 and Table 2 for an explanation of the connectivities given under each structure.

O- sites, such as [1a/3 - 2H]2- (cf. Figure 11 and Table 2). In the presence of sufficient monomers, this nucleophile should

substitute faster the F of [1 - H]- than the Cl of 2 due to the superior electrophilicity of an aromatic C-atom attached to F than Cl (Table 2). A copolymer with separated sulfone and bisphenolate subunits would emerge (structure II in Chart 3). At high concentrations of the PES monomers 2 and 3, on the other hand, the condensation kinetics of these two species would be favorable, leading to the combined PES monomer [2/3 - H]- (Figure 11) or an oligomer thereof, which later copolymerizes with the quinoxaline phenolate to yield a copolymer with structure I. The slowest and, hence, least probable initial copolymerization step is SNAr of 2’s Cl-atom by a quinoxaline phenolate; the product of such a reaction with the syn isomer, 1a/2, is shown in Figure 11. This intermediate possesses two electrophilic sites, viz. C(F) and C(Cl), and can yield either I or II, depending on which site reacts first with the more reactive bisphenolate A nucleophile [3 - 2H]2- (SNAr at C(F) should predominate, vide supra). Overall, the formation of PPQ/PES copolymers with separated sulfone and bisphenolate units (II) should predominate at high molar concentrations of the PPQ monomer and at the beginning of the polymerizations, when sufficient PPQ monomer is available. Reversely, copolymers with contiguous PES units (I) are expected to be the major products at high molar concentrations of 2 and 3 and toward the end of the polymerizations. The same macromolecule may contain segments with structures I and II as the monomer concentrations change during propagation. Each time the growing oligomers reach the same number of sulfone and bisphenolate units, there is a nucleophilic and an electrophilic chain end, and cyclization via intramolecular SNAr becomes competitive, leading to termination. After the monomers have been consumed, chains with two nucleophilic or two electrophilic ends, analogous to 1a/2 and [1a/3 - 2H]2- (cf. Table 2 and Figure 11), may still be present, in approximately equal concentrations because 2 and 3 are always mixed in a 1:1 molar ratio. Intermolecular SNAr of these oligomers can take place, giving rise to linear copolymers with one halogen atom at one and one Osite at the other chain end; the latter can now cyclize via intramolecular SNAr to yield the macrocyclic PPQ/PES copolymers identified in the MALDI mass spectra. Macrocycle Structures. The structures of several small PPQ and PPQ/PES oligomers were optimized by empirical calculations using the MM2 force field. Figure 12 shows the geometries of

Figure 12. Optimized structures (molecular mechanics) of the smallest possible PPQ and PPQ/PES macrocycles. The H-atoms are omitted for simplicity. (a) All-syn PPQ 3-mer; (b) PPQ/PES dimer with the syn PPQ monomer; and (c) PES dimer. See Chart 3 and Table 2 for an explanation of the connectivities given under each structure. 1956

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Figure 13. Optimized structures (molecular mechanics) of cyclic PPQ 6-mers. The H-atoms are omitted for simplicity. (a) All-anti isomer; (b) 5 syn/1 anti isomer. See Chart 3 and Table 2 for an explanation of the connectivities given under each structure.

Figure 14. Optimized structures (molecular mechanics) of PPQ/PES 3-mers (cyclic) containing one PES and two PPQ repeat units. (a) Contiguous arrangement of the PES components and anti orientation of the PPQ substituents; (b) separate PES components between two anti PPQ units; (c) separate PES components between one anti and one syn PPQ unit. See Chart 3 and Table 2 for an explanation of the connectivities given under each structure. The energy levels of these isomers lie within 15 kJ/mol, increasing in the order (b) < (c) < (a). Replacing both anti with syn PPQ monomers in (a) barely changes the energy. Any other contiguous or separate connectivity leads to structures of considerably higher energy.

the smallest possible macrocycles, viz. the PPQ 3-mer, the PPQ/ PES dimer, and the PES dimer. At these sizes, the most stable arrangement involves the syn isomer of 1 (i.e., 1a). The quinoxaline phenyl rings and the bisphenol A methyl groups point out away from the ring cavity to minimize atom-atom repulsions. It is further noticed that the phenyl pendants of the PPQ 3-mer are arranged in a helical manner, suggesting that the macrocycles may be formed as enantiomeric mixtures. The optimized structure of the PPQ 4-mer closely resembles that of the 3-mer, containing only syn repeat units; structures

carrying one or more anti quinoxalines have higher strain energies. In contrast, the most stable PPQ 5-mer carries one anti and four syn units. With larger sizes, several isomers are essentially isoenergetic. For example, the all-syn, all-anti, and 5 syn/1 anti combinations of the 6-mer lie within 3 kJ/mol. Nonetheless, the corresponding conformations may differ substantially, as demonstrated in Figure 13 for the all-anti and 5 syn/1 anti isomers of the 6-mer; the former forms a large cavity (note, again, the helical arrangement of the phenyl pendants), while the latter has a twisted structure with two smaller cavities. Analytical Chemistry, Vol. 73, No. 9, May 1, 2001

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For the copolymeric macrocycles, the effect of incorporating the PES unit contiguously (-S-O-E-O-) or in separate ether (-O-E-O-) and sulfone (-S-) segments was investigated for the simplest system, i.e., the PPQ/PES 3-mer composed of two PPQ and overall one PES monomers. Structures of comparable strain energy can be constructed with either connectivity if the proper PPQ isomer (syn or anti) is selected (cf. Figure 14). In the low-energy structures, the phenyl and methyl substituents generally point outside the cavity, whose shape depends on both the type of PPQ monomer inserted (1a or 1b) and the contiguity of the PES unit (Figure 14). CONCLUSIONS This study confirms the usefulness of MALDI mass spectrometry for the microstructural characterization of industrially important polymers. The spectra indicate that the self-polymerizable PPQ and PPQ/PES monomers used have a high predilection for producing macrocycles (at least up to molecular weights of ∼15 000). Parallel calculations point out that the growing chains emerging during propagation still bear highly reactive nucleophilic

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and electrophilic sites, making cyclization, which removes these reactive groups, the most efficient means of termination. This study also shows the value of theory in providing the mechanistic information necessary to explain the experimental results. The interesting array of cavities present in PPQ and PPQ/PES macrocycles could be useful in the formation of inclusion complexes. ACKNOWLEDGMENT The National Science Foundation is gratefully thanked for generous support. We also acknowledge financial assistance from the Ohio Board of Regents and the University of Akron and thank Dr. Robert P. Lattimer for his suggestions and review of the manuscript.

Received for review October 23, 2000. Accepted January 25, 2001. AC001247H