Synthesis of Poly(phenylquinoxaline)s via Self-Polymerizable Quinoxaline Monomers Daniel J. Klein, †,§ David A. Modarelli, ‡ and Frank W. Harris* ,† The Maurice Morton Institute and Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, and Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601 Received June 27, 2000; Revised Manuscript Received February 9, 2001 ABSTRACT: The development of three new self-polymerizable quinoxaline monomers was pursued in an attempt to increase the susceptibility of the monomers toward aromatic nucleophilic substitution reactions. The polymerizations were carried out in N-methyl-2-pyrrolidinone in the presence of potassium carbonate, to yield high molecular weight poly(phenylquinoxaline)s (PPQs). Replacement of the 1,4- phenylene group of the phenol in the isomeric monomer mixture of 2-(4-hydroxyphenyl)-3-phenyl-6- fluoroquinoxaline and 3-(4-hydroxyphenyl)-2-phenyl-6-fluoroquinoxaline with the 2,6-naphthylene, 4,4′- biphenylene, and 4,4′-oxydiphenylene groups resulted in more reactive monomers as evidenced by shorter polymerization times and lower polymerization temperatures needed to obtain PPQs with high intrinsic viscosities. Polymerizations of the 1,4-phenylene-, 2,6-naphthylene-, 4,4′-biphenylene-, and 4,4′-oxydiphen- ylene-containing monomers led to PPQs with intrinsic viscosities ranging from 1.4 to 2.5 dL/g and glass transition temperatures ranging from 221 to 287 °C. All of the PPQs exhibited high tensile properties, with tensile strengths of g92 MPa, tensile moduli of g2.6 GPa, and elongations of g88%. Introduction Poly(phenylquinoxaline)s (PPQs) are a class of high- temperature/high-performance thermoplastics that have many desirable properties such as high glass transition temperatures (T g ’s), low dielectric constants, high chemi- cal resistance, excellent mechanical properties, and high thermooxidative stability. Since the first synthesis of conventional PPQs, which were prepared from bis(- diamine) and bis(R-diketone) monomers, extensive re- search has been carried out in their synthesis and characterization. 1-5 A major barrier to widespread commercial use of PPQs has been the relatively high cost of the monomers involved. An alternate approach to the synthesis of PPQs is through the polymerization of monomers containing preformed quinoxaline rings via aromatic nucleophilic substitution (S N Ar) reactions. Difluoroquinoxaline mono- mers were synthesized and polymerized with a series of aromatic diols to afford poly(aryl ether phenylquin- oxaline)s with high intrinsic viscosities. 6-8 The electron- withdrawing pyrazine ring activates the electrophilic fluorocarbon toward substitution. The aryl-ether link- age imparts properties such as better solution and melt- processing characteristics and improved toughness com- pared to PPQs without aryl-ether linkages. 7 Diol quinoxaline monomers have also been polymer- ized to high intrinsic viscosities with a series of activated difluoro monomers. 9,10 Highly activated difluoro mono- mers were necessary due to the deactivated nature of the quinoxaline monomers. This arises from the electron- withdrawing ability of the pyrazine ring, which stabi- lizes the phenoxide intermediate during polymerization, resulting in lower nucleophilicity. In our laboratory, self-polymerizable quinoxaline monomers have been developed, which offers the ad- vantage of having an inherent 1:1 stoichiometry. 5,11,12 Thus, the isomeric monomer mixture 3-(4-hydroxyphen- yl)-2-phenyl-6-fluoroquinoxaline and 2-(4-hydroxyphen- yl)-3-phenyl-6-fluoroquinoxaline was synthesized and polymerized to high intrinsic viscosities in N-methyl- 2-pyrrolidinone (NMP). The resulting PPQ has a T g of 251 °C and polymer decomposition temperatures above 500 °C in both nitrogen and air. This PPQ had a tensile strength of 107 MPa, tensile modulus of 3.18 GPa, and an elongation to break of approximately 4%. Experimental Section Materials. Anisole, phenylacetyl chloride, copper(II) bro- mide (CuBr2), hydrobromic acid (HBr) (48%), fluorobenzene, 4-methoxyphenol, pyridine hydrochloride, potassium fluoride, trifluoroacetic acid, trifluoroacetic acid, Raney nickel, ni- trobenzene, 2-methoxynaphthalene, 4-fluoro-2-nitroaniline, 4-(4-bromophenyl)phenol, palladium(II) acetate, palladium(II) chloride, phenylacetylene, triphenylphosphine, and copper(I) iodide (Aldrich Chemical Co.) were used as received. NMP (Fisher Chemical Co.) was distilled from phosphorus pentoxide under reduced pressure. Aluminum chloride (AlCl 3), glacial acetic acid, ethyl acetate, dimethyl sulfoxide (DMSO), chloro- form (CHCl 3), hydrochloric acid (HCl), and methylene chloride (CH2Cl2) (Fisher Chemical Co.) were used as received. Potas- sium carbonate (Fisher Chemical Co.) was ground and dried at 100 °C under reduced pressure overnight before use. Toluene (Fisher Chemical Co.) was dried with magnesium sulfate overnight before use. All other chemicals were used as received. 1-(4-Fluorophenyl)-2-phenylethanone (13) 13 and 1-(4-fluorophenyl)-2-phenylethane-1,2-dione (14) 14 were pre- pared according to the literature. Characterization. Differential scanning calorimetric (DSC) analyses were performed under nitrogen at heating rate of 10 °C/min using a DuPont model 2910 thermal analyzer equipped with a DSC cell. Thermogravimetric analyses (TGA) were performed in nitrogen and air at a heating rate of 10 °C/min using a TA Instruments model 2950 thermogravimetric ana- lyzer. Proton nuclear magnetic resonance ( 1 H NMR) spectra were obtained with a Varian Gemini 200 NMR spectrometer † The Maurice Morton Institute and Department of Polymer Science. ‡ Department of Chemistry. § Current address: Advanced Materials and Processing Branch, NASA Langley Research Center, Hampton, VA 23681-2199. * To whom correspondence should be addressed. E-mail: harris@polymer.uakron.edu. 2427 Macromolecules 2001, 34, 2427-2437 10.1021/ma0011082 CCC: $20.00 © 2001 American Chemical Society Published on Web 03/14/2001