Chemistry and Biochemistry Faculty Publications Chemistry and Biochemistry

7-3-2019

Phosphine Oxide Containing Poly(pyridinium salt)s as Fire Retardant Materials

Maksudul M. Alam InnoSense LLC

Bidyut Biswas University of Nevada, Las Vegas

Alexi K. Nedeltchev University of Nevada, Las Vegas

Haesook Han University of Nevada, Las Vegas, [email protected]

Asanga D. Ranasinghe University of Nevada, Las Vegas

SeeFollow next this page and for additional additional works authors at: https:/ /digitalscholarship.unlv.edu/chem_fac_articles Part of the Polymer Science Commons

Repository Citation Alam, M. M., Biswas, B., Nedeltchev, A. K., Han, H., Ranasinghe, A. D., Bhowmik, P. K., Goswami, K. (2019). Phosphine Oxide Containing Poly(pyridinium salt)s as Fire Retardant Materials. Polymers, 11(7), MDPI. http://dx.doi.org/10.3390/polym11071141

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This Article has been accepted for inclusion in Chemistry and Biochemistry Faculty Publications by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. Authors Maksudul M. Alam, Bidyut Biswas, Alexi K. Nedeltchev, Haesook Han, Asanga D. Ranasinghe, Pradip K. Bhowmik, and Kisholoy Goswami

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Article Phosphine Oxide Containing Poly(pyridinium salt)s as Fire Retardant Materials

Maksudul M. Alam 1, Bidyut Biswas 2, Alexi K. Nedeltchev 2, Haesook Han 2, Asanga D. Ranasinghe 2, Pradip K. Bhowmik 2,* and Kisholoy Goswami 1

1 InnoSense LLC, 2531 West 237th Street, Torrance, CA 90505, USA 2 Department of Chemistry and Biochemistry, University of Nevada Las Vegas, 4505 Maryland Parkway Box 454003, Las Vegas, NV 89154-4003, USA * Correspondence: [email protected]; Tel.: +1-(702)-895-0885; Fax: +1-(702)-895-4072



Received: 12 June 2019; Accepted: 1 July 2019; Published: 3 July 2019


Abstract: Six new rugged, high-temperature tolerant phosphine oxide-containing poly(4,40-(p-phenylene)- bis(2,6-diphenylpyridinium)) polymers P-1, P-2, P-3, P-4, P-5, and P-6 are synthesized, characterized, and evaluated. Synthesis results in high yield and purity, as confirmed by elemental, proton (1H), and carbon 13 (13C) nuclear magnetic resonance (NMR) spectra analyses. High glass transition temperatures (Tg > 230 ◦C) and high char yields (>50% at 700 ◦C) are determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively. These new ionic polymers exhibit excellent processability, thin-film forming, high-temperature resistance, fire-resistance and retardation, coating, adhesion, mechanical and tensile strength, and n-type (electron transport) properties. The incorporation of phosphine oxide and bis(phenylpyridinium) moieties in the polymer backbones leads to high glass transition temperatures and excellent fire retardant properties, as determined by microcalorimetry measurements. The use of organic counterions allows these ionic polymers to be easily processable from several common organic solvents. A large variety of these polymers can be synthesized by utilizing structural variants of the bispyrylium salt, phosphine oxide containing diamine, and the counterion in a combinatorial fashion. These results make them very attractive for a number of applications, including as coating and structural component materials for automobiles, aircrafts, power and propulsion systems, firefighter garments, printed circuit boards, cabinets and housings for electronic and electrical components, construction materials, mattresses, carpets, upholstery and furniture, and paper-thin coatings for protecting important paper documents.

Keywords: phosphine oxide; poly(pyridinium salt)s; fire retardant polymers; atomic force microscopy; luminescence; UV-Vis spectroscopy; microcalorimetry

1. Introduction Polymers are extensively used in our everyday lives due to their tunable properties and ease of processing. These qualities render polymers useful for applications ranging from adhesives and lubricants to structural components and windows for aircrafts. However, common polymers are also highly combustible, and produce toxic gases and smoke during combustion. Thus, the development of polymers with fire retardant properties is a major challenge. In recent years, regulations have required significant improvement of the fire performance of materials used in construction, transportation, and clothing. At the same time, some fire resistant materials containing bromine or chlorine moieties are being phased out due to their detrimental effects on health and the environment during combustion. Thus, the development of effective fire resistant polymers that can be produced and utilized safely and in an environmentally conscious manner is of great interest [1–11].

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Over the past few years, nitrogen-containing polymers have received unabated attention in the design and synthesis of ionic polymers that have desirable electroluminescent (EL) [12–25], conducting [26], and liquid-crystalline (LC) properties [27–41], which make them attractive materials in many technological applications. Among the nitrogen-containing EL polymers, poly(pyridinium salt)s have received considerable interest and attention for applications in electronic and optoelectronic devices for displays and lighting, solar light harvesting, sensors, photonic devices, automobile and aircraft parts, and for packaging in the electronic industry. These ionic EL polymers exhibit many interesting properties, including redox behavior, electrical conductivity, electrochromism, photochromism, thermochromism, and LC properties [42–58]. The ionic polymers are also the appropriate polymers for the construction of functional multilayer assemblies by a sequential layer-by-layer deposition technique through electrostatic interactions [59–66]. The ionic EL polymers, which are also known as electroactive polymers (EAPs), render the ability to induce strains that are as high as two orders of magnitude greater than the movements that are possible with rigid and fragile electroactive ceramics [67]. EAP materials also have a higher response speed, lower density, and improved resilience when compared to shape memory alloys [67]. The synthesis of this class of photoactive and electroactive ionic polymers by the ring-transmutation polymerization reaction, and the characterization of their properties have been reported by several groups [42–58]. These ionic polymers have a high glass transition temperature, Tg, and are thermally and thermo-oxidatively stable. They are highly crystalline and have excellent film-forming properties. Although poly(pyridinium) salts have very attractive properties and are thermally stable, they lack the robust, high-temperature, high-performance, high mechanical-strength, low-cost, and lightweight properties of ionic EL polymer materials. Over the course of recent decades, conventional materials such as metals, wood, glass, and ceramic have been increasingly replaced by synthetic polymers due to their versatility, low density, mechanical properties, and the ease with which they can be molded. However, these advantages are shadowed by their flammability and low stability in high temperatures in comparison to metals. In recent years, considerable attention has been focused on preparing flame-retardant polymers; among these, phosphorus-containing polymers are the most widely used [1–11,68–70]. Phosphorus moieties have been incorporated into different polymeric backbones, including epoxy resin, poly(amic acid), polycarbonate, poly(vinyl chloride), polyester, polyimide, and poly(methyl methacrylate) [71–78]. Among the polymers with phosphorus-containing moieties, the polymers with phosphine oxide moieties have improved flame-retardant properties, thermal oxidative stability, solubility in organic solvents, miscibility, and adhesion to other compounds [79–83]. Although the incorporation of phosphine oxide moieties in the polymer backbone results in improved properties [84–92], the manufacturing costs with these polymers are prohibitive, because they are difficult to process. Currently, there is no robust high-temperature, high-performance, and lightweight phosphine oxide-containing electroactive and photoactive ionic polymer that can be manufactured and processed cost-effectively,and whose properties can be fine-tuned using easy synthetic routes. Thus, the development of easily processable and phosphine oxide-containing ionic EL polymers that have high temperature resistance and photoluminescence efficiency is highly desirable. The combination of ionic polymeric materials with a high-temperature resistant phosphine oxide group in the form of macromolecular architecture has the potential to deliver such a novelty. In this study, we report on the design, synthesis, characterization, and evaluation of properties and the performance of six new phosphine oxide-containing poly(4,40-(p-phenylene)-bis(2,6- diphenylpyridinium)) ionic polymers—P-1, P-2, P-3, P-4, P-5, and P-6 (Figure1)—using the ring-transmutation polymerization reaction. The thermal stability, fire-resistant, char yields, mechanical strength, coating and film forming, optical absorption, photoluminescence, electrochemical, and morphological properties of these ionic polymers have been studied to establish them as potential high-temperature and high-performance materials. The synthetic approach is versatile and amenable to low-cost mass production. This versatility stems from the ability of a large variety of these polymers to be synthesized by utilizing structural variants of the bispyridinium salt, phosphorous Polymers 2019, 11, 1141 3 of 25 Polymers 2019, 11, x FOR PEER REVIEW 3 of 26 polymers to be synthesized by utilizing structural variants of the bispyridinium salt, phosphorous oxide-containing diamine, and the counterion in a combinatorial fashion. Thus, the chemical structural oxide-containing diamine, and the counterion in a combinatorial fashion. Thus, the chemical modifications allow the fine-tuning of their fire retardant properties. The purpose for using both structural modifications allow the fine-tuning of their fire retardant properties. The purpose for using phenylated bispyridinium ditosylate and phenyl phosphine oxide moieties in the backbone is to both phenylated bispyridinium ditosylate and phenyl phosphine oxide moieties in the backbone is develop advanced polymeric structural materials with high thermal stability, high glass transition to develop advanced polymeric structural materials with high thermal stability, high glass transition temperature, and enhanced photophysical, electrochemical, thermal, and mechanical properties. temperature, and enhanced photophysical, electrochemical, thermal, and mechanical properties.

FigureFigure 1. 1. ChemicalChemical structures structures of of rugged rugged high high temper temperatureature tolerant tolerant phosphine phosphine oxide-containing oxide-containing

poly(4,4poly(4,4′-(0-(pp-phenylene)--phenylene)-bisbis(2,6-diphenylpyridinium))(2,6-diphenylpyridinium)) polymers polymers P-1P-1 toto P-6P-6. .

TheThe bispyrylium bispyrylium ditosylate ditosylate monomer monomer (M (M in Figurein Figure 2) is2 )an is excellent an excellent building building block blockfor the for design the anddesign development and development of ionic polymers of ionic polymerswith both withliquid-crystalline both liquid-crystalline (LC) and light-emitting (LC) and light-emitting properties [42–55].properties It [contains42–55]. It a contains stable phenyl a stable group phenyl and group heteroatom and heteroatom in the inaromatic the aromatic ring that ring increase that increase the thermalthe thermal stability. stability. Since Since these these ionic ionic polymers polymers are cationic, are cationic, they they have have great great potential potential for building for building up multilayerup multilayer assemblies assemblies with withanionic anionic polymers polymers by sequential by sequential electrostatic electrostatic deposition deposition technique. technique. The phenylThe phenyl phosphine phosphine oxide oxidecontaining containing aromatic aromatic diamines diamines (bis(3-aminophenyl)phenyl (bis(3-aminophenyl)phenyl phosphine phosphine oxide (oxidem-DAPPO (m-DAPPO), bis(3-aminophenyl)-3,5-bis(trifluoromethyl)phenyl), bis(3-aminophenyl)-3,5-bis(trifluoromethyl)phenyl phosphine phosphine oxide oxide(BATFPO (BATFPO), and), bis(4-aminophenoxy-4-phenyl)and bis(4-aminophenoxy-4-phenyl) phenyl phenyl phosphine phosphine oxide oxide (p-BAPPO (p-BAPPO) as) asshown shown in in Figure Figure 2),2), were selectedselected because of of their their high high thermo-oxidative thermo-oxidative stability, stability, high high glass glass transition transition temperature, temperature, and the potentialand the potentialfire retardant fire retardant behavior ofbehavior phosphorous-containing of phosphorous-containing polymers. Thepolymers. initial exposureThe initial of exposure such polymers of such to polymersatomic oxygen to atomic is expected oxygen to result is expected in a passivating to result phosphate in a passivating glassy layer phosphate on the surface, glassy preventing layer on further the surface,the erosion preventing/corrosion further caused bythe atomic erosion/corrosion oxygen in a harsh caused thermal by environmentatomic oxygen [78 –in83 ,93a –harsh95]. thermal environment [78–83,93–95].

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Figure 2. Chemical structures of 4,4′-(1,4-phenylene)-bis(2,6-diphenylpyrylium) ditosylate monomer Figure 2. Chemical structures of 4,4 -(1,4-phenylene)-bis(2,6-diphenylpyrylium) ditosylate monomer (M), and phosphine oxide containing0 diamines, bis(3-aminophenyl)phenyl phosphine oxide (m- (M), andDAPPO phosphine), bis(3-aminophenyl)-3,5-bis(trifl oxide containing diamines,uoromethyl)phenyl bis(3-aminophenyl)phenyl phosphine oxide phosphine (BATFPO), oxide and bis(4- (m-DAPPO), bis(3-aminophenyl)-3,5-bis(trifluoromethyl)phenylaminophenoxy-4-phenyl) phenyl phosphine oxide phosphine (p-BAPPO oxide). (BATFPO), and bis(4-aminophenoxy- 4-phenyl) phenyl phosphine oxide (p-BAPPO). 2. Materialsand Methods 2. MaterialsandTerephthalaldehyde, Methods acetophenone, triphenylmethanol, p-toluenesulfonic acid monohydrate, Terephthalaldehyde,3,5-bis(trifluoromethyl)bromobenzene, acetophenone, triphenylmethanol,diphylphosphinicp -toluenesulfonicchloride, acid4-aminophenol, monohydrate, 3,5- phenylphosphonic dichloride, acetic acid, acetic anhydride, p-bromofluorobenzene, ethyl acetate, bis (trifluoromethyl)bromobenzene,triphenyl phosphine oxide, nitric diphylphosphinicchloride, acid, sulfuric acid, hydrochloric 4-aminophenol, acid, potassium phenylphosphonicdichloride, hydroxide, sodium acetic acid,bicarbonate, acetic anhydride, Na2SO4, SnClp-bromofluorobenzene,2·2H2O, Pd/C, and Mg ethylturnings acetate, were triphenyl purchased phosphine from TCI oxide,America nitric acid, sulfuric acid,(Portland, hydrochloric OR, USA) acid,and used potassium as received. hydroxide, For synthesis sodium and purification bicarbonate, purposes, Na SO reagent, SnCl grade2H O, Pd/C, 2 4 2· 2 and Mg turningssolvents wereincluding purchased ethanol, from propanol, TCI America toluene, (Portland, benzene, OR, chloroform, USA) and diethyl used as ether, received. hexane, For synthesis and purificationtetrahydrofuran purposes, (THF), reagent dichloromethane, grade solvents ethylenechloride, including ethanol, acetonitrile, propanol, dimethyl toluene, sulfoxide benzene, (DMSO), chloroform, and N,N-dimethylacetamide (DMAc) were used as obtained from Sigma-Aldrich (Milwaukee, WI, diethyl ether,USA). hexane,Spectrophotometric tetrahydrofuran grade (THF),solvents dichloromethane,obtained from Sigma-Aldrich ethylenechloride, were used acetonitrile, for optical dimethyl sulfoxideabsorption (DMSO), and and fluorescenceN,N-dimethylacetamide measurements. High-pur (DMAc)ity acetonitrile were used (purity as obtained >99.9%) obtained from Sigma-Aldrich from (Milwaukee,Sigma-Aldrich WI, USA). was Spectrophotometric used for electrochemical grade measurements. solvents obtained from Sigma-Aldrich were used for optical absorption and fluorescence measurements. High-purity acetonitrile (purity >99.9%) obtained from 2.1. General Characterization Methods Sigma-Aldrich was used for electrochemical measurements. The (proton) 1H and (carbon 13) 13C (nuclear magnetic resonance) NMR spectra of monomers 2.1. Generaland Characterizationpolymers were recorded Methods on a Varian NMR J 400 spectrometer, 400 MHz at 298 K using both CDCl3 and d6-DMSO as solvents. Their elemental analyses were performed from Numega Resonance TheLaboratory, (proton) 1 HCA. and To (carbonassess the 13) molecular13C (nuclear weight magnetic of the polymer, resonance) gel permeation NMR spectra chromatography of monomers and polymers(GPC) were was recorded run at 50 on °C a with Varian a flow NMR rate J of 400 1 mL/min. spectrometer, The GPC 400 instrument MHz at had 298 a K Water using 515 both pump CDCl 3 and simultaneously with a Viscotek Model 301 Triple Detector Array. The array contained a laser d6-DMSO as solvents. Their elemental analyses were performed from Numega Resonance Laboratory, refractometer, a differential viscometer, and a light scattering detector both right angle laser light CA. To assess the molecular weight of the polymer, gel permeation chromatography (GPC) was run at scattering (RALS) and low angle laser light scattering (LALS) in a single instrument with a fixed 50 ◦C withinterdetector a flow rate system of 1 and mL temperatur/min. Thee GPCcontrol instrument that can be regulated had a Water up to 51580 °C. pump The instrument simultaneously was with a Viscotekcalibrated Model with 301 a Triple pullulan Detector standard Array. of P-50 The obtained array from contained Polymer a Standard laser refractometer, Services USA, a Inc. diff erential viscometer,Separations and a light were scatteringaccomplished detector using ViscoGel both right I-MBHMW-3078 angle laser columns light scattering purchased (RALS) from Viscotek. and low angle laser lightAn scattering aliquot of 100 (LALS) to 200 inmL a of single 2 mg/mL instrument polymer solution with a fixedin DMSO interdetector containing 0.01 system M of andLiBr temperaturewas injected. The dn/dc (refractive index increment) values were corrected by injecting different volumes control thatto assess can bethe regulatedtrend. All the up data to 80 analyses◦C. The were instrument performed was by using calibrated Viscotek with TriSEC a pullulan software. standard of P-50 obtainedThermogravimetric from Polymer analysis Standardof the polymer Services was conducted USA, Inc. with Separationsa Universal V3.0G were TA accomplished Instruments. using ViscoGelA I-MBHMW-3078 heating rate of 10 °C/min columns in N purchased2 was used with from runs Viscotek. being cond Anucted aliquot from of room 100 totemperature 200 mL of to 2 mg/mL polymer solution in DMSO containing 0.01 M of LiBr was injected. The dn/dc (refractive index increment) values were corrected by injecting different volumes to assess the trend. All the data analyses were performed by using Viscotek TriSEC software. Thermogravimetric analysis of the polymer was conducted with a Universal V3.0G TA Instruments. A heating rate of 10 ◦C/min in N2 was used with runs being conducted from room temperature to 800 ◦C. Differential scanning calorimetry (DSC) measurements were run on a TA Instruments Q100 DSC in nitrogen. The heat–cool–heat method was used with an initial heating rate of 40 ◦C/min, a cooling rate of 20 ◦C/min, and a final Polymers 2019, 11, 1141 5 of 25

Polymers 2019, 11, x FOR PEER REVIEW 5 of 26 heating rate800 °C. of 10Differential◦C/min inscanni nitrogen.ng calorimetry All the (DSC) transitions measurements were recordedwere run on from a TA the Instruments second heating Q100 cycles of DSC thermograms.DSC in nitrogen. The heat–cool–heat optical absorption method andwas used fluorescence with an initial properties heating rate of theseof 40 °C/min, ionic polymersa were studiedcooling using rate of a20 Shimadzu °C/min, and UV-2401PCa final heating UV-Visrate of 10 spectrophotometer °C/min in nitrogen. All andthe transitions PTI QuantaMaster were ™ Model QM-4recorded/2005 from spectrofluorometer, the second heating respectively. cycles of DSC Microcalorimetry thermograms. The measurements optical absorption were and done with fluorescence properties of these ionic polymers were studied using a Shimadzu UV-2401PC UV-Vis a microscale combustion calorimeter (Govmark MCC-2) using 5 to 10-mg samples with a heating spectrophotometer and PTI QuantaMaster™ Model QM-4/2005 spectrofluorometer, respectively. rate of 1 ◦MicrocalorimetryC/sec to 750 ◦C measurements in the pyrolysis were zone.done with The a combustionmicroscale combustion zone was calorimeter set to 900 (Govmark◦C; oxygen and nitrogenMCC-2) flow rates using were 5 to 10-mg set at samples 20 mL/ minwith anda heating 80 mL rate/min, of 1 °C/sec respectively. to 750 °C Thein the microcalorimetry pyrolysis zone. The data are the averagecombustion of three zone measurements was set to 900 on °C; each oxygen sample. and nitrogen flow rates were set at 20 mL/min and 80 mL/min, respectively. The microcalorimetry data are the average of three measurements on each 2.2. Synthesissample. of 4,4’-(1,4-Phenylene)-bis(2,6-diphenylpyrylium)ditosylate Monomer (M)

A mixture2.2. Synthesis of terephthalaldehyde of 4,4’-(1,4-Phenylene)-bis(2,6-diphenylpyrylium)ditosylate (10.0 g) and acetophenone (54.3 Monomer g) was (M) stirred in 250 mL of 95% ethanol at 65 ◦AC. mixture After theof terephthalaldehyde starting compounds (10.0 were g) and dissolved, acetophenone a solution (54.3 ofg) KOHwas stirred (10.5 in g) 250 in 10mL mL of of water was added95% dropwise ethanol at over 65 °C. 30 After min the with starting vigorous compounds stirring. were A dissolved, yellow precipitate a solution formedof KOH (10.5 immediately. g) in 10 Then, the heterogeneousmL of water reaction was added mixture dropwise was heatedover 30 atmin reflux with untilvigorous it turned stirring. pink A yellow over a precipitate period of 5formed h. During this time, the immediately.p-bischalcone Then, was the redissolved heterogeneous and reaction reacted mixture with was two heated additional at reflux equivalents until it turned of pink acetophenone over to a period of 5 h. During this time, the p-bischalcone was redissolved and reacted with two additional form the desiredequivalents tetraketone, of acetophenoneI (Scheme to1 ),form which the was desired also precipitatedtetraketone, I out. (Scheme The reaction 1), which mixture was also was filtered hot, and theprecipitated tan solid out. was The collected reaction bymixture filtration was filtered to afford hot, 41.0 and gthe of tan the solid crude was product. collected It by was filtration recrystallized from tolueneto afford to afford 41.0 g of 38.0 the g crude (yield: produc 89%)t. It of was off-white recrystallized crystals from of tolu compoundene to affordI (Scheme 38.0 g (yield:1). The 89%) chemical structureof and off-white purity ofcrystalsI were of confirmedcompound byI (Scheme elemental 1). andThe 1chemicalH NMR structure analyses and [54 ].purity of I were confirmed by elemental and 1H NMR analyses [54].

SchemeScheme 1. Synthesis 1. Synthesis of 4,4’-(1,4-phenylene)- of 4,4’-(1,4-phenylene)-bisbis(2,6-diphenylpyrylium)(2,6-diphenylpyrylium) ditosylate ditosylate monomer monomer (M). (M).

For theFor conversion the conversion of I ofto I Mto M(the (the second second step step in inScheme Scheme 1), triphenylmethanol1), triphenylmethanol (7.8 g) and (7.8 p- g) and toluenesulfonic acid monohydrate (5.8 g) were added to 100 mL of acetic anhydride, and stirred at p-toluenesulfonicroom temperature acid monohydrate for 3 h. Then, solid (5.8 tetraketone g) were added I (7.2 g) to was 100 added mL to of the acetic reaction anhydride, mixture, and and the stirred at room temperaturemixture was for heated 3 h. to Then, 100 °C solid for 1 h. tetraketone The heterogeIneous(7.2 g)mixture was addedbecame toclear. the Upon reaction cooling, mixture, yellow and the mixture wascrystals heated appeared to 100 and◦ wereC for collected 1 h. The by heterogeneousfiltration, after which mixture they were became washed clear. carefully Upon with cooling, acetic yellow crystals appearedanhydride and ethanol. were collected Then, the byproduct filtration, was recrystallized after which from they acetic were acid washed and dried carefully in vacuum with acetic 1 + anhydrideto andafford ethanol. 7.9 g of M Then, (yield the 75%) product [54]. H wasNMR recrystallized of M in d6-DMSO, from dH: acetic9.35 (4H, acid s, aromatic and dried meta in O vacuum), to 9.21 (4H, s, 1,4-phenylene), 7.58–8.93 (20H, m, phenyl), 7.46–7.47 (4H, d, J = 6.7 Hz, tosylate), 7.09– afford 7.9 g of M (yield 75%) [54]. 1H NMR of M in d -DMSO, d : 9.35 (4H, s, aromatic meta O+), 9.21 7.10 (4H, d, J = 7.7 Hz, tosylate), 2.27 (6H, s, CH3). 6 H (4H, s, 1,4-phenylene), 7.58–8.93 (20H, m, phenyl), 7.46–7.47 (4H, d, J = 6.7 Hz, tosylate), 7.09–7.10 (4H, d, J = 7.72.3. Hz, Bis(3-aminophenyl)phenyl tosylate), 2.27 (6H, s,Phosphine CH3). Oxide (m-DAPPO) An amount of 11.14 g of triphenyl phosphine oxide (TPO), II, was placed in a 250-mL flask 2.3. Bis(3-aminophenyl)phenylequipped with a stirrer, Phosphinenitrogen inlet, Oxide and (m-DAPPO)a thermometer. A volume of 60 mL of 96% H2SO4 was added, and once the reactant was dissolved, the solution was cooled to −5 °C with an ice/salt bath. A An amount of 11.14 g of triphenyl phosphine oxide (TPO), II, was placed in a 250-mL flask solution of 5.80 g of 90% fuming HNO3 in 40 mL of H2SO4 was added dropwise over a period of 1 h. equippedThe with samples a stirrer, were maintained nitrogen inlet,at −5 °C and during a thermometer. the addition, increased A volume to room of 60temperature mL of 96% (rt), Hand2SO 4 was added, andleft at once this thetemperature reactant for was 8 h. dissolved, The obtained the pale solution yellow solution was cooled was poured to 5 intoCwith 400 mL an of ice ice/salt bath. − ◦ A solutionwater, of 5.80 resulting g of 90%in a sticky fuming solid HNO that 3wasin collected 40 mL of by H decantation.2SO4 was addedThen, this dropwise solid was over dissolved a period in of 1 h. CHCl3 and washed with aqueous sodium bicarbonate solution until it was neutral. The organic layer The samples were maintained at 5 ◦C during the addition, increased to room temperature (rt), and left was dried over Na2SO4 overnight− and filtered. The solvent was removed by rotary evaporation. The at this temperature for 8 h. The obtained pale yellow solution was poured into 400 mL of ice water, resulting in a sticky solid that was collected by decantation. Then, this solid was dissolved in CHCl3 and washed with aqueous sodium bicarbonate solution until it was neutral. The organic layer was dried over Na2SO4 overnight and filtered. The solvent was removed by rotary evaporation. The crude product was dried in vacuo and recrystallized twice from ethanol to produce 9.6 g (26.1 mmol, yield 65%) of pure product [96,97]. Elemental and 1H NMR analyses were performed to confirm the chemical structure of m-DNPPO. In the second step, m-DNPPO (5.0 g) was added to a solution of SnCl 2H O 2· 2 Polymers 2019, 11, 1141 6 of 25 in 50 mL of concentrated HCl and 100 mL of ethanol. This mixture was left stirring at rt for 2 h and then heated for an additional 2 h. The reaction mixture was cooled down to rt and poured in 60 g of KOH in 200 mL of ice water and stirred vigorously. The solid crude product was filtered, washed with H2O until neutral, and recrystallized from chloroform to give 3.5 g (11.4 mmol, yield 84%) [98]. The chemical structure of m-DAPPO was confirmed by elemental, 1H and 13C NMR spectra analyses. Anal. Calcd. for C18H17N2OP (308.32): C 70.12, H 5.56, N 9.09; found: C 70.17, H 5.82, N 9.43.

2.4. Bis(3-aminophenyl)-3,5-bis(trifluoromethyl)phenyl Phosphine Oxide (BATFPO) An amount of 1.05 g of Mg turnings and 50 mL of ether were charged into a 250-mL three-necked, round-bottomed flask, equipped with magnetic stirrer, condenser, drying tube, thermometer, and nitrogen inlet. The solution was cooled to below 5 ◦C in an ice bath; then, 9.71 g of 3,5- bis(trifluoromethyl)bromobenzene was added dropwise over a period of 1 h, while stirring vigorously. Then, the mixture was allowed to react for an additional 5 h. Then, 7.00 g of diphylphosphinic chloride was added dropwise over a period of 1 h. After reacting for an additional 18 h, a brown solution was obtained. Next, 10% aqueous sulfuric acid was added to the solution to a pH of 1, followed by the addition of 250 mL of water and diethyl ether to form aqueous and organic layers. After decanting the ether layer, the aqueous phase was washed twice with diethyl ether, and all the organic layers were combined and dried by evaporation, resulting in 3,5-bis(trifluoromethyl)phenyl diphenyl phosphine oxide (TFPO), which was a light brown solid. Then, TFPO was dissolved in chloroform and washed several times with 10% sodium bicarbonate and three times with water. The organic layer was condensed by rotary evaporator and stored at room temperature for 12 h and another 12 h in a freezer. The fibrous off-white crystals of TFPO were collected by vacuum filtration and further purified by recrystallization in hexane [99]. In the second step, bis(3-nitrophenyl)-3,5-bis(trifluoromethyl)phenyl phosphine oxide (DNTFPO) was prepared by the nitration of TFPO using concentrated sulfuric and concentrated nitric acid. Purified TFPO (10.0 g) was added into a 250-mL three-necked flask equipped with a nitrogen inlet, thermometer, drying tube, and mechanical stirrer. Concentrated sulfuric acid (23 mL) was added into the flask to dissolve the compound TFPO at room temperature. The solution was cooled down to 0 ◦C with an ice-water bath. Nitric acid (4.7 mL) was added dropwise to the solution over a period of 1 h, while stirring vigorously and maintaining 0 ◦C. Then, the mixture was allowed to react for 8 h and poured into 650 g of finely divided ice. The resulting yellowish solid was extracted with chloroform, and then followed by washing with sodium bicarbonate aqueous solution until the pH reached 7. The solvent was removed with a rotary evaporator, and the remaining solid was recrystallized twice from absolute ethanol, which afforded 10.0 g of pale-yellow crystals of DNTFPO [99]. Then, bis(3-aminophenyl)-3,5-bis(trifluoromethyl)phenyl phosphine oxide (BATFPO) was prepared by the reduction of DNTFPO by the stannous chloride method [30]. An amount of 4.0 g of BATFPO was added to a solution of 10.8 g of SnCl 2H O in 30 mL of concentrated HCl and 60 mL of ethanol, and left stirring at 2· 2 rt for 2 h and heated for an additional 2 h. The reaction mixture was cooled down to room temperature, poured into a solution of 40 g of KOH in 200 mL of ice water, and stirred vigorously. The solid crude product was filtered and washed with copious amount of H2O until neutral, giving the crude product of BATFPO, and it was air-dried. Then, it was further purified by sublimation to afford 3.0 g (yield 86%). The 1H and 13C NMR spectra, and elemental analysis confirmed its chemical structure and purity. Anal. Calcd. for C20H15N2OF6P (444.32): C 54.07, H 3.40, N 6.30; found: C 53.88, H 3.80, N 6.39. It showed a Tm at 228 ◦C with ∆H = 9.0 kcal/mol in the first heating cycle of the DSC thermogram (lit. mp = 226–227 ◦C) [99].

2.5. Bis(4-aminophenoxy-4-phenyl) Phenyl Phosphine Oxide (p-BAPPO) First, a solution of p-bromofluorobenzene (15 g) in THF (40 mL) was added dropwise to a slurry of 2.2 g of magnesium in 50 mL of THF in an ice bath over a period of 3 h. This was allowed to stir overnight at room temperature. During this time, a gray-colored solution appeared. The mixture was again cooled to 0 ◦C, and a solution of phenylphosphonic dichloride (8.35 g) in 20 mL of THF was added dropwise over a period of 3-4 h with stirring. Then, the mixture was warmed to room temperature and stirred Polymers 2019, 11, 1141 7 of 25

for 12 h. The mixture was quenched with 10% H2SO4 and stirred for 1 h. Ether (200 mL) was added to separate the organic layer, and the aqueous layer was extracted with ether (3 75 mL). The combined × ethereal extract was washed with NaHCO3 solution and water, and dried over sodium sulfate. The solvent was removed under reduced pressure to furnish a light brown oil. The crude product was purified by crystallization from THF/hexane (1:1) to afford bis(4-fluorophenyl)-phenylphosphineoxide (p-FPPO) as an off-white solid (89% yield), mp 128–130 ◦C. In the second step, a magnetically stirred mixture of p-FPPO (4 g), 4-aminophenol (3.06 g), and K2CO3 (5.28 g) in DMAc (15 mL) was heated to reflux for 24 h in nitrogen. Then, the reaction mixture was cooled to rt and poured into ice water with vigorous stirring. The precipitated off-white solid product was collected in a Buchner funnel using vacuum filtration, and washed well with water to remove the salt and unreacted 4-aminophenol. The crude product was finally purified by column chromatography over silica gel eluting with 2% methanol/98% ethyl acetate (v/v) to furnish p-BAPPO as an off-white solid powder with 82% yield. 1 13 The chemical structure was confirmed by H and C NMR and DSC analysis (mp 98-100 ◦C) [100].

2.6. Phenylphosphine Oxide-Containing Poly(4,40-(p-phenylene)-bis(2,6-diphenylpyridinium)) Ionic Polymers

The first phenylphosphine oxide-containing poly(4,40-(p-phenylene)-bis(2,6-diphenylpyridinium)) ionic polymer (P-1) was prepared by the ring-transmutation polymerization reaction [42–58] as follows: 6.5873 g of M was polymerized with m-DAPPO (2.3000 g) on heating in DMSO at 130–140 ◦C for 24 h (Scheme 5). The water generated during the polymerization was distilled out from the reaction medium as a toluene/water azeotrope. The yellowish solid P-1 polymer was isolated with 80% yield by two cycles of precipitation with distilled water and dissolution in methanol. P-1 was fully characterized by 1 13 elemental analysis, and H and C NMR spectra analyses. Anal. Calcd. for C72H55N2O7S2P (1155.33): C 74.85, H 4.80, N 2.42, S 5.55; found: C 73.36, H 4.82, N 2.44, S 5.45. Using the identical ring-transmutation polymerization reaction (Scheme 5) and purification processes, an ionic polymer P-2 (77% yield; yellow solid) was obtained by polymerizing M (8.4800 g, 6.56 mmol) with BATFPO (2.918 g, 6.56 mmol) in DMSO at 130-140 ◦C for 24 h. The polymer was essentially isolated in a quantitative yield by precipitation with distilled water. It was further purified by redissolving in methanol and by subsequent re-precipitation with the addition of distilled water. Similarly, an ionic polymer P-3 (79% yield; dark brown solid) was obtained by polymerizing M (5.3787 g, 6.09 mmol) with p-BAPPO (3.0000 g, 6.09 mmol) in DMSO at 130–140 ◦C for 24 h. The polymer was isolated in a quantitative yield by precipitation with distilled water. It was further purified by redissolving in methanol and by subsequent re-precipitation with the addition of distilled water to yield ionic polymer P-3. The chemical structures of both ionic polymers P-2 and P-3 were also confirmed by elemental, 1H, and 13C NMR spectra analyses (see Supplementary Materials). Elemental analysis for P-2 polymer, Anal. Calcd. for C79H53N2O7F6S2P (1291.34): C 68.83, H 4.14, N 2.17, S 4.97; found: C 66.38, H 4.69, N 2.28, S 5.74. Elemental analysis for P-3 polymer, Anal. Calcd. for C84H63N2O9S2P (1339.55): C 75.32, H 4.74, N 2.09, S 4.79; found: C 71.87, H, 5.24, N 2.05, S 5.00. Ionic polymer P-4 was prepared by the metathesis reaction of ionic polymer P-1 with lithium triflimide in DMSO (Scheme 6) [42–55] First, 1.40 g (1.21 mmol) of polymer P-1 was dissolved in 50 mL of DMSO. To the DMSO solution of polymer P-1, lithium triflimide (0.73 g, 2.54 mmol) was slowly added. The resulting solution was kept at 50 ◦C for 48 h on continuous stirring. After reducing the volume of DMSO solution by a rotary evaporator, the reaction mixture was added to distilled water, affording the desired ionic polymer P-4. It was collected by vacuum filtration, washed several times with a large quantity of hot distilled water, and dried in vacuum at 100 ◦C for 72 h and weighed to give 1.58 g (1.50 mmol) of polymer P-4 (yield 95%). Anal. Calcd. for C62H41N4O9F12S4P (1373.24): C 54.23, H 3.01, N 4.08, S 9.34; found: C 54.41, H 3.19, N 4.02, S 9.05. Using a similar metathesis reaction, ionic polymer P-5 was prepared from ionic polymer P-2. First, 1.40 g (1.08 mmol) of P-2 was dissolved in 50 mL of DMSO. Then, lithium triflimide (0.73 g, 2.28 mmol) was added to the DMSO solution of this polymer. The resulting solution was kept at 50 ◦C for 48 h. After reducing the DMSO solution by a rotary evaporator, the reaction mixture was added to distilled Polymers 2019, 11, 1141 8 of 25 water, affording the desired ionic polymer P-5. It was collected by vacuum filtration and washed several times with a large quantity of hot distilled water. This procedure was repeated once more; then, the collected polymer was dried in vacuum at 100 ◦C for 72 h and weighted to give 1.54 g (1.02 mmol) of polymer P-5. Unfortunately, the 1H NMR spectrum of this polymer showed a pair of doublets [δ = 7.09 (d) and 7.46 ppm (d)], suggesting the incomplete exchange of tosylate ions by triflimide ions. To complete the exchange of tosylate ions to produce polymer P-5, a metathesis reaction was carried out for the third time yielding the successful metathesis reaction. Using the identical procedure, ionic polymer P-6 was prepared by the metathesis reaction of P-3 with lithium triflimide in DMSO. The chemical structures and purities of ionic polymers P-5 and P-6 were confirmed by elemental, 1H, and 13C NMR spectra analyses (see Supplementary Materials). Elemental analysis for polymer P-5, Anal. Calcd. for C64H39N4O9F18S4P (1509.23): C 50.93, H 2.60, N 3.71, S 8.50; found: C 50.26, H 2.71, N 3.75, S 10.25. Elemental analysis for polymer P-6, Anal. Calcd. for C74H49N4O11F12S4P (1557.43): C 57.07, H 3.17, N 3.60, S 8.23; found: C 54.57, H 3.77, N 3.55, S 8.88.

2.7. Cyclic Voltammetry Measurements Cyclic voltammetry experiments were done on an EG&G Princeton Applied Research potentiostat/ galvanostat (model 263A). Data were collected and analyzed by the model 270 Electrochemical Analysis System software. A three-electrode electrochemical cell was used in all the experiments, as previously described [101,102]. Platinum wire electrodes were used as both counter and working electrodes, and silver/silver ion (Ag in 0.1 M AgNO3 solution, Bioanalytical System, Inc.) was used as a reference electrode. The Ag/Ag+ (AgNO3) reference electrode was calibrated at the beginning of the experiments by running cyclic voltammetry on ferrocene/ferrocenium ions as the internal standard. By means of the internal ferrocenium/ferrocene (Fc+/Fc) standard, the potential values, which were obtained in reference to the Ag/Ag+ electrode, were converted to the saturated calomel electrode (SCE) scale. The films of all the ionic polymers were coated on the Pt working electrode by dipping the Pt wire into the viscous solution in methanol, and then drying it in a vacuum oven at 80 ◦C for 8 h. An electrolyte solution of 0.1 M of TBAPF6 in a mixed water/acetonitrile solvent was used in all the experiments. All the solutions in the three-electrode cell were purged with ultrahigh-purity nitrogen for 10–15 min before each experiment, and a blanket of N2 was used during the experiment.

3. Results and Discussion

3.1. Synthesis and Characterization of Monomers

4,40-(1,4-phenylene)-bis(2,6-diphenylpyrylium) ditosylate monomer (M) was synthesized according to the known procedures in two steps (Scheme1)[ 54]. In the first step, terephthalaldehyde was condensed with acetophenone to afford the desired tetraketone, I. Then, the product was recrystallized from toluene to afford 38.0 g (yield: 89%) of off-white crystals of compound I. The chemical structure and purity of I were confirmed by elemental, 1H NMR, and differential scanning calorimetry (DSC) (melting endotherm at 206 ◦C) analyses. In the second step (Scheme1), tetraketone I was subsequently cyclodehydrated to monomer M by treatment with triphenylmethyl tosylate, which is a hydride acceptor. This hydride acceptor was generated in situ from triphenylmethanol and tosic acid. The product was collected by filtration, washed carefully with (CH3CO)2O and ethanol, recrystallized from acetic acid, and finally obtained as yellow crystals of the desired 4,40-(1,4-phenylene)-bis(2,6-diphenylpyrylium) ditosylate monomer with a yield of 75%. The chemical structure and purity of M were confirmed by 1H and 13C NMR and elemental analyses. The NMR peak positions and the integration ratio between the aromatic protons and the aliphatic protons are in excellent agreement with the calculated values of the compound. Several endotherms by DSC analyses were observed at 161 ◦C(Tm), 195 ◦C, and 304 ◦C (Ti), which match with the previously reported values [54]. Bis(3-aminophenyl)phenyl phosphine oxide (m-DAPPO) was synthesized by a two-step process (Scheme2)[ 96,97]. In the first step, triphenyl phosphine oxide (TPO), II, was reacted with 90% Polymers 2019, 11, x FOR PEER REVIEW 9 of 26

Polymers 2019peak, 11 positions, 1141 and the integration ratio between the aromatic protons and the aliphatic protons are 9 of 25 Polymersin excellent 2019, 11agreement, x FOR PEER with REVIEW the calculated values of the compound. Several endotherms by 9DSC of 26 analyses were observed at 161 °C (Tm), 195 °C, and 304 °C (Ti), which match with the previously peak positions and the integration ratio between the aromatic protons and the aliphatic protons are fuming HNOreported3 and values 96% [54]. H2SO4 to afford bis(3-nitrophenyl)phenyl phosphine oxide (m-DNPPO). The crudein productexcellentBis(3-aminophenyl)phenyl wasagreement dried with in vacuum the phosphine calculated and recrystallized oxide valu es(m-DAPPO of the twicecompound.) was from synthesized Several ethanol byendotherms and a two-step obtained by process DSCm-DNPPO analyses were observed at 161 °C (Tm), 195 °C, and 304 °C (Ti), which match with the previously (yield: 65%).(Scheme Elemental 2) [96,97]. and In the1H first NMR step, analyses triphenyl confirmed phosphine oxide the chemical (TPO), II, structurewas reacted and with high 90% purity of reportedfuming HNO values3 and [54]. 96% H2SO4 to afford bis(3-nitrophenyl)phenyl phosphine oxide (m-DNPPO). The m-DNPPOcrude. InBis product the(3-aminophenyl)phenyl second was dried step, inbis vacuum(3-aminophenyl)phenyl phosphine and recrystallized oxide (m-DAPPO twice phosphine) fromwas synthesized ethanol oxide and (by m-DAPPOobtained a two-step m-DNPPO process) was prepared by the reduction(Scheme(yield: 65%). 2) of[96,97]. Elementalm-DNPPO In the and first 1byH NMRstep, stannous triphenylanalyses chloride confirmedphosphine method the oxide chemic [(TPO98al]. ),structure ThisII, was method andreacted high waswith purity used90% of for the synthesisfumingm of-DNPPO this HNO diamine,. In3 and the 96% sincesecond H2SO the 4step, toreduction afford bis(3-aminophenyl)phenyl bis(3-nitrophenyl)phenyl with hydrogen phosphine over phosphine Pd/C oxide inoxide the ((m-DAPPOm-DNPPO hydrogenation)). wasThe shaker crudeprepared product by the was reduction dried in of vacuum m-DNPPO and recrystallized by stannous chloridetwice from method ethanol [98]. and This obtained method m-DNPPO was used produced a low yield of 28% [96]. The solid crude product was filtered, washed with H2O until neutral, (yield:for the 65%).synthesis Elemental of this diamine,and 1H NMR since analyses the reduct confirmedion with hydrogenthe chemic overal structure Pd/C in andthe hydrogenationhigh purity of and recrystallizedm-DNPPO. fromIn the chloroform second step, to givebis(3-aminophenyl)phenylm-DAPPO (yield: phosphine 84%). The oxide chemical (m-DAPPO structure) was and high shaker produced a low yield of 28% [96]. The solid crude1 product13 was filtered, washed with H2O purity ofprepareduntilm-DAPPO neutral, by the wereand reduction recrystallized also confirmed of m-DNPPO from bychloroform by elemental, stannous to chloridegiveH, andm-DAPPO methodC NMR [98].(yield: analyses.This 84%). method The DSC waschemicalthermograms used of m-DAPPOforstructure the weresynthesis and obtainedhigh of purity this diamine, at of heating m-DAPPO since and the were coolingreduct alsoion confirmed rates with hydrogen of by 10 elemental,◦C/ overmin Pd/C in1H, nitrogen. and in the13C NMRhydrogenation In analyses. the first heating shaker produced a low yield of 28% [96]. The solid crude product was filtered, washed with H2O cycle, theDSC peak thermograms maximum of ofm-DAPPO the melting were endothermobtained at heating was 208and cooling◦C, which rates agreesof 10 °C/min well in with nitrogen. thereported untilIn the neutral, first heating and cycle,recrystallized the peak maximumfrom chloroform of the melting to give endotherm m-DAPPO was (yield: 208 °C, 84%). which The agrees chemical well value of the melting point (mp = 203 ◦C) [96,97]. structurewith the reported and high value purity of of the m-DAPPO melting point were (mpalso confirmed= 203 °C) [96,97]. by elemental, 1H, and 13C NMR analyses. DSC thermograms of m-DAPPO were obtained at heating and cooling rates of 10 °C/min in nitrogen. In the first heating cycle, the peak maximum of the melting endotherm was 208 °C, which agrees well with the reported value of the melting point (mp = 203 °C) [96,97].

SchemeScheme 2. Synthesis 2. Synthesis of bis of (3-aminophenyl)bis(3-aminophenyl) phenyl phenyl phosphine phosphine oxide oxide (m-DAPPO) (m-DAPPO. ).

Scheme 3 outlines the synthesis of bis(3-aminophenyl)-3,5-bis(trifluoromethyl)phenyl phosphine Scheme3 outlines the synthesis of bis(3-aminophenyl)-3,5-bis(trifluoromethyl)phenyl phosphine oxide (BATFPOScheme). This 2. Synthesis diamine of wasbis(3-aminophenyl) synthesized viaphenyl the phosphine Grignard oxide reaction (m-DAPPO) prepared. from 3,5- oxide (BATFPObis(trifluoromethyl)bromobenzene). This diamine was with synthesized diphenylphophinic via thechloride, Grignard followed reaction by nitration prepared and from 3,5-bis(trifluoromethyl)bromobenzenereductionScheme reactions 3 outlines [98,99]. the synthesis The solid of with bis crude(3-aminophenyl)-3,5- diphenylphophinic product was filteredbis(trifluoromethyl)phenyl chloride, and washed followed with the phosphine by copious nitration and reductionoxideamount reactions (BATFPO of water [98,). 99until This]. Theneutral, diamine solid which was crude gavesynthesized product a crude wasproductvia the filtered ofGrignard BATFPO, and reaction washed and was prepared with air-dried the from. copious Then, 3,5- it amount bis(trifluoromethyl)bromobenzene with diphenylphophinic chloride, followed1 by13 nitration and of waterwas until further neutral, purified which by gavesublimation a crude to productafford pure of BATFPO (yield, and 86%). was air-dried.H and C NMR Then, spectra it was further reductionand elemental reactions analyses [98,99]. confirmed The solid its chem crudeical prod structureuct was and filtered purity. and It showed washed a Twithm at the228 copious°C with purified by sublimation to afford pure BATFPO (yield 86%). 1H and 13C NMR spectra and elemental amountΔH = 9.0 of kcal/mol water until in the neutral, first heating which cycle gave ofa crudethe DSC product thermogram of BATFPO, (lit. mp and = 226–227 was air-dried °C) [99].. Then, it analyseswas confirmed further purified its chemical by sublimation structure to and afford purity. pure BATFPO It showed (yield a T 86%).m at 2281H and◦C with13C NMR∆H spectra= 9.0 kcal/mol and elemental analyses confirmed its chemical structure and purity. It showed a Tm at 228 °C with in the first heating cycle of the DSC thermogram (lit. mp = 226–227 ◦C) [99]. ΔH = 9.0 kcal/mol in the first heating cycle of the DSC thermogram (lit. mp = 226–227 °C) [99].

Scheme 3. Synthesis of bis(3-aminophenyl)-3,5-bis(trifluoromethyl) phenyl phosphine oxide (BATFPO).

Scheme 3.SchemeTheSynthesis third 3. diamineSynthesis of bis(3-aminophenyl)-3,5-bis(trifluoromethyl) compoundof bis(3-aminophenyl)-3,5-bis(trifluoro bis(4-aminophenoxy-4-phenyl)methyl) phenyl phenylphenyl phosphine phosphinephosphine oxide oxide (BATFPO (p- ). BAPPO(BATFPO).) was synthesized in a two-step method (Scheme 4). First, bis(4-fluorophenyl)- Thephenylphosphineoxide third diamine compound (p-FPPObis) as(4-aminophenoxy-4-phenyl) an off-white solid with a yield of phenyl 89% and phosphine mp = 128–130 oxide °C was (p-BAPPO ) The third diamine compound bis(4-aminophenoxy-4-phenyl) phenyl phosphine oxide (p- was synthesizedprepared by in areacting two-step p-bromofluorobenzene method (Scheme with4). First,phenylphosphonicbis(4-fluorophenyl)-phenylphosphineoxide dichloride in the presence of BAPPOmagnesium) was in tetrahydrofuransynthesized in (THF)a two-step [100]. Inmethod the second (Scheme step, 4).p-FPPO First, wasbis (4-fluorophenyl)-reacted with 4- (p-FPPO) as an off-white solid with a yield of 89% and mp = 128–130 C was prepared by phenylphosphineoxideaminophenol in the presence (p-FPPO of K) as2CO an3 inoff-white N,N-dimethyl solid with acetamide a yield of(DMAc) 89% and to affordmp = ◦128–130 p-BAPPO °C. wasThe reacting preparedcrudep-bromofluorobenzene product by reacting was finally p-bromofluorobenzene purified with phenylphosphonic by column with chromatography phenylphosphonic dichloride over dichloride insilica the gel presencein eluting the presence with of magnesium2% of in tetrahydrofuranmagnesiummethanol/98% in (THF) ethyltetrahydrofuran acetate [100]. (v/v) In the (THF)to furnish second [100]. p-BAPPO step, In thep-FPPO assecond an off-white wasstep, reactedp solid-FPPO powder withwas reacted 4-aminophenolwith 82% with yield. 4- in the aminophenol in the presence of K2CO3 in N1 ,N-dimethyl13 acetamide (DMAc) to afford p-BAPPO. The presenceThe of KchemicalCO in structureN,N-dimethyl was confirmed acetamide by H and (DMAc) C NMR to and aff DSCord analysesp-BAPPO (mp. = The 98–100 crude °C) [100]. product was crude 2product3 was finally purified by column chromatography over silica gel eluting with 2% finally purifiedmethanol/98% by column ethyl acetate chromatography (v/v) to furnish over p-BAPPO silica gelas an eluting off-white with solid 2% powder methanol with /82%98% yield. ethyl acetate (v/v) to furnishThe chemicalp-BAPPO structureas was an confirmed off-white by solid 1H and powder 13C NMR with and DSC 82% analyses yield. The(mp = chemical 98–100 °C) structure[100]. was 1 13 confirmed by H and C NMR and DSC analyses (mp = 98–100 ◦C) [100]. Polymers 2019, 11, x FOR PEER REVIEW 10 of 26

Scheme 4. Synthesis of bis(4-aminophenoxy-4-phenyl) phenyl phosphine oxide (p-BAPPO). Scheme 4. Synthesis of bis(4-aminophenoxy-4-phenyl) phenyl phosphine oxide (p-BAPPO). 3.2. Synthesis and Characterization of Ionic Polymers The phenyl phosphine oxide-containing poly(4,4′-(p-phenylene)-bis(2,6-diphenylpyridinium)) ionic polymer (P-1) was prepared by a ring-transmutation polymerization reaction (Scheme 5) [42– 58]. This polymerization reaction was essentially a polycondensation reaction between 4,4′-(1,4- phenylene)-bis(2,6-diphenylpyrylium) ditosylate monomer (M) and bis(3-aminophenyl)phenyl phosphine oxide (m-DAPPO) liberating water as a condensation product. M was polymerized with m-DAPPO simply on heating in non-volatile, non-toxic, water-miscible solvent dimethyl sulfoxide (DMSO) at 130–140 °C for 24 h. Regarding the water, the condensation product that was generated during the polymerization reaction was distilled out from the reaction medium as a toluene/water azeotrope. The yellowish solid ionic polymer P-1 was isolated by two cycles of precipitation with distilled water and dissolution in methanol. Its yield of 77–80% with high purity was achieved after purification. It should be noted that the reaction medium was maintained as a homogeneous solution throughout the entire polymerization reaction period, thus permitting the production of a high molecular weight polymer. This polycondensation reaction was essentially in contrast to other polycondensation reactions in which polymers usually precipitate out of solutions prematurely, thus limiting the maximum molecular weights of the polymers. Polymer P-1 was fully characterized by elemental analysis, 1H and 13C NMR, and gel permeation chromatography (GPC) analyses. Figure 3 shows the 1H and 13C NMR spectra of ionic polymer P-1 in d6-DMSO at room temperature. Its 1H NMR spectrum showed unique resonances at δ = 8.93 and 8.70 ppm for the protons of the aromatic moieties of bispyridinium salts and a set of resonances at δ = 7.44 and 7.05 ppm and 2.25 ppm for the protons of the aromatic moiety and methyl group in the tosylate counterion. The relative integration ratio of all the aromatic protons (49H) and the aliphatic protons (6H) is in excellent agreement with the calculated value obtained from those in the repeating unit of this polymer. Its 13C NMR spectrum contained both aliphatic and aromatic carbon signals at appropriate chemical shifts, as expected. Using similar ring-transmutation polymerization reaction (Scheme 5) and purification processes, ionic polymers P-2 and P-3 were obtained by polymerizing M with the corresponding diamines (BATFPO and p-BAPPO) in DMSO at 130-140 °C for 24 h with yields of 77% (yellow solid) and 79% (dark brown solid), respectively. The chemical structures of both the ionic polymers P-2 and P-3 were also confirmed by elemental, 1H, and 13C NMR spectra analyses. Figure 4 shows the 1H and 13C NMR spectra of polymer P-2 in d6-DMSO taken at room temperature, and the 1H and 13C NMR spectra of polymer P-3 are given in Figure S1. Similar to the analysis of 1H NMR spectrum of polymer P-1 (vide supra), its relative integration ratio of the aromatic protons (47H) and aliphatic protons (6H) is in excellent agreement with the calculated value obtained from those in the repeating unit of this polymer. Its 13C NMR spectrum also contained both aliphatic and aromatic carbon signals at appropriate chemical shifts, as expected.

Polymers 2019, 11, 1141 10 of 25

3.2. Synthesis and Characterization of Ionic Polymers

The phenyl phosphine oxide-containing poly(4,40-(p-phenylene)-bis(2,6-diphenylpyridinium)) ionic polymer (P-1) was prepared by a ring-transmutation polymerization reaction (Scheme5) [ 42–58]. This polymerization reaction was essentially a polycondensation reaction between 4,40-(1,4-phenylene)- bis(2,6-diphenylpyrylium) ditosylate monomer (M) and bis(3-aminophenyl)phenyl phosphine oxide (m-DAPPO) liberating water as a condensation product. M was polymerized with m-DAPPO simply on heating in non-volatile, non-toxic, water-miscible solvent dimethyl sulfoxide (DMSO) at 130–140 ◦C for 24 h. Regarding the water, the condensation product that was generated during the polymerization reaction was distilled out from the reaction medium as a toluene/water azeotrope. The yellowish solid ionic polymer P-1 was isolated by two cycles of precipitation with distilled water and dissolution in methanol. Its yield of 77–80% with high purity was achieved after purification. It should be noted that the reaction medium was maintained as a homogeneous solution throughout the entire polymerization reaction period, thus permitting the production of a high molecular weight polymer. This polycondensation reaction was essentially in contrast to other polycondensation reactions in which polymers usually precipitate out of solutions prematurely, thus limiting the maximum molecular weights of the polymers. Polymer P-1 was fully characterized by elemental analysis, 1H and 13C NMR, and gel permeation chromatography (GPC) analyses. Figure3 shows 1 13 1 the H and C NMR spectra of ionic polymer P-1 in d6-DMSO at room temperature. Its H NMR spectrum showed unique resonances at δ = 8.93 and 8.70 ppm for the protons of the aromatic moieties of bispyridinium salts and a set of resonances at δ = 7.44 and 7.05 ppm and 2.25 ppm for the protons of the aromatic moiety and methyl group in the tosylate counterion. The relative integration ratio of all the aromatic protons (49H) and the aliphatic protons (6H) is in excellent agreement with the calculated value obtained from those in the repeating unit of this polymer. Its 13C NMR spectrum contained both aliphatic and aromatic carbon signals at appropriate chemical shifts, as expected. Using similar ring-transmutation polymerization reaction (Scheme5) and purification processes, ionic polymers P-2 and P-3 were obtained by polymerizing M with the corresponding diamines (BATFPO and p-BAPPO) in DMSO at 130–140 ◦C for 24 h with yields of 77% (yellow solid) and 79% (dark brown solid), respectively. The chemical structures of both the ionic polymers P-2 and P-3 were also confirmed by elemental, 1H, and 13C NMR spectra analyses. Figure4 shows the 1H and 13 1 13 C NMR spectra of polymer P-2 in d6-DMSO taken at room temperature, and the H and C NMR spectra of polymer P-3 are given in Figure S1. Similar to the analysis of 1H NMR spectrum of polymer P-1 (vide supra), its relative integration ratio of the aromatic protons (47H) and aliphatic protons (6H) is in excellent agreement with the calculated value obtained from those in the repeating unit of this polymer. Its 13C NMR spectrum also contained both aliphatic and aromatic carbon signals at appropriate chemical shifts, as expected. Polymers 2019, 11, x FOR PEER REVIEW 11 of 26

Scheme 5. Synthesis of poly(4,4′-(p-phenylene)-bis(2,6-diphenylpyridinium)) polymer P-1 using a Scheme 5. Synthesis of poly(4,40-(p-phenylene)-bis(2,6-diphenylpyridinium)) polymer P-1 using ring-transmutation polymerization reaction. a ring-transmutation polymerization reaction.

Figure 3. 1H and 13C NMR spectra of ionic polymer P-1 in d6-DMSO taken at room temperature.

Figure 4. 1H and 13C NMR spectra of ionic polymer P-2 in d6-DMSO taken at room temperature.

PolymersPolymers 2019 2019, ,11 11, ,x x FOR FOR PEER PEER REVIEW REVIEW 1111 of of 26 26

Polymers 2019, 11, 1141 11 of 25 ′ SchemeScheme 5.5. SynthesisSynthesis ofof poly(4,4poly(4,4-(′-(p-p-phenylene)-phenylene)-bisbis(2,6-diphenylpyridinium))(2,6-diphenylpyridinium)) polymerpolymer P-1P-1 usingusing aa ring-transmutationring-transmutation po polymerizationlymerization reaction. reaction.

1 13 Figure1 3. H1 and13 13C NMR spectra of ionic polymer P-1 in d6-DMSO taken at room temperature. FigureFigure 3. H 3. and H andC NMRC NMR spectra spectra of of ionic ionic polymerpolymer P-1P-1 inin d6d-DMSO6-DMSO taken taken at room at room temperature. temperature.

Figure 4. 1H and 13C NMR spectra of ionic polymer P-2 in d6-DMSO taken at room temperature. Figure1 4. 1H and13 13C NMR spectra of ionic polymer P-2 in d6-DMSO taken at room temperature. Figure 4. H and C NMR spectra of ionic polymer P-2 in d6DMSO taken at room temperature.

Additional ionic polymers were prepared by a simple metathesis reaction of the respective primary ionic polymers in the presence of a desired ion (Scheme6)[ 42–55]. This reaction leads to the exchange of the original tosylate ions with newly introduced ions. Using this metathesis reaction, ionic polymers P-4, P-5, and P-6 were prepared with high yield (95%) and purity from the respective ionic polymers P-1, P-2, and P-3 by exchanging the tosylate ions with triflimide ions. It ought to be noted here that although ionic polymers with the triflimide ion can be made directly from a triflimide-modified monomer M, handling the very acidic bistrifluoromethanesulfonyl imide counter ion that would be required is very inconvenient. The chemical structures and purities of P-4, P-5, and P-6 ionic polymers were confirmed by elemental, 1H, and 13C NMR spectra analyses (Figures S2–S4). The facile ring-transmutation polymerization eliminates the need for extreme purification of the monomers or the final polymers. The purification of this class of polymers was conveniently performed by simple dissolution and precipitation in benign solvents, including water. Furthermore, this polymerization reaction requires no special pieces of glassware, no special catalysts, and no rigorous exclusion of moisture, which will enable the scale up for the synthesis of this class of ionic polymers. Thus, these high-temperature tolerant ionic polymers can represent an attractive alternative to reduce their impact on the environment pollution. The metathesis reaction offers excellent potential for the low-cost mass production of a range of ionic polymers with tunable properties. Polymers 2019, 11, x FOR PEER REVIEW 12 of 26

Additional ionic polymers were prepared by a simple metathesis reaction of the respective primary ionic polymers in the presence of a desired ion (Scheme 6) [42–55]. This reaction leads to the exchange of the original tosylate ions with newly introduced ions. Using this metathesis reaction, ionic polymers P-4, P-5, and P-6 were prepared with high yield (95%) and purity from the respective ionic polymers P-1, P-2, and P-3 by exchanging the tosylate ions with triflimide ions. It ought to be noted here that although ionic polymers with the triflimide ion can be made directly from a triflimide- modified monomer M, handling the very acidic bistrifluoromethanesulfonyl imide counter ion that would be required is very inconvenient. The chemical structures and purities of P-4, P-5, and P-6 ionic polymers were confirmed by elemental, 1H, and 13C NMR spectra analyses (figures S2–S4). The facile ring-transmutation polymerization eliminates the need for extreme purification of the monomers or the final polymers. The purification of this class of polymers was conveniently performed by simple dissolution and precipitation in benign solvents, including water. Furthermore, this polymerization reaction requires no special pieces of glassware, no special catalysts, and no Polymers 2019rigorous, 11, 1141 exclusion of moisture, which will enable the scale up for the synthesis of this class of ionic 12 of 25 polymers.

Scheme 6. Metathesis reaction for the conversion of ionic polymer P-1 to P-4. Scheme 6. Metathesis reaction for the conversion of ionic polymer P-1 to P-4.

3.3. MolecularThus, Weight these and high-temperature Solubility tolerant ionic polymers can represent an attractive alternative to reduce their impact on the environment pollution. The metathesis reaction offers excellent potential Thefor number-average the low-cost mass production molecular of weights a range of (M ionicns) polymers of the ionicwith tunable polymers properties.P-1 to P-6 ranging from 36 to 65 kDa are summarized in Table1. Importantly, the polydispersity index (PDI) is low ( <1.73) 3.3. Molecular Weight and Solubility in comparison to other commercially available polymers that have much wider molecular weight The number-average molecular weights (Mns) of the ionic polymers P-1 to P-6 ranging from 36 distributions. This property provides the polymers with very specific properties that can be tailored for to 65 kDa are summarized in Table 1. Importantly, the polydispersity index (PDI) is low (<1.73) in various applications.comparison to Basedother commercially on the molecular available weight poly datamers ofthat the have ionic much polymers, wider molecular the ring- weight transmutation polymerizationdistributions. reaction This property appeared provid toes be the quite polymers eff ectivewith very for specific synthesizing properties phosphorousthat can be tailored with high molecularfor weight variouscontaining applications. ionic Based polymers. on the molecular weight data of the ionic polymers, the ring- transmutation polymerization reaction appeared to be quite effective for synthesizing phosphorous Tablewith 1. Thermalhigh molecular properties weight and containing gel permeation ionic polymers. chromatography (GPC) data of ionic polymers P-1 to P-6.

Table 1. Thermal properties and gel permeation chromatography (GPC) dataGPC of ionic Results polymers P-1 a to P-6. Polymer Tg (◦C) Td (◦C) Char Yield at 700 ◦C (%) Mn (Da) PDI P-1 270 348 52 36,559 1.49 P-2 275 352 53 51,261 1.29 P-3 258 343 49 64,545 1.31 P-4 253 439 54 43,933 1.73 P-5 231 418 47 53,783 1.50 P-6 243 437 45 64,264 1.42 a Td is the decomposition temperature at which the mass of the polymer was reduced by 5 wt% of the original.

The molecular weight of the polymers can easily be adjusted by utilizing non-stoichiometric ratios of M and diamines in the polymerization reaction. In terms of solubility, P-1, P-2, and P-3 ionic polymers were found to be readily soluble in methanol, ethanol, and acetonitrile. Polymers P-4, P-5, and P-6, on the other hand, are soluble in acetone and acetonitrile. However, they have no or very poor solubility in water, propanol, toluene, chloroform, tetrahydrofuran, and dichloromethane, suggesting that these polymers have high resistance to water, water vapor, and organic gases, as required for their use as coatings and structural component materials.

3.4. Thermal Properties To determine the thermal properties of the ionic polymers, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed at heating and cooling rates of 10 ◦C/min and at a heating rate of 10 ◦C/min in nitrogen, respectively. The DSC thermograms of ionic polymers P-1, P-2, Polymers 2019, 11, 1141 13 of 25

and P-3 are shown in Figure5. A broad endotherm at 112 ◦C due to solvent loss was observed in the first heating cycle of the DSC thermogram of P-1 (Figure5a), while no such endotherm was observed in the second heating cycle. The high glass transition temperature of P-1—Tg = 276 ◦C—was evident in its DSC thermograms. However, with further heating, an exotherm was observed after the Tg at 320 ◦C, which was due to cold crystallization. A glass transition temperature of 275 ◦C was observed for P-2 (Figure5b), whereas no cold crystallization endotherm was observed at a higher temperature up to 350 ◦C. For P-3, a glass transition temperature of Tg = 243 ◦C was observed (Figure5c). For triflimide counterions containing ionic polymers P-4, P-5, and P-6, Tg in the range of 230 to 253 ◦C was observed by DSC analysis. Depending on the chemical structures of the polymers, the Tg ranged from 231 to 275 ◦C (Table1). These results indicate that these ionic polymers have high Tg values. Polymers 2019, 11, x FOR PEER REVIEW 14 of 26

Figure 5.FigureDiff erential5. Differential scanning scanning calorimetry calorimetry (DSC) (DSC) thermograms thermograms ofof ionicionic polymers P-1, (a) P-2,P-1 ,(andb) P-2P-3 , and (c) obtained at heating and cooling rates of 10 °C/min in nitrogen. P-3 obtained at heating and cooling rates of 10 ◦C/min in nitrogen. Figure 6 shows the TGA plots of two representative ionic polymers P-2 and P-4 obtained at a Figure6 shows the TGA plots of two representative ionic polymers P-2 and P-4 obtained at heating rate of 10 °C/min in nitrogen. The decomposition temperatures (Tds) of the ionic polymers a heatingare rate also ofcompiled 10 ◦C/ minin Table in nitrogen.1. The Td values The decompositionwere found to be temperaturesin the temperature (Td ranges) of theof 343 ionic to 352 polymers are also°C compiled for the tosylate in Table containing1. The ionic T polymersd values (P-1 were, P-2, found and P-3 to), and be 418 in theto 439 temperature °C for the triflimide range of 343 to 352 ◦containingC for the ionic tosylate polymers containing (P-4, P-5, and ionic P-6 polymers), at which only (P-1 a, 5%P-2 weight, and lossP-3 ),of andall six 418 ionic to polymers 439 ◦C for the triflimideoccurred. containing These ionicdecomposition polymers temperatures (P-4, P-5, and wereP-6 mo),re at than which 100 only°C higher a 5% than weight that lossof a poly( of allp- six ionic phenylene-diphenylpyridinium) ionic polymer (245 °C) that does not contain the phosphine oxide polymers occurred. These decomposition temperatures were more than 100 ◦C higher than that of moiety. In general, polymers containing the tosylate counterions have a higher Tg but a lower Td p a poly( compared-phenylene-diphenylpyridinium) to those containing triflimide ionic counteri polymerons due (245 to◦C) the that high does thermal not containstability theof phosphinethe oxide moiety.fluorinated In general, anion. The polymers TGA analyses containing also revealed the tosylate that all six counterions ionic polymers have (P-1 a higherto P-6) haveTg but a high a lower Td comparedchar to yield those at 700 containing °C (52% for triflimide P-1, 53% counterionsfor P-2, 49% for due P-3 to, 54% the for high P-4, thermal 47% for P-5 stability, and 45% of thefor P-6 fluorinated). anion. TheThese TGA results analyses suggest alsothat these revealed ionic thatpolymers all six have ionic high polymers Td values (andP-1 charto P-6 yields.) have a high char yield at 700 ◦C (52% for P-1, 53% for P-2, 49% for P-3, 54% for P-4, 47% for P-5, and 45% for P-6). These results suggest that these ionic polymers have high Td values and char yields.

Polymers 2019, 11, 1141 14 of 25 Polymers 2019, 11, x FOR PEER REVIEW 15 of 26

FigureFigure 6.6. ThermogravimetricThermogravimetric analysis (TGA) plots of two representative ionic polymers P-2 andand P-4P-4

obtainedobtained atat aa heatingheating raterate ofof 1010 ◦°C/minC/min inin nitrogen.nitrogen.

3.5.3.5. Processing, Moisture, andand VaporVapor Resistance,Resistance, OpticalOptical/Fluorescence/Fluorescence Properties,Properties,Thin ThinFilms, Films, Adhesion, Adhesion, Mechanical and Tensile Strength Mechanical and Tensile Strength The ionic polymers P-1, P-2, and P-3 were found to be readily soluble in methanol, ethanol, The ionic polymers P-1, P-2, and P-3 were found to be readily soluble in methanol, ethanol, and and acetonitrile, whereas P-4, P-5, and P-6 are soluble in acetone and acetonitrile because of different acetonitrile, whereas P-4, P-5, and P-6 are soluble in acetone and acetonitrile because of different triflimide counterions. However, these ionic polymers have no or very poor solubility in Water, triflimide counterions. However, these ionic polymers have no or very poor solubility in propanol, toluene, chloroform, tetrahydrofuran, and dichloromethane. The solubility profiles of these water, propanol, toluene, chloroform, tetrahydrofuran, and dichloromethane. The solubility profiles ionic polymers are summarized in Table2. These results indicate that the phosphine oxide-containing of these ionic polymers are summarized in Table 2. These results indicate that the phosphine oxide- ionic polymers have high resistance to moisture, water vapor, and organic gases, which are prerequisite containing ionic polymers have high resistance to moisture, water vapor, and organic gases, which criteria for coatings and structural component materials. are prerequisite criteria for coatings and structural component materials. Table 2. Solubility profiles of ionic polymers P-1 to P-6 in different solvents. Table 2. Solubility profiles of ionic polymers P-1 to P-6 in different solvents. Solvent P-1 P-2 P-3 P-4 P-5 P-6 Solvent P-1 P-2 P-3 P-4 P-5 P-6 H2O H2O + – + ––––––– – – – – MeOH MeOH + + ++ + + –––– – – EtOH + + + ––– EtOH + + + – – – n-Propanol + –– + – + ––– n-Propanol + – – + – + – – – Acetone + – + – + – + + + Acetone + – + – + – + + + Toluene – – – – – – Toluene – – – – – – CHCl3 – + – + –– + –– CHCl3 THF – – + + – + –– + – + – –– + – – THF CH2Cl2 – – + + – + –––– + – – + – – CH2Cl2 CH3CN – + + + – + + – + +– + – – CH3CN + = Soluble, + + – = Slightly+ soluble,+ – = Insoluble.+ + + + = Soluble, + – = Slightly soluble, – = Insoluble. The optical absorption and fluorescence properties of these ionic polymers were examined usingThe a Shimadzu optical absorption UV-2401PC and UV-Vis fluorescence spectrophotometer properties of and these PTI ionic QuantaMaster polymers were™ Model examined QM-4 using/2005 spectrofluorometer,a Shimadzu UV-2401PC respectively. UV-Vis spectrophotometer Figure7 shows the and absorption PTI QuantaMaster™ and fluorescence Model spectra QM-4/2005 of P-1 inspectrofluorometer, methanol. The P-1 respectively.polymer absorbsFigure 7 inshow thesUV-visible the absorption region and (200–400 fluorescence nm) withspectra absorption of P-1 in methanol. The P-1 polymer absorbs in the UV-visible* region (200–400 nm) with absorption maximum maximum (λmax) at 342 nm, which is due to π–π transition. P-1 has a high molar absorption coefficient (λmax5) at 3421 nm,1 which is due to π–π* transition. P-1 has a high molar absorption coefficient (~105 (~10 M− cm− ) and shows linear dependence with concentration. In methanol solution, the P-1 −1 −1 polymerM cm ) emitsand shows blue light linear with dependence a peak maximum with concentration. at 458 nm. Similar In methanol optical absorption solution, andthe P-1 fluorescence polymer propertiesemits blue in light methanol with solutiona peak weremaximum also observed at 458 nm. for ionic Similar polymers opticalP-2 absorptionto P-6. These and polymers fluorescence were foundproperties to absorb in methanol light in thesolution wavelength were also range observed of 200 to for 400 ionic nm, polymers and emit lightP-2 to in P-6. the UV-visibleThese polymers range (350were to found 650 nm). to absorb Both absorptionlight in the and wavelength emission range intensities of 200 increase to 400 nm, linearly and withemit thelight increased in the UV-visible polymer concentrationrange (350 to 650 between nm). Both 0.1–5 absorptionµM. These and ionic emission polymers intensities have excellent increase thin-film linearly forming with the properties. increased Figurepolymer8 shows concentration the representative between absorption0.1–5 μM. andThese fluorescence ionic polymers spectra have of P-1 excellentpolymer thin-film thin films forming coated onproperties. glass substrates Figure 8 by shows spin the coating. representative Thin films absorption of P-1 to P-6 andalso fluorescence absorb in spectra the 200 of to P-1 400-nm polymer regions thin films coated on glass substrates by spin coating. Thin films of P-1 to P-6 also absorb in the 200 to 400- nm regions with peaks at ~245 nm. They exhibit weak fluorescence, as expected. The thin films of the

Polymers 2019, 11, 1141 15 of 25

PolymersPolymers 2019 2019, ,11 11, ,x x FOR FOR PEER PEER REVIEW REVIEW 1616 of of 26 26 with peaks at ~245 nm. They exhibit weak fluorescence, as expected. The thin films of the ionic ionicpolymersionic polymers polymers form form form aligned aligned aligned structures, structures, structures, as observed as as observed observed by opticalby by optical optical microscopy microscopy microscopy (Figure (Figure (Figure9). 9). The 9). The The formation formation formation of ofalignedof alignedaligned structures structuresstructures of the ofof thin thethe filmsthinthin films offilms ionic ofof polymers ionicionic polymerspolymers was further waswas examined furtherfurther examinedexamined by atomic by forceby atomicatomic microscope forceforce microscope(AFM)microscope analysis, (AFM) (AFM) as discussedanalysis, analysis, as below.as discussed discussed Due tobelow. below. the aligned Du Duee to to structure, the the aligned aligned these structure, structure, ionic polymers these these ionic ionic can bepolymers polymers used for canenhancingcan be be used used thefor for performanceenhancing enhancing the the of perf optoelectronicperformanceormance of of devices.optoelectronic optoelectronic devices. devices.

FigureFigure 7. 7. Optical OpticalOptical absorption absorption and and fluorescence fluorescence fluorescence spectra spectra of of ionic ionic polymer polymer P-1 P-1 in in methanol methanol methanol (concentration (concentration −6 6 −4 4 == 10 1010−−–106–10–10− 4M).− M).M).

FigureFigure 8. 8.8. Absorption Absorption (left) ((left)left )and and fluorescence fluorescence fluorescence (right) (right) (right )spectra spectra of of P-1 P-1 thin thin films films films on on glass glass substrates. substrates.

Polymers 2019, 11, 1141 16 of 25 PolymersPolymers 2019 2019, ,11 11, ,x x FOR FOR PEER PEER REVIEW REVIEW 1717 of of 26 26

FigureFigure 9. 9. Optical OpticalOptical micrographs micrographsmicrographs of of thin thin films filmsfilms of of P-1 P-1 on on glass glass substrates. substrates.

ToTo understand understandunderstand the thethe adhesion, adhesion,adhesion, delamination, delamination,delamination, mechanical, mechmechanical,anical, and tensile andand tensile strengthtensile strengthstrength of the ionic ofof polymers,thethe ionicionic polymers,large-areapolymers, large-area thinlarge-area films thin ofthin the filmsfilms ionic ofof polymersthethe ionicionic polymerspolymers on metal on substrateson metalmetal substratessubstrates were prepared werewere preparedprepared by spray byby coating. sprayspray coating.Figurecoating. 10 FigureFigure shows 1010 the showsshows large-area thethe large-arealarge-area thin films thinthin of the filmsfilms ionic ofof the polymerthe ionicionic polymerP-1polymeron tin P-1P-1 metal onon tin substratestin metalmetal substratessubstrates (without (withoutany(without surface anyany treatment surfacesurface treatmenttreatment of the metal ofof the substrate)the metalmetal subssubs preparedtrate)trate) prepared byprepared spray coatingbyby sprayspray from coatingcoating methanol fromfrom methanol solutions.methanol solutions.Thesolutions. thin films The The thin retainedthin films films their retained retained quality their their even quality quality after even even bending–unbending after after bending–unbending bending–unbending the metal the substratesthe metal metal substrates substrates at different at at differentangles.different They angles.angles. also TheyThey retained alsoalso the retainedretained quality thethe of thequalityquality thin filmsofof thethe over thinthin a films longfilms period overover aa of longlong time. periodperiod No delamination ofof time.time. NoNo delaminationanddelamination flecking of andand the thinfleckingflecking films of wereof thethe observed thinthin filmsfilms while werewere bending–unbending observedobserved whilewhile bending–unbending thebending–unbending metal substrates (Figure thethe metalmetal 10). substratesSimilarsubstrates properties (Figure(Figure were10).10). SimilarSimilar also observed propertiesproperties for ionicwerewere polymers alsoalso observedobservedP-2 to forforP-6 ionicionic. These polymerspolymers results P-2 suggestP-2 toto P-6P-6 that. . TheseThese they resultsshowresults excellent suggestsuggest adhesion thatthat theythey to showshow metal excellentexcellent substrates, adhesionadhesion high mechanical toto metalmetal substrates,substrates, and tensile highhigh strength, mechanicalmechanical and have andand excellent tensiletensile strength,applicationstrength, and and potential have have excellent excellent as coating application application and structural potential potential component as as coating coating materials and and structural structural for automobiles, component component aircrafts, materials materials engines, for for automobiles,andautomobiles, power/propulsion aircrafts, aircrafts, engines, systems.engines, and and power/propulsion power/propulsion systems. systems.

FigureFigure 10.10.10. Photographs PhotographsPhotographs of of bending–unbendingof bending–unbendingbending–unbending of large-area ofof large-arealarge-area thin films thinthin offilmsfilmsP-1 onofof tinP-1P-1 substrates onon tintin substratessubstrates prepared preparedbyprepared spray coatingby by spray spray from coating coating methanol from from methanol solution.methanol solution. solution. 3.6. High-Temperature Heat Treatment and Morphologies of Thin Films 3.6.3.6. High-Temperature High-Temperature Heat Heat Treatment Treatment and and Morphologies Morphologies of of Thin Thin Films Films The thermal stability of the ionic polymers was also determined by heat treating their thin films at TheThe thermal thermal stability stability of of the the ionic ionic polymers polymers was was also also determined determined by by heat heat treating treating their their thin thin films films 550 C in air for 30 min, and probing their morphologies before and after the heat treatment, as shown atat 550550◦ °C°C inin airair forfor 3030 min,min, andand probingprobing theirtheir morpmorphologieshologies beforebefore andand afterafter thethe heatheat treatment,treatment, asas in Figure 11. After heat treatment at 550 C in air for 30 min, a change of color of the thin films from shownshown in in Figure Figure 11. 11. After After heat heat treatment treatment ◦at at 550 550 °C °C in in air air for for 30 30 min, min, a a change change of of color color of of the the thin thin films films yellow to brown was observed, which may be due to the phosphine oxide group forming a glassy fromfrom yellowyellow toto brownbrown waswas observed,observed, whichwhich maymay bebe duedue toto thethe phosphinephosphine oxideoxide groupgroup formingforming aa phosphate layer [79–83,93–95]. However, no significant changes in the structural alignment of thin glassyglassy phosphate phosphate layer layer [79–83,93–95]. [79–83,93–95]. However, However, no no significant significant changes changes in in the the structural structural alignment alignment of of films for ionic polymers were observed (Figure 11). thinthin films films for for ionic ionic polymers polymers were were observed observed (Figure (Figure 11). 11).

Polymers 2019, 11, 1141 17 of 25 Polymers 2019, 11, x FOR PEER REVIEW 18 of 26

Polymers 2019, 11, x FOR PEER REVIEW 18 of 26

Figure 11. Optical micrographs of thin-films thin-films of ionic polymer P-1 before and after heat treatment at

550 ◦°CFigureC in air 11. for Optical 30 min micrographs at different different of magnifications.magnifications. thin-films of ionic polymer P-1 before and after heat treatment at 550 °C in air for 30 min at different magnifications. For morphologicalmorphological studies, studies, the the thin thin films films of ionic of ionic polymers polymers weresolvent were solvent cast from cast their from methanol their For morphological studies, the thin films of ionic polymers were solvent cast from their methanolsolution onto solution polished onto silicon polished substrates silicon (1 substrates1 cm), and (1 dried × 1 cm), at 60 and◦C under dried vacuum at 60 °C for under 12 h. Heat-treatedvacuum for methanol solution onto polished silicon× substrates (1 × 1 cm), and dried at 60 °C under vacuum for 12samples h. Heat-treated were prepared samples by heatingwere prepared the ionic by polymer heating thinthe filmsionic polymer at 550 ◦C thin for 30films min at utilizing550 °C for a hot 30 12 h. Heat-treated samples were prepared by heating the ionic polymer thin films at 550 °C for 30 plate.min utilizing AFM images a hot wereplate. obtained AFM images using awere Nano-Scope obtained III using microscope a Nano-Scope (Digital III Instruments microscope Inc., (Digital Veeco min utilizing a hot plate. AFM images were obtained using a Nano-Scope III microscope (Digital InstrumentsMetrology group, Inc., Veeco Santa Metrology Barbara, CA) group, in standardSanta Barb tappingara, CA) mode in standard with a tapping single silicon-crystal mode with a single tip as Instruments Inc., Veeco Metrology group, Santa Barbara, CA) in standard tapping mode with a single silicon-crystala nanoprobe.silicon-crystal The tip tapping-modetipas aas nanoprob a nanoprob AFMe.e. The The height, tapping-mode tapping-mode 3D, and cross-section AFMAFM height, height, images3D 3D, and, andof cross-section the cross-section surface images morphologies images of of theof P-1 surfacetheionic surface polymermorphologies morphologies thin films of ofP-1 on P-1 siliconionic ionic polymer substratespolymer thin before filmsfilms and on on aftersilicon silicon heat substrates substrates treatment before arebefore shownand andafter in afterheat Figure heat 12. Thetreatment roottreatment mean are squareareshown shown (RMS) in Figurein Figure surface 12. 12. roughnessThe The root root mean 0.08913mean squaresquare/0.61544, (RMS) (RMS) average surface surface height roughness roughness of domains 0.08913/0.61544, 0.08913/0.61544, 0.52786/1.81704, averageand maximumaverage height height height of domainsof ofdomains domains 0.52786/1.81704, 0.52786/1.81704, 1.07694/3.11783 andand were maximum observed height height from of the ofdomains analysisdomains 1.07694/3.11783 of 1.07694/3.11783 AFM results were obtained were observedfromobserved thin filmsfrom from ofthe the theanalysisP-1 analysisionic of polymerof AFM AFM results beforeresults / afterobtained heat from treatment.from thin thin films Afilms higher of ofthe RMS theP-1 P-1ionic surface ionic polymer roughness, polymer before/afteraveragebefore/after height heat of heat domains,treatment. treatment. and A maximum Ahigher higher RMSRMS height surfsurf oface domains roughness,roughness, were average observed average height afterheight heatof domains,of treatment domains, and for and the maximumP-1 filmsmaximum compared height height of to domainsof that domains before were were heat observed treatmentobserved afterafter due heat to the treatment treatment deformation for for the ofthe P-1 the P-1 films surface films compared morphology compared to that to by that the before heat treatment due to the deformation of the surface morphology by the formation of glassy beforeformation heat of treatment glassy phosphate due to the layers. deformation Similar AFM of th morphologicale surface morphology changes by were the also formation observed of forglassy the phosphate layers. Similar AFM morphological changes were also observed for the thin films of other phosphate layers. Similar AFM morphological changes were also observed for the thin films of other thin filmsionic polymers of other ionic P-2 to polymers P-6 in theP-2 series.to P-6 in the series. ionic polymers P-2 to P-6 in the series.

FigureFigure 12. Tapping-mode 12. Tapping-mode atomic atomic force force microscope microscope (AFM) (AFM height,) height, three-dimensional three-dimensional (3D), (3D), and and cross-section cross- section images and data for thin films of ionic polymer P-1 before and after heat treatment at 550 °C images and data for thin films of ionic polymer P-1 before and after heat treatment at 550 ◦C in air for 30 min. in air for 30 min. Figure 12. Tapping-mode atomic force microscope (AFM) height, three-dimensional (3D), and cross- section images and data for thin films of ionic polymer P-1 before and after heat treatment at 550 °C in air for 30 min.

Polymers 2019, 11, 1141 18 of 25

3.7. Fire Resistant and Retardant Properties The fire resistant and retardant properties of the ionic polymers P-1 to P-6 and a reference polymer (RP) (polythiophene) were investigated by direct flaming/firing them with a propane torch and probing their burning and/or flame ignition and retardant behavior (Figure 13). A fire test for each of the ionic polymersPolymers was 2019, 11 a, verticalx FOR PEER flame REVIEWtest based on the NASA Upward Flame Propagation19 of Test26 (NASA

Standard 60013.7. Fire and Resistant ASTM and D6413). Retardant AllProperties the tests were conducted in a chemical hood utilizing an open design setup in a 20% ambient oxygen environment. All the polymer samples were held firmly by The fire resistant and retardant properties of the ionic polymers P-1 to P-6 and a reference a clean stainlesspolymer steel(RP) (polythiophene) hook. A clean were piece investigated of paper by direct below flaming/firing the polymer them sample with a propane was placed torch for easy viewing ofand any probing dripping. their Then,burning the and/or polymer flame ignition samples and were retardant exposed behavior to an (Figure ignition 13). sourceA fire test at for their bottom edge for 10each s toof overthe ionic 5 min.polymers A propanewas a vertical torch flame was test utilizedbased on the as NASA an igniter. Upward All Flame the Propagation flame studies were videotapedTest so (NASA that the Standard afterflame, 6001 and afterglow, ASTM D6413). and charAll the formation tests were times,conducted as wellin a chemical as flame hood propagation, utilizing an open design setup in a 20% ambient oxygen environment. All the polymer samples were melting, andheld drippingfirmly by a couldclean stainless be observed steel hook. by A directclean piece flaming of paper with below a propanethe polymer torch sample for was over 5 min, although deformationplaced for easy andviewing color of any change dripping. from Then, yellow the polymer to dark samples due towere char exposed formation to an ignition were observed (Figure 13source). These at their results bottom indicate edge for that10 s to the over phenyl 5 min. A phosphine propane torch oxide-containing was utilized as an igniter. ionic All polymers the (P-1 to P-6) developedflame studies in were this videotaped study exhibit so that excellent the afterflame, fire afterglow, resistant and and char retardant formation propertiestimes, as well (videoas clips flame propagation, melting, and dripping could be observed by direct flaming with a propane torch in Supplementaryfor over 5 min, Materials). although Additionally,deformation and Figurecolor change 14 shows from yellow the heat to dark release due to ratechar (HRR),formation in units of W/g of polymerswere observedP-1 to (FigureP-6 over 13). These time inresults seconds indicate as that measured the phenyl by phosphine microcalorimetry. oxide-containing It provides ionic useful informationpolymers about ( theP-1 to combustion P-6) developed of in the this polymers study exhibit and excellent is effective fire resistant for the and laboratory retardant properties evaluation of the flame retardant(video clips properties in Supplementary of polymers. Materials). It measures Additional notly, Figure only 14 the shows HRR, the butheat alsorelease the rate peak (HRR), heat release in units of W/g of polymers P-1 to P-6 over time in seconds as measured by microcalorimetry. It rate (PHRR);provides both useful are considered information about significant the combustion parameters of the polymers for evaluating and is effective the fire for retardant the laboratory properties of materials.evaluation Total heat of releasedthe flame retardant (THR) isproperties another of relevantpolymers. parameter,It measures not which only the represents HRR, but also the the sum of heat released untilpeak theheat flame release is rate extinguished. (PHRR); both Theare consider data obtaineded significant from parameters microcalorimetry for evaluating for the polymers fire P-1 to P-6 are compiledretardant in properties Table3, of which materials. suggest Total thatheat thisreleased class (THR) of ionic is another polymers relevant are parameter, excellent which fire retardant represents the sum of heat released until the flame is extinguished. The data obtained from polymers,microcalorimetry since they exhibit for polymers relatively P-1 lowto P-6 HRR, are compiled PHRR in and Table THR 3, which values. suggest Additionally, that this class both of P-4 and P-5 showedionic very polymers low PHRR are excellent values fire when retardant compared polymers, withsince theyP-6, exhibit which relatively was presumably low HRR, PHRR related to the flexible etherand THR linkages values. present Additionally, in P-6 boththat P-4 facilitate and P-5 showed the combustion very low PHRR processes, values when as compared opposed with to the rigid aromatic moietiesP-6, which present was presumably in P-4 and relatedP-5 to. the flexible ether linkages present in P-6 that facilitate the combustion processes, as opposed to the rigid aromatic moieties present in P-4 and P-5.

Figure 13. Photographs taken during direct flaming/combustion of a reference polymer (RP; top row) and ionic polymers (P-1, P-2, and P-3) (bottom three rows). Polymers 2019, 11, x FOR PEER REVIEW 20 of 26

PolymersFigure2019 ,13.11, Photographs 1141 taken during direct flaming/combustion of a reference polymer (RP; top row)19 of 25 and ionic polymers (P-1, P-2, and P-3) (bottom three rows).

FigureFigure 14. 14. HeatHeat release release rate rate (HRR) (HRR) of polymerspolymersP-1 P-1to toP-6 P-6vs. vs. time. time. Table 3. Microcalorimetry data of polymers P-1 to P-6. Table 3. Microcalorimetry data of polymers P-1 to P-6. Sample HRR (J/g.K) Peak HRR (W/g) THR (kJ/g) TPHRR (◦C) Char (%) Sample HRR (J/g.K) Peak HRR (W/g) THR (kJ/g) TPHRR (oC) Char (%) P-1 81 75.83 11.3 392.9 51.45 P-1 81 75.83 11.3 392.9 51.45 80 77.87 10.8 391.8 52.17 8580 84.4177.87 10.8 10.4 391.8 393.8 52.17 51.53 85 84.41 10.4 393.8 51.53 P-2P-2 9393 92.8 7.1 7.1 389.1 389.1 51.91 51.91 90 89.88 7.5 390.3 51.19 90 89.88 7.5 390.3 51.19 76 75.69 7.3 394.6 52.31 76 75.69 7.3 394.6 52.31 P-3P-3 103103 103.4 12.0 12.0 579.5 579.5 50.99 50.99 9393 93.7 11.8 11.8 581.8 581.8 51.61 51.61 103103 103.2 12.5 12.5 565.8 565.8 51.09 51.09 P-4P-4 19.9 55.76 55.76 21.3 57.07 57.07 20.7 53.87 53.87 P-5P-5 16.3 48.63 48.63 14.5 48.07 48.07 16.5 47.27 47.27 P-6P-6 9898 99.67 3.9 3.9 488.5 488.5 45.64 45.64 7171 71.79 3.6 3.6 485.5 485.5 46.73 46.73 9292 92.64 3.6 3.6 483.5 483.5 45.28 45.28

3.8. Electrochemical Electrochemical Properties To understand the electronic structures of the ionic polymers P-1 to P-6 in relation to the charge transport processes in optoelectronic devices, we performed cyclic voltammetry (CV) measurements on the filmsfilms of thethe ionicionic polymers.polymers. The CV measurements were performed on an EG&G Princeton Applied Research Research potentiostat/galvanostat potentiostat/galvanostat instrument instrument (model (model 263A) 263A) in inan anelectrolyte electrolyte solution solution of 0.1 of M of tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile. Platinum (Pt) wire 0.1 M of tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile. Platinum (Pt) wire electrodes were used used as as both both counter counter and and working working electrodes, electrodes, and and the the Ag/Ag+ Ag/Ag electrode+ electrode was was used used as as the reference. A ferrocene/ferrocenium (Fc/Fc+) redox couple was used as an internal standard.

Polymers 2019, 11, 1141 20 of 25

The potential values obtained in reference to the Ag/Ag+ electrode were converted to the saturated calomel electrode (SCE) scale. Films of ionic polymers P-1 to P-6 were coated on the Pt working electrode by dipping the Pt wire into 0.5–1 wt% methanol solutions, and dried under vacuum at 80 ◦C for 6 h. All the ionic polymers showed reversible reductions with onset reduction potentials of –1.83 to –1.80 V versus SCE. The formal reduction potentials were also very similar, being in the range of –1.95 to –1.91 V versus SCE. We estimated the electron affinity (EA, LUMO level) of P-1 to P-6 to be virtually identical, 2.57–2.59 eV. Irreversible oxidation was observed for all six ionic polymers. By rough estimation, the ionization potential (IP, HOMO level) for these ionic polymers was found to be ca. 5.40 eV. These results suggest that the electrochemical properties of these ionic polymers are determined primarily by the bis(2,6-diphenylpyridinium) backbone structure. The observed reversible reduction, high electron affinity, and irreversible oxidation suggest that these ionic polymers are intrinsic n-type (electron transport) and hole-blocking materials.

4. Conclusions Our study focused on synthesizing and characterizing six new phosphine oxide-containing ionic polymers with tunable properties via ring-transmutation polymerization and metathesis reactions. The development of these materials aimed to meet the need for safe, easily processed, tunable materials. The approach followed enables the production of ionic polymers through a simple polymerization reaction with high yields and purity utilizing DMSO as solvent for the polymerization reaction and methanol-water for their purification. This process also permitted the easy adjustment of properties by a simple step such as counterion exchange. Polymers with high glass transition temperatures (Tg > 230 ◦C) and relatively high decomposition temperatures greater than 340 ◦C were achieved. The polymers were found to avoid ignition even after 5 min of exposure to direct fire, demonstrating their ability to act as high-temperature, fire retardant materials. Additionally, microcalorimetry measurements showed that they had relatively low HRR, PHRR, and THR values, which suggested that this class of ionic polymers had excellent fire retardant properties. These polymers were readily resistant to moisture and common organic solvents, and were found to have excellent film-forming properties. We also demonstrated the ability of these polymers to absorb light in the UV range of the spectrum and produce photoluminescence in the visible region. Photoactive, electroactive, and robust, high-temperature tolerant ionic polymers have high application potentials in the areas of electronics, optoelectronics, fire and corrosion resistant coatings, and structural components for automobiles, aircraft, engines, power, and propulsion systems. In addition, they can be used for firefighter garments, printed circuit boards, construction materials, and paper-thin coatings for protecting bonds, securities, stock certificates, real estate titles, and deeds.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/11/7/1141/s1, Figures S1–S4: 1H and 13C NMR spectra of polymers P-3, P-4, P-5 and P-6, Video: polymers P-1, P-2, P-3, P-4 and reference polythiophene. Author Contributions: Conceptualization, M.M.A. and P.K.B.; Data curation, H.H. and P.K.B.; Formal analysis, M.M.A. and H.H.; Funding acquisition, M.M.A. and P.K.B.; Investigation, B.B.; Methodology, A.K.N.; Project administration, M.M.A. and P.K.B.; Resources, P.K.B.; Software, M.M.A. and P.K.B.; Supervision, M.M.A. and P.K.B.; Validation, M.M.A., H.H. and P.K.B.; Visualization, A.D.R. and K.G.; Writing—original draft, M.M.A.; Writing—review & editing, M.M.A. and P.K.B. Funding: This work is in part supported by the NSF under Grant No. 0447416 (NSF EPSCoR RING-TRUE III), NSF-Small Business Innovation Research (SBIR) Award (Grant OII-0610753), NSF-STTR Phase I Grant No. IIP-0740289, and NASA GRC Contract No. NNX10CD25P. Acknowledgments: M.M.A, H.H. and P.K.B. sincerely thank Thirumal Mariappan and Prof. Charles A. Wilkie at Marquette University, Milwaukee for providing us the microcalorimetry data for the ionic polymers. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. Polymers 2019, 11, 1141 21 of 25

References

1. Negrell-Guirao, C.; Boutevin, B.; David, G.; Fruchier, A.; Sonnier, R.; Lopez-Cuesta, J.-M. Synthesis of polyphosphorinanes Part II. Preparation, characterization and thermal properties of novel flame retardants. Polym. Chem. 2011, 2, 236–243. 2. Negrell-Guirao, C.; Boutevin, B. Synthesis of Polyphosphorinanes. 1. Homopolymerization and Telomerization of Allyl Ether Phosphite. Macromolecules 2009, 42, 2446–2454. [CrossRef] 3. Laoutid, F.; Bonnaud, L.; Alexandre, M.; Lopez-Cuesta, J.-M.; Dubois, P. New prospects in flame retardant polymer materials: From fundamentals to nanocomposites. Mater. Sci. Eng. Rep. 2009, 63, 100–125. [CrossRef] 4. Weil, E.D.; Levchik, S.V. Flame Retardants in Commercial Use or Development for Textiles. J. Fire Sci. 2008, 26, 243–281. [CrossRef] 5. Levchik, S.V.; Weil, E.D. A Review of Recent Progress in Phosphorus-based Flame Retardant. J. Fire Sci. 2006, 24, 345–364. [CrossRef] 6. Levchik, S.V.; Weil, E.D. Flame retardancy of thermoplastic polyesters—a review of the recent literature. Polym. Int. 2005, 54, 11–35. [CrossRef] 7. Levchik, S.V.; Weil, E.D. Thermal decomposition, combustion and flame-retardancy of epoxy resins— A review of the recent literature. Polym. Int. 2004, 53, 1901–1929. [CrossRef] 8. Lu, S.-Y.; Hamerton, I. Recent developments in the chemistry of halogen-free flame retardant polymers. Prog. Polym. Sci. 2002, 27, 1661–1712. [CrossRef] 9. Zaikov, G.E.; Lomakin, S.M. Polymer Flame Retardancy: A New Approach. J. Appl. Polym. Sci. 1998, 68, 715–725. [CrossRef] 10. Cullis, C.F.; Hirschler, M.M. The Combustion of Organic Polymers; Oxford University: New York, NY, USA, 1980. 11. Lyons, J.W. The Chemistry and Uses of Fire Retardant; Wiley: New York, NY, USA, 1970. 12. Wang, C.; Kilitziraki, M.; MacBride, J.A.H.; Bryce, M.R.; Horsburgh, L.E.; Sheridan, A.K.; Monkman, A.P.; Samuel, I.D.W. Tuning the Optoelectronic Properties of Pyridine-Containing Polymers for Light-Emitting Devices. Adv. Mater. 2000, 12, 217–222. [CrossRef] 13. Hong, H.; Sfez, R.; Vaganova, E.; Yitzchaik, S.; Davidov, D. Electrostatically self-assembled poly(4- vinylpyridine-co-vinylpyridinium-chloride)-based LED. Thin Solid Films 2000, 366, 260–264. [CrossRef] 14. Wang, Y.Z.; Epstein, A.J. Interface Control of Light-Emitting Devices Based on Pyridine-Containing Conjugated Polymers. Acc. Chem. Res. 1999, 32, 217–224. [CrossRef] 15. Zhang, X.; Shetty, A.S.; Jenekhe, S.A. Efficient electroluminescence from a new n-type conjugated polyquinoline. Acta Polym. 1998, 49, 52–55. [CrossRef] 16. Jenekhe, S.A.; Alam, M.M.; Zhu, Y.; Jiang, S.; Shevade, A.V. Single-Molecule Nanomaterials from π-Stacked Side-Chain Conjugated Polymers. Adv. Mater. 2007, 19, 536–542. [CrossRef] 17. Tonzola, C.J.; Alam, M.M.; Jenekhe, S.A. A new synthetic route to soluble polyquinolines with tunable photophysical, redox, and electroluminescent properties. Macromolecules 2005, 38, 9539–9547. [CrossRef] 18. Tonzola, C.J.; Alam, M.M.; Jenekhe, S.A. New silicon-containing polyquinolines: Synthesis, characterization, and electroluminescence. Macromol. Chem. Phys. 2005, 206, 1271–1279. [CrossRef] 19. Kwon, T.W.; Alam, M.M.; Jenekhe, S.A. n-type conjugated dendrimers: Convergent synthesis, photophysics, electroluminescence, and use as electron-transport materials for light-emitting diodes. Chem. Mater. 2004, 16, 4657–4666. [CrossRef] 20. Eichen, Y.; Nakhmanovich, G.; Gorelik, V.; Epshtein, O.; Poplawski, J.M.; Ehrenfreund, E. Effect of Protonation-Deprotonation Processes on the Electrooptical Properties of Bipyridine-Containing Poly(p-phenylene-vinylene) Derivatives. J. Am. Chem. Soc. 1998, 120, 10463–10470. [CrossRef] 21. Epstein, A.J.; Wang, Y.Z.; Jessen, S.W.; Blatchford, J.W.; Gebler, D.D.; Lin, L.-B.; Gustafson, T.L.; Swager, T.M.; MacDiarmid, A.G. Pyridine-based conjugated polymers: Photophysical properties and light-emitting devices. Macromol. Symp. 1997, 116, 27–38. [CrossRef] 22. Wang, Y.Z.; Gebler, D.D.; Fu, D.K.; Swager, T.M.; MacDiarmid, A.G.; Epstein, A.J. Light-emitting devices based on pyridine-containing conjugated polymers. Synth. Met. 1997, 85, 1179–1182. [CrossRef] 23. Bunten, K.A.; Kakkar, A.K. Synthesis, Optical Absorption, Fluorescence, Quantum Efficiency, and Electrical Conductivity Studies of Pyridine/Pyridinium Dialkynyl Organic and Pt(II)-σ-Acetylide Monomers and Polymers. Macromolecules 1996, 29, 2885–2893. [CrossRef] Polymers 2019, 11, 1141 22 of 25

24. Gebler, D.D.; Wang, Y.Z.; Blatchford, J.W.; Jessen, S.W.; Lin, L.-B.; Gustafson, T.L.; Wang, H.L.; Swager, T.M.; MacDiarmid, A.G.; Epstein, A.J. Blue electroluminescent devices based on soluble poly(p-pyridine). J. Appl. Phys. 1995, 78, 4264–4266. [CrossRef] 25. Yamamoto, T.; Muruyama, T.; Zhou, Z.-H.; Ito, T.; Fukuda, T.; Yoneda, Y.; Begum, F.; Ikeda, T.; Sasaki, S.;

Takezoe, H.; et al. π-Conjugated Poly(pyridine-2,5-diyl), Poly(2,20/-bipyridine-5,50-diyl), and Their Alkyl Derivatives. Preparation, Linear Structure, Function as a Ligand to Form Their Transition Metal Complexes, Catalytic Reactions, n-Type Electrically Conducting Properties, Optical Properties, and Alignment on Substrates. J. Am. Chem. Soc. 1994, 116, 4832–4845. 26. Kawai, T.; Yamaue, T.; Onoda, M.; Yoshino, K. Photoinduced charge separation in multilayered heterostructures of conducting polymers and their doping effects. Synth. Met. 1999, 102, 971–972. [CrossRef] 27. Han, H.; Vantine, P.R.; Nedeltchev, A.K.; Bhowmik, P.K. Main-Chain Ionene Polymers Based on trans-1,2-Bis(4-pyridyl)ethylene Exhibiting Both Thermotropic Liquid-Crystalline and Light-Emitting Properties. J. Polym. Sci. Part A Polym. Chem. 2006, 44, 1541–1554. [CrossRef] 28. Bhowmik, P.K.; Han, H.; Nedeltchev, I.V.Main-Chain Viologen Polymers with Triflimide Counterion Exhibiting Lyotropic Liquid-Crystalline Properties in Polar Organic Solvents. J. Polym. Sci. Part A Polym. Chem. 2002, 40, 2015–2024. [CrossRef] 29. Bhowmik, P.K.; Han, H.; Cebe, J.J.; Burchett, R.A.; Sarker, A.M. Main-Chain Viologen Polymers with Organic Counterions Exhibiting Thermotropic Liquid-Crystalline and Fluorescent Properties. J. Polym. Sci. Part A Polym. Chem. 2002, 40, 659–674. [CrossRef] 30. Bhowmik, P.K.; Molla, A.H.; Han, H.; Gangoda, M.E.; Bose, R.N. Lyotropic Liquid Crystalline Main-Chain

Viologen Polymers: Homopolymer of 4,40-Bipyridyl with the Ditosylate of trans-1,4-Cyclohexanedimethanol and Its Copolymers with the Ditosylate of 1,8-Octanediol. Macromolecules 1998, 31, 621–630. [CrossRef] 31. Bhowmik, P.K.; Akhter, S.; Han, H. Thermotropic Liquid Crystalline Main-Chain Viologen Polymers. J. Polym. Sci. Part A Polym. Chem. 1995, 40, 1927–1933. [CrossRef] 32. Bhowmik, P.K.; Han, H. Lyotropic Liquid Crystalline Main-Chain Viologen Polymers. J. Polym. Sci. Part A Polym. Chem. 1995, 40, 1745–1749. [CrossRef] 33. Han, H.; Bhowmik, P.K. Liquid Crystalline Main-chain Ionene Polymers. Trends Polym. Sci. 1995, 3, 199–206. 34. Bhowmik, P.K.; Xu, W.; Han, H. Thermotropic Liquid Crystalline Main-Chain Viologen Polymers: Homopolymer of 4,4’-Bipyridyl with Ditosylate of trans-l,4-Cyclohexanedimethanol and Its Copolymers with Ditosylate of 1,8-0ctanediol. J. Polym. Sci. Part A Polym. Chem. 1994, 32, 3205–3209. [CrossRef] 35. Masson, P.; Gramain, P.; Guillon, D. Thermotropic liquid crystalline behaviour of pyridinium and poly(4-vinylpyridinium) salts quaternized with ω-alkylphenylbenzoate derivatives. Macromol. Chem. Phys. 1999, 200, 616–620. [CrossRef] 36. Zheng, W.-Y.; Wang, R.-H.; Levon, K.; Rong, Z.Y.; Taka, T.; Pan, W. Self-assembly of the electroactive complexes of polyaniline and surfactant. Macromol. Chem. Phys. 1995, 196, 2443–2462. [CrossRef] 37. Cao, Y.; Smith, P. Liquid-crystalline solutions of electrically conducting polyaniline. Polymer 1993, 34, 3139–3143. [CrossRef]

38. Merz, A.; Reitmeier, S. Poly-[N,N-(1,4-divinylbenzene-β,β0diyl)-4,4’-bipyridinium dibromide], a Novel Conducting Viologen Polymer. Angew. Chem. Int. Ed. Engl. 1989, 28, 807–808. [CrossRef] 39. Moore, J.S.; Stupp, S.I. Charge-Transfer and Thermochromic Phenomena in Solid Polyelectrolytes. Macromolecules 1986, 19, 1815–1824. [CrossRef] 40. Akahoshi, H.; Toshima, S.; Itaya, K. Electrochemical and Spectroelectrochemlcal Properties of Polyviologen Complex Modified Electrodes. J. Phys. Chem. 1981, 85, 818–822. [CrossRef] 41. Simon, M.S.; Moore, P.T. Novel Polyviologens: Photochromic Redox Polymers with Film-Forming Properties. J. Polym. Sci., Part A Polym. Chem. 1975, 13, 1–16. [CrossRef] 42. Jo, T.S.; Nedeltchev, A.K.; Biswas, B.; Han, H.; Bhowmik, P.K. Synthesis and characterization of poly(pyridinium salt)s derived from various aromatic diamines. Polymer 2012, 53, 1063–1071. [CrossRef] 43. Nedeltchev, A.K.; Han, H.; Bhowmik, P.K.; Ma, L. Solution, thermal and optical properties of new poly(pyridinium salt)s derived from conjugated quinoline diamines. J. Polym. Sci. Part A Polym. Chem. 2011, 49, 1907–1918. [CrossRef] Polymers 2019, 11, 1141 23 of 25

44. Nedeltchev, A.K.; Han, H.; Bhowmik, P.K. Solution, thermal, and optical properties of poly(pyridinium salt)s derived from an ambipolar diamine consisting of diphenylquinoline and triphenyl amine moieties. J. Polym. Sci. Part A Polym. Chem. 2010, 48, 4611–4620. [CrossRef] 45. Nedeltchev, A.K.; Han, H.; Bhowmik, P.K. Solution, thermal and optical properties of novel poly(pyridinium salt)s derived from conjugated pyridine diamines. J. Polym. Sci. Part A Polym. Chem. 2010, 48, 4408–4418. [CrossRef] 46. Nedeltchev, A.K.; Han, H.; Bhowmik, P.K. Solution, thermal and optical properties of new poly(pyridinium salt)s derived from bisquinoline diamines. Polym. Chem. 2010, 1, 908–915. [CrossRef] 47. Bhowmik, P.K.; Han, H.; Nedeltchev, A.K.; Mandal, H.D.; Jimenez-Hernandez, J.A.; McGannon, P.M. Poly(pyridinium salt)s with organic counterions derived from an aromatic diamine containing tetraoxyethylene units exhibiting amphotropic liquid-crystalline and photoluminescence properties. J. Appl. Polym. Sci. 2010, 116, 1197–1206. [CrossRef] 48. Bhowmik, P.K.; Han, H.; Nedeltchev, A.K.; Mandal, H.D.; Jimenez-Hernandez, J.A.; McGannon, P.M. Poly(pyridinium salt)s with organic counterions derived from an aromatic diamine containing oxyethylene unit exhibiting amphotropic liquid-crystalline and photoluminescence properties. Polymer 2009, 50, 3128–3135. [CrossRef] 49. Bhowmik, P.K.; Kamatam, S.; Han, H.; Nedeltchev, A.K. Synthesis and characterization of poly(pyridinium salt)s with oxyalkylene units exhibiting amphotropic liquid-crystalline and photoluminescence properties. Polymer 2008, 49, 1748–1760. [CrossRef] 50. Bhowmik, P.K.; Han, H.; Nedeltchev, A.K. Synthesis and characterization of poly(pyridinium salt)s with anthracene moieties exhibiting both lyotropic liquid-crystalline and UV light-emitting properties. Polymer 2006, 47, 8281–8288. [CrossRef] 51. Bhowmik, P.K.; Han, H.; Nedeltchev, A.K. Synthesis and characterization of poly(pyridinium salt)s with organic counterions exhibiting both thermotropic liquid-crystalline and light-emitting properties. J. Polym. Sci. Part A Polym. Chem. 2006, 44, 1028–1041. [CrossRef] 52. Bhowmik, P.K.; Han, H.; Cebe, J.J.; Nedeltchev, I.K.; Kang, S.-W.; Kumar, S. Synthesis and characterization of poly(pyridinium salt)s with organic counterions exhibiting both thermotropic liquid-crystalline and light-emitting properties. Macromolecules 2004, 37, 2688–2694. [CrossRef] 53. Bhowmik, P.K.; Burchett, R.A.; Han, H.; Cebe, J.J. Synthesis and characterization of poly(pyridinium salt)s with organic counterion exhibiting both lyotropic liquid-crystalline and light-emitting properties. Polymer 2002, 43, 1953–1958. [CrossRef] 54. Bhowmik, P.K.; Burchett, R.A.; Han, H.; Cebe, J.J. Synthesis and characterization of poly(pyridinium salt)s with organic counterion exhibiting both lyotropic liquid-crystalline and light-emitting properties. Macromolecules 2001, 34, 7579–7581. [CrossRef] 55. Bhowmik, P.K.; Burchett, R.A.; Han, H.; Cebe, J.J. Synthesis and characterization of poly(pyridinium salt)s with organic counterion exhibiting both lyotropic liquid-crystalline and light-emitting properties. J. Polym. Sci. Part A Polym. Chem. 2001, 39, 2710–2715. [CrossRef] 56. Spiliopoulos, I.K.; Mikroyannidis, J.A. Poly(pyridinium salt)s with stilbene or distyrylbenzene chromophores. J. Polym. Sci. Part A Polym. Chem. 2001, 39, 2454–2462. [CrossRef] 57. Harris, F.W.; Chuang, K.C.; Huang, S.A.X.; Janimak, J.J.; Cheng, S.Z.D. Aromatic poly(pyridinium salt)s: Synthesis and structure of organo-soluble, rigid-rod poly(pyridinium tetrafluoroborates)s. Polymer 1994, 35, 4940–4948. [CrossRef] 58. Huang, S.A.X.; Chuang, K.C.; Cheng, S.Z.D.; Harris, F.W. Aromatic poly(pyridinium salt)s Part 2. Synthesis and properties of organo-soluble, rigid-rod poly(pyridinium triflate)s. Polymer 2000, 41, 5001–5009. [CrossRef] 59. Decher, G. Fuzzy nanoassemblies: Toward layered polymeric multicomposites. Science 1997, 277, 1232–1237. [CrossRef] 60. Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Ultrathin polymer coatings by complexation of polyelectrolytes at interfaces: Suitable materials, structure and properties. Macromol. Rapid Commun. 2000, 21, 319–348. [CrossRef] 61. Arys, X.; Laschewsky, A.; Jonas, A.M. Ordered polyelectrolyte “multilayers”—1. Mechanisms of growth and structure formation: A comparison with classical fuzzy “multilayers”. Macromolecules 2001, 34, 3318–3330. [CrossRef] Polymers 2019, 11, 1141 24 of 25

62. Arys, X.; Fischer, P.; Jonas, A.M.; Koetse, M.M.; Laschewsky, A.; Legras, R.; Wischerhoff, E. Ordered polyelectrolyte multilayers. Rules governing layering in organic binary multilayers. J. Am. Chem. Soc. 2003, 125, 1859–1865. [CrossRef] 63. Tang, Z.; Kotov, N.A.; Magonov, S.; Ozturk, B. Nanostructured artificial nacre. Nat. Mater. 2003, 2, 413–418. [CrossRef] 64. Laschewsky, A.; Mallwitz, F.; Baussard, J.-F.; Cochin, D.; Fischer, P.; Habib Jiwan, J.-L.; Wischerhoff, E. Aggregation phenomena in polyelectrolyte multilayers made from polyelectrolytes bearing bulky functional, hydrophobic fragments. Macromol. Symp. 2004, 211, 135–155. [CrossRef] 65. Holder, E.; Marin, V.; Alexeev,A.; Schubert, U.S. Greenish-Yellow-, Yellow-, and Orange-Light-Emitting Iridium(III) Polypyridyl Complexes with Poly(ε-caprolactone)–Bipyridine Macroligands. J. Polym. Sci. Part A Polym. Chem. 2005, 43, 2765–2776. [CrossRef] 66. Dobrawa, R.; Würthner, F. Metallosupramolecular Approach toward Functional Coordination Polymers. J. Polym. Sci. Part A Polym. Chem. 2005, 43, 4981–4995. [CrossRef] 67. Bar-Cohen, Y. Electroactive polymers as artificial muscles-reality and challenges. In Proceedings of the 42nd AIAA Structures, Structures Dynamics and Materials Conferences (SDM), Gossamer Spacecraft Forum (GSF), Seattle, WA, USA, 16–19 April 2001. 68. Chen-Yang, Y.W.; Yuan, C.Y.; Li, C.H.; Yang, H.C. Preparation and Characterization of Novel Flame Retardant (Aliphatic phosphate)Cyclotriphosphazene-Containing Polyurethanes. J. Appl. Polym. Sci. 2003, 90, 1357–1364. [CrossRef] 69. Wu, W.; Yang, C.Q. Correlation Between Limiting Oxygen Index and Phosphorus/Nitrogen Content of Cotton Fabrics Treated with a Hydroxy-Functional Organophosphorus Flame-Retarding Agent and Dimethyloldihydroxyethyleneurea. J. Appl. Polym. Sci. 2003, 90, 1885–1890. [CrossRef] 70. Rosy, A. Synthesis, Characterization, and Thermal Studies of Cardanol-Based Polyphosphate Esters. J. Polym. Sci. Part A Polym. Chem. 1993, 31, 3187–3191. 71. Liu, Y.-L.; Hsiue, G.-H.; Lan, C.-W.; Chiu, Y.-S. Flame-Retardant Polyurethanes from Phosphorus-Containing Isocyanates. J. Polym. Sci. Part A Polym. Chem. 1997, 35, 1769–1780. [CrossRef] 72. Kim, H.-J.; Choi, J.-K.; Jo, B.-W.; Chang, J.-H.; Farris, R.J. Flame retarding properties of phosphorus containing poly(amic acid). Korea Polym. J. 1998, 6, 84–90. 73. Tian, C.; Wang, H.; Liu, X.; Ma, Z.; Guo, H.; Xu, I. Flame Retardant Flexible Poly(vinyl chloride) Compound for Cable Application. J. Appl. Polym. Sci. 2003, 89, 3137–3142. [CrossRef] 74. Galip, H.; Hasipo˘glu,H.; Gündüz, G. Flame-Retardant Polyester. J. Appl. Polym. Sci. 1999, 74, 2906–2910. [CrossRef] 75. Liu, J.; Gao, Y.; Wang, F.; Wu, M. Preparation and Characteristics of Nonflammable Polyimide Materials. J. Appl. Polym. Sci. 2000, 75, 384–389. [CrossRef] 76. Kashiwagi, T.; Morgan, A.B.; Antonucci, J.M.; VanLandingham, M.R.; Harris, R.H., Jr.; Awad, W.H.; Shields, J.R. Thermal and Flammability Properties of a Silica–Poly(methylmethacrylate) Nanocomposite. J. Appl. Polym. Sci. 2003, 89, 2072–2078. [CrossRef] 77. Wu, Q.; Lu, J.; Qu, B. Preparation and characterization ofmicrocapsulated red phosphorus and itsflame-retardant mechanism in halogen-freeflame retardant polyolefins. Polym. Int. 2003, 52, 1326–1331. [CrossRef] 78. Liu, Y.-L.; Hsiue, G.-H.; Chiu, Y.-S. Synthesis, Characterization, Thermal, and Flame Retardant Properties of Phosphate-Based Epoxy Resins. J. Polym. Sci. Part A Polym. Chem. 1997, 35, 565–574. [CrossRef] 79. Gravalos, K.G. Synthesis and Flammability of Copolyisophthalamides. I. With Phosphorus Groups in the Main Chain. J. Polym. Sci. Part A Polym. Chem. 1992, 30, 2521–2529. [CrossRef] 80. Liu, Y.L.; Liu, Y.L.; Jeng, R.J.; Chiu, Y.-S. Triphenylphosphine oxide-based bismaleimide and poly(bismaleimide): Synthesis, characterization, and properties. J. Polym. Sci. Part A Polym. Chem. 2001, 39, 1716–1725. [CrossRef] 81. Mateva, R.P.; Dencheva, N.V. Flammability and Thermal Behavior of Phosphorus-Containing Polyamide-6. J. Appl. Polym. Sci. 1993, 47, 1185–1192. [CrossRef] 82. Ma, Z.; Zhao, W.; Liu, Y.; Shi, J. Synthesis and Properties of Intumescent, Phosphorus-Containing, Flame-Retardant Polyesters. J. Appl. Polym. Sci. 1997, 63, 1511–1515. [CrossRef] 83. Faghihi, K.; Hajibeygi, M. Synthesis and Properties of New Poly(amide imide)s Containing Trimellitic Rings and Hydantoin Moieties in the Main Chain under Microwave Irradiation. J. Appl. Polym. Sci. 2004, 92, 3447–3453. [CrossRef] Polymers 2019, 11, 1141 25 of 25

84. Connell, J.W.; Watson, K.A. Space environmentally stable polyimides and copolyimides derived from bis(3-aminophenyl)-3,5-di(trifluoromethyl)phenylphosphine oxide. High Perform. Polym. 2001, 13, 23–34. [CrossRef] 85. Connell, J.W.; Hergenrother, P.M.; Smith, G., Jr. The USA as Represented by the Administrator of the NASA. U.S. Patent Specification 5245044, 14 September 1994. 86. Connell, J.W.; Hergenrother, P.M.; Smith, G., Jr. The USA as Represented by the Administrator of the NASA. U.S. Patent Specification 5 317078, 31 May 1994. 87. Connell, J.W.; Hergenrother, P.M.; Smith, G., Jr. The USA as Represented by the Administrator of the NASA. U.S. Patent Specification 5412059, 2 May 1995. 88. Smith, C.D.; Grubbs, H.; Webster, H.F.; Gungör, A.; Wightman, J.P.; McGrath, J.E. Unique Characteristics Derived From Poly(Arylene Ether Phosphine Oxide)s. High Perform. Polym. 1991, 3, 211–229. [CrossRef] 89. Smith, G., Jr.; Connell, J.W.; Hergenrother, P.M. Oxygen plasma resistant phenylphosphine oxide-containing poly(arylene ether)s. Polymer 1994, 35, 2834–2839. [CrossRef] 90. Connell, J.W.; Smith, G., Jr.; Hergenrother, P.M. Oxygen plasma-resistant phenylphosphine oxide-containing polyimides and poly{arylene ether heterocycle)s: 1. Polymer 1995, 36, 5–11. [CrossRef] 91. Connell, J.W.; Smith, G., Jr.; Hedrick, J.L. Oxygen plasma-resistant phenylphosphine oxide-containing polyimides and poly(arylene ether heterocycle)s: 2. Polymer 1995, 36, 13–19. [CrossRef] 92. Lennhoff, J.; Harris, G.; Vaughn, J.; Edwards, D.; Zwiener, J. Electrically conductive space-durable polymeric films for spacecraft thermal and charge control. High Perform. Polym. 1999, 11, 101–111. [CrossRef] 93. Wilson, D.; Stenzenberg, H.D.; Hergenrother, P.M. Polyimides; Blackie: Glasgow, UK, 1990. 94. Liu, Y.-L.; Hsiue, G.-H.; Chiu, Y.-S.; Jeng, R.-J. Phosphorus Containing Epoxy for Flame Retardant II: Curing Reaction of Bis-(3-Clycidyloxy) Phenylphosphine Oxide. J. Appl. Polym. Sci. 1996, 61, 1789–1796. [CrossRef] 95. Schuler, P.; Mojazza, H.B.; Haghighat, R. Atomic oxygen resistant films for multi-layer insulation. High Perform. Polym. 2000, 12, 113–123. [CrossRef] 96. Martínez-Núñez, M.F.; Sekharipuram, V.N.; McGrath, J.E. Synthesis and characterization of polyimides using a novel phosphorus containing diamine. Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem. 1994, 35, 709–710.

97. Kizilkaya, C.; Karata¸s,S.; Apohan, N.-K.; Güngör, A. Synthesis and Characterization of Novel Polyimide/SiO2 Nanocomposite Materials Containing Phenylphosphine Oxide via Sol-Gel Technique. J. Appl. Polym. Sci. 2010, 115, 3256–3264. [CrossRef] 98. Brock, T.; Sherrington, D.C.; Swindell, J. Synthesis and Characterisation of Porous Particulate Polyimides. J. Mater. Chem. 1994, 4, 229–236. [CrossRef] 99. Jeong, K.U.; Kim, J.-J.; Yoon, T.-H. Synthesis and characterization of novel polyimides containing fluorine and phosphine oxide moieties. Polymer 2001, 42, 6019–6030. [CrossRef] 100. Hergenrother, P.M.; Havens, S.J. Polyimides Containing Carbonyl and Ether Connecting Groups. I1. J. Polym. Sci. Part A Polym. Chem. 1989, 27, 1161–1174. [CrossRef] 101. Agrawal, A.K.; Jenekhe, S.A. Electrochemical Properties and Electronic Structures of Conjugated Polyquinolines and Polyanthrazolines. Chem. Mater. 1996, 8, 579–589. [CrossRef] 102. Yang, C.-J.; Jenekhe, S.A. Conjugated Aromatic Polyimines. 2. Synthesis, Structure, and Properties of New Aromatic Polyazomethines. Macromolecules 1995, 28, 1180–1196. [CrossRef]

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