Synthesis of New Polymer Ionomers via Ring-Opening Metathesis Polymerization

The N-pentafluorophenyl-exo-endo-norbornene-5,6-dicarboximide (2a) and N-phenyl-exo-endo-norbornene-5,6-dicarboximide (2b) monomers were synthesized and polymerized via ring-opening metathesis polymerization (ROMP) using bis(tricyclohexylphosphine) benzylidene ruthenium(IV) dichloride (I) and tricyclohexylphosphine [1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2ylidene][benzylidene] ruthenium dichloride (II). Both catalysts were used to synthesize random and block high molecular weight copolymers which were further hydrogenated using a Wilkinson’s catalyst. Then, the saturated copolymers were modified by reacting with sodium 4-hydroxybenzenesulfonate dihydrate to generate new ionomers with fluoro-sulfonic acid pendant groups.


Introduction
The ring-opening metathesis polymerization (ROMP) of fluorinated norbornenes using classical metathesis catalysts is well established [1] [2].Furthermore, the presence of fluorine containing moieties in the polynorbornene dicarboximide structures becomes important for their gas transport properties due to an increase of the interactions between the gases and the polar fluorinated moieties as well as in the free volume which in turns fa-cilitates the diffusion of the gas molecules through the polymer [3] [4].The pentafluorophenyl moieties also provide the possibility of further modifications.They are highly reactive towards the nucleophilic aromatic substitutions and multiblock copolymers have been successfully prepared by a polycondensation reaction between fluorinated oligomers and hydroxyl-terminated telechelics [5].
On the other hand, we reported the synthesis and ionic transport performance of a ionic polynorbornene dicarboximide [6] [7], therefore we have envisioned the synthesis of high molecular weight polymers, their homogenous post-hydrogenations and even further sulfonations to obtain new polymeric ionomers.

Techniques
1 H NMR, 13 C NMR and 19 F NMR spectra were recorded on a Varian spectrometer at 300, 75 and 300 MHz, respectively, in deuterated chloroform CDCl 3 , N,N-dimethylformamide (DMF-d 7 ) and dimethylsulfoxide (DMSOd 6 ).Tetramethylsilane (TMS) and trifluoroacetic acid (TFA) were used as internal standards, respectively.Glass transition temperatures, T g , were determined in a DSC-7 Perkin Elmer Inc., at scanning rate of 10˚C/min under nitrogen atmosphere.The samples were encapsulated in standard aluminum DSC pans.Each sample was run twice on the temperature range between 30˚C and 300˚C under nitrogen atmosphere.The T g values obtained were confirmed by TMA from the first heating cycle conducted at a rate of 10˚C/min under nitrogen atmosphere with a TA Instruments Thermomechanical Analyzer TMA 2940.Onset of decomposition temperature, T d , was determined using thermogravimetric analysis, TGA, which was performed at a heating rate of 10˚C/min under nitrogen atmosphere with a DuPont 2100 instrument.FT-IR spectra were obtained on a Thermo Nicolet 6700 spectrometer.Molecular weights and molecular weight distributions were determined with reference to polystyrene standards on a Waters 2695 ALLIANCE GPC at 35˚C in tetrahydrofuran using a universal column and a flow rate of 0.5 mL•min −1 .X-ray diffraction measurements of copolymer films as cast were carried out in a Siemens D-5000 diffractometer between 4 and 70 degrees 2θ, at 35 KV 25 mA, using CuK α radiation (1.54 Å).Tapping mode atomic force microscopy (TM-AFM) was performed in air using a Scanning Probe Microscope Jeol JSPM-4210 with a NSC12 µmasch needle.The samples were imaged at ambient conditions.

Metathesis Copolymerization of Monomers
Copolymerizations were carried out in glass vials under dry nitrogen atmosphere.They were inhibited by adding a small amount of ethyl vinyl ether and the solutions were poured into an excess of methanol.The copolymers were purified by solubilization in chloroform containing a few drops of 1 N HCl and precipitation into methanol.The obtained copolymers were dried in a vacuum oven at 40˚C to constant weight.

Results and Discussion
Monomers 2a and 2b were prepared in high yields according to literature [8] [9].2,3,4,5,6-Pentafluoroaniline and aniline reacted with NDA to the corresponding amic acids which were cyclized to imide using acetic anhydride as dehydrating agent (Scheme 1). 1 H, 13 C and 19 F NMR spectra as well as elemental analysis confirmed monomers structure and purity.The high molecular weight copolymers were synthesized via ROMP using bis(tricyclohexylphosphine) benzylidene ruthenium(IV) dichloride (I) and tricyclohexylphosphine [1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene] [benzylidene] ruthenium dichloride (II) (Scheme 2).Table 1 summarizes the results of the high conversion copolymerizations of 2a with 2b.It is observed that catalyst II produced random high molecular weight copolymers in the early minutes of reaction in high yield at room temperature with almost complete incorporation of 2a in the copolymer (Entry 1).On the contrary, catalyst I was not able to incorporate monomer 2a in copolymer in the same time even at 45˚C (Entry 3) and more reaction time was needed for the incorporation of 2a to take place and it could be detected by 1 H NMR (Entries 4 and 5).In fact, complete incorpo-Scheme 3. Hydrogenation and further sulfonation of polynorbornene based copolymers bearing pentafluorophenyl moieties.ration of monomer 2a in copolymer, in high yield and using catalyst I, was only achieved by increasing temperature to 65 ˚C (Entry 6).Block copolymers of 2a with 2b were synthesized in high yields using catalyst I (Entries 7-9).In the first case, the copolymerizations were conducted at 65˚C and the monomer 2b was added initially polymerizing completely within 0.33 h (Entries 7-9).Immediately, monomer 2a was added to the reaction and complete incorporation of 2a in copolymer was detected after 1.33 h of being added to the growing polymer (Entry 9). Figure 1 shows the thermomechanical analysis performed on the random and block copolymers synthesized.As expected, a single transition is observed, at 205˚C, and interpreted as the glass transition temperature, T g , of the random copolymer 3 whereas two transitions are observed for the block copolymer 4, at 170˚C and 224˚C, and attributed to the corresponding T g of 2a and 2b homopolymer regions, respectively.The thermo-oxidative stabilities of block copolymer 4 were enhanced by quantitative hydrogenation according to the methodology previously reported for this kind of polymers (Scheme 3) [10].
Figure 2 shows the X-ray diffraction patterns of the as cast random copolymer 3 as well as block copolymer 4 films.
Polymeric ionomer was synthesized by reaction of hydrogenated copolymer 5 with sodium 4-hydroxybenzenesulfonate dehydrate (Scheme 3).Ionomer films were cast from sulfonated copolymer solutions in DMF and DMSO, respectively.The films were quite flexible when fully hydrated and became somewhat brittle as they dried out.The substitution reaction was monitored by 19 F NMR and 1 H NMR spectroscopy and the degree of sulfonation was controlled both by the nucleophilic agent amount and the time of reaction.According to Figure 3, it is appreciated that as the pentafluorophenyl moiety is sulfonated, the signal corresponding to the fluorine atom in meta position of unsulfonated copolymer 5 (b, −160.2 ppm) becomes weak and a new meta signal corresponding to those pentafluorophenyl moieties which have already been sulfonated (6) become to grow (d, −153.1 ppm) until a unique meta signal is observed at complete sulfonation of copolymer 5 (d, −153.1 ppm).
The signal corresponding to the fluorine atom in para position (c, −150.1 ppm) decreases until its complete disappearance when a fully sulfonated copolymer is obtained.From this analysis we conclude that only the carbon in para position has undergone the nucleophilic aromatic substitution.In addition, FT-IR allowed us to confirm the introduction of sulfonate groups in the copolymers by observing the characteristic bands around 1033 and 1165 cm −1 assigned to symetric and asymmetric stretching of sulfonate groups.

Conclusion
The random and block high molecular weight copolymers of 2a with 2b using ruthenium alkylidene catalysts   were synthesized.The main chains were hydrogenated and the perfluoroaromatic moieties were further sulfonated quantitatively to yield thermally enhanced film forming new polymeric ionomers.

Table 1 .
General conditions for copolymerization of monomer 2a.Molar ratio of monomer to catalyst = 1000, 1,2-Dichloroethane as solvent, Mol% of 2a in the feed = 50; b Determined by 1 H NMR; c Methanol insoluble polymer; d Reaction time for monomer 2b; e Reaction time for monomer 2a; f GPC analysis in tetrahydrofuran with polystyrene calibration standards. a