Bis-Perfluorocycloalkenyl (PFCA) Aryl Ether Monomers Towards a Versatile Class of Semi-Fluorinated Aryl Ether Polymers

A unique class of perfluorocycloalkenyl (PFCA) aryl ether monomers was synthesized from commercially available perfluorocycloalkenes (PFCAs) and bisphenols in good yields. This facile one pot reaction of per- fluorocycloalkenes, namely, octafluorocyclopentene (OFCP), and decafluorocyclohexene (DFCH), with bisphenols occurs at room temperature via an addition–elimination reaction in the presence of a base. The synthesis of PFCA monomers and their condensation with bisphenols lead to perfluorocycloalkenyl (PFCA) aryl ether homopolymers and copolymers with random and/or alternating polymer architectures.

With few exceptions such as, fluorinated vinylene derived Teflon, perfluorocyclco-olefins, in general, do not undergo homopolymerization via free radical mechanism. 36   Due to the volatility of perfluorocycloalkenes, the stochiometry ratio is difficult to sustain during polymerization. However, bis-PFCA aryl ether monomers reported here, not only precludes the stochiometry ratio issue, but also provides a new synthetic versatility which can afford a variety of random and/or alternating copolymers with specific design control to be published elsewhere (Scheme 2). 5 These monomers also provide the option of synthesizing copolymers containing both, OFCP and DFCP moieties.
Attack of the nucleophile on the PFCA double bond generates a carbanion which can eliminate fluoride resulting in vinyl substituted and allyl substituted products (Scheme 3). The ratio of vinylic to allylic products depends on the ring size, reaction conditions and nucleophile. 26 The vinylic product would be the major product unless the allyl position has a more favored leaving group. 26 The allylic product was noticeable in case of DFCH (M3-M6) as evident by three signals with identical molecular weight in the GC-MS spectra (Fig. S23). Allyl substitution was not observed in case of OFCP (M1-M3) as evidenced by NMR and GC-MS. The presence of allylic products for M4-M6 also explains the unaccounted signals in the 1 H, 19 F and 13 C NMR (Figs. 3 and S13-S20). Interestingly, there were no other addition products, as has previously been reported with fluorinated arylene vinylene ether (FAVE) polymers. 37,38 13 Carbon ( 19 F-coupled) NMR has been proved to be a very helpful technique to determine structural properties of M1-M6. 19 Fluorine coupled 13 Carbon NMR of perfluorocycloalkenes and their products show short range as well as long range coupling, and each carbon of PFCA ring shows a splitting pattern due to each fluorine atom present on the ring. For example, in OFCP the b carbon, a, with two fluorine atoms shows a tquint.t with J = 1375.0, 120.0, 25.4 Hz, and the a carbon, b, shows a tqm, J = 1287.5, 116.8 Hz (Fig. S2). As expected, DFCH and its products show a more complicated and unresolved splitting pattern in their 13 C NMR spectra due to the higher number of fluorine atoms (Figs. S4, S15, S18 and S20).
In the case of M1, the 1 H NMR spectrum (Fig. S5) shows a doublet of doublet (at 7.41 and 7.67 with J = 8.2 Hz), representing a symmetric environment. The 19 F NMR (Fig. S6) shows well integrated four fluorine peaks, where peak at À150.45 ppm represents the terminal vinyl fluorine indicating the addition-elimination reaction product. The 13 C NMR ( 19 F coupled) spectrum (Fig. 4) shows singlets for the phenylene carbons whereas PFCP ring carbon signals undergo short range as well as long range coupling and therefore a complex pattern is observed. For example, the allylic carbon, f, shows ttd with J = 1032.1, 98.5, 27.7 Hz whereas the b carbon, e, shows a broad splitting pattern overlapping with adjacent carbon, g's, signals.
To further establish the structures and understand the solidstate nature of these monomers, X-ray analysis was conducted. Bis-PFCA monomers were purified using column chromatography with hexanes as the eluent. The purified monomers, M1, M4 and M6 were obtained as white crystalline solids, while other monomers were highly viscous clear liquids. Attempts to crystallize M1, M4 and M6 from a multitude of polar and non-polar solvents were unsuccessful. Finally, recrystallization of M1 and M6 from chlorobenzene and a mixture of toluene and bromobenzene, respectively, with a slow evaporation, resulted in the afforded Xray quality crystalline solids. Crystal structures for M1 and M6 are shown in Figures 2 and S22, and selected crystal properties including dihedral bond angles (°) for M1 and M6 are reported in Tables 2 and S1, respectively. Figure 2 shows the ORTEP representation for M1. Crystal structure measurements were carried out at 193 K. The crystal shows the monoclinic, P21/n space group with no sign of thermal atomic displacement. Both PFCP rings show a presence of a double bond (C1-C2 and C18-C19) with a bond length of 1.32 Å. Two phenyl rings stand with a torsion angle (C8-C9-C12-C13) of 30.8°. The PFCP ring (with higher numbered carbon) inclined to the phenyl ring with 90.3°, but the other PFCP ring inclined with 56.6°to the corresponding phenyl ring. Interestingly, the former PFCP ring is almost planar, but PFCP ring (with lowered numbered carbon) shows an envelope conformation with the flap atom, C4. Figure S22 shows the ORTEP representations for molecule A and molecule B, two conformation structures of M6 crystal. Crystal data were recorded at 188 K. The crystal shows triclinic, P-1 space group and a significant amount of thermal displacements, resulting in the asymmetry in molecules. M6 crystal properties are very different from M1 crystal structural properties because of two extra trifluoromethyl groups and a bigger PFCA rings. Both molecules A and B show the C @ @ C bond length of 1.32 Å in PFCH rings as we   These PFCA monomers (M1-M6) were also characterized with ATR-FTIR. Figures 5 and S21 show the IR spectrum for monomers M1-M6. A strong peak at 830 cm À1 represents the typical, out of plane, aromatic C-H bending mode. 39 These spectra show the characteristic bands from 1080 to 1285 cm À1 due to CF 2 and C-O-C asymmetric stretching. 35,40 Peaks at 1499 and 1600 cm À1 (strong and medium intensities respectively) are associated with C@C stretching for the aromatic rings in these spectra. Figures 5 and    Figure 5 shows a small peak at 1747 cm À1 (easily visible for M4 and M6) which belongs to characteristic C@C stretching for a allylic substituted PFCH ring (minor products). 18 Further, the GC-MS and elemental analyses showed the expected molecular weight and elemental composition for all the products M1-M6.
In conclusion, we have developed a new class of PFCA aryl ether monomers from commercially available feedstocks via an addition-elimination reaction, in a good isolated yield. These monomers are a substantial step for the synthesis of a wide range of PFCA homo and copolymers of variable randomness or blockiness.