In Situ EPR Spectroscopy of Aromatic Diyne Cyclopolymerization

Abstract

Electron paramagnetic resonance (EPR) spectroscopy was successfully used for the first time to follow the Bergman cyclization of bis-ortho-diynyl arene (BODA) compounds. Five BODA monomers with different spacer (X) and terminal groups (R) were compared. In situ polymerization via EPR spectroscopy yielded first-order rate expressions. Monomers with spacer -O- or -C(CF(3))(2) and terminal group R = Ph exhibited similar kinetic behavior upon thermal polymerization, whereas monomers with pyridine and thiophene terminal groups gave significantly higher rates of polymerization over phenyl-terminated derivatives. A model compound, 1,2-bis(phenylethynyl)benzene, was used to probe the polymerization mechanism, and radical intermediates were found to be stable indefinitely at room temperature.

Polyarylene Networks via Bergman Cyclopolymerization of Bis‐ortho‐diynyl Arenes

“…[37] Tetraphenyl pendant substitution in BODA monomers provides large melt processing windows ranging from 4-5 h at 210°C as found by dynamic mechanical spectroscopy. [24][25][26] Exothermic polymerization of BODA monomers 1-5 in the melt are detected by differential scanning calorimetry (DSC, 10°C min -1 ) at 200-210°C, giving thermal reaction profiles consistent with known phenyl substituted arenediynes (Fig. 2).…”

“…Branched polyarylene networks derived from BODA monomers 1-5 (Scheme 2), however, overcome the processability-performance trade-off issues common to linear polyarylenes. [24][25][26] When heated, BODA monomers 1-5 undergo initial Bergman cyclization to initiate polymerization to form reactive branched oligomers which can be melt-or solution-processed and thermally cured providing an interesting class of polymer networks. As in linear polymerization of enediynes, this polymerization scheme does not require catalysts or initiators and does not produce side products.…”

“…We have previously reported the synthesis and preliminary polymerization studies of easily prepared tetraynes, or bis-ortho-diynylarene (BODA) monomers, as a route to high performance aromatic polymers via readily processable intermediates. [24][25][26][27][28] Our synthetic methodology utilizes thermal intramolecular cyclization of aromatic enediynes (Scheme 1). [29][30][31][32] Enediyne research, in general, has enjoyed rapid progress with primary impetus stemming from their biological activity as antitumor agents.…”

“…material or a different enediyne, chain addition to the starting material to propagate a polymer, coupling with other radical species, or termination by atom abstraction from protic or halogenated sources. [24][25][26] The application of the Bergman cyclization to prepare thermally stable linear poly(phenylenes) and polynaphthalenes by the thermolysis of substituted enediynes and 1,2-dialkynylbenzenes has been reported in recent years, [11,14,35,36] although the most thorough structural investigation of these polymerizations indicate that the products are not simply polynaphthalenes but rather more complex copolymers of naphthalenyl, five-membered fulvenyl structures, and possibly polyacetylenyl repeat units. [37] The polymerization of BODA monomers leads to crosslinked network versions of these polymers (Scheme 2 illustrates the idealized polynaphthalenyl structure for clarity of the branching scheme, although the actual backbones are probably more akin to the poly(naphthalene-co-benzofulvene) structure illustrated in Scheme 1).…”

“…The intrinsically branched architecture resulting from the tetra-functionality of BODA monomers leads to oligomers with excellent melt and solution processability, which can be utilized in molding and coating applications prior to final cure. [24][25][26][27][28] The cured BODA network polymers also serve as excellent precursors to glassy carbonaceous material. Here we detail the polymerization, characterization, processability, solid-state photoluminescence, and thermal stability of selected BODA products derived from commercial bisphenols.…”