Poly(phenylene methylene) Synthesis, Characterization, Processing

Abstract

Hydrocarbon polymers belong to the basic structures in polymer architecture. Poly(phenylene methylene) (PPM) represents one of the simplest structured polymer of this category. Before this PhD thesis, however, poly(phenylene methylene) was not available with molar masses sufficiently high to attract interest in materials science, in spite of numerous attempts to synthesize this polymer. Yet, based on the attractive properties of related polymers such as polyethylene, poly(para-phenylene), and pol(para-xylylene), it has been expected that poly(phenylene methylene) possesses interesting properties, such as high hydrophobicity, excellent thermal stability, and good barrier properties. This dissertation demonstrates the successful isolation of poly(phenylene methylene)s with broad range of molar masses by optimization of the catalytic polymerization of benzyl chloride with SnCl4, FeCl3, or organometallic tungsten(II) compounds, followed by fractionation. Low molar mass products were also obtained by quenching the reaction at moderate monomer conversions. Thus, products with number average molar masses (Mn) ranging from 200 g mol−1 – 167,900 g mol−1 were isolated. The glass transition temperature (Tg) of these polymers with different molar masses follows the Flory-Fox equation. The onset of decomposition temperature of higher molar mass products proceeds above 450 °C, according to TGA. Furthermore, the substitution pattern of PPM was discussed by study of chemical shifts of the methylene group by extensive NMR spectroscopy (1H, 13C, DEPT and HSQC) and by comparison with two mono-substituted derivatives of PPM – poly(2,4,6-trimethylphenylene methylene) and poly(2,3,5,6-tetramethylphenylene methylene). The analysis revealed that the signal group of the methylene unit in 13C NMR spectra are sensitive to the substitution pattern of the two adjacent phenylene rings, which is utilized to compare the substitution patterns of different PPM samples. Moreover, it was demonstrated that poly(phenylene methylene) exhibits pronounced blue fluorescence in solutions as well as in the solid state despite its non-p-conjugated nature. Optical spectroscopy was used to explore the characteristics and the physical origin of its unexpected optical properties, namely absorption in the 350 nm – 450 nm and photoluminescence in the 400 nm – 600 nm spectral regions. PPM possesses two discrete optically-active species, and a relatively long photoluminescence lifetime (>8 ns) in the solid-state. Phenomena like p-stacking and aggregation/crystallization, as well as impurities, are excluded as the origins of the optical properties. Instead there is sufficient evidence that PPM supports homoconjugation. Poly(2-methylphenylene methylene) and poly(2,4,6- trimethylphenylene methylene) – two derivatives of PPM – were synthesized and found to exhibit comparable spectroscopic properties, confirming the generality of the findings reported for PPM. Finally, the processability of fluorescent poly(phenylene methylene) has been investigated by employing various processing techniques. Thin fibers over one kilometer in length and short thick fibers (diameter of ~1 mm) were produced by melt spinning. The fibers exhibited birefringence and the ability to guide light waves (red light). Furthermore, a broad thickness range for films (few nm to few mm) was achieved with different approaches such as spin- coating, hot pressing, and die casting. The films featured crack-free and very smooth surfaces. Additionally, freestanding foams of the polymer were obtained by foaming highly concentrated solutions and near quasi-monodisperse microspheres were prepared by a microfluidic high-throughput emulsification. The materials properties of these morphologies were investigated and discussed for implementation as potential products from plastic optical fibers and light emitting diodes, to protective coatings and packaging, to insulators and separation membranes.