Diacetylenic polymers, thermally

Sundar and Keller have reported the synthesis of linear boron-silicon-diacetylene polymers (31) (Fig. 21) using phenylboron dichloride (PBD) as the source for boron.50 The compositions of the thermally stable boron-silicon-diacetylene copolymers 31a-d are summarized in Figure 21. These polymers were observed to possess exceptional thermooxidative stabilities. 

P. Gruner-Bauer, E. Dormann The dielectric properties of diacetylenes during thermal solid state polymerization. Second International Conference Electrical, Optical and Acoustic Properties of Polymers, Canterbury, UK, (The Plastics and Rubber Instimte, London, UK), 15/1 (1990)... 

Chance, R.R. and Patel, G.N. (1978) Solid-state polymerization of a diacetylene crystal thermal, ultraviolet, and y-ray polymerization of 2,4-hexadiyne-l,6-diol bis-(p-toluene sulfonate). /. Polym. Sci., Polym. Phys. Ed., 16, 859. 

Our research efforts have focused on the synthesis of hybrid inorganic-organic diacetylenic polymers as precursors to materials with high thermal and oxidative Corresponding author... 

The synthesis of block polymers of diacetylene-silarylene and diacetylene-carboranylenesiloxane polymers (99a-e) (Fig. 61) by the polycondensation reaction of 1,4-dilithiobutadiyne with l,4-bis(dimethylchlorosilyl) benzene and/or l,7-bis(tetramethylchlorodisiloxane)-m-carborane have been reported by Sundar and Keller.129 These polymers are a hybrid between the carboranylenesiloxane and silarylene-siloxane polymers and have high char yields (up to 85%) at 1000°C in N2 and in air, reflecting the thermal stabilizing effects of the carborane and aromatic units in the polymeric backbone. 

The polymerization proceeds under photo- [49,50],X-ray [51], and y-ray [52] irradiation in the dark in vacuo, in air, or even in water or organic solvent as the dispersant (nonsolvent) for the crystals, similar to the solid-state polymerization of diacetylene compounds [ 12]. The process of topochemical polymerization of 1,3-diene monomers is also independent of the environment surrounding the crystals. Recently, the thermally induced topochemical polymerization of several monomers with a high decomposition and melting point was confirmed [53]. The polymer yield increases as the reaction temperature increases during the thermal polymerization. IR and NMR spectroscopies certified that the polymers obtained from the thermally induced polymerization in the dark have a stereoregular repeating structure identical to those of the photopolymers produced by UV or y-ray irradiation. 

Diacetylenes of the type shown in Scheme 13 can react under thermal or photochemical conditions to give crystalline polymers [80]. The change from colorless monomer to brightly colored polymer crystals of the same shape can occur... 

In course of subsequent work Bubeck, Tieke, and Wegner discovered that the action spectrum for photopolymerization of undoped diacetylene multilayers extends into the visible provided some polymer formed in course of previous UV-irradiation is present. Since obviously excitation of the polymer can sensitize the reaction this effect has been termed self-sensitization. Checking the absorption spectrum of the polymer produced via self-sensitization assured that the final product is identical with the product obtained under UV excitation of the monomer. Later work by Braunschweig and Bassler demonstrated, that the effect is not confined to multilayer systems but is also present in partially polymerized single crystalline TS-6, albeit with lower efficiency. Interestingly, the action spectrum of self-sensitization follows the action spectrum for excitation of an electron from the valence band of the polymer backbone to the conduction band rather than the excitonic absorption spectrum of the polymer which is the dominant spectral feature in the visible (see Fig. 21). The quantum yield is independent of the electric field, whereas in a onedimensional system the yield of free carriers, determined by thermal dissociation of optically produced, weakly bound geminate electron-hole pairs, is an linear function of an applied electric field 29.30,32,129) Apparently, the sensitizing action does not... 

A direct measure of the optical absorption coefficient a is the optical density OD defined by OD = log Iq/I = 0,434od (Iq and I are the incident and the transmitted light intensities and d is the thickness of the crystal). Weak polymer absorption in the range from 600 to 400 nm is present in the original monomer crystals due to weak thermal polymerization reactions. The absorption of the linear polymer molecules (which are homogeneously distributed within the partially polymerized monomer diacetylene crystals) increase during UV-irradiation at room temperature due to photopolymerization reactions. In contrast to the monomer absorption, the polymer... 

Fig. 4a and b. Optical absorption spectra of a diacetylene crystal (TS-6) at low temperatures, a Spectrum of the low temperature photoproducts obtained after UV-irradiation b Spectrum of the polymer filaments obtained after thermal polymerization at 300 K at low degree of conversion (about 100 ppm)... 

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