Thermal degradation of alkyne-containing polystyrenes

The synthesis and thermal behavior of several alkyne-containing polystyrenes was studied. The introduction of the alkynyl group was performed by coupling reaction of poly(p-bromostyrene) with alkynes using palladium/copper catalyst. Some polymers were synthesized by radical polymerization using a,a'-azobisisobutyronitrile as initiator. The thermal properties of the polymers were studied using thermogravimetry and differential scanning calorimetry. In general, these analyses showed that the alkyne-containing polystyrenes were all high char-yielding materials. The polymers were subjected to pyrolysis-gas chromatography/mass spectrometry in the range 500-550 °C for 10-15 s. Pyrolysis of poly(p-trimethylsilylethynylstyrene) gave monomer as main product (ca. 80 wt.%). Significative quantities of the corresponding trimethylsilylethynyl derivatives of benzene, toluene, ethylbenzene, and methylstyrene, were also detected. The random scission, which leads essentially to the formation of monomer, was the major reaction pathway also in the pyrolysis of poly(p-phenylethynylstyrene). In fact, p-phenylethynylstyrene contributed ca. 40% of the degradation products. Poly(p-ethynylstyrene) showed somewhat different degradation behavior from those of other investigated alkyne-containing polymers. The terminal alkyne was so susceptible to cross-linking that the monomer was recovered in very low amount (1-3 wt.%, depending on the pyrolysis conditions). The strong cross-linking tendency of poly(p-ethynylstyrene) was confirmed by the presence, among the degradation products, of p-xylene, p-ethyltoluene, and p-methylstyrene.