The radical polymerizations of 2-, 3-, and 4-(phenylethynyl)styrenes (1a-c) and the copolymerizations of 1a-c (M-1) with styrene (M-2) were carried out using AIBN as the initiator in toluene at 60 degrees C. The number-average molecular weights (M(n)s) were extremely low for poly(2-phenylethynylstyrene) (2a) and poly[(phenylethynyl)styrene-co-styrene] (3a), and increased in the order of 2a, 3a << 2b, 3b < 2c, 3c. Monomer reactivity ratios were determined as r(1) = 1.80 and r(2) = 0.51 for 1a, r(1) = 1.72 and r(1) = 0.53 for 1b, and r(1) = 3.17 and r(2) = 0.24 for 1c. Polymers 2a-c and 3a-c underwent an exothermic reaction at elevated temperature to form organic solvent-insoluble polymers. Although the decomposition of 2a was observed from 200 degrees C, 2b and 2c exhibited a high heat-resistance property in both nitrogen and air atmospheres, in particular, 2b showed no significant weight loss below 450 degrees C.