Free radical copolymerization of 4-phenylbut-1-en-3-yne (PB) with methyl methacrylate(MMA) was studied. The polymerization of MMA was inhibited by the presence of small amounts of PB, but the copolymerization yield incresed with increase in the PB concentration, and PB-rich copolymers were obtained. The monomer reactivity ratios, r(MMA) and r(PB), were found to be 0.096 and 2.83, respectively. The Q and e values of PB were calculated by using the values of MMA and were found to be 2.69 and 0.74, respectively. The slow polymerization and low molecular weights were attributed to the low propagating activity of PB radicals.