SELECTIVE DIMERIZATION BY A RADICAL-CHAIN REACTION IN GAMMA-IRRADIATED CRYSTALS OF IS(3-ACETOXYPROP-1-YNYL)-4-METHOXY-6-METHYLBENZENE

A single dimer is formed by a radical chain reaction upon gamma-irradi ation of is(3-acetoxyprop-1-ynyl)-4-methoxy-6-methylbenzene (1) crysta ls, sealed in a glass ampoule under vacuum at room temperature, The di merization is initiated by the addition of the prop-2-ynyl-type radica l attached at the 1-position of the benzene ring to the beta-carbon of the triple bond attached at the 1-position of the benzene ring. The r esulting vinyl radical undergoes H-abstraction from the prop-2-ynyl me thylene of the 3-acetoxyprop-1-ynyl group attached at the 1-position r esulting in a chain reaction. The dimerization proceeds after the gamm a-irradiation is stopped. The kinetic chain length of the dimerization may be more than 2 x 10(3). Such an exclusive formation of a dimeric product by the gamma-irradiation was observed neither for benzene solu tions of 1 nor for crystals of other disubstituted bis(3-acetoxyprop-1 -ynyl)benzenes. The crystal structure of 1 is responsible for the sele ctive dimerization by the chain reaction. A mechanism for the chain re action is proposed on the basis of the crystal structure.