The positron annihilation lifetime measurements have been performed on a number of amorphous styrene-methyl acrylate copolymers and styrene-butyl methacrylate copolymers. The densities of copolymers were obtained with immersion method by using a capillary pyenometer and the average molecular weights were determined by gel chromatography. The lifetime tau(3) of ortho-positronium (o-Ps) pick-off annihilation have been found to correlate with side group volume and polarity of macromolecular chains in the copolymers, and relative intensity I-3 is attributed mainly to the electron-attracting groups trapping the spur electrons and positrons. The experimental results have been discussed on the basis of the structural variation of macromolecular chains. In addition, the PALS measurement as a function of time for polystyrene and several styrene-methyl acrylate copolymers has also been performed. The result shows that an electric field is built in polymers during extended positron annihilation spectroscopy measurement, and the field effect is a main factor which causes the decrease in I-3 with time.