Slow positron beam and optical absorption measurements are carried out to study structural defects and positronium formation in 40 keV B+-implanted polymethylmethacrylate (B:PMMA) with ion doses from 6.25 x 10(14) to 5.0 x 1016 ions/cm(2). Detailed depth-selective information on defects in implanted samples was obtained by measuring of Doppler broadening of positron annihilation gamma rays as a function of incident positron energy and these experimental results were compared with SRIM (stopping and range of ions in matter) simulation results. Two general processes, appearance of free radicals at lower ion doses (<10(16) ions/cm(2)) and carbonization at higher ion doses (>10(16) ions/cm(2)), are considered from the Doppler S E and W E dependences in the framework of the concept of defects formation during radiation damage of polymer structure. Probabilities of ortho-positronium (o-Ps) formation are analyzed using S W plot and slow positron annihilation lifetime measurements. Dose dependence of o-Ps lifetime tau(3) and intensity I-3 at the incident positron energy of 2.15 keV correlates well with the dose dependence of S-parameter and seems to account for the existence of the expected two processes, i.e., scission of polymer chains and appearance of free radicals preceding the aggregation of the clusters resulting in the formation of network of conjugated bonds at lower ion doses and carbonization at higher ion doses. The increase of optical absorption observed with increasing ion implantation dose also suggests a formation of carbonaceous phase in the ion-irradiated PMMA.