Copolymerization propagation rate coefficients, k(p,copo), have been measured for the binary systems methyl acrylate (MA)-dodecyl methaerylate (DMA), butyl acrylate (BA)-methyl methaerylate (MMA), dodecyl acrylate (DA)-DMA, and DA-MMA at 40 degreesC and 1000 bar by the pulsed laser polymerization (PLP)-size-exclusion chromatography (SEC) technique. These acrylate-methaerylate systems are interesting because of the significant difference, by more than 1 order of magnitude, between the homopropagation rate coefficients of the two families. Reactivity ratios, r(i), are determined from monomer feed compositions and the NMR spectroscopically measured copolymer compositions. The resulting r(i) values for the four acrylate-methacrylate copolymerizations agree within experimental accuracy. Moreover, these r(i) data are surprisingly close to reactivity ratio data estimated from individual addition rate coefficients to MA and MMA, respectively, of appropriate small (meth)acrylate-type free radicals. Such addition rate coefficients have been determined via EPR in liquid solution by the Hanns Fischer group. The terminal model allows for excellent individual fits of composition and of k(p,copo) for each of the four systems. The implicit penultimate unit effect (IPUE) model (and the explicit penultimate unit effect (EPUE) model) are capable of simultaneously fitting composition and rate data for the MMA-BA and DMA-MA systems whereas both models fail to provide a satisfactory representation of the two DA-containing systems. The data suggest that, with DA being one of the comonomers, individual propagation rate coefficients are not adequately described by consideration of only terminal and penultimate units at the free-radical terminus. On the other hand, ratioing individual propagation rate coefficients of free radicals with the same penultimate units seems to eliminate most of the impact of the penultimate units. For this reason the resulting and widely used "terminal model" reactivity ratios are reasonable and meaningful kinetic quantities although penultimate effects on the individual propagation rate coefficients undoubtedly operate.