Rate coefficients of propagation, kp,copo, for MA-MMA, MA-DMA, DA-MMA, and DA-DMA bulk copolymerizations have been determined via PLP-SEC. The kp,copo values are analyzed in terms of the terminal and penultimate unit effect models. Alkyl acrylate and alkyl methacrylate reactivity ratios, as obtained from copolymer composition, are not significantly dependent on the size of the alkyl ester moiety, whereas kp,copo clearly depends on the size of the alkyl group. For MA-DMA, an almost perfect simultaneous fit of copolymer composition and kp,copo is obtained via the terminal model. For DA-MMA, even the penultimate unit effect model affords no adequate simultaneous fit. The observed effects are assigned to hindrance of internal rotational motion in the transition state due to intermolecular and intramolecular mobility terms.