Chains of the copolymers formed by styrene (S) with two different comonomers, vinylmethylether (VME)and methylmethacrylate (MMA), are studied to see how these two comonomers influence the expansion of the coil and the segregation between blocks (the comonomers differ in that homopolymer PS forms miscible blends with PVME and is incompatible with PMMA). Two comonomer sequences are considered: di-block and alternating. Their chains are simulated by molecular dynamics, at two coil densities: the unperturbed random coil state (attained by use of a cut-off for the non-bonded interactions), and a more dense, collapsed coil state (with no cut-off). Properties analysed are: radius of gyration, scattering form factor, separation between block' centres of mass, and pair distribution function between blocks' monomer units. The alternating copolymers (and the corresponding homopolymers) are divided into two parts and treated as 'block' copolymers, for comparison. The di-block copolymer chains are no more expanded than the corresponding homopolymer chains, and no clear distinction between the VME-S and MMA-S pairs can be established. The analysis of 'copolymer' form factors show a slightly larger global segregation of the MMA-S blocks. On the other hand, the alternating copolymer chains of VME-S and MMA-S can be clearly differentiated. Compared to their corresponding homopolymer chains, the VME-S alternating chain is more contracted, and its two blocks are in closer proximity, while the MMA-S alternating chain is more expanded, and its two blocks are more segregated. Thus, a correlation between the compatibility of the homopolymer pair and the degree of segregation of the alternating copolymer chain has been found. (C) 2003 Elsevier Ltd. All rights reserved.