Modeling of radical copolymerization near a selectively adsorbing surface: Design of gradient copolymers with long-range correlations

A new approach to synthesis of copolymers with long-range correlations is proposed. Using Monte Carlo simulations and the lattice bond-fluctuation model, we perform the computer-aided sequence design of a two-letter (AB) copolymer with quenched primary structure near a chemically homogeneous impenetrable surface. We simulate an irreversible radical copolymerization of selectively adsorbed A and B monomers with different affinity to the surface, allowing for a strong short-range monomer- (A-) surface attraction. To describe the chain growth analytically, we introduce and investigate a simple theoretical model based on stochastic processes and probabilistic statistics. We find that this model provides a close approximation to the simulation data and explains a number of statistical properties of copolymer sequences. It is shown that, under certain conditions, the chain propagation near the adsorbing surface proceeds as a randomly alternating growth, leading to a copolymer with a specific quasi-gradient primary structure and power-law long-range correlations in distribution of different monomer units along the chain. The gradient extends along the entire chain for any chain length. We find that the statistical properties of the copolymer sequences correspond to those of a one-dimensional fractal object with scale- invariant correlations. Thus, just by radical copolymerization of two monomers with different affinity to a certain plane surface, it is possible to obtain copolymers with a gradient primary structure.