A process for preparing non-naturally-occurring defined monomer sequence polymers is provided, and in which a high degree of synthetic control is obtained by the use of solvent resistant diafiltration membranes. Also provided is a process for separating non-naturally-occurring defined monomer sequence polymers from synthetic by-products or excess reagents using solvent resistant diafiltration membranes, and a use of a solvent resistant diafiltration membrane in processes for preparing and separating non-naturally-occurring defined monomer sequence polymers.