The invention relates to a process of preparing regioregular polymers, in particular head-to-tail (HT) poly-(3-substituted) thiophenes with high regioregularity, to novel polymers prepared by this process, to the use of the novel polymers as semiconductors or charge transport materials in optical, electrooptical or electronic devices including field effect transistors (FETs), electroluminescent, photovoltaic and sensor devices, to FETs and other semiconducting components or materials comprising the novel polymers, to a process of endcapping polymers and to endcapped polymers obtained thereof and their use in the above devices.