What is the fine-grained topic of the following text: Our experimental colleagues, led by Michael Sommer from the University of Freiburg, showed that exclusive α-C-H selectivity is feasible in the DA of π-extended monomers having unsubstituted thiophene or furan units, leading to fully linear materials. Only return the topic.
α-C-H selectivity in the Diels-Alder reaction of π-extended monomers