Complex sentence: Sergei Yesenin never knew his son by Volpin, but Alexander Esenin-Volpin grew up to become a prominent poet and activist in the Soviet Union's dissident movement of the 1960s with Andrei Sakharov and others.
Split into simpler sentences: Sergei Yesenin never knew his son by Volpin, but Alexander Esenin-Volpin grew up to become a prominent poet. He was also an activist in the Soviet Union's dissident movement of the 1960s with Andrei Sakharov and others.