Complex sentence: His choice of career was attributed to or influenced by John W. Draper, under whom he chose to study as the most outstanding chemist of the time; this man also, later, became a founder of the American Chemical Society.
Split into simpler sentences: His choice of career was attributed to, or influenced by John W. Draper, under whom he chose to study. Draper was considered an outstanding chemist who later became a founder of the American Chemical Society.