Complex sentence: Commonwealth Edison was founded by Samuel Insull, who had been Thomas Edison's secretary and a founder of Edison General Electric, which became General Electric, before he came to Chicago to head Commonwealth Edison, which was a merger of Commonwealth Electric and Chicago Edison.
Split into simpler sentences: Commonwealth Edison was founded by Samuel Insull, who had been Thomas Edison's secretary. He was a founder of Edison General Electric, which became General Electric, before he came to Chicago to head Commonwealth Edison: a merger of Commonwealth Electric and Chicago Edison.