After S.H.I.E.L.D. attempted to replicate Hank's shrinking formula, he resigned and started his own company Pym Technologies, and took in Darren Cross as a protégé, but stopped when Cross became obsessed with learning the secrets of Pym Particles.
He resigned. He started his own company Pym Technologies. He took in Darren Cross as a protégé. He stopped. Cross became obsessed with learning the secrets of Pym Particles. S.H.I.E.L.D. attempted to replicate Hank's shrinking formula.