Chinese chemical companies are making more ingredients for illegal fentanyl than ever.Strained relations between Beijing and Washington are undermining efforts to stop the flow.Among the available products are compounds with obscure names such as N-Phenyl-4-piperidinamine, which Mexican cartels purchase to make into fentanyl.The opioid has become the most deadly illegal drug the U.S. has ever seen.Continue reading your article witha WSJ membershipa WSJ membershipAlready a member?"