Novel Zinc and Magnesium Alkyl and Amido Cations for Ring-Opening Polymerization Reactions

The reaction of [H(OEt 2 ) 2 ][B(C 6 F 5 ) 4 ] with Zn[N(SiMe 3 ) 2 ] 2, MgBu 2, and {Mg[N(SiMe 3 ) 2 ] 2 } 2 in diethyl ether proceeds in very high yields to give the salts [(Et 2 O) 3 ZnN(SiMe 3 ) 2 ][B(C 6 F 5 ) 4 ] ( 2 ), [(Et 2 O) 3 Mg n Bu][B(C 6 F 5 ) 4 ] ( 3 ), and [(Et 2 O) 3 MgN(SiMe 3 ) 2 ][B(C 6 F 5 ) 4 ] ( 4 ), respectively. The structures of 2 and 4 were determined by X-ray crystallography; the compounds contain distorted-tetrahedral metal centers and are isostructural. [(Et 2 O) 3 ZnCH 2 CH 3 ][B(C 6 F 5 ) 4 ] ( 1 ) as well as 2 − 4 catalyze the ring-opening polymerization of epoxides and ε-caprolactone. Cyclohexene oxide is polymerized extremely rapidly to high molecular weight materials, while the polymerization of propene oxide gives low molecular weight materials via a cationic mechanism. The zinc species are more stable than their magnesium analogues and show higher productivities. ε-Caprolactone is polymerized efficiently to high molecular weight polymers.