Remarkable Lewis acid effects on polymerization of functionalized alkenes by metallocene and lithium ester enolates

Abstract

Drastic effects of Lewis acids E(C 6F 5) 3 (E = Al, B) on polymerization of functionalized alkenes such as methyl methacrylate (MMA) and N, N-dimethyl acrylamide (DMAA) mediated by metallocene and lithium ester enolates, Cp 2Zr[OC(O i Pr) CMe 2] 2 ( 1) and Me 2C C(O i Pr)OLi, are documented as well as elucidated. In the case of metallocene bis(ester enolate) 1, when combined with 2 equiv. of Al(C 6F 5) 3, it effects highly active ion-pairing polymerization of MMA and DMAA; the living nature of this polymerization system allows for the synthesis of well-defined diblock and triblock copolymers of MMA with longer-chain alkyl methacrylates. In sharp contrast, the 1/2B(C 6F 5) 3 combination exhibits low to negligible polymerization activity due to the formation of ineffective adduct Cp 2Zr[OC(O i Pr) CMe 2] +[O C(O i Pr)CMe 2B(C 6F 5) 3] − ( 2). Such a profound Al vs. B Lewis acid effect has also been observed for the lithium ester enolate; while the Me 2C C(O i Pr)OLi/2Al(C 6F 5) 3 system is highly active for MMA polymerization, the seemingly analogous Me 2C C(O i Pr)OLi/2B(C 6F 5) 3 system is inactive. Structure analyses of the resulting lithium enolaluminate and enolborate adducts, Li +[Me 2C C(O i Pr)OAl(C 6F 5) 3] − ( 3) and Li +[Me 2C C(O i Pr)OB(C 6F 5) 3] − ( 4), coupled with polymerization studies, show that the remarkable differences observed for Al vs. B are due to the inability of the lithium enolborate/borane pair to effect the bimolecular, activated-monomer anionic polymerization as does the lithium enolaluminate/alane pair.