Coordination addition polymerization and kinetic resolution of methacrylamides by chiral metallocene catalysts

Abstract

This contribution reports the first successful coordination addition polymerization of N,N- dialkylmethacrylamides and the first example of kinetic resolution of a racemic methacrylamide by chiral metallocene catalysts. The polymerization of methacryloyl-2-methylaziridine (MMAz) by rac-(EBI)Zr - (THF)[OC(Oi Pr)dCMe2][MeB(C6F5)3] (1) is stereospecific and also exhibits a high degree of control over polymerization. This polymerization follows first-order kinetics in both concentrations of monomer and catalyst, consistent with a monometallic propagation mechanism involving the fast step of intramolecular conjugate addition within the catalyst monomer coordination complex leading to the eight-membered-ring resting intermediate. Substituents on the highly strained aziridine ring stabilize the aziridine moiety against thermally induced cross-linking through its ring-opening reaction; thus, the polymer derived from methacryloyl-tetramethyleneaziridine (MTMAz) exhibits greatly enhanced resistance toward thermal cross-linking over poly(MMAz), marking 57 and 42 °C higher onset cross-linking and maximum cross-linking temperatures, respectively. Enantiomeric catalyst (S,S)-1 demonstrates experimentally and theoretically its ability to kinetically resolve the racemic MMAz monomer with a low stereoselectivity factor s of 1.8. Polymerizability of several methacrylamide monomers has been investigated via a combined experimental and theoretical (DFT) study that examines the degree of conjugation between the vinyl and carbonyl double bonds, relative polymerization reactivity, and relative energy for the formation of amide enolate intermediates. © 2009 American Chemical Society.