Controlled or High-Speed Group Transfer Polymerization by Silyl Ketene Acetals without Catalyst

Group
transfer polymerization (GTP) is an important ambient-temperature
living polymerization method using silyl ketene acetal (SKA) or related
initiators. Although several different GTP systems have been developed
for polymerizing acrylic monomers, they all require the use of a catalyst
to activate the SKA initiator, commonly believed to be ineffective
on its own. Now, this work shows that, in fact, the neutral SKA alone
mediates either controlled or extremely rapid polymerization of acrylic
monomers such as methyl methacrylate (MMA) in polar donor solvents
such as DMF, depending on the nuclearity of the SKA and the chelating
pendant group on Si. In the case of a mono-SKA such as Me2CC­(OMe)­OSiMe3, the GTP of MMA in DMF is relatively
slow (several hours to completion) but is controlled and remarkably
efficient, producing PMMA with Mn values
close to those predicted on the basis of the [M]/[I] ratio, low Đ values (≤1.2), and high
initiation efficiencies (≥80%). In sharp contrast, the di-SKAs
linked by an oxo, ferrocenyl, or binaphthyl bridge, as well as the
mono-SKA with a donor chelating methoxy pendant group on Si, mediate
extremely rapid polymerization (a few seconds to completion), affording
an extremely high turnover frequency up to 1.92 × 105 h–1, but the polymerization is uncontrolled. Several
lines of evidence obtained through mechanistic studies indicate that
the polymerization by the mono-SKA and di-SKA in DMF proceeds through
a dissociative pathway with the released enolate anion being the highly
active species and the polymerization characteristics are highly dependent
on the amount of free enolate anions in solution. In this mechanism,
the donor ability of the solvent plays a critical role in promoting
the activity through activation of the Si site of the neutral SKA
by forming the pentacoordinate Si intermediate.