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

• Published in: Macromolecules 2016-11, Vol.49 (21), p.8075-8087
• Main Authors: Chen, Jiawei, Gowda, Ravikumar R, He, Jianghua, Zhang, Yuetao, Chen, Eugene Y.-X
• Format: Article
• Language: English
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.6b01654

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 M n 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.