Mechanistic aspects of the metal catalyzed alternating copolymerization of epoxides and carbon monoxide.
The cobalt-catalyzed alternating copolymerization of epoxides and CO is a novel, direct approach to aliphatic polyesters, such as poly(hydroxybutyrate) (PHB). This reaction was found to be catalyzed by Ph3Si[Co(CO)4] (4) and pyridine affording in a first step the stable mono-insertion product Ph3Si-O-CH(CH3)-CH2-CO-Co(CO)4 (5). However, a profound mechanistic understanding, especially of the role of pyridine as the key component for the polymerization reaction was missing. ATR-IR online monitoring under catalytic conditions and DFT calculations were used to show that an acylpyridinium cation is formed by cleavage of the cobalt-acyl bond of 5 in the presence of pyridine. The Lewis acid thus generated activates the next incoming epoxide monomer for ring opening through [Co(CO)4]-. The catalytic cycle is completed by a subsequent CO insertion in the new cobalt-alkyl bond. The calculations are used to explore the energetic hypersurface of the polymerization reaction and are complemented by extended experimental investigations that also support the mechanistic hypotheses.
Allmendinger
Markus
M
Department of Materials and Catalysis, University of Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany.
Molnar
Ferenc
F
Zintl
Manuela
M
Luinstra
Gerrit A
GA
Preishuber-Pflügl
Peter
P
Rieger
Bernhard
B