Mechanistic Aspects of the Metal Catalyzed Alternating Copolymerization of Epoxides and Carbon Monoxide

✍ Scribed by Markus Allmendinger; Ferenc Molnar; Manuela Zintl; Gerrit A. Luinstra; Peter Preishuber-Pflügl; Bernhard Rieger

• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: English
• Weight: 213 KB
• Volume: 11
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/chem.200401077

✦ Synopsis

Abstract

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 Ph~3~Si[Co(CO)~4~] (4) and pyridine affording in a first step the stable mono‐insertion product Ph~3~Si‐O‐CH(CH~3~)‐CH~2~‐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.