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

• Published in: Chemistry : a European journal Vol. 11; no. 18; pp. 5327 - 5332
• Main Authors: Allmendinger, Markus, Molnar, Ferenc, Zintl, Manuela, Luinstra, Gerrit A., Preishuber-Pflügl, Peter, Rieger, Bernhard
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 05.09.2005; WILEY‐VCH Verlag
• Subjects: carbonylation; density functional calculations; epoxides; polymerization
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200401077

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. A powerful analytical tool to gain insight into reaction mechanisms is online ATR‐IR spectroscopy. Until now a profound mechanistic understanding of the cobalt catalyzed alternating copolymerization of epoxides and CO was missing. Online ATR‐IR investigations (see figure), model compound chemistry and DFT calculations allowed us to ascertain the reaction mechanism, especially the role of pyridine as co‐catalyst in the catalytic cycle.