The Cobalt-Catalyzed Alternating Copolymerization of Epoxides and Carbon Monoxide:  A Novel Approach to Polyesters

Abstract

The metal-catalyzed synthesis of polyolefins, polyketones, and polycarbonates is well-known in academia and is already successfully applied in industrial processes. Still missing, however, is the metal-catalyzed synthesis of aliphatic polyesters, as one of the most important biodegradeable polymer families. We report here on the cobalt-catalyzed alternating copolymerization of propylene oxide and carbon monoxide, affording atactic and isotactic polyhydroxybutyrates (PHB). The postulated mechanism is supported by online ATR-IR analytics.

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

“…[16] A mechanistic proposal for the occurrence of multiple, alternating PO/CO insertions starts with the cleavage of the cobaltÀacyl bond in 5 by pyridine to give an acylium cation and tetracarbonyl cobaltate (6, Scheme 4). [17][18][19][20] Interaction of the Lewis-acidic acylium cation with PO (7) activates the 4 ] À (6 ! 7).…”

“…[3] Our initial forays into that field surprisingly revealed that besides the ring opening of b-butyrolactone, there is a direct alternating copolymerization reaction of propylene oxide and CO to give PHB. [4][5][6] This direct copolymerization reaction, which combines a Lewis acid (LA) induced epoxide ring opening process with a metal centered CO insertion, was initiated from 2 (Scheme 2), [7] simply by adding pyridine. However, a detailed mechanistic understanding, including the identification of the catalytically important intermediates, was lacking.…”

“…+ [Co(CO) 4 ] À -type compounds. This basic group of carbonylation catalysts can generate b-butyrolactone, a reaction recently intensively investigated by Coates and Alper, [8] as well as polyester.…”

“…[7,9] The intermediate species LA-O-CH(CH 3 )-CH 2 -CO-Co(CO) 4 (2), that 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 -COCo(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 cobaltÀacyl 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 ]…”