Cobaltoporphyrin-Catalyzed CO₂/Epoxide Copolymerization: Selectivity Control by Molecular Design

Abstract

A series of cobalt­(III) chloride porphyrin complexes of the general formula 5,10,15,20-tetra­(<i>p</i>-alkoxy)­phenylporphyrin cobalt chloride (<b>4b</b>–<b>e</b>) and the related 5,10,15,20-tetra­(<i>p</i>-nitro)­phenylporphyrin cobalt chloride (<b>4f</b>) are presented and their reactivity toward propylene oxide (PO)/CO₂ coupling/copolymerization is explored. While the nitro-substituted complex (<b>4f</b>), in conjunction with an onium salt, shows moderate activity toward cyclization, the <b>4b</b>–<b>e</b>/onium systems show superior copolymerization activity in comparison to tetraphenylporphyrin Co­(III) chloride (<b>4a</b>) with high selectivity and conversion to poly­(propylene carbonate) (PPC). A comprehensive copolymerization behavior study of the alkoxy-substituted porphyrin complexes <b>4b</b>–<b>e</b> in terms of reaction temperature and CO₂ pressure is presented. Complexes bearing longer alkoxy-substituents demonstrate the highest polymerization activity and molecular weights, however all substituted catalyst systems display a reduced tolerance to increased temperature with respect to PPC formation. Studies of the resulting polymer microstructures show excellent head-to-tail epoxide incorporation and near perfectly alternating poly­(carbonate) character at lower polymerization temperatures