Highly Active Salen-Supported, Cobalt-Based Catalysts For The Synthesis of Regio- and Stereoregular Polycarbonates

Abstract

Synthetic routes to new (salen)CoX (salen = N,N'-bis(salicylidene)-1,2-diaminoalkane; X = halide or carboxylate) epoxide/CO2 copolymerization catalysts are described, and the X-ray crystal structures of (R,R)-(salen-1)CoCl (2.4) and rac-(salen-1)CoI are presented. The (salen)CoX series are highly active catalysts for the alternating copolymerization of propylene oxide (PO) and CO2, yielding poly(propylene carbonate) (PPC) with no detectable propylene carbonate byproduct. The PPC generated using these catalyst systems is highly regioregular with 92 - 99% carbonate linkages and a narrow molecular weight distribution. Inclusion of organic-based, ionic or Lewis basic cocatalysts with (salen)CoX catalysts results in a remarkable activity enhancement for the copolymerization. In the case of (R,R)-(salen-1)CoOBzF5 (2.3) with [PPN][OBzF5] (3.1), an unprecedented catalytic activity exceeding 700 turnovers per hour is achieved for the copolymerization of rac-PO and CO2, yielding iso-enriched, regioregular PPC. When the rac-PO/CO2 copolymerization is carried out with catalyst system 2.3/[PPN]Cl at -20 C, a krel of 9.7 for (S)- over (R)-PO is observed. The stereochemistry of the monomer and catalyst used in the copolymerization has dramatic effects on catalytic activity and the PPC microstructure. Using catalyst (R,R)-(salen-1)CoBr (2.5) with (S)-PO/CO2 generates highly regioregular, isotactic PPC, whereas using (R)-PO/CO2 with the same catalyst gives an almost completely regiorandom copolymer. The rac-PO/CO2 copolymerization catalyzed by rac-(salen-1)CoBr (2.30) yields syndio-enriched PPC, a novel PPC microstructure. The (salen)CoX systems are also successful catalysts for the alternating copolymerization of cyclohexene oxide (CHO) and CO2, yielding syndiotactic poly(cyclohexene carbonate) (PCHC), a previously unreported PCHC microstructure. Variation of the salen ligand and reaction conditions, as well as the inclusion of organic-based, ionic cocatalysts, has dramatic effects on the polymerization rate and the resultant PCHC tacticity. Catalyst rac-(salen-2)CoBr (4.6) has the highest activity for CHO/CO2 copolymerization, yielding syndiotactic PCHCs with 81% r-centered tetrads. Using Bernoullian statistical methods, PCHC tetrad and triad sequences were assigned in the 13C{1H} NMR spectra of these polymers in the carbonyl and methylene regions, respectively.