New Insights into the Ring-Opening Polymerization of β-Butyrolactone Catalyzed by Chromium(III) Salphen Complexes

The heterogeneous nature of beta-butyrolactone (BL) polymerization towards tactic poly(3-hydroxybutyrate) (PHB) in the presence of chromium(III) salphen (salphen=N,N'-disalicylidene-o-phenylenediamine) complexes is supported by a number of experimental observations. Depending on the substitution pattern, initially soluble chromium(III) salphen chloride complexes can generate microcrystalline agglomerates under the polymerization conditions, driven by formation of m-OH bridges between metal centers. Coordinated water molecules are suggested to be the source of such bridging ligands. The formation of these/this heterogeneous species is a prerequisite for the stereocontrolled ring-opening polymerization (ROP) of BL, whereas both iso- and syndioselective enchainment occurs simultaneously. According to the analysis of the C-13 NMR spectra of the polymers, the ratio of the corresponding triads depends on a number of parameters in a not yet understood manner. Besides dual stereoselectivity, the heterogeneous chromium(III) salphen species feature catalytic sites with different activities, which is reflected in the very broad molecular mass distribution of the produced PHB. Highly active catalytic sites cause the formation of polymer chains with a high molecular mass at the beginning of polymerization. The described behavior is not inherent to truly homogeneous chromium salphen complexes and is more in line with a bimetallic ROP mechanism proposed earlier, which requires a particular mutual spatial orientation of two salphen complexes for the efficient catalysis of BL polymerization.