Mono- and Dinuclear Bioxazoline-Palladium Complexes for the Stereocontrolled Synthesis of CO/styrene Polyketones

The coordination chemistry of the chiral bioxazoline ligand (4S,4′S)-2,2′-bis(4-isopropyl-4,5-dihydrooxazole) to PdII provides evidence that the ligand bonding can occur either through chelation of one PdII ion leading to a mononuclear species with the expected cis geometry, or by double bridging of two PdII ions giving a dinuclear complex with trans geometry. The species in solution are identified by 1H NMR spectroscopy. Both the mononuclear and the dinuclear complexes promote the CO/styrene copolymerization, yielding the corresponding polyketone with a fully or a predominantly isotactic microstructure, depending on the reaction medium. The nature of the anion present in the palladium precatalysts affects the polyketone stereochemistry. MALDI-TOF analysis of the copolymers synthesized reveals the presence of p-hydroxyphenolic end-groups, thus confirming and explaining the role of 1,4-hydroquinone as a molecular weight regulator.