Cationic Palladium(II) Complexes for CO/Vinyl Arene Copolymerization in the Presence of Carbon Dioxide

Abstract

New dicationic and monocationic palladium complexes with 2,2′-bipyridine ligands bearing perfluorinated ponytails have been prepared and used as catalysts for CO/4-tert-butylstyrene (TBS) and CO/styrene (ST) copolymerization in supercritical carbon dioxide and in CO2-expanded liquids. Best catalytic performances were achieved carrying out the copolymerization in carbon dioxide expanded substrates in the presence of 2,2,2-trifluoroethanol. The dicationic systems with 5,5′-substituted ligands [Pd(N–N)2][X]2 generated the most active and productive catalysts with up to 532 g copolymer CP/g Pd·h for CO/TBS and led to polyketones with high molecular weight and narrow polydispersity (molecular weight up to Mw = 692 000 for CO/ST; Mw/Mn = 1.2–3.3). MALDI-TOF mass spectrometry analysis of copolymer end groups revealed that the main initiation and termination steps involved the insertion of the alkene into the Pd–H bond and the β-H elimination on the Pd–alkyl bond, respectively.