Pd-Salen and Pd-Salan complexes: Characterization and application in styrene polymerization

Two new Pd2+ complexes based on the Salen-type H2L1 or Salan-type H2L2 ligand (H2L1 = N,N′-Bis(3-methoxy-salicylidene)phenylene-1,2-diamine), H2L2 = 1,2-Bis(2-hydroxyl-3-methoxy-phenyl)methylamino-benzene), were synthesized and characterized by FT-IR, 1H NMR and X-ray crystallography. They were shown to catalyze the polymerization of styrene, giving the syndio-enriched atactic polystyrenes with moderate molecular weights and narrow molecular weight distributions. The detailed stereoregular analysis of the polymeric products shows that the slight increase in electrophilicity or Lewis acidity of active species by the reduction from Salen-type to Salan-type ligand endows the distinctive increase of the stereoselectivities besides the increase of the catalytic activities.