Stereo-and Enantioselective Alternating Copolymerization of α-Olefins with Carbon Monoxide. Synthesis of Chiral Polymers

Palladium(II) compounds of the type [PdL₂(MeCN)₂] (BF₄)₂ (L₂ = chiral bidentate ligand) have been used as catalysts for the synthesis of chiral alternating α-olefin-carbon monoxide copolymers. For the synthesis of the chiral propylene–carbon monoxide copolymer and the ethylene-propylene–carbon monoxide terpolymer, chiral bidentate phosphines were employed as the ligand. The alternating styrene–carbon monoxide copolymers made thus far have been highly syndiotactic and since isotactic segments in the polymer backbone were required for chirality, a new ligand system was needed. When 2-pyridinecarboxaldehydeimine derivatives were used as the bidentate ligand, the ratio of isotactic to syndiotactic segments in the styrene–carbon monoxide copolymers was found to increase with increasing steric size of the substituent on the imine nitrogen, and when the enantiomerically pure 2-pyridinecarboxaldehyde-N-α-methylbenzylimine was employed, chiral alternating styrene–carbon monoxide copolymers were obtained. Finally, it appears that the optical rotations of the chiral copolymers were primarily due to the presence of stereogenic tertiary carbon centers in the polymer backbone with only minimal contribution from polymer conformation.