Efficient and controlled ring-opening copolymerization of cyclohexene oxide with cyclic anhydrides catalyzed by proline-based chiral alkyl Al(III) compounds.

Abstract

A set of new chiral monomeric (S)- and (R)-isomers of Al(III) compounds were synthesized using a proline-based ligand moiety. The reaction of AlMe₃ with one equivalent of pro-ligands (1a-d) in dry toluene yielded the corresponding dimethyl Al(III) compounds (2a-d) in high yields. The molecular structures of compounds 2a, 2c and 2d were confirmed using single-crystal X-ray diffraction analysis. These compounds exhibited a distorted tetrahedral geometry at the Al(III) center. These compounds were tested as catalysts for the ring-opening copolymerization (ROCOP) of cyclohexane oxide (CHO) with cyclic anhydrides to produce copolyesters with tetraphenyphosphonium chloride (TPPCl) as a cocatalyst. The (S)-isomer was found to be more reactive than the (R)-isomer towards the ROCOP. Alternating polyesters were obtained from the ROCOP of cyclohexene oxide (CHO) with phthalic anhydride (PA) and CHO with succinic anhydride (SA), producing copolyesters with moderate to high M_n (9589 g mol^-1) and with narrow dispersity (Đ) (1.23). The (S)-isomer substituted with fluorine atoms at the phenyl ring was most active for the polymerization reactions. The kinetics studies of ROCOP performed using 2a (S-isomer) as catalyst showed higher rate than 2d (R-isomer). 2a produced an isotactic-enriched semicrystalline polyester, whereas 2d formed an atactic amorphous polyester. The DSC traces revealed that the glass transition temperature (T_g) increases with increasing molecular weight (M_n) of these polyesters.