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)-isomer 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 by single-crytal 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 cyclohene 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. Perfectly 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 Mn (9589 D) and with narrow dispersity (Ð) (1.23). The (S)-isomer substitued with fluorine atoms at the phenyl ring was most active for the polymerization reactions. The kinetics studies of ROCOP performed using 2a as catalyst showed first-order dependence on the catalyst, cocatalyst and cyclohexene oxide and and a zero order dependence on phthalic anhydride. The DSC trace revealed that the glass tansition temperature (Tg) increases with increasing molecular weight (Mn) of these polyesters.