Complexities in the ring-opening polymerization of lactide by chiral salen aluminum initiators

The preparation of (R, R)- and (S, S)-salen Al(OR) complexes, where R = Et, CH2(i)Pr, CH2(t)Bu, and CH2CH(S)MeCl, are reported, along with their reactions with rac-lactide (salen = N, N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino). Rapid, reversible coordination of LA to the salen metal complex is observed, and it is shown that the relative rates of alcohol/alkoxide exchange are comparable to the NMR time scale while the rate of chain transfer involving (R, R)-salenAl(O-R-R) and (S, S)-salenAl(O-S-R) is much faster than the initial rate of ring opening of the LA monomer. For a primary [Al-OR] moiety, the ring opening of rac-LA is much faster than the ring-opening polymerization/enchainment of LA, and in the initial ring-opening event, the diastereoselectivity is dependent on the solvent, the chirality of the salen ligand, and the OR group. Irrespective of the initiator group OR or the solvent, the system moves to a pseudostatic equilibrium concentration of L- and D-LA which is dependent on the nature of the chirality of the salen ligand. Further studies show that the relative rate of trans-esterification is slower than the rate of LA enchainment and that the rate of epimerization is the slowest reaction in the system. Adventitious water leads to loss of catalytic activity and formation of the inert oxo-bridged compound [(salen)Al]2(mu-O) which has been structurally characterized.