Macromolecular engineering of polylactones and polylactides. 5. Synthesis and characterization of diblock copolymers based on poly-ε-caprolactone and poly(L,L or D,L)lactide by aluminum alkoxides

Aluminum isopropoxide is known to be an effective initiator for the ring-opening polymerization of lactides (D,L and L,L) and epsilon-caprolactone (epsilon-CL). For a question of mutual reactivity, the sequential polymerization of these two comonomers can only be achieved when epsilon-CL is first polymerized followed by the lactide. Formation of a large amount of homo PLA is however observed and has been attributed to a great difference in the mean degree of association of aluminum alkoxides in toluene in the presence of epsilon-CL and LA. In toluene, the mean number (n) of active sites per Al(O(i)Pr)3 molecule jumps from 1 to 3 when epsilon-CL is substituted by LA. The addition of a small amount of an alcohol, like 2-propanol, is effective in preventing the aluminum alkoxide from associating, and n is then 3 whatever the monomer used. 2-Propanol participates in the polymerization of epsilon-CL and LA as supported by a decrease in the molecular weight of the final polymer in relation to the molar amount of added alcohol. This means that the alcohol molecules are rapidly exchanged with the alkoxide groups on Al. Under such conditions, the formation of pure diblock copolymers P(epsilon-CL-b-LA) is reported. Another successful strategy for the synthesis of the diblock copolymer consists in using an Al derivative that bears only one alkoxide group to initiate the polymerization of the two cyclic esters. In this regard, a diethylaluminum alkoxide is of great value, particularly the ethoxide one. A third, conceptually similar, approach to the synthesis of P(epsilon-CL-b-LA) copolymers relates to the reaction of a performed hydroxy-terminated PCL with triethylaluminum and the use of that macroinitiator in the ring-opening polymerization of the lactide.