Coordination-Insertion Polymerization of Lactide

The three-step coordination-insertion mechanism for the ROP of cyclic esters was first formulated in 1971 by Dittrich and Schulz [60]. The first experimental 

The first step of the coordination-insertion mechanism (I) consists of the coordination of the monomer to the Lewis-acidic metal center (Fig. 3.7). The monomer subsequently inserts into one of the aluminum-alkoxide bonds via nucleophilic addition of the alkoxy group on the carbonyl carbon (11) followed by ring opening via acyl-oxygen cleavage (1) hydrolysis of the active metal-alkoxide bond leads to the formation of a hydroxyl end group, while the second chain end is capped with an isopropyl ester, as indicated by NMR characterization of the resulting polymers [48]. 

Calculations have revealed the critical influence of the aluminum substituents. Significantly lower activation barriers are predicted for trialkoxides relative to the corresponding monoalkoxides [66]. The sensitivity of aluminum initiators toward steric bulk has also been noted and is attributed to the shortness of the aluminum-oxygen bonds [67]. Accordingly, the activation barrier for the insertion step is found to be about 40% higher for lactide than for glycolide for the 

Comparatively, the mechanism of Zn(Lact)2-catalyzed ROP has been much less studied. However, the combination of Zn(Lact)2 with a primary alcohol is demonstrated to increase its activity and allows for a better control of the polymerization, as in the case of Sn(Oct)2 [83]. Thus, it seems rather likely that the polymerization proceeds with Zn(Lact)2 in a similar way to that discussed above for Sn(Oct)2. In such coordination-insertion polymerizations the efficiency 

Much interest has flius been devoted to zinc derivatives as potential nontoxic catalysts. Zinc powder itself is a relatively good polymerization catalyst that is used industrially [87, 88]. With reaction times of several days at 140°C in bulk, it is roughly as active as Al(Oi-Pr)3. Numerous zinc salts have also been investigated [89,90]. So far, the best results regarding lactide conversion and 

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