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The theoretical description of the Sn(Oct)₂‐catalyzed ROP of cyclic esters has been studied by many authors, but there does not appear to be a theory that consistently explains all experimental results of the coordination–insertion polymerization [3, 4,82–84]. Different polymerization mechanisms may dominate, depending on polymerization conditions, catalyst and initiator concentration, and the presence of a solvent.

Here, it is assumed that lactide is polymerized in bulk with Sn(Oct)₂—a Lewis acid—and that the mechanism follows the model proposed by Kowalski et al. [84]. Since lactide is a cyclic ester, its ring can be opened by nucleophilic attack on the ester bond to start polymerization. Suitable initiators (nucleophiles) are water and alcohols, including the hydroxyl group of lactic acid. One ester linkage of a lactide ring is cleaved by reaction of the OH group of the initiator R—OH, creating a new R—O—C(O)— ester end group and an OH end group (Figure 1.8).

Every initiating molecule is covalently bonded as an end group to each polymer chain [84]. Via transesterification reactions, the 2‐ethylhexanoate ligands of the SnOct₂ catalyst will also end up as octanoic ester groups in the polymer. In some papers, the Sn(II) catalyst is indicated as the initiator, presumably because lactide also polymerizes upon addition of that substance, and the effect of impurities is overlooked. An initiator—or coinitiator—is a substance that can start polymerization, in the case of lactide by opening the lactide ring, and thus offers control over molecular weight. This has to be a nucleophile and cannot be the Sn catalyst itself, as supported by the excellent work of Kowalski et al. who proved that SnOct₂ needs activation with R—OH (Figure 1.9) [84].

FIGURE 1.8 Ring‐opening polymerization of lactide to PLA initiated by an alcohol.

FIGURE 1.9 Equilibrium reaction of tin octoate with alcohol initiator or impurities to form catalytically active tin alkoxide bonds Sn─O─R [76].

In a nutshell, the total hydroxyl content, including R—OH initiator and lactic acid impurities, determines the maximum attainable M _n (number‐average molecular weight) [4, 6]. The rate of polymerization is controlled by factors such as temperature and catalyst content, with the remark that a tin(II) octoate catalyst requires traces of the initiator to become active.