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A vast number of catalysts have been utilized in the ROP of lactide, of which the most studied are the carboxylates and alkoxides of Sn [117–126] and Al [127–133]. Of these, stannous 2‐ethylhexanoate (tin octanoate) is the most intensively studied. The polymerization mechanism is believed to involve a preinitiation step, in which stannous 2‐ethylhexanoate is converted to a stannous alkoxide by reaction with a hydroxyl‐bearing compound. Then, the polymerization proceeds on the tin–oxygen bond of the alkoxide ligand, whereas the carboxylate itself is inactive in the polymerization [120]. Reviews with emphasis on Sn‐ and Al‐catalyzed ring‐opening polymerization have been published by Stridsberg et al. [134] and Slomkowski et al. [135]. Some kinetic studies were also included in the reviews. However, the highly active catalysts based on, for example, tin compounds are toxic [136], and efficient catalysts showing less toxicity based on Ca [137–143], Fe [95,144–154], Mg [155–158], and Zn [158–165] have, therefore, been developed for lactide and lactone polymerization. Many of these, however, tend to cause racemization of PLA, especially when polymerizing at high temperatures. In addition to the aforementioned metals, Kricheldorf et al. [166] used other salts prepared from cations and anions belonging to the human metabolism in the ROP. Zinc lactate was found to be the most efficient of the tested catalysts with regard to reactivity and obtaining high‐molecular‐weight PLA. More recently, however, a potassium‐based catalyst been reported to be more efficient in the ROP of polylactide to high molecular weight [167]. Other catalyst/initiator systems of low‐toxicity metals for ROP have been discussed in a study by Okada [168].

Catalysts have been developed for the stereoselective ROP of lactides. In the early publications, semicrystalline PLAs were prepared from both meso‐lactide (yielding syndiotactic PLA) and racemic lactide (yielding stereoblock isotactic PLA) using chiral aluminum catalysts containing bulky ligands [131, 132, 169]. After annealing, a T _m of 152°C was obtained for the syndiotactic PLA and the racemic PLA was reported to have a T _m of 191°C. The high T _m for the latter PLA was believed to result from stereocomplex formation of synthesized stereoblock PLA. The work using aluminum catalysts in stereoselective polymerization has continued [133,170–172], and other metal complexes have been utilized as well [158, 164,173–175]. Many of the studies, though, were conducted only in solution; therefore, the selectivity of the catalyst in, for example, melt polymerization remains unclear. A review including discussion on the stereocontrolled ROP of rac‐ and meso‐lactides has been published [176].

Metal‐free catalysis of ROP was reviewed [177, 178]. Both organocatalytic (nucleophilic, cationic, and bifunctional) and enzymatic approaches were discussed.