Coordination-insertion chain lactide

Generalized coordination-insertion chain growth mechanism of lactide to PLA R = growing polymer chain. 

Figure 2.10 Coordination-insertion chain growth reaction scheme of lactide to PLA using tin octoate R, growth of polymer chain (Henton et al., 2005).

Application of metal salts and well-defined metal complexes in ROP has enabled the exploitation of a three-step coordination-insertion mechanism, first formulated in 1971 by Dittrich and Schulz [17]. This proceeds through coordination of lactide by the carbonyl oxygen to the Lewis acidic metal center, leading to the initiation and subsequent propagation by a metal alkoxide species. This species can be either isolated or generated in situ by addition of an alcohol to a suitable metal precursor to result in the formation of a new chain-extended metal alkoxide, as shown in Scheme 3 [16]. 

ROP of lactides is very well suited for end-chain functionalization of PLA. In the anionic initiation pathway, a functional anionic initiator (e.g. a functional metal alkoxide) can be used to start the polymerization reaction. Even better suited is the coordination-insertion pathway using an or-ganometalhc compound and a functional alcohol coinitiator that is easier to obtain than the corresponding anionic initiating agent. The anionic initiator and the alcohol coinitiator. 

The polymerization undergoes coordination-insertion mechanism. The initiation step involves nucleophilic attack of the active group, such as hydride, alkyl, amide, or alkoxide group, on the carbonyl carbon atom of a lactide or lactone to form a new lanthanide alkoxide species via acyl-oxygen cleavage. The continued monomer coordination and insertion into the active metal-alkoxo bond formed completes the propagation step as shown in Scheme 27. The reactions with stereoselectivity proceed by means of a chain-end control mechanism , which is the chirality of the propagating... 

Scheme 10.2 Coordination-insertion mechanism for the metal-catalyzed ROP of lactide (RO refers either to the initiating alkoxy group or to the growing polymer chain).

ROP of lactide in solution at room temperature [11]. AH of the experimental data support a coordination-insertion mechanism, with three active chains growing per metaUic center however, the practical use of these simple alkoxides may also suffer from aggregation phenomena [12]. 

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