Coordination insertion, mechanism for

Figure 2 Coordination-insertion mechanism for the polymerization of lactide by a metal complex with an alkoxide initiator...

Figure 10.4 The coordination-insertion mechanism for ring-opening polymerization of e-caprolactone with trialkoxy aluminum. (After Deshayes et al., 2003.)...

Figure 21.2 Coordination-insertion mechanism for the ring-opening pol3rmerisation of lactones. Adapted from [30].

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... 

Figure 3.7 Coordination-insertion mechanism for the Al(Oz-Pr)3-catalyzed ROP of lactide [48]...

This scheme is remarkably close to the coordination insertion mechanism believed to operate in the metal alkoxide-catalyzed ring-opening polymerization of cyclic esters (see Section 2.3.6). It shares many features with the mechanism proposed above for the metal alkoxide-catalyzed direct polyesterification (Scheme 2.18), including the difficulty of defining reaction orders. 

More success has been had with Ir complexes incorporating permelhylcyclopentadiene and NHC ligands. Complexes 18-20 (Fig. 4.7) were evalnated for norbomene polymerisation following activation with MAO [22]. Complex 19 was the most active, giving a TOF of 12 220 h over 10 min, followed by 18 (TOF = 3 220 h" ), while 20 was inactive, indicating that a hemilabile pendant group seems essential. Analysis (NMR) of the polymers formed with 18 and 19 shows that polymerisation proceeds via an addition (coordination-insertion) mechanism. 

Lewis acids were also screened for the ROP of lactones [65]. The polymerization takes place according to a cationic mechanism provided that the counterion is not too nucleophilic. Conversely, when Lewis acids with a nucleophilic counterion are used, several examples are reported where the polymerization takes place according to the usual coordination-insertion mechanism (Fig. 12). This coordination-insertion mechanism was indeed reported for the ROP initiated by ZnCl2 [66], TiCU, and AICI3 [67]. 

While the exact mechanism remains under debate, a coordination-insertion mechanism was generally accepted for the Sn(II) initiators [20]. In addition, SnOct2 has been demonstrated to be an efficient catalyst in the presence of alcohols, especially at elevated temperatures. However, it is also known to undergo inter- and... 

Thus, in hydrogen-transfer reactions, most of the catalysts do prefer the outer-sphere mechanism instead of the MPV or the insertion mechanisms. For instance, the high stability of the intermediate formed, alkoxide in the case of carbonyl hydrogenation, is a major drawback for the inner-sphere mechanism. Nevertheless, in some particular cases, the inner-sphere mechanism may be competitive with the outer-sphere one. In these cases, some requirements must be accomplished, such as the high lability of one of the metal ligands in order to allow easily the substrate coordination or the formation of not very stable intermediates. 

Scheme 4. The proposed reaction pathway for the ROP of a cyclic ester by the coordination-insertion mechanism...

The most effective, and commercially applied, method to produce polylactide is via the ring-opening polymerization of lactide. This process is initiated by metal complexes and proposed to occur via a coordination-insertion mechanism, as illustrated in Fig. 2. The most common initiators for this polymerization are Lewis acidic metal alkoxide or amide complexes. Key initiator criteria are sufficient Lewis acidity to enable binding and activation of the lactide unit and a labile metal alkoxide (or amide) bond so as to enable efficient insertion. 

The generally accepted mechanism for the metal-catalyzed ROP of lactide is a coordination-insertion mechanism (Figure 2), which was proposed by Dittrich and Schulz. " In this mechanism, the lactide is activated after coordination to a metal center through the carbonyl oxygen. Then an initiator, such as an alkoxide, attacks the... 

Other methods have also been used for the synthesis of macromonomers. For example the synthesis of poly(D,L)-lactide macromonomers was reported [225]. It is known that aluminum alkoxides can be used as initiators for the polymerization of lactides via a coordination-insertion mechanism. A functional initiator was prepared by the reaction of triethylaluminum with 2-hydroxyethyl-methacrylate (HEMA) as shown in the following Scheme 71. 

Much of the recent activities in anionic ring-opening polymerization involve the use of anionic coordination initiators. For these initiators, the metal coordinates with the carbonyl (C=0) oxygen of the monomer, which is followed by cleavage of the acyl-oxygen (CO-0) bond of the monomer and insertion into the metal-oxygen (M-O) bond of the initiator. The experimental evidence for this coordination-insertion mechanism comes from end group analysis of the polymer formed. 

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