Coordination-insertion mechanism

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. 

Considerable effort has been carried out by different groups in the preparation of amphiphihc block copolymers based on polyfethylene oxide) PEO and an ahphatic polyester. A common approach relies upon the use of preformed co- hydroxy PEO as macroinitiator precursors [51, 70]. Actually, the anionic ROP of ethylene oxide is readily initiated by alcohol molecules activated by potassium hydroxide in catalytic amounts. The equimolar reaction of the PEO hydroxy end group (s) with triethyl aluminum yields a macroinitiator that, according to the coordination-insertion mechanism previously discussed (see Sect. 2.1), is highly active in the eCL and LA polymerization. This strategy allows one to prepare di- or triblock copolymers depending on the functionality of the PEO macroinitiator (Scheme 13a,b). Diblock copolymers have also been successfully prepared by sequential addition of the cyclic ether (EO) and lactone monomers using tetraphenylporphynato aluminum alkoxides or chloride as the initiator [69]. 

The coupling of epoxides and carbon dioxide as catalyzed by metal complexes is generally believed to occur via a coordination-insertion mechanism involving either one or two metal centers. The array of reaction processes that are frequently associated with this transformation are indicated below (Schemes 1 ). 

ROP of p-lactones is highly prone to numerous side reactions, such as transester-fication, chain-transfer or multiple hydrogen transfer reactions (proton or hydride). Specifically, the latter often causes unwanted functionalities such as crotonate and results in loss over molecular weight control. Above all, backbiting decreases chain length, yielding macrocyclic structures. All these undesired influences are dependent on the reaction conditions such as applied initiator or catalyst, temperature, solvent, or concentration. The easiest way to suppress these side reactions is the coordination of the reactive group to a Lewis acid in conjunction with mild conditions [71]. p-BL can be polymerized cationically and enzymatically but, due to the mentioned facts, the coordinative insertion mechanism is the most favorable. Whereas cationic and enzymatic mechanisms share common mechanistic characteristics, the latter method offers not only the possibility to influence... 

Later, Kricheldorf and coworkers extended the concept of the aluminum alkox-ide-initiated ROP of lactones to a set of other metal alkoxides such as tin(lV) [23-25], titanium, and zirconium alkoxides. As a rule, the polymerization takes place according to the same coordination-insertion mechanism shown in Fig. 12. 

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]. 

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]. 

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... 

Abstract. This paper reviews ring-opening polymerization of lactones and lactides with different types of initiators and catalysts as well as their use in the synthesis of macromolecules with advanced architecture. The purpose of this paper is to review the latest developments within the coordination-insertion mechanism, and to describe the mechanisms and typical kinetic features. Cationic and anionic ring-opening polymerizations are mentioned only briefly. 

Keywords. Ring-opening polymerization, Lactones, Lactides, Kinetics, Coordination-insertion mechanism, Macromolecular architecture... 

Depending on the initiator, the polymerization proceeds according to three different major reaction mechanisms [18], viz., cationic, anionic, or coordination-insertion mechanisms [19-21]. In addition, radical, zwitterionic [22], or active hydrogen [18] initiation is possible, although such techniques are not used to any great extent. The focus in this review is on the coordination-insertion mechanism and the other methods are described only briefly. 

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

Investigations of the coordination-insertion mechanism have resulted in two slightly different reaction pathways. Kricheldorf and coworkers have proposed a mechanism [11,57] where the co-initiating alcohol functionality and the mon-... 

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. 

Fig. 2 Coordination- insertion mechanism M metal, OR alkoxide group, L ligand(s)...

T7177>. Ring-opening polymerization can proceed by two mechanisms depending on the initiator used the coordination-insertion mechanism or the anionic mechanism. While most studies focused on the synthesis of poly(/3-hydroxy butyrate), alternative PHAs were synthesized from a-methyl-/3-pentyl-/3-propiolactone... 

The homoleptic iron(II) phenoxy functionalised carbene complex was used in the ROP of e-caprolactone [64]. The polymerisation proceeds via a coordination-insertion mechanism [65-67] and the final product has the functionalised imidazolium unit as end group. Performance was riddled by poor control over number-average molecular weight and decreasing molecular weight with decreasing feedstock levels. This was attributed to inter-and/or intramolecular transesteriflcation processes [65,66],... 

The polymerization undergoes a coordination-insertion mechanism. The initiation step involves nucleophilic attack of the active group, such as a hydride, alkyl, amide or alkox-ide 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 Figure 8.50. 

It was discovered that the addition of 1,3-cyclohexadiene to the Rh -catalyzed reactions increased the rate of butadiene polymerization by a factor of over 20 [20]. Considering the reducing properties of 1,3-cyclohexadiene, this effect could be due to the reduction of Rh to Rh and stabilization of this low oxidation state by the diene ligands. With neat 1,3-cyclohexadiene, Rh is reduced to the metallic state. These emulsion polymerizations are sensitive to the presence of Lewis basic functional groups. A stoichiometric amount of amine (based on Rh) is sufficient to inhibit polymerization completely. It was also discovered that styrene could be polymerized using the Rh catalyst. However, the atactic nature of the polymer, along with the kinetic behavior of the reaction, indicated that a free-radical process, rather than a coordination-insertion mechanism, was operative. 

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... 

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

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. 

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