Metal-alkoxide bonds

The insertion of C02 into a metal alkoxide bond is unique in that it can occur into either the M—O bond or the O—R bond leading in both cases to the same product, (75). 

Metal-catalyzed reactions of C02 and epoxides that give polycarbonates and/or carbonates have been extensively investigated as a potentially effective C02 fixation (Beckman, 1999 Inoue, 1987). The possible reaction mechanism is illustrated in Figure 3.8 (Darensbourg et al., 1999). The repetition of the reaction sequence in which C02 inserts into a metal-alkoxide bond, followed by ring-opening of the epoxide with the metal carbonate forms the alternating copolymer. In 1969, this copolymerization was first reported by Inoue and Tsuruta who used a Zn catalyst derived from... 

The putative mechanism involves coordination and activation of the lactide by the metal complex (1, Fig. 2). The lactide, once activated, is subsequently attacked by the metal alkoxide group (another way to view this is that lactide inserts into the metal alkoxide bond) (2, Fig. 2). The putative intermediate then undergoes ring opening of the lactide, by an acyl bond cleavage, and a new metal alkoxide bond is... 

The effect of the substitution on the phenyl ring can be illustrated by considering two parallel effects (1) the steric obstacle created by both the chloride and the methyl groups, which hinder the approaching of an aldehyde to the metal-alkoxide bond when disposed in the ortho position and (2) the electrostatic interaction between the metal and the chloride, which may facilitate the approach of the aldehyde to the metal center, and hence the activity (Fig. 1). 

This greater strength of the late-metal-thiolate bond compared to the early-inetal-thi-olate bond or the late-metal-alkoxide bond has been rationalized in two ways. First, the late-metal-thiolate bond involves the favorable match of a soft ligand with a soft metal. Second, this bond involves a larger covalent component than the bond of a late metal to an alkoxide or an early metal to a thiolate. ... 

Additives that coordinate to Al perturb the polymerization kinetics in toluene. For instance, THF and cyclohexanone compete e-CL for coordination to aluminum (18), which decreases the propagation rate and may even inhibit polymerization. In contrast, kinetics is faster upon addition of a Lewis base, such as 4-picoline (19), because the coordination of this Lewis base onto the Al atom polarizes the metal-alkoxide bond and facilitates the monomer insertion. Although the reactivity of the active sites is increased, the extent of transesterification reactions is reduced, more likely for steric reasons. 

The first step of the coordination-insertion mechanism (I) consists of the coordination of the monomer to the Lewis-acidic metal center (Fig. 3.7). The monomer subsequently inserts into one of the aluminum-alkoxide bonds via nucleophilic addition of the alkoxy group on the carbonyl carbon (11) followed by ring opening via acyl-oxygen cleavage (1) hydrolysis of the active metal-alkoxide bond leads to the formation of a hydroxyl end group, while the second chain end is capped with an isopropyl ester, as indicated by NMR characterization of the resulting polymers [48]. 

Unfortunately, anionic ROP promoted by metal alkoxides is often accompanied by significant intra- and intermolecular transesterification and termination reactions, resulting in the formation of cyclic oligomers and broadening the molecular weight distribution. The occurrence of side reactions along anionic ROP is ascribed to the high ionicity of the metal alkoxide bond. 

Since the initial reaction step represents a hydrolysis of the metal alkoxide bond, it may be defined as hydrolytic sol-gel synthesis in order to point out the similarity but also distinct difference to Hhefluorolytic sol-gel synthesis that will be reflected in more detail here. As a matter of fact, both the hydrolytic and the nonhydrolytic sol-gel syntheses result in the formation of M-X-M (X O or F) bridges that - when properly performed - results in the formation of nanoscopic particles. 

The faster kinetics is aceounted for the coordination of the Lewis base onto the metal, which polarises the metal alkoxide bond and makes the monomer insertion easier (Fig. 4.12). An excess of triphenylphosphine is however not beneficial to polymerisation. Worse, this excess can compete with the monomer for coordination to aluminium, which is detrimental to the kinetics. 

Both Sn(Oct)2 and Al(Oi-Pr)3 have been extensively studied in terms of activity, polymerization control and mechanism [8, 9]. According to experimental and theoretical data, the polymerization proceeds via a three-step coordination-insertion mechanism (Scheme 10.2). With Sn(Oct)2, the key alkoxide complex is generated in situ upon reaction with the exogenous alcohol. The nature of the ester chain-end is intimately related to the initiating alkoxide, and it is classically determined experimentally by H NMR and/or mass spectrometry, using electrospray ionization (ESI) or matrix-assisted laser desorption ionization time-of-flight (MALDl-ToF) techniques. When all of the monomer has been consumed, the active metal-alkoxide bond is hydrolyzed and a hydroxyl end-group is Uberated. 

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