Lead alkoxides, coordination polymers

This method exclusively yields macrocyclic polyesters without any competition with linear polymers. Furthermore, the coordination-insertion ROP process can take part in a more global construction set, ultimately leading to the development of new polymeric materials with versatile and original properties. Note that other types of efficient coordination initiators, i.e., rare earth and yttrium alkoxides, are more and more studied in the framework of the controlled ROP of lactones and (di)lactones [126-129]. These polymerizations are usually characterized by very fast kinetics so as one can expect to (co)polymerize monomers known for their poor reactivity with more conventional systems. Those initiators should extend the control that chemists have already got over the structure of aliphatic polyesters and should therefore allow us to reach again new molecular architectures. It is also important to insist on the very promising enzyme-catalyzed ROP of (di)lactones which will more likely pave the way to a new kind of macromolecular control [6,130-132]. 

The nucleophilic addition of enol silanes with aldehydes to produce P-silyloxy carbonyl adducts 47 is an example of a group-transfer process (Scheme 2), for applications in polymer synthesis, see [64a, 64b, 64cj. In its simplest mechanistic rendition the reaction proceeds upon coordination of the aldehyde to Lewis acid MX4 to afford an activated electrophilic species 42. Addition of the nucleophilic enol silane 43 to 42 leads to C-C bond formation and generation of the aldol adduct. Various intermediate structures 44,45,46 have been postulated to be formed concomitant with or following C-C bond formation. The generation of intermediates 45 and 46 necessitates subsequent silylation of the P-alkoxide furnishing aldol adduct 47 and regenerating catalyst MX4. 

Methoxyethanol displays numerous coordination modes (Fig. 2) but is basically a bidentate and assembling ligand [Af(0C2H40Me)n]m alkoxides are thus large oligomers (w = 9, Af = Ca m = 10, Af = Y), sometimes infinite polymers [Af = Bi, Pb(II)]. Despite the large values of the nuclearity, however, they are soluble (with the exception of lead) because of cyclic (Af = Y) or compact structures (Af = Ca) or dissociation of the polymer in solution, as observed for Bi [32]. Aryloxides are, with the exception of barium, either monomers or dimers [30,38]. 

ROP is carried out in solution, in the melt, in the bulk or in suspension. The involved mechanism can be ionic (anionic or cationic), coordination-insertion or free-radical polymerization [19].The cationic pol)rmerization is initiated by only two catalysts, trifluoromethane-sulphonic acid and its methyl ester [10, 15]. Initiators such as potassium methoxide, potassium benzoate, zinc stearate, n-, sec-, fer-butyl lithium or 18-crown-6-ether complexes are added for the anionic polymerization to induce a nucleophilic reaction on the carbonyl to lead to an acyl-oxygen link cleavage. According to Jedkinski et al. only the primary alkoxides, such as the first mentioned catalyst, can yield polymers with negligible racemization, transesterification and termination [10]. 

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]. 

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