Lactides metal catalysts

Keywords Metal catalysts Poly(glycolide) Poly(lactide) Poly(lactide-co-glycolide) Ring-opening polymerization Stereocontrol... 

Fig. 1 Synthesis of lactide monomer from natural resources, lactide polymerization in the presence of a metal catalysts, and biodegradation of PLA. MOR metal alkoxide...

Recently, dithio-diolate ligands have been employed for construction of group 4 metal catalysts for the ROP of lactide. These metal dithiolate complexes form mononuclear species of the type [(OSSO)M(OR)2] with an octahedrally coordinated metal center. These fluxional compounds acted as highly active catalysts in the ROP of L- and rac-lactide. Hafnium complexes were also introduced as initiators for the ROP of L-lactide and rac-lactide (vide infra) in very limited cases. To our knowledge, the hafnium derivative 146 displayed the highest activity among the group 4 catalysts reported to date (complete conversion of 300 equiv. of... 

The mechanical properties of PLA rely on the stereochemistry of insertion of the lactide monomer into the PLA chain, and the process can be controlled by the catalyst used. Therefore, PLAs with desired microstructures (isotactic, heterotactic, and S3mdiotactic) can be derived from the rac- and W50-Iactide depending on the stereoselectivity of the metal catalysts in the course of the polymerization (Scheme 15) [66]. Fundamentally, two different polymerization mechanisms can be distinguished (1) chain-end control (depending on stereochemistry of the monomer), and (2) enantiomorphic site control (depending on chirality of the catalyst). In reality, stereocontrolled lactide polymerization can be achieved with a catalyst containing sterically encumbered active sites however, both chain-end and site control mechanisms may contribute to the overall stereocontrol [154]. Homonuclear decoupled NMR analysis is considered to be the most conclusive characterization technique to identify the PLA tacticity [155]. Homonuclear... 

Conventional ring-opening polymerization of cyclic anhydrides, carbonates, lactones, and lactides require extremely pure monomers and anhydrous conditions as well as metallic catalysts, which must be completely removed before use, particularly for medical applications. To avoid these difficult restrictions, an enzymatic polymerization may be one of the more feasible methods to obtain the polyesters. This method was first reported by two independent groups (Kobayashi [152] and Gutman [153]) who showed that lipases, enzymes capable of catalyzing the hydrolysis of fatty acid esters, can polymerize various medium-sized lactones. 

Keywords Carbon dioxide and epoxides copolymerization Group 13 metal catalysts Group 3 metal catalysts Lactide polymerization Polycarbonate Polylactide... 

Figure 15 Metal catalysts bearing tripodal nitrogen donor ligands for lactide ROP...

In 2011 Okuda and co-workers reported a series of metal catalysts supported by a tetradentate cyclen-derived ligand for lactide polymerization (Figure 18). Notably, these catalysts were highly active at room temperature for the ROP of meso-LA and polymerized 100 equiv. of monomer in 30 min. However, the reactivity decreased when rac- or L-LA was used. The magnesium variant achieved modest isoselectivity with rac-LA (P =0.64). ... 

Figure 18 Examples of macrocyclic metal catalysts used for lactide polymerization...

Group 4 metals have also been used widely in conjunction with salen-type ligands (Figure 25). In 2006 Gregson et reported several chiral and achiral titanium salen alkoxide complexes for the ROP of lactide. All catalysts reported were modestly active and heteroselective (P 0.51-0.57). Several achiral Ti and Zr salan catalysts were reported by Gendler et for melt polymerization of lactide. While no stereoselectivity has been reported for either system, the Zr complexes were more active towards lactide ROP than the Ti analogs. 

Figure 26 Variations of salen-type ligands used in metal-based lactide ROP catalysts...

As one-pot reactions by the simultaneous initiation of both polymerizations always affect one another, control of the overall process is often very difficult to achieve. Another fairly new strategy, the AROP of lactones [210-213], lactides [214] or benzyl-L-glutamate [215] and the controlled radical polymerization of vinyl monomer, which take place in one-pot but in consecutive fashion, has been introduced by several groups. In this strategy, the AROP of lactones, lactides, or benzyl-L-glutamate can st be initiated by either an enzymatic or a metal catalyst at low temperature. In a second step, ATRP of MMA [210-213], tBMA [212], or 2-hydroxyethyl MA [214] and NMRP of styrene [211, 215] can be activated by increasing the reaction temperature and injecting the ATRP catalyst, respectively (Scheme 11.48). The reaction was conducted in one-pot, without any intermediate work-up and purification. 

Metal hahdes in imidazolium ionic hquids offer unique enviromnents able to facihtate dehydration reactions. Under such conditions certain metal halides are able to catalyze formal hydride transfer reactions that otherwise do not occur in the ionic liquid media. We have now discovered two systems in which this transformation has been observed. The initial system involves the conversion of glucose to fractose followed by dehydration the second system involves the dehydration of glycedraldehyde dimer followed by isomerization to lactide. CrCls" anion is the only catalyst that has been effective for both systems. VCI3" is effective for the glyceraldehyde dimer system but not for glucose. 

Zinc compounds have recently been used as pre-catalysts for the polymerization of lactides and the co-polymerization of epoxides and carbon dioxide (see Sections 2.06.8-2.06.12). The active catalysts in these reactions are not organozinc compounds, but their protonolyzed products. A few well-defined organozinc compounds, however, have been used as co-catalysts and chain-transfer reagents in the transition metal-catalyzed polymerization of olefins. 

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