Polymerize lactide

The general issues from literature surveys dealing with lanthanide initiators reveal the following (1) catalyst precursors with larger lanthanide metals polymerize lactide faster than metals of smaller radii, (2) lanthanide catalysts polymerize lactide at slower rates than cyclic esters such as s-caprolactone and, in most cases. 

Poly(L-lactide) has been derivatized by such straightforward reactions as esterification, acetylation, and etherification. Often, the optical rotation of a monomeric analog or of some other small, related molecule is similar to that of a polymer after making due allowances for the concentration of the asymmetric unit in a polymer chain. The monomeric lactide was not considered a satisfactory model for the ORD of the polymeric lactide. However, the derivatives mentioned above, did have ORDs that were similar to each other and to the poly(lactide) [91]. 

The less active bases MTBD and DBU were also found to polymerize lactide reasonable fast, but with the advantage that after monomer consumption, hardly any transesterification occurred. The reason for the reduced activity of these bases compared to TBD are their structural differences, while TBD contains two accessible nitrogen atoms as illustrated in Fig. 3.12a, MTBD and DBU are essentially monofunctional. 

It has been found that trifluoromethane sulphonic acid and its methyl ester are the only cationic initiators known to polymerize lactide [15], and the mechanism of this process has been outlined in different papers [2, 3, 15]. 

Alkyl and alcohol adducts of saturated N-heterocyclic carbenes (NHCs) are also used in the ROP of lactide. In these systems, NHC catalysts are generated in situ at elevated temperatures (65-144 °C) to polymerize lactide in the presence of an alcohol initiator. " " For example, alcohol adducts of SIMes act as single-component catalyst/initiators for the ROP of lactide. As shown in Scheme 1.14, they reversibly liberate the alcohol initiator with the carbene... 

Cyclic Diesters. Cyclic diesters were subjected to the lipase-catalyzed ringopening polymerization. Lactide, cyclic dimer of lactic acid, was polymerized by lipase PC in bulk at high temperature (80-130°C) to produce poly(lactic acid) with Mw up to 2.7 x 10 (189,190). Protease (proteinase K) also induced the polymerization of lactide however, the catalytic activity was relatively low. 

Many of the catalyst systems can be used to polymerize lactide, including transition metals such as aluminum, zinc, tin and the... 

In an effort to adjust the interface properties of polylactide (PLA) nanoparticles used for drug delivery, copolymers were synthesized having a polypeptide block in between PEG and PLA. One copolymer was prepared by polymerization of O-protected l serine-NCA with an amino-terminated PEG. Subsequently, the deprotected serine groups were used to polymerize lactide to produce a comb-like copolymer block. This copolymer was compared with a triblock copolymer PEG-h-poly(alanine)-h-PLA. In this case, the endgroup of the poly(alanine) block was used as initiator [54]. Both types of copolymers form nanoparticles, but the copolymers with the comb-like structure show particularly interesting properties. Zeta-potential measurements indicate that the poly (serine) backbone covers the PLA core. 

Similar to PGA, polymers obtained from the synthesis of lactide have also been widely used in the biomedical field. As lactide is a chiral molecule, polymeric lactide exists as three isomers L-lactide, o-Iactide, and meso-lactide. Therefore, four different types of polylactic acid are available poly(L-lactic acid), poly(D-lactic acid), poly(DL-lactic acid), and mesopolylactic acid. 

While the vast majority of Al-based ROP initiators of cyclic esters are neutral derivatives, several reports have recently highlighted that ligand-supported cationic or anionic A1 complexes may mediate the polymerization of cycUc esters such as LA and e-CL. For instance, the A1 cations 2(1-22 (Fig. 5), thought to be of interest for their enhanced Lewis acidity, indeed exhibit an excellent activity in the ROP of e-CL, but are all inactive in the ROP of rac-lactide [19, 103, 104]. In cmitrast, the A1 alkoxide anion 23 (Fig. 5), which incorporates two nucleophilic alkoxide moieties, was found to polymerize lactide in a controlled manner and under milder conditions (room temperature) than those typically required for neutral A1 alkoxide analogues [105]. 

DP 200 polymers being accessed within 6h and displaying predictable molecular weights and narrow polydispersities (<1.08). The ROP of e-caprolactone proceeded much more slowly, with DPs in excess of 75 being achieved after five days, albeit with excellent control over the final polymer. In the application of these more active bases, it was found that DBU and MTBD were able to efficiently polymerize lactide in the absence of a thiourea cocatalyst (see Section 14.2.4). 

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