Lactides polymerization, reversible

Scheme 1.14 Polymerization of lactide hy reversible activation and deactivation of SIMes.

Scheme 1.15 Polymerization of lactide by reversible activation of triazolylidene carbenes.

Spassky and coworkers discovered a remarkable stereocontrol of an enantiomerically pure A1 complex (7 )-161a for the ROP of rac-lactide resulting in a tapered stereoblock PLA microstructure with high melting point =187 °C) (Fig. 26) [160]. Structurally analogous, racemic salen-Al complex 162 resulted in highly isotactic PLA [161]. Feijen s enantiopure chiral complex (RJ )-163 (Fig. 26) exhibited an excellent reverse stereocontrol by preferential polymerization of L-lactide over D-lactide monomer (Kss/Krr = 14) that resulted in PLA with... 

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... 

It was earlier mentioned that the reversible lactide formation from polycondensated lactic acid was initially explored by Carothers. He furthermore observed that manipulation of the temperature and pressure could be utilized for pushing the equilibrium toward the lactide product. This was utilized later for the preparation of lactide, but the presence of other species (e.g., lactic acid, water, lactoyllactic acid, lactoyl-lactoyllactic acid, and higher oligomers) necessitates further purification of the crude lactide to make it useful for polymerization purposes. 

A block copolymer of poly(A-isopropylacrylamide) (PNI-PAAm) and PLA may combine the thermosensitive property of PNIPAAm and the biodegradable property of PLA. Polymeric micelles from such polymers can improve protein release properties. Temperature change can alter the hydro-philicity and conformation of PNIPAAm, which may affect the physicochemical properties of microspheres of the polymer. Amphiphilic block copolymers of P(NIPAAm-Z -LA) have been prepared by ROP of LA in toluene using Sn(Oct)2 as a catalyst and hydroxyl-terminated PNIPAAm, as depicted in Figure 4.6 [62]. Such copolymers have recently been synthesized by ROP of lactide using the two hydroxyl groups of 5, 5 -bis(2-hydroxyethyl-2 -butyrate)trithiocarbonate (BHBT). PLA-Z -PNIPAAm-Z -PLA was prepared by reversible addition-fragmentation chain transfer (RAFT) polymer-... 

Segmented terpolymers of poly(alkyl methacrylate)-g-poly(D-lactide)/poly(dimethylsiloxane) (PLA/PDMS) were prepared by combination of a grafting through technique (macromonomer method) and controlled/living radical polymerization such as atom transfer radical polymerization (ATRP) or reversible addition-fragmentation chain transfer polymerization. In a single-step approach, the low molecular weight methacrylate monomer (methyl methacrylate... 

Lactide production technologies have been in use since the 1930s, with the related pubhcation by Carothers et al. (1932) about the reversible polymerization of six-membered cyclic esters. After that, lactide technology underwent a period of inactivity because the purity of lactide was insufficient for large-scale production. Lactide technology did well after DuPont developed a purification technique. This ultimately led towards mass-scale production by NatureWorks. This section mainly focuses on the mass-scale lactide production as developed by Cargill—DuPont (currently known as NatureWorks) in the early phases, as well as some related lactide technologies. 

In addition to traditional anionic polymerization techniques for the ROP of cyclic esters, the metal-free anionic ROP of lactide has also been reported (60). Bredereck s reagent [tert-butoxybis (dimethylamino) methane] and related compounds were shown to be latent anionic initiators capable of dissociation to a formatni-dinium cation and an alkoxide anion (Scheme 14.16). These species are then able to propagate PLA formation, resulting in a well-controlled polymerization at 70 °C that produces polymers with predictable molecular weights by monomer conversion and monomer initiator ratio, with narrow polydispersities (<1.18). Both, NMR and mass spectroscopic investigations have demonstrated incorporation of the alcohol initiator at the end of the polymer chain. The authors proposed that, as a consequence of the electrophilic nature of the formamidinium counterion, a reversible capture of the propagating anion (Scheme 14.16) was possible, and may be a hkely source of the excellent control observed. 

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