Lactide polymerization

Anionically prepared hydroxy-terminated PBd was reacted with AlEt3 to form the corresponding aluminum alkoxide macroinitiator, capable of initiating the polymerization of L-lactide [117]. Using ratios [PBd-OH]/[AlEt3] between 1 and 6, reaction temperatures between 70 and 120 °C and maintaining the conversion of the lactide polymerization below 90%, products with narrow molecular weight distribution were obtained. 

Activated magnesium, 72 835 Activated monomer mechanism, for lactide polymerization, 20 300 Activated sludge, 72 24... 

Kowalski A, Libiszowski J, Majerska K, Duda A, Penczek S (2007) Kinetics and mechanism of E-caprolactone and LJ--lactide polymerization coinitiated with zinc octoate or aluminum acetylacetonate The next proofs for the general alkoxide mechanism and synthetic applications. Polymer 48 3952-3960... 

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...

In most cases, the metal-catalyzed ROP activity is best assessed by determining the polymerization rate constant (kp). A second order rate law is being followed by most of the catalytic lactide polymerizations ... 

Fig. 3 Bis(phenolate)-supported dimeric lithium and potassium L-lactide polymerization initiators [36-43]...

The successful utilization of alkoxo Zn- and Mg-tris(pyrazolyl) borate initiators in the lactide polymerization inspired the synthesis of sterically bulky B-diketiminates (BDls) (Fig. 5) and their zinc and magnesium derivatives [60-62]. Replacing the ancillary ligands resulted in the production of several mono- and dinuclear complexes of Mg" and Zn" 24-39 (Fig. 6), which demonstrated excellent catalytic activity for the polymerization of l- and rac-lactide [62-66]. 

Table 2 Mg and Zn complexes and their activity in lactide polymerization...

Fig. 13 Calcium complexes supported by N,0-donor Schiff base ligands Table 4 Ca complexes in rac-lactide and L-lactide polymerization...

Similarly, a turn-over frequency (TON) of 227 of the polymerization process was distinctly low for 77d with [M]/[I] = 350, at 110 °C for 6 h, using in the melt polymerization conditions. Biocompatible calcium complex 77a used as catalyst at 110 °C produced in 30 min PLAs with high molecular weight (65,000-110,600) and narrow polydispersities (1.02-1.05) using [M]/[I] = 350-700. It is worthy of note that complex 77a displayed a notable heteroselectivity (probability of racemic linkages between monomers, = 0.73, see Sect. 4.2) in polymerization of rac-lactide in THF at 33 °C. Data on the aforementioned calcium initiators and their lactide polymerization are listed in Table 4. 

A series of bis(thiophosphinic amido) yttrium complexes 86a-h was promising due to their high rates and enhanced control of the L-lactide polymerization activity with high polymerization rates A app = 2.2 x lO to 1.1 x 10 s at [LA]q = 1 M, [l]o = 5 mM [104]. It is worth noting that the phosphorous substituents greatly influenced the rate of the reaction, and that the rate followed the order isopropyl > phenyl > ethoxy. 

In the past, several aluminum-alkyl, halide, and alkoxide complexes supported by multidentate ligands were examined for their catalytic lactide polymerization activities. To this end, monomeric aluminum complexes 148a, b (Fig. 21) were synthesized in our laboratory for producing polyesters with thiolate end groups [137]. These complexes initiated polymerizations under reflux condition in toluene and xylene forming PLAs with narrow molecular weight distributions (PDIs 1.15-1.25). 

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... 

Recently, Williams et al. have reported the ring-opening polymerization of carbohydrate lactones [43, 44] and the lactide polymerization coinitiated by carbohydrate esters and pyranoses [45] (Scheme 4). 

Early-on it was discovered that these Salen compounds, and the related six-coordinate cations [6], were useful as catalysts for the polymerization of oxiranes. These applications were anticipated in the efforts of Spassky [7] and in the substantial work of Inoue [8]. Subsequently, applications of these compounds in organic synthesis have been developed [9]. Additional applications include their use in catalytic lactide polymerization [10], lactone oligomerization [11], the phospho-aldol reaction [12], and as an initiator in methyl methacrylate polymerization [13]. 

ROP of lactones and lactides using lanthanide alkoxide-based initiators is a relatively recent discovery. The first example of lactone polymerization by lanthanide alkoxide complexes was reported in a DuPont patent written by McLain and Drysdale in 1991 [89]. In general, the activity of these catalysts is much higher than that determined for aluminum alkoxides, especially in lactide polymerization [90-92]. Polymers of relatively high molecular weight and narrow MWD are formed. The negative side-reactions such as macrocycle formation, transesterification, and racemization are absent. 

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

Redox active ancillary ligands were first used to control lactide polymerizations using titanium complexes [51]. Very recently, Diaconescu and co-workers have reported derivatives of salen complexes incorporating a ferrocene unit which can be oxidized and reduced to switch the polymerization OFF and ON , respectively (Fig. 11) [52, 53],... 

Vert, M, 2000. Lactide polymerization faced with therapeutic application requirements. Macromolecular Symposia 153 333-42. 

The simple homoleptic complex Sn(Oct)2 catalyzes lactide polymerization both in solution and in the melt at temperatures >130°C and is the most widely used catalyst for lactide polymerization industrially. As previously mentioned, commercially available lactide with >90% L-LA polymerized with Sn(Oct)2 will generate -90% isotactic PLA. While this material has commercial applications, its thermal and mechanical properties are not suitable for many applications where polyolefins are typically used. One way to improve the polymer properties is by increasing the isotacticity or forming isotactic stereoblock PLA through stereoselective polymerization of L- and D-LA mixtures. However, this cannot be achieved with Sn(Oct)2 and other simple initiators. ... 

Since the bulk properties of PLA are highly dependent on the stereoregularity or the tacticity of the polymer, i the development of catalysts for rac-lactide (or meso-lactide) polymerization has been focused on achieving stereoselectivity. 2 2 " Numerous organocatalysts such as A-heterocyclic carbenes (NHCs) and phosphine-based compounds " have been investigated for the controlled ROP of lactide. 

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