Lactide monomer

Scheme 1. Currently proposed insertion mechanisms in ROP of (di)lactones (schematized here for lactide monomers)...

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...

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... 

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... 

The kinetics of the l-LA polymerization have been investigated in chloroform at 60 °C [81] Fig. 3 shows the semi-logarithmic plot of -ln([M]/[M]0) versus reaction time, t. [M]0 is the initial lactide monomer concentration and [M] the lactide concentration at a given reaction time t. 

The linearity of the plot shows that the propagation was first order with respect to lactide monomer. The absence of an induction period indicates that the initiator was reactive from the beginning and that no rearrangement of initiator aggregates was necessary to form the active species. The nature of the metal, alkoxide groups, solvent, and temperature does not generally influence the first order in monomer [71,104]. 

Due to the presence of two chiral centers, there are three forms of the lactide monomer (Fig. 9). Repeating units with different configurations have been used to produce stereocopolymers where the physical and mechanical properties and the rate of degradation are easily adjusted. 

Fig. 9. Structure of the different stereoforms of the lactide monomer and the resulting repeating unit, the chiral center marked with. a) l-LA, b) D,D-lactide, and c) meso-lactide...

Random copolymerization of MMA with other polar monomers proceeds in a living fashion with relative monomer reactivity ratios in the order BuA > MMA = EtMA > /-PrMA when mediated by 4(Sm Me)/THF [60, 89]. Block polymerization of MMA with other polar monomers as lactone yields ideal living copolymers (PDI = 1.11-1.34) under these conditions. Similarly, ABA triblock copolymers were obtained by sequential addition of MMA, BuA, and MMA [89]. AB block copolymers could be obtained by sequential addition of (L,L)-lactide and (D,D)-lactide (PDI = 1.38) as well as -caprolactone and (l,l)- lactide monomers (PDI = 1.36) in the presence of Y(OCH2CH2NMe2) [82]. 

Biodegradable plastics based on lactic acid have been available on a small scale for many years. They have been used In applications such as medical implants, but their high price was a deterrent to widespread use in lower value applications such as packaging. However, new technologies for production of lactide monomers greatly lowered costs, making the polymers much more competitive. Generally, the lactic acid is obtained from corn or other biobased materials by a fermentation process, and then chemical synthesis is used to produce the polymer from the lactic acid or lactide monomers. 

Adequate amounts of the (macro)initiator, monomer, and thiourea were added to the vial and dissolved in anhydrous dichloromethane (DCM) or amylene stabilized chloroform (TCM). Typically, the mass concentration of the solid reagents, i.e. lactide monomer and macroinitiator, was around 10 %. After stirring the solutions for a couple of minutes the DBU was added with an accurate micropipette. The vials were sealed and the reaction solution was stirred at room temperature. Once the reaction time had elapsed the DBU was neutralized by adding equimolar amounts of benzoic acid. Then the polymer was precipitated into cold methanol, collected by filtration, and dried under vacuum at 80 °C for 24 h. To reduce the risk of poly(lactic acid) degradation in the protic methanol, fast filtration is essential. This was achieved by allowing the precipitate to sediment and removing the excess methanol before filtration. 

Bacterial fermentation is used to produce lactic acid from corn starch or cane sugar which is further processed to produce lactide monomer. Because lactic acid is difficult to polymerize directly to high polymers in a single step on a commercial scale, most companies used a two-step process. Lactic acid is first oligomerized to a linear chain with a MW of less than 3,000 by removing water. 

General Aspects of Lactide Monomer and Lactide 9.2.2.4 Functionalization of PCL via Statistical Copolymerization 175... 

Modification along the PLA chain by copolymerization of lactide with modified lactide monomers or other functional comonomers... 

Figure 2.1 Lactide monomer (left) and polymer (right)...

Scheme 1.10 Mechanism of the cationic ROP of lactide (monomer activation mechanism).

The above thermodynamic analysis of the polycondensation reveals that PLIAs having high molecular weights may be produced when the condensed water is efficiently removed to a level of 1 ppm from the polymerization system without evaporation of the L-lactide monomer present in equilibrium. The ordinary reaction conditions that may allow the effective removal of the water may involve (1) a temperature range of 180-200 °C (2) a low pressure below 5 torr and (3) a long reaction time in the presence of an appropriate catalyst and, in some cases, azeotropic solvent for removing water efficiently. ... 

Figure 4.1 Stereoisomers of lactide monomers (reproduced with Elsevier s permission from Ref. 9).

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