Depolymerization of PLLA

Table 8.7 shows the fractions of LLA, DLA, and MLA formed by the thermal degradation (i.e. depolymerization) of PLLA at different temperatures and reaction times [272]. The yields of LA and LLA increase with an increase in degradation time within a degradation temperature range of 250-290 °C and become saturated at 12 and 6%, respectively, when degradation proceeds for over 10 hours. The initial LA formation rate and the saturation... 

As a consequence of a decrease in the physical dimensions of the MgO particles, thermogravimetric profiles of PLLA/MgO composites can be shifted to lower temperatures due to an increase in the catalytic surface area [59]. However, decreasing the dimensions can cause other side reactions with unfavorable products (e.g., cyclic oligomers and m o-lactide) owing to the presence of different chemical structures/species on the MgO surface. Heat treatment of the MgO particles effectively suppressed oligomer production and enhanced the formation of L,L-lactide, indicating that the surface chemical properties of MgO also considerably influence the depolymerization of PLLA. 

Fan et al. [38,54] found that Ca ion-catalyzed depolymerization of PLLA with PLLA-Ca caused considerable racemization at temperatures lower than 250°C, forming a large amount of meso- diCi dQ as a by-product. They proposed a novel racemization mechanism based on a Sn2 reaction at an... 

The depolymerization of PLLA at a high temperature induces the chain-transfer intra- and inter-transesterification and depolymeiization reactions by the evident change of the specific optical rotation number. In other words, the high capacity of a transition metal is able to coordinate ester groups and accelerate reactions. 

Polylactide (PLA) is prepared by the ring-opening polymerization of lactides, such as L,L-lactide, D,D-lactide, and meso-lactide, that are cyclic dimers of lactic acid, with poly (L-lactide) (PLLA), an especially well-known crystalline polymer, being prepared from L,L-lactide [1-3]. This ringopening polymerization is an equilibrium reaction in which the concentration of cyclic monomer is temperature dependent [4]. Therefore, the lactides are regenerated through the thermal depolymerization of PLA. 

The thermal degradation of PLA has been claimed to mainly occur via random scission based on a linear relationship between inverse of the number-average degree of polymerization P and time as shown in Equation 23.2 [28]. Recently, Aoyagi et al. [9] and Abe et al. [29] suggested that the isothermal degradation of PLLA at 220, 290, and 330°C proceeded not only via simple random scission, but also via an unzipping depolymerization of the polymer chain based on the nonlinear relationships of l/P and P with time. 

To clarify the effects of chain-end structures of PLA, Lee et al. [34] synthesized C1-, NH2-, and COOH-terminated PLLAs from OH-terminated PLLAs. The thermal stability of OH-terminated PLLAs was poor, whereas NH2- and Cl-terminated PLLAs were more resistant to thermal degradation. The main mechanisms of PLLA thermal degradation are transesterification and backbiting reactions that cause random degradation and unzipping depolymerization, respectively, starting from the carboxyl and/or hydroxyl chain ends [7, 8, 10, 29, 49]. 

MgO even in the lower temperature range. This characteristic antiracemization effect of MgO is due to the lower basicity of Mg compared to Ca. At temperatures lower than 270°C, the pyrolysis of PLLA/MgO (5 wt%) composite occurred causing unzipping depolymerization, resulting in selective L,L-lactide production. 

To achieve selective depolymerization of the PLLA component, some polymer blends of PLLA with linear low-density polyethylene (LLDPE) [74, 75], PS [76], PBS [77], and PBSA [77] were prepared and thermally degraded with... 

Moreover, the molecular weight remained around 100 000 Da, being much lower than that of the PLLA obtained by the ring-opening polymerization of Z-lactide. Therefore, they examined the melt/solid polycondensation of lactic acid in which the melt polycondensation of Z,-lactic acid was subjected to solid-state polycondensation below Tm of PLLA [8]. In solid state, the polymerization reaction can be favored over the depolymerization or other side reactions. Particularly, in the process of crystallization of the resultant polymer, both monomer and catalyst can be segregated and concentrated in the noncrystalline part to allow the polymer formation to reach 100% [9]. Figure 3.2 shows the whole process of this melt/solid polycondensation of Z-lactic acid. In this process, a polycondensation with a molecular weight of 20 000 Da is first prepared by... 

Direct polycondensation of PLA involves first a dehydration equilibrium for esterification and then a ring-chain equilibrium where depolymerization of poly-L-lactide (PLLA) into L-lactide (L-LA)... 

Fan et al. [32,47] found that Ca-ion-catalyzed depol5mierization of PLLA with PLLA-Ca caused considerable racemization at temperatures lower than 250 C, forming a large amount of meso-lactide as a by-product They proposed a novel racemization mechanism based on a 5 2 reaction at an asymmetrical methine carbon, which occurred as a back-biting reaction from an active chain end structure R-COO" Ca of PLLA (Scheme 9.4) [8, 47]. At temperatures over 320 C, side reactions such as the ester-semiacetal tautomerization, caused the formation of meso-lactide, but not dominantly. At 250-320°C, L,L-lactide is produced exclusively, because unzipping depolymerization proceeds as the main reaction. 

Polylactides, 18 Poly lactones, 18, 43 Poly(L-lactic acid) (PLLA), 22, 41, 42 preparation of, 99-100 Polymer age, 1 Polymer architecture, 6-9 Polymer chains, nonmesogenic units in, 52 Polymer Chemistry (Stevens), 5 Polymeric chiral catalysts, 473-474 Polymeric materials, history of, 1-2 Polymeric MDI (PMDI), 201, 210, 238 Polymerizations. See also Copolymerization Depolymerization Polyesterification Polymers Prepolymerization Repolymerization Ring-opening polymerization Solid-state polymerization Solution polymerization Solvent-free polymerization Step-grown polymerization processes Vapor-phase deposition polymerization acid chloride, 155-157 ADMET, 4, 10, 431-461 anionic, 149, 174, 177-178 batch, 167 bulk, 166, 331 chain-growth, 4 continuous, 167, 548 coupling, 467 Friedel-Crafts, 332-334 Hoechst, 548 hydrolytic, 150-153 influence of water content on, 151-152, 154... 

Some reports about the effect of the Sn-based catalyst on PLLA thermal degradation have been published. Noda [40] evaluated the activity of mono-, di-, tetraalkyl and aryl tin(IV) compounds as intramolecular transesterification catalysts and found that monobutyltin trichloride was the most active catalyst with some other compounds indicating almost the same depolymerization activity as Sn(Oct)2. Degee et al. [41] also reported that PLLA prepared with Sn(Oct)2 was less stable thermally than that prepared with Al alk-oxides based on TGA data. However, Cam and Mamed [33] found a contrary effect on thermal decomposition onset temperature in the order of Al > Zn > Sn. 

As mentioned above, Al(OH)3 and MgO are effective degradation catalysts that selectively depolymerized PLLA into L,L-lactide. For MgO in particular, only a small amount (<5 wt%) is required for the depolymerization [55], although the basicity of MgO is poor when compared to other alkaline earth oxides, as shown in the following order BaO > SrO > CaO>MgO [56]. 

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