Rings lactide

Figure 9.2. Lactide ring formation by depolymerization of low-molecular weight PLA. Reprinted with permission from J. Lunt, Polymer Degradation and Stability, Vol. 59, p. 145,1998, 1998, Elsevier Science Ltd.

Industrially this process is catalysed with tin(ii) octanoate and there is a desire to move away from tin and use less-toxic metals or even metal-free conditions. The polymer itself is used in a diverse range of applications from commodity plastics (packaging) to high-end biological uses (tissue engineering, sutures and stents). " In the mechanism initially the ester coordinates to the Lewis-acidic metal centre, the alkoxide then attacks the carbonyl carbon and the lactide ring opens and new alkoxide is formed and... 

L-lactide ring opening polymerization CORI Kinetics model based on Ludovic using axial dispersion and compartment model (series of ideal perfectly mixed and plug flow) Banu et al. 2010... 

A1 (8.103) and Zn (8.104 and 8.105) complexes are efficient initiators for l-LACTIDE ring-opening polymerization in the presence of benzyl alcohol, and polymerization reactions take place in an immortal manner, that is, their polydisper-sity index (PDI) is almost 1 (PDI=1.02,MW= 13,000-14,000) [147].Bis(phosphino) carbazole (PNP-carbazolide) rare-earth metal bis(alkyl) complexes (8.106) initiated... 

The molar ratio between hydroxyl groups and aluminum cations was crucial in promoting a controlled polymerization. Thus, a defect of aluminum cations should lead to more efficient aluminum triaUcoxides, but this was not the case when polymerizations were performed from lactide rings [46]. It was claimed that the probability to form a large amount of triaUcoxide species from the anchored hydroxyl groups was rather low, and most probably a mixture of aluminum mono-. 

Schwach, G., Coudane, J., Engel, R., Vert, M., 1996. Zn lactate as initiator of OL-lactide ring opeiung polymerization and comparison with Sn octoate. Polymer Bulletin 37, 771-776. 

It was also shown that lactide ends would not initiate ring-opening polymerisation of 8-caprolactone in the presence of aluminium triiospropoxide caprol-actone ends 10 would open L-lactide rings [117]. 

Scheme 8.2 Comprehensive kinetic scheme of lactide ring-opening polymerization.

One biopolymer that is already used as a matrix for composite production is poly(lactic acid) (PLA). The PLA is a thermoplastic biopolymer with lactic acid which is derived from starch by a fermentation process as its basic monomer. High molecular weight PLA is polymerized by the lactide ring opening polymerization to PLA (Garlotta, 2001 Gupta et al., 2007 Lim et al., 2008). 

Currently, NatureWorks is largest producer of PLA for domestic applications. NatureWorks employs lactide ring-opening polymerization for the mass production of 140,000 MT per year of PLA, which is branded as Ingeo . NatureWorks PLA is produced mainly for biodegradable packaging, containers, clothing, fibers, etc. Purac is the major producer of PLA for the biomedical and pharmaceutical industries. 

After fermentation, the resulting lactic acid is formed into cyclic lactide through dimerization using reactive distillation. The lactide ring is then polymerized to produce PLA. 

In contrast to the more traditional polycondensation, lactide ring opening polymerization provides a direct and easy access to the corresponding high molecular weight polylactide. Thus lactic acid can be transformed into lactides, by means of a combined process of oligomerization and cyclization. The racemic mixture of L and D lactic acid generate three different types of lactides two optically active forms and a mesoform. 

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