Polylactide Synthesis: Ring Opening Polymerization of Lactide

Polylactic acid

Polylactic acid is a widely used family of bioplastics that emerged in commercial utility with over six decades of research. Carothers et al, were the first to document the polymerization and depolymerization of oligomeric polylactic acid in the year 1932. However, until the 1960s, researchers struggled to get high molecular weight polylactic acid, and later found methods to produce highmolecullar weight polymer which was still relatively expensive to make.

With extensive research on polymerization catalysts, and polymerization mechanisms and comonomers, in the early 1990s, Cargill Corporation successfully started to produce lactide and polylactide (polylactic acid) in a continuous process. The  polylactic acid production is now owned by Nature Works ( a subsidiary of Cargill), and produces and sells the resin as well as its products including packaging materials, fibers, films and clothing.

Polylactic acid synthesis

Commercial production of polylactic acid is done using the corn as the main feed stock.  The process involves producing unrefined glucose by milling the corn, followed by fermentation with microorganisms to make lactic acid.

Lactic acid is also a natural product, and is the smallest molecule with an asymmetric carbon.  As shown in following figure, lactic acid exists with two optical isomers – L (+) lactic acid which is the naturally occurring isomer  and D(-) Lactic acid.

L-lactic-acid-molecule-structure

L-lactic-acid-molecule-structure

D-lactic-Acid_Structure

D-lactic-Acid_Structure

Thus, when the lactic acid is converted in to its dimer, lactide, 3 different stereoisomers of lactide is obtained – L- lactide, D- Lactide and Meso Lactide–. This is usually done through reactive distillation process. The two step process involves converting lactic acid to polylactic acid oiligomers, and cyclizing the oligomers to get the cyclic ester of two molecules of lactic acid.

 

Lactide Biomonomer manufacturers:

The world’s major lactide manufacturers include Cargill, Dupont, Boehringer, and Purac Biochem.

Polylactic acid can be synthesized by two different routes – polycondensation of lactic acid, or by ring opening polymerization of lactide–.  The former method is not widespread industrially due to the difficulty in getting the high molecular weight polymer.

Polylactide Synthesis: Ring Opening Polymerization of Lactide

The ring opening polymerization of lactide is the preferred method industrially to produce high molecular weight polylactide in high yield.

Ring opening polymerization (ROP) of lactides yields polylactide acid polymer which exhibit distinctive properties depending on the molecular weight, chain architecture (linear vs. branched), and the degree of crystallinity. Unlike the polycondensation process of lactic acid, the ROP process yields high molecular weight polymer. The branching depends mainly depends on the catalyst and the purity of the monomer.

Polylactide Synthesis: Ring Opening Polymerization of Lactide

Polylactide Synthesis: Ring Opening Polymerization of Lactide

Catalysis of Ring Opening Polymerization (ROP) of Lactide

stannous_Octoate-tin(II)-ethyl-hexanoate-Structure

stannous_Octoate-tin(II)-ethyl-hexanoate-Structure

The ring opening polymerization of lactide can be catalyzed by various compounds including metal alkoxides, metal oxides and enzymes.    One of the most widely used catalysts is stannous octoate.

It’s proven to yield high molecular weight polylactic acid with narrow molecular weight distribution.

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Lactide Monomer : Precursor for the Polylactide Biopolymer

Lactide

Lactide, the dimer of lactic acid, is derived from renewable agricultural resources such as beet sugar, cane sugar, corn starch etc.  The process involves the fermentation of the plant sugars to get lactic acid.  In commercial production of lactide, first, lactic acid is polymerized to get oliomers of polylactic acid.  Then through the controlled depolymerization, oligomers are converted to the cyclic dimer, lactide.

Stereoisomers of Lactide

As lactic acid is a chiral molecule, a racemic mixture of lactic acid could produce two optically active enantiomeric forms L-lactide, D- Lactide, and the meso form D, L-lactide.  The properties of

D-lactic-Acid_Structure

D-lactic-Acid_Structure

L Lactide Structure and Properties

L-lactic-acid-molecule-structure

L-lactic-acid-molecule-structure

D Lactide Structure and Properties

 

(D,L Lactide) (Meso Lactide) Strcture and Properties

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Lactic Acid: Precursor for Polylacticacid

What is Lactic acid:

Lactic acid is the precursor for the synthesis of polylactic acid through the polycondensation route.   L-lactic acid is a natural material that occurs in the metabolism of all animals and microorganisms, and therefore, non toxic to the living beings in smaller concentrations.

L Lactic acid molecular structure

Lactic acid is derived from renewable resource starches including corn starch, tapioca and potatoes.  Process starts with milling the starch to get glucose.   Sugar feed stocks are also can be obtained directly from agricultural resources such as sugar beets or sugar cane.  Then, lactic acid is produced by anaerobic fermentation of dextrose to produce lactic acid.

Chirality of Lactic acid:

Lactic acid is the smallest natural material with a chiral center.  As shown below, it’s stereoisomers  L-lactic acid and D-lactic acid could produce polylactic acid with range of properties.

Polylactide:

Poly(lactic acid) can be synthesized through the polycondensation reaction of lactic acid.  However, in order to drive the reaction forward to obtain high molecular weight polymer, the water produced as a byproduct has to be removed.  The process involves high temperatures and longer reaction times, thus, usually; ring opening polymerization of lactide is the preferred method of producing high molecular weight polylactide (PLA).

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