2. PLGA (chemical structure below) is a random block copolymer of PLA and PGA. P

Question

2. PLGA (chemical structure below) is a random block copolymer of PLA and PGA. Please explain (1) why PGA, PLA, or PLGA have different crystallinity (ability to crystallize). For example, PGA has a high crystallinity around 45-55%, PLA has a crystallinity around 37%, and all PLGAs are amorphous (3 points); and (2) Why 50:50 (monomer ratio of PLA to PGA) PLGA is the fastest degrading (2 points)? 6 CH 0 0 PLA 100 x - Number of units of Lactic Acid y - Number of units of Glycolic Acid Copolymer ratio

Explanation / Answer

Answer : 1)

PLGA, PLA, and PGA have different crystallinity because of a (different) high melting point and variable solubility in organic solvents that are generally low and depends on polymer molecular weight.

PGA is a highly crystalline material which has a high melting point (225-230)degree and variable solubility.

PLA is a chiral molecule and exists in two distinct optically active forms – L-lactide and D-lactide. When each of these monomers is polymerized, the resulting polymer is semi-crystalline.  Polymerization of a racemic mixture of L- and D-lactides forms poly-D, L-lactic acid (PDLLA), which is amorphous and has a glass transition temperature of 55-60°C. The degree of crystallinity can be tuned by altering the ratio of D to L enantiomers within the polymer.

The ratio of glycolide to lactic acid at different compositions allows control of the degree of crystallinity of the polymers.4 When the crystalline PGA is copolymerized with PLA, the degree of crystallinity is reduced.

2)

The higher the lactic acid the slower the degradation. This is partially due to the increased hydrophobicity of the methyl group on the lactic acid monomer which prevents what from penetrating into the polymer and subsequently hydrolyzing the ester bond.

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