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

(a) The difference in crystallinity of PGA, PLA and PLGA arises due to their crystal structure. The crystalinity depends on size of side group (bulky side group will reduce crystallinity), the extent of chain branching (brancing will reduce crystallinity) and the complexity of repeat unit ( more complex repeating unit will reduce crystallinity), As out of three polymers PGA, PLA and PLGA, the complexity in the structure increaes from PGA to PLA and to PLGA, thus the crystalinity also reduced in similar manner.

(b) PLGA 50:50 exhibits fastest degradation rate due to preferential degradation of glycolic acid proportion assigned by higher hydrophilicity as the  increase in glycolic acid percentage in the oligomers accelerates the weight loss of polymer.

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