Catalytic Synthesis and Characterization of Biodegradable ...

Chapter 1
Polylactide and poly(lactic acid) are same chemical product. PLA is used as abbreviation
for both poly(lactic acid) and polylactide. Lactide is the cyclic dimer of lactic acid, which is
produced by means of a combined process of oligomerization and cyclization of lactic acid.
Ring-opening polymerization (ROP) of lactide (Figure 1.3.2) is a preferred route to prepare
PLA because of the higher controllability of the polymerization. 42 Indeed, high
molecular-weight PLA can be obtained by ROP of lactide. 43 Because one lactide molecule
has the two chiral carbon atoms, three stereoisomers of lactide can be generated (L, L-, D, D-,
and meso-lactide) (Figure 1.3.3). The 1:1 mixture of L, L-, and D, D-lactide is
racemic-lactide (rac-LA). Since lactide monomer is chiral, the control of PLA
stereochemistry is easily achieved by the polymerization of L, L-, D, D- and meso-lactide
stereochemical forms. Modulation of the polymer stereochemistry leads to PLA with
dramatically different properties. For example, PLLA is a semicrystalline polymer (Tg at 67
, melting transition at 180 ), while poly(rac-lactide) is an amorphous material (Tg at 58
). The degree of crystallinity of PLA decreases dramatically as the L,L- content decreases
over narrow compositional range from 100 to 92%. The degree of crystallinity of PLA with
85% L,L- content was amorphous. 44
Figure 1.3.2 Synthesis of PLA by the ROP of lactide.
O
O
O
O
L,L-lactide
O
O
O
‐ 12 ‐
O
O
O
O
O
D,D-lactide meso-lactide
Figure 1.3.3 Three stereoisomers of lactide.
The driving force for the ROP of lactide is the ring strain of the monomer. As a
polymerizable six-membered ring, the ring strain of lactide is modest. Therefore, high