Environmental Engineering Reference
In-Depth Information
is then filtered, and the acid evaporated, crystallized, and acidified [190]. Further
purification steps are taken for pharmaceutical or food applications.
Direct polycondensation polymerization of lactic acid is possible, though this
typically results in only low-molecular-weight polymers [194]. To produce
polymers with higher molecular weight, an intermediate (lactide) is typically
formed. This cyclic dimer of lactic acid can then be polymerized by ring-opening
polymerization to form high-molecular-weight polymers, as shown in Scheme 5.7
[194]. In order to produce lactide, lactic acid is oligomerized by polycondensation,
CH
3
H
H
H
3
C
OH
HO
HO
Polycondensation
of lactic acid
O
D
- Lactic acid
O
L
- Lactic acid
O
O
O
O
O
HO
n
OH
O
O
Depolymerization
at reduced pressure
PLA M
n
< 5,000 Da
CH
3
H
CH
3
O
O
O
O
O
O
H
CH
3
H
H
3
C
H
H
O
O
O
O
O
O
H
H
3
C
H
3
C
L,L
- Lactide
L,D
- Lactide
D,D
- Lactide
Polymerization
through ROP
O
O
O
O
O
HO
n
OH
O
O
PLA M
n
~10
4
Da
Scheme 5.7
Production of high-molecular-weight PLA polymers by ring-opening polymerization
of lactide. Lactide, in various stereoisomers, is produced by depolymerization of low-molecular-
weight PLA at reduced pressure [194].
