Green Route to Prepare Renewable Polyesters from Monomers: Enzymatic Polymerization 221
γ-Butyrolactone
O O O O OO
OO
OO O O
O
O
γ-Valerolactone
δ-Valerolactone ε-Caprolactone
Pentadecalactone
Ambrettolide
Globalide
Figure 7.1The chemical structure of some lactones derived from renewable resources.
(γ-BL), for instance, can be produced from succinic acid, which is in turn produced
from glucose via fermentation [21–23]. Furthermore,γ-valerolactone (γ-VL) can be
prepared directly from levulinic acid (LA), which is another platform of chemicals pre-
pared from chemical transformation of cellulose and hemicelluloses, by hydrogenation
process [24].δ-Valerolactone (DVL) can be produced in two synthesis steps starting
from furfural [25]. Interestingly, it has been recently reported thatε-caprolactone
(ε-CP) can also be obtained using bio-based synthetic route in four steps starting from
hydroxymethylfurfural (HMF) [26].
Utilization of lipase as catalyst for ring-opening polymerization (ROP) of lactones
was discovered by two independent research groups, that is, Kobayashi and Knani,
in 1993 [27, 28]. Since then, enzyme-catalyzed ROP of lactones have been extensively
investigated to find the effect of different reaction conditions, for example, enzyme
origin [29–31], water content [32, 33], solvent [34, 35], temperature [36, 37], and
concentration [34], on polymerization kinetic and the final product,
The overall synthesis is shown in Figure 7.2.
It is well accepted that the lipase-catalyzed ROP of lactones proceeds via several
steps including first activation of the carbonyl ester by formation of an acyl-enzyme
intermediate between the lactone and the OH (hydroxyl groups) of serine residue in the
active center of the enzyme. This is followed by the attack of the initiator, for example,
water or alcohol, to deacylate the acyl-enzyme intermediate and form the correspond-
ingω-hydroxycarboxylic acid/ester. The later step is known as the initiation step where
the type of initiator will determine the end groups of resulting polyester chains. In the
propagation step, the deacylation of acyl-enzyme intermediate is carried out usually by
the terminal hydroxyl group of the growing chains to increase the length of resulting
chain by one monomer unit. The polymerization and copolymerization of substituted
and unsubstituted lactones with different ring sizes have been widely investigated [16].
Figure 7.2Lipase-catalyzed ROP of lactones.
Lipase
O
O
(CH)n
(CH)nm
R
R
O
O
R: H, alkyl...
n: 2, 3, 4, ..., 15
