222 Introduction to Renewable Biomaterials
Among them, lipase-catalyzed ROP of seven-membered lactone (ε-CP) is the most
investigated one due to the importance of its polymer in many applications [35, 38–40].
The best results were observed when immobilized lipase fromCandida antarctica
lipase B (N435) was used as a catalyst. In this case, poly(ε-caprolactone) (poly(ε-CP)) of
molecular weight 25 kDa was obtained in toluene at 70∘C [41]. Thurechtet al. [42] were
able to prepare poly(ε-CP) of molecular weight up to 50 kDa when the polymerization
process was carried out in supercritical carbon dioxide (scCO 2 ). The polymer in this
case has, however, a polydispersity of about 2 due to enzyme-catalyzed transesterifi-
cation reaction.γ-BL showed only low activity toward lipase-catalyzed ROP, as only
polymer with molar mass of 888 Da was obtained after 430 h of polymerization in
n-hexane at a temperature of 60∘CusinglipasefromBurkholderia cepacia(lipase PC)
as a catalyst [43]. The copolymerization ofγ-BL withε-CP was carried out in toluene at
70 ∘C using N435 as catalyst for 4–48 h, which resulted in copolymers ofMn,basedon
theγ-BL/ε-CP feed ratio, in the range between 9600 and 16,100 Da. Many attempts have
been made to enzymatically polymerizeδ-VL to produce poly(δ-valerolactone) using
wide varieties of enzymes as catalyst [40, 44]. The highest molar mass (Mn=3200 Da)
was obtained at 45∘C using isooctane as solvent and lipase CC as catalyst [45].
Interestingly, the kinetic investigation of lipase-catalyzed ROP of unsubstituted lac-
tones of various ring sizes revealed that the reactivity of monomer and the achievable
molecular weight increases by increasing the ring size, which contrast to the trend
observed for the metal–organic catalyzed ROP of lactones due to the remarkably
decrease of ring strain in large lactones [46, 47]. Consequently, increasing attention
has been paid toward lipase-catalyzed ROP of macrolactones that can be derived from
biomass. Of particular interest is the lipase-catalyze ROP of pentadecalactone (PDL),
a naturally produced material with worldwide consumption in the range of 100–1000
metric tons per year, mainly used as fragrance ingredient of many finished consumer
product categories [48]. Poly(pentadecalactone) (PPDL) has been obtained withMw
up to 143 kDa at reaction temperature of 85∘C for 72 h using N435 as catalyst [49].
Tadenet al. [50] reported the synthesis of PPDL with molecular weight of 200 kDa at
45 ∘C in miniemulsion using lipase PC as a catalyst. In fact, PPDL showed mechanical
and crystallization properties similar to those of polyethylene (PE) [51–54]. The
potential applications of this polymer were investigated by preparation fibers from
high-molecular-weight PPDL, which revealed a tensile strength up to 0.74 GPa [49].
Another interesting example is the lipase-catalyzed ROP of unsaturated macrolac-
tones, namely, ambrettolide and globalide, to investigate their potential application as
biomaterials. The resulting polymers revealed high crystallinity, nontoxicity, and were
able to cross-link in the melt to yield fully amorphous materials [55].
Lactide is a cyclic diester of lactic acid, which is produced through bacterial fermenta-
tion of glucose and other biomass sources [56]. The monomer is used mainly to produce
PLA, the second largest produced biopolymer, through Sn(II)-catalyzed ROP. Utiliza-
tion of lipase for the ROP of lactide resulted in PLA with low molecular weights and/or
low reaction yield. While lipase PS was able to catalyze the ROP of lactide at a tempera-
ture of 80–130∘CtogivePLAofMwup to 12,600 Da, the yield of resulting polyester was
significantly low (3–16%) [57]. Under these conditions, DLLA (DL-Lactide) resulted
in higher molecular weight compared to LLA or DLA. When N435, however, was
used as catalyst instead of lipase PS, only DLA could participate in the polymerization
reaction in toluene under mild conditions to give PLA withMnvalue 3300 Da, while
