4.1 Ring-Opening Polymerization in Water
and in Miniemulsion
ROP of lactones to various polyesters has been widely studied [19, 68]. Lipasecatalyzed ROP is normally carried out in bulk or in an organic solvent like toluene,
1,4-dioxane, or dibutyl ether [17, 19, 20].
Water was used as solvent for the first time in the lipase-catalyzed ROP of five
lactone monomers, ε-CL, OL, UDL, DDL, and PDL (Scheme 5) [69, 70].
Macrolides of UDL, DDL, and PDL are less reactive than lactones of smaller
ring size due to lower ring strain when using a usual chemical catalyst [71]. However, they showed higher reactivity in enzyme catalysis and were polymerized by
lipase in water to produce the corresponding polyesters; typically, UDL gave
polyUDL with M n 1,300 (M w /M n ¼ 2.1) in 79% yields at 60
C for 72 h. DDL is
hardly soluble in water; however, addition of the lipase gave a white emulsion-like
solution, which allowed the ROP. In contrast, a mixture of the lipase and ε-CL or
OL did not form an emulsion-like solution, and thus failed to induce the ROP.
Therefore, it seems that the enzyme protein behaved like a surfactant [69–71].
A second example of the use of water as medium is the lipase-catalyzed ROP of
a lactone in miniemulsions [72]. Typically, a mixture of PDL monomer, water,
nonionic surfactant having a PEG chain of molecular weight 2,000, and hexadecane
was vigorously stirred for 1 h at 45
C to give a miniemulsion system. To the
mixture, a suspension of lipase PS in surfactant solution was added, and the
resulting miniemulsion consisting of PDL nanodroplets was subjected to ROP
with stirring at 45 or 60
C for up to 24 h to reach a full conversion of PDL.
PolyPDL nanoparticles were obtained, which is considered to be a direct synthesis
of biodegradable polymer nanoparticles (size < 100 nm). PolyPDL showed a
bimodal molecular weight distribution; the majority was of high molecular weight
(>2.0 Â 10
5 ). It was possible to introduce a reactive group in the presence of an
unsaturated alcohol or acid such as linoleic acid in the reaction system via esterification reactions.
4.2 Lipase-Catalyzed Polyester Synthesis and Degradation
in Other Green Solvents
Supercritical carbon dioxide (scCO 2 ) was employed for the first time to prepare
polyesters via ROP of lactones. Lipase-catalyzed ROP of ε-CL proceeded to give a
polymer (PCL) with molecular weight higher than 10
4 . Copolymerization of ε-CL
with DDL afforded a random copolyester. The enzymatic polycondensation
between divinyl adipate and 1,4-butane diol also took place to produce the
corresponding polyester [73]. Later, a similar study on ROP of ε-CL in scCO 2
followed [74].
Green Polymer Chemistry: Recent Developments
161
and in Miniemulsion
ROP of lactones to various polyesters has been widely studied [19, 68]. Lipasecatalyzed ROP is normally carried out in bulk or in an organic solvent like toluene,
1,4-dioxane, or dibutyl ether [17, 19, 20].
Water was used as solvent for the first time in the lipase-catalyzed ROP of five
lactone monomers, ε-CL, OL, UDL, DDL, and PDL (Scheme 5) [69, 70].
Macrolides of UDL, DDL, and PDL are less reactive than lactones of smaller
ring size due to lower ring strain when using a usual chemical catalyst [71]. However, they showed higher reactivity in enzyme catalysis and were polymerized by
lipase in water to produce the corresponding polyesters; typically, UDL gave
polyUDL with M n 1,300 (M w /M n ¼ 2.1) in 79% yields at 60
C for 72 h. DDL is
hardly soluble in water; however, addition of the lipase gave a white emulsion-like
solution, which allowed the ROP. In contrast, a mixture of the lipase and ε-CL or
OL did not form an emulsion-like solution, and thus failed to induce the ROP.
Therefore, it seems that the enzyme protein behaved like a surfactant [69–71].
A second example of the use of water as medium is the lipase-catalyzed ROP of
a lactone in miniemulsions [72]. Typically, a mixture of PDL monomer, water,
nonionic surfactant having a PEG chain of molecular weight 2,000, and hexadecane
was vigorously stirred for 1 h at 45
C to give a miniemulsion system. To the
mixture, a suspension of lipase PS in surfactant solution was added, and the
resulting miniemulsion consisting of PDL nanodroplets was subjected to ROP
with stirring at 45 or 60
C for up to 24 h to reach a full conversion of PDL.
PolyPDL nanoparticles were obtained, which is considered to be a direct synthesis
of biodegradable polymer nanoparticles (size < 100 nm). PolyPDL showed a
bimodal molecular weight distribution; the majority was of high molecular weight
(>2.0 Â 10
5 ). It was possible to introduce a reactive group in the presence of an
unsaturated alcohol or acid such as linoleic acid in the reaction system via esterification reactions.
4.2 Lipase-Catalyzed Polyester Synthesis and Degradation
in Other Green Solvents
Supercritical carbon dioxide (scCO 2 ) was employed for the first time to prepare
polyesters via ROP of lactones. Lipase-catalyzed ROP of ε-CL proceeded to give a
polymer (PCL) with molecular weight higher than 10
4 . Copolymerization of ε-CL
with DDL afforded a random copolyester. The enzymatic polycondensation
between divinyl adipate and 1,4-butane diol also took place to produce the
corresponding polyester [73]. Later, a similar study on ROP of ε-CL in scCO 2
followed [74].
Green Polymer Chemistry: Recent Developments
161