Hydrolytic degradation of PCL by lipase CA catalyst was studied in scCO 2 [75].
The addition of acetone (5 vol%) accelerated the degradation of high molecular
weight PCL to produce smaller molecular weight (<500) linear and cyclic
oligomers, which could be repolymerized by the same catalyst. It is useful that
scCO 2 is easy to remove after the reaction to recover the catalyst, and the reaction
can be recycled.
Ionic liquids are often used as reaction solvent for the synthesis and modification
of polymers due to their green character [76]. The first paper on ionic liquids as
solvent for enzymatic polymerization appeared in 2002. Lipase-catalyzed ROP of
ε-CL and the polycondensation between diethyl adipate or sebacate and 1,4-butane
diol were achieved in an ionic liquid such as 1-butyl-3-methyl-imidazolium salts
([bmim][PF 6 ]). The ROP gave rise to PCL with M n of 4,200 (M w /M n ¼ 2.7) in 97%
yields at 60
C after 7 days [77]. Lipase CA-catalyzed ROP of ε-CL in three ionic
liquids, [bmim][BF 4 ], [bmim][PF 6 ], and [bmim][(CF 3 SO 2 ) 2 N], at 60
C for 24 h
produced PCL with a higher M n of 7,000–9,500 (M w /M n ~ 2.4) in good yields. In
the polycondensation of the above combinations, M n was up to 5,400 [78]. Since
ionic liquids have high boiling points, with tunable nature for hydrophobicity and
solubility, the polymerization of polar monomers (which are less soluble in an
organic solvent) is suggested as an appropriate way.
A more recent paper reported that by using four kinds of ionic liquid, the ROP of
lactide by lipase CA catalyst at room temperature for 24 h produced PLA having
molecular weight values reaching 55,000 in 35% yields [79].
5 Conclusions
For conducting “green polymer chemistry”, the following aspects are stressed and it
is very important that attention is paid to them. Typically, (1) starting materials are
biobased renewable resources to mitigate use of fossil-based law materials; (2) synthesis or modification reactions are catalytic, not molar reactions; (3) catalysts are
nontoxic and re-usable; (4) reaction solvents are environmentally benign to
decrease use of organic solvents; and (5) product polymer structures are subjected
to material recycling. The present article is concerned mainly with our recent
research results performed in this direction. In particular, results employing lactic
acid and itaconic anhydride suggest future materials, as shown in Sect. 2. Also, the
characteristics of enzyme catalysis shown in Sect. 3 are to be noted.
It is important to keep paying attention to climate change and global warming,
consumption of natural resources, and the method of energy generation and consumption; all of these issues are directly connected with the future environment. As
polymer scientists, we are very much required to conduct green polymer chemistry
to preserve the environment as well as we can.
162
S. Kobayashi
The addition of acetone (5 vol%) accelerated the degradation of high molecular
weight PCL to produce smaller molecular weight (<500) linear and cyclic
oligomers, which could be repolymerized by the same catalyst. It is useful that
scCO 2 is easy to remove after the reaction to recover the catalyst, and the reaction
can be recycled.
Ionic liquids are often used as reaction solvent for the synthesis and modification
of polymers due to their green character [76]. The first paper on ionic liquids as
solvent for enzymatic polymerization appeared in 2002. Lipase-catalyzed ROP of
ε-CL and the polycondensation between diethyl adipate or sebacate and 1,4-butane
diol were achieved in an ionic liquid such as 1-butyl-3-methyl-imidazolium salts
([bmim][PF 6 ]). The ROP gave rise to PCL with M n of 4,200 (M w /M n ¼ 2.7) in 97%
yields at 60
C after 7 days [77]. Lipase CA-catalyzed ROP of ε-CL in three ionic
liquids, [bmim][BF 4 ], [bmim][PF 6 ], and [bmim][(CF 3 SO 2 ) 2 N], at 60
C for 24 h
produced PCL with a higher M n of 7,000–9,500 (M w /M n ~ 2.4) in good yields. In
the polycondensation of the above combinations, M n was up to 5,400 [78]. Since
ionic liquids have high boiling points, with tunable nature for hydrophobicity and
solubility, the polymerization of polar monomers (which are less soluble in an
organic solvent) is suggested as an appropriate way.
A more recent paper reported that by using four kinds of ionic liquid, the ROP of
lactide by lipase CA catalyst at room temperature for 24 h produced PLA having
molecular weight values reaching 55,000 in 35% yields [79].
5 Conclusions
For conducting “green polymer chemistry”, the following aspects are stressed and it
is very important that attention is paid to them. Typically, (1) starting materials are
biobased renewable resources to mitigate use of fossil-based law materials; (2) synthesis or modification reactions are catalytic, not molar reactions; (3) catalysts are
nontoxic and re-usable; (4) reaction solvents are environmentally benign to
decrease use of organic solvents; and (5) product polymer structures are subjected
to material recycling. The present article is concerned mainly with our recent
research results performed in this direction. In particular, results employing lactic
acid and itaconic anhydride suggest future materials, as shown in Sect. 2. Also, the
characteristics of enzyme catalysis shown in Sect. 3 are to be noted.
It is important to keep paying attention to climate change and global warming,
consumption of natural resources, and the method of energy generation and consumption; all of these issues are directly connected with the future environment. As
polymer scientists, we are very much required to conduct green polymer chemistry
to preserve the environment as well as we can.
162
S. Kobayashi