302
M. Hong
8.3 Lewis Pair-Mediated Ring-Opening
(Co)Polymerization
Ring-opening (co)polymerization [RO(C)P] of cyclic esters or epoxides generally
affords biodegradable and/or biocompatible polymers that have been recognized
as greener or more sustainable polymeric materials relative to the conventionally
nondegradable petroleum-based polymers [18, 19, 23, 62–68]. To achieve competitive physical and mechanical performance with petroleum-based polymers, one of
the critical and practical approaches is to control the microstructure (e.g., MW,
chemo-, regio-, and stereoselectivity) of ring-opened polymers via the development
of powerful polymerization catalysis. The recent advances in the employment of LP
catalysts for furnishing a high degree of control over RO(C)Ps are updated in this
section of the chapter.
O-Carboxyanhydrides (OCAs) can be prepared with a rich variety of side-chain
functionalities from natural α-hydroxy acids or amino acids, thus considered as an
alternative class of the monomers to lactide and its derivatives for ROP to produce
poly(α-hydroxy acids) with diverse physical properties. In 2018, Yang, Meng, and
co-workers found that the CLA comprising Zn(C 6 F 5 ) 2 and organic amines can
efficiently initiate the controlled/living ROP of OCAs [69]. Screening the polymerization solvents [DCM, CHCl 3 , dioxane, tetrahydrofuran (THF)] and temperatures (25–50 °C) by using Zn(C 6 F 5 ) 2 /1-hexanamine as the catalyst indicated that
performing the polymerization in THF at 50 °C represented the optimized conditions for Phe-OCA (Scheme 8.13) ROP, because of the favored dissociation of CLA
under these conditions. Increasing [Phe-OCA] 0 :[Zn(C 6 F 5 ) 2 ] 0 :[1-hexanamine] 0 ratio
from 25:1:1 to 200:1:1, complete monomer consumption can be accomplished with
1–24 h (TOF: 7–40 h
−1 ), affording well-defined poly(Phe-OCA)s with M n values
N
H
R
H
Zn(C 6 F 5 ) 2
O
RNH
O
Zn
C 6 F 5
Coordination-insertion
-C 6 F 5 H
O
RNH
OH
Zn(C 6 F 5 ) 2
Phe-OCA
Zn-alkoxide
Propagation
O
RNH
O
H
n
N
R
H
Zn(C 6 F 5 ) 2
H
O
O
O
O
Ph
+
-CO 2
Initiation
Scheme 8.13 Proposed mechanism for ROP of Phe-OCA by Zn(C 6 F 5 ) 2 /RNH 2 CLA
M. Hong
8.3 Lewis Pair-Mediated Ring-Opening
(Co)Polymerization
Ring-opening (co)polymerization [RO(C)P] of cyclic esters or epoxides generally
affords biodegradable and/or biocompatible polymers that have been recognized
as greener or more sustainable polymeric materials relative to the conventionally
nondegradable petroleum-based polymers [18, 19, 23, 62–68]. To achieve competitive physical and mechanical performance with petroleum-based polymers, one of
the critical and practical approaches is to control the microstructure (e.g., MW,
chemo-, regio-, and stereoselectivity) of ring-opened polymers via the development
of powerful polymerization catalysis. The recent advances in the employment of LP
catalysts for furnishing a high degree of control over RO(C)Ps are updated in this
section of the chapter.
O-Carboxyanhydrides (OCAs) can be prepared with a rich variety of side-chain
functionalities from natural α-hydroxy acids or amino acids, thus considered as an
alternative class of the monomers to lactide and its derivatives for ROP to produce
poly(α-hydroxy acids) with diverse physical properties. In 2018, Yang, Meng, and
co-workers found that the CLA comprising Zn(C 6 F 5 ) 2 and organic amines can
efficiently initiate the controlled/living ROP of OCAs [69]. Screening the polymerization solvents [DCM, CHCl 3 , dioxane, tetrahydrofuran (THF)] and temperatures (25–50 °C) by using Zn(C 6 F 5 ) 2 /1-hexanamine as the catalyst indicated that
performing the polymerization in THF at 50 °C represented the optimized conditions for Phe-OCA (Scheme 8.13) ROP, because of the favored dissociation of CLA
under these conditions. Increasing [Phe-OCA] 0 :[Zn(C 6 F 5 ) 2 ] 0 :[1-hexanamine] 0 ratio
from 25:1:1 to 200:1:1, complete monomer consumption can be accomplished with
1–24 h (TOF: 7–40 h
−1 ), affording well-defined poly(Phe-OCA)s with M n values
N
H
R
H
Zn(C 6 F 5 ) 2
O
RNH
O
Zn
C 6 F 5
Coordination-insertion
-C 6 F 5 H
O
RNH
OH
Zn(C 6 F 5 ) 2
Phe-OCA
Zn-alkoxide
Propagation
O
RNH
O
H
n
N
R
H
Zn(C 6 F 5 ) 2
H
O
O
O
O
Ph
+
-CO 2
Initiation
Scheme 8.13 Proposed mechanism for ROP of Phe-OCA by Zn(C 6 F 5 ) 2 /RNH 2 CLA