8 Lewis Acid−Base Pairs for Polymerization Catalysis …
303
close to the calculated ones (M n up to 26.8 kg/mol) and low Ð values below 1.10.
As a control, no polymer or only oligomer was obtained when the polymerization was initiated by 1-hexanamine or Zn(C 6 F 5 ) 2 alone. Switching to manOCA
(Scheme 8.14), the controlled ROP was also achieved by Zn(C 6 F 5 ) 2 /1-hexanamine
([man-OCA] 0 :[Zn(C 6 F 5 ) 2 ] 0 :[1-hexanamine] 0 = 50:5:1, M n = 8.1 kg/mol, Ð = 1.10,
TOF = 17 h
−1 ), thus giving access to the preparation of poly(Phe-OCA-b-man-OCA)
diblock copolymer via sequential addition. The combination of Zn(C 6 F 5 ) 2 with the
other primary amines (e.g., cyclohexamine, benzylamine, phenylamine) or secondary
amine (diethylamine) all exhibited good control over MW for Phe-OCA ROP with
identical activities. Experimental mechanistic studies and the theoretical calculations
reveal that both amine and Zn(C 6 F 5 ) 2 are involved in the cooperative catalysis of initiation, which is followed by the ring opening and decarboxylation to give an intermediate with a hydroxyl group that can react with Zn(C 6 F 5 ) 2 to generate a Zn-alkoxide
terminus for subsequent coordination–insertion chain propagation (Scheme 8.13).
Correspondingly, poly(Phe-OCA) with an amide group at the α-terminus of the
polymer chain was yielded. In addition, ROPs of enantiomerically pure Phe-OCA
and manOCA by Zn(C 6 F 5 ) 2 /1-hexanamine were also attempted, which resulted in
polymers with moderate isotacticities (66–77%) caused by the epimerization side
reaction due to high C–H acidity at chiral center.
To avoid the epimerization side reaction in manOCA ROP, in 2020, Wu et al. developed a selective catalyst system based on a weak LP composing of 3-fluoropyridine
(3-F-Py) LB and aminobisphenolate zinc complex LA (Zn-1: R 1 = R 2 =
t Bu, R 3
= Me; Zn-2: R 1 = R 2 =
t Bu, R 3 = Et; Zn-3: R 1 = Ad, R 2 =
t Bu, R 3 = Me; Zn4: R 1 =
t Bu, R 2 = OMe, R 3 = Me, Scheme 8.14), which can smoothly convert
enantiomerically pure L-manOCA into poly(L-manOCA) in presence of L-methyl
lactate (L-ML) initiator at RT in toluene with Zn-1/3-F-Py as the most selective LP
R 1
R 2
O
N
R 3
Zn
O
R 1
R 2
N
F
O
O
O
O
Ph
O
O
O
O
Ph
O
O
Ph
n
O
O
Ph
n
poly(L-manOCA)
poly(D-manOCA)
O
O
O
O
Ph
O
O
O
O
Ph
O
O
Ph
x
O
O
Ph
y
Isotactic
poly(rac-manOCA)
-CO 2
R 1
R 2
O
N
R 3
Zn
O
R 1
R 2
N
F
O
O
O
O
Ph
H
O
R
manOCA
Scheme 8.14 Synthesis of enantiopure poly(L-manOCA) and poly(D-manOCA) as well as
isotactic poly(rac-manOCA) via the ROP of L-manOCA, D-manOCA, and rac-manOCA, respectively, by 3-F-Py/aminobisphenolate zinc complex
303
close to the calculated ones (M n up to 26.8 kg/mol) and low Ð values below 1.10.
As a control, no polymer or only oligomer was obtained when the polymerization was initiated by 1-hexanamine or Zn(C 6 F 5 ) 2 alone. Switching to manOCA
(Scheme 8.14), the controlled ROP was also achieved by Zn(C 6 F 5 ) 2 /1-hexanamine
([man-OCA] 0 :[Zn(C 6 F 5 ) 2 ] 0 :[1-hexanamine] 0 = 50:5:1, M n = 8.1 kg/mol, Ð = 1.10,
TOF = 17 h
−1 ), thus giving access to the preparation of poly(Phe-OCA-b-man-OCA)
diblock copolymer via sequential addition. The combination of Zn(C 6 F 5 ) 2 with the
other primary amines (e.g., cyclohexamine, benzylamine, phenylamine) or secondary
amine (diethylamine) all exhibited good control over MW for Phe-OCA ROP with
identical activities. Experimental mechanistic studies and the theoretical calculations
reveal that both amine and Zn(C 6 F 5 ) 2 are involved in the cooperative catalysis of initiation, which is followed by the ring opening and decarboxylation to give an intermediate with a hydroxyl group that can react with Zn(C 6 F 5 ) 2 to generate a Zn-alkoxide
terminus for subsequent coordination–insertion chain propagation (Scheme 8.13).
Correspondingly, poly(Phe-OCA) with an amide group at the α-terminus of the
polymer chain was yielded. In addition, ROPs of enantiomerically pure Phe-OCA
and manOCA by Zn(C 6 F 5 ) 2 /1-hexanamine were also attempted, which resulted in
polymers with moderate isotacticities (66–77%) caused by the epimerization side
reaction due to high C–H acidity at chiral center.
To avoid the epimerization side reaction in manOCA ROP, in 2020, Wu et al. developed a selective catalyst system based on a weak LP composing of 3-fluoropyridine
(3-F-Py) LB and aminobisphenolate zinc complex LA (Zn-1: R 1 = R 2 =
t Bu, R 3
= Me; Zn-2: R 1 = R 2 =
t Bu, R 3 = Et; Zn-3: R 1 = Ad, R 2 =
t Bu, R 3 = Me; Zn4: R 1 =
t Bu, R 2 = OMe, R 3 = Me, Scheme 8.14), which can smoothly convert
enantiomerically pure L-manOCA into poly(L-manOCA) in presence of L-methyl
lactate (L-ML) initiator at RT in toluene with Zn-1/3-F-Py as the most selective LP
R 1
R 2
O
N
R 3
Zn
O
R 1
R 2
N
F
O
O
O
O
Ph
O
O
O
O
Ph
O
O
Ph
n
O
O
Ph
n
poly(L-manOCA)
poly(D-manOCA)
O
O
O
O
Ph
O
O
O
O
Ph
O
O
Ph
x
O
O
Ph
y
Isotactic
poly(rac-manOCA)
-CO 2
R 1
R 2
O
N
R 3
Zn
O
R 1
R 2
N
F
O
O
O
O
Ph
H
O
R
manOCA
Scheme 8.14 Synthesis of enantiopure poly(L-manOCA) and poly(D-manOCA) as well as
isotactic poly(rac-manOCA) via the ROP of L-manOCA, D-manOCA, and rac-manOCA, respectively, by 3-F-Py/aminobisphenolate zinc complex