8 Lewis Acid−Base Pairs for Polymerization Catalysis …
307
value was also observed when the polymerization was performed at a relatively
low initiator loading ([PO] 0 :[BDM] 0 :[
t Bu-P 2 ] 0 :[BEt 3 ] 0 = 16000:1:10:30, M nGPC =
210.3 kg/mol, Ð = 1.07, M nCal = 902.1 kg/mol).
t Bu-P 2 /BEt 3 can also mediate
controlled ROP of BO, tert-butyl glycidyl ether (TBGE), and AGE, but was ineffective for the polymerizations of styrene oxide (SO) and cyclohexene oxide (CHO)
(Scheme 8.15). PEO-b-PPO-b-PEO, PPO-b-PEO-b-PPO, PAGE-b-PTBGE-b-PAGE
triblock, PTBGE-b-PBO-b-PPO-b-PBO-b-PTBGE pentablock, as well as one-pot
one-catalyst to PPO-based polyurethane, were successfully synthesized by
t Bu-P 2 (or
t Bu-P 1 )/BEt 3 . The formation of both three-component base-H
+ –O(R)–BEt
–
3 complex
and BEt 3 -activated epoxide enabled the occurrence of cooperative initiation, while
the fast activity exchange between dormant and active hydroxy species has been
proven essential for controlled polymerization (Scheme 8.16, bottom).
Meanwhile, Zhang et al. also reported the living and efficient ROP of epoxides (e.g., PO, EO, BO) at 0 °C by using the same BEt 3 /organic base LP
catalytic system [73].Among the alcohol initiators [e.g., BnOH, MeOH, EtOH,
BDM, HO(CH 2 ) 3 OH] and organic bases (e.g.,
t Bu-P 1 ,
t Bu-P 2 , DBU, MTBD)
evaluated, BEt 3 /
t Bu-P 2 /BnOH was found to be the most active catalyst for PO
ROP, in which rapid monomer consumption was observed within 2 min (TOF =
7500 h
−1 , [PO] 0 :[BnOH] 0 :[BEt 3 ] 0 :[
t Bu-P 2 ] 0 = 250:1:3:1). A PPO with a high M n
up to 80.5 kg/mol and a very low Ð of 1.03 was obtained. Regioselectivity in
the ROPs of rac-PO, S-PO and R-PO can be achieved by this catalytic system,
showing the presence of head-to-tail structure without any detectable head-tohead or tail-to-tail linkage as a result of the selective attack at the methylene site
of PO by the growing active species. Taking advantage of the living nature of
this catalyst, a triblock copolymer poly(COS-alt-PO)-b-PPO-b-poly(COS-alt-PO)
as well as diblock copolymers PEO-b-PPO and poly(styrene-b-PO) starting from αmethoxy/ω-OH PEO and hydroxyl-end functionalized polystyrene macroinitiators
were synthesized accordingly.
When BEt 3 was added in excess of the
t Bu-P 2 (e.g., BEt 3 :
t Bu-P 2 = 3:1) to
the mixed monomers of epoxide and cyclic ester, only ROP of epoxide occured
selectively with cyclic ester remained unreacted, while
t Bu-P 2 alone or
t Bu-P 2 in
excess of BEt 3 allowed the selective ROP of cyclic ester. On the basis of this disclosure, in 2019, Zhang, Ling, and co-workers established a switchable polymerization to ether-ester-type multiblock copolymer by switching the monomer selectivity
in the copolymerization of epoxide and cyclic ester through varying the amount
of BEt 3 or
t Bu-P 2 during the polymerization [74]. For example,
t Bu-P 2 was first
added to a mixture of δ-VL, PO, and BDM to turn “on” selective ROP of δ-VL
([δ-VL] 0 :[PO] 0 :[BDM] 0 :[
t Bu-P 2 ] 0 = 110:150:1:0.1, Scheme 8.17, top).
When near quantitative conversion of δ-VL (94%) was accomplished after 30 min,
BEt 3 was added to the mixture ([BEt 3 ] 0 :[
t Bu-P 2 ] 0 = 3:1) to turn “off” ROP of δ-VL
and turn “on” selective ROP of PO for [12 h, Conv. (PO) = 84%]. Correspondingly, well-defined PPO-b-PVL-b-PPO triblock copolymer (M n = 19.0 kg/mol, Ð =
1.09) was successfully produced without random or tapered sequence (simultaneous
enchainment of two monomers). The PVL-b-PPO-b-PVL triblock copolymer (M n =
30.7 kg/mol, Ð = 1.10) with ether-first order was also feasible (Scheme 8.17, top),
307
value was also observed when the polymerization was performed at a relatively
low initiator loading ([PO] 0 :[BDM] 0 :[
t Bu-P 2 ] 0 :[BEt 3 ] 0 = 16000:1:10:30, M nGPC =
210.3 kg/mol, Ð = 1.07, M nCal = 902.1 kg/mol).
t Bu-P 2 /BEt 3 can also mediate
controlled ROP of BO, tert-butyl glycidyl ether (TBGE), and AGE, but was ineffective for the polymerizations of styrene oxide (SO) and cyclohexene oxide (CHO)
(Scheme 8.15). PEO-b-PPO-b-PEO, PPO-b-PEO-b-PPO, PAGE-b-PTBGE-b-PAGE
triblock, PTBGE-b-PBO-b-PPO-b-PBO-b-PTBGE pentablock, as well as one-pot
one-catalyst to PPO-based polyurethane, were successfully synthesized by
t Bu-P 2 (or
t Bu-P 1 )/BEt 3 . The formation of both three-component base-H
+ –O(R)–BEt
–
3 complex
and BEt 3 -activated epoxide enabled the occurrence of cooperative initiation, while
the fast activity exchange between dormant and active hydroxy species has been
proven essential for controlled polymerization (Scheme 8.16, bottom).
Meanwhile, Zhang et al. also reported the living and efficient ROP of epoxides (e.g., PO, EO, BO) at 0 °C by using the same BEt 3 /organic base LP
catalytic system [73].Among the alcohol initiators [e.g., BnOH, MeOH, EtOH,
BDM, HO(CH 2 ) 3 OH] and organic bases (e.g.,
t Bu-P 1 ,
t Bu-P 2 , DBU, MTBD)
evaluated, BEt 3 /
t Bu-P 2 /BnOH was found to be the most active catalyst for PO
ROP, in which rapid monomer consumption was observed within 2 min (TOF =
7500 h
−1 , [PO] 0 :[BnOH] 0 :[BEt 3 ] 0 :[
t Bu-P 2 ] 0 = 250:1:3:1). A PPO with a high M n
up to 80.5 kg/mol and a very low Ð of 1.03 was obtained. Regioselectivity in
the ROPs of rac-PO, S-PO and R-PO can be achieved by this catalytic system,
showing the presence of head-to-tail structure without any detectable head-tohead or tail-to-tail linkage as a result of the selective attack at the methylene site
of PO by the growing active species. Taking advantage of the living nature of
this catalyst, a triblock copolymer poly(COS-alt-PO)-b-PPO-b-poly(COS-alt-PO)
as well as diblock copolymers PEO-b-PPO and poly(styrene-b-PO) starting from αmethoxy/ω-OH PEO and hydroxyl-end functionalized polystyrene macroinitiators
were synthesized accordingly.
When BEt 3 was added in excess of the
t Bu-P 2 (e.g., BEt 3 :
t Bu-P 2 = 3:1) to
the mixed monomers of epoxide and cyclic ester, only ROP of epoxide occured
selectively with cyclic ester remained unreacted, while
t Bu-P 2 alone or
t Bu-P 2 in
excess of BEt 3 allowed the selective ROP of cyclic ester. On the basis of this disclosure, in 2019, Zhang, Ling, and co-workers established a switchable polymerization to ether-ester-type multiblock copolymer by switching the monomer selectivity
in the copolymerization of epoxide and cyclic ester through varying the amount
of BEt 3 or
t Bu-P 2 during the polymerization [74]. For example,
t Bu-P 2 was first
added to a mixture of δ-VL, PO, and BDM to turn “on” selective ROP of δ-VL
([δ-VL] 0 :[PO] 0 :[BDM] 0 :[
t Bu-P 2 ] 0 = 110:150:1:0.1, Scheme 8.17, top).
When near quantitative conversion of δ-VL (94%) was accomplished after 30 min,
BEt 3 was added to the mixture ([BEt 3 ] 0 :[
t Bu-P 2 ] 0 = 3:1) to turn “off” ROP of δ-VL
and turn “on” selective ROP of PO for [12 h, Conv. (PO) = 84%]. Correspondingly, well-defined PPO-b-PVL-b-PPO triblock copolymer (M n = 19.0 kg/mol, Ð =
1.09) was successfully produced without random or tapered sequence (simultaneous
enchainment of two monomers). The PVL-b-PPO-b-PVL triblock copolymer (M n =
30.7 kg/mol, Ð = 1.10) with ether-first order was also feasible (Scheme 8.17, top),