8 Lewis Acid−Base Pairs for Polymerization Catalysis …
289
The livingness of (BHT)Al
i Bu 2 /IAP-1 FLP-mediated polymerization is proposed
to originate from exclusive zwitterionic initiating species, and no problematic side
reactions. However, this FLP catalyst was less active and became sluggish toward
γ MMBL ([γ MMBL] 0 :[IAP-1] 0 :[(BHT)Al
i Bu 2 ] 0 = 800:1:2, TOF = 141 h
−1 , M n
= 85.6 kg/mol, Ð = 1.12, I
*
= 105%) and MS polymerization ([MS] 0 :[IAP1] 0 :[(BHT)Al
i Bu 2 ] 0 = 400:1:2, TOF = 2.67 h
−1 ), respectively, and lost the controlled
ability in DMAA polymerization by producing polymers with a bimodal distribution.
In addition, compared with (BHT)Al
i Bu 2 /IAP-1 FLP, other FLP catalytic systems
comprising sterically more demanding LA and IAP-1 LB resulted in either less effective polymerization ([MMA] 0 :[IAP-1] 0 :[(BHT) 2 Al
i Bu] 0 = 800:1:2, TOF = 141 h
−1 ,
M n = 72.0 kg/mol, Ð = 1.13, I
*
= 112%) or uncontrolled/nonliving polymerization
[MeAl(BHT) 2 /IAP-1] accompanied by backbiting chain termination [39, 40]. The
combination of strong acidic Al(C 6 F 5 ) 3 with IAP-1 also led to a uncontrolled polymerization due to the existence of the backbiting termination, though Al(C 6 F 5 ) 3 /IAP1–4 (Scheme 8.2) CLAs all showed high-speed polymerizations (TOF: 96000 h
−1 ,
[MMA] 0 :[LB] 0 :[LA] 0 = 800:1:2) with moderate to high initiation efficiencies (I
* :
52–92%) and produced PMMAs with narrow dispersities (Ð: 1.11–1.21). Low polymerization activity was observed under the same conditions when switching to less
nucleophilic IAP-5 (Scheme 8.2) LB to pair with Al(C 6 F 5 ) 3 (TOF = 13.7 h
−1 ). The
screening of LA and LB scope indicated that matched Lewis acidity, basicity, and
steric effect are critical for achieving efficient and controlled polymerization.
The livingness and robustness of (BHT)Al
i Bu 2 /IAP-1 FLP provided a practical
approach for rapid and scalable synthesis of sequence-controlled methacrylic
multiblock copolymers at RT with double high [molecular weight and degree of
polymerization per block value (dp n )] and double multiple [monomers (k) and block
numbers (n)] (DHDM) features as reported by Zhang et al. very recently [41]. At
the outset, the authors tested the multiply chain-extension polymerization of MMA.
Even after adding 10 batches of 400 equiv. of MMA or 1 batch of 400 equiv. of MMA
followed by 28 batches of 100 equiv. of MMA (the initial addition of 400 equiv.
of MMA was to ensure quantitative I
* value), all of monomers can be rapidly and
quantitatively converted by (BHT)Al
i Bu 2 /IAP-1 FLP (2 min per block), and a high
degree of control over MW still maintained, which resulted in a high-MW decablock
or nonacosablock PMMA (274–374 kg/mol, Scheme 8.3a, b) with a relatively
low Ð (1.18–1.26) and a near quantitative I
* value (107–117%). Based on the
successful multiply chain-extension polymerization, (BHT)Al
i Bu 2 /IAP-1-mediated
multiblock copolymerizations were then performed by choosing four different
monomers, including MMA, EMA, 2-methoxyethyl mechacrylate (MEMA), and
2-ethyoxyethyl mechacrylate (EEMA). Through sequential addition of monomers,
a pentablock copolymer (n = 5, k = 4, dp n = 400, Scheme 8.3c) with a high M n of
146 kg/mol and a low Ð of 1.18 was achieved. Noteworthy is that scaling up this
multiblock copolymerization was also feasible, and multigram quantity of pentablock
copolymer can be obtained (114.6 g, 98%) with full conversion accomplished within
2 min for each batch monomer. Moreover, block numbers (n) were further extended
to 25 with dp n value of 100, and even to 53 with dp n value of 50 for the multiblock
copolymerizations (k = 4). Remarkably, it took only 25 min (1 min per block) to
289
The livingness of (BHT)Al
i Bu 2 /IAP-1 FLP-mediated polymerization is proposed
to originate from exclusive zwitterionic initiating species, and no problematic side
reactions. However, this FLP catalyst was less active and became sluggish toward
γ MMBL ([γ MMBL] 0 :[IAP-1] 0 :[(BHT)Al
i Bu 2 ] 0 = 800:1:2, TOF = 141 h
−1 , M n
= 85.6 kg/mol, Ð = 1.12, I
*
= 105%) and MS polymerization ([MS] 0 :[IAP1] 0 :[(BHT)Al
i Bu 2 ] 0 = 400:1:2, TOF = 2.67 h
−1 ), respectively, and lost the controlled
ability in DMAA polymerization by producing polymers with a bimodal distribution.
In addition, compared with (BHT)Al
i Bu 2 /IAP-1 FLP, other FLP catalytic systems
comprising sterically more demanding LA and IAP-1 LB resulted in either less effective polymerization ([MMA] 0 :[IAP-1] 0 :[(BHT) 2 Al
i Bu] 0 = 800:1:2, TOF = 141 h
−1 ,
M n = 72.0 kg/mol, Ð = 1.13, I
*
= 112%) or uncontrolled/nonliving polymerization
[MeAl(BHT) 2 /IAP-1] accompanied by backbiting chain termination [39, 40]. The
combination of strong acidic Al(C 6 F 5 ) 3 with IAP-1 also led to a uncontrolled polymerization due to the existence of the backbiting termination, though Al(C 6 F 5 ) 3 /IAP1–4 (Scheme 8.2) CLAs all showed high-speed polymerizations (TOF: 96000 h
−1 ,
[MMA] 0 :[LB] 0 :[LA] 0 = 800:1:2) with moderate to high initiation efficiencies (I
* :
52–92%) and produced PMMAs with narrow dispersities (Ð: 1.11–1.21). Low polymerization activity was observed under the same conditions when switching to less
nucleophilic IAP-5 (Scheme 8.2) LB to pair with Al(C 6 F 5 ) 3 (TOF = 13.7 h
−1 ). The
screening of LA and LB scope indicated that matched Lewis acidity, basicity, and
steric effect are critical for achieving efficient and controlled polymerization.
The livingness and robustness of (BHT)Al
i Bu 2 /IAP-1 FLP provided a practical
approach for rapid and scalable synthesis of sequence-controlled methacrylic
multiblock copolymers at RT with double high [molecular weight and degree of
polymerization per block value (dp n )] and double multiple [monomers (k) and block
numbers (n)] (DHDM) features as reported by Zhang et al. very recently [41]. At
the outset, the authors tested the multiply chain-extension polymerization of MMA.
Even after adding 10 batches of 400 equiv. of MMA or 1 batch of 400 equiv. of MMA
followed by 28 batches of 100 equiv. of MMA (the initial addition of 400 equiv.
of MMA was to ensure quantitative I
* value), all of monomers can be rapidly and
quantitatively converted by (BHT)Al
i Bu 2 /IAP-1 FLP (2 min per block), and a high
degree of control over MW still maintained, which resulted in a high-MW decablock
or nonacosablock PMMA (274–374 kg/mol, Scheme 8.3a, b) with a relatively
low Ð (1.18–1.26) and a near quantitative I
* value (107–117%). Based on the
successful multiply chain-extension polymerization, (BHT)Al
i Bu 2 /IAP-1-mediated
multiblock copolymerizations were then performed by choosing four different
monomers, including MMA, EMA, 2-methoxyethyl mechacrylate (MEMA), and
2-ethyoxyethyl mechacrylate (EEMA). Through sequential addition of monomers,
a pentablock copolymer (n = 5, k = 4, dp n = 400, Scheme 8.3c) with a high M n of
146 kg/mol and a low Ð of 1.18 was achieved. Noteworthy is that scaling up this
multiblock copolymerization was also feasible, and multigram quantity of pentablock
copolymer can be obtained (114.6 g, 98%) with full conversion accomplished within
2 min for each batch monomer. Moreover, block numbers (n) were further extended
to 25 with dp n value of 100, and even to 53 with dp n value of 50 for the multiblock
copolymerizations (k = 4). Remarkably, it took only 25 min (1 min per block) to