8 Lewis Acid−Base Pairs for Polymerization Catalysis …
295
polymerization of DEVP at –30 °C led to a PDEVP with a relatively broad Ð of 1.33
[26]. In contrast to these intermolecular LP catalysts, intramolecular Al-P BLP-1 not
only exhibited high activity toward DEVP and DIVP polymerizations at RT, but also
afforded high-MW polymers with narrow to extremely narrow dispersities, though I
*
values of these polymerizations were relatively low ([monomer] 0 :[BLP-1] 0 = 500,
DEVP: TOF = 3700 h
−1 , M n = 250 kg/mol, Ð = 1.19, I
*
= 33%; DIVP: TOF =
450 h
−1 , M n = 250 kg/mol, Ð = 1.05, I
*
= 38%). Moreover, living characteristics
of their LPPs has been confirmed by the increase of polymer M n versus monomer
conversion and monomer-to-initiator ratio with low Ðs maintained. Accordingly, the
preparation of poly(DEVP-b-DIVP) diblock copolymer was accessible with BLP1, whereas the combination with reverse monomer order was unsuccessful. ESI-MS
analysis revealed that no methyl or any other end group was detectable in the obtained
PDEVP oligomer, and in the
31 P NMR spectrum of unquenched polymerization
mixture, the appearance of the signal of free phosphine MeP
i Pr 2 and the disappearance of the signal of BLP-1 can be observed. These results indicated that the polymerization is most likely initiated via the deprotonation of the acidic α-position of
the monomer by the methylene bridge of BLP-1 followed by its cleavage and release
of the free MeP
i Pr 2 (Scheme 8.7a), thus ruling out the possible conjugate addition
(Scheme 8.7b) and nucleophilic transfer pathways (Scheme 8.7c). The exclusive
deprotonation initiation mechanism should be responsible for the good control of
the polymerization process. Theoretical calculations provided further evidence for
a deprotonation mechanism, as the natural bonding orbital charges for the bridging
methylene showed negative values (ca. –1e) for BLP-1 as well as the other BLPs
2–5. In comparison to BLP-1, more sterically demanding BLPs 2–5 were less active
or ineffective toward DEVP polymerization ([DEVP] 0 :[BLP] 0 = 500, BLP-2: TOF
= 900 h
−1 , M n = 290 kg/mol, Ð = 1.12, I
*
= 28%; BLP-3 and BLP-4: TOF =
0 h
−1 ; BLP-5 (neat): TOF = 1200 h
−1 ). The observed higher TOF for BLP-1 is
presumably attributed to its low Gibbs free reaction enthalpies because of the formation of a neutral, thermodynamically favorable η
2 -ylene deprotonation intermediate
over a zwitterionic η
1 -ylide intermediate (Scheme 8.7a). Different from efficient and
controlled polymerization of DEVP by BLP-1, this BLP was unable to polymerize
MMA, and the control for DMAA polymerization (Ð = 1.52) was lost, despite its
remarkable activity ([DMAA] 0 :[BLP-1] 0 = 500, TOF = 10000 h
−1 ).
In place of conventionally sequential addition method, in 2020, Chen et al. established LPP as a unique and convenient methodology to synthesize well-defined block
copolymers from one-pot comonomer mixture (mixed addition) [53]. The authors
found that the combination of sterically unhindered PMe 3 LB with MeAl(BHT) 2 LA
can differentiate n-butyl acrylate (
n BA) and tert-butyl acrylate (
t BA) (Scheme 8.8,
top), where
n BA was depleted first which then followed by the consumption of
t BA, thus affording P
n BA-b-P
t BA diblock copolymer with a M n of 127 kg/mol
and a unimodal distribution (Ð = 1.02) from one-pot comonomer mixture within
15 s ([
t BA] 0 :[
n BA] 0 :[LA] 0 :[LB] 0 = 100:900:2:1). This achievement was proposed
to originate from three distinctive features of LPP. First, zeroth-order kinetics dependence on monomer concentration in LPP substantially suppresses the tapering
effect that commonly presents in conventional polymerizations due to the first-order
295
polymerization of DEVP at –30 °C led to a PDEVP with a relatively broad Ð of 1.33
[26]. In contrast to these intermolecular LP catalysts, intramolecular Al-P BLP-1 not
only exhibited high activity toward DEVP and DIVP polymerizations at RT, but also
afforded high-MW polymers with narrow to extremely narrow dispersities, though I
*
values of these polymerizations were relatively low ([monomer] 0 :[BLP-1] 0 = 500,
DEVP: TOF = 3700 h
−1 , M n = 250 kg/mol, Ð = 1.19, I
*
= 33%; DIVP: TOF =
450 h
−1 , M n = 250 kg/mol, Ð = 1.05, I
*
= 38%). Moreover, living characteristics
of their LPPs has been confirmed by the increase of polymer M n versus monomer
conversion and monomer-to-initiator ratio with low Ðs maintained. Accordingly, the
preparation of poly(DEVP-b-DIVP) diblock copolymer was accessible with BLP1, whereas the combination with reverse monomer order was unsuccessful. ESI-MS
analysis revealed that no methyl or any other end group was detectable in the obtained
PDEVP oligomer, and in the
31 P NMR spectrum of unquenched polymerization
mixture, the appearance of the signal of free phosphine MeP
i Pr 2 and the disappearance of the signal of BLP-1 can be observed. These results indicated that the polymerization is most likely initiated via the deprotonation of the acidic α-position of
the monomer by the methylene bridge of BLP-1 followed by its cleavage and release
of the free MeP
i Pr 2 (Scheme 8.7a), thus ruling out the possible conjugate addition
(Scheme 8.7b) and nucleophilic transfer pathways (Scheme 8.7c). The exclusive
deprotonation initiation mechanism should be responsible for the good control of
the polymerization process. Theoretical calculations provided further evidence for
a deprotonation mechanism, as the natural bonding orbital charges for the bridging
methylene showed negative values (ca. –1e) for BLP-1 as well as the other BLPs
2–5. In comparison to BLP-1, more sterically demanding BLPs 2–5 were less active
or ineffective toward DEVP polymerization ([DEVP] 0 :[BLP] 0 = 500, BLP-2: TOF
= 900 h
−1 , M n = 290 kg/mol, Ð = 1.12, I
*
= 28%; BLP-3 and BLP-4: TOF =
0 h
−1 ; BLP-5 (neat): TOF = 1200 h
−1 ). The observed higher TOF for BLP-1 is
presumably attributed to its low Gibbs free reaction enthalpies because of the formation of a neutral, thermodynamically favorable η
2 -ylene deprotonation intermediate
over a zwitterionic η
1 -ylide intermediate (Scheme 8.7a). Different from efficient and
controlled polymerization of DEVP by BLP-1, this BLP was unable to polymerize
MMA, and the control for DMAA polymerization (Ð = 1.52) was lost, despite its
remarkable activity ([DMAA] 0 :[BLP-1] 0 = 500, TOF = 10000 h
−1 ).
In place of conventionally sequential addition method, in 2020, Chen et al. established LPP as a unique and convenient methodology to synthesize well-defined block
copolymers from one-pot comonomer mixture (mixed addition) [53]. The authors
found that the combination of sterically unhindered PMe 3 LB with MeAl(BHT) 2 LA
can differentiate n-butyl acrylate (
n BA) and tert-butyl acrylate (
t BA) (Scheme 8.8,
top), where
n BA was depleted first which then followed by the consumption of
t BA, thus affording P
n BA-b-P
t BA diblock copolymer with a M n of 127 kg/mol
and a unimodal distribution (Ð = 1.02) from one-pot comonomer mixture within
15 s ([
t BA] 0 :[
n BA] 0 :[LA] 0 :[LB] 0 = 100:900:2:1). This achievement was proposed
to originate from three distinctive features of LPP. First, zeroth-order kinetics dependence on monomer concentration in LPP substantially suppresses the tapering
effect that commonly presents in conventional polymerizations due to the first-order