8 Lewis Acid−Base Pairs for Polymerization Catalysis …
287
Since the concept of LPP was first introduced in 2010, it opens up
an unprecedented pathway to effectively polymerizing polar vinyl monomers
(Scheme 8.1b), and ring-opening (co)polymerizing [RO(C)P] cyclic esters and
epoxides (Scheme 8.1c), as demonstrated by its relatively broad monomer scope
(Scheme 8.1d), high activity, control or livingness, and complete chemo- or regioselectivity. In the area of LPP of polar vinyl monomers, Chen et al. developed metal-free
phosphine/borane-based LP catalysts in 2014 for exceptionally active polymerization of biomass-derived γ MMBL [24]. In the same year, the chemoselective polymerization of divinyl acrylic monomers, such as vinyl methacrylate (VMA), allyl
methacrylate (AMA), and 4-vinylbenzyl methacrylate (VBMA), has been achieved
by Lu and co-workers through using Al(C 6 F 5 ) 3 /N-heterocyclic olefin (NHO) CLAs,
which led to soluble polymers bearing pendant vinyl groups for the subsequent
postfunctionalization [25].
In 2016, Rieger et al. realized the relatively controlled polymerization of sterically demanding methacrylates and functionalized monomers, such as furfuryl
methacrylate (FMA), n-butyl methacrylate (
n BuMA), tert-butyl methacrylate (
t BuMA), 4-vinylpyridine (4-VP), N,N-dimethyl acrylamide (DMAA), by
employing CLAs comprising weaker acidic LAs (e.g., AlMe 3 , AlEt 3 , AlPh 3 )
and weaker LBs (e.g., PMe 3 , PEt 3 ) with less steric hindrance [26]. In 2017,
Takasu et al. utilized the 1,3-di-tert-butylimidazolin-2-ylidene (I
t Bu) LB and a
sterically encumbered methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide)
[MeAl(BHT) 2 ] LA for the exclusive 1,4-addition polymerization of (E,E)-methyl
sorbate (MS) to produce a cyclic polymer [27]. Recently, Zhang, Chen, and coworkers established the first living LPP of MMA via the development of a noninteracting FLP catalyst based on MeAl(BHT) 2 LA and NHO LB [28]. The successful
extension from main-group LA to rare-earth LA for LPP has also been achieved by
Xu and co-workers [29, 30].
Moreover, in the area of LP-mediated RO(C)P, the controlled ROP of L-lactide
(L-LA) and ε-caprolactone (ε-CL) to cyclic poly(co)esters was reported by Amgoune
and Bourissou et al. in 2013 through combining Zn(C 6 F 5 ) 2 LA with an organic base
(e.g., 1,2,2,6,6-pentamethylpiperidine), [31] while in 2015, Dove, Naumann et al.
developed a simple LP based on a metal halide [MgX 2 (X = Cl, Br, I), YCl 3 , AlCl 3 ,
etc.] LA and an organic base [NHCs, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),
etc.] for the ROP of macrolactone ω-pentadecalactone (PDL) [32]. In 2017, Zhang
and Chen et al. disclosed that Al(C 6 F 5 ) 3 /NHO CLA can promote the living ROP of
δ-valerolactone (δ-VL) or ε-CL, [33] while Yang et al. reported the utilization of the
borane/amine LPs for the controlled ROP of N-carboxy anhydrides (NCAs) [34].
LP catalyst based on BEt 3 /LB or Zn(C 6 F 5 ) 2 /LB was also successfully employed
for alternating/regioselective copolymerization of CO 2 (COS and anhydrides) with
epoxides [35–37].
The above achievements and breakthroughs have already been comprehensively
reviewed in our 2018 Chemical Reviews [9]. Nevertheless, over the past 2 years,
a number of creative and insightful contributions to this area continued to emerge,
bringing about the expanding applications of LP in polymerization catalysis. Accordingly, these recent advances in this time frame are highlighted in this chapter, which
287
Since the concept of LPP was first introduced in 2010, it opens up
an unprecedented pathway to effectively polymerizing polar vinyl monomers
(Scheme 8.1b), and ring-opening (co)polymerizing [RO(C)P] cyclic esters and
epoxides (Scheme 8.1c), as demonstrated by its relatively broad monomer scope
(Scheme 8.1d), high activity, control or livingness, and complete chemo- or regioselectivity. In the area of LPP of polar vinyl monomers, Chen et al. developed metal-free
phosphine/borane-based LP catalysts in 2014 for exceptionally active polymerization of biomass-derived γ MMBL [24]. In the same year, the chemoselective polymerization of divinyl acrylic monomers, such as vinyl methacrylate (VMA), allyl
methacrylate (AMA), and 4-vinylbenzyl methacrylate (VBMA), has been achieved
by Lu and co-workers through using Al(C 6 F 5 ) 3 /N-heterocyclic olefin (NHO) CLAs,
which led to soluble polymers bearing pendant vinyl groups for the subsequent
postfunctionalization [25].
In 2016, Rieger et al. realized the relatively controlled polymerization of sterically demanding methacrylates and functionalized monomers, such as furfuryl
methacrylate (FMA), n-butyl methacrylate (
n BuMA), tert-butyl methacrylate (
t BuMA), 4-vinylpyridine (4-VP), N,N-dimethyl acrylamide (DMAA), by
employing CLAs comprising weaker acidic LAs (e.g., AlMe 3 , AlEt 3 , AlPh 3 )
and weaker LBs (e.g., PMe 3 , PEt 3 ) with less steric hindrance [26]. In 2017,
Takasu et al. utilized the 1,3-di-tert-butylimidazolin-2-ylidene (I
t Bu) LB and a
sterically encumbered methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide)
[MeAl(BHT) 2 ] LA for the exclusive 1,4-addition polymerization of (E,E)-methyl
sorbate (MS) to produce a cyclic polymer [27]. Recently, Zhang, Chen, and coworkers established the first living LPP of MMA via the development of a noninteracting FLP catalyst based on MeAl(BHT) 2 LA and NHO LB [28]. The successful
extension from main-group LA to rare-earth LA for LPP has also been achieved by
Xu and co-workers [29, 30].
Moreover, in the area of LP-mediated RO(C)P, the controlled ROP of L-lactide
(L-LA) and ε-caprolactone (ε-CL) to cyclic poly(co)esters was reported by Amgoune
and Bourissou et al. in 2013 through combining Zn(C 6 F 5 ) 2 LA with an organic base
(e.g., 1,2,2,6,6-pentamethylpiperidine), [31] while in 2015, Dove, Naumann et al.
developed a simple LP based on a metal halide [MgX 2 (X = Cl, Br, I), YCl 3 , AlCl 3 ,
etc.] LA and an organic base [NHCs, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),
etc.] for the ROP of macrolactone ω-pentadecalactone (PDL) [32]. In 2017, Zhang
and Chen et al. disclosed that Al(C 6 F 5 ) 3 /NHO CLA can promote the living ROP of
δ-valerolactone (δ-VL) or ε-CL, [33] while Yang et al. reported the utilization of the
borane/amine LPs for the controlled ROP of N-carboxy anhydrides (NCAs) [34].
LP catalyst based on BEt 3 /LB or Zn(C 6 F 5 ) 2 /LB was also successfully employed
for alternating/regioselective copolymerization of CO 2 (COS and anhydrides) with
epoxides [35–37].
The above achievements and breakthroughs have already been comprehensively
reviewed in our 2018 Chemical Reviews [9]. Nevertheless, over the past 2 years,
a number of creative and insightful contributions to this area continued to emerge,
bringing about the expanding applications of LP in polymerization catalysis. Accordingly, these recent advances in this time frame are highlighted in this chapter, which