8 Lewis Acid−Base Pairs for Polymerization Catalysis …
285
Mg(HMDS) 2 Magnesium bis(hexamethyldisilazide) (Scheme 8.15)
TOF
Turnover frequency = moles of substrate (monomer) consumed per
mole of catalyst (initiator) per hour
8.1 Introduction
The development of powerful catalysis or methodologies for new or more sustainable, efficient, controlled, and selective polymerizations is a long-standing scientific challenge in the field of polymer synthesis, which brings about the advanced
polymeric materials with notable properties and specific functions [1–5]. Lewis
pair polymerization (LPP), a recently emerged polymerization methodology, has
attracted an increasing level of interest and achieved remarkable successes in polymerizing heteroatom-containing polar monomers [6–9]. Inspired by the seminal work
of Stephan and Erker on “frustrated Lewis pair (FLP)” chemistry for the cooperative activation of small molecules, [10–14] Chen and co-workers reported the first
application of FLP chemistry for polymer synthesis in 2010 through uncovering
highly efficient polymerization of polar vinyl monomers, such as methyl methacrylate (MMA), α-methylene-γ -butyrolactone (MBL), and γ -methyl-α-methylene-γ -
butyrolactone (γ MMBL), by a Lewis pair (LP) catalyst comprising a superacidic
and sterically encumbered Al(C 6 F 5 ) 3 Lewis acid (LA) and a sterically encumbered
Lewis base (LB) [e.g., P
t Bu 3 , N-heterocyclic carbenes (NHC)] [15].
According to the degree of interaction between LA and LB, LP catalysts
utilized in LPPs can be classified into FLPs, interacting LPs (ILPs), and classical Lewis (acid−base) adducts (CLAs) (Scheme 8.1a). It should be noted that
only those CLAs and ILPs, which can dissociate into the “frustrated” free LA
+ LB form in the presence of a suitable solvent or monomer, are capable of
providing sufficient unquenched reactivity for promoting efficient polymerization. In a typical LPP, both LB and LA sites of the LP catalyst synergistically/cooperatively participate in the chain initiation process to generate active
zwitterionic intermediates for the following chain-growth process that consists
of the repeating fundamental steps of nucleophilic attack of zwitterionic intermediates with LA-activated monomers and the recapture of the LA from the
growing polymer chain by incoming monomer via coordination (Schemes 8.1b, c).
Compared to conventional polymerization techniques, LPP has shown several unique
advantages or intriguing opportunities: (1) the synergy and cooperativity of both LA
and LB, which makes the LPPs significantly different from the classic anionic [16,
17] and zwitterionic [18–20] polymerizations that are typically initiated by a negative
charge (Nu
– ) and an LB or an LA, respectively, can render the polymerization with
enhanced activity or allow the polymerization that would be generally inaccessible
by an LA or LB alone. (2) Readily available LAs and LBs, as well as easily adjustable
Lewis acidity, basicity, and steric effects of LPs afford a straightforward approach to
285
Mg(HMDS) 2 Magnesium bis(hexamethyldisilazide) (Scheme 8.15)
TOF
Turnover frequency = moles of substrate (monomer) consumed per
mole of catalyst (initiator) per hour
8.1 Introduction
The development of powerful catalysis or methodologies for new or more sustainable, efficient, controlled, and selective polymerizations is a long-standing scientific challenge in the field of polymer synthesis, which brings about the advanced
polymeric materials with notable properties and specific functions [1–5]. Lewis
pair polymerization (LPP), a recently emerged polymerization methodology, has
attracted an increasing level of interest and achieved remarkable successes in polymerizing heteroatom-containing polar monomers [6–9]. Inspired by the seminal work
of Stephan and Erker on “frustrated Lewis pair (FLP)” chemistry for the cooperative activation of small molecules, [10–14] Chen and co-workers reported the first
application of FLP chemistry for polymer synthesis in 2010 through uncovering
highly efficient polymerization of polar vinyl monomers, such as methyl methacrylate (MMA), α-methylene-γ -butyrolactone (MBL), and γ -methyl-α-methylene-γ -
butyrolactone (γ MMBL), by a Lewis pair (LP) catalyst comprising a superacidic
and sterically encumbered Al(C 6 F 5 ) 3 Lewis acid (LA) and a sterically encumbered
Lewis base (LB) [e.g., P
t Bu 3 , N-heterocyclic carbenes (NHC)] [15].
According to the degree of interaction between LA and LB, LP catalysts
utilized in LPPs can be classified into FLPs, interacting LPs (ILPs), and classical Lewis (acid−base) adducts (CLAs) (Scheme 8.1a). It should be noted that
only those CLAs and ILPs, which can dissociate into the “frustrated” free LA
+ LB form in the presence of a suitable solvent or monomer, are capable of
providing sufficient unquenched reactivity for promoting efficient polymerization. In a typical LPP, both LB and LA sites of the LP catalyst synergistically/cooperatively participate in the chain initiation process to generate active
zwitterionic intermediates for the following chain-growth process that consists
of the repeating fundamental steps of nucleophilic attack of zwitterionic intermediates with LA-activated monomers and the recapture of the LA from the
growing polymer chain by incoming monomer via coordination (Schemes 8.1b, c).
Compared to conventional polymerization techniques, LPP has shown several unique
advantages or intriguing opportunities: (1) the synergy and cooperativity of both LA
and LB, which makes the LPPs significantly different from the classic anionic [16,
17] and zwitterionic [18–20] polymerizations that are typically initiated by a negative
charge (Nu
– ) and an LB or an LA, respectively, can render the polymerization with
enhanced activity or allow the polymerization that would be generally inaccessible
by an LA or LB alone. (2) Readily available LAs and LBs, as well as easily adjustable
Lewis acidity, basicity, and steric effects of LPs afford a straightforward approach to