Lewis pair polymerization by classical and frustrated Lewis pairs: acid, base and monomer scope and polymerization mechanism

Abstract

Classical and frustrated Lewis pairs (LPs) of the strong Lewis acid (LA) Al(C(6)F(5))(3) with several Lewis base (LB) classes have been found to exhibit exceptional activity in the Lewis pair polymerization (LPP) of conjugated polar alkenes such as methyl methacrylate (MMA) as well as renewable α-methylene-γ-butyrolactone (MBL) and γ-methyl-α-methylene-γ-butyrolactone (γ-MMBL), leading to high molecular weight polymers, often with narrow molecular weight distributions. This study has investigated a large number of LPs, consisting of 11 LAs as well as 10 achiral and 4 chiral LBs, for LPP of 12 monomers of several different types. Although some more common LAs can also be utilized for LPP, Al(C(6)F(5))(3)-based LPs are far more active and effective than other LA-based LPs. On the other hand, several classes of LBs, when paired with Al(C(6)F(5))(3), can render highly active and effective LPP of MMA and γ-MMBL; such LBs include phosphines (e.g., P(t)Bu(3)), chiral chelating diphosphines, N-heterocyclic carbenes (NHCs), and phosphazene superbases (e.g., P(4)-(t)Bu). The P(4)-(t)Bu/Al(C(6)F(5))(3) pair exhibits the highest activity of the LP series, with a remarkably high turn-over frequency of 9.6 × 10(4) h(-1) (0.125 mol% catalyst, 100% MMA conversion in 30 s, M(n) = 2.12 × 10(5) g mol(-1), PDI = 1.34). The polymers produced by LPs at RT are typically atactic (P(γ)MMBL with ∼47% mr) or syndio-rich (PMMA with ∼70-75% rr), but highly syndiotactic PMMA with rr ∼91% can be produced by chiral or achiral LPs at -78 °C. Mechanistic studies have identified and structurally characterized zwitterionic phosphonium and imidazolium enolaluminates as the active species of the current LPP system, which are formed by the reaction of the monomer·Al(C(6)F(5))(3) adduct with P(t)Bu(3) and NHC bases, respectively. Kinetic studies have revealed that the MMA polymerization by the (t)Bu(3)P/Al(C(6)F(5))(3) pair is zero-order in monomer concentration after an initial induction period, and the polymerization is significantly catalyzed by the LA, thus pointing to a bimetallic, activated monomer propagation mechanism. Computational study on the active species formation as well as the chain initiation and propagation events involved in the LPP of MMA with some of the most representative LPs has added our understanding of fundamental steps of LPP. The main difference between NHC and PR(3) bases is in the energetics of zwitterion formation, with the NHC-based zwitterions being remarkably more stable than the PR(3)-based zwitterions. Comparison of the monometallic and bimetallic mechanisms for MMA addition shows a clear preference for the bimetallic mechanism.

Frustrated Lewis Pair Chemistry: Development and Perspectives

“…These data together with computation work support a bimolecular, activated-monomer propagation mechanism. [178] In a similar fashion, Chen and coworkers have also reported the polymerization of 2-vinyl pyridine and 2-isopropenyl-2-oxazoline using FLPs based on Al(C 6 F 5 ) 3 and NHCs. [179] These are also proposed to form by a bimolecular, activated-monomer polymerization mechanism (Scheme 122).…”

Sustainable Polymers from Biomass‐Derived α‐Methylene‐γ‐Butyrolactones

“…This polymerization is proposed to proceed via zwitterionic phosphonium or imidazolium enolaluminate active propagating species (Fig. 18), and the structures of such active species derived from the stoichiometric reaction of the FLP with MMA (21) and MBL (22) have been isolated, characterized by NMR, and confirmed by X-ray diffraction analysis (53). Intriguingly, although the borane congener B(C 6 F 5 ) 3 can also form analogous zwitterionic species such as t Bu 3 P-CH 2 C(Me)=C(OMe)O-B(C 6 F 5 ) 3 , they are inactive for such polymerization reactions (52).…”

“…Al(C 6 F 5 ) 3 -based Lewis pairs with phosphines and NHCs were found to be highly active for polymerization of MBL and γ MMBL at RT to high MW polymers (52,53). This polymerization is proposed to proceed via zwitterionic phosphonium or imidazolium enolaluminate active propagating species (Fig.…”

“…Free-radical copolymerizations of γ MMBL with MMA and styrene have been investigated in detail through photoinitiation using the pulsed-laser polymerization/size-exclusion chromatography technique (43,44). Recently reported literature for the polymerization of γ MMBL included coordination polymerization with metallocene complexes (36,45), half-sandwich indenyl rare-earth metal dialkyls (46), ansarare-earth metal catalysts (27), and group 4 nonmetallocene benzyl complexes (47); GTP with bifunctional silicon propagators (48) and dinuclear silyliumenolate bifunctional catalysts (49); anionic polymerization with potassium salts (KH/Al(C 6 F 5 ) 3 ) (50); and zwitterionic polymerization with N-heterocyclic carbenes (51) and alane-based classical and frustrated Lewis pairs (52,53). Several patents were filed based on MBL/ γ MMBL/ β MMBL polymerization, copolymerization, and their applications (54-57), for example, as plastic optical fiber materials due to their excellent transparency and heat resistance (58).…”

Sterically Bulky NHC Adducts of GaMe3 and InMe3 for H2 Activation and Lactide Polymerization

“…More recently, the polymerization of polar/unsaturated monomers, including cyclic esters, initiated by simple Al-based Lewis adducts has attracted attention [28,29]. Such a Lewis pair polymerization (LPP) relies on the cooperative activation of the monomer by the Lewis acid/base fragments and may be related to FLP reactivity.…”