No 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.

Under relatively mild conditions (≤140 °C, 1 atm) and in the absence of added acid catalysts typically employed in biomass conversion, cellulose dissolved in certain ionic liquids (ILs) has been converted into water-soluble reducing sugars in high total reducing sugar yield (up to 97%), or directly into the biomass platform chemical 5-hydroxymethyl furfural (HMF) in high conversion (up to 89%) when CrCl2 is added. The combined study of experimental methods and ab initio calculations demonstrates that the significantly increased K w by ILs in the IL−water mixture is responsible for the catalysis seen in the current efficient biomass conversion system without added acid catalysts. The finding that the water in ILs under mild conditions can exhibit high K w values (up to 3 orders of magnitude higher than the pure water under ambient conditions) is significant because such high K w values are typically achievable by the water under harsh high-temperature or subcritical water conditions.

Oxidative activation of group transfer polymerization initiator silyl ketene acetals with a catalytic amount of the olefin polymerization activator Ph 3 CB(C 6 F 5 ) 4 directly affords the first methyl methacrylate addition product, the highly active propagating species that contains both the nucleophilic and electrophilic catalyst sites to promote the controlled methacrylate polymerization via cooperative catalysis.

The living polymerization of conjugated polar alkenes such as methacrylates by a noninteracting, authentic frustrated Lewis pair (FLP) has remained elusive ever since the report on FLP-promoted polymerization in 2010. Here we report that the polymerization of alkyl methacrylates by a FLP system based on a strongly nucleophilic N-heterocyclic olefin (NHO) Lewis base and sterically encumbered but modestly strong Lewis acid MeAl(4-Me-2,6-t Bu 2 -C 6 H 2 O) 2 is not only rapid but also living. This living polymerization was indicated by the formation of a linear, living chain, capped with NHO/H chain ends, without backbiting-derived cyclic chain ends. The true livingness of this FLP-promoted polymerization has been unequivocally verified by five lines of evidence, including the predicted polymer number-average molecular weight (M n , up to 351 kg•mol −1 ) coupled with low dispersity (Đ = 1.05) values; obtained high to quantitative initiation efficiencies; an observed linear increase of polymer M n vs monomer conversion and the monomer-to-initiator ratio; found precision in multiple chain extensions; and formed well-defined diblock and ABA triblock copolymers with narrow molecular weight distributions (Đ = 1.09−1.13), regardless of the comonomer addition order.