Lewis pair polymerization by classical and frustrated Lewis pairs: Acid, base and monomer scope and polymerization mechanism
John, Mallory G.
Chen, Eugene You Xian
KAUST DepartmentBiological and Environmental Sciences and Engineering (BESE) Division
KAUST Catalysis Center (KCC)
Physical Sciences and Engineering (PSE) Division
Chemical Science Program
Permanent link to this recordhttp://hdl.handle.net/10754/562005
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AbstractClassical and frustrated Lewis pairs (LPs) of the strong Lewis acid (LA) Al(C 6F 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 6F 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 6F 5) 3, can render highly active and effective LPP of MMA and γ-MMBL; such LBs include phosphines (e.g., P tBu 3), chiral chelating diphosphines, N-heterocyclic carbenes (NHCs), and phosphazene superbases (e.g., P 4- tBu). The P 4- tBu/Al(C 6F 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 γMMBL 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 6F 5) 3 adduct with P tBu 3 and NHC bases, respectively. Kinetic studies have revealed that the MMA polymerization by the tBu 3P/ Al(C 6F 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. This journal is © 2012 The Royal Society of Chemistry.
SponsorsThis work was supported by the National Science Foundation (CHE 1150792) for the study carried out at Colorado State University. MGJ thanks NSF-REU (CHE 1004924) for support of summer research. LC thanks the HPC team of Enea (www.enea.it) for using the ENEA-GRID and the HPC facilities CRESCO (www.cresco.enea.it) in Portici, Italy. We thank Boulder Scientific Co. for the research gifts of B(C<INF>6</INF>F<INF>5</INF>)<INF>3</INF> and [Ph<INF>3</INF>C][B(C<INF>6</INF>F<INF>5</INF>)<INF>4</INF>], and Dr. Brian Newell for the help on the X-ray structural analysis.
PublisherRoyal Society of Chemistry (RSC)