ansa‐Rare‐Earth‐Metal Catalysts for Rapid and Stereoselective Polymerization of Renewable Methylene Methylbutyrolactones

Abstract

Two ansa-half-sandwich rare-earth-metal (REM) dialkyl complexes supported by an ethylene-bridged fluorenyl (Flu)-N-heterocyclic carbene (NHC) ligand, [M{C(2)H(4)(η(5)-Flu-κ(1)-NHC)}(CH(2)SiMe(3))(2)] (M=Y, 1; Lu, 2), and a chiral ansa-sandwich samarocene incorporating a C(2) ligand, [Sm(η(5)-C(12)H(8))(2)(thf)(2)] (3), have been investigated for the coordination-addition polymerization of renewable methylene butyrolactones, α-methylene-γ-butyrolactone (MBL) and γ-methyl-α-methylene-γ-butyrolactone ((γ)MMBL). Both ansa-half-sandwich complexes 1 and 2 exhibit exceptional activity for the polymerization of (γ)MMBL at room temperature in dimethylformamide (DMF); with a 0.25 mol% catalyst loading, quantitative monomer conversion can be achieved under 1 min, giving a high turn-over frequency (TOF) of 24,000 h(-1). This TOF value represents a rate enhancement, by a factor of 8, 22, or 2400, over the polymerizations by unbridged samarocene [Sm(Cp*)(2)(thf)(2)] (Cp*=η(5) -pentamethylcyclopentadienyl), by bridged ansa-samarocene 3 with C(2) ligation, or by the corresponding REM trialkyls without the ansa-Flu-NHC ligation, respectively. Complexes 1 and 2 are also highly active for the polymerization of β-methyl-α-methylene-γ-butyrolactone ((β)MMBL), realizing the first example of the metal-mediated coordination polymerization of this monomer and its copolymerization with (γ)MMBL. More remarkably, the resulting P(β)MMBL homopolymer is highly stereoregular (91% mm) and exhibits a high T(g) of 290 °C. In sharp contrast, catalysts 1 and 2 have poor activity and efficiency in the polymerization of the parent MBL or the acyclic analog methyl methacrylate. Polymerization and kinetic studies using the most active catalyst (1) of the series have uncovered characteristics of its (γ)MMBL polymerization and yielded a unimolecular propagation mechanism. A surprising chain-initiation pathway for the polymerization in DMF by 1 has been revealed, and catalytic polymerization in the presence of an organoacid has also been examined.

Rare-Earth Half-Sandwich Dialkyl and Homoleptic Trialkyl Complexes for Rapid and Stereoselective Polymerization of a Conjugated Polar Olefin

“…The assignments of the triads were based on the backbone quaternary carbon resonances, following the literature. 5 Computational Details. Calculations followed the procedures described in our prior publications.…”

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

“…The 1 H nuclear magnetic resonances (NMR) of the polymers were too broad to unravel the tacticity of the polymer, and no T g was reported. A later study showed that the polymer by radical initiation was essentially atactic (27,45). The study of the polymerization kinetics revealed that the rate of polymerization (R p ) followed the kinetic expression R p = k[AIBN] 0.54 [ β MMBL] 1.04 .…”

“…Chemically, MBL can be produced from biomass sugar-based itaconic anhydride (20), whereas γ MMBL can be readily prepared via a two-step process developed by DuPont (21,22) from the biomass-derived levulinic acid, which is produced at 450 tons/year (23). β MMBL is currently prepared by a multistep synthesis (24,25) but can be potentially prepared from condensation of 3-methyl-γbutyrolactone, available from hydrogenation of the biomass-derived itaconic acid (26), with formaldehyde (27). These biomass-derived methylene butyrolactones are of special interest in exploring the outlook of substituting the petroleumbased methacrylate monomers for specialty chemicals production (14,28).…”

“…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).…”

“…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). β MMBL was initially polymerized radically to atactic materials (24) but more recently through metalcatalyzed coordination polymerization to highly stereoregular (isotactic) materials (25,27,45). Stereoselective polymerization of β MMBL into stereo-defect-free P β MMBL by single-site chiral metallocene catalysts (45) that are known to promote stereospecific coordination polymerization of polar vinyl monomers (59) has also been reported.…”