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ansa ‐Rare‐Earth‐Metal Catalysts for Rapid and Stereoselective Polymerization of Renewable Methylene Methylbutyrolactones
Author(s) -
Hu Yangjian,
Miyake Garret M.,
Wang Baoli,
Cui Dongmei,
Chen Eugene Y.X.
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201102677
Subject(s) - polymerization , methylene , monomer , chemistry , carbene , ligand (biochemistry) , polymer chemistry , catalysis , ethylene , ring opening polymerization , metal , copolymer , medicinal chemistry , organic chemistry , polymer , biochemistry , receptor
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.