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Olefin polymerization In an attempted distillation of ethyl lithium, Ziegler observed α-olefins. He reasoned that the following process was occurring involving partial decomposition of C2H5Li and formation of ethylene : β-H elimination insertion Li + LiH Li Organoaluminum compounds such as Et2AlH displayed even higher activities towards ethylene resulting in higher aluminum alkyls that could be readily hydrolyzed to produce higher alcohols. excess Al H Al n Al m 1 Ziegler-Natta polymerization Ziegler wondered what other metals may do. An exploration of this curiosity led to the TiCl3/Et2AlCl catalyzed Zeigler-Natta polymerization (Nobel Prize, 1963) which is currently used commercially to produce ~15 million tons of polyethylene and polypropylene annually. Ziegler's original process for ethylene polymerization: TiCl4 / AlR3 n ethylene polyethylene Natta extended this to propylene polymerization. He found that a greater stereospecificity could be achieved by using crystalline TiCl3, with the amount of the desired isotactic polypropylene increased to 90%. TiCl3 / AlR3 n propylene Ziegler, Angew. Chem. 1955, 67, 541. Natta, Angew. Chem. 1956, 68, 393. polypropylene 2 General features of TM-catalyzed polymerization Oligomer, n = 3 – 100 Polymer, n > 1000 Termination via β-H elimination n LnM R R R = H, alkyl R H LnM Ln M R insertion n Propagation via insertion R LnM R LnM R R LnM LnM insertion 3 Isotactic polypropylene Polymerization of alkenes is of great industrial importance, and one key issue is the production of stereoregular polymers. The most important isotactic polypropylene has a higher melting point, density and tensile strength than the soft and elastic atactic form. Also of commercial importance is syndiotactic polypropylene. isotactic: stereoregular material, long sequences having the same stereochemistry at adjacent carbons. Physical properties: crystalline with a melting temperature ~165°C. syndiotactic: long sequences having the opposite stereochemistry at adjacent carbons. Physical properties: semi-crystalline with a melting temperature ~100°C. atactic: stereo-random polymer that behaves as an amorphous gum elastomer. 4 Cossee mechanism for polypropylene According to the Cossee mechanism, propagation of the polymer occurs at the Ti centers. The role of the alkyl aluminum species is that of an initiator, by alkylating TiCl3. The δ-form of TiCl3 is stereoselective below 100°C. In the more recent form of the catalyst (used since the 1980s), TiCl4 is supported on MgCl2, and AlEt3 may be used for alkylation. Cl Ti Cl Cl Cl Polymer Ti Cl Cl Ti disfavored Me H Ti Cl Cl Cl Polymer Ti Cl Cl Ti Me HMe H Ti Cl Cl Ti Cl Cl Ti Polymer favored Representation of a Ti center on the edge of a TiCl3 crystal. The growing polymer occupies one open site. Propene preferentially binds with the methyl substituent anti to the polymer chain. Cossee TL 1960 (17) 17. Brintzinger Angewandte 1995 (34) 1143. 5 Metallocene polymerization catalysts The development of Ziegler-Natta catalysts has, since the 1980’s, included the use of metallocenes – complexes of early transition metals with Cp ligands or ligands derived from cyclopentadiene. Ti Cl Cl n polyethylene Et2AlCl ethylene Natta JACS 1957 (79) 2975. Breslow JACS 1957 (79) 5072. Mechanism: Ti Cl Cl Et2AlCl Ti Cl Al Cl Cl Cl Ti Al Cl Cl alkylation Ti Cl migratory insertion Unlike the heterogeneous Ziegler-Natta polymerization catalysts, the Ti catalysts are ineffective for propylene polymerization. Cl Al Cl Cl Ti H Ti H Cl n n propagation Al Cl Cl Ti n Breslow JACS 1959 (81) 81. Al Cl Cl β-H elimination H Cl Al Cl Cl n 6 Metallocene polymerization catalysts Trace amounts of water cause a significant increase in the rates of ethylene polymerization by Cp2TiEtCl / AlEt2Cl system. It was later found that water also activated analogous Zr complexes (which are unreactive in the pure form) to highly active catalysts for both ethylene and propylene polymerization. Zr ethylene n polyethylene X X = Cl or Me AlR3 n atactic polypropylene propylene Hydrolysis of AlMe3 results in the formation of a mixture of oligomeric methylaluminoxanes. Preformed MAO is equally effective as an activator of Cp2ZrMe2 and Cp2ZrCl2 catalysts towards olefin polymerization. Al Me Me Me H2O Me Al O n or MAO Barron JACS 1995 (117) 6465. Me O Al Me Al O O Al Al O Me Me n 7 Activation by MAO It is postulated that the highly Lewis acidic Al centers in MAO "abstract" CH3 resulting in a cationic Zr complex and a [CH3-MAO]− counterion that may be weakly associated with the metal. Zr Cl Cl MAO polymer β-H elimination H3 C Zr Zr Me Me MAO H3 C Al(MAO) Zr Al(MAO) Me n propagation H3 C Zr CH3 Al(MAO) Al(MAO) Zr Me insertion Kaminsky ACIEE 1976 (15) 630. Kaminsky ACIEE 1980 (19) 390. Brintzinger ACIEE 1995 (34) 1143. 8 Cationic metallocene catalysts [Cp2ZrMe(THF)]+ was one of the first cationic complexes used for ethylene polymerization. The low polymerization activity was attributed to the coordinated THF which competes with ethylene for binding. Cl Zr Me AgBPh4 (1 eq) Zr THF O Me BPh4 Jordan JACS 1986 (108) 7410. First well-characterized cationic zirconocene catalyst capable of propylene polymerization at high rates: Zr-C 2.25, 2.56 Å Zr Me Me B(C6F5)3 (1 eq) Zr C6H6 Marks JACS 1991 (113) 3623. H3 C B(C6F5)3 Me 9 C2-symmetrical metallocene catalysts Brintzinger's C2-symmetric catalysts: 1-Naphthyl Me Cl Zr Cl Ethylene-bis-(indenyl) zirconium dichloride MAO 50 ºC Cl Zr Cl Ethylene-bis(tetrahydroindenyl) zirconium dichloride MAO 60 ºC Me2Si Me Cl Zr Cl Naphthyl-1 MAO 50 ºC n 78% isotacticity,188 activity (kg pol/mol Zr·h), Mw=24,000. 91% isotacticity, 7700 activity (kg pol/mol Zr·h), Mw = 12,000. >99% isotacticity, 875 activity (kg pol/mol Zr·h), Mw = 920,000. Brintzinger ACIEE 1985 (6) 507. Paulus OM 1994 (13) 954. Paulus OM 1994 (13) 954. 10 C2-symmetrical metallocene catalysts Proposed model for isospecific polymerization: The C2-symmetrical ligand controls the stereochemistry of monomer addition. Olefin binds with the Me-substituent anti to the growing polymer chain. Stabilizing C-H agostic interaction is thought to rigidify the TS thereby increasing the stereospecificity of insertion. polymer is in open quadrant Polym Polym Zr H H H Me is down in open quadrant H Zr∗ H Zr H H n C-H agostic TS The meso form of the catalyst gives atactic polypropylene Ewan JACS 1984 (106) 6355. Grubbs Acc. Chem. Res. 1996 (29) 85. Coates Chem. Rev. 2000 (100) 1223. ∗ Cl Zr Cl 11 Torsional isomers Removal of the bridge between the indenyl ligands allows rotation about the metal ligand bond and formation of C2- and meso- isomers, which have similar energies. The phenyl substituents were incorporated to slow down the ligand rotation. The result of this was production of an isotactic-atactic stereoblock copolymer. R Zr C2 -isomer isotactic block, < 28 % Waymouth Science 1995 267 217. R Zr meso-isomer atactic block 12 Shell higher olefin process - SHOP SHOP process is operated on a 1 million ton capacity and constitutes one of the largest applications of homogeneous catalysis. Ph Ph Ph P Ni PPh3 Ph O activity: 6,000 mol ethylene/ mol Ni ethylene, 40 atm Mechanism: n 50 ºC, toluene Ph Ph P Ph Ni PPh3 O Ph -PPh3 Ph Ph P Ph Ni O Ph insertion Ph Ph P H Ni O Ph β-H elimination Keim and Kruger Angewandte 1978 (17) 466. Keim Angewandte 1990 (29) 235. H Ph Ph Ph Ph P Ni H O Ph Ph Ph P Ni O Ph n Ph Ph P Ni O Ph Ph Ph P Ni O Ph β-H elimination Ph Ph P H Ni O Ph associative displacement 99% linear 98% α-olefins up to C30 H propagation n 13 Brookhart’s cationic Ni(II) catalyst The rate of associative displacement leading to chain termination is retarded in these systems by the steric bulk of the ligand which blocks the axial positions above and below the plane of the Ni complex, and thus hinders addition of C2H4 and formation of 5-coordinate species. N N Ni Br 71 methyl branches per 1000 carbon atoms Br MAO ethylene, 1 atm n 25 ºC, toluene m branched polyethylene Mw = 410,000 Catalyst activation: Ar N N Ar Ni Br Br MAO Ar N N Ar Ni Ar N Me Brookhart JACS 1995 (117) 6414. Brookhart Chem. Rev. 2000 (100) 1169. N Ar + Ni Me 14 Mechanism of branching catalyst activation N N Ni MAO Br N N Br Ni N N CH3 Ni insertion propagation Linear high Mw polymer N N CH3 H N Ni N N Ni N Ni branched high Mw polymer n branching β-H elimination N N re-insertion Ni propagation N H n N Ni N n N H Ni m n 15 Grubbs’ neutral Ni(II) catalyst Unlike heterogeneous Ziegler-Natta and homogeneous cationic metallocene polymerization catalysts (poisoned by O, N, and S heteroatom functionality), the neutral Ni(II) catalyst is highly tolerant of oxygenated functionality. Olefin polymerizations can be run in the presence of ether, ketone, and ester additives without significantly inhibiting catalyst activity. O Ph Ni 3 N PPh Pri Pri ethylene, 7 atm 10 ºC, toluene n linear PE Mw > 250,000 activity: 3,700 kg PE/h⋅mol Ni Grubbs Science 2000 (287) 460. 16 Proposed catalytic cycle O Ph Ni 3 N PPh Pri The rate of associative displacement of the polymer leading to chain termination is retarded in these systems (as in the Brookhart system) by the steric bulk of the ligand, which blocks the axial positions above and below the plane of the Ni complex. Pri N O Ni Ph N PPh3 O -PPh3 N O associative displacement N O Ni Ni Ph O Ni O O n O Ni Ph Ph N H N N H insertion H β-H elimination N Ni H Ni propagation H Ni n 17