M.C. White, Chem 153 EMPolymerization-297- Week of november 18. 2002 Ziegler natta polymerization What has guided my research has been solely the wish to do something that gave me jo that is a joy from finding, somehow or somewhere, something really novel.At least at the utset, the only thing of value aimed for is an accretion in knowledge, rather than new In an attempted distillation of ethyllithium, Ziegler such as Et2AlH displayed even higher activities observed ethylene and higher a-olefins. He reasoned towards ethylene n higher aluminum alkyls that could be readily that the following process was occuring hydrolyzed to pre alcohols - + Lih CAH tPC Ziegler found that traces of Ni salts (accidently Lih incorporated during cleaning the reactor)resulted only in butene and r,Alh AH+ Eisch Chem. Edu. 1983(60)1009 If traces of Ni salts could make such a dramatic impact on the The stereochemistry of polypropylene significantly influences its physical properties. Isotactic course of ethylene oligermerizations, Ziegler wondered what other polymers are the most useful commercially with such physical properties as high tensile etals may do. An exploration of this curiosity led to the strength and high melting points (165C) Cl/Et AlCI catalyzed Zeigler Natta polymerization(Nobel Prize, 63)which is currently used commercially to produce 15 million tons of polyethylene and polypropylene annually Ziegler's original process for ethylene polymerization: 人人人人人人 TICI/AIR3 polyethylene stereochemistry at adjacent carbons bons Physical properties line with a melting Ziegler Angew. Chem. 1955 (67)54 Natta extends this to propylene polymerization. He finds that by regularity of the surface of the heterogeneous catalyst is increased. This results in a greater specificity in polymerization with the amount of desired isotactic polypropylene inreasing from 40% to 90%. atactic: stereorandom polymer that behaves as an amorphous 人 For other polymer tacticities see: Coates Chem. Rev. 2000(100)1223 Natta Angew. Chem. 1956(68)393
M.C. White, Chem 153 EM Polymerization -297- Week of November 18, 2002 Ziegler Natta Polymerization Natta Angew. Chem. 1956 (68) 393. "What has guided my research has been solely the wish to do something that gave me joy, that is a joy from finding, somehow or somewhere, something really novel...At least at the outset, the only thing of value aimed for is an accretion in knowledge, rather than new applications." Karl Ziegler. Li ∆ LiH Li ∆ LiH Al H 100oC Al Al Al Al H If traces of Ni salts could make such a dramatic impact on the course of ethylene oligermerizations, 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 polyethylene Ziegler Angew. Chem. 1955 (67) 541. Natta extends this to propylene polymerization. He finds that by using crystalline TiCl3, the regularity of the surface of the heterogeneous catalyst is increased. This results in a greater stereospecificity in polymerization with the amount of desired isotactic polypropylene inreasing from 40% to 90%. TiCl3/AlR3 n polypropylene The stereochemistry of polypropylene significantly influences its physical properties. Isotactic polymers are the most useful commercially with such physical properties as high tensile strength and high melting points (~165oC). In an attempted distillation of ethyllithium, Ziegler observed ethylene and higher α-olefins. He reasoned that the following process was occuring: + β-hydride elimination propagation β-hydride elimination + 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. Ziegler found that traces of Ni salts (accidently incorporated during cleaning the reactor) resulted only in butene and R2AlH. Ni salts + Eisch J. Chem. Edu. 1983 (60) 1009. isotactic: stereoregular material, long sequences having the same stereochemistry at adjacent carbons. Physical properties: crystalline thermoplastic. syndiotactic: long sequences having the opposite stereochemistry at adjacent carbons. Physical properties: semicrystalline with a melting temperature ~ 100oC. atactic: stereorandom polymer that behaves as an amorphous gum elastomer. For other polymer tacticities see: Coates Chem. Rev. 2000 (100) 1223
M.C. White, Chem 153 EMPolymerization-298- Week of november 18. 2002 Cossee mechanism for Ziegler Natta polymerizations According to the Cossee mechanism, propagation of the polymer occurs exclusively at the Ti center. The role of the alkyl aluminum pecies is thought to be that of initiator by alkylating the TiCI olefin coordination CHT cHI Representation of a TiCl lattice with an cis-carbometalation via a concerted 4-membered TS open coordination site on the surface Cossee190(17)12 Cossee stereochemical model for isotactic polypropylene formation Polvmer Polymer CHT si-face re-face Representation of a stereogenic Ti center on the edge of a chiral TiCl crystal. The growing polymer occupies the open quadrant. The olefin preferentially binds via its si-face placing its methyl substituent trans to the bulky polymer chain. Modem MgCl2-supported Ziegler Natta Cossee tz1960(17)17 catalysts are highly stereoselective resulting in formation of essentially Brintzinger ACIEE 1995(34)1143. only the isotactic polymer
M.C. White, Chem 153 EM Polymerization -298- Week of November 18, 2002 Cossee mechanism for Ziegler Natta polymerizations Cl Cl Ti Cl Cl Ti Ti Cl Cl Cl Ti Cl Cl Ti Ti Cl Cl Cl Ti Cl Cl Ti Ti Cl Cl Cl Ti Cl Cl Ti Ti Cl Representation of a TiCl3 lattice with an open coordination site on the surface According to the Cossee mechanism, propagation of the polymer occurs exclusively at the Ti center. The role of the alkyl aluminum species is thought to be that of initiator by alkylating the TiCl3. olefin coordination cis-carbometalation via a concerted 4-membered TS. Cossee TL 1960 (17) 12. Cossee mechanism for Ziegler Natta heterogeneous polymerization. Cossee stereochemical model for isotactic polypropylene formation: Cl Cl Ti Cl Cl Ti Ti Cl Polymer si-face favored Cl Cl Ti Cl Cl Ti Ti Cl Polymer re-face disfavored Representation of a stereogenic Ti center on the edge of a chiral TiCl3 crystal. The growing polymer occupies the open quadrant. The olefin preferentially binds via its si-face placing its methyl substituent trans to the bulky polymer chain. Modern MgCl2-supported Ziegler Natta catalysts are highly stereoselective resulting in formation of essentially only the isotactic polymer. Cossee TL 1960 (17) 17. Brintzinger ACIEE 1995 (34) 1143
M.C. White, Chem 153 EM Polymerization -299- Week of november 18. 2002 Metallocenes as homogeneous polymerization catalysts No reaction is observed in the absence of Et2 AICl or Et AL. Both EtAlCl and Et Al alone produce only oligomers. Unlike the heterogeneous Ziegler-Natta polymerization catalysts, these catalysts are ineffective at polymerizing a-olefins( propylene) EtAICI polyethylene Natta JACS 1957(79)2975 Breslow JACS 1957(79)5072 Breslow's proposed mechanism cr 8. CI EtAICl T代 Insertion Polarization of the Ti-CI bond by the Lewis acidic Al center promotes H Breslow JACS 1959(81)81
M.C. White, Chem 153 EM Polymerization -299- Week of November 18, 2002 Metallocenes as homogeneous polymerization catalysts TiIV Cl Cl Et2AlCl n polyethylene No reaction is observed in the absence of Et2AlCl or Et3Al. Both Et2AlCl and Et3Al alone produce only oligomers. Unlike the heterogeneous Ziegler-Natta polymerization catalysts, these catalysts are ineffective at polymerizing α-olefins (propylene). Natta JACS 1957 (79) 2975. Breslow JACS 1957 (79) 5072. Breslow's proposed mechanism: Breslow JACS 1959 (81) 81. TiIV Cl Al Cl δ+ δ- TiIV Cl Cl Et2AlCl σ-bond metathesis? Cl TiIV Cl Al Cl Cl TiIV Cl Al Cl δ+ δ- Cl cis- migratory insertion propagation TiIV Cl Al Cl Cl P β-hydride H elimination P TiIV H Cl Al Cl Cl TiIV H Cl Al Cl δ+ Cl (termination) Polarization of the Ti-Cl bond by the Lewis acidic Al center promotes ethylene coordination/insertion. P
M.C. White, Chem 153 EM Polymerization-300- Week of november 18. 2002 Activation by MA0 considered a poison arly transition metal Activation by MAO sts. Trace amounts of water were reported to cause a in the rates of ethylene polymerization by Dimetyl=iconium complex Cp2TiEtCI/AlEtCl2 system. It was later found that water activated It is postulated that the highly Lewis acidic Al centers in MAO"abstract"CH3-resulting analogous Zr complexes which were typically unreactive towards even in a cationic Zr complex and a weakly coordinating(CH3-MAO) counterion that may or ethylene polymerizations to highly active catalysts for both ethylene and may not be weakly associated with the metal H3CAI(MAO Me mAo RiAl H3G-AI(MAO +H,o atactic polypropylene Me H3C一 AlMAC) In sit formation of mao(methylalumino oxane). Hydrolysis of AIMe3 by water results in the formation of a mixture of oligomeric aluminoxanes(exact compositions and structures are still not known) polypropylene Preformed MAO is equally effective as an activator of Cp2ZrMer and Cp2ZrCl2 catalysts towards olefin polymerizations MAO MAO nAMEs Kaminsky ACIEE 1976(15)630 MAo(methlylalumino oxane Kaminsky ACIEE 1980( 19)390 Barron JACS 1995(117)6465 Brintzinger ACIEE 1995(34)1143
M.C. White, Chem 153 EM Polymerization -300- Week of November 18, 2002 Activation by MAO In situ formation of MAO (methylalumino oxane). Hydrolysis of AlMe3 by water results in the formation of a mixture of oligomeric aluminoxanes (exact compositions and structures are still not known). Preformed MAO is equally effective as an activator of Cp2ZrMe2 and Cp2ZrCl2 catalysts towards olefin polymerizations. nAlMe3 nH2O Al Me O n O Al Al O O Al Al O Me Me Me Me n MAO (methlylalumino oxane) Barron JACS 1995 (117) 6465. Activation by MAO: ZrIV Me Me It is postulated that the highly Lewis acidic Al centers in MAO "abstract" CH3_ resulting in a cationic Zr complex and a weakly coordinating (CH3-MAO)- counterion that may or may not be weakly associated with the metal. MAO ZrI V Me H3C Al(MAO) ZrI V Me H3C Al(MAO) ZrI V H3C Al(MAO) δ- δ+ δ- δ+ polypropylene Dichlorozirconium complex Dimethylzirconium complex ZrI V Cl Cl MAO ZrI V Me Me ligand exchange (via σ-bond metathesis?) MAO as above Kaminsky ACIEE 1976 (15) 630. Kaminsky ACIEE 1980 (19) 390. Brintzinger ACIEE 1995 (34) 1143. atactic polypropylene ZrIV R' R' R3Al + H2O n R' = Me or Cl No polymerization activity Water is generally considered a poison for early transition metal polymerization catalysts. Trace amounts of water were reported to cause a significant increase in the rates of ethylene polymerization by Cp2TiEtCl/AlEtCl2 system. It was later found that water activated analogous Zr complexes which were typically unreactive towards even ethylene polymerizations to highly active catalysts for both ethylene and propylene polymerization. or or n
M.C. White, Chem 153 EMPolymerization-301 Week of november 18. 2002 Cationic metallocene catalysts First preformed and spectroscopically characterized cationic complex capable of ethylene polymerization. This work supports the proposal that cationic Zr and Ti complexes formed upon olefin inding are the active polymerization catalysts. The low polymerization activity was attributed to the coordinated THF which competes with ethylene for binding -BPh CH3 AgBPhg(l eq) THE H3 Jordan JACS 1986(108)7410 First well-characterized cationic zirconocene catalyst capable of propylene polymerization at high rates. H3C-B(C6 F5)3 B(C6F5)(1 eq). Marks JACS 1991(113)3623
M.C. White, Chem 153 EM Polymerization -301- Week of November 18, 2002 Cationic metallocene catalysts ZrI V CH3 CH3 AgBPh4 (1 eq) THF ZrIV CH3 O BPh4 First preformed and spectroscopically characterized cationic complex capable of ethylene polymerization. This work supports the proposal that cationic Zr and Ti complexes formed upon olefin binding are the active polymerization catalysts.The low polymerization activity was attributed to the coordinated THF which competes with ethylene for binding. Jordan JACS 1986 (108) 7410. First well-characterized cationic zirconocene catalyst capable of propylene polymerization at high rates. ZrIV CH3 CH3 B(C6F5)3 (1 eq) C6H6 ZrI V CH3 H3C B(C6F5)3 Marks JACS 1991 (113) 3623