1、ZieglersreactorHow important is the catalytic polymerization of olefin ?Polyolefins: world production: 70 million tons (2003), 130 million tons (2009) 170 Million tons (2012)ca. 50% of the whole production of plastics. Mostlypolymerized by Ziegler-Natta and metallocene catalysts.Tailoring the struct
2、ure of polyolefins is extremely Impor-tant to yield properties which can be used in various conditions. (some examples)Metallocene and late transition metal catalysts are powerful in controlling the structure of the resulting polyolefins.Research in both University and Company: thousandsof patents a
3、nd publications.Ziegler-Natta Catalyst1953: Ziegler, HDPE by AlRn/Transition TiCln at low pressure in the Mlheim Max Planck Institute for Coal Research1954: Natta, Isotactic poly(-olefin) by the same mixture Milan Polytechnic 1954-: world-wide research and commercialization“It is doubtful that it ca
4、n be challenged by any othercatalyst for its versatility.”Huang, J.; Rempel, G. L. Prog. Polym. Sci. 1995, 20, 459-526The year when Staudinger was awarded Nobel prizeZiegler-Natta Catalyst“Ziegler Natta catalysts became prominent in a specialperiod in the history of polymer science, a period that no
5、tonly produced many new commercial polymers but alsoenhanced our basic knowledge of polymer properties andstructure as well as polymerization processes”.Boor, J. Ziegler-Natta Catalysts and Polymerizations, p.1 Academic Press,New York, 1979.Nobel Prize for Ziegler-Natta1963:“Nature synthesizes many
6、stereoregular polymers, for example, cellulose and rubber. This ability has so far thought to be a monopoly of Nature operating with biocatalysts known as enzymes. But now professor Natta has broken this monopoly”.Nobel e-Museum: http:/www.nobel.se/chemistryZiegler-Natta catalystTwo components:Compo
7、unds of group IV-VIII transition metal: TiCl4, TiR2Cl, VCl4, ZrCl4Compounds of group I-III metal: AlEt3, Al-i-Bu3, AlEt2Cl, AlEtCl2, AlEt2ORThe third component: electron donors as reactivity promoterAmine, ether, etc.MgCl2 Supported catalystThe early situation: colored products due to metal residues
8、, removal of solvents.Breakthrough in catalyst engineering:Z.-N./ MgCl2 with phthalates in 1960sZ.-N./ MgCl2 with diethers in 1990sAdvantages: very high activities; solvent free; controlled morphology of the final PP; possible for in-situ polymer alloysKey point: well-defined microporous in catalyst
9、.MgCl2 supported catalystBasell Reactor Granule Technology:BASF-Shell:StereospecificityConsider a monosubstituted ethylene: CH2=CHRThe polymerization leads to the formation of stereocenterat the tertiary carbons:Consider a fully extended planar zigzag conformation,two different configurations are po
10、ssible for each stereocenter. Note: it cannot tell which is R and which isS because we do not know the length at two sides of thecenter. They should be so and we name them in arbitraryStereospecificityisotacticsyndiotacticatacticStereospecific or stereoselective polymerizationMechanismThe Cossee mod
11、el:Two steps: 1) complexation; 2) migratory insertion.The growing terminus flips from site to siteMechanismThe trigger model by Ystenes: TerminationZiegler-Natta catalyst: living active site (lifetime hours)but non-living polymer chains (seconds to minutes).Therefore, each active site produces many
12、polymer chains.Termination by hydride to metal center:or to monomer:Termination by hydrogen (important in industry for MW control):Nattas Nobel prize lectureNattas Nobel prize lecture1963: The first highly stereoregular isotactic polymers were obtained in the presence of heterogeneous catalyst; howe
13、ver, it soon became clear that the heterogeneity of the catalyst is an essential factor for the polymerization of alphatic olefins to isotactic polymers, The vision was proved to be wrong after the discovery of metallocene catalyst.Chronicle of the metallocene catalystChronicle of the metallocene ca
14、talyst in olefin polymerization in olefin polymerization1952, Determination of ferrocene structure;1955, Metallocene as a component of Ziegler-Natta catalyst; Cp2TiCl2/AlR2Cl, homogenous Polymerization reactivity low: 104 g PE /(mol Ti hatm)1973, Water effect: increasing activity;1975, Kaminsky, tre
15、mendous increase in activity at larger amount of water; Cp2TiR2/AlR3 (halogen-free)1976, Inspired by AlMe3/H2O, (BASF and Kaminsky, DE patent) MAO (methylaluminoxane) was prepared; Cp2TiMe2/AlMe3/MAO: 107 g PE /(mol Ti hatm) but low stereospecificity.1982, Synthesis of ansa metallocene with C2-symme
16、try by Brintzinger;1984, isotactic polypropylene.Why study metallocene ?1. Homogenous solution;2. Single site active center;3. Lower polydispersity of the product;4. Uniform incorporation of -olefin comonomers;5. Improved properties of the product: clarity, strength, etc.The feature of metallocene c
17、atalysts: originally inorder to better understand the mechansim of Z-N catalystMetallocene structureBrintzinger: ansa-metallocene, the idea was once heavily criticized by his colleagues due to Nattas vision.Traditional:ConstrainedgeometrycatalystMetallocenesZirconocenes: mostly for ethene polymeriza
18、tionZrClClXR2R1R2R1R3R3RXMClClXZrClClX(Ind)2ZrCl2X(Flu)(Cp)ZrCl2X(Flu)2ZrCl2What is MAO: the cocatalystMethylalumoxanes:Possible structure:networklinearcyclicThe exact structureis still unclear!What is MAO: the cocatalystFunction of the cocatalystMClClCpCpMMeMeCpCpMMeClCpCpMAOMAO2. The formation of
19、the cationic metallocene complex:MMeMeCpCpCpMMeCpMeAlMAOMMeCpCpMeAlMAO+3. Reactivation of inactive complexes formed by Hydrogen transfer reactions.1. Alkylation of metallocene:Feature of ethene polymerizationMW: 104 106 g/mol, depends on ratio of metallocene to monomer, temperature and the additiona
20、l hydrogen.MWD: 3Mixing of different metallocene: bimodal distributionActivity: much higher than Z.-N. catalystTurnover:LLDPE: copolymerization of ethene and -olefin, uniform distributionDependence of PE morphology on SCBRegiospecificity: enchainment Head to tail, head to head, tail to tailStereospe
21、cificity: stereoregularity, tacticity Atactic, syndiotactic, isotactic.Two mechanisms of stereocontrol:1. Catalytic site control: errors are corrected2. Chain end control: stereoblock 1. Chain propagation:2. Chain termination and transfer:LnMRHLnM HR+ZrCl2Me2CZrMe2XZrClClMAOYTiZrFeRuCo NiPdNickel ca
22、talyst are used in the SHOP (Shell High Olefin Process) for the production of high olefin from ethene. (ethene oligomerization)Late transition metal catalyzed olefin polymerizationNNRRMBrBrM = Ni or PdNNNiPNNNiHPPropagation bymigratory insertionNNNiHPBeta-hydrideeliminationchain transfer byethylene
23、displacement NNNiH+Pnew chainpropagationPropagation and chain transfer:Mechanism of late transition metal catalyzed polymerizationChain transfer is greatlyretarded by the extremesteric bulk of the diimine ligand (the approach of ethene is blocked by the bulky substituents).Branching by chain walking:Producing methyl branchesProducing longer branchesNNNiHPchainmigrationNNNiPNNNiHpropagationpropagationPNNNiPFeature of the catalyst and the productPP melting point vs. regularityLiving polymerization by late transition metal
