Coordination Polymerization Ziegler Natta Processes

Stereoregular Polymerization Cationic Initiation of Vinyl Ethers Schildknecht et al. Ind. Eng. Chem. 39, 180, (1947) Isotactic vinyl ether

Stereoregular Polymerization Anionic Polymerization of Methyl Methacrylate, H. Yuki, K. Hatada, K.Ohta, and Y. Okamoto, J. Macromol. Sci. A9, 983 (1975) Isotactic Syndiotactic

POLYETHYLENE (LDPE) Molecular Weights: 20,000-100,000; MWD = 3-20 density = 0.91-0.93 g/cm3 Highly branched structure—both long and short chain branches Tm ~ 105 C, X’linity ~ 40% 15-30 Methyl groups/1000 C atoms Applications: Packaging Film, wire and cable coating, toys, flexible bottles, housewares, coatings

Ziegler’s Discovery 1953 K. Ziegler, E. Holzkamp, H. Breil and H. Martin Angew. Chemie 67, 426, 541 (1955); 76, 545 (1964). + Ni(AcAc) Same result + Cr(AcAc) White Ppt. (Not reported by Holzkamp) + Zr(AcAc) White Ppt. (Eureka! reported by Breil)

Polypropylene (atactic) Formation of allyl radicals via chain transfer limits achievable molecular weights for all a-olefins

Natta’s Discovery Isotactic Syndiotactic 1954 Guilio Natta, P. Pino, P. Corradini, and F. Danusso J. Am. Chem. Soc. 77, 1708 (1955) Crystallographic Data on PP J. Polym. Sci. 16, 143 (1955) Polymerization described in French Isotactic Syndiotactic Ziegler and Natta awarded Nobel Prize in 1963

Polypropylene (isotactic) Density ~ 0.9-0.91 g/cm3—very high strength to weight ratio Tm = 165-175C: Use temperature up to 120 C Copolymers with 2-5% ethylene—increases clarity and toughness of films Applications: dishwasher safe plastic ware, carpet yarn, fibers and ropes, webbing, auto parts

Polyethylene (HDPE) Essentially linear structure Few long chain branches, 0.5-3 methyl groups/ 1000 C atoms Molecular Weights: 50,000-250,000 for molding compounds 250,000-1,500,000 for pipe compounds >1,500,000 super abrasion resistance—medical implants MWD = 3-20 density = 0.94-0.96 g/cm3 Tm ~ 133-138 C, X’linity ~ 80% Generally opaque Applications: Bottles, drums, pipe, conduit, sheet, film

Polyethylene (LLDPE) Copolymer of ethylene with a-olefin Density controlled by co-monomer concentration; 1-butene (ethyl), or 1-hexene (butyl), or 1-octene (hexyl) (branch structure) Applications: Shirt bags, high strength films

UNIPOL Process N. F. Brockman and J. B. Rogan, Ind. Eng. Chem. Prod. Res. Dev. 24, 278 (1985) Temp ~ 70-105°C, Pressure ~ 2-3 MPa

CATALYST PREPARATION Ball mill MgCl2 (support) with TiCl4 to produce maximum surface area and incorporate Ti atoms in MgCl2 crystals Add Al(Et)3 along with Lewis base like ethyl benzoate Al(Et)3 reduces TiCl4 to form active complex Ethyl Benzoate modifies active sites to enhance stereoselectivity Catalyst activity 50-2000 kg polypropylene/g Ti with isospecificity of > 90%

Catalyst Formation AlEt3 + TiCl4 → EtTiCl3 + Et2AlCl Et2AlCl + TiCl4 → EtTiCl3 + EtAlCl2 EtTiCl3 + AlEt3 → Et2TiCl2 + EtAlCl2 EtTiCl3 → TiCl3 + Et. (source of radical products) Et. + TiCl4 → EtCl + TiCl3 TiCl3 + AlEt3 → EtTiCl2 + Et2AlCl

General Composition of Catalyst System Group I – III Metals Transition Metals Additives AlEt3 TiCl4 H2 Et2AlCl EtAlCl2 a,g, d TiCl3 MgCl2 Support O2, H2O i-Bu3Al VCl3, VoCL3, V(AcAc)3 R-OH Phenols Et2Mg Et2Zn Titanocene dichloride Ti(OiBu)4 R3N, R2O, R3P Aryl esters Et4Pb (Mo, Cr, Zr, W, Mn, Ni) HMPA, DMF

Adjuvants used to control Stereochemistry Phenyl trimethoxy silane Ethyl benzoate 2,2,6,6-tetramethylpiperidine Hindered amine (also antioxidant)

Nature of Active Sites Bimetallic site Monometallic site Active sites at the surface of a TiClx crystal on catalyst surface.

Monometallic Mechanism for Propagation Monomer forms π -complex with vacant d-orbital Alkyl chain end migrates to π -complex to form new σ-bond to metal

Monometallic Mechanism for Propagation Chain must migrate to original site to assure formation of isotactic structure If no migration occurs, syndiotactic placements will form.

Enantiomorphic Site Control Model for Isospecific Polymerization Stereocontrol is imposed by initiator active site alone with no influence from the propagating chain end, i.e. no penultimate effect Demonstrated by: 13C analysis of isotactic structures not Stereochemistry can be controlled by catalyst enantiomers

Modes of Termination 1. β-hydride shift 2. Reaction with H2 (Molecular weight control!)

Types Of Monomers Accessible for ZN Processes 1. -Olefins 2. Dienes, (Butadiene, Isoprene, CH2=C=CH2) trans-1,4 cis-1,4 iso- and syndio-1,2 1.2 Disubstituted double bonds do not polymerize

Ethylene-Propylene Diene Rubber (EPDM) S Ethylene-Propylene Diene Rubber (EPDM) S. Cesca, Macromolecular Reviews, 10, 1-231 (1975) Catalyst soluble in hydrocarbons Continuous catalyst addition required to maintain activity Rigid control of monomer feed ratio required to assure incorporation of propylene and diene monomers

Development of Single Site Catalysts Z-N multisited catalyst, multiple site reactivities depending upon specific electronic and steric environments Single site catalyst—every site has same chemical environment

Kaminsky Catalyst System W. Kaminsky et. al. Angew. Chem. Eng. Ed Kaminsky Catalyst System W. Kaminsky et.al. Angew. Chem. Eng. Ed. 19, 390, (1980); Angew. Chem. 97, 507 (1985) Linear HD PE Al:Zr = 1000 Activity = 107 g/mol Zr Me = Tl, Zr, Hf Atactic polypropylene, Mw/Mn = 1.5-2.5 Activity = 106 g/mol Zr

Methylalumoxane: the Key Cocatalyst MAO Proposed structure

Nature of active catalyst Transition metal alkylation MAO Ionization to form active sites Noncoordinating Anion, NCA

Homogeneous Z-N Polymerization Advantages: High Catalytic Activity Impressive control of stereochemistry Well defined catalyst precursors Design of Polymer microstructures, including chiral polymers Disadvantages: Requires large excess of Aluminoxane (counter-ion) Higher tendency for chain termination: β-H elimination, etc. Limited control of molecular weight distribution

Evolution of single site catalysts Date Metallocene Stereo control Performance 1950’s None Moderate Mw PE Some comonomer incorporation Early 1980’s High MW PE Better comonomer incorporation

Synthesis of Syndiotactic Polystyrene N. Ishihara et. al Synthesis of Syndiotactic Polystyrene N. Ishihara et.al. Macromolecules 21, 3356 (1988); 19, 2462 (1986) Styrene syndiotactic polystyrene m.p. = 265C

Evolution of single site catalysts Date Late 1980’s Metallocene Stereo control Slight Performance Very High Mw PE, excellent comonomer incorporation Highly Syndio- tactic Used commercially for PP Early 1990’s Isotactic

Technology S-curves for polyolefin production