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Anatomy of Addition Polymerizations

Anatomy of Addition Polymerizations. Initiation Generation of active initiator Reaction with monomer to form growing chains Propagation Chain extension by incremental monomer addition Termination Conversion of active growing chains to inert polymer Chain Transfer

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Anatomy of Addition Polymerizations

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  1. Anatomy of Addition Polymerizations • Initiation • Generation of active initiator • Reaction with monomer to form growing chains • Propagation • Chain extension by incremental monomer addition • Termination • Conversion of active growing chains to inert polymer • Chain Transfer • Transfer of active growing site by terminating one chain and reinitiating a new chain.

  2. Polymerizability of Vinyl Monomers Active Centers must be stable enough to persist though multiple monomer additions • Typical vinyl monomers

  3. Polymerizability of Vinyl Monomers

  4. Polymerizability of Vinyl Monomers

  5. Types of Vinyl Polymerization

  6. Commodity Vinyl Polymers Polystyrene (1920) PS Styrofoam, clear plastic cups envelop windows, toys Poly(vinyl chloride) (1927) PVC garden hose, pipe, car trim, seat covers, records, floor tiles

  7. Semi-Commodity Polymers Poly(methyl methacrylate) (1931) PMMA plexiglas, embedding resin, resist for X-ray applications Polytetrafluoroethylene. (1943) teflon, non stick cookware, no grease bearings, pipe-seal tape

  8. Suspension Polymerization Equivalent to a "mini-bulk" polymerization Advantages • Aqueous (hydrocarbon) media provides good heat transfer • Good particle size control through agitation and dispersion agents • Control of porosity with proper additives and process conditions • Product easy to recover and transfer Disadvantages • Suspending Agents contaminate product • Removal of residual monomer necessary

  9. Suspension (Pearl) Polymerization

  10. Suspension Polymerization of Styrene Monomer Phase 16.6 Kg. Styrene (0.5 kg Methacrylic Acid) 0.012 kg AIBN 0.006 kg Benzoyl Peroxide 0.015 kg tert-Butyl Perbenzoate Aqueous Phase: 16.6 Kg of H2O 0.24 kg Ca3PO4 0.14 kg Na+ Naphthalene sulfonate 0.077 kg. 15% Sodium Polyacrylate Polymerization Time. Hours

  11. EMULSION POLYMERIZATION • Advantages: • High rate of polymerization ~ kp[M] Npart/2 • High molecular weights, ()  of particles/  R. sec-1 = N kp [M] / Ri • Few side reactions High Conversion achieved • Efficient heat transfer • Low viscosity medium Polymer never in solution • Low tendancy to agglomerate • Emulsified polymer may be stabilized and used directly Disadvantages: Polymer surface contaminatedby surface active agents Coagulation introduces salts; Poor electrical properties

  12. Components of Emulsion Polymerization R. Water soluble initiator

  13. POLYMERS PRODUCED USING EMULSION PROCESSES

  14. Ziegler-Natta (Metal-Coordinated) Polymerization • Stereochemical Control • Polydisperse products • Statistical Compositions and Sequences • Limited set of useful monomers, i.e. olefins • SINGLE SITE CATALYSTS

  15. Polyolefins • Polypropylene (1954) • PP • dishwasher safe plastic ware, carpet yarn, fibers and ropes, webbing, auto parts

  16. Tacticity Isotactic All asymmetric carbons have same configuration • Methylene hydrogens are meso • Polymer forms helix to minimize substituent interaction Syndiotactic • Asymmetric carbons have alternate configuration • Methylene hydrogens are racemic • Polymer stays in planar zig-zag conformation Heterotactic (Atactic) • Asymmetric carbons have statistical variation of configuration

  17. 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)

  18. Natta’s Discovery • 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

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

  20. 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

  21. 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

  22. Polyethylene (LLDPE) • Copolymer of ethylene witha-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

  23. 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%

  24. 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

  25. 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

  26. General Composition of Catalyst System

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

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

  29. 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

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

  31. 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

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

  33. 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

  34. 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

  35. 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

  36. Kaminsky Catalyst SystemW. 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

  37. Methylalumoxane: the Key Cocatalyst n = 10-20 MAO Proposed structure

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

  39. 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

  40. Evolution of single site catalysts

  41. Synthesis of Syndiotactic PolystyreneN. Ishihara et.al. Macromolecules21, 3356 (1988); 19, 2462 (1986) Styrene syndiotactic polystyrene m.p. = 265C

  42. Evolution of single site catalysts

  43. Technology S-curves for polyolefin production

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