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Ziegler-Natta Polymerization: Synthesis of tacticity specific polypropylene

Ziegler-Natta Polymerization: Synthesis of tacticity specific polypropylene. S.C.S. Lai (s.lai@chem.LeidenUniv.nl) Leiden University April 8th, 2004. Table of contents. Overview Mechanism (general) Structure of catalyst Stereospecifity Role of ß-TiCl 3 Conclusion.

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Ziegler-Natta Polymerization: Synthesis of tacticity specific polypropylene

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  1. Ziegler-Natta Polymerization: Synthesis of tacticity specific polypropylene S.C.S. Lai (s.lai@chem.LeidenUniv.nl) Leiden University April 8th, 2004

  2. Table of contents • Overview • Mechanism (general) • Structure of catalyst • Stereospecifity\ • Role of ß-TiCl3 • Conclusion Ziegler-Natta Polymerization

  3. Overview, polymerization (1) • Three possible polymer syntheses mechanisms: • Free radicals • ions • metalorganic complexes • Polymers of specific tacticity wanted in industries: Isotactic Syndiotactic Atactic Ziegler-Natta Polymerization

  4. Overview, polymerization (2) • Linear vs. branched polymers Ziegler-Natta catalyst generally used to produce linear, isotactic polypropylene! Ziegler-Natta Polymerization

  5. Overview, history (1) • First report in September 1955 using “purple phases” of TiCl3 (α-TiCl3 and γ-TiCl3) and AlEt3 (higher activity) orAlEt2Cl (higher stereoselectivity). • Solvay 1973: Added TiCl4, which acted as a catalyst to convert β-TiCl3 into an active phase of TiCl3 (higher activity due to smaller particles). Ziegler-Natta Polymerization

  6. Overview, history (2) • Shell 1980: TiCl4 supported on MgCl2 in presence of AlEt3 orAlEt2Cl. Active species still TiCl3 . • Other remarks: • Awarded Nobel price in 1963. • 1980’s: Process attributed to Robert Banks and J. Paul Hogan Cerutti, L; International Journal for Philosophy of Chemistry, 1999 (5), 3-41 Ziegler-Natta Polymerization

  7. Mechanism • Two complications • Why Cl-vacancy? • Why stereospecific? Cossee-Arlman postulate (1964) Ziegler-Natta Polymerization

  8. Structure of the catalyst, overview • Three phases of TiCl3 Ziegler-Natta Polymerization

  9. Structure of the catalyst, overview • Schematic view of the structures of α-TiCl2, α-TiCl3 and ß-TiCl3 Ziegler-Natta Polymerization

  10. Structure of the catalyst, Cl-vacancies (1) Ion count: (2m2 – 2) Cl- (m - 1)2 Ti2+ ----------------------------- Surplus of 4(m - 1) negative charges Offsetting by Cl- vacancies Sheet of α-TiCl2, consisting of 2 layers of Cl with Ti in the octahedral holes. Ziegler-Natta Polymerization

  11. Structure of the catalyst, Cl-vacancies (2) Thus: Surplus of 4 (m – 1) Cl- on (m – 1)2 Ti2+ Number of vacancies: Typical crystal of ~1μm has about than 1-2 vacancies per 1000 Ti2+-ions. Analogous calculation for α-TiCl3 yields the same result. Ziegler-Natta Polymerization

  12. Structure of the catalyst, active site (1) • Cl-vacancies on the edges of the crystal. • Electron Microscopy: active sites are on the edges • Ti at the active sites in a square of Cl Ziegler-Natta Polymerization

  13. Structure of the catalyst, active site (2) • Square makes an angle of 55° with the base plane. • Cl-’s not equivalent: • 3 stuck in crystal • 1 bound by 2 Ti3+ • 1 loosely bound (to 1 Ti3+) • Vacancy and L not equivalent sites Ziegler-Natta Polymerization

  14. Stereospecifity, bonding of propylene Two possibilities: 1. Alkalyne moves back to vacancy 2. Alkalyne doesn’t move back Ziegler-Natta Polymerization

  15. Stereospecifity, Polymerization (1) Polymer moves back to vacancy  isotactic polypropylene Ziegler-Natta Polymerization

  16. Stereospecifity, Polymerization (2) Polymer doesn’t back to vacancy  syndiotactic polypropylene Experimental: Some syndiotactic PP at -70° Ziegler-Natta Polymerization

  17. ß-TiCl3, Structure (1) β-TiCl3 has a needle structure: Cl Cl Cl Cl Cl Ti3+ Cl Ti3+ Cl Ti3+ Cl Cl Cl Cl Cl Actual structure Cl Cl Cl Cl Cl Ti3+ Cl Ti3+ Cl Ti3+   Cl Cl  ß1 ß2 Charges: 3(m+2) + 3(m+9) -  3 vacancies per chain Ziegler-Natta Polymerization

  18. ß-TiCl3, Structure (2) ß1 site: TiCl3FCl2L  TiCl3FClLR Charge - 1/2 ß2 site: TiCl3FClL2 TiCl3FR2 Charge +1/2 Ziegler-Natta Polymerization

  19. ß-TiCl3, Reactivity • ß1 site: 1 vacancy, limited space  1,4 trans-polymers • ß2 site: 2 vacancies, both forming pi-bonds with diene  1,4 cis-polymers • Experimental: • butadiene: mixture of trans and cis • isoprene: only cis Reactive sites for diene-polymerization: Ziegler-Natta Polymerization

  20. Conclusion • Three phases of TiCl3 • Only α-TiCl3 andγ-TiCl3 active in stereospecific Ziegler-Natta polymerization • Active sites are the Cl--vacancies, located at the edges of the catalyst. • Stereospecifity are due stereometric interactions, forcing the same orientation for each propagation step • ß-TiCl3 has 2 different active sites, one forcing dienes to polymerize 1,4-cis, one 1,4-trans, if molecule is flexible. Ziegler-Natta Polymerization

  21. Final remarks • Slides: http://home.wanadoo.nl/scslai • Questions? Ziegler-Natta Polymerization

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