Ziegler-Natta Polymerization: Synthesis of tacticity specific polypropylene S.C.S. Lai Leiden University April 8th, 2004

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization2 Table of contents  Overview  Mechanism (general)  Structure of catalyst  Stereospecifity\  Role of ß-TiCl 3  Conclusion

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization3 Overview, polymerization (1)  Three possible polymer syntheses mechanisms:  Free radicals  ions  metalorganic complexes  Polymers of specific tacticity wanted in industries: Isotactic Syndiotactic Atactic

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization4 Overview, polymerization (2)  Linear vs. branched polymers Ziegler-Natta catalyst generally used to produce linear, isotactic polypropylene!

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization5 Overview, history (1)  First report in September 1955 using “purple phases” of TiCl 3 ( α -TiCl 3 and γ -TiCl 3 ) and AlEt 3 (higher activity) or AlEt 2 Cl (higher stereoselectivity).  Solvay 1973: Added TiCl 4, which acted as a catalyst to convert β -TiCl 3 into an active phase of TiCl 3 (higher activity due to smaller particles).

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization6 Overview, history (2)  Shell 1980: TiCl 4 supported on MgCl 2 in presence of AlEt 3 or AlEt 2 Cl. Active species still TiCl 3.  Other remarks:  Awarded Nobel price in  1980’s: Process attributed to Robert Banks and J. Paul Hogan Cerutti, L; International Journal for Philosophy of Chemistry, 1999 (5), 3-41

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization7 Mechanism  Two complications  Why Cl-vacancy?  Why stereospecific? Cossee-Arlman postulate (1964)

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization8 Structure of the catalyst, overview Three phases of TiCl 3 ColorStuctureActivity α -TiCl 3 PurpleHexagonal layered structure Isotactic β -TiCl 3 BrownNeedle structureLittle stereospecifity γ -TiCl 3 PurpleCubic layered structure Like α -TiCl 3

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization9 Structure of the catalyst, overview Schematic view of the structures of α -TiCl 2, α -TiCl 3 and ß -TiCl 3

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization10 Structure of the catalyst, Cl- vacancies (1) Sheet of α -TiCl 2, consisting of 2 layers of Cl with Ti in the octahedral holes. Ion count: (2m 2 – 2) Cl - (m - 1) 2 Ti Surplus of 4(m - 1) negative charges Offsetting by Cl - vacancies

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization11 Structure of the catalyst, Cl- vacancies (2) Thus: Surplus of 4 (m – 1) Cl - on (m – 1) 2 Ti 2+ Number of vacancies: Typical crystal of ~1μm has about than 1-2 vacancies per 1000 Ti 2+ -ions. Analogous calculation for α -TiCl3 yields the same result.

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization12 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

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization13 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 Ti 3+ 1 loosely bound (to 1 Ti 3+ ) Vacancy and L not equivalent sites

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization14 Stereospecifity, bonding of propylene Two possibilities: 1. Alkalyne moves back to vacancy 2. Alkalyne doesn’t move back

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization15 Stereospecifity, Polymerization (1) Polymer moves back to vacancy  isotactic polypropylene

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization16 Stereospecifity, Polymerization (2) Polymer doesn’t back to vacancy  syndiotactic polypropylene Experimental: Some syndiotactic PP at -70°

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization17 ß-TiCl 3, Structure (1) β -TiCl 3 has a needle structure: ClClClCl Cl Ti 3+ Cl Ti 3+ Cl Ti 3+ Cl ClClClCl Actual structure ClClClCl Cl Ti 3+ Cl Ti 3+ Cl Ti 3+  ClCl  ß 1 ß 2 Charges: 3(m+2) + 3(m+9) -  3 vacancies per chain

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization18 ß-TiCl 3, Structure (2) ß 1 site: TiCl 3 F Cl 2 L   TiCl 3 F Cl L R Charge - 1/2 ß 2 site: TiCl 3 F Cl L  2  TiCl 3 F R 2 Charge +1/2

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization19 ß-TiCl 3, Reactivity Reactive sites for diene-polymerization:  ß 1 site: 1 vacancy, limited space  1,4 trans-polymers  ß 2 site: 2 vacancies, both forming pi-bonds with diene  1,4 cis-polymers  Experimental:  butadiene: mixture of trans and cis  isoprene: only cis

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization20 Conclusion  Three phases of TiCl 3  Only α -TiCl 3 and γ -TiCl 3 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-TiCl 3 has 2 different active sites, one forcing dienes to polymerize 1,4-cis, one 1,4-trans, if molecule is flexible.

S.C.S. Lai, April 8th 2003Ziegler-Natta Polymerization21 Final remarks  Slides:  Questions?