Catalysis & Sustainable Processes The Polymers Story 8 lectures http://www.kcpc.usyd.edu.au/cem3113.html username: chem3 password: carbon12 Lecturer: Associate Professor Sébastien Perrier s.perrier@chem.usyd.edu.au; tel: 9351 3366, room 351 KCPC 1 assignment Given out in Lecture 5 (Friday 20/03/09) Worth 20% Course structure Understanding the problem: - Making Polymers Step Growth Polymerisation (bio)degradable polymers Addition polymerisation recyclable polymers (?) - The raw material Solving the problem: - Sustainable Process - Sustainable Synthesis - Sustainable Materials
Ziegler-Natta Polymerisation Nobel Prize Chemistry 1963 Ziegler-Natta catalysts 1953 Ziegler-Natta catalyst Karl Ziegler (Max Planck Institute for Coal esearch) Giulio Natta (Polytechnic Institute of Milan) 1963 Nobel Prize => Production of low density poly(ethylene) (LDPE) i.e. branched polyethylene.
Ziegler-Natta catalysts Karl Ziegler discovered that high-density poly(ethylene), DPE, could be produced using organometallic catalysts. Such catalysts form co-ordination complexes. The catalyst is based on a mixture of: (early) transition metal salt + aluminium alkyl Ziegler-Natta Catalysts Giulio Natta discovered that 1-alkenes could be polymerised to produce stereoregular polymers, with catalysts of the type described by Ziegler. The Ziegler-Natta catalyst is not consumed during the reaction. Many different types of Ziegler-Natta catalysts have been developed. They can be either: heterogeneous, or homogeneous. Ziegler-Natta catalysts: (early) transition metal salt + aluminium alkyl Ethene Ti 3 AlEt 3 Propene Ti 3 Al 2 1-Butene Ti 3 AlEt 2 Ethene VO 3 Al 2 + propene + non-conjugated diene
Advantages of OM-catalytically produced polymers No branching: e.g. DPE (high density polyethylene) greater strength/heat resistance than LDPE (plastic squeeze bottles, moulded housewares) Stereoselective: e.g. PP Tacticity describes the relative stereoisomerism of macromolecules. e.g. Polypropylene: atactic: methyl groups randomly oriented No order (atactic): No packing isotactic: all methyl groups oriented towards one side syndiotactic: methyl groups oriented in an alternating way egular arrangement (Isotactic, syndiotactic): Easy to pack into crystals and fibers. Limitations Poly(vinyl chloride) Free radical polymerisation! Acrylates Anionic polymerisation!
Metallocene Polymerisation Metallocenes An important class of Ziegler-Natta catalysts are the metallocenes. They consist of a transition metal sandwiched between two cyclopentadiene rings. A co-catalyst is also employed. This is often derived from the reaction of aluminium alkyl with water. Make polyethylene up to 6Million g/mol (DP ~200,000)! Tune tacticities (block copolymers) Fe What is a Metallocene? - + - Aromatic ring 2 - + Fe 2+ Fe Fe Ferrocene Zr Zr bis-chlorozirconocene
bis(2-phenylindenyl)zirconium dichloride Zr + Zr + ac form meso form Elastomeric polypropylene Isotactic blocks form crystals tied together by soft rubbery tethers of atactic polypropylene => more strength (No crosslinked necessary) thermoplastic elastomer Ziegler-Natta vs. Metallocenes Ziegler-Natta Catalysts: Several metal sites causes less control of polymer branching Metallocenes: Single metal site allows for more control of branching and molecular weight distribution Versatility of metallocenes with countless variations (ie. bridging atoms, overcrowding)
Metathesis Polymerisation Nobel Prize Chemistry 2005 Metathesis C C + C C C C + C C Metathesis 1950s by industrial chemists: 1956, erbert S Eleuterio, (DuPont): Propylene-ethylene copolymer from a propylene feed passed over a molybdenum-onaluminum catalyst. Analysis showed that the output gas was a mixture of propylene, ethylene, and 1-butene. Cyclopentene: "the polymer I got looked like somebody took a pair of scissors, opened up cyclopentene, and neatly sewed it up again n http://pubs.acs.org/cen/coverstory/8051/8051olefin2.html
1971: Chauvin et al. (French Petroleum Institute) suggested that olefin metathesis is initiated by a metal carbene. The metal carbene reacts with an olefin to form a metallacyclobutane intermediate that breaks apart to form a new olefin and a new metal carbene, which propagates the reaction Makromol. Chem., 141, 161 (1971) (because of a typographical error in the journal's running head, this paper is sometimes erroneously cited with a 1970 publication date!!!) L M L M M n n L n ' ' ' ' ' ' n n Poly(dicyclopentadiene) Properties: (PDCP) - Thermoset - Very good impact resistance at low temperatures Process: reaction injection molding or IM Applications: - Snowmobiles cowlings - Satellite dish antenna - Storage tank for chemicals Catalysts Schrock W, Mo and e Catalysts (80 s-90 s) Arylimido complexes of Mo: Prof ichard Schrock Massachusetts Institute of Technology igh tolerance for functionality 100% active fully characterized by NM and X-ray crystallography. air and water-sensitive Coordinative and electronic unsaturation (making them electrophilic) Bulky ligands (prevents bimolecular decomposition).
Grubbs u Catalysts(1990's): Metal is not in its highest oxidation state Supported by phosphine ligands Prof obert. Grubbs California Institute of Technology Very tolerant of functionality (operate in water) Lower metathesis rates than the Schrock catalysts Metathesis polymerisation Polymerization of Acetylenes Acyclic Diene Metathesis Polymerisation 1 2 1 2 3 4 3 4 ing Opening Metathesis Polymerisation ing Opening Metathesis Polymerisation (OMP) Cyclic olefin:"new" generated olefin remains attached to the catalyst Driving force = relief of ring strain Second step shown below is essentially irreversible. M M M 2 M 1
Back biting If the Catalyst is too active, it can metathesize the unstrained olefinic bonds in the growing polymer chain educe the molecular weight Increase the polydispersity Applications Poly(acetylene) n Semi-conducting polymers (Nobel Prize Chemistry 2000) Alan eeger, Alan G. MacDiarmid, ideki Shirakawa