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Imperial College London

Imperial College London. 4I-11 Case studies in Inorganic Chemistry. Lecture 7 Biorenewable Polymers 1: The Stereoselective Polymerisation of Lactide Dr. Ed Marshall Rm: M220, Mezzanine Floor, RCS 1 e.marshall@imperial.ac.uk www.ch.ic.ac.uk/marshall/4I-11.html & WebCT.

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Imperial College London

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  1. Imperial College London 4I-11 Case studies in Inorganic Chemistry Lecture 7 Biorenewable Polymers 1: The Stereoselective Polymerisation of Lactide Dr. Ed Marshall Rm: M220, Mezzanine Floor, RCS 1 e.marshall@imperial.ac.uk www.ch.ic.ac.uk/marshall/4I-11.html & WebCT 4I-11 - Lectures 7 - Slide 1

  2. Imperial College London Learning objectives • Over the next two lectures you should acquire the knowledge to allow you to: • 1. Describe why the polymerisation of lactide is so intensely researched. • 2. Explain how chiral and achiral (salen)-supported Al complexes may be used to prepare isotactic and syndiotactic polylactide. • 3. Explain how b-diketiminate supported complexes of Zn and Mg may be used to prepare heterotactic polylactide. • 4. Understand how computational chemistry may be used to investigate polymerisation mechanisms and to shed light onto the causes of stereoselectivity. 4I-11 - 7 - 2

  3. Imperial College London Polylactide or Poly(lactic acid) - PLA (www.natureworksllc.com) The first mainstream polymer to be made from renewable resources. PLA is: biodegradable,biocompatibleandbioresorbable enzymatic degradation fermentation corn starch lactic acid step-growth condensation (-H2O) ring-opening polymerisation * heat * * (chain growth) * polylactic acid, PLA lactide oligomers These 2 lectures focus on this step 4I-11 - 7 - 3

  4. Imperial College London Initiators Typical initiators for lactide polymerisation are metal-alkoxides, e.g. Al(OiPr)3: Acyl-oxygen bond breaks Industrially, the initiator used is a tin(II) carboxylate - in the presence of alcohol, this is believed to form tin(II) alkoxides, and these are the actual initiating species: tin(II) bis(2-ethylhexanoate) "SnOct2" true initiator 4I-11 - 7 - 4

  5. Imperial College London Mechanism of propagation Initiation involves nucleophilic attack of the alkoxide at the lactide carbonyl. This leaves the monomer heterocycle intact. In order to open the ring, the monomer rolls around to place the acyl oxygen nearer the metal centre. NB: Every step is reversible. Coordinative-insertion Mechanism other lactones open in a similar way 4I-11 - 7 - 5

  6. Imperial College London Lactide - three different stereoisomers (S,S)-lactide "L-lactide" (R,R)-lactide "D-lactide" (S,R)-lactide "meso-lactide" 50:50 mix = rac-lactide Since L-lactic acid is the naturally occurring form, (S,S)-lactide is much cheaper than (R,R)-lactide. Rac-lactide is even cheaper (the industrial enzymatic conversion of starch into lactic acid is aspecific). Meso-lactide is not commercially available and must be separated from the (R,R) and (S,S) monomers by a steam distillation. Most commonly studied: L-lactide and rac-lactide 4I-11 - 7 - 6

  7. Imperial College London PLA - stereoregular microstructures (tacticities) isotactic -(SSSSSSSS)- "poly(L-lactide)" S,S isotactic -(RRRRRRRR)- "poly(D-lactide)" R,R heterotactic -(SSRRSSRR)- rac syndiotactic -(SRSRSRSR)- meso 4I-11 - 7 - 7

  8. Imperial College London PLA - physical properties depend on tacticity There is one other important form of PLA known as an isotactic stereocomplex: isotactic poly(L-lactide) mix and co-crystallise Tm > 235 °C isotactic poly(D-lactide) 4I-11 - 7 - 8

  9. Imperial College London Single-site catalysts Ln X M e.g. Cp2ZrMe+ for the polymerisation of ethene sterically bulky ancillary ligand(s) substrate approaches vacant coordination site and may then react with X PLA stereochemistry potentially controlled by the sterics / chirality of Ln 4I-11 - 7 - 9

  10. Imperial College London First report of stereoregular polymerisation of rac-lactide Spassky rac-LA isotactic bias Pm = 0.68 60 °C Stereoselectivity presumably arises from a chain-end control mechanism: (S,S)-LA k(S,S) / k(R,R) = 2.8 [Al]-(S,S)-OMe [Al](OMe) k(R,R) / k(S,S) = 2.8 (R,R)-LA [Al]-(R,R)-OMe [Al](OMe) Macromolecular Chemistry and Physics 1997, 198, 1227-1238 4I-11 - 7 - 10

  11. Imperial College London Subsequent modifications to salen Al initiators Numerous salen ligands have been investigated (24 examples are reported in a recent paper), but the most notable one is: Nomura rac-LA highly isotactic Pm = 0.92 60 °C Tm = 192 °C But does this catalyst produce a PLA stereocomplex? J. Am. Chem. Soc. 2002, 124, 5938-5939 - WebCT Nomura2002.pdf Proc. Nat. Acad. Sci. 2006, 103, 15343-15348 - WebCT Gibson2006.pdf 4I-11 - 7 - 11

  12. Imperial College London Enantiomorphic site control with chiral salen Al initiators? Spassky rac-LA k(R,R) / k(S,S) = 20 60 °C enantiopure initiator The polymer produced is a tapered stereoblock copolymer: increasing R content -RRRRRRRRRRR- -RR---------------SS- -SSSSSSSSSSSS- increasing S content Tm = 185 °C Macromolecular Chemistry and Physics 1996, 197, 2627 4I-11 - 7 - 12

  13. Imperial College London Dispelling the stereocomplex myth Smith & Baker / Coates rac-LA isotactic PLA Tm = 191 °C 60 °C Smith and Baker proposed (i) (correctly) that each initiator enantiomer consumes just one monomer enantiomer and (ii) (incorrectly) that the chains then form a stereocomplex. racemic initiator However, subsequent studies by Coates showed that the racemic initiator actually gives short chains of -RRRR- and -SSSS- but these then exchange between different metal centres. The PLA product is not a stereocomplex - it is a stereoblocky copolymer: -(RR…RR)-(SS…SS)-(RR…RR)-(SS…SS)- Elevated Tm values arise from cocrystallisation of short isotactic domains J. Am. Chem. Soc. 2000, 122, 1552-1553 - WebCT Baker&Smith2000.pdf J. Polym. Sci. Polym. Chem. 2000, 38, 4686 - WebCT Coates2000.pdf 4I-11 - 7 - 13

  14. Imperial College London Formation of isotactic PLA from chiral salen Al initiators (S)-selective ligand (S)-selective ligand (R)-selective ligand (R)-selective ligand 4I-11 - 7 - 14

  15. Imperial College London First report of syndiotactic PLA Coates Preferentially consumes (R,R)-LA versus (S,S)-LA enantiopure initiator meso-LA selectively cleaved at acyl bond adjacent to (R)-methyl J. Am. Chem. Soc. 2002, 124, 1316 - WebCT Coates2002.pdf 4I-11 - 7 - 15

  16. Imperial College London Lecture 7 Conclusions 1. Salen-supported Al-based initiators nearly always give highly isotactic PLA (from the racemic monomer). The highest isotacticities are usually observed with chiral salen ligands, although the Nomura initiator is actually achiral. ligand-assisted chain end stereocontrol 2. The isotactic product is actually a stereoblock, not a stereocomplex. Tm values are therefore higher than for isotactic poly(L-lactide), but lower than for the stereocomplex. 3. Syndiotactic PLA may be prepared from meso-lactide using a chiral salen ligand (with a 2,2'-diaminobinaphthyl backbone). 4I-11 - 7 - 16

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