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Imperial College London

Imperial College London. 4I-11 Case studies in Inorganic Chemistry. Lecture 8 Biorenewable Polymers 2: 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/4I11.html. 4I-11 - Lecture 8 - Slide 1.

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Imperial College London

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  1. Imperial College London 4I-11 Case studies in Inorganic Chemistry Lecture 8 Biorenewable Polymers 2: 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/4I11.html 4I-11 - Lecture 8 - Slide 1

  2. Imperial College London Recap of Lecture 7: Mechanism of propagation Every step is reversible. Coordinative-insertion Mechanism 4I-11 - 8 - 2

  3. Imperial College London Last lecture (salen)Al(OR) and derivatives convert rac-lactide into isotactic poly(lactide) Product is believed to be a stereoblock copolymer, with short sequences of all R alternating with short sequences of all S (as opposed to a stereocomplex, formed from complete all R chains and all S chains) correct structure incorrect structure 4I-11 - 8 - 3

  4. Imperial College London Remaining learning outcomes • Over these 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 - 8 - 4

  5. Imperial College London b-Diketiminate ligands diketimine canonicals: Deprotonation results in a monoanionic bidentate ligand - known as NacNac or BDI. e.g. [(BDI)MgiPr] Ar = 2,6-diisopropylphenyl Dalton Trans. 2003, 3088 - WebCT Gibson2003.pdf 4I-11 - 8 - 5

  6. Imperial College London First report of heterotactic PLA Coates rac-LA (R) (S) (R) (S) CH2Cl2 25 °C Heterotactic PLA • 100 equiv rac-LA consumed in 20 mins • Highly stereoselective - Pr = 0.90 (0.94 at 0 °C) R = iPr, Pr = 0.90 R = nPr, Pr = 0.76 R = Et, Pr = 0.79 steric bulk of iPr groups is essential for stereocontrol J. Am. Chem. Soc. 2001, 123, 3229 - WebCT Coates2001.pdf 4I-11 - 8 - 6

  7. Imperial College London The Mg analogue Under the same conditions - i.e. CH2Cl2, 25 °C - [(BDI)Mg(m-OiPr)2] gives atactic PLA But the Mg initiator is heteroselective in coordinating solvents: Chisholm rac-LA Heterotactic PLA, Pr = 0.90 THF 25 °C Inorg. Chem. 2002, 41, 2785 - WebCT Chishiolm2002.pdf 4I-11 - 8 - 7

  8. Imperial College London However, magnesium BDI initiators can be heteroselective NMR studies reveal that in THF, the Mg-propagating species is mononuclear, but in CH2Cl2 it is dimeric. The Zn analogue is monomeric even in CH2Cl2: Propagating Mg species in THF Propagating Zn species in CH2Cl2 Heterotactic PLA formed when the propagating species are mononuclear J. Am. Chem. Soc. 2005, 127, 6048 - WebCT Rzepa2005.pdf 4I-11 - 8 - 8

  9. Imperial College London Computational studies • Goals of this project: • to understand the mechanism of ring-opening better. • to explain why the Mg and Zn initiators give heterotactic PLA. • to explain why reduction in the N-aryl ortho substituents (e.g. from iPr to Et) leads to a loss in stereoselectivity. • Method employed: • Reaction coordinate mapped out for the insertion of two LA units (LA1 and LA2) using (BDI)Mg(OMe)(THF) as the initiator. • Free energies of competing transition states (i.e. R,R or S,S-lactide insertion) calculated. • All calculations performed at a very high level [B3-LYP 6-311G(3d)] - many of the calculated geometries took 7 - 10 days to converge. 4I-11 - 8 - 9

  10. Imperial College London The Reaction Coordinate - calculated for LA1 = (R,R) & LA2 = (S,S) Two transition states, TS1 and TS2 TS2 is higher in energy than TS1 4I-11 - 8 - 10

  11. Imperial College London Revised mechanism TS1: Formation of new M-O bond and cleavage of M-OR bond TS2: Formation of new M-O bond and cleavage of heterocycle Both transition states involve bond breaking / forming 4I-11 - 8 - 11

  12. Imperial College London Computing the origin of stereocontrol Although initially calculated for LA1 = R,R-lactide and LA2 = S,S-lactide, we have to consider several other possible assemblies. Total number of assembly modes: LA1 = R,R or S,S; LA2 = R,R or S,S; LA2 may approach either face of the ring-opened LA1 8 possibilities However, the 8 possible assembly modes exist as 4 enantiomeric pairs: 4I-11 - 8 - 12

  13. Imperial College London e.g. Consider the approach of S,S-LA2 to R,R-LA1 S R R S S R R S mirror image 8 possible assembly modes = 4 enantiomeric pairs ∴ only 4 calculations required 4I-11 - 8 - 13

  14. Imperial College London Curtis-Hammett Principle Since every stage of the ring-opening mechanism is reversible, the product distribution (i.e. whether R,R or S,S lactide is inserted) depends only on the competing geometries for the rate-determining step. Calculated transition state free energies (kcal mol-1): lowest barrier for heterotactic PLA In every case TS2 is rate-determining Therefore, the reason for heterotactic stereocontrol must lie within the four competing geometries for TS2 - RR,RR - SS,SS - RR,SS - and SS,RR. 4I-11 - 8 - 14

  15. Imperial College London Competing TS2 geometries - the origin of stereocontrol RR,SS: 18.9 kcalmol-1 4I-11 - 8 - 15

  16. Imperial College London Competing TS2 geometries - the origin of stereocontrol RR,SS: 18.9 kcalmol-1 RR,RR: 20.2 kcalmol-1 4I-11 - 8 - 16

  17. Imperial College London Competing TS2 geometries - the origin of stereocontrol SS,RR: 18.9 kcal mol-1 RR,RR: 20.2 kcal mol-1 SS,SS: 25.4 kcal mol-1 4I-11 - 8 - 17

  18. Imperial College London Competing TS2 geometries - the origin of stereocontrol SS,RR: 18.9 kcal mol-1 RR,RR: 20.2 kcal mol-1 SS,SS: 25.4 kcal mol-1 SS,RR: 28.1 kcal mol-1 Heterotactic PLA formed via LA1= R,R and LA2 = S,S. Next R,R then inserts via the enantiomer of the SS,RR transition state 4I-11 - 8 - 18

  19. Imperial College London Summary of the origin of stereocontrol Heterotactic PLA formed via LA1 = R,R and LA2 = S,S. R,R-LA3 then inserts via the enantiomer of the SS,RR transition state 4I-11 - 8 - 19

  20. Imperial College London Conclusions • Heterotactic PLA may be prepared using b-diketiminate Zn and Mg alkoxides, but the Mg initiators must be used in THF. • The propagating species responsible for heterotactic PLA formation is mononuclear. • Computational analysis reveals that the rate determining step is TS2, i.e. the cleavage of the monomer heterocycle. • Heterotactic PLA arises because of the minimisation of Me - Me steric clashes in the competing geometries of TS2. 4I-11 - 8 - 20

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