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Imperial College London Lecture 7 Biorenewable Polymers 1: The Stereoselective Polymerisation of Lactide Dr. Ed Marshall Rm: M220, Mezzanine Floor, RCS.

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Presentation on theme: "Imperial College London Lecture 7 Biorenewable Polymers 1: The Stereoselective Polymerisation of Lactide Dr. Ed Marshall Rm: M220, Mezzanine Floor, RCS."— Presentation transcript:

1 Imperial College London Lecture 7 Biorenewable Polymers 1: The Stereoselective Polymerisation of Lactide Dr. Ed Marshall Rm: M220, Mezzanine Floor, RCS 1 & WebCT 4I-11 - Lectures 7 - Slide 1 4I-11 Case studies in Inorganic Chemistry

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  -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

3 These 2 lectures focus on this step Imperial College London Polylactide or Poly(lactic acid) - PLA ( cornstarch lactic acid oligomerslactidepolylactic acid, PLA fermentation enzymatic degradation step-growth condensation (-H 2 O) heat ring-opening polymerisation (chain growth) The first mainstream polymer to be made from renewable resources. PLA is:biodegradable, biocompatible and bioresorbable 4I * * * *

4 Imperial College London Initiators Typical initiators for lactide polymerisation are metal-alkoxides, e.g. Al(O i Pr) 3 : 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) "SnOct 2 " true initiator 4I Acyl-oxygen bond breaks

5 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 Imperial College London Mechanism of propagation 4I

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

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

8 Imperial College London PLA - physical properties depend on tacticity PolymerT m (°C)T g (°C)Modulus (GPa) Degradation time (months) isotactic PLA >24 syndiotactic PLA15345n/a heterotactic PLAamorphous49n/a atactic PLAamorphous There is one other important form of PLA known as an isotactic stereocomplex: isotactic poly(L-lactide) isotactic poly(D-lactide) mix and co-crystallise T m > 235 °C 4I

9 Imperial College London Single-site catalysts LnLn M X sterically bulky ancillary ligand(s) substrate approaches vacant coordination site and may then react with X e.g. Cp 2 ZrMe + for the polymerisation of ethene PLA stereochemistry potentially controlled by the sterics / chirality of L n 4I

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

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 P m = °C But does this catalyst produce a PLA stereocomplex? T m = 192 °C J. Am. Chem. Soc. 2002, 124, WebCT Nomura2002.pdf Proc. Nat. Acad. Sci. 2006, 103, WebCT Gibson2006.pdf 4I

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

13 Imperial College London Dispelling the stereocomplex myth racemic initiator rac-LA 60 °C Smith & Baker / Coates isotactic PLA T m = 191 °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. 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 T m values arise from cocrystallisation of short isotactic domains 4I J. Am. Chem. Soc. 2000, 122, WebCT Baker&Smith2000.pdf J. Polym. Sci. Polym. Chem. 2000, 38, WebCT Coates2000.pdf

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

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

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. T m 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

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