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Biorenewable Polymers 1: The Stereoselective Polymerisation of Lactide

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1 Biorenewable Polymers 1: The Stereoselective Polymerisation of Lactide
4I-11 Case studies in Inorganic Chemistry 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

2 Learning objectives Imperial College London 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

3 polylactic acid, PLA lactide oligomers
Polylactide or Poly(lactic acid) - PLA (www.natureworksllc.com) Imperial College London The first mainstream polymer to be made from renewable resources. PLA is: biodegradable, biocompatible and bioresorbable 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

4 tin(II) bis(2-ethylhexanoate)
Initiators Imperial College London 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

5 Coordinative-insertion Mechanism other lactones open in a similar way
Mechanism of propagation Imperial College London 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

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

8 Degradation time (months)
PLA - physical properties depend on tacticity Imperial College London Polymer Tm (°C) Tg (°C) Modulus (GPa) Degradation time (months) isotactic PLA 170 60 2.7 >24 syndiotactic PLA 153 45 n/a heterotactic PLA amorphous 49 atactic PLA 55 1.9 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

9 vacant coordination site and may then react with X
Single-site catalysts Imperial College London 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

10 First report of stereoregular polymerisation of rac-lactide
Imperial College London 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](OMe) [Al]-(S,S)-OMe (R,R)-LA k(R,R) / k(S,S) = 2.8 [Al](OMe) [Al]-(R,R)-OMe Macromolecular Chemistry and Physics 1997, 198, 4I

11 Subsequent modifications to salen Al initiators
Imperial College London 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, WebCT Nomura2002.pdf Proc. Nat. Acad. Sci. 2006, 103, WebCT Gibson2006.pdf 4I

12 -RR---------------SS-
Enantiomorphic site control with chiral salen Al initiators? Imperial College London 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

13 Dispelling the stereocomplex myth
Imperial College London 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, WebCT Baker&Smith2000.pdf J. Polym. Sci. Polym. Chem. 2000, 38, WebCT Coates2000.pdf 4I

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

15  meso-LA selectively cleaved at acyl bond adjacent to (R)-methyl
First report of syndiotactic PLA Imperial College London 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, WebCT Coates2002.pdf 4I

16 ligand-assisted chain end stereocontrol
Lecture 7 Conclusions Imperial College London 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


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