Imperial%20College

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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_at_imperial.ac.uk www.ch.ic.ac.uk/mars
hall/4I-11.html WebCT
4I-11 - Lectures 7 - Slide 1
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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.

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Polylactide or Poly(lactic acid) - PLA
(www.natureworksllc.com)
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)

oligomers
lactide
polylactic acid, PLA
These 2 lectures focus on this step
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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
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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
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Lactide - three different stereoisomers
(S,S)-lactide "L-lactide"
(R,R)-lactide "D-lactide"
(S,R)-lactide "meso-lactide"
5050 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
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PLA - stereoregular microstructures (tacticities)
isotactic -(SSSSSSSS)- "poly(L-lactide)"
S,S
isotactic -(RRRRRRRR)- "poly(D-lactide)"
R,R
heterotactic -(SSRRSSRR)-
rac
syndiotactic -(SRSRSRSR)-
meso
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PLA - physical properties depend on tacticity
Polymer Tm (C) Tg (C) Modulus (GPa) Degradation time (months)
isotactic PLA 170 60 2.7 gt24
syndiotactic PLA 153 45 n/a n/a
heterotactic PLA amorphous 49 n/a n/a
atactic PLA amorphous 55 1.9 12 - 16
There is one other important form of PLA known as
an isotactic stereocomplex
isotactic poly(L-lactide)
mix and co-crystallise
Tm gt 235 C
isotactic poly(D-lactide)
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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
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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
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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
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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
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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 -(RRRR)-(SSSS)-(RRRR)-(
SSSS)-
Elevated Tm values arise from cocrystallisation
of short isotactic domains
J. Am. Chem. Soc. 2000, 122, 1552-1553 - WebCT
BakerSmith2000.pdf J. Polym. Sci. Polym. Chem.
2000, 38, 4686 - WebCT Coates2000.pdf
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Formation of isotactic PLA from chiral salen Al
initiators
(S)-selective ligand
(S)-selective ligand
(R)-selective ligand
(R)-selective ligand
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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
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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).
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