Transcript Slide 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
[email protected]
www.ch.ic.ac.uk/marshall/4I-11.html & WebCT
4I-11 - Lectures 7 - Slide 1
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.
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London
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
corn
fermentation
starch
lactic acid
step-growth
condensation
ring-opening
polymerisation
*
*
polylactic acid, PLA
heat
*
(chain growth)
(-H2O)
*
lactide
oligomers
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
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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"
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
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PLA - stereoregular microstructures (tacticities)
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London
isotactic
-(SSSSSSSS)"poly(L-lactide)"
S,S
isotactic
-(RRRRRRRR)"poly(D-lactide)"
R,R
heterotactic
-(SSRRSSRR)rac
meso
syndiotactic
-(SRSRSRSR)4I-11 - 7 - 7
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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
>24
syndiotactic PLA
153
45
n/a
n/a
heterotactic PLA
amorphous
49
n/a
n/a
atactic PLA
amorphous
55
1.9
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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)
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Single-site catalysts
X
Ln
sterically bulky
ancillary ligand(s)
M
e.g. Cp2ZrMe+ for the
polymerisation of ethene
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
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London
Spassky
rac-LA
60 °C
isotactic bias
Pm = 0.68
Stereoselectivity presumably arises from a chain-end control mechanism:
[Al](OMe)
[Al](OMe)
(S,S)-LA
(R,R)-LA
[Al]-(S,S)-OMe
k(S,S) / k(R,R) = 2.8
[Al]-(R,R)-OMe
k(R,R) / k(S,S) = 2.8
Macromolecular Chemistry and Physics 1997, 198, 1227-1238
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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
60 °C
highly isotactic
Pm = 0.92
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?
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Spassky
rac-LA
60 °C
k(R,R) / k(S,S) = 20
enantiopure initiator
The polymer produced is a tapered stereoblock copolymer:
increasing R content
-RRRRRRRRRRR-
-RR---------------SSincreasing S content
Macromolecular Chemistry and Physics 1996, 197, 2627
-SSSSSSSSSSSS-
Tm = 185 °C
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Dispelling the stereocomplex myth
Smith & Baker / Coates
rac-LA
60 °C
racemic initiator
isotactic PLA
Tm = 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)-
J. Am. Chem. Soc. 2000, 122, 1552-1553 - WebCT Baker&Smith2000.pdf
J. Polym. Sci. Polym. Chem. 2000, 38, 4686 - WebCT Coates2000.pdf
Elevated Tm values arise
from cocrystallisation of
short isotactic domains
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Formation of isotactic PLA from chiral salen Al initiators
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(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
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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).
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