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Imperial College London 4A3 Advanced Polymer Synthesis Biorenewable Polymers 2: The Heterotactic Polymerisation of rac-Lactide 4A3 - Slide 19
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  • Imperial College

    London4A3 Advanced Polymer Synthesis

    Biorenewable Polymers 2:

    The Heterotactic Polymerisation of rac-Lactide

    4A3 - Slide 19

  • Imperial College

    London

    Every step is reversible

    Coordinative-Insertion Mechanism

    Recap of Lecture 1: Mechanism of propagation

    4A3 - Slide 20

  • Imperial College

    LondonLast lecture

    (salen)Al(OR) and derivatives convert

    rac-lactide into isotactic poly(lactide)

    Product is a stereoblock copolymer, with short sequences of all R and

    short sequences of all S stereocentres.

    correct structure

    incorrect structure

    4A3 - Slide 21

  • Imperial College

    LondonLearning outcomes

    Over lectures 1 and 2 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.

    4A3 - Slide 22

  • Imperial College

    Londonb-Diketiminate ligands

    Diketimine canonicals

    Deprotonation results in a monoanionic bidentate ligand:

    e.g.

    Ar = 2,6-diisopropylphenyl[(BDI)MgiPr]

    Dalton Trans. 2003, 3088 - WebCT Gibson2003.pdf 4A3 - Slide 23

  • Imperial College

    London

    R = iPr, Pr = 0.90

    R = nPr, Pr = 0.76

    R = Et, Pr = 0.79

    First report of heterotactic PLA

    Coates

    rac-LA

    CH2Cl225 C

    (S)

    (S)

    (R)

    (R)

    Heterotactic PLA

    100 equiv rac-LA consumed in 20 mins Highly stereoselective - Pr = 0.90 (0.94 at 0 C)

    steric bulk of iPr groups is

    essential for stereocontrol

    J. Am. Chem. Soc. 2001, 123, 3229 - WebCT Coates2001.pdf 4A3 - Slide 24

  • Imperial College

    London

    Under the same conditions (CH2Cl2, 25 C)

    [(BDI)Mg(m-OiPr)2] gives atactic PLA

    The Mg analogue

    rac-LA

    THF

    25 C

    Heterotactic PLA, Pr = 0.90

    Chisholm

    However, the Mg initiator is heteroselective in coordinating solvents:

    Inorg. Chem. 2002, 41, 2785 - WebCT Chishiolm2002.pdf

    Prof. Chisholm is lecturing on Thursday 18th

    October at 12pm on lactide polymerisation

    4A3 - Slide 25

  • Imperial College

    LondonHowever, 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:

    Heterotactic PLA formed when the

    propagating species are mononuclear

    Propagating Mg

    species in THF

    Propagating Zn

    species in CH2Cl2

    4A3 - Slide 26

  • Imperial College

    LondonComputational studies J. Am. Chem. Soc. 2005, 127, 6048-6051

    Goals of this research project:

    to understand the mechanism of ring-opening better.

    to explain why the Mg and Zn initiators give heterotactic PLA.

    Method:

    (i) Reaction coordinate mapped out for the insertion of two LA units (LA1 and

    LA2) using (BDI)Mg(OMe)(THF) as the initiator.

    (ii) Free energies of competing transition states (i.e. R,R or S,S-lactide

    insertion) calculated.

    J. Am. Chem. Soc. 2005, 127, 6048 - WebCT Rzepa2005.pdf 4A3 - Slide 27

  • Imperial College

    LondonThe Reaction Coordinate - calculated for LA1 = (R,R) & LA2 = (S,S)

    Two transition states, TS1 and TS2

    TS2 is higher in energy than TS1

    4A3 - Slide 28

  • Imperial College

    LondonRevised 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

    4A3 - Slide 29

  • Imperial College

    LondonComputing the origin of stereocontrol

    We have to consider several possible ways in which the PLA may form:

    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

    LA2

    LA2

    LA1

    4A3 - Slide 30

  • Imperial College

    LondonBut the 8 assembly modes exist as 4 enantiomeric pairs

    8 possible assembly modes = 4 enantiomeric pairs

    only 4 calculations required

    S

    S SS

    R

    R R

    R

    mirror image

    e.g. Consider the approach of S,S-LA2 to R,R-LA1

    4A3 - Slide 31

  • Mg

    LA1

    LA2

    Mg

    Mg

    Mg

    Mg

    Mg

    MgMg

    mirror plane

    SS

    RR

    RR

    SS

    SS

    R

    RR

    SS

    S

    R

    S

    S

    SS

    RR

    R

    S

    R

    RR

    RR

    SS

    mirror plane

    Imperial College

    LondonOnly four sets of calculations required

    4A3 - Slide 32

  • Imperial College

    LondonCurtis-Hammett Principle

    All stages of the ring-opening mechanism is reversible, so the cause of

    stereoselectivity arises from the energetics of the rate-determining step.

    Calculated transition state free energies (kcal mol-1):

    LA1 LA2 TS1 TS2

    RR RR 13.5 20.2

    SS SS 6.7 25.4

    RR SS 10.5 18.9

    SS RR 12.5 28.1

    In every case TS2 is rate-determining

    Therefore, the reason for heterotactic stereocontrol must lie in TS2

    lowest barrier for

    heterotactic PLA

    4A3 - Slide 33

  • Imperial College

    LondonCompeting TS2 geometries - the origin of stereocontrol

    RR,SS:

    18.9 kcalmol-1

    4A3 - Slide 34

  • Imperial College

    LondonCompeting TS2 geometries - the origin of stereocontrol

    RR,SS:

    18.9 kcalmol-1

    RR,RR:

    20.2 kcalmol-1

    4A3 - Slide 35

  • Imperial College

    LondonCompeting TS2 geometries - the origin of stereocontrol

    RR,SS:

    18.9 kcal mol-1

    RR,RR:

    20.2 kcal mol-1

    SS,SS:

    25.4 kcal mol-1

    4A3 - Slide 36

  • Imperial College

    LondonCompeting TS2 geometries - the origin of stereocontrol

    RR,SS:

    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

    4A3 - Slide 37

  • Imperial College

    LondonSummary 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

    4A3 - Slide 38

  • Imperial College

    LondonConclusions

    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.

    4A3 - Slide 39


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