Structure. Extremely interesting BPs (very high Tm ~ C) Excellent thermic and mechanical properties

Transcription

1 tructure Extremely interesting BPs (very high Tm ~ C) Excellent thermic and mechanical properties Limitations: no straigthforward synthetic pathways available (Expensive material) Applications: biomedical devices Left-handed Helicoidal chain ight-handed Helicoidal chain Ikada et al. Macromolecules 1987

2 Through polymerization catalysis of lactide with a well-defined catalyst Possibility of a controlled polymerization process affording polymers with well-defined chain length and stereoregularity 1) M- Catalyst 2) Hydrolysis n H Dimers of lactic acid Poly(lactic acid)

3 General Mechanism for the ing-opening Polymerization of Lactides by Metal Alkoxides Complexes LM LM LM n M = oxophilic and electropositive (Mg, Ca, Al, Y, Ti, Zr, Zn,) Important features of the catalyst: - Lewis acidity of M - ucleophilicity de - - ature of the ligand L LM Living polymerization (in the ideal case) n

4 (D,L)-LA Al( i Pr) 3 (1 mol%) H 2n Atatic Poly(lactic) acid Amorphous Biodegradable Positive Aspect: Living Polymerization (control of the polymer chain length) egative Aspect: o discrimination between the two lactide enantiomers o stereoregularity in the prepared PLA

5 tereoselective Polymerization of rac-lactide: Access to stereoregular PLAs Al Me cat.: 1% mol. (L)-lactide (D)-lactide rac-lactide Isotactic tereoblock Poly(lactide) Properties: ew material, extremely high melting point (T m up to 190 C) And highlyresistant - Importance of the use of a well-defined and single-site catalyst Le Borgne, A.; Vincens, V.; Jouglard, M.; passky,. Makromol. Chem., Macromol. ymp. 1993, 73, 37. omura,.; Ishii,.; Akakura, M.; Aoi, K. J. Am. Chem. oc. 2002, 124, 5938.

6 LAl n - 1 LAl n initial insertion of (,)-lactide LAl n - 1 LAl n initial insertion of (,)-lactide The last inserted lactide unit stereocontrols the insertion of the subsequent unit transesterification hydrolysis Isotactic tereoblock PLA H n n'

7 H n Commercial Poly(lactic acid) T m = 162 C Extremely interesting BPs (very high Tm ~230 C) Excellent mechanical properties Isotactic tereoblock Poly(lactic acid) (Tm~ 190 C) Inter-molecular Chain interactions Intra-molecular Chain interactions The «tereocomplex Character» improves its thermal properties

8 Enantioselective Polymerization Principle: In an enantioselective polymerization, one enantiomer of a racemic monomer mixture is preferentially polymerized to give an optically active polymer (with a specific chain stereochemistry) Enantiopure Catalyst A A racemic 50% conversion Major advantage: - btention of a polymer with a controlled stereochemistry

9 The use of enantiomerically pure Al Complexes: Enantioselective Polymerization of Lactides Interest: Access to stereoregular PLA polymers Al Me ()-L*Al (L)-LA (D)-LA racemic ()-L*Al (D)-lactide polymerized preferentially (optically active isotactic Poly(D-lactide) At 50% conversion, ee (unreacted monomer) = 80% ee k D /k L = 20 passky Macromol. Chem. Phys. 1996, 197, 2627.

10 Formation of new materials via the formation of enantiomeric poly(l-la) and poly(d-la) block polymers Al Me ()-L*Al ()-L*Al racemic (L)-LA (D)-LA racemic mith et al. JAC 2000, 122, ()-L*Al ()-L*Al Due to chain transfer during the polymerization process Poly(L-lactide) (isotactic) Poly(D-lactide) (isotactic) Isotactic tereoblock Poly(lactide) Properties: ew material, extremely high melting point (T m = 187 C) And highly resistant

11 L* () Al L* () Al initial insertion of (,)-lactide initial insertion of (,)-lactide n - 1 n'-1 L* () Al L* () Al n n' transesterification L*Al = "chiral pocket" that induces stereocontrol L*Al n n'

12 Importance of the Metal Catalyst Activity: Y >, n > Al > Ti, Zr tereoregularity: Al >, n, Y > Ti, Zr > Toxicity: n whereas are biocompatible Cost: Y is an expensive metal Lactide Monomer High purity required for high activity (it costs!!!)

13 Interest: - use of metal-free catalysts (because of toxicity problems) - no need to remove metal traces prior to polymer processing (costly) Approach: - use of very nucleophilic organic species to ring-open lactide - the ring-opened species ring-opens another lactide unit and so on. Most efficient ucleophiles to date: dimethylamino- -pyridine (DMAP) -heterocyclic carbenes (highly nucleophilic)

14 Atactic PLA - H H Waymouth, Hedrick et al. JAC 2005, 127, 9079.

15 «H»

16 Cyclic PLAs are more thermally stable linear PLAs Waymouth, Hedrick et al. Angew. Chem. Int. Ed. 2007, 46, 2627

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18 rganocatalysis: P of lactide thanks to supramolecular recognition Waymouth et al. J. Am. Chem. oc Use of a thiourea-amine Bifunctional catalyst such as 1

19 From:

20 Poly(hydroalkanoates) : PHAs PHAs are naturally produced by micro-organisms from various carbon sources (typically from the sugar family) ome micro-organismsmay accumulate PHA From 30% to 80 % of cellular dry weight Depending on the carbon source, different monomers and thus Polymers may be obtained. Polyhydrobutyrate (PHB) is the main polymer of this family PHB PHB n PHV m (Biopol, Monsanto, 20% HV) Major problem: the extraction and recovery steps are expensive

21 - First observed in bacteria by Lemoigne et al. in the 1920s ptically active macromolecules (PHBs) used as a «carbon reserves» by bacteria

22 Biosynthesis of PHB Acetic acid Implicated enzymes: (1) ketothiolase: dimerizes acetyl-coa (2) reductase: hydrogenation to ()-3-hydroxy-butyryl-CoA (3) ynthase (or polymerase): polymerisation: access to PHB

23 Two thiol groups within the polymerase enzyme are believed to be involved in the initiation and propagation polymerisation process.

24 PHB is a highly crystalline biodegradable and biocompatible polymer (Tg = 5 C, Tm = 153 C)

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