Chiral polyesters

More recently, Heise and coworkers have shown that DKR can be combined with enzymatic polymerization for the synthesis of chiral polyesters from racemic secondary diols in one pot [34] (Figure 4.12). 

Fig. 42.16 Chiral polyester-polyamide composite containing supported BINAP.

Extension of DKR to polymer chemistry would readily result in chiral polyesters, polycarbonates, or polyamides from an optically inactive monomer mixture. Scheme 10 describes three variants of chemoenzymatic catalysis applied in polymer chemistry that recently appeared in the literature. Route A uses AA and BB monomers to prepare chiral polymers from racemic/diasteromeric diols. Route B converts an enantiomer mixture of AB monomers to homochiral polymers. Route C is the enzymatic ring-opening polymerization of co-methylated lactones to homochiral polyesters. Details will be given in Sect. 3.4.2. 

In the present chapter, the current status of enzymatic polyester synthesis is described. For information on the enzymatic synthesis of chiral polyesters and polyester block copolymers using enzymatic polymerizations please refer to Chapters 11 and 12 respectively. 

Chiral polyesters by dynamic kinetic resolution polymerization. Angew. Chem. Int. Ed., 45 (13), 2130-2132. 

While the polymerization of optically inactive AA-BB and AB monomers under DKR conditions leads to chiral polyesters, these approaches always result in limited molecular weights since a condensation product is formed that needs to be effectively removed. A solution for this would be to use the eROP of lactones, where no condensation products are formed during polymerization. In principle, the eROP of lactones can lead to very high MW polyesters (>80kgmol 1) [57]. Addition of a methyl substituent at the ro-position of the lactone introduces a chiral center. Peeters et al. conducted a systematic study of substituted e-caprolactones which revealed that monomers with a methyl at the 3-, 4-, or 5-position could be polymerized enantioselectively while a methyl at the 6-position (a-methyl-e-caprolactone, 6-MeCL) could not [58]. The lack of reactivity of the latter monomer in a Novozym 435-catalyzed polymerization reaction was attributed to the formation of S-secondary alcohol end-groups. These cannot act as a nucleophile in the propagation reaction since the lipase-catalyzed transesterification is highly R-selective for secondary alcohols. 

Hilker et al (44) combined dynamic kinetic resolution with enzymatic polycondensation reactions to synthesize chiral polyesters from dimethyl adipate and racemic secondary diols. The concept offered an efficient route for the one-pot synthesis of chiral polymers from racemic monomers. Palmans at al (18,43) generalized the approach to Iterative Tandem Catalysis (ITC), in which chain growth during polymerization was effected by two or more intrinsically different catalytic processes that were compatible and complementary. 

Chiellini E., GaUi G., Po R., Chiral liquid-crystalline polymers. XII. New chiral polyesters by chemical modification of a nonmesophasic polymer, Polym. Bull., 23, 1990, 397-402. 

In 2006, Heise s group reported a novel concept for the synthesis of chiral polyesters based on a lipase-catalysed DKR polymerisation of racemic diols." As shown in Scheme 4.25, a mixture of stereoisomers of a secondary diol was enzymatically polymerised with a difunctional acyl donor (dicarboxylic acid... 

As reported by Kanca and co-workers, the enantioselective transesterification polycondensation of racemic AB-type monomers containing a secondary hydroxy group and a methyl ester moiety led to chiral polyesters by iterative tandem catalysis. The concurrent actions of an enantioselective acylation catalyst (such as CALB) and a racemisation catalyst (Ru(Shvo)) resulted in the high conversion of the racemic monomers to enantioenriched polymers. AB-type monomers used were typically methyl 6-hydroxyheptanoate, methyl 8-hydroxynonanoate and methyl 13-hydroxytetradecanoate. The polycondensation at 70 "C in toluene gave a polyester of high yield with a M of around several thousand and an enantiomeric excess higher than 74% [18]. 

Scheme 8.31 Synthesis of chiral polyesters via enz5miatic DKR polymerisation with a ruthenium catalyst.

Copolymerization of racemic lactones with an achiral lactone is an effective way to prepare a chiral polyester. The enantioselective copolymerization of racemic lactones with achiral lactones, such as s-CL and DDL, has been established by lipase. As a typical example, fi-BL and 6 -CL were copolymerized by lipase in isooctane to produce the S-enriched optically active copolyester with 69% e.e. of the -BL unit, and R - P-BL with 100% e.e. remained unreacted. This indicated that the S-isomer of -BL preferentially reacted during the copolymerization (Scheme 16a) [114]. Also, during the copolymerization of the racemic y0-BL with DDL, the (S)-isomer of -BL preferentially reacted to give the (S)-enriched optically active copolymer with an enantiomeric excess of the y0-BL unit of 69%, which is much higher than that for the homopolymerization of yS-BL. (5-CL was also enantioselectively copolymerized... 

Figure 4.12 Chiral polyester-supported BINAP ligands for Ru-catalyzed asymmetric...

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