Polyesters, optically active

The enantioselectivity was greatly improved by the copolymerization with 7- or 13-membered non-substituted lactone using lipase CA catalyst (Scheme 8) the ee value reached ca. 70% in the copolymerization of (3-BL with DDL. ft is to be noted that in the case of lipase CA catalyst, the (5 )-isomer was preferentially reacted to give the (5 )-enriched optically active copolymer. The lipase CA-catalyzed copolymerization of 8-caprolactone (6-membered) with DDL enan-tioselectively proceeded, yielding the (/ )-enriched optically active polyester with ee of 76%. 

Optically active polyesters were synthesized by lipase CA-catalyzed ring-opening polymerization of racemic 4-methyl or ethyl-e-caprolactone. The (5 )-isomer was enantioselectively polymerized to produce the polyester with >95% ee. Quantitative reactivity of 4-substituted e-caprolactone using lipase CA as catalyst was analyzed. The polymerization rate decreased by a factor of 2 upon the introduction of a methyl substitutent at the 4-position. Furthermore, 4-ethyl-8-caprolactone polymerized five times slower than the 4-methyl-8-caprolactone. This reactivity difference is strongly related to the enantioselectivity. Interestingly, lipase CA displayed 5 -selectivity for 4-methyl or ethyl-8-caprolactone, and the enantioselectivity was changed to the (f )-enantiomer in the case of 4-propyl-8-caprolactone. 

PPF catalyzed an enantioselective polymerization of bis(2,2,2-trichloroethyl) tra 5-3,4-epoxyadipate with 1,4-butanediol in diethyl ether to give a highly optically active polyester (Scheme 9). °° The molar ratio of the diester to the diol was adjusted to 2 1 to produce the (-) polymer with enantiomeric purity of >96%. The polymerization of racemic bis(2-chloroethyl) 2,5-dibromoadipate with excess of 1,6-hexanediol using lipase A catalyst produced optically active trimer and pentamer. The polycondensation of 1,4-cyclohexanedimethanol with fumarate esters using PPL catalyst afforded moderate diastereoselectivity for the cis/trans monocondensate and markedly increased diastereoselectivity for the dicondensate product. 

Enzymatic enantioselective oligomerization of a symmetrical hydroxy diester, dimethyl /Lhydroxyglutarate, produced a chiral oligomer (dimer or trimer) with 30-37% ee [24]. PPL catalyzed the enantioselective polymerization of e-substituted-e-hydroxy esters to produce optically active oligomers (DP < 6) [25]. The enantioselectivity increased with increasing bulkiness of the monomer substituent. Optically active polyesters with molecular weight of more than 1000 were obtained by the copolymerization of the racemic oxyacid esters with methyl 6-hydroxyhexanoate. 

The same catalyst system was applied to the condensation of racemic a,a -dimethyl-l,4-benzenedimethanol and dimethyl adipate. Optically active polyesters (Mw = 3400g/mol Mn = 2100g/mol) were obtained [24] (Scheme 1.18). 

Interestingly, enzymes are chiral catalysts and their potential for enantio-selective polymerization has been investigated [93]. Several examples are reported where a racemic mixture of lactones is polymerized by enzymatic polymerization to afford the corresponding optically active polyester [93]. For instance, lipase CA (Novozym 435) catalyses the ROP of racemic 4-methyl-s-caprolactone and 4-ethyl-s-caprolactone in bulk at 45 °C and 60 °C to afford (S )-eiuiched poly(4-methyl-e-caprolactone) and poly(4-ethyl- -caprolactone) with an enantiomeric purity higher than 95% [153]. 

Shirahama H, Shomi M, Sakane M, Yasuda H (1996) Biodegradation of novel optically active polyesters synthesized by copolymerization of (R)-MOHEL with lactone. Macromolecules 29 4821 828... 

The ring-opening polymerization of dilactide (dimeric cyclic ester of lactic acid) allows the preparation of high molecular weight, optically active polyesters of lactic acid. The configuration of the asymmetric carbon atoms of the monomer is retained when the polymerization is initiated with SnCl4 or Et2Zn, for example ... 

Lipase-catalyzed ring-opening polymerization of nine-membered lactone, 8-octanolide (OL), has been reported.165 Lipases CA and PC showed the high catalytic activity for the polymerization. Racemic fluorinated lactones with a ring size from 10 to 14 were enantioselectively polymerized by lipase CA catalyst to give optically active polyesters.166... 

Chart 3.4 Chemical structures of an electro-optically active polyester and a chemically related monomer. 

Poly(3-hydroxyalkanoates) (PHAs) are optically active polyesters that are produced as an intracellular energy source and carbon storage materials by a wide variety of bacteria, and have great importance as biocompatible and biodegradable thermoplastic materials. ... 

Tsuji, H., Yamamoto, S., Okumura, A. and Sugiura, Y. (2010) Hetero-stereocomplexation between biodegradable and optically active polyesters as a versatile preparation method for biodegradable materials. Biomacromolecules, 11,252-258. 

He, Z. and R.E. Prud homme. 1999. Conformational and packing modeling of optically active polyesters. 2. Helical structure of an isotactic polyactone. Macromolecules 32 7655. 

Polyesters with high optical purity were synthesized by the lipase CA-catalyzed copolymerization of racemic /3-BL with e-CL or DDL (175). (S)- -BL was preferentially reacted with DDL to give the (S)-enriched optically active copolymer with ee of /3-BL unit = 69%. 5-CL was also enantioselectively copolymerized by the lipase catalyst to give the (R)-enriched optically active polyester with ee up to 76%. 

Optically active polymers are important functional materials for several industrial and bio-m ical applications and are extensively used as chiral catalysts for asymmetric synthesis, packing materials of chromatographic columns and chiral materials for the preparation of liquid crystal polymers (7). Polymers such as poly hydroxy alkanoates (PHAs), naturally occurring microbial optically active polyesters, are important materials in biomedical applications owing to their biodegradability (2). In synthetic polymer chemistry, synthesis of optically active polymers has been one of the most challenging tasks. Most synthetic chiral polymers are prepared from optically pure starting materials which are, except when isolated from nature, in limited supply and difficult to prepare (7, 3). 

Optically active lactones are valuable building blocks in organic synthesis (4) and in the preparation of optically active biodegradable polymers (7,5). Several chemical methods for producing these compounds and their corresponding polymers have been explored (6) but unfortunately all of these methods are either experimentally cumbersome or afford the lactones with only modest enantioselectivities. Examples of chemically prepared optically active polyesters include poly(a-phenyl-P-propiolactone) (7), poly(a-ethy(-a-phenyl-P -propiolactone) (S, 9), poly(a-methyl-a-ethyl-P-propiolactone) (70) and poly(lactic acid) (77, 72). Use of enantioselective polymerization catalysts to carry out stereoelective polymerizations of racemic lactones has produced mixed results. For example, stereoelective polymerization of [/ ,S]- P-methyl-P-propiolactone with a catalyst from Zn ( 2115)2 and [7 ]-(-)-3,3-dimethyl-l,2-butanediol showed only a small enantiomeric enrichment in the final polymer (75). Stereoselective copolymerizations of racemic (LL/DD monomers) and meso (LD monomer) lactides using chiral catalyst that gives heterotactic and syndiotactic PLA, respectively have also been studied (77). 

When the vine-twining polymerization was conducted using optically active polyesters, poly(lactide)s (PLAs), which had three kinds of the stereoisomers, i.e., poly(L-lactide) (PLLA), poly(D-lactide) (PDLA), and poly(DL-lactide) (PLDLA), as guest polymers, the author found that amylose, produced by the enzymatic polymerization, perfectly recognized the chirality in PLAs on complexation and... 

For the synthesis of an optically active polyester from a racemic monomer, a new method of dynamic kinetic resolution was used. A mixture of stereoisomers of a secondary diol, a,a -dimethyl-l,4-benzenedimethanol, were enzymatically polymerised with dimethyl adipate (Scheme 12.6, [1]) [32]. 

Fluorinated lactones, lO-fluorodecan-9-olide, 11-fluoroundecan-10-olide, 12-fluorododecan-ll-olide, and 14-fluorotetradecan-13-olide, were polymerized by lipase to produce the optically active polyesters with an of 3000 to 8000, while 10-fluorodecan-ll-olide gave an optically inactive polymer with an of 11000 by lipase-catalyzed ring-opening polymerization (Scheme 17) [115]. 

Examples of stereoelective copolymerization have been reported. Matsuura, Tsuruta, Terada, and Inoue (156) prepared a polyester from 3-phenyl-A -tetrahydrophthalic acid anhydride and propylene oxide using diethyl zinc-(+)borneol as an optically active catalyst and obtained an optically active polyester. The authors reported that the stereoelective copolymerization of 3-phenyl-tetrahydrophthalic acid anhydride had occurred, but that of propylene oxide was not observed. Kximata, Furukawa, and Saegusa (157) copolymerized racemic propylene oxide with ethylene oxide using the ZnEt2/(+)-borneol system, and found that stereoelectivity of propylene oxide was not hindered by the occurrence of an ethylene oxide unit at the end of the growing chain. 

When the substituent is in the 3 position one encounters a family of naturally occuring optically active polyesters of which poly-3-hydroxybutyrate (PHB) is the most common. 

Efforts to synthesize P(3HB) without impurities of natural origin have led to the development of chemical routes to PHAs, which include the ROP of butyrolactone and other different lactones. However, due to the inability to easily obtain optically active polyesters and the lower molecular weights of the obtained products compared to those achievable in the biosynthetic approaches, these routes to PHAs are not very significant, except for the preparation of otherwise unattainable copolymers. On the other hand, racemic P(3HB) obtained by ROP is less crystalline, with elastomeric properties, and can be of interest in some medical applications, such as drug delivery. 

Poly(3-hydroxbutyric acid) [P(3HB)] is the most famous member in the family of PHAs. This homopolymer is an optically active polyester of (R)-specific units and has a high crystallinity up to 60 70%. The particular advantage is that P(3HB) is a thermoplastic and therefore can be processed by using the existing equipment. However the drawbacks are also serious... 

Kikuchi, H., Uyama, H. and Kobayashi, S. (2000) lipase-catalyzed enantioselective copolymerization of substituted lactones to optically active polyesters. Macromolecules, 33, 8971-5. 

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