Chiral from optically pure monomers

The synthetic synthesis of known chiral polymers mostly starts from optically pure monomers obtained form the chiral pool. The optically pure fermentation product L-lactic acid, for example, is the starting material for the synthesis of poly(L-lactide). However, converting a racemic or achiral monomer quantitatively into a homochiral polymer is less straightforward [3]. This is surprising considering the enormous potential of biocatalysis and tandem catalysis that has emerged in the past decades to prepare optically active intermediates [4]. 

One of the easiest ways to prepare chiral polymers is the polymerization of optically pure monomers. These monomers can be derived or isolated from the chiral pool, be synthesized from prochiral substrates using asymmetric catalysis, or be obtained by lipase-catalyzed resolution of a racemate followed by further synthetic manipulation. 

Since PHAs are isotactic, they can serve as a feedstock for enantiomeric compounds, which can be used in the synthesis of chiral chemicals such as antibiotics, vitamins, fragrances, and pheromones. Optically pure monomers may be obtained from PHAs by chemical hydrolysis at 80-160°C using a titanate catalyst (25) or by enzymatic hydrolysis using extracellular bacterial depolymerases. It is possible to synthesize the optically active monomers by using bacteria, which can make the polymer but lack the 3-hydroxyalkanaote polymerase (Fig. 3a) or possess a high activity of intracellular PHA depolymerase. Monomers such as 4HB have therapeutic applications as an intravenous anaesthetic, for the treatment of narcolepsy, alcohol, heroin, and nicotine addiction (26). 

The first one is that of polymers obtained from optically active monomers for which the optical purity of the polymer is that of the original monomer. In this case, the polymerization reaction does not modify the chirality of the monomer unit as compared to that of the initial monomer. Macromolecules obtained from the polymerization of ethylenic monomers having an optically active lateral group correspond to this case for example, optically active 4-methylhex-l-ene generates an optically pure polymer whose chiral centers are located on the side-chain ... 

Polymers derived from natural sources such as proteins, DNA, and polyhy-droxyalkanoates are optically pure, making the biocatalysts responsible for their synthesis highly appealing for the preparation of chiral synthetic polymers. In recent years, enzymes have been explored successfully as catalysts for the preparation of polymers from natural or synthetic monomers. Moreover, the extraordinary enantioselectivity of lipases is exploited on an industrial scale for kinetic resolutions of secondary alcohols and amines, affording chiral intermediates for the pharmaceutical and agrochemical industry. It is therefore not surprising that more recent research has focused on the use of lipases for synthesis of chiral polymers from racemic monomers. 

One may have a question of whether an optically active helical polymer obtained from an enantiopure monomer adopts a purely P- (or M-) screw sense helical main chain in solution at a given temperature, or is composed of an ensemble of pseudo-diastereomeric mixed helical motifs containing P- and M-screw senses. Fluorescence (FL) studies combined with circular dichroism (CD), UV, and NMR spectra of the main chain constitute a powerful probe in identifying the main chain chirality (screw sense, uniformity, and rigidity) and optical purity of helical polymers, since the photoexcited energy above... 

There are several alternative methods for the synthesis of optically active polymers from achiral or racemic monomers that do not involve polymerization catalysts. Optically active polymers have been formed from achiral dienes immobilized in a chiral host lattices [ 106]. In these reactions, the chiral matrix serves as a catalyst and can be recovered following the reaction. For example, 1,3-penta-dienes have been polymerized in perhydrotriphenylene and apochoUc acid hosts, where asymmetric induction occurs via through-space interactions between the chiral host and the monomer [107,108]. The resultant polymers are optically active, and the optical purities of the ozonolysis products are as high as 36%. In addition, achiral monomers have been found to pack in chiral crystals with the orientations necessary for topochemical soHd-state polymerization [109]. In these reactions, the scientist is the enantioselective catalyst who separates the enantiomeric crystals. The oligomers, formed by a [27H-27i] asymmetric photopolymerization, can be obtained in the enantiomeric pure form [110]. 

Photocycloaddition of chiral crystals of the optically pure (S)-(-l-) or ( )-(—) monomer ethyl 2-cyano-3-[p-sec-butyl-3 -( )-propenoate]-phenyl-(E)-propenoate (237), provided photodimers and polymers with quantitative diastereomeric yields from ]2tc -l- 2tc topochemical photocycloaddition [183] (Scheme 37). 

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