Synthesis of chiral polymers

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. 

Chiral polymers can be prepared using a one-pot system, i.e., all reactants and catalysts are present at the start of the reaction and both catalysts work simultaneously. However, one can also envisage the synthesis of chiral polymers using catalysts in sequence, either in one pot or even completely independent of each other. This section will deal with the synthesis of chiral block copolymers using different catalysts in sequence. An interesting example of the synthesis of chiral polymers using catalysts in sequence is the synthesis of chiral block copolymers in a sequential approach. Both ATRP and nitroxide-mediated LFRP were evaluated for this purpose. 

Chiral (acyloxy)borane (CAB) is known as an effective chiral Lewis acid catalyst for enantioselective allylation of aldehydes. Marshall applied the CAB complex 1 to the addition of crotylstannane to achiral aldehydes and found that the CAB catalyst gives higher syn/anti selectivity than BINOL/Ti catalysts in the reaction [4]. CAB complex 2 was utilized in asymmetric synthesis of chiral polymers using a combination of dialdehyde and bis(allylsilane) [5] or monomers possessing both formyl and allyltrimethylsilyl groups [6]. 

These empirical guidelines have been applied in the following topics (/) the planning and execution of an absolute asymmetric synthesis of chiral polymers with quantita-... 

The use of lipase-catalyzed kinetic resolution as key step in monomer synthesis is a powerful approach for the synthesis of optically-enriched chiral polymers. An alternative route towards the synthesis of chiral polymers consists of the enzymatic polymerization of optically pure monomers. 

A more elegant approach towards lipase-catalyzed synthesis of chiral polymers is to take advantage of the intrinsic ability of lipases to discriminate between ena-tiomers. Chiral polymers can be formed when one of the two enantiomers of a chiral monomer preferentially reacts during the polymerization reaction. This is referred to as a kinetic resolution polymerization (KRP) and allows the optically active polymer to be directly procured from a racemic mixture, albeit in a maximum of 50% yield. 

Scheme 11.22 Synthesis of chiral polymer precursors for the formation of chiral crosslinked...

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. 

It has been demonstrated that the combination of metal-catalysed racemisation and enzymatic kinetic resolution is a powerful method for the synthesis of optically active compounds from racemic alcohols and amines. There are many metal complexes active for racemisation, but the conditions for enzymatic acylation often limit the application of the metal complexes to DKR. In the case of DKR of alcohols, complementary catalyst systems are now available for the synthesis of both (R)- and (5)-esters. Thus, (R)-esters can be obtained by the combination of an R-selective lipase, such as CAL-B or LPS, and a racemisation catalyst, whereas the use of an A-selective protease, such as subtilisin, at room temperature provides (5)-esters. The DKR of alcohols can be achieved not only for simple alcohols but also for those bearing various additional functional groups. The DKR of alcohols has also been applied to the synthesis of chiral polymers and coupled to tandem reactions, producing various polycyclic compounds. 

High molecular weight stereoregular vinyl polymers contain mirror planes of S5mimetry perpendicular to the molecular axis (Fig. 15) and thus do not have inherent chirality associated with the main chain. Synthesis of chiral polymers from vinyl monomers, with the exception of low molecular weight oligomers. 

In polymer chemistry, one of the most challenging tasks is to efficiently synthesize optically active synthetic polymers. The extraordinary enantioselectivity of lipases offers new perspectives towards these materials, and it is therefore not surprising that some research efforts have focused on the use of lipases to synthesize chiral polymers from racemic monomers. Methodologies like kinetic resolution and even chemoenzymatic dynamic kinetic resolution (DKR) have already been exploited on the industrial scale to afford chiral intermediates for the pharmaceutical and agrochemical industry. Recently, these methodologies have been successfully applied in the synthesis of chiral polymers. 

Following the dctectkm of spontaneous polarizatioa in FLC polymers, a large effort has been launched directed at the synthesis of chiral polymers exhibiling smectic C phases. Deoobert et al. [11] synthesized structural modifications of the Shibaev polymer (2a,2b). 

Figure 9 Synthesis of chiral polymers from (/ )-/V-(1-phenyl-ethyl)methacrylamide and methacrylic acid and (/ )-1-phenylethylamine.

Abstract. The asymmetric synthesis of chiral polymers by topochemically controlled polymerization in chiral crystals is discussed. Following a short survey of topochemical polymerization in the solid state and some comments on chiral crystals, we present the requirements for the performance of asymmetric polymerization based on [2+2]-photocycloaddition. The planning and execution of two successful polymerizations of this sort are described. In the first, we start with a chiral non-racemic monomer and obtain optically active cyclobutane oligomers. The optical yields of the dimer and trimer were quantitative on the scale of N.M.R. sensitivity. In the second reaction we start with a racemate, and by the processes of crystallization in a chiral structure and solid-state reaction we generate an optically active polymer, in the absence of any outside chiral agent. The possible application of this novel method to additional systems is discussed. 

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