Racemic monomers

An intermediate case between the polymerization of enantiomerically pure and racemic monomers is the polymerization of a partially resolved mixture of enantiomers with variable optical purity. Two processes are distinguishable, depending on whether polymerization occurs in the presence of an achiral (or racemic) or of a chiral (optically active) catalyst. 

A somewhat different situation arises in the copolymerization of a racemic monomer with an optically active monomer of similar structure in the presence of a conventional stereospecific (or stereoselective) catalyst (299, 321). Examples concern the copolymerization of racemic 3,7-dimethyl-1-octene with (5)-3-methyl-l-pentene and of racemic jec-butyl vinyl ether with various optically active vinyl ethers. In all cases there was preferential copolymerization of one of the two enantiomers of the racemic monomer with the second monomer and simultaneous formation of an optically active homopolymer containing predominantly the noncopolymerized antipode, according to Scheme 20. The two products are easily separated, due to their different solubilities. 

The isoselective polymerization of a racemic mixture of monomers can proceed in two ways depending on initiator, monomer, and reaction conditions. Racemate-forming enantiomer-differentiating polymerization involves both the R and 5 monomers polymerizing at the same rate hut without any cross-propagation [Hatada et al., 2002]. A racemic monomer mixture polymerizes to a racemic mixture of all-5 and all-5 polymer molecules [Pino, 1965 ... 

A special case of asymmetric enantiomer-differentiating polymerization is the isoselective copolymerization of optically active 3-methyl-1-pentene with racemic 3,7-dimethyl-1-octene by TiCl4 and diisobutylzinc [Ciardelli et al., 1969]. The copolymer is optically active with respect to both comonomer units as the incorporated optically active 3-methyl-l-pentene directs the preferential entry of only one enantiomer of the racemic monomer. The directing effect of a chiral center in one monomer unit on the second monomer, referred to as asymmetric induction, is also observed in radical and ionic copolymerizations. The radical copolymerization of optically active a-methylbenzyl methacrylate with maleic anhydride yields a copolymer that is optically active even after hydrolytic cleavage of the optically active a-methylbenzyl group from the polymer [Kurokawa and Minoura, 1979]. Similar results were obtained in the copolymerizations of mono- and di-/-menthyl fumarate and (—)-3-(P-styryloxy)menthane with styrene [Kurokawa et al., 1982],... 

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. 

In the next year, Price and his coworkers (6,7) found that the crystalline polymer obtained by Pruitt and Baggett was isotactie. The fact that the crystalline polymer obtained from racemic monomer with the iron catalyst had the same X-ray pattern as the optically active crystalline polymer obtained from the optically active monomer under the same condition showed that these polymers were isotactic, and that the asymmetric carbon atoms in this polymer had the same configuration as in the monomer from which it was derived, i.e., propylene oxide polymerized with retention of configuration of its asymmetric carbon atom. 

The above arguments do not disprove the reality of helical polymerisation they show only that the kinetic evidence may not be sound. A stronger proof for such a growth is provided by the study of the effects exerted by the presence of one enanthiomorph on the propagation of the other. This approach was also explored by Lundberg and Doty, and their results are given in Table 8. The racemic monomer, as well as the mixture of both isomers, polymerises more slowly than the pure enanthiomorph. The identical rates found for d,l and d + l mixture prove that... 

Chromatographic separation of polymers obtained from racemic monomers. 433... 

Polymerization of non-asymmetric or racemic monomers with the aid of optically active catalysts or initiators ... 

The third method (106), which has been so far adopted only to produce optically active polymers from poly-a-olefins obtained from racemic monomers, is of great interest, owing to its simplicity and to the relatively high optical purity attained in one step. 

An appreciable optical activity, depending on the degree of polymer stereoregularity, may be expected and, as will be shown below, has been actually observed in vinyl polymers derived from racemic monomers, when one of the two antipodes is preferentially polymerized (104). 

Table 4. Physical properties of different fractions of polymers of 3-methyl-1-pmtene and 3.7-dimethyl-l-octene obtained by polymerisation of the racemic monomers with (+ )-bis-[(S -2-methyl-butyl]-zinc and TiClt or TiCl3 "ARA ...

Thus the stereospecific polymerization of a racemic monomer will yield optically active polymers only if it is accomplished stereoelectively, namely in the presence of catalysts capable of polymerizing preferentially one of the two antipodes of the racemic monomer. 

The synthesis of optically active polymers from racemic monomers could in principle be ascribed i) to asymmetric induction by optically active terminal groups of the macromolecules originally present in the catalyst, ii) to a steric control of the propagation exerted by the intrinsically asymmetric active centers of the catalyst, or iii) to both these factors. 

Further experimental data are needed to draw reasonable conclusions about the differences in stereoisomeric composition or in solubility of polymers from optically active and racemic monomers. 

Preliminary data on the crystalline structure of some poly-a-olefins obtained from optically active and racemic monomers were obtained by G. Natta, P. Corradini, I. W. Bassi (80) and are reported in Table 6. 

I. R. spectra of polymers of optically active and racemic monomers (12) having similar stereoregularity are identical in the case of poly-5-methyl-l-heptene, but slightly different in the case of poly-3-methyl-l-pentene and poly-4-methyl- 1-hexene. A very characteristic crystallinity band has been found in the I. R. spectrum of poly-5-methyl-l-heptene at 12.06 fi bands which seem connected with stereoregularity have been found in the I. R, spectrum of poly-4-methyl-l-hexene at 10.06 fi the nature of these bands has been proved when preparing a practically atactic sample by hydrogenation of poly-4-methyl-l-hexyne (24). 

Investigations were made on the I. R. spectra of poly-[(S)-l-methyl-propylj-vinyl-ether and poly-[(S)-2-methyl-butyl]-vinyl-ether (65) a band at 911 cm-1 related to stereoregularity was detected in the spectrum of the former while a crystallinity band at 827 cm-1 was found in the latter. The spectra of polymers obtained from an optically active or a racemic monomer did not reveal any remarkable difference. 

It is long since that the resolution of low-molecular-weight racemic compounds by elution chromatography on optically active supports was known (58) however only recently vinyl-polymers obtained from racemic monomers have been separated in fractions having optical activity of opposite sign by this technique (106). 

Therefore, this method appears very promising not only to investigate the structure of the polymers of racemic monomers, but also to prepare polymers of moderate optical purity, particularly when the preparation of optically active monomers is troublesome. 

As shown in Table 22 in most examples, the prevailing absolute configuration of the asymmetric carbon atoms of the lateral chains of the first eluted fractions is opposite to the one of the support this indicates that the polymer having the same structure as the support is more strongly adsorbed. However, this is not a general phenomenon, as it is shown by the chromatography of poly-3.7-dimethyl-l-octene obtained from the racemic monomer using poly-(S)-3-methyl-l-pentene as support (118). 

Polymer from the racemic monomer Supporting polymer Prevailing absolute configuration of the asymmetric carbon atom of the lateral chains of the first eluted polymer fractions Ref. 

Beside the structure of the monomer, also the type of catalyst used should play an important role in favouring the synthesis of either prevailingly (R) and (S) separable polymers or random (R) (S) copolymers from racemic monomers until now all the separable polymers have been produced by heterogeneous coordination catalysts. 

The investigations on the mechanism of chromatograph ic separation and on the production of different types of polymers of racemic monomers should lead to interesting improvements in this method for the preparation of optically active polymers. 

In the field of the stereospecific heterogeneous polymerization of a-olefins, the stereoselective (106,118) and stereoelective (103) polymerization of racemic monomers, having an asymmetric carbon atom in a. 

In this case, in the polymerization of a racemic monomer, the antipode forming the first monomeric unit of a macromolecule should be responsible for the type of antipode prevailingly included in that particular macromolecule. 

Section II. A new method of synthesizing optically active polymers has been found based on the copolymerization of a racemic monomer with an optically active monomer having similar structure. In fact the optically active monomers copolymerize preferentially with the antipode of the racemic monomer having the... 

Olefin polymerization using heterogeneous catalysts is a very important reaction and stereochemical aspects have been studied extensively. For a review on this topic see Pino et al. [9], Briefly, the origin of stereoregularity in polyolefins (47) is explained by the chiral nature of the acdve site during polymerization. If the absolute configuration of the first intermediate can be controlled by chiral premodification then we should obtain a non-racemic mixture of R - and "S"-chains. This has indeed been observed e.g. with catalyst M4 for the polymerization (partial kinetic resolution) of racemic 3,7-dimethyl-l-octene (ee 37%) and also for the racemic monomer 46 using Cd-tartate M5. 

The extremely slow ROMP of 1,7,7-trimethylnorbornene is initiated by 8 (R = Ph) to give an all-fra/w, all-HT polymer which is necessarily isotactic when made from a single enantiomer. It is atactic when made from racemic monomer showing that the two enantiomers then add randomly to the growing chain305. 

Some systems which have been recently studied are summarized in Table 8. Polymers of the racemic monomers show little sign of HT bias for any value of the cis double-bond... 

R = C6Fs, (198) R = CHMeCCbMe (optically active and racemic monomers)... 

The investigation of platinum(II)-chiral olefin complexes has shown that, when the diastereomeric equilibrium is reached, which diastereoface of the olefin is preferentially bound to the metal depends on the type of chirality of the olefin used61-63. When an optically active asymmetric ligand is present in the complex and a racemic olefin, is used, one diastereoface will be preferred for complexation and correspondingly one of the antipodes is preferentially complexed61 63). Let us suppose that with a certain catalytic system (e.g., Rh/(—)-DIOP), the re-re enantioface of a prochiral a-olefin reacts preferentially. With the same catalytic system the same face of all a-olefins, including the racemic a-olefins, is expected to react preferentially. However, when a racemic olefin is used, two diastereomeric transition states (e.g. a and b in Fig. 11) can form for each of the transition states shown in Fig. 7, depending on which one of the antipodes of the racemic monomer approaches the catalyst. 

Name heteroatom-containing cyclic and acyclic monomers capable of undergoing coordination polymerisation. Indicate racemic monomers among them. 

Indeed, MALDI-TOF MS analyses of the oligopeptide products demonstrated the preferential formation of racemic mixtures of oligopeptides with homochiral sequences, Fig. 20, generated from deuterium enantiolabeled racemic monomer [206]. The degree of stereospecificity observed in this reaction increased as the homochiral oligopeptide length increased, as shown in Fig. 21. 

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