In stereoelective polymerization

In stereoelective polymerization the number of catalytic sites having opposite type of chirality is different, and therefore the rate of polymerization of the two antipodes of the monomer is different. 

Variation of the percentage of the crystalline part as a function of temperature in stereoelective polymerization of methyl oxirane. 

Fig. 17. Carbon 13 NMR spectra of the amorphous part of poly(methyl oxirane) obtained in stereoelective polymerization. Optical activity of the polymer = 6.3 (C H, C = 1).

Stereoelective polymerization (type 3) requires the presence of a chiral catalyst with an excess of active centers of a given configuration or with a differential reactivity of the centers that catalyze polymerization of one or the other of the two enantiomers (299). With regard to racemic a-olefins, the best results were obtained with 3,7-dimethyloctene in the presence of TiCl, + Zn[(S)-2-methylbutyl]2 as catalyst (309). The resulting polymer is dextrorotatory, la o = -1-16.1, and the residual monomer is levorotatory, a o = —0.63, (310). These values indicate a rather ihodest degree of stereoelectivity. 

Further investigations are necessary to clarify the mechanism of stereoregulation in the polymerization of olefins and diolefins probably the stereoelective polymerization of vinyl monomers and the asymmetric polymerization of diolefinic compounds will give further interesting contributions to the future progresses in this field. 

A similar scheme was reported by Pino, based on the stereoregulating effect of electron donors and on the stereoelectivity obtained in the polymerization of racemic a-olefins in the presence of chiral bases 109). 

Sepulchre has reported the use of an easily prepared zinc-binaphthol (1) complex which gives a high degree of stereoelectivity in the polymerization of episulfides. In the polymerization of rac-ethylthiirane, the ee of the unreacted monomer is 66% at 46% conversion (Rsyj =15) (Scheme 1) [30]. Spassky and Sepulchre have previously reported the use of this compound for the highly selective of polymerization of rac-methylthiirane, where at 50% conversion, the optical purity of the unreacted monomer is 80% (Rs/r=20) [31]. 

P. Pino, A. Oschwald, F. Ciardelli, C. Carlini, and E. Chiellini, Stereoselection and stereoelection in a-olefin polymerization, in Coordination Polymerization, J. C. W. Chien, (ed.). Academic Press, New York, 1975. 

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). 

Most of the work in the field of stereoselective and stereoelective polymerization of oxiranes and thiiranes was carried out on monosubstituted monomers and was reviewed in some publications... 

Influence of the nature of the initiator Initiators resulting from the reaction between an organometallic derivative and a chiral compound containing an acidic hydrogen were the most usually employed in the stereoelective polymerization of oxiranes and thiiranes. 

The temperature showed a strong effect in the polymerization of t-butyl thiirane (26) The stereoelectivity Pj, doubled in va lue when temperature Towered from 20° to -3" and on the contrary Pn decreased with raising of T and at temperatures higher than 1T5° the choice of the enantiomer was inverted as shown in table IV. The limit value of the optical purity of monomer was also modified. 

Second, monomers of high optical purity could be Isolated In limited amounts starting from racemic mixtures. In such a case the stereoelective polymerization can be considered as an original resolution method of special interest for monomers which are not easily prepared by conventional synthetic ways under their optically active form. Increase in stereoelectivity is observed when using chiral media, i.e. enantiomerically enriched monomers or external chiral additives. 

It is clear that stereoselectivity implies stereoelectivity, and that a relationship must exist between the actual stereoselectivity and stereoelectivity observed when a racemic monomer is polymerized using a given chiral catalyst. If the relative overall rate constants of polymerization of the two enantiomers on a single chiral catalytic center are known (e.g, from a stereoelective polymerization experiment), the average relative cunounts of the two enantiomeric monomeric units in a polymer chain formed in the stereoselective polymerization can, with certain assumptions, be evaluated. 

Stereoselective or stereoelective polymerization of racemic monomers. Stereoregular polymers have been obtained by stereoselective polymerization from many cyclic, racemic monomers (100,121,127-140). Some of these monomers are indicated in Table 2. The identification of the stereoselective character of a polymerization process, which leads to isotactic optically inactive polymers, requires a very accurate characterization of the polymer obtained. For the polymers reported in Table 2, elution chromatography on an optically active support, spectroscopic (IR, or NMR) measurements, comparison of the X-ray pattern with that of the corresponding optically active polymer, and enzymic degradation have been used. In many cases the stereoselectivity of the polymerization process has been confirmed by the stereo-electivity observed when using the catalyst in an optically active form. 

Stereoselective or stereoelective polymerization of racemic monomers. The stereoselective polymerization of racemic monomers was first investigated in the case of racemic a-olefins (184). When the asymmetric carbon atom was in the a-position with respect to the double bond, essentially stereoregular polymers were obtained, the single macromolecules having isotactic main chain and carbon atoms with mostly the seune configuration in the lateral chains. The stereoselectivity decreases when the asymmetric carbon atom of the monomer is in the 3-position with... 

The stereoelective polymerization of a racemic vinyl ether in the presence of an optically active catalyst has been attempted (192), but not achieved. On the other hand, the cationic copolymerization of racemic 1-methylpropyl vinyl ether with (S)-l-phenylethyl vinyl ether or (R)-1-phenylethyl vinyl ether was shown (193) to be stereoelective in the presence of the heterogeneous catalyst A1(O-i.C H )3/H2SO4. 

A remarkably high stereoelectivity has recently been reported in the polymerization of a-methylbenzyl methacrylate (194,195). For the anionic polymerization of this monomer by a Grignard reagent/ (-) sparteine system in toluene at -78°C, the optical purity of the unreacted monomer reached nearly 100% at about 65% conversion. Additionally, the polymer obtained was shown by NMR to have a very high content of isotactic triads. 

Stereoselectivity and stereoelectivity in the polymerization of racemic a-olefins have probably the same origin as in the polymerization of epoxides (Section 3.2.2.), auid are determined by the chiral character of the catalysts used. Chiral catalytic centers of a given configuration attack prevailingly one face of the double bond of the monomer and centers of the opposite configuration attack prevailingly the other face. In the case of chiral olefins, the two diastereofaces of a monomer molecule have different reactivities, and whether the re-re face or the si-si face of this molecule is more reactive depends on the type of chirality of the asymmetric carbon atom. For instance, on the basis of the investigation on Pt-complexes (179), the si-si face is the more reactive face in an (S)-olefin. Thus chiral catalytic centers which attack the si-si face may prefer the (S)-antipode, whereas those which attack the re-re face may prefer the (R)-antipode (Scheme 23). 

The stereoelectivity ratio was not modified when running a copolymerization reaction in the presence of achiral monomer e.g, ethylene sulfide. This confirms again the strong catalyst control process involved in these polymerization. 

In a previous paper presented by Dr, Leborgne and Spassky, from the Universite Pierre et Marie Curie, in Paris, it has been shown that it is now possible to prepare optically active polylactones by a "stereoelective" polymerization process. It is the purpose of the present paper to indicate important differences in properties between the optically active and the racemic poly(a-methyl-a-n-propyl-3-propiolactone) (PMPPL). Results will be presented concerning the crystallization and melting properties, as well as the dynamic and transient mechanical properties of these polymers. 

In another approach, a combination of the stereoelective polymerization and chiral ligand exchange resulted in the formation of a stereocomplex showing also T = 210 This... 

Thus, in stereoelective or stereoselective polymerizations of rac-LA initiated with salen-Al (e.g., structures 6a-6d) or Al-pyrrole Schiff base complexes 6e, predominantly isotactic PLA has been formed. Similar results were obtained with lanthanide L3 complexes of a chiral alkoxides (e.g., structure 6f). ... 

Optical rotation (OR) readings increased with polymerization time and eventually leveled off. GPC measurement showed approximately 50 mol.% consumption of rac-LA. In the second step, an equimolar quantity of (R)-SB(OH)2 (with respect to the S enantiomer) was introduced. In subsequent polymerization, a gradual decrease in OR was observed. Taking into account the determined stereoelectivity coefficient. Pm = 0.96, for the final poly(rac-LA), the gradient poly[(S,S)-lA-grad- R,R)-lA] rather than the block copolymer structure was expected. Indeed, homodecoupled NMR spectra showed, apart from the strong signal of the isotactic mmm... 

Other thiiranes that have heen polymerized with stereoselective and/or stereoelective initiator systems include isopropylthiirane, t-hutylthiirane, ° and cis- and tram-2,S-dimethylthiirane. ° ° The polymerization of t-butylthiirane gives pure isotactic chains with most stereospedfic initiators. An exceptionally high stereoelection was observed in the polymerization of methyl- or ethylthiirane with optically active atropisomeric initiator systems. For example, zinc (S)-l,l-hi-2-naphtholate gave an r-value of 15-20. ° ° At 67% conversion a residual monomer with an optical activity of [a]D24 = 51.77° (neat, 1 dm) was obtained (Scheme 28). 

Most of the stereoselective initiators are heterogeneous systems that have a low effidency, and therefore a predse control of molecular weight and polydispersity is not possible. A number of stereoselective and stereoelective polymerizations of MT have also been studied in a homogeneous phase using chiral cadmium thiolates of cysteine esters and cadmium carboxylates of cysteine and methionine (Figure 4). 

Since 1962, OA polypropylene oxide has been obtained from racemic monomers by asymmetric-selective or a stereoelective polymerization, which was extensively studied by Tsuruta and Furukawa and reviewed recently by these authors [106]. If the epoxide is substituted by an amino group, the polymerization gives only oligomers [110]. Some general observations on this subject are given in Section LB. 

From monomers with two chiral centers, optically active or racemic, (both in the heterocycle and in the lateral chain) different diastereoisomeric polymers can be obtained by classical or stereoelective processes. OA polythiiranes with two OA centers have been synthesized by a stereoelective polymerization of (+) or (-)A -methyl,A -sec-butyl,A-thiiranylamine using ZnEt2/( ) dimethyl-3,3-butanediol-l,2. The presence of an R) or (S) lateral chain has no influence on the stereoelection. In this case the thioether chromo-phore near the asymmetric carbon atom of the main chain becomes optically active and its contribution to the ORD curves is preponderant with no special conformational effect their characterization is now in progress [1 lOd]. 

Modern stereoelective polymerizations were also done on racemic propylene sulfide and superior homolog derivatives using optically active complex catalysts as for propylene oxide the resulting polymers possess monomeric units of a given configuration in excess over the opposite one because of the preferential conversion of one of the two antipodes of the racemic mixture. This is detailed by Dr Spassky etal. in a next chapter. 

New results on stereoelective polymerization of i/,/-oxiranes and if,/-thiiranes are reported by Dr Spassky et al in this book, and stereoelective polymerization of asymmetric a-olefins by Dr Ciardelli et al. Here some information about the synthesis will be briefly described. 

Stereoelective polymerization of a racemic monomer is a sterical control polymerization process in which one of the monomeric antipodes is preferentially polymerized. If the conversion is not complete, both polymer and recovered monomer are optically active. It is synonymous with asymmetric-selection polymerization the yield can be simply evaluated from the optical activity of unreacted monomer. This theoretically important method of synthesis concerns, for the moment, mainly the polymerization of racemic three-membered heterocycles (oxiranes, thiiranes and aziridines) and of a-amino-7V-carboxy acid-anhydrides (or Leuch s salts). 

---