Racemic Monomer Research Articles

Computer experiment for selection of optical isomer during prebiotic polymerization.

Computer experiments simulating prebiotic polymerization were carried out using the Monte Carlo method. A high degree of single optical isomers from racemic monomers occurred by assuming a small asymmetry in their chemical natures, which was included to investigate an amplification of the asymmetry due to parity non-conservation. The asymmetry developed much more slowly than the growth of polymers, and the degree of asymmetry in the polymers increased with the degree of polymerization. Competition between the preferential selection and the disturbance by the statistical fluctuation are discussed.

Polymerisation stereoelective en phase homogene du methyl-2 thiiranne amorcee par des semences optiquement actives

Optically active seeds have been prepared by polymerization of the laevorotatory enantiomer of 2-methylthiirane with cadmium thiolate; they have been used for the polymerization of the racemic monomer, which is stereoelective. These results are explained by the formation of two types of chiral sites in unequal numbers, each type of site choosing only one enantiomer.

Préparation et étude des propriétés chiroptiques de poly(tert‐butyloxi‐ranne)s optiquement actifs

Optically active poly(tert-butyloxirane)s of various molecular weights were prepared by polymerization of R(–)-tert-butyloxirane with different initiators or by stereoelective polymerization of the racemic monomer. Using identical conditions of polymerization (initiator, temperature etc.) it was found that the pure enantiomer polymerizes much faster than the racemic monomer. The chiroptical properties of the obtained polymers showed similarities in behaviour with those of a synthetized linear model molecule representative for a monomeric unit of the polymer. The enhanced optical activities of low molecular weight polymers were correlated with the presence of chelated hydroxylic end-groups.

Préparation et étude des propriétés chiroptiques de poly(tert‐butyloxiranne)s optiquement actifs

AbstractOptically active poly(tert‐butyloxirane)s of various molecular weights were prepared by polymerization of R(–)‐tert‐butyloxirane with different initiators or by stereoelective polymerization of the racemic monomer. Using identical conditions of polymerization (initiator, temperature etc.) it was found that the pure enantiomer polymerizes much faster than the racemic monomer. The chiroptical properties of the obtained polymers showed similarities in behaviour with those of a synthetized linear model molecule representative for a monomeric unit of the polymer. The enhanced optical activities of low molecular weight polymers were correlated with the presence of chelated hydroxylic end‐groups.

Polymerization of α,α‐disubstituted β‐propiolactones and lactams. VII. Stereoregular polymers from optically active lactones

AbstractOptically active α‐phenyl‐ααethyl‐β‐propiolactone of high optical purity was prepared and polymerized by homogeneous anionic initiation to the isotactic polyester. The racemic and isotactic polymers had apparently different crystalline properties suggesting that the former may be syndiotactic or may crystallize with unit cells containing both R and S blocks. Similar attempts to prepare α‐methyl‐α‐isopropyl‐β‐propiolactone of high optical purity were unsuccessful although a partially crystalline polymer was obtained from the racemic monomer.

Preparation de polymethylglycidylethers et de polymethylglycidylthioethers optiquement actifs

Optically active polymethylglycidyl ethers and polymethylglycidyl thioethers were prepared by two ways, viz polymerization of optically active monomers by anionic initiation and stereoelective polymerization of racemic monomers. From chiroptical properties, it was shown that in both case dextrorotatory monomers have R configuration and that the sign of optical activity of polymers is not solvent dependent. Using the same chiral initiator, the stereoelectivity ratio found is higher for the oxirane derivative ( r = 2.0) than for the thiirane ( r = 1.6). For the latter, it was possible to perform an antisteric stereoelective process as in the case of propylene sulphide.

13C n.m.r. study of optically active polymers: poly(4-methyl-1-hexene)

Isotactic polymers of optically active and racemic 4-methyl-1-hexene, obtained by polymerization with Ziegler—Natta catalysts, were studied by 13C n.m.r. Polymer fractions with different stereoregularity and molecular weight, derived from the monomer with high optical purity, show in their 13C n.m.r. spectra differences which are tentatively associated with possible conformation effects. Stereoselectivity is observed in polymers obtained from the racemic monomer.

Structural Studies of Polyesters. VII. Molecular and Crystal Structures of Racemic Poly(β-ethyl-β-propiolactone)

The molecular and crystal structures of the synthesized racemic poly-(β-ethyl-β-propiolactone) [—CH(C2H5)CH2COO—]n were studied by X-ray diffraction. The unit cell is orthorhombic, P212121(D24), with a=9.32A, b=10.02A, and c(fiber axis)=5.56A; two molecules pass through the unit cell. The molecule has a conformation shown by the Fischer projection for the case of the rectus polymer, giving a left-handed (2/1) helix: the O—C bond is nearly skew, 136°, and the C—C(O) bond is far from the gauche form, −21°. The internal rotation angle about the sequence O—CH—CH2—CH3is 64°. From the results of the X-ray analysis, it may be concluded that (1) the crystalline polymer prepared from the racemic monomer, β-ethyl-β-propiolactone, is isotactic and (2) the bulk sample of this racemic polymer consists of two kinds of crystallites, one composed only of left-handed helices of rectus polymer chains, the other only of right-handed helices of sinister polymer chains.

Preparation and polymerization of racemic and optically active tert‐butyl thiirane

AbstractPreparation and polymerization of tert‐butyl thiirane are described. The racemic monomer was obtained by reaction in aqueous solution of tert‐butyl oxirane and potassium thiocyanate. The levorotatory enantiomer was synthesized from R(−) 3.3‐dimethyl 1.2‐butane diol passing through the intermediate of the cyclic carbonate. Polymerization of the racemic monomer with coordinate type initiators give crystalline, soluble products of high molecular weights and high melting points (close to 205°C). Crystalline, soluble, high molecular weight polymers but of lower melting points (close to 160°C) could also be obtained with the levorotatory enantiomer, either by coordiante or purely anionic initiation. These polymers have high optical activities of positive sign in all the solvents used. NMR measurements show that racemic and optically active crystalline polymers have the same isotactic structure. The differences in melting point and X‐ray diagrams show the existence of several crystalline modifications. These results indicate a strong difference in the behaviour of tert‐butyl thiirane as compared to methyl thiirane.

Some peculiarities of the asymmetric synthesis of macromolecules by ionic polymerization

The asymmetric polymerizations of a non-dissymmetric monomer (1-phenyl-4-methyl-butadiene-1,3) and racemic monomers (N- n-butyl-propylenimine and propylene sulphide) have been studied using optically active catalytic systems of ionic character. The cationic polymerization of PHMB gives polymers having ca. 85–90 per cent of the trans-1,4 structure. It is suggested that the main factor responsible for the lower stereo specificity of the process is the structure of the growing carbonium ion, which can readily assume a planar configuration. This also explains the failure of attempts at asymmetric synthesis of polyindene, poly-1-methylcyclopentene, etc., in cationic polymerization. Using d,1-BPI as an example, the possibility of realizing the stereoselective polymerization with optically active cationic catalysts (BF 3/R∗OH and d-tartaric acid) has been shown. Data on the control polymerization of ethylenimine and N- n-butylethylenimine on the same catalysts, as well as the formation of polymers (on d-tartaric acid) with rotation sign opposite to that of the d-tartaric acid, may serve as a proof of the asymmetric synthesis of poly BPI. In the asymmetric polymerization of d,1-PS catalysed by anionic catalysts ( n- BuLi.2R∗OEt and n- BuLi.LiOR∗ ), optically active polymers are formed only for the latter catalyst. The unusual character of the course of the stereoselective PS polymerization under the action of this catalyst leads to macromolecules with disulphide bonds (proved by u.v. and i.r. spectroscopy, elemental analysis and chemical methods). A mechanism is suggested.

Synthesis and characterization of poly‐β‐hydroxybutyrate. I. Synthesis of crystalline DL‐poly‐β‐hydroxybutyrate from DL‐β‐butyrolactone

AbstractThe polymerization of β‐butyrolactone was investigated as a possible monomer for a proposed synthesis of the naturally occurring polyester, D‐poly‐β‐hydroxybutyrate (D‐PHB). The racemic DL‐monomer was used in this initial study to determine the best conditions and catalyst system for use in a subsequent study of the polymerization of optically active β‐butyrolactone. In so doing it was found that certain organometallic catalysts (Et2Zn and Et3Al) plus a cocatalyst of water produced highly crystalline samples of polyester from the racemic monomer. This paper describes the synthesis and characterization of the racemic polymer obtained using these catalyst systems, and compares the results obtained with certain other catalysts that were also investigated for this purpose. Examination of the DL‐PHB by infrared, NMR, x‐ray, and electron microscopy shows that it is possible to synthesize a crystalline racemic polymer that is virtually identical (excepting optical activity) to the naturally occurring polymer, D‐PHB.

Stereoelective polymerization of propylene sulphide

Systems based on diethylzinc with optically active α-glycols were found to be very efficient initiators in the stereoelective polymerization of racemic propylene sulphide. The enantionter, the absolute configuration of which corresponds to that of the chiral glycol used in the initiator, is preferentially incorporated into the polymer chain. Starting from a racemic monomer with R/S = 50/50, a distribution as high as R/S = 80/20 could be obtained in the polymer. Stereoelection is much higher in a non-polar than in a polar solvent.

Condensation polymerization of N‐dithiocarbonyl alkoxycarbonyl‐amino acids. Part 4. Effect of optical isomer on condensation polymerization of N‐dithiocarbonyl ethoxycarbonyl‐alanine

AbstractCondensation polymerization of N‐dithiocarbonyl ethoxycarbonyl‐L‐alanine (DTE‐L‐alanine) was carried out in various solvents and the effect of an optical isomer was examined. The rate of polymerization of the L‐ as well as the racemic monomer increased with increasing monomer concentration, polarity of solvent, and polymerization temperature. However, the molecular weight of the polymer was independent of the polarity of solvent. High polymers were obtained in 1.4‐dioxane, ethylene glycol diacetate, and n‐butyl acetate. Poly‐alanine having a high molecular weight (ηsp/c > 1.5) in dichloroacetic acid at 30°C was prepared under profitable conditions.The rate of polymerization of the L‐monomer and the molecular weight of the polymer obtained were much larger than that of the DL‐monomer. Addition of a small amount of DL‐monomer to the L‐monomer remarkably decreased both the rate of polymerization and the molecular weight of the polymer. In the polymerization of DTE‐L‐alanine without catalyst no racemization was observed. L‐Monomer was preferentially incorporated into the growing polymer produced from L‐monomer, i.e. asymmetric selection was observed as was the case with N‐carboxy‐alanine anhydride.

Poly‐(α‐methyl benzyl methacrylate). Preparation and preliminary observations

AbstractPoly‐(α‐methyl benzyl methacrylate) (PMBMA) polymers were prepared in isotactic, syndiotactic, and atactic forms using both optically active and racemic monomers. The isotactic material was prepared by anionic polymerization in a hydrocarbon solvent at a low temperature (∼−60°C), the syndiotactic and atactic polymers were synthesized by ultraviolet initiated free radical polymerization at –65 and 32°C, respectively. From the proton magnetic resonance spectrum, it could be determined that the isotactic polymer was of high stereochemical purity. The syndiotactic material was less pure, and the atactic polymer was largely syndiotactic. Racemic and optically active α‐methyl benzyl pivalates were prepared to serve as monomeric analogues of the polymer. The optical activity of the isotactic polymer in dioxane is one‐third greater than that of the monomeric analogue at 5890 Å; the syndiotactic polymer is slightly less optically active than the monomeric analogue, and the atactic polymer is between the syndio‐ and iso‐polymers but much nearer the syndio material. The enhancement or decrease of the optical activity is associated with the orientation correlation of successive chain segments, the specific chemical structure of the optically active group, and on solvent‐polymer interactions. The orientation correlations persist over a short range only.

Polymerization in the crystalline state. VIII. Polymerization in N‐carboxy anhydrides of γ‐benzyl glutamate, γ‐methyl glutamate, and ϵ‐carbobenzoxylysine

AbstractThe kinetics of the solid‐state polymerization of the N‐carboxy anhydrides (NCA) of the L‐ and racemic forms of γ‐benzyl glutamate (BG), γ‐methyl glutamate (MG), and ϵ‐carbobenzoxylysine (CL) were studied as a function of temperature and aqueous vapor pressure. The reaction of the L‐forms of BG and MG was characterized by an induction period, while the CL derivative reached its maximum polymerization rate at the outset of the reaction. Water vapor had only a minor effect in accelerating the reaction and reducing the chain length of the polypeptides formed. The racemic monomers were found to have different crystal structures from those of the L‐isomers and the racemic MG and CL derivatives polymerized much more slowly than the corresponding optically active crystals. All polymers gave diffuse x‐ray diffraction patterns. Infrared spectra of the L‐polypeptides showed that they were largely in the α‐helical form. The polymer derived from the racemic BG–NCA had a content of α‐helical material which suggested that it consisted of polypeptides with long blocks of D and L residues.

Optically Active Vinyl Polymers. X. Synthesis of Optically Active Poly-α-olefins from Racemic Monomers by Asymmetric Catalysts

<b>Optically Active Vinyl Polymers. X. Synthesis of Optically Active Poly-α-olefins from Racemic Monomers by Asymmetric Catalysts</b>

Effect of asymmetric centers on free radical polymerization and the properties of polymers: Methacrylyl alanine, methacrylyl glutamic acid, acrylyl glutamic acid, and their polymers

AbstractMethacrylyl‐D‐alanine, methacrylyl‐L‐glutamic acid, and acrylyl‐L‐glutamic acid were homopolymerized or copolymerized with their enantiomorphs in dioxane, aqueous 0.1N HCl, and in water at 90% neutralization of the monomer. The homopolymerization of the optical isomer and copolymerization of racemic mixtures proceeded at identical rates for the first two monomers at 90% neutralization and for methacrylalanine in 0.1N HCl and in butanol, while with methacrylyl glutamic acid in 0.1N HCl the racemic monomer polymerized slightly more slowly than the pure optical isomer. The optical activities of the homopolymers in water varied with the polymerization medium, probably because of differences in the degree of stereoregulation. Optical activities of copolymers prepared from a mixture of the enantiomorphic monomers containing 85% of L and 15% of D isomer were mostly lower than 70% of that of the L homopolymer, which suggests that the growing chain radical has a bias toward an alternation of the steric configurations of the monomer units. Changes in the optical activities of the homopolymers with degree of ionization were compared with the behavior of pivalyl‐L‐alanine, pivalyl‐L‐glutamic acid, and isobutyryl‐L‐glutamic acid. Poly(methacrylyl‐D‐alanine) behaved in a manner similar to that of its analogue, but the changes in the optical activities of the methacrylyl‐ and acrylyl glutamic acid polymers followed a complex pattern quite different from that of their analogues. The results were interpreted on the assumption that (1) in the polymer the intrinsically more weakly acidic γ carboxyls ionize first since their ionization leads to a lower charge density than the ionization of α‐carboxyl groups and (2) the extension of the polymer backbone affects the optical activity by modifying the conformational distribution in the side chains. It is not believed that, in the cases investigated, helix formation makes an appreciable contribution to the difference between the optical activities of the polymer and their analogues.