Synthesis and Hydrolysis of α,ω‐Perfluoroalkyl‐Functionalized Derivatives of Poly(ethylene oxide)

Summary: Two series of novel α,ω‐perfluoroalkyl terminated esters of poly(ethylene oxide) (PEO) (R_F‐PEO) having the general structure C_mF_2m+1? COO? (CH₂? CH₂? O)_n? OC? C_mF_2m+1, with m = 1,2,3,4 or 5 have been synthesized. The influences of the PEO molar mass, the length of the perfluoroalkyl group (R_F) and temperature on the cleavage of the ester bridge in aqueous solution and the effect of the hydrolysis process on the size of aggregates formed in water were studied. According to ¹H and ¹⁹F NMR measurements the degree of functionalization obtained (up to 96 mol‐%) increases with the decrease in the length of the R_F group. All of the derivatives showed ester cleavage in water in short time scales. The rates of hydrolysis of the ester bridge in aqueous solution were determined from pH‐measurements. It was verified that the rate law for hydrolysis corresponds to a pseudo‐first order type. The hydrolysis kinetic constant k increased with a decrease in the length of the R_F group ranging from 0.2 × 10^?3 s^?1 for the longest R_F group (C₅F₁₁? ) up to 1.2 × 10^?2 s^?1 for the shortest R_F group (CF₃? ). The value of k depended almost exclusively on the length of the perfluoroalkyl chain and was independent of the length of the PEO backbone (1 000 or 2 000 g · mol^?1), as long as no additional phenomenon such as phase separation was present. It was also found that the change in the value of k with temperature followed a non‐Arrhenius pattern and there was an evident relationship between the non‐linearity in the ln k vs. 1/T relation with increasing temperatures and the occurrence of a macroscopic phase separation of LCST type. Dynamic light scattering measurements showed the coexistence of unimers with associated species with apparent hydrodynamic radii (R_h) of approximately 20–45 nm for all samples in aqueous solutions. These species might correspond to aggregates of a few micelles. For some samples also larger aggregates were found with R_h in the 100–500 nm range, which might be attributed to clusters of micelles.

     

Asymmetric induction in the cyclopolymerization of divinyl ethers derived from (R)- or (S)-3,3′-dimethyl-1,1′-bi-2-naphthol

Optically active divinyl ethers, (?)-(R)- and (+)-(S)-2,2′-bis[2-(2-vinyloxyethoxy)ethoxy]-3,3′-dimethyl-1,1′-binaphthyl [(R)- 1b and (S)- 1b ] were polymerized to produce chiral poly(crown ether)s. Their optical rotation was found to be profoundly influenced by the polymerization conditions. When increasing the monomer concentration from 0,1 to 0,3 mol · 1^?1, after polymerization with SnCl₄ in CH₂Cl₂ at 0°C, the optical rotation of the resulting polymers is drastically changed from +44,5° to ?17,3° for (R)- 1b and from ?35,6° to +20,9° for (S)- 1b . The analysis of ¹H NMR showed that the polymers have changed their optical rotation due to a configuration which has a tendency to be preferentially racemic diad at higher monomer concentrations in a nonpolar solvent. There are indications that the twist of 2,2′-binaphtyl moieties, the methyl groups in 3-, and 3′-positions as steric barrier, and the intramolecular solvation of the growing carbo-cation cooperatively control the propagation to induce the asymmetry in the main chain.     

Polymérisation de thiirannes différemment substitués à l'aide du système chiral ZnEt2/(S)-dihydroxy-2,2′ binaphtyl-1,1′, 1. Stéréoélectivité de la polymérisation

Differently substituted thiiranes were polymerized using an atropoisomeric chiral initiator: ZnEt₂/(S)-2,2′-dihydroxy-1,1′-binaphthyl (DHBN) (mole ratio 1/1). It was observed that high stereoelectivities are obtained with monomers bearing small alkyl substituents and that the resolution efficiency of the initiator decreases as the bulkiness of the substituent increases. From the comparison of the results obtained with another initiator system, ZnEt₂/(R)-3,3-dimethyl-1,2-butanediol (DMBD) (mole ratio 1/1), the use of ZnEt₂/(S)-DHBN could be shown to be more suitable for the polymerization of thiiranes with linear alkyl substituents. Contrary to the case of polymerization initiated by the system ZnEt₂/(R)-DMBD, the polymerization reaction of thiiranes with ZnEt₂/(S)-DHBN was usually accompanied by the production of the corresponding olefins in various amounts.     

Über oligomere und polymere 2.5-dimethoxyphenylen-vinylene

Poly-p-2,5-dimethoxyphenylene-vinylene and a number of oligomers with p-phenylene units of the general formula 2.5-(CH₃O)₂-4-R? C₆H₂? CH?CH? [2,5-(CH₃O)₂? C₆H₂? CH?CH]_n? C₆H₂? 2.5-(CH₃O)₂-4-R (n = 0 to 4; R ? CH₃, H, CHO) were synthesized. Poly-m-2.5-dimethoxyphenylene-vinylene and oligomers with m-phenylene units (n = 0, 1; R ? H, CHO) have also been prepared. The synthesis were performed by the WITTIG reaction of triphenylphosphonium salts for example 2.5-(CH₃O)₂? C₆H₃? CH₂P(C₆H₅)₃Br, 2.5-(CH₃O)₂? C₆H₂-1.3-[CH₂P(C₆H₅)₃Br]₂ or 2.5-(CH₃O)₂-4-CH₃? C₆H₂? CH?CH? 2.5-(CH₃O)₂? C₆H₂-4-CH₂P(C₆H₅)₃Br and aldehydes, for example 2.5-dimethoxyisophthalaldehyde, 4.4′-diformyl-2.2′.5.5′-tetramethoxystilbene or 4-formyl-4′-methyl-2.2′.5.5′-tetramethoxystilbene. The hydrogenation of the vinylene units in the compounds with n = 1 gave the 2.5-dimethoxyphenylenethylenes. The demethylation of 2.5-dimethoxyphenylene-vinylenes respectively -ethylenes (n = 1) leads in most of the cases to the corresponding hydroxy-compounds.     

Réaction époxide amine: Réactivité du sulfonyl-4,4′ dianiline vis-à-vis du groupement époxy

Mechanism and kinetics of the polycondensation of 4,4′-sulfonyldianiline ( 2 ) with 2,3-epoxy-1-phenoxypropane ( 1 ) were analysed in the absence or presence of a catalyst [N,N-(dimethyl)-benzylamine or BF₃ complex]. Chemical data obtained by ¹³C NMR and HPLC show that secondary reactions (homopolymerisation, reaction of hydroxyl groups with the epoxy groups) take a prominent part.     

Photocontrolled Living Anionic Polymerization of Silicon‐Bridged [1]Ferrocenophanes with Fluorinated Substituents: Synthesis and Characterization of Fluorinated Polyferrocenylsilane (PFS) Homopolymers and Block Copolymers

Photocontrolled living anionic ring‐opening polymerization (ROP) of strained silicon‐bridged [1]ferrocenophanes Fe(η‐C₅H₄)₂SiMe(CH₂CH₂CF₃) ( 1 ), Fe(η‐C₅H₄)₂SiMe(CH₂CH₂(CF₂)₇CF₃) ( 2 ) and Fe(η‐C₅H₄)₂SiMe(CH₂CH₂CH₂C₆F₅) ( 3 ) with 3,3,3‐trifluoropropyl, heptadecafluoro‐1,1,2,2‐tetrahydrodecyl, and pentafluorophenylpropyl hydrofluorocarbon substituents, respectively, on silicon, have been explored. Thus well‐defined fluorine‐containing homopolymers [Fe(η‐C₅H₄)₂Si(Me)R_F]_n ( 4–6 ) (R_F = hydrofluoro‐ carbon substituents), with controlled molecular weights and narrow molecular weight distributions ( < 1.10) were synthesized from the photocontrolled ROP of the respective [1]ferrocenophanes 1–3 , initiated by Na[C₅H₅] in THF at 5 °C. The sequential living photolytic anionic ROP allowed the synthesis of well‐defined diblock copolymers, polyferrocenylmethylphenyl‐ silane‐block‐polyferrocenylmethyl(3,3,3‐trifluoropropyl)silane (PFMPS‐b‐PFS_F) ( 7a–c ), with different ratios of the two blocks. DSC analysis of the resulting fluorinated diblock copolymers showed the presence of two distinct glass transition temperatures suggesting the immiscibility of the two blocks. The diblock copolymers 7a–c undergo predictable self‐assembly in the bulk as revealed by TEM analyses. The increased hydrophobicity of the fluorinated organometallic homopolymers was also studied by measuring the water contact angles on the homopolymer films.

     

Etude du comportement d'une série de polystyrènes linéaires dans des mélanges de solvants

The preferential solvation of anionically polymerised polystyrene by different solvent/non solvent mixtures (CCl₄/CH₃OH, C₆H₆/CH₃OH, dioxane/CH₃OH, C₆H₆/Heptane) was studied by light scattering. The parameter λ′ of preferential solvation depends on molecular weight M according to where λ′_∞ is the preferential solvation at infinite molecular weight and A a quantity characteristic for the composition of the mixed solvent. A increases with decreasing thermodynamic goodness of the solvent mixture. The above equation can be derived semi-empirically by assuming that the solvent composition near the chain depends on the density of chain segments. This hypothesis furthermore explains the observed anomalous behavior of the intrinsic viscosity/molecular weight relationship below molecular weights of about 160,000 in solvent/non solvent mixtures.     

Systèmes réactifs polyuréthannes bloqués, 1. Dissociation thérmique en fonction du type d'isocyanate et du type d'agent de blocage. etude par DSC et IRTF

The preparation of blocked isocyanates and their thermal dissociation into isocyanate and blocking agent was studied by DSC and IR spectroscopy. Thermal deblocking was found to be possible only with isocyanates blocked with ε-caprolactame or 2-butanone oxime, but not with β-ketoesters. With 2-butanone oxime — the most convenient blocking agent — the dissociation of blocked aromatic isocyanates starts at ≈ 80°C and is fast at 120°C. With blocked aliphatic isocyanates, the temperature of dissociation (T_d) is higher (≈ 120°C). Ether or tertiary amines have only a poor catalytic effect on the dissociation, and metal naphthenates have no apparent effect. The thermal and viscosity properties of prepolymers with terminal blocked isocyanate groups were investigated. The molecular interactions decrease in the order 1,4-phenylene diisocyanate (7) ? 4,4′-methylenediphenylene diisocyanate > trans-1,4-cyclohexylene diisocyanate ≈ 2,4- and 2,6-toluene diisocyanate.     

Reactive poly(2-vinyl-4,4-dimethyl-5-oxazoione) and poly[(2-vinyl-4,4-dimethyl-5-oxazolone)-co-(methyl methacrylate)]s. Synthesis,characterization and chemical modification with 4-methoxy-4′-(β-aminoethoxy)biphenyl

Poly(2-vinyl-4,4-dimethyl-5-oxazolone) ( P₀ ) and poly[(2-vinyl-4,4-dimethyl-5-oxazolone)-co-(methyl methacrylate)]s with increasing content of methyl methacrylate units ( P₁–P₄ ) were synthesized and characterized. NMR spectra were discussed in terms of monomer sequence distribution and tacticity effects. The reaction of 4-methoxy-4′-hydroxybiphenyl ( 1 ) with 2-ethyl-2-oxazoline was utilized to prepare 4-methoxy-4′-(β-aminoethoxy)biphenyl ( 3 ) through the intermediate 4-methoxy-4′-[(N-propanoyl)-β-aminoethoxy]biphenyl ( 2 ). The homopolymer P ₀ and two copolymers P ₂ and P ₃ were functionalized with 4-methoxybiphenyl side groups by reaction with 3 via a ring-opening process in N,N-dimethylformamide (DMF) or 1,2-dichloroethane. The resulting copolymers P₅–P₈ were characterized by ¹H and ¹³C NMR. The highest degree of functionalized units was obtained in DMF at 80°C.     

Polymeric chiral crown ethers, 4. Substituent effects of methyl and phenyl groups in the 3,3′-positions on chiral recognition of poly(crown ether)s synthesized by cyclopolymerization of divinyl ethers derived from 1,1′-bi-2-naphthol

Chiral poly(crown ether)s were synthesized by cationic cyclopolymerization of (S)-2,2′-bis(2-vinyloxyethoxy)-3,3′-dimethyl-1,1′-binaphthyl [(S)- 1b ] and (R)-2,2′-bis[2-(2-vinyloxyethoxy)-ethoxy]-3,3′-dimethyl (or 3,3′-diphenyl)-1,1′-binaphthyl [(R)- 3b or (R)- 3c ]. The substituents in the 3,3′-positions of binaphthyl moiety disturb the intramolecular cyclization in the polymerization of monomer (S)- 1b , but have no influence on the cyclopolymerization tendency of monomers (R)- 3b and (R)- 3c . The polymers from (R)- 3b and (R)- 3c [(R)- 4b and (R)- 4c ] have a higher ability of chiral recognition towards a-amino acids, such as phenylglycine, phenylalanine, valine, and methionine, than the polymer from (R)- 3a [(R)- 4a ], which has no substituent in 3,3′-positions. The methyl and the phenyl substituents in the 3,3′-positions undoubtedly act as additional barrier causing an increase in the ability of chiral recognition, though the effect is less than expected from the result of model crown ethers.     

Zur kristallstruktur der polycarbonate aus 4,4′-dihydroxydiphenyl-2,2-propan, 4,4′-dihydroxydiphenylsulfid und 4,4′-dihydroxydiphenylmethan

The polycarbonate from 4,4′-dihydroxydiphenyl-2,2-propane crystallizes monoclinically in the space group C_s² with four chains each in the lattice cell, but the polycarbonate from 4,4′-dihydroxydiphenyl sulfide and from 4,4′-dihydroxydiphenyl methane crystallizes orthorhombically in the space group C_2v⁹ with two chains each in the lattice cell. In all three substances, the fibre identity period always comprises two monomeric units, so that always eight and always four monomeric units, respectively, are located in the lattice cell. The crystal densities are 1.315 g./ccm. for the polycarbonate from 4,4′-dihydroxydiphenyl-2,2-propane, 1.50 g./ccm. for the polycarbonate from 4,4′-dihydroxydiphenyl sulfide, and 1.303 g./ccm. for the polycarbonate from 4,4′-dihydroxydiphenyl methane. The fibre identity period of the three substances, as compared with their theoretical maximum length in the case of fully stretched chains, has been shortened by 15 per cent to 20 per cent. Accordingly, the crystal elasticity modulus in the direction of the chain only amounts to about 450 to 600 kp./mm². The crystal structures considered do not contain any glide planes, so that the crystal domains possess a high shear strength. The low crystallization tendency of the polycarbonate from 4,4′-dihydroxydiphenyl-2,2-propane is easily and plausibly accounted for by its crystal structure.     

Synthèse de polysiloxanes acyloxyméthylés par réaction des acides acétique,benzoïque et méthacrylique,en présence de triéthylamine,sur des polysiloxanes chlorométhylés, 2. Etude de la réaction sur trisiloxane et tétrasiloxane modèles

The reaction of acetic, benzoic and methacrylic acids with 3-chloromethylheptamethyltrisiloxane and 3,5-bis(chloromethyl)octamethyltetrasiloxane in toluene in the presence of triethylamine was studied. Beside the expected transformation of oxy(chloromethyl)methylsilanediyle ( A ) units into oxy(acyloxymethyl)methylsilanediyle ( C ) units, redistributions of A , C and trimethylsilyl/oxytrimethylsilyl (M) were observed. The conversion versus time curves of distribution of the former oligomers (mole fractions of trisiloxanes: F₁ = m_{M- X -M} = m_{M- A -M} + m_{M- C -M} and tetrasiloxanes: F₂ = m_{M- X - X -M} = m_{M- A - A -M} + m_{M- A - C -M} + m_{M- C - C -M}) were followed. The reaction rates were not only controlled by the acid used but also by the starting oligosiloxane (trisiloxane or tetrasiloxane). The distributions of the homogeneous and heterogeneous oligomers seem to result from controlled combinations of A , C and M according to orientations of cleavages and to relative affinities of RCOOH/NEt₃ towards various siloxanes. For the total conversions the mole fractions F₁ and F₂ fit with those calculated by a law based on a hazard combination of C and M. This result may be explained by the compensation of electronic and steric factors.     

Synthèse et copolymérisation avec l'acrylamide de macromonomères d'acrylate de dodécyle

Graft copolymers of well-defined structure and composition were prepared by radical copolymerization of acrylamide and poly(dodecyl acrylate) macromonomers. The macromonomers were prepared by telomerization of dodecyl acrylate in the presence of 2-mercaptoethanol as a transfer agent, followed by reaction with acryloyl chloride. Poly(dodecyl acrylate) macromonomers with M _n = 1100, 1100, 2200 and 4600 were synthesized. The kinetics of the radical telomerization of dodecyl acrylate with 2-mercaptoethanol and AIBN was studied at 66°C in THF. Under these conditions we obtained the transfer constant C_T = 0,9. The graft copolymerization of acrylamide with the poly(dodecyl acrylate) macromonomer was carried out in the presence of the transfer agent C₆F₁₃C₂H₄SH. If C₀ = [AIBN]/[Acrylamide] = 0,01 and R₀ = [C₆F₁₃C₂H₄SH]/[Acrylamide] = 0,01, for acrylamide a DP _n = 160 was obtained in all cases, for the macromonomer of M _n = 1100 a DP _n = 2,5, and for the macromonomer of M _n > 1100 a DP_n = 1.     

Propene‐Ethene Copolymers Synthesised with Cs‐Symmetric Metallocenes and Different Cocatalysts

Summary: Propene‐ethene copolymers with low to moderate amounts of ethene were synthesised with two C_s‐symmetric metallocene complexes, [(p‐CH₃OPh)₂C(2,7‐di‐^tBuFlu)(Cp)]ZrCl₂ ( I ) and [(p‐CH₃OPh)₂C(2,7‐di‐^tBuFlu)(Cp)]HfCl₂ ( Ia ). Methylalumoxane (MAO), N,N‐dimethylanilinium tetra(pentafluorophenyl)borate {[Me₂HNPh][B(C₆F₅)₄]}, trityl tetra(pentafluorophenyl)borate {[Ph₃C][B(C₆F₅)₄]} and tris(pentafluorophenyl)borane [B(C₆F₅)₃] were used as cocatalysts in the polymerisations. The differences in polymerisation activity, ethene incorporation rate and molecular weight as well as the microstructure and the thermal behaviour of the homo‐ and copolymers were studied. The different cocatalysts have a strong influence on the polymerisation activity and the syndiotacticity of the polymers, but only a minor influence on the ethene incorporation rate and the molecular weight. At 30 °C, the incorporation rate of ethene is ca. 25 times faster for complex I and ca. 16 times faster for complex Ia than the incorporation rate of propene. The thermal behaviour of the homo‐ and copolymers is dependent of the polymerisation temperature and the cocatalyst. I /MAO produces homo‐ and copolymers at 30 °C with more regular microstructure than I /[Me₂HNPh][B(C₆F₅)₄], I /[Ph₃C][B(C₆F₅)₄] and I /B(C₆F₅)₃ leading to homo‐ and copolymers with higher melting and crystallisation temperatures. On the contrary, at 0 °C I /[Me₂HNPh][B(C₆F₅)₄] and I /[Ph₃C][B(C₆F₅)₄] produce copolymers with higher propene sequence syndiotacticity and higher melting and crystallisation temperatures than I /MAO. The influence of the cocatalyst on the catalyst coordination environment is temperature‐dependent and thereby the structure of the forming homo‐ or copolymers varies.

Effect of different cocatalysts and the amount of ethene in the feed on the polymerisation activity of complex I.     

Laser light scattering studies of soluble high performance fluorine-containing polyimides, 1. Polyimide synthesized from 2,2′-bis(3,4-dicarboxyphenyl)-hexafluoropropane dianhydride and 2,2′-(trifluoromethyl)-4,4′-biphenyldiamine

Five fractions of a polyimide synthesized from 2,2′-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 2,2′-(trifluoromethyl)-4,4′-biphenyldiamine (PFMB) (6FDA/PFMB) in tetrahydrofuran (THF) at 30°C were investigated by a combination of static and dynamic laser light scattering (LLS). The relations of 〈R_h〉 (nm) = 2.38 x 10^?2 M _w^0.560 and A₂ (mol · cm^?3 · g^?2) = 2.1 × 10^?1 M _w^?0.43 were established, where M _w, 〈R_h〉 and A₂ are weight-average molecular weight, average hydrodynamic radius and second virial coefficient, respectively. A combination of M _w and the translational diffusion coefficient distribution G(D) leads to a relation of D(cm²/s) = 2.41 × 10^?4 M ^0.564. With this relation, we successfully convert each G(D) into a corresponding molecular weight distribution (MWD). On the basis of the Benoit-Doty theory, we found that the persistence length and the Flory characteristic ratio C_∞ of the 6FDA-PFMB chain are ～3.3 nm and ～40, respectively, indicating that the 6FDA-PFMB chain is more extended than typical random-coil chains. On the other hand, the ratio of the radius of gyration to the hydrodynamic radius, i.e., 〈R_g〉/〈R_h〉 ～ 1.8, together with the values of the exponents (～0.56) indicate that the 6FDA-PFMB chain has a coil chain conformation. Therefore, the 6FDA-PFMB chain has an extended coil conformation in THF at 30°C.     

Synthesis and characterization of some new polyamides containing azomethine groups in the polymer backbone, 2

New aromatic polyamides containing azomethine groups in the backbone and also having different linking groups like ? CH₂? , ? CH₂? CH₂? , and ? O? were synthesized by reacting 4,4′-[1,4-phenylenebis(methylidynenitrilo)]dibenzoyl dichloride (2) with some diamines 1a – d, 4, and 6 by the low temperature solution polycondensation method. The physical, spectral, and thermal properties of the synthesized polyamides were investigated. The effect of the nature of different linking groups on the properties of these aromatic polymides was explored by comparing their spectral and thermal data.     

Synthesis of new polynaphthalimidophenylquinoxalines and their copolymers

A series of novel polyphenylquinoxalines containing naphthalimide structures in the chain were prepared by reacting 3,3′-diaminobenzidine, 3,3′,4,4′-tetraaminodiphenyl ether and 3,3′,4,4′-tetraaminodiphenyl sulfone with the two new monomers of N,N′-bis(4-benzoylcarbonylphenyl)-4,4′-(1,3-phenylenedioxy)-di-1,8-naphthalenedicarboximide ( 3a ) and N,N′-bis(4-benzoylcarbonylphenyl)-4,4′-[isopropylidenebis(1,4-phenyleneoxy)]-di-1,8-naphthalenedicarboximide ( 3b ). The obtained polymers exhibit high decomposition temperatures, their T_g values being, however, below 300°C. Their solubility in common solvents, e.g. chloroform, is excellent, but their molecular weight is low. Higher molecular weight products were obtained by copolymerization of 3a, b with N,N′-bis(4-benzoylcarbonylphenyl)-1,8 : 4,5-naphthalenetetracarboxdiimide ( 3c ) yielding films and fibers with good mechanical properties.     

Azo polymers — syntheses and reactions, 3. Photolysis of monomeric and polymeric azo compounds

Unsymmetrical azo compounds of the general structure m-R? C₆H₄? N?N? C(OCOCH₃) (R¹) (R²) ( 2–4 ), in which R stands for H, CH₃, CH?CH₂ or a polymeric residue, were irradiated with light of λ ≥ 330 nm. Depending on the substituents R¹ and R² and the reaction temperature trans-cis isomerization or/and decomposition was observed. Polymers containing azo groups and CC-double bonds can be crosslinked by irradiation.     

Liquid crystalline poly(β-aminoester)s containing different mesogenic groups

Four series of new poly(β-aminoester)s based on different mesogenic units and amino spacers were prepared and their properties examined. The polymers were obtained by a Michael-type polyaddition reaction of the diamines piperazine ( 5a ), 2-methylpiperazine ( 5b ), N,N′-dimethylhexamethylenediamine ( 5c ), and 4,4′-trimethylenedipiperidine ( 5d ) to diacrylates containing the anisotropic group, trans-4,4′-vinylenedi-p-phenylene diacrylate ( 1 ), 4-(4-acryloyloxybenzylideneamino)phenyl acrylate ( 2 ), 4,4′-diphenylylene diacrylate ( 3 ), and 4,4′-azoxydi-p-phenylene diacrylate ( 4 ). The mesophase behaviour of the resulting polymers 6–9 was strongly influenced by the nature of both the anisotropic and spacer groups. The 4,4′-azoxydi-p-phenylene unit appeared to be the most efficient in promoting liquid crystal properties, whereas no mesophases could be observed in polymers incorporating the 4,4′-biphenylylene unit. Doping of poly(β-aminoester)s, based on 4,4′-azoxydi-p-phenylene units, with a low-molar-mass cholesterogen allowed to obtain cholesteric structures extending over wide ranges of temperature. The molecular weight of the polymers was found to play a role in determining thermotropic mesomorphism by affecting the melting temperature of the polymer. In this context, some non-macromolecular model compounds were analyzed in respect of their thermal behaviour.     

Polyoxirannes et polythiirannes comportant un centre chiral dans la chaîne latérale, 3. Polymérisation de (2RS,3S)- et (2R,3S)-épithio-1,2 méthyl-3 pentanes

Different mixtures of (2R,3S)- and (2S,3S)-1,2-epithio-3-methylpentanes ( 1 ) were polymerized using anionic (sodium), stereoselective (ZnEt₂/H₂O, cadmium tartrate) and stereoelective (ZnEt₂/R(?)-3,3-dimethyl-1,2-butanediol (R(?)DMBD)) initiator systems. In all cases a preferential consumption of the (2R,3S)-diastereoisomer was observed. The reactivity ratios of the two diastereoisomers (2R,3S) and (2S,3S) in the copolymerization, initiated with sodium, were determined: r₁ = 1,40 ± 0,08 and r₂ = 0,54 ± 0,05. A low but significant diastereoisomeric enrichment was observed in stereoselective polymerization. It results from different consumption rates for the two diastereoisomers on enantiomorphic sites of the initiator. The election obtained with a stereoelective system is much higher. It is, however, not very different from that observed with isopropylthiiranne, which does not bear an additional chiral center in the alkyl substituant.