Synthesis and liquid-crystalline properties of thermotropic homopolycarbonates

Six series of polycarbonates with flexible spacers have been prepared by melt condensation of aromatic diols such as 4,4′-bis[(2-hydroxyethyl)oxy]biphenyl, 4,4′-bis [(6-hydroxyhexyl)oxy]biphenyl, 1,5-bis[(2-hydroxyethyl)oxy]naphthalene, 1,5-bis[(6-hydroxyhexyl)oxy]naphthalene, 2,6-bis[(2-hydroxyethyl)oxy]naphthalene, 2,6-bis[(6-hydroxyhexyl)oxy]naphthalene, 4,4′-bis[(6-hydroxyhexyl)oxy]stilbene, 3,3′-dimethyl-4,4′-bis[(2-hydroxyethyl)oxy]stilbene, 3,3′-dimethyl-4,4′-bis[(6-hydroxyhexyl)oxy]stilbene, 4,4′-bis[(2-hydroxyethyl)oxy]-α-methylstilbene and 4,4′-bis[(6-hydroxyhexyl)oxy]-α-methylstilbene

1 For systematic IUPAC nomenclature see Experimental part.

with several alkylene diphenyl dicarbonates. Polycarbonates with biphenyl and stilbene mesogenic groups show liquid-crystalline behavior with the formation of a nematic phase over a temperature range of 46°C and 21°C, respectively. Amorphous polycarbonates were obtained when 1,5-naphthalene or α-methylstilbene groups are incorporated. Polymers with 2,6-naphthalene and 3,3′-dimethylstilbene groups show mesophase formation up to a certain spacer length, a transition range of 15°C and 28°C, respectively, being observed.     

Synthesis and properties of liquid-crystalline side-chain polymers containing β-diketonato transition metal complexes

Polymers ( 5-7 ) containing β-diketonato metal complexes of Cu²⁺, Ni²⁺ and Co³⁺ were synthesized from new thermotropic liquid-crystalline poly(acrylate ester)s ( 4 ) with a mesogenic bidentate β-diketone in the side chain, and the liquid-crystalline properties were examined by differential scanning calorimetry (DSC), X-ray diffraction and polarizing microscopy. The mesophase of 4 is an enantiotropic smectic A phase showing a batônnet and focal conic fan texture in polarizing microscopy. Polymers 5a-c containing a bis(β-diketonato) copper(II) complex also exhibit an enantiotropic smectic A phase, and polymer 5d prepared from the vinyl monomer 8 shows a nematic phase. Concerning the complexes of polymers 6 and 7 with nickel(II) and cobalt(III), only 6a and 7a containing a small amount of metal-chelating β-diketonate units exhibit liquid-crystalline behavior like the parent polymer 4 . It was found that the fourcoordinated square planar bis(β-diketonate)copper(II) acts as the mesogen; both nickel(II) and cobalt(III) complexes do not work as mesogens and rather hinder the liquid crystallinity of 4 . The electron spin resonance (ESR) spectra of 5a and 5b change markedly along with the transition from the smectic A phase to the isotropic one. The broadening and simplification of the spectra for the isotropic phase indicates increased interaction between neighboring bis(β-diketonato)copper(II) complexes and substantial electron spin-spin interaction. The ESR spectra of 5c and 5d also imply this increased interaction between neighbouring complexes and spin-spin interaction regardless of the phase transition.     

Synthesis and thermotropic liquid-crystalline properties of polyurethane-polystyrene graft copolymers

New polyurethane-graft-polystyrenes (4) of well-defined structures and compositions were synthesized by macromonomer technique and the structure-liquid-crystalline (LC) property relationship is discussed. The expected graft copolymers with polystyrene grafts were prepared by melt polycondensation of a mixture of a dihydroxy derivative of biphenyl as a mesogenic moiety (1) and a dihydroxy-terminated styrene macromonomer (2) with diphenyl N,N′-hexamethylenedi-carbamate (3) . Of the resulting polymers mesogenic unit-rich graft copolymers retain the LC mesophases in spite of introduction of the polystyrene segments onto the polyurethane backbones and possess microphase-separated structures. Polystyrene-rich copolymers reveal no well-defined LC mesophases and analogous thermal properties to the polystyrene segments.     

Synthesis of polystyrene containing transition metals

The synthesis of styrene polymers containing transition metals was achieved by reaction of different polystyrenes with Mt(CO)₆ or (CH₃? C?N)₃Mt(CO)₃, (Mt = Cr, Mo, or W). Atactic, isotactic, and cross-linked polystyrene (with divinylbenzene) were employed. Organometallic polymers with good stability were only obtained by coordination of tricarbonylchromium groups to the phenyl rings of the polystyrene. Tricarbonylmolybdenum- and tricarbonyltungsten complexed polystyrene showed very poor stability and could not be isolated. The amount of chromium incorporated depends on the molecular structure of the polymers, the chromiumcarbonyl complex used in the reaction, and the reaction time. The reactivity of the tris(acetonitrile)tricarbonylmetal-complexes is higher than that of the hexacarbonylmetal-complexes and the addition reactivity of the polymers is in the order: atactic > isotactic > cross-linked polystyrene     

Synthesis and properties of thermotropic liquid-crystalline polymers linked through bis(ß-diketonato)copper(II) complex

Thermotropic liquid-crystalline polymers ( 2a–2e, 3a–3c ) linked via the bis(ß-diketonato)-copper(II) complex were prepared by mixing the bifunctional bidentate ligand ( 1a–1c ) with copper(II) chloride. The homopolymers 2c–2e and the copolymers 2a–3c were ascertained to exhibit a stable mesophase by means of polarizing microscopic observation. The homopolymers show at least a smectic mesophase and the copolymer exhibit both smectic and nematic ones. Fibers were prepared by melt-drawing of the molten polymers at the mesophase temperature into air at room temperature except for 2a and 2b . Electron spin resonance (ESR) spectra of the melt drawn fibers indicated that the plane of the square planar bis(ß-diketonato)copper(II) complex in the fiber is oriented parallel to the fiber axis. X-ray diffraction patterns from the melt-drawn fibers are characterized by several sharp inner reflections at meridional position and a diffuse one at equatorial position, suggesting that the fibers possess an axial orientation with smectic A phase which is frozen in during melt-drawn spinning.     

Side-chain liquid-crystalline polymers containing a siloxane spacer, 1. Synthesis of side-chain liquid-crystalline polymers containing siloxane bond in the spacer unit

Side-chain liquid-crystalline polymers containing siloxane bond in the spacer units were prepared. For the synthesis of the desired monomers, novel silanol compounds carrying a mesogenic group were prepared as the intermediates. The polymers were obtained by ordinary free-radical polymerization. As the mesogenic groups, p- or p′-cyano-substituted biphenyl group, phenyl and biphenylyl benzoates were chosen, and the structural effects on the thermal properties were investigated. Introduction of a disiloxane group in the side chain resulted in a decrease of T_g which was more marked for longer siloxane units. Furthermore, only when a terphenylene-type mesogen was introduced, a stable smectic mesophase was observed with wide range of phase stability.     

Side-Chain liquid-crystalline polymers containing a siloxane spacer, 2. Effects of mesogenic groups on the thermal properties of side-chain liquid-crystalline polymers containing a disiloxane unit in the spacer

A series of side-chain Iiquid-crystalline polymers was prepared containing disiloxane units in the spacer. The polymers consist of a polymethacrylate backbone and several kinds of mesogenic side groups. For the synthesis of the desired monomers, silanol compounds carrying a mesogenic groups were prepared as the intermediates. The polymers were obtained by ordinary free-radical polymerization. As the mesogenic groups, linear-type p- or p′-substituted phenyl and biphenylyl benzoates, p- or m-substituted benzoyloxybiphenyl groups and laterally attached biphenylyl benzoate-type mesogens were chosen, and the structural effects on the thermal properties of the polymers were investigated. In the case of linear-type mesogens, only when triple-core mesogens were introduced a stable smectic mesophase was observed with a wide range of phase stability, while on mesophase was observed for the linear double-core and the laterally attached mesogens. On the other hand, a highly ordered smectic phase was observed for polymers having linear-type triple-core mesogens with p-alkoxy and m-cyano end groups. Furthermore, it was revealed that the introduction of a disiloxane unit in the spacer lowered the temperature range of the mesophase, in comparison with polymers having an alkylene spacer with the same backbone component and mesogens.     

Synthesis and thermotropic properties of novel side‐chain dimer liquid crystalline polymers

A new class of side‐chain liquid crystalline polymers with polymethacrylate or polyether backbones and two different mesogenic moieties in one side‐chain, i.e. a biphenyl group and a cholesterol group, have been synthesized. The thermotropic behavior of both, the monomeric precursors and the polymers, has been investigated. The monomers exhibit smectic A and chiral nematic phases and the polymers exhibit smectic A mesophases. For both, monomers and polymers, the isotropization temperatures and the corresponding dimensionless entropy changes ΔS/R depend on the parity of the spacer between the two mesogens. Replacement of the biphenyl group in the polymethacrylates by a phenyl group lowers the isotropization temperatures, but the smectic A phase is retained.     

Side-chain liquid-crystalline polymers containing siloxane spacer, 3. Thermal properties of copolymers containing different types of mesogens

Copolymerization of monomers containing the disiloxane unit in the spacer component and linear triple-core mesogens with comonomers containing linear double-core mesogens, or laterally attached triple-core mesogens was carried out radically. The effect of copolymer composition and monomer structure on the mesomorphic properties of the obtained copolymers was investigated in detail. The copolymers with a comonomer content up to 50 mol-%, exhibit an enantiotropic nematic phase, whereas the parent homopolymers containing triple-core mesogens exhibit a smectic phase. The copolymer containing more than 50 mol-% of the comonomer shows no mesophase. The isotropization temperature of the copolymers decreases with increasing comonomer content. However, the glass transition temperature is almost unchanged upon introduction of the comonomer unit. In case of copolymers containing laterally attached mesogens, a smectic phase was observed below the temperature range of the nematic phase. Consequently, the mesophase and the temperature range of the liquid-crystalline state can be controlled by the introduction of the comonomer unit whose parent homopolymer does not exhibit any mesophase.     

Transition temperatures of thermotropic liquid-crystalline copolyesters

A series of thermotropic copolyesters with glass transition temperatures (T_g) ranging from 70°C up to 180°C based on terephthaloyl chloride ( 1 ), chlorohydroquinone ( 2a ) or hydroquinone ( 2b ), 4,4′-isopropylidenediphenol (bisphenol-A) ( 3 ) and ethylene glycol ( 4 ) was synthesized. The mesomorphic properties of the copolyesters were examined by visual observation of stiropalescence and birefringence under a polarizing microscope. The T_g values of the copolyesters depend mainly on the composition of non-mesogenic or spacer segments and are practically independent of the content of mesogenic units. The structure of mesogenic units also shows a relatively small influence on the T_g behaviour.     

Synthesis and investigation of the properties of polymers containing dicarbaundecaborate

Polymides containing dicarbundecaborate moieties ( 1 ) are formed by nucleophilic degradation of polyamides containing 1,7-dicarba-closo-dodecaborane(12) (m-carborane) moieties in the polymeric chain. The formation of polyamides 1 is accompained by decomposition of the polymeric chain. Potassium cations in the dicarbundecaborate moieties were shown to be easily replaced by protons. The structure of polymers containing exclusively dicarbaundecaborate units ( 2 ) was investigated by IR and ¹H NMR spectroscopy. Some chemical and physico-chemical properties of these polyamides were studied.     

Synthesis and properties of processable rigid polymers containing spiroacetal moieties

A series of rigid rod-like polyspiroacetals bearing flexible side chains was prepared by interfacial polycondensation of 4,4′-(2,4,8,10-tetraoxaspiro[5,5]undecane-3,9-diyl)diphenol with 2,5-dialkoxy-substituted terephthaloyl dichlorides. The majority of these polymers were soluble in polar solvents like tetrahydrofuran (THF), dimethylformamide (DMF), CHCl₃, phenol/CCl₄ (1 : 1 by vol.) mixtures, etc. The molecular weights ranged from 7 600 to 15 000 and the melting temperature from 182 to 228°C. Both of the above values decreased with increasing length of the side chains. The TG curves revealed 5% weight loss at 350–370°C under N₂ gas atmosphere. Wide-angle X-ray diffractograms showed that all of the polymers have considerable crystallinity at room temperature.     

Preparation and mesomorphic properties of new thermotropic liquid-crystalline copoly(urethane-carbonate)s

New thermotropic liquid-crystalline (LC) copoly(urethane-carbonate)s (4) with high inherent viscosities were prepared by melt polycondensation from 6,6′-(4,4′-biphenylylenedioxy)dihexanol (1) and a mixture of aliphatic bis(phenyl carbamate)s (2) and bis(phenyl carbonate)s (3) in high yields. The polymers characterized by FTIR spectra and elemental analyses show good solubilities in organic solvents such as chloroform. Flexible films were cast from the polymer solutions. Their thermotropic LC properties were evaluated by differential scanning calorimetry and observation under the polarizing microscope equipped with a hot stage. These measurements indicate that the polymers form stable LC mesophases and possess block-like sequence distributions.     

Synthesis and characterization of some new photochromic side-chain liquid-crystalline polymers

The synthesis and characterization of a series of liquid-crystalline copolymethacrylates containing side chains with both azobenzene and benzanilide moieties is described. The phase behaviour of the polymers is discussed in dependence of their copolymer composition. The copolymers are suitable for optical investigations on the photochemical trans-cis isomerization of the azo group.     

Thermoelastic and photoelastic properties of crosslinked liquid-crystalline side chain polymers

Measurements of stress and birefringence of crosslinked liquid-crystalline side chain polymers are reported. The results are compared with those obtained for conventional elastomers. It is found that the thermoelastic behaviour in the nematic state completely differs from that observed for common polymer networks due to the anistropic ordering of the mesogenic side chains. The retractive force in the nematic state is found to be independent of path, indicating that the nematic phase is a homogeneous phase. Photoelastic investigations prove the propotionality of stress and birefringence above the clearing temperature T_c which is similar to common elastomers. The stress-optical coefficient, however, strongly depends on temperature in the pretransformation region. X-ray measurements show a uniform orientation of the mesogenic side groups in the nematic state. The diffraction patterns observed contain many more reflexes than observed for common nematics.     

Synthesis and magnetic properties of mesogenic side-chain polymers containing stable radicals

An amorphous and a liquid-crystalline side-chain polymer containing stable nitroxyl radicals are synthesized, the free radical being located either in a piperidine N-oxyl or a 1,3-oxazolidine N-oxyl moiety. These polymers are available by reaction of precursor polymer having a formyl group in every repeating unit with low-molecular-weight nitroxyl free radicals functionalized with an amino group. The conversion of the polymer-analogous reaction is nearly quantitative. Both the amorphous and the liquid-crystalline polymer show paramagnetic behaviour in the temperature range from 6 to 300 K.     

Synthesis and liquid crystalline properties of thermotropic copolycarbonates

Copolycarbonates with flexible spacers have been prepared by melt condensation of 4,4′-bis[(2-hydroxyethyl)oxy]biphenyl, 1,5-bis[(2-hydroxyethyl)oxy]naphthalene and pentarespectively octamethylene diphenyl dicarbonate in varying concentrations. These random copolycarbonates show liquid crystalline behavior, with the formation of nematic phase over a temperature range of maximum 22°C respectively 47°C, this for at least 75 mol-% of biphenyl units. Incorporation of 1,5-bis[(6-hydroxyhexyl)oxy]naphthalene does not lead to liquid crystallinity. Copolymers with 2,6-bis[(6-hydroxyhexyl)oxy]naphthalene and 4,4′-bis[(6-hydroxyhexyl)oxy]biphenyl show mesomorphic behavior, with the formation of a mesophase with a maximum width of 20°C, for incorporation up to 75 mol-% of either of the mesogenic units. Copolycarbonates which contain only biphenyl mesogenic units and which differ in the length of the flexible spacers were also obtained; they behave liquid crystalline over the entire composition range with maximum 34°C for the nematic phase region.     

Crosslinkable liquid-crystalline combined main-chain/side-group polymers with low glass transition temperatures

Crosslinkable and crosslinked liquid-crystalline polymers with elastic properties were prepared from main-chain and combined main-chain/side-group polymers. These polymers exhibit nematic, smectic A, and smectic C phases. Polymers of these types with azoxybenzene as mesogen do not crystallize upon cooling. As a consequence, the liquid-crystalline phases are frozen in glassy around room temperature.     

Synthesis and thermotropic properties of new mesogenic diacrylate monomers

A new class of mesogenic α,ω-alkylene bis[4-(4-acryloyloxybenzoyloxy)benzoate]s 1a — 1g were synthesized and studied for their mesophase behaviour. The thermodynamic parameters of the isotropization transition T_i, ΔH_i and ΔS_i show distinct odd-even alternations and demonstrate that the thermotropic liquid crystalline properties in homologous series of mesogens can not be a continuous function of the length of the inner spacer segment. It is anticipated that the investigated diacrylates are of value as precursors of thermotropic semiflexible polymers, crosslinking agents for mesomorphic polymer networks and low molecular weight twin model compounds.