Ring‐opening metathesis polymerization of cis‐cyanocyclooct‐4‐ene: Search for active catalysts,variation of monomer to catalyst ratios and monomer concentrations

The ring‐opening metathesis polymerization (ROMP) of cis‐cyanocyclooct‐4‐ene initiated by ruthenium‐based catalysts of the first, second, and third generation was studied. For the polymerization with the second generation Grubbs catalyst [RuCl₂(?CHPh)(H₂IMes)(PCy₃)] (H₂IMes = N,N′‐bis(mesityl)‐4,5‐dihydroimidazol‐2‐ylidene), the critical monomer concentration at which polymerization occurs was determined, and variation of monomer to catalyst ratios was performed. For this catalyst, ROMP of cis‐cyanocyclooct‐4‐ene did not show the features of a living polymerization as M_n did not linearly increase with increasing monomer conversion. As a consequence of slow initiation rates and intramolecular polymer degradation, molar masses passed through a maximum during the course of the polymerization. With third generation ruthenium catalysts (which contain 3‐bromo or 2‐methylpyridine ligands), polymerization proceeded rapidly, and degradation reactions could not be observed. Contrary to ruthenium‐based catalysts of the second and third generation, a catalyst of the first generation was not able to polymerize cis‐cyanocyclooct‐4‐ene. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Controlling thermomechanical behavior of semicrystalline hydrogenated polynorbornene through the cis- to trans-cyclopentylene ratio

Though polynorbornene synthesized by ring-opening metathesis polymerization has an intrinsically all-cis configuration of the cyclopentylene backbone rings, a fraction of these rings can be epimerized to the trans configuration during hydrogenation over suitable catalysts. By varying the method of hydrogenation, semicrystalline hydrogenated polynorbornenes (hPNs) with trans levels between 0 and 36% were obtained. With increasing trans content, the glass transition temperature, melting point, and enthalpy of melting decrease modestly. By contrast, the temperature at which the hPN crystal transitions into a rotationally disordered polymorph varies strongly with trans content, ranging from 126 °C (all-cis) to 71 °C at 27% trans; at trans contents of 34% and above, no rotationally-ordered phase is observed at any temperature. The room-temperature Young's modulus shows no dependence on trans content, while the yield stress drops by 20% at 1% trans content and slowly decreases further with additional epimerization. The temperature dependence of the Young's modulus differs for trans-containing versus all-cis polymers, while the temperature dependence of the yield stress is set by the polymorph type (rotationally ordered vs. disordered).     

A 1H NMR study of tautomerism in some 1-arylamino-3-aryliminopropenes

¹H n.m.r. spectra at ambient temperatures reveal that an equilibrium exists between the ‘all-trans’ and ‘all-cis’ isomers of some of the 1-arylamino-3-aryliminopropenes. The ‘all-cis’ isomer predominates in nonpolar solvents, whereas the ‘all-trans’ isomer is favoured in hydrogen bonding solvents. From a consideration of the magnitudes of the ³J coupling constants, it is reported that the ‘cis-trans’ isomer is the most stable form of the 4-nitrophenyl derivative in dimethyl sulphoxide.     

Synthesis of soluble,all-trans poly(2,5-diheptyl-p-phenylenevinylene) via metathesis polycondensation

The synthesis of substituted poly(p-phenylenenvinylene) (PPV) was carried out via metathesis polycondensation of 2,5-diheptyl-1,4-divinylbenzene (DHepDVB). A stable molybdenumcarbene complex served as catalyst. The preparation of the educt employed (DHepDPV, 3a ) is described. The obtained poly(2,5-diheptyl-1,4-phenylenevinylene) (DHepPPV, 5 ), an intensely yellow product, has an all-trans-configuration and, with a degree of polymerization of ≈ 10 is soluble in conventional organic solvents.     

A novel polar polymer via romp

We have synthesised poly(2,3-bis(trifluoromethyl)norbornadiene) (PBTFMND) via ring-opening metathesis polymerisation (ROMP) to yield a glassy, highly polar polymer. The high trans polymer is ∼92% tactic and the high cis polymer is ∼75% tactic but the type of tacticity cannot be determined by NMR techniques. Thermally Stimulated Depolarisation (TSD) measurements give relaxed susceptibilities as high as 45 for the high trans material but as low as 3 for the high cis material. These data suggest that both the trans and cis materials are syndiotactic. The high trans material can be poled to yield a maximum polarisation of about 20 mC m⁻² and a pyroelectric coefficient of 6 μC m⁻². Though this is less than that of PVDF, the low room temperature permittivity and loss of PBTFMND results in a pyroelectric figure of merit comparable with that of PVDF.     

Reactivities of cis- and trans-3-substituted acrylic acids on copolymerization with acrylamide

Several pairs of cis- and trans-3-substituted acrylic acids (3SAA) were copolymerized with acrylamide in order to determine the major factors affecting the relative reactivities of geometrical isomers of 1,2-disubstituted ethylenes (1,2-DE). The results were that the relative reactivity of cis isomer is larger than that of trans isomer when one substituent is electron-withdrawing and the other is electron-donating. The trans isomer is more reactive than the cis isomer when both substituents are electron-withdrawing. A new method of reactivity comparison of cis- and trans-1,2-DE is proposed in regard to the inductive substituent constant.     

Stereoselective Syntheses of Benz[f]isoindoline-Derivatives by Intramolecular Cycloadditions of Styrenes to Olefins

The N-allyl-N-cinnamyl amide 10 undergoes thermal cyclization to a 2:1-mixture of the trans- and cis-benz(f)isoindolines 11a and 12a . By comparison, the thermolysis of the corresponding bis-cinnamylamide 14 proceeds in a highly stereoselective manner to give the cis-fused[4+2]-adduct 16a . Similarly, the trans-fused stereoisomeric adducts 30a and 31a were obtained with high stereochemical control on heating the N-allyl-N-diphenylallyl amide 28 . The thermal transformations 4 → 5 + 6a and 17 → 18a + 20a show the competitive formation of [2+2]-adducts. An alternative approach to (substituted) benz[f]isoindolines 16 via the all-cis-isomer 24a has been developed. The described structures have been assigned on the basis of spectral evidence, chemical correlations and by X-ray-diffraction study of the isomer 16b . These results illustrate the utility of substituent interactions in order to direct intramolecular cyclo-additions at will towards either endo- or exo-products.     

Computer Automated Structure Evaluation (CASE) of the teratogenicity of retinoids with the aid of a novel geometry index

Summary The CASE (Computer Automated Structure Evaluation) program, with the aid of a geometry index for discriminating cis and trans isomers, has been used to study a set of retinoids tested for teratogenicity in hamsters. CASE identified 8 fragments, the most important representing the non-polar terminus of a retinoid with an additional ring system which introduces some rigidity in the isoprenoid side chain. The geometry index helped to identify relevant fragments with an all-trans configuration and to distinguish them from irrelevant fragments with other configurations.     

Cis/trans ISOMERIZATION OF CAROTENOIDS BY THE TRIPLET CARBONYL SOURCE 3-HYDROXYMETHYL-3,4,4-TRIMETHYL-1,2-DIOXETANE*

Abstract— The interaction of biological carotenoids with 3-hydroxymethyl-3,4,4-trimethyl-1,2-dioxetane (HTMD), a thermodissociable source of electronically excited ketones, was investigated using reversed-phase high-performance liquid chromatography. Incubation of the all-trans isomers of β-carotene, lycopene and canthaxanthin with HTMD led to significant trans-to-cis isomerization, with cis isomers accounting for 20–50% of products formed (the balance assigned as oxidation products). The isomers forming from all-trans-β-carotene were identified as 9-cis-, 13-cis- and 15-cis-β-carotene by cochromatography of cis isomer standards and by on-line diode array absorbance spectroscopy. An HTMD-dependent cis-to-trans isomerization was observed in incubations started with 15-cis-β-carotene, and it occurred more rapidly and to a greater extent than the isomerization of all-trans-β-carotene. The isomer patterns generated from lycopene and β-carotene are generally similar to those reported recently for various human tissues (Stahl et al, 1992, Arch. Biochem. Biophys. 294 , 173–177).     

Regiospecific cationic polymerization of spiroorthoesters with different cyclic ether ring sizes

Spiroorthoesters (SOEs), cis‐2,3‐tetramethylene‐1,4,6‐trioxaspiro[4,5]decane ( I ) and cis‐2,3‐tetramethylene‐1,4,6‐trioxaspiro[4,6]undecane ( II ), with different cyclic ether ring sizes were synthesized, and their stereostructure and steric energy were determined. With steric‐hindrance‐sensitized 9‐phenyl‐9,10‐dihydro‐anthracen‐10‐ylium cation as an initiator, I and II underwent regiospecific polymerization to yield trans form of stereoregular poly(ether esters)—poly(trans‐2‐oxycyclohexyl pentanoate) (? [trans‐2‐OCHP]_n? ) ( III ) and poly(trans‐2‐oxycyclohexyl hexanoate) (? [trans‐2‐OCHH]_n? ) ( V ), respectively. With SnCl₄ as another initiator, I and II underwent regiospecific polymerization through different mechanisms to afford cis form poly(cis‐2‐oxycyclohexyl pentanoate) (? [cis‐2‐OCHP]_n? ) ( IV ) and trans form (? [trans‐2‐OCHH]_n? ) ( VI ) stereoregular poly(ether esters). The polymerization mechanisms of SOEs proceeded in the regiospecific manner were determined by the relationship among the sterostructures of SOEs and its subsequently formed polymers, the steric energy of monomers, and the free energy difference in the transition state of reaction. Owing to the conversion of cis substitution at C‐2 and C‐3 in I or II to the trans form during polymerization, polymers III , V , and VI exhibited a higher volume of expansion during polymerization than IV , which showed high volume shrinkage. Group contributions of divalent trans‐ and cis‐1.2‐cyclohexyl groups were derived and confirmed by measuring the densities of the corresponding stereoregular polymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

NMR signal assignment of cis/trans‐conformational segments in MEH‐CN‐PPV

Poly[2‐(2′‐ethylhexyloxy)‐5‐methoxy‐1,4‐phenylene‐(1‐cyanovinylene)] MEH‐CN‐PPV and its all‐trans model compound 1,4‐bis(α‐cyanostyryl)‐2‐(2‐ethylhexyloxy)‐5‐methyloxybenzene were synthesized via Knoevenagel condensation. All‐cis isomer and cis–trans isomer of 1,4‐bis(α‐cyanostyryl)‐2‐(2‐ethylhexyloxy)‐5‐methyloxybenzene were prepared by the photoisomerization reaction. Comparison of the ¹H NMR spectra between MEH‐CN‐PPV and three model compounds proved the occurrence of cis‐vinylene in the backbone of MEH‐CN‐PPV. According to the ratio between the cis‐vinylene signal and trans‐vinylene signal, the content of the cis‐vinylene could be estimated to be 15% in MEH‐CN‐PPV. This large cis‐vinylene content came from the rapid photochemical isomerization of cyanovinylene and was likely relative to the poor electroluminescence property of MEH‐CN‐PPV. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1105–1113, 2008     

Molecular conformation of cyclenes: Part IX. Asymmetrical 1,3,6-cyclononatriene and 1,3,6-cyclodecatriene

The strain energy of the most probable conformers of 1,3,6-cyclononatriene and 1,3,6-cyclodecatriene, which are asymmetrical compounds, has been calculated as a function of various geometrical parameters by means of the semiempirical Hendrickson's treatment, partially modified. The most stable conformation of 1,3,6-cyclononatriene is a member of the all-cis family, while a transcis-trans structure is the most stable among the 1,3,6-cyclodecatriene conformations. In this latter compound however, the energy minima of the all-cis, trans-cis-cis, and cis-cis-trans conformers also appear to be close to each other. Some possible interconversions are also discussed.     

Olefin metathesis polymerization: Some recent developments in the precise polymerizations for synthesis of advanced materials (by ROMP,ADMET)

Recent results for synthesis of end-functionalized polymers (EFP) by using olefin metathesis polymerization have been introduced including basic characteristics in ring-opening metathesis polymerization (ROMP) of cyclic olefins and acyclic diene metathesis (ADMET) polymerization for synthesis of conjugated polymers. Several approaches were demonstrated for synthesis of EFP by living ROMP using molybdenum (exclusive coupling with aldehyde) and ruthenium catalysts (sacrificial ROMP, chain transfer). Cis specific (Z selective) ROMPs were achieved by molybdenum, ruthenium, and vanadium catalysts by the ligand modification. The catalytic synthesis of EFP with high cis selectivity has been achieved by combined ROMP with chain transfer by V(CHSiMe₃)(N-2,6-Cl₂C₆H₃)[OC(CF₃)₃](PMe₃)₂. The ADMET polymerization using molybdenum and ruthenium catalysts afforded defect-free, high molecular weight poly(arylene vinylene)s containing all trans olefinic double bonds. The methods for precise synthesis of EFPs, exhibiting unique optical properties combined with the end groups, were developed. The catalytic one-pot syntheses for EFPs have also been developed.     

A systematic study of stereochemical effects in homologous poly(alkenamer)s: Dewar benzene versus norbornene

Two diastereomeric derivatives of norbornene, dimethyl (1R,2R,3S,4S)-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate and dimethyl (1R,2S,3S,4S)-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate, were synthesized and polymerized using ring-opening metathesis polymerization (ROMP). For comparative purposes, diastereomeric derivatives of Dewar benzene, dimethyl (1R,2S,3R,4S)-bicyclo[2.2.0]hex-5-ene-2,3-dicarboxylate and dimethyl (1R,2S,3S,4S)-bicyclo[2.2.0]hex-5-ene-2,3-dicarboxylate, were also synthesized and polymerized using ROMP. The polymerization reactions proceeded in a controlled manner as evidenced in part by linear relationships between the monomer-to-catalyst feed ratios and the molecular weights of the polymer products. Chain extension experiments were also conducted which facilitated the formation of block copolymers. Although the poly(norbornene) derivatives exhibited glass transition temperatures that were dependent on their monomer stereochemistry (cis: 115°C vs. trans: 125°C), more pronounced differences were observed upon analysis of the polymers derived from Dewar benzene (cis: 70°C vs. trans: 95°C). Likewise, microphase separation was observed in block copolymers that were prepared using the diastereomeric monomers derived from Dewar benzene but not in block copolymers of the norbornene-based diastereomers. The differential thermal properties were attributed to the relative monomer sizes as reducing the distances between the polymer backbones and the pendant stereocenters appeared to enhance the thermal effects.     

Living ring-opening metathesis polymerization of norbornenes bay-functionalized perylene diimides

Ring-opening metathesis polymerization (ROMP)-derived poly(oxanorbornene imide)s bearing bay-linked mono - alkoxy -M1 and 1,7-di-alkoxy M2 functionalized perylene diimides (PDIs) were synthesized using Grubb's third ( G3 ) and Hoveyda-Grubbs second generation ( HG2 ) ruthenium-alkylidene metathesis initiators. The mono-alkoxy-derived PDI-based non-ladderphane polymer poly M1 displayed 67% to 77% of the trans olefin content in the polymer chain depending on the initiator used for the polymerization. When using the symmetrical 1,7-di-alkoxy-derived PDI-based polymer poly M2 having the ladderphane type-structure, this displayed a significant amount of cis and trans olefin contents in the polymer chains, irrespective of the type of initiators used for the polymerization. ROMP of both monomers M1 and M2 proceeded in a well-controlled manner with a linear dependence of molecular weight on the monomer/initiator ratio using G3 as initiator. Optical properties of the ladderphane-based poly M2 and non-ladderphane-based poly M1 were characterized in both solution and the film state. X-ray diffraction (XRD) analysis for all the polymers showed significant π-stacking in the thin film state with ordered molecular packing and closer values of d-spacing for both poly M1 and poly M2 . Film morphology examined by AFM elucidated homogenous smooth polymer surface for both polymers in general, but with some irregularities observed for poly M1 . In addition, CV analysis revealed both polymers could be good candidates as electron-accepting materials, with excellent film-forming ability, and thermal stability.     

The photoisomerization of retinal

Abstract— –Quantum efficiencies have been measured for the photoisomerization of four stereoisomers of retinal (all-trans, 13-cis, 11 cis, and 9-cis) in two solvents at different wavelengths of irradiation and at various temperatures. In heane at 25°C the quantum efficiencies for isomerization at 365 nm are: 9-cis to trans, 0.5; 13-cis to trans, 0.4; 11-cis to trans, 0.2; all-trans to monocis isomers, 0.2-0.06, depending upon assumptions made regarding the stereo-isomeric composition of the product. These values vary somewhat with the wavelength of the irradiating light. The quantum efficiency for the photoisomerization of all-trans retinal in hexane decreases by a factor of 30 when the temperature is lowered from 25° to – 65°C; the activation energy for this photoisomerization is about 5 kcal/mole. The quantum efficiencies for the isomerization of the monocis isomers to all-trans retinal in hexane are virtually independent of temperature. In ethanol the rates of photoisomerization from trans to cis or cis to trans depend only slightly on the temperature between 25° and – 65°C. The photosensitivities of the stereoisomers of retinal are of the same order of magnitude as those of the retinylidene chromophores of rhodopsin (11 -cis), metarhodopsin I (all-trans), and isorhodopsin (9-cis); but it is not yet possible to derive the photochemistry of rhodopsin uniquely and quantitatively from that of retinal.     

Photoinduced Gilch Polymerization: A Basically New Access to Poly(p‐phenylene vinylenes) (PPVs) of Exciting Constitutional Homogeneity

A novel procedure has been developed for the Gilch reaction leading to poly(p‐phenylene vinylenes) (PPVs). In the first step, selective activation of the starting material is achieved at low temperature. Subsequently, controlled chain growth is induced by lighting the α‐halo‐p‐quinodimethane monomer. In contrast to the thermal Gilch polymerization, the photoinduced process allows adjusting crucial parameters such as intensity and energy of light. The progress of PPV formation can be followed visually or by in situ UV–vis spectroscopy. If the polymers are formed under appropriate conditions, they show very high molar masses, polydispersities in the common range, and higher constitutional homogeneity than thermally grown PPVs.

     

Trennung und Charakterisierung von β-Carotin-Isomeren

Separation and Characterization of the cis-Isomers of β,β-Carotene A stable HPLC. system is described allowing the excellent separation of 11 different cis-isomers of β,β-carotene from the all-trans compound. The system is applied to the analysis of cis/trans mixtures obtained from plant extracts and by photoisomerization of the all-trans isomer. Al₂O₃ is used as the stationary phase while hexane with controlled H₂O content is utilized as the mobile phase. With the aid of the optimum conditions 8 sufficiently stable cis isomers were isolated and their structures shown to be the 9-, 13- and 15-cis, the 9,9′-, 9, 13-, 9, 13′- and 13,13′-di-cis and, tentatively, the 9,13,13′-tri-cis β,β-carotenes by application of 270-MHz-FT.-¹H-NMR. spectroscopy.     

Transition Metal Compounds in Combination with Alkylaluminoxanes as Catalysts for Olefin and Diene Polymerization

Catalytic systems based on Zr, Co, Ti, V, and Nd in combination with –(–RAlO–)– _n alkylaluminoxanes, where R = Me or iso-Bu, were used for the polymerization of olefins and dienes. The structure of methylaluminoxane (MAO) was studied with the use of theoretical calculations and experimentally by vibrational spectroscopy. It was found that only cis and trans conformations of linear MAO oligomers actually occurred at the active centers of olefin polymerization. The stereospecificity of catalytic systems for diene polymerization depends on the nature of the substituent at aluminum, as well as on the nature of the transition metal and its ligand environment.