Aryl Cyanates Research Articles

An Atom‐Economical Approach to 2‐Aryloxypyridines and 2,2′/2,3′‐Diaryloxybipyridines via Ruthenium‐Catalyzed [2+2+2] Cycloadditions

AbstractAn efficient, ruthenium(II)‐catalyzed, solvent‐free approach for the synthesis of 2‐aryloxypyridines with good to excellent yields has been developed by the reaction of 1,6‐diynes with aryl cyanates. This atom‐economical catalytic strategy offers a mild and practical approach to access a variety of such cycloadducts with excellent regioselectivities. The protocol was further extended to the synthesis of 2,2′‐ and 2,3′‐diaryloxybipyridines by the reaction of tetraynes with aryl cyanates.magnified image

Kinetic and Mechanistic Insights into Thermally Initiated Polymerization of Cyanate Esters with Different Bridging Groups

AbstractThis study investigates the kinetics of thermally initiated polymerization of aryl cyanates with different bridging fragments ((CH3)2C, S, and O). To minimize the effect of impurities on the kinetics, the aryl cyanates are synthesized in a highly pure form. The rates of polymerization are measured by differential scanning calorimetry (DSC) and treated by an advanced isoconversional method. The DSC measurements indicate that the ((CH3)2C‐ and S‐bridging monomers have, respectively, the lowest and highest reactivities. The observed variation of the activation energy with conversion and temperature is determined to be in agreement with the two‐step kinetic model by Kamal. Estimating the parameters of the Kamal model has provided mechanistic insights into the polymerization process. The enhanced reactivity of the S‐bridged monomer is found to be associated with acceleration of the first step that demonstrates an increased value of the preexponential factor. The acceleration is linked to higher polarizability of this monomer that can promote stronger intermolecular interaction and preferred orientation of the reacting species.

Effect of Segmental Configuration on Properties of n-Propyl-Bridged Polycyanurate Networks

The effect of the chemical configuration of network segments on the physical properties, cure properties, mechanical performance, and chemical stability of polycyanurate networks was investigated via synthesis, network formation, and characterization of an isomeric series of n-propyl-bridged cyanate ester monomers. Configurations that provide cyanurate oxygen atoms with either nearby methyl groups or nearby bridge groups exhibited decreased moisture uptake by up to 50%, along with a roughly 20–40 °C reduction in the loss in glass transition temperatures due to hydrolysis, for networks immersed in 85 °C water for 96 h. In ortho,para-linked aryl cyanates, dry glass transition temperatures of cured networks were reduced compared to analogous para,para-linked networks by only about 10 °C, compared to a reduction of 30 °C in ortho-methylated cyanate ester networks, leading to higher “wet” glass transition temperatures in the ortho,para-linked networks. Neither methyl groups nor bridge groups in a position orth...

Improving the hydrolytic stability of aryl cyanate esters by examining the effects of extreme environments on polycyanurate copolymers

Three cyanate ester monomer or oligomer species: 2,2-bis(4-cyanatophenyl)propane 1, 1–1-bis(4-dicyanatophenyl)ethane 2, and the oligomeric phenolic cyanate 3, are blended in various ratios to form binary mixtures (18 in total), formulated with copper(II) acetylacetonate (200ppm) in dodecylphenol (1% w/v active copper suspension) and cured (3Kmin−1 to 150°C+1h; 3Kmin−1 to 200°C+3h) followed by a post cure (3Kmin−1 to 260°C+1h). Cured copolymers were exposed to environments of elevated relative humidity (75% RH) and parallel immersion testing in H2O, H2SO4 (10%) and NaOH (10%) at 25°C for a period of up to 2years and accelerated ageing in boiling water (14days). Periodic measurements are made of moisture gain along with infrared spectra and compared with cured homopolymers. Changes in mass are recorded periodically throughout exposure, prior to destructive thermo-mechanical analyses. Dynamic mechanical thermal analysis data comparing neat and exposed blends demonstrate the detrimental effect of moisture ingress whilst data from thermogravimetric analysis demonstrate no change in degradation onset between neat and exposed materials. An optimised blend of 1:1 of monomer units 1 and 2 was found to absorb less moisture than blends of different stoichiometry or between other respective monomeric units, consequently limiting the deleterious effect of moisture ingress.

Spectroscopic study of chemical modifications induced by swift heavy ions on polymers: the contribution of the CIRIL Platform and the CIMAP Laboratory

This paper gathers results obtained on the chemical ageing of polymers, at the CIRIL platform, using Swift heavy ions (SHI) from the GANIL accelerator. Swift heavy ions induce high values of electronic stopping power or LET (Linear Energy Transfer) and deposit their energy in the polymer through electronic processes, in a few nanometer size cylinder centered on the ion path. This results in huge local doses and dose rates. Both defects created in the polymer chain and gas release were quantified using spectroscopic methods (FTIR and Residual gas analysis (RGA)). Defects created in polymers submitted to SHI can be separated in two main series: defects common to all ionizing radiations and defects specific to SHI. A common trend of the evolution of these defects, under inert environment, is the following: 1) for the first group of defects, in most of the polyolefins, there is a limited (if inexistent) effect of LET on the radiation chemical yield of creation at low doses. Among defects of this first series, the behavior of vinyl groups is particular, 2) LET effect on SHI specific defects (triple bonds and cumulenes) is tremendous. Triple bonds (alkynes, alkyl or aryl cyanates) are created after a LET threshold value, depending on the polymer chemical structure. The dose effect on macromolecular defects, under inert environment, is also presented. The study of the LET effect on gas release, in various polyolefines, gives an insight on the mechanism of bond cleavage in presence of high ionization and excitation densities. Finally, few results on radiation-induced oxidation are presented. Compared to low-ionizing radiations, oxidation is reduced and unsaturated bonds are created under SHI.

ChemInform Abstract: Solvent‐Free Preparation of 5‐Aryloxytetrazoles via [2 + 3] Cycloaddition of Cyanates and Sodium Azide Using Silica Supported Sulfuric Acid as an Effective Heterogeneous Catalyst.

A simple and efficient method is described for the preparation of 5-aryloxytetrazoles in excellent yields and high purity from aryl cyanates using silica sulfuric acid as catalyst. The rate of product formation was enhanced by introduction of electron-donating substituents. The 1H NMR spectral characteristics of the products are also discussed.

Solvent-free Preparation of 5-Aryloxytetrazoles via [2+3] Cycloaddition of Cyanates and Sodium Azide Using Silica Supported Sulfuric Acid as an Effective Heterogeneous Catalyst

A simple and efficient method is described for the preparation of 5-aryloxytetrazoles in excellent yields and high purity from aryl cyanates using silica sulfuric acid as catalyst. The rate of product formation was enhanced by introduction of electron-donating substituents. The 1H NMR spectral characteristics of the products are also discussed.

Studying the co-reaction of propenyl-substituted cyanate ester-bismaleimide blends using model compounds

Reactive modifiers, bearing cyanate and/or alkenyl groups have been shown to improve the thermo-mechanical and water uptake properties of cured bismaleimides and BMI/cyanate ester blends. The allyl-substituted modifiers have been the subject of much study, but the reaction mechanism of the more reactive propenyl analogues (for which lower moisture absorptions have been recorded) have not received much attention until the present study. The synthesis and full characterisation of model maleimide and propenyl-substituted aryl cyanate compounds is reported. Infrared and Raman spectroscopy and thermal analysis techniques are used to examine the thermally initiated co-reaction between blends of the two model compounds. Raman spectroscopy reveals that as the thermal reaction proceeds, there is a pronounced decrease in the alkenyl CC stretch band at 1655cm−1 as a function of temperature and this is accompanied by a concomitant decrease in the vinylidene band at 3010cm−1. In the absence of a dedicated catalyst, the cyanate cyclotrimerization is slow and follows the co-reaction between the alkenyl group and the maleimide ring. Molecular modelling experiments using semi-empirical and ab initio methods support the formation of the trans ene adduct which is consistent with calculated and observed vibrational frequencies.

Synthesis, Characterization and Applications of Polyurethane Aryl Cyanate Ester Resin

Polyurethane-based aryl cyanate ester resin (PU-CER) with a biphenyl backbone obtained from polymeric 4,4‘-diphenylmethanediisocyanate, bisphenol A and cyanogen bromide was synthesized to get a polymer with better functional properties such as better adhesion and impact resistance. Synthesis of polyurethane based 4,4‘-(oxy biphenyl propane) cyanate ester involved two steps of polyurethane hydroxyl-terminated prepolymer (PU-PP) formation and esterification (PU-CER). PU-CER was cyclotrimerized to yield a triazine ring-containing polymer that possesses better adhesion at high temperature and has greater impact strength. PU-PP and PU-CER were characterized by wet chemical analysis, spectral methods and thermal methods. PU-CER showed better thermal stability and adhesion with single part polyurethane lamination adhesive and also functioned as a better toughening agent in two-part epoxy laminate system..

The development of novel functionalised aryl cyanate esters. Part 2. Mechanical properties of the polymers and composites

Carbon fibre preimpregnated tape (prepreg) was prepared from a range of prepolymers comprising several different blends of alkenyl-functionalised cyanate ester (CE) monomers with commercial CE and bismaleimide (BMI) monomers and blends. Carbon fibre composites were prepared by autoclave moulding. Dynamic mechanical thermal analysis (DMTA) was used to assess cure cycles by the measurement of glass transition temperatures (Tg). Mechanical properties are reported for neat resins and compared with the corresponding composite properties such as interlaminar shear strength, flexural strength, compressive strength and mode I fracture toughness (GIC). By incorporation of alkenyl-functionalised CE monomers into CE/BMI blends it is possible to raise the value of Tg while maintaining levels of GIC (when compared with an unmodified BMI/CE blend) and without leading to a degradation in other mechanical properties. This enhancement in neat resin fracture toughness was translated into the corresponding composite. For example, a novel modifier 5rPOCN, at an incorporation level of 15% by weight, produces a composite with Tg=170°C, interlaminar shear strength 113MPa and GIC=513J/m2, comparable with reported thermoplastic-toughened CE composites.

A new synthetic route for the preparation of alkenyl functionalized aryl cyanate ester monomers

A new, simplified synthetic route is reported for a novel family of functionalized cyanate monomers. Importantly, the synthetic route has fewer purification requirements, making it a potentially cheaper and more efficient route than previously reported syntheses. The products have been characterized extensively using nuclear magnetic resonance spectroscopy, infrared spectroscopy, and elemental analysis. Bearing alkenyl groups, these materials have the ability to form copolymeric networks with a wide variety of monomers, to yield high performance composite matrices.

Synthesis and characterization of new phosphorus and other heteroatom containing aryl cyanate ester monomers and networks

Several aromatic dicyanate monomers have been synthesized bearing para-linked strong electron withdrawing groups, such as phenylphosphine oxide, sulfone, and carbonyl. These groups increased the reactivity of the cyanate functional groups and eliminated the need for curing catalysts. However, an undesirable decrease in the processing window between the monomer melting point and the onset of cure was also generally observed. An arylene ether phenyl phosphine oxide system was designed that displayed several attractive characteristics such as a low softening point, a wide processing window, cure with no catalyst, high Tg and high char yield in air, suggesting that these new thermosets might show good fire resistance. The dicyanate ester monomers were synthesized in high yield by reacting various bisphenols with cyanogen bromide in the presence of triethylamine. The high reactivity of the cyanate functional groups required that the cyanation reaction be conducted at temperature below 0°C in order to prevent imidocarbonate side reactions. Proton NMR and FT-IR were used to characterize these monomers. The cyclotrimerization curing process was monitored by the disappearance of the carbon-nitrogen triple bond stretch (2270 cm−1). An optimal cure schedule was determined and the cured polycyanurate networks were characterized by DSC, DMTA, and TGA. Tg values were typically > 250°C and 5% weight loss values were observed by TGA in air above 400°C. Several of the dicyanate monomers with sufficiently large processing windows were cured into single lap shear adhesive bonds onto titanium 6/4 and the measurements are reported herein. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 977–987, 1997

Synthesis and characterization of new phosphorus and other heteroatom containing aryl cyanate ester monomers and networks

Several aromatic dicyanate monomers have been synthesized bearing para-linked strong electron withdrawing groups, such as phenylphosphine oxide, sulfone, and carbonyl. These groups increased the reactivity of the cyanate functional groups and eliminated the need for curing catalysts. However, an undesirable decrease in the processing window between the monomer melting point and the onset of cure was also generally observed. An arylene ether phenyl phosphine oxide system was designed that displayed several attractive characteristics such as a low softening point, a wide processing window, cure with no catalyst, high Tg and high char yield in air, suggesting that these new thermosets might show good fire resistance. The dicyanate ester monomers were synthesized in high yield by reacting various bisphenols with cyanogen bromide in the presence of triethylamine. The high reactivity of the cyanate functional groups required that the cyanation reaction be conducted at temperature below 0°C in order to prevent imidocarbonate side reactions. Proton NMR and FT-IR were used to characterize these monomers. The cyclotrimerization curing process was monitored by the disappearance of the carbon-nitrogen triple bond stretch (2270 cm−1). An optimal cure schedule was determined and the cured polycyanurate networks were characterized by DSC, DMTA, and TGA. Tg values were typically > 250°C and 5% weight loss values were observed by TGA in air above 400°C. Several of the dicyanate monomers with sufficiently large processing windows were cured into single lap shear adhesive bonds onto titanium 6/4 and the measurements are reported herein. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 977–987, 1997

Synthesis and characterization of new phosphorus and other heteroatom containing aryl cyanate ester monomers and networks

Synthesis and characterization of new phosphorus and other heteroatom containing aryl cyanate ester monomers and networks

Mechanical properties of tough, high temperature carbon fibre composites from novel functionalized aryl cyanate ester polymers

Carbon fibre pre-impregnated tape (prepreg) was prepared from a range of prepolymers comprising blends of propenyl-functionalized cyanate ester (CE) oligomers with commercial bis-maleimide (BMI) monomers. Cured neat resin plaques and carbon fibre composites were prepared by autoclave moulding. Various blends of a commercial BMI mixture, a CE and a co-monomer with either allyl or propenyl and cyanate pendant groups were prepared. Differential scanning calorimetry (d.s.c.) data were used to monitor cure of neat resin, while dynamic mechanical thermal analysis (d.m.t.a.) was used to assess cure cycles of composite materials by the measurement of glass transition temperatures (Tg). Composite properties measured were interlaminar shear strength, flexural strength, compressive strength and Mode 1 Fracture toughness (GIC).

Development of Novel Functionalized Aryl Cyanate Ester Oligomers. 1. Synthesis and Thermal Characterization of the Monomers

A range of allyl-functionalized cyanate ester oligomers (and their precursors) containing four and six phenylene groups in the backbone were prepared in high yield and characterized using 1 H and 13 C NMR spectroscopy and microanalysis. The thermally-initiated polymerization of the monomers was studied using DSC and the kinetic parameters of the reaction obtained: the polycyclotrimerization was extremely well-fitted to a first-order model over the apparent conversion range 5-50% (yielding Arrhenius parameters as apparent activation energies of 211.7 kJ mol -1 and ln A= 39.8 for 6 and 148.1 kJ mol -1 and ln A= 27.0 for 12)

Kinetics and mechanism of the titanium tetrachloride-catalysed cyclotrimerisation of aryl cyanates

Aryl cyanates are converted cleanly at 25 °C to 1,3,5-triazines by catalytic amounts of titanium tetrachloride in dichloromethane. Based on IR spectroscopic, kinetic and product analysis, a mechanism is proposed involving rate-limiting nucleophilic attack of the cyanate nitrogen on the cyanato carbon of a cyanate–titanium tetrachloride complex. Subsequent steps are fast, with no evidence for dimeric or acyclic trimeric intermediates; it is suggested that these steps involve a series of fast stereoselective reactions of nitrillium ion intermediates.

ChemInform Abstract: A New Modified Electrophilic Cyanation of Aromatics, e.g. (I), with Activated Aryl Cyanates (II), (VI).

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

A study of the thermal and dynamic mechanical properties of functionalized aryl cyanate esters and their polymers

AbstractDifferential scanning calorimetry (DSC) was employed to examine the thermally initiated and catalysed polymerizations of a number of cyanate ester monomers. Rigorous purification by preparative HPLC was found to have marked effects on the thermal polymerization characteristics of these compounds as determined by DSC. The effects on the thermal behaviour of varying the heating rate and level of catalysis were also examined. The thermal polymerization of the cyanate ester monomers was found to obey a first order model over wide apparent conversion limits. Functionalized cyanate ester/bis‐maleimide copolymers were prepared and the thermal stabilities of these were evaluated by thermogravimetric analysis (TGA) alongside a variety of commercial blends and homopolymers. The dynamic mechanical properties of the cured neat resins were also investigated.

A New Modified Electrophilic Cyanation of Aromatics with Activated Aryl Cyanates

Abstract The selective cyanation of electron-rich aromatics succeeds in moderate to good yields with the activated aryl cyanates 1a-d using AlCl3/HCl. The formation of p-isomeres is preferred.