Thermal latency of novel chelating borate catalysts as latent catalysts for epoxy–phenolic resins

Novel quaternary ammonium bis(2‐oxybenzoyloxy)borate salts ( 1a – 1c ) or quaternary ammonium bis(1,2‐benzenedioxy)borate salts ( 2a and 2b ) with tetra‐n‐butylammonium (TBA⁺), tetra‐n‐octylammonium (TOA⁺), or bis(triphenylphosphoranylidene)ammonium (PNP⁺) cations were synthesized as latent catalysts of epoxy/phenol–novolac resins by the complexation between boric acid and salicylic acid or catechol, followed by neutralization with quaternary ammonium hydroxide. Polyaddition reactions of diglycidyl ether of bisphenol A (DGEBA) and 4,4′‐bisphenol F (44BPF) or bisphenol F (BPF‐D) with the ammonium borates were investigated as model reactions of epoxy/phenol–novolac resin systems with respect to the thermal latency and storage stability of the catalyst. The polyaddition of DGEBA/44BPF with 1a – 1c in diglyme at 150 °C for 6 h proceeded up to 85–96% conversions and gave polymers with number‐average molecular weights of 4180–10,500, whereas the polyaddition at 80 °C for 6 h gave less than 8% conversions. However, the polyaddition with 2a containing TBA⁺ cation proceeded to only a 32% conversion at 150 °C for 6 h in diglyme and to a 64% conversion even at 180 °C for 6 h in triglyme and only gave low molecular weight oligomers, and no reaction proceeded in the polyaddition at 80 °C. However, polyaddition with 2b containing PNP⁺ cation proceeded up to a 96% conversion at 150 °C for 6 h in diglyme and gave a higher molecular weight polymer with a number‐average molecular weight of 8050, whereas the polyaddition at 80 °C for 6 h gave only a 5% conversion. The catalytic activity of ammonium borates 1a – 1c and 2a and 2b depended on the borate anion structure: 1a and 1c with bis(2‐oxybenzoyloxy)borate anion revealed higher activity than 2a and 2b with bis(1,2‐benzenedioxy)borate anion, respectively. In comparison with tetra‐n‐butylammonium bromide (TBAB) as a conventional ammonium salt or tetra‐n‐butylammonium tetrakis(benzoyloxy)borate (TBA‐TBB), 1a – 1c and 2b revealed better thermal latency. The catalytic activity of ammonium borates also depended on the bulkiness of the ammonium cation, and the order of activity was 1c (PNP⁺) > 1b (TOA⁺) ≧ 1a (TBA⁺) and 2b (PNP⁺) > 2a (TBA⁺). The storage stability of DGEBA/BPF‐D with the ammonium borate catalysts 1a – 1c and 2a and 2b in bulk at 40 °C was much better than that with TBAB and TBA‐TBB. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2702–2716, 2002     

Synthesis of poly(cyanurate)s by a novel polyaddition of bis(epoxide)s with triazine dichloride

Poly(cyanurate)s (P‐1–P‐4) containing triazine groups in the main chain and pendant chloromethyl groups in the side chain were synthesized by the polyaddition of bis(epoxide)s with 2,4‐dichloro‐6‐(diphenylamino)‐s‐triazine (DPAT) using quaternary onium salts as catalysts. The polyaddition of diglycidyl ether of bisphenol‐A (DGEBA) with DPAT proceeded smoothly in chlorobenzene at 100 °C for 12 h to give P‐1 with M_n = 19,000 in a 92% yield, when tetrabutylammonium chloride (TBAC) was used as a catalyst. However, no reaction occurred without a catalyst or with triethylamine alone under the same reaction conditions. Polyadditions of other bis(epoxide)s with DPTA also proceeded smoothly using 5 mol % of TBAC as a catalyst in chlorobenzene to produce corresponding polymers (P‐2≈P‐4) in high yields under similar reaction conditions. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4006–4012, 2000     

Synthesis of poly(silyl ether)s containing pendant chloromethyl groups by the polyaddition of bis(oxetane)s with dichlorosilanes

The polyaddition of bis(3‐ethyl‐3‐oxetanylmethyl) terephthalate (BEOT) with dichlorodiphenylsilane (CPS) using tetrabutylammonium bromide (TBAB) as a catalyst proceeded under mild reaction conditions to afford a polymer containing silicon atoms in the polymer main chain. A poly(silyl ether) (P‐1) with a high molecular weight (M_n = 53,200) was obtained by the reaction of BEOT with CPS in the presence of 5 mol % of TBAB in toluene at 0 °C for 1 h and then at 50 °C for 24 h. The structure of the resulting polymer was confirmed by IR and ¹H NMR spectra. Furthermore, it was proved that the polyaddition of certain bis(oxetane)s with dichlorosilanes proceeds smoothly to give corresponding poly(silyl ether)s with TBAB as the catalyst. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2254–2259, 2000     

Synthesis and characterization of fluoropolymers by the polyaddition of bis(epoxide)s with dicarboxylic acids and diols

The synthesis and characterization of the fluoropolymers poly 1a – 1d and poly 2a – 2d with pendant hydroxyl groups were examined. The polyaddition of bis(epoxide)s [2,2′‐bis(4‐glycidyletherphenyl)hexafluoropropane and bisphenol A diglycidyl ether] with dicarboxylic acids (tetrafluoroterephthalic acid and terephthalic acid) and diols [2,2′‐bis(4‐hydroxyphenyl)hexafluoropropane, 2,2′,3,3′,5,5′,6,6′‐octafluoro‐4,4′‐biphenol, 1,4‐bis(hexafluorohydroxyisopropyl)benzene, and 1,3‐bis(hexafluorohydroxyisopropyl)benzene] was carried out at 50–100 °C for 6–48 h in the presence of quaternary onium salts (tetrabutylammonium bromide, tetrabutylammonium chloride, tetrabutylphosphonium bromide, and tetrabutylphosphonium chloride; 2.5 mol %) as catalysts in dimethyl sulfoxide, N‐methylpyrrolidone, dimethylformamide, dimethylacetamide, dioxane, diglyme, o‐dichlorobenzene, chlorobenzene, and toluene to afford the corresponding polymers, poly 1a – 1d and poly 2a – 2d , with number‐average molecular weights of 11,000–59,400 in 45–97% yields. The solubility of the obtained polymers was good, and their thermal stability might be assumed from their structures. A linear relationship was observed between the contents of the fluorine atoms and the refractive indices. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1395–1404, 2002     

Synthesis of polyethers with unsymmetrical structures by the polyaddition of 3‐ethyl‐3‐(glycidyloxymethyl)oxetane with diacyl chlorides

Polyethers with unsymmetrical structures in the main chains and pendant chloromethyl groups were synthesized by the polyaddition of 3‐ethyl‐3‐(glycidyloxymethyl)oxetane (EGMO) with certain diacyl chlorides with quaternary onium salts or pyridine as catalysts. The unsymmetrical polyaddition of EGMO containing two different cyclic ether moieties such as oxirane and oxetane groups with terephthaloyl chloride proceeded smoothly in toluene at 90 °C for 6 h to give polymer 1 with a number‐average molecular weight (M_n) of 51,700 in a 93% yield when tetrabutylammonium bromide (TBAB) was used as a catalyst. The polyaddition also proceeded smoothly under the same conditions when other quaternary onium salts, such as tetrabutylammonium chloride, tetrabutylammonium iodide, tetrabutylphosphonium chloride, and tetrabutylphosphonium bromide, and pyridine were used as catalysts. However, without a catalyst no reaction occurred under the same reaction conditions. Polyadditions of EGMO with isophthaloyl chloride and adipoyl chloride gave polymer 2 (M_n = 28,700) and polymer 3 (M_n = 25,400) in 99 and 65% yields, respectively, under the same conditions. The chemical modification of the resulting polymer, polymer 1 , which contained reactive pendant chloromethyl groups, was also attempted with potassium 3‐phenyl‐2,5‐norbornadiene‐2‐carboxylate with TBAB as a phase‐transfer catalyst, and a polymer with 65 mol % pendant norbornadiene moieties was obtained. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 368–375, 2001     

Studies on epoxy resins cured by cationic latent thermal catalysts: The effect of the catalysts on the thermal,rheological, and mechanical properties

To investigate the effect of catalysts on the thermal, rheological, and mechanical properties of an epoxy system, a resin based on diglycidyl ether of bisphenol‐A (DGEBA) was cured by two cationic latent thermal catalysts, N‐benzylpyrazinium hexafluoroantimonate (BPH) and N‐benzylquinoxalinium hexafluoroantimonate (BQH). Differential scanning calorimetry was used for the thermal characterization of the epoxy systems. Near‐infrared spectroscopy was employed to examine the cure reaction between the DGEBA and the latent thermal catalysts used. The rheological properties of the blend systems were investigated under an isothermal condition with a rheometer. To characterize the mechanical properties of the systems, flexure, fracture toughness (K_IC), and impact tests were performed. The phase morphology was studied with scanning electron microscopy of the fractured surfaces of mechanical test samples. The conversion and cure activation energy of the DGEBA/BQH system were higher than those of the DGEBA/BPH system. The crosslinking activation energy showed a result similar to that obtained from the cure kinetics of the blend systems. The flexure strength, K_IC, and impact properties of the DGEBA/BQH system were also superior to those of the DGEBA/BPH system. This was a result of the substituted benzene group of the BQH catalyst, which increased the crosslink density and structural stability of the epoxy system studied. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 187–195, 2001     

Novel synthesis of polyphosphonates by the polyaddition of bis(epoxide) with diaryl phosphonates

The polyaddition of bisphenol A diglycidyl ether (BPGE) with bis(4‐chlorophenyl) phenylphosphonate was carried out using quaternary onium salts or crown ether complexes as catalysts. When the polyaddition was performed using tetrabutylammonium chloride, tetrabutylphosphonium chloride, or 18‐crown‐6/KCl in N‐ methyl‐2‐pyrrolidone at 110°C for 48 h, the corresponding polyphosphonate with moderated molecular weights was obtained in 88–96% yields. The structure of the resulting polyphosphonate was confirmed by IR and ¹H‐NMR spectra. The polyaddition of BPGE with various diaryl phosphonates also proceeded very smoothly to produce the corresponding polyphosphonates with moderate molecular weights. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 959–965, 1999     

Pd(0)‐catalyzed polyaddition of bifunctional vinyloxiranes with phenol derivatives: The synthesis of polymers containing an allyl aryl ether moiety in the main chain

The palladium(0)‐catalyzed polyaddition of bifunctional vinyloxiranes [1,4‐bis(2‐vinylepoxyethyl)benzene ( 1a ) and 1,4‐bis(1‐methyl‐2‐vinylepoxyethyl)benzene ( 1b )] with oxygen nucleophiles such as hydroquinone and bisphenol A gave new unsaturated polyethers containing an allyl aryl ether moiety and pendant hydroxy groups. The polyaddition with 1a was largely affected by the phosphine ligands employed and the reaction temperature. The polyaddition with hydroquinone and bisphenol A was conducted at room temperature for 24 h in tetrahydrofuran in the presence of PPh₃ and gave the desired polyethers in good yields, the number‐average molecular weights (M_n) of which were 5700 and 7700, respectively. 1,2‐Bis(diphenylphosphino)ethane (dppe) was not effective in the polyaddition with 1a . The polyaddition of 1b with hydroquinone and bisphenol A gave the corresponding polyethers despite the kinds of ligands employed (PPh₃ and dppe), contrary to the polyaddition with 1a . The polyaddition of 1b with 4,4′‐biphenol was also carried out in the presence of Pd₂(dba)₃ · CHCl₃/dppe as a catalyst (where dba is dibenzylideneacetone) and afforded the expected polyether with a high M_n value (M_n = 24,900). In addition, vinyloxirane 1b could reacted with racemic 1,1′‐bi‐2‐naphthol to give the corresponding polyether in a good yield. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 476–482, 2003     

Novel synthesis of polyethers by the polyaddition of bis(epoxide)s with bis(phosphinate)s

The polyaddition of bisphenol A diglycidyl ether with bis[4‐(P,P‐diphenylphosphinyloxy)phenyl] sulfone catalyzed by quaternary onium salt, such as tetrabutylammonium chloride afforded a new phosphorus‐containing polyether with good solubility in common organic solvents. Having studied various factors affecting the reaction, such as temperature, catalyst concentration, reaction time, etc., an appropriate polyaddition condition was suggested as using 5 mol % of suitable quaternary ammonium or phosphonium salt in polar solvent at 150°C within 25 h in an ampule for producing high molecular weight polymer. A number of polyethers bearing pendent phosphinate ester groups from the polyaddition of certain bis(epoxide)s and bis(phosphinate)s were synthesized under the above condition and characterized by GPC, IR, and NMR. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1009–1016, 1999     

Synthesis of polyester by novel addition reaction of active di-ester with di-epoxy compound

Some polyesters having pendant ether groups were synthesized with high yield by polyaddition of active di-esters such as di(S-phenyl)thioisophthalate (PTIP) and di(S-phenyl)thiosebacate with di-epoxy compounds such as diglycidyl ether of bisphenol A (BPGE) and diglycidyl ether of ethylene glycol using some quaternary ammonium salts as a catalyst. The degree of polymerization in the reaction of PTIP with BPGE was strongly affected by the kind of reaction solvent, monomer concentration, the reaction temperature, and the kind of catalyst. As a result, it is found that tetrabutylammonium chloride has the highest catalytic activity for the polyaddition of PTIP with BPGE, and dimethylsulfoxide is a suitable solvent for the reaction system.     

Copolymerization of diglycidyl ether of bisphenol A with γ‐butyrolactone catalyzed by ytterbium triflate: Shrinkage during curing

Diglycidyl ether of bisphenol A (DGEBA) was cured with γ‐butyrolactone (γ‐BL) with ytterbium triflate as a catalyst. The curing was studied with differential scanning calorimetry, Fourier transform infrared (FTIR), and thermomechanical analysis. FTIR studies confirmed that four elemental reactions took place during the curing process: the formation of a spiroorthoester (SOE) by the reaction of DGEBA with γ‐BL, the homopolymerization of SOE, the homopolymerization of DGEBA, and the copolymerization of SOE and DGEBA. Moderate proportions of γ‐BL produced materials with higher glass‐transition temperatures, and the curing occurred with lower shrinkage after gelation because of the polymerization of SOE, with near‐zero shrinkage during the final stages of the curing. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2794–2808, 2003     

Cage-shaped borate esters with tris(2-oxyphenyl)methane or -silane system frameworks bearing multiple tuning factors: geometric and substituent effects on their Lewis acid properties

Boron complexes that contain new tridentate ligands, tris(o‐oxyaryl)methanes and ‐silanes, were prepared. These complexes had a cage‐shaped structure around a boron center and showed higher Lewis acidity and catalytic activity than open‐shaped boron compounds. The cage‐shaped ligands determined the properties of the borates by altering the geometry and were consistently bound to the metal center by chelation. The synthesized compounds were L?B(OC₆H₄)₃CH, L?B(OC₆H₄)₃SiMe, and its derivatives (L=THF or pyridine as an external ligand). Theoretical calculations suggested that the cage‐shaped borates had a large dihedral angle (C_ipso‐O‐B‐O) compared with open‐shaped borates. The geometric effect due to the dihedral angle means that compared with open‐shaped, the cage‐shaped borates have a greater Lewis acidity. The introduction of electron‐withdrawing groups on the aryl moieties in the cage‐shaped framework increased the Lewis acidity. Substitution of a bridgehead Si for a bridgehead C decreased the Lewis acidity of the boron complexes because the large silicon atom reduces the dihedral angle of C_ipso‐O‐B‐O. The ligand‐exchange rates of the para‐fluoro‐substituted compound B(OC₆H₃F)₃CH and the ortho‐phenyl‐substituted compound B(OC₆H₃Ph)₃CH were less than that of the unsubstituted borate B(OC₆H₄)₃CH. The ligand‐exchange rate of B(OC₆H₄)₃SiMe was much faster than that of B(OC₆H₄)₃CH. A hetero Diels–Alder reaction and Mukaiyama‐type aldol reactions were more effectively catalyzed by cage‐shaped borates than by the open‐shaped borate B(OPh)₃ or by the strong Lewis acid BF₃?OEt₂. The cage‐shaped borates with the bulky substituents at the ortho‐positions selectively catalyzed the reaction with less sterically hindered substrates, while the unsubstituted borate showed no selectivity.     

Molecular motion,morphology, and thermal properties of multiwall carbon nanotube/polysilsesquioxane composite

Diglycidyl ether of bisphenol A (DGEBA)‐bridged polyorganosiloxane precursors have been prepared successfully by reacting diglycidyl ether of bisphenol A epoxy resin with 3‐aminopropyltriethoxysilane. Acid‐modified and unmodified multiwalled carbon nanotube (MWCNT) were dispersed in the diglycidyl ether of bisphenol A‐bridged polyorganosiloxane precursors and cured to prepare the carbon nanotube/diglycidyl ether of bisphenol A‐bridged polysilsesquioxane (MWCNT/DGEBA‐PSSQ) composites. The molecular motion of MWCNT/DGEBA‐PSSQ nanocomposites was studied by high‐resolution solid‐state ¹³C NMR. Acid‐modification can improve the affinity between MWCNT and the polymer matrix. The molecular motion of the DGEBA‐PSSQ decreased with acid‐modified MWCNT content. However, when unmodified MWCNT was used, the molecular motion of the DGEBA‐PSSQ was increased. SEM and TEM microphotographs confirm that acid‐modified MWCNT exhibits better dispersion than unmodified MWCNT in DGBEA‐PSSQ. The dynamic mechanical properties of acid‐modified MWCNT/DGBEA‐PSSQ composites are more favorable than those of unmodified MWCNT. T_g of the DGEBA‐PSSQ decreased from 174.0 °C (neat DGEBA‐PSSQ) to 159.0 °C (1 wt % unmodified MWCNT) and 156.0 °C (1 wt % acid‐modified MWCNT). The storage modulus (at 30 °C) of the DGEBA‐PSSQ increased from 1.23 × 10⁹ Pa (neat DGEBA‐PSSQ) to 1.65 × 10⁹ Pa (1 wt % acid‐modified MWCNT). However, when unmodified MWCNT was used, the storage modulus of the DGEBA‐PSSQ decreased to 6.88 × 10⁸ Pa (1 wt % unmodified MWCNT). At high temperature, above 150 °C, storage modulus of nanocomposites was higher than that of neat polymer system. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 472–482, 2008     

Curing behavior and toughening performance of epoxy resins containing hyperbranched polyester

The effects of the hyperbranched polyester with hydroxyl end groups (HBPE‐OH) on the curing behavior and toughening performance of a commercial epoxy resin (diglycidyl ether of bisphenol A, DGEBA) were presented. The addition of HBPE‐OH into DGEBA strongly increased its curing rate and conversion of epoxide group due to the catalytic effect of hydroxyl groups in HBPE‐OH and the low viscosity of the blend at curing temperature. The improvements on impact strength and critical stress intensity factor (or fracture toughness, K_1c) were observed with adding HBPE‐OH. The impact strength was 8.04 kJ m^?1 when HBPE‐OH reached 15 wt% and the K_1c value was approximately two times the value of pure epoxy resin when HBPE‐OH content was 20 wt%. The morphology of the blends was also investigated, which indicated that HBPE‐OH particles, as a second phase in the epoxy matrix, combined with each other as the concentration of HBPE‐OH increased. Copyright © 2004 John Wiley & Sons, Ltd.     

Calorimetric studies on an anhydride‐cured epoxy resin from diglycidyl ether of bisphenol‐A and diglycidyl ether of poly(propylene glycol). I. Onset of diffusion control during isothermal polymerization

Calorimetric studies on a series of anhydride‐cured epoxy resins, in which the epoxy oligomer is a mixture of diglycidyl ether of bisphenol‐A (DGEBA) and diglycidyl ether of poly(propylene glycol) (DGEPPG) in different mole ratios, were carried out. DGEPPG is a flexible epoxy oligomer that was used to tune glass transition temperature for the fully reacted epoxy resin. Conversion versus time curves for the systems with different DGEBA/DGEPPG mole ratios (not including the neat DGEPPG system) were found to overlap with each other in mass‐controlled reaction regime, indicating similar reactivities of epoxy groups in both epoxy oligomers. Onset of diffusion‐controlled reaction regime for different systems was estimated by fitting the conversion versus time data using a phenomenological kinetic equation, as well as from direct comparison of the conversion versus time curves. For the systems (i.e., 0, 10, and 30% DGEPPG) that vitrify during reaction, the crossover from mass‐controlled to diffusion‐controlled reaction occurs close to the onset of the vitrification, where T_g is about 25–30 K below the reaction temperature. For the system (i.e., 50% DGEPPG system) that does not vitrify during the reaction, such crossover still occurs when the T_g of the mixture reaches a value about 25 K below the reaction temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2155–2165, 2008     

Synthesis and characterization of fluorine‐containing polyesters by the polyaddition of bis(epoxide)s with active diesters

The synthesis and optical properties of polyesters with pendant fluorinated phenoxy groups were examined. The polyaddition of bisphenol AF diglycidyl ether ( 1 ) with fluorine‐containing terephtalates ( 2a–f ) was carried out with tetrabutylphosphonium chloride (TBPC) as the catalyst in chlorobenzene to afford the corresponding polyesters with number‐average molecular weights (M_n's) ranging from 15,200 to 30,000 in 88–96% yields. Furthermore, the polyaddition of 1 with isophthalate 2g and phthalate 2h also produced high‐molecular‐weight polyesters with M_n's = 22,700 and 22,600 in 88 and 84% yields, respectively. The linear relationship was observed between the fluorine contents and refractive indices of the obtained polyesters. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 213–222, 2003     

Flame retardant epoxy resins based on diglycidyloxymethylphenylsilane

Silicon‐containing epoxy resins were prepared from diglycidyloxymethylphenyl silane (DGMPS) and diglycidylether of bisphenol A (DGEBA) by crosslinking with 4,4′‐diaminodiphenylmethane (DDM). Several DGMPS/DGEBA molar ratios were used to obtain materials with different silicon contents. Their thermal, dynamomechanical, and flame‐retardant properties were evaluated and related to the silicon content. The weight loss rate of the silicon‐containing resins is lower than that of the silicon free resin. Char yields under nitrogen and air atmospheres increase with the silicon content. The LOI (limited oxygen index) values increased from 24 for a standard commercial resin to 36 for silicon‐containing resins, demonstrating improved flame retardancy. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5580–5587, 2006     

High resolution solid‐state 13C‐NMR study of as‐cured and irradiated epoxy resins

This article presents the effects of strong ionizing radiations on the physico‐chemical modifications of aliphatic or aromatic amine‐cured epoxy resins based on diglycidyl ether of bisphenol A (DGEBA). Such epoxy resins have a considerable number of applications in the nuclear industrial field and are known to be very stable under moderate irradiation conditions. Using extensively high resolution solid‐state ¹³C‐NMR spectroscopy we show that the aliphatic amine‐cured resin (DGEBA‐TETA) appears much more sensitive to gamma rays than the aromatic amine‐cured one (DGEBA‐DDM). On the one hand, qualitative analyses of the high resolution solid‐state ¹³C‐NMR spectra of both epoxy resins, irradiated under similar conditions (8.5 MGy), reveal almost no change in the aromatic amine‐cured resin whereas new resonances are observed for the aliphatic amine‐cured resin. These new peaks were interpreted as the formation of new functional groups such as amides, acids and/or esters and to alkene groups probably formed in the aliphatic amine skeleton. On the other hand, molecular dynamics of these polymers are investigated by measuring the relaxation times, T_CH, T_1ρH and T_1C , before and after irradiation. The study of relaxation data shows the formation, under irradiation, of a more rigid network, especially for the aliphatic amine‐cured system and confirms that aromatic amine‐cured resin [DGEBA‐4,4′‐diaminodiphenylmethane(DDM)] is much less affected by ionizing radiations than the aliphatic amine‐cured resin [DGEBA‐triethylenetetramine(TETA)]. Moreover, it has been shown that the molecular modifications generated by irradiation on the powder of the aliphatic‐amine‐cured resin appear to be homogeneously distributed inside the polymers as no phase separations can be deduced from the above analyses. Copyright © 2001 John Wiley & Sons, Ltd.     

Stannylium ions, a tin(II) arene complex, and a tin dication stabilized by weakly coordinating anions

The reactivity of aryl‐substituted stannylenes, Ar₂Sn ( 4 ), towards silylarenium borates, [R₃SiArH][B(C₆F₅)₄] ( 3 ), was investigated. The reaction with 2,3,4‐trimethyl‐6‐tert‐butylphenyl (mebp)‐substituted stannylene gave silyl‐substituted stannylium ions 2 a , b , which were characterized by NMR spectroscopy supported by the results of quantum‐mechanical computations of molecular structures and magnetic properties. The tri‐iso‐propylphenyl‐substituted stannylium ions 2 c , d undergo a decomposition reaction in toluene to give the dicationic tin–arene complex [Sn(C₇H₈)₃]²⁺ ( 5 ) in the form of the [B(C₆F₅)₄] salt in high yields. The 5 [B(C₆F₅)₄]₂ salt was identified by single crystal X‐ray diffraction analysis and by Mössbauer spectroscopy. The bonding situation was investigated by using natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) calculations. The substitution of the weakly coordinating borate anion by the carboranate [CB₁₁H₆Br₆]^? results in replacement of the toluene ligands and formation of tin(II) carboranate with only weak Sn²⁺–anion interactions as suggested by the solid‐state structure of the isolated salt.     

Radical Cyclization Reactions via Manganese(III) Acetate Leading to 2‐Thienyl‐Substituted Dihydrofuran Compounds

The 2‐thienyl‐substituted 4,5‐dihydrofuran derivatives 3 – 8 were obtained by the radical cyclization reaction of 1,3‐dicarbonyl compounds 1a – 1f with 2‐thienyl‐substituted conjugated alkenes 2a – 2e by using [Mn(OAc)₃] (Tables 1–5). In this study, reactions of 1,3‐dicarbonyl compounds 1a – 1e with alkenes 2a – 2c gave 4,5‐dihydrofuran derivatives 3 – 5 in high yields (Tables 1–3). Also the cyclic alkenes 2d and 2e gave the dihydrobenzofuran compounds, i.e., 6 and 7 in good yields (Table 4). Interestingly, the reaction of benzoylacetone (=1‐phenylbutane‐1,3‐dione; 1f ) with some alkenes gave two products due to generation of two stable carbocation intermediates (Table 5).