Thermal cis–trans isomerization and decomposition of polyacetylene

Thermal cis-trans isomerization and decomposition of polyacetylene film prepared with a Ti(OC₄H₉)₄–Al(C₂H₅)₃ (Al/Ti = 4) system were investigated under inert gas or in vacuum by means of thermal analysis and infrared spectroscopy. Thermograms of differential thermal analysis of cis-polyacetylene revealed the existence of two exothermic peaks at 145 and 325°C and one endothermic peak at 420°C which were assigned to cis-trans isomerization, hydrogen migration accompanied with crosslinking reaction, and thermal decomposition, respectively. The isomerization was followed by infrared spectroscopy over the temperature range 75–115°C. The reaction did not obey simple kinetics. The apparent activation energy for the cis-trans isomerization was 17.0 kcal/mole for the polymer containing 88% cis configuration and increased with increasing trans content up to 38.8 kcal/mole for 80% trans content.     

Polymerization of acetylenic derivatives. XXX. Isomers of polyphenylacetylene

Cis-transoidal (orange, soluble, and of low crystallinity) and cis-cisoidal (red, insoluble, and highly crystalline) polyphenylacetylenes (PPA) were prepared by Ziegler-Natta catalysts and trans-cisoidal (yellow, soluble, and amorphous) polyphenylacetylenes were prepared by using phosphine complexes, TiCl₄ and by thermal initiation. The cis-transoidal and cis-cisoidal structures isomerize thermally in the solid state above 100°C. In solution the cis-transoidal structure isomerizes above 80°C. The polymers obtained by thermal isomerization are soluble, amorphous, and have a trans-cisoidal structure. At temperatures higher than 120°C the cis–trans isomerization is accompanied by cyclization and by scission of the polymer chain. A method was developed for determination of cis content of cis-transoidal and cis-cisoidal polyphenylacetylenes.     

Dramatic decrease of the cis content and molecular weight of cis‐transoidal polyphenylacetylene at 23 °C in solutions prepared in air

The thermal cis–trans isomerization of cis‐transoidal polyphenylacetylene (PPA) synthesized with Noyori's catalyst [Rh(C?CPh)(norbornadiene)(PPh₃)₂] has been investigated under both ambient and inert atmospheres in solution and in bulk. In all cases, an intramolecular cyclization results in cis–trans isomerization, and subsequent chain cleavage produces 1,3,5‐triphenylbenzene. This reaction is accelerated by the presence of air, particularly when the reaction takes place in solution. Decreases in the cis content and molecular weight show that the intramolecular cyclization process takes place at 23 °C in solution. The mechanism of this reaction is identical to that reported previously for cis‐cisoidal and cis‐transoidal PPA synthesized with Ziegler–Natta catalysts. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3212–3220, 2002     

Polysulfone–polydimethylsiloxane block copolymers

Alternating block copolymers have been synthesized from dihydroxyl-terminated polysulfone and bis(dimethylamine)-terminated polydimethylsiloxane oligomers. The products are soluble, amorphous, and transparent, and display excellent thermal and hydrolytic stability. Elastomeric and rigid compositions can be prepared by varying oligomer molecular weight. Copolymers made with oligomers of ≥ 5000 molecular weight are two-microphase systems which display glass transition temperatures at ?120°C and at +160°C, and therefore have a wide useful temperature range.     

Radiation-induced cis–trans isomerization of polyisoprenes and temperature dependence of the equilibria

The cis-trans interconversion of polyisoprenes in solutions induced by γ-radiation in the presence of a sensitizer, which is any one of organic bromides or n-butyl mercaptan, was studied by using hevea and gutta percha as starting substances. The percentage cis remaining or converted after irradiation were determined by the infrared absorption. The equilibrium constants for the interconversion at 22, 60, and 100°C. were found to be 3.00, 5.25, and 7.33, respectively. The first-order rate constants for cis → trans and trans → cis isomerizations at 22°C. were calculated to be 9.05 and 2.91, respectively. The results were interpreted by the mechanism proposed by Golub, according to which the double bonds from π complexes with radiolytic fragments from sensitizers give a radical transition state capable of interconversion. However, our results showing that heating shifts the equilibrium toward trans isomer are not in accord with the mechanisms of the radiation-induced isomerization of polybutadiene of Golub and those for photoisomerization of aromatic azo compounds.     

Structure–initiator activity dependence of aminimides as thermally latent initiators in the polymerization of glycidyl phenyl ether

Novel aliphatic aminimides were synthesized from the corresponding carboxylic acid esters, 1,1‐dimethylhydrazine, and epoxides in 54–95% yields. Bulk polymerization of glycidyl phenyl ether (GPE) with 3 mol % of the aminimides was evaluated by DSC as a model process for curing of epoxy resin. All the aminimides showed no exothermic DSC peak below 120 °C but showed sharp exothermic peaks above 137 °C, indicating good thermal latency. Good relationships were observed between the calculated bond length from the carbonyl carbon to the α‐carbon of the aliphatic group (R C), DSC onset temperatures, and the thermal dissociation temperatures (T_d 's) of the aminimides. The aminimide with a longer R C bond length showed lower T_d and DSC onset temperature, that is, higher activity. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3428–3433, 2000     

Synthesis and thermal properties of thermosetting bis-benzocyclobutene–terminated arylene ether monomers

A series of new bis-benzocyclobutene-endcapped arylene ether monomers was prepared and characterized. Whereas 2,6-bis(4-benzocyclobutenyloxy)benzonitrile (BCB-EBN) could be prepared in good yield using the standard procedure (K₂CO₃/NMP/toluene/Dean–Stark trap/120°C), other bis(benzocyclobutene) (BCB)-terminated monomers containing ether-benzophenone (BCB-EK), ether-phenylsulfone (BCB-ES), and ether-6F-benzoxazole (BCB-EBO) moieties were invariably contaminated by mono-endcapped products under similar reaction conditions. This can be attributed to a much greater activating effect of the nitrile group on the ortho-fluorides in the aromatic nucleophilic displacement reaction than the carbonyl, sulfonyl, and benzoxazolyl groups. However, the latter monomers could be synthesized (70–80%) from 4-trimethylsiloxybenzocyclobutene and respective aromatic fluorides in the presence of CsF at 140°C. Similar curing behaviors under N₂ (DSC: extrapolated onset and peak temperatures at 227–230° and 260–262°C, respectively) characterized all four monomers. BCB-EK, BCB-ES, and BCB-EBN showed melting transitions at 108, 119, and 146°C, in that order. As BCB-EBO contained more rigid benzoxazole segments, it only exhibited a glass transition (T_g) at 85°C prior to curing exotherm, after it had been previously heated to 125°C. The following T_gs were observed for the cured materials: BCB-EK (201°C), BCB-EBN (224°C), BCB-ES (264°C), and BCB-EBO (282°C). The relative thermal stability according to TGA (He) results is: BCB-ES < BCB-EBN < BCB-EK < BCB-EBO. Finally, the results from thermal analysis, infrared spectroscopic, and variable temperature microscopic studies indicated that the nitrile group plays an important role in the cure chemistry, thermal, and microstructural properties of BCB-EBN. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2637–2651, 1998     

Polymerization of o‐quinodimethanes. V. Ionic polymerization of o‐quinodimethanes generated by thermal isomerization of corresponding benzocyclobutenes

The ionic polymerization of substituted o‐quinodimethanes via thermal isomerization of benzocyclobutenes is described. In the cationic polymerizations of 1‐methoxy‐o‐quinodimethane in the presence of various cationic initiators at 110 °C for 12 h, chain transfer reactions also considerably underwent besides the polymerization. Meanwhile, cationic polymerizations of 1‐trimethylsilyloxy‐o‐quinodimethane under the same conditions gave good yields of the corresponding polymer. Anionic polymerizations of 1‐cyano‐o‐quinodimethane in the presence of anionic initiators such as n‐BuLi or t‐BuOK were performed at various temperatures for 12 h. Good yields of hexane‐insoluble polymer, which was produced by anionic polymerization of corresponding o‐quinodimethane as an intermediate, were obtained above 120 °C. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 844–850, 2008     

Cationic polymerization of L,L‐lactide

Cationic bulk polymerization of L ,L‐ lactide (LA) initiated by trifluromethanesulfonic acid [triflic acid (TfA)] has been studied. At temperatures 120–160 °C, polymerization proceeded to high conversion (>90% within ～8 h) giving polymers with M_n ～ 2 × 10⁴ and relatively high dispersity. Thermogravimetric analysis of resulting polylactide (PLA) indicated that its thermal stability was considerably higher than the thermal stability of linear PLA of comparable molecular weight obtained with ROH/Sn(Oct)₂ initiating system. Also hydrolytic stability of cationically prepared PLA was significantly higher than hydrolytic stability of linear PLA. Because thermal or hydrolytic degradation of PLA starting from end‐groups is considerably faster than random chain scission, both thermal and hydrolytic stability depend on molecular weight of the polymer. High thermal and hydrolytic stability, in spite of moderate molecular weight of cationically prepared PLA, indicate that the fraction of end‐groups is considerably lower than in linear PLA of comparable molecular weight. According to proposed mechanism of cationic LA polymerization growing macromolecules are fitted with terminal ? OH and ? C(O)OSO₂CF₃ end‐groups. The presence of those groups allows efficient end‐to‐end cyclization. Cyclic nature of resulting PLA explains its higher thermal and hydrolytic stability as compared with linear PLA. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2650–2658, 2010     

Effects of thermal stability on the crystallization behavior of poly(vinylidene chloride)

Thermal analysis based on TGA (thermal gravimetric analysis) and DSC (differential scanning calorimeter) shows no significant degradation for PVDC which has been annealed at 210°C for less than 2 min. And the following recrystallization behavior at lower temperature (120°C) is also independent of the thermal treatment and is not affected by the difference of molecular weight. The degradation which includes dehydrochlorination at lower temperature and intramolecular cyclization or intermolecular crosslinking of the polyenes at higher temperature starts when the melting time at 210°C is more than 2 min, which also causes weight loss and heat exchange in the TGA and DSC thermograms. The recrystallization behavior of the degraded PVDC (staying at 210°C for more than 2 min) shows a strong dependence on the molecular weight. The crystallinity is decreased with the melting time at 210°C due to the increase of the degree of crosslinking. However, the POM (polarized optical microscopy) pictures and IR spectra show a favorable nucleation effect is present due to the formation of trichlorobenzene from the cyclization of the polyenes as nuclei. The crystallinity of the PVDC recrystallized at 120°C after staying at 210°C for more than 2 min is actually dependent on the molecular weight, melting time at 210°C, and cyclized or crosslinking types of degradation. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3269–3276, 1999     

Consecutive isomerization and cyclization of 3‐(2‐aminophenyl)‐1‐arylprop‐2‐yn‐1‐ols leading to 2‐arylquinolines in the presence of potassium hydroxide

3‐(2‐Aminophenyl)‐1‐arylprop‐2‐yn‐1‐ols are readily converted to 2‐arylquinolines in good yields in ethanol at 80° in the presence of potassium hydroxide via domino isomerization and cyclization.     

Aromatic polybenzoxazoles containing ether–sulfone linkages

A series of poly(o‐hydroxy amide)s having both ether and sulfone linkages in the main chain were synthesized via the low‐temperature solution polycondensation of 4,4′‐[sulfonylbis(1,4‐phenylene)dioxy]dibenzoyl chloride and 4,4′‐[sulfonylbis(2,6‐dimethyl‐1,4‐phenylene)dioxy]dibenzoyl chloride with three bis(o‐aminophenol)s including 4,4′‐diamino‐3,3′‐dihydroxybiphenyl, 3,3′‐diamino‐4,4′‐dihydroxybiphenyl, and 2,2‐bis(3‐diamino‐4‐hydroxyphenyl)hexafluoropropane. Subsequent thermal cyclodehydration of the poly(o‐hydroxy amide)s afforded polyethersulfone benzoxazoles. Most of the poly(o‐hydroxy amide)s were soluble in polar organic solvents such as N‐methyl‐2‐pyrrolidone; however, the polybenzoxazoles without the hexafluoroisopropylidene group were organic‐insoluble. The polybenzoxazoles exhibited glass‐transition temperatures (T_g) in the range of 219–282 °C by DSC and softening temperatures (T_s) of 242–320 °C by thermomechanical analysis. Thermogravimetric analyses indicated that most polybenzoxazoles were stable up to 450 °C in air or nitrogen. The 10% weight loss temperatures were recorded in the ranges of 474–593 °C in air and 478–643 °C in nitrogen. The methyl‐substituted polybenzoxazoles had higher T_g's but lower T_s's and initial decomposition temperatures compared with the corresponding unsubstituted polybenzoxazoles. For a comparative purpose, the synthesis and characterization of a series of sulfonyl polybenzoxazoles without the ether group that derived from 4,4′‐sulfonyldibenzoyl chloride and bis(o‐aminophenol)s were also reported. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2262–2270, 2001     

Isomerization of n-Hexane over Chlorinated Pt/γ-Al2O3 Catalysts

Chlorided 0.57 wt. % Pt/γ-Al₂O₃ is a moderately active and highly selective catalyst for the isomerization of n-hexane at 300 °C when an excess of H₂ is present. Chlorine, which is continuously lost from the catalyst, may be replaced by adding a small amount of chloroform to the reactant stream. The catalyst exhibits activity and high isomerization selectivity even at 150 °C. Under the conditions of these lower temperature experiments, the equilibrium concentration of C₆ alkenes would be only 8 × 10^?7 Torr. This suggests that the classical mechanism, which involves the formation of n-hexene on the Pt, is not applicable. Rather, it is proposed that isomerization of the alkane occurs at acid sites on the catalyst, and the role of Pt is to catalyze the hydrogenation of any alkenes that might be formed as a result of cracking reactions. At temperatures ≤ 300 °C Brönsted acid sites are present in the catalyst, and presumably are responsible for the isomerization and cracking activity, but at higher temperatures Lewis acid sites play a dominant role.     

Ring‐opening polymerization of 1,3‐benzoxazines by p‐toluenesulfonates as thermally latent initiators

p‐Toluenesulfonic acid (TsOH) and several alkyl p‐toluenesulfonates, that is, methyl p‐toluenesulfonate (TsOMe), cyclohexyl p‐toluenesulfonate (TsOCH), and neopentyl p‐toluenesulfonate (TsONP), were evaluated as initiators for the ring‐opening polymerization of benzoxazines. TsOH and TsOMe were highly efficient initiators that induced the polymerization at 60 and 80 °C, respectively. In contrast, TsOCH and TsONP did not initiate the polymerization below 100 °C, while they induced the polymerization at elevated temperatures, 120 and 150 °C, respectively. When TsOCH was used as an initiator, the corresponding polymerization rate was comparable to that observed for the polymerization with using TsOH as an initiator. These results suggested that neutral TsOCH and TsONP can be regarded as “thermally latent initiators,” which underwent the thermal dissociation at the elevated temperatures to generate the corresponding alkyl cations and/or TsOH as the initiators of the polymerization. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Cyclopolymerization. VI. Polymerization mechanism of N-methyl-N-allylmethacrylamide at lower temperature

Mechanistic investigations on the polymerization of N-methyl-N-allylmethacrylamide (MAMA) at lower temperature were carried out based upon the ESR studies of MAMA and its monofunctional counterparts irradiated with ⁶⁰Co γ rays. Cyclopolymerizability of MAMA was also studied in connection with the hindered rotation about its amide C? N bond. The propagating radical observed is only related to the methacryl group but not to the allyl group both in MAMA and its monofunctional counterparts. Polymerization at ?78°C yielded a polymer with a lower degree of cyclization(88.8%) as compared with that of polymers formed at higher temperatures (93.5% above 0°C). A structural study revealed that the increment of the unsaturation in the poly-MAMA obtained at ?78°C is due to the allyl group and the content of pendant methacryl group is almost unchanged over the temperature range from ?78 to 120°C. These results led to the conclusion that the polymerization of MAMA at ?78°C proceeds mainly through the methacryl group, the rate-determining step is the cyclization reaction, and, in addition, cyclization reaction scarcely occurs when it polymerizes through the allyl group. Since MAMA is frozen into a glassy state, the effect of glass transition temperature (T_g) has been studied and it was suggested that the polymerization of MAMA proceeds only above T_g.     

Polymerization of o‐quinodimethanes. III. Polymerization of o‐quinodimethanes bearing electron‐withdrawing groups formed in situ by thermal ring‐opening isomerization of corresponding benzocyclobutenes

The substituent effect on the radical polymerization of o‐quinodimethanes, generated by thermal isomerization of benzocyclobutenes, was investigated. Polymerizations of three benzocyclobutenes bearing electron‐withdrawing groups were studied, namely 1‐cyanobenzocyclobutene (1), 1‐chlorobenzocyclobutene (2), and 1‐bromobenzocyclobutene (3). While radical polymerizations of 2 and 3 did not afford any polymer, radical polymerization of 1 afforded n‐hexane‐insoluble polymer(M_n = 5000) in moderate yields at temperatures above 120°C. The structure of the obtained polymer was confirmed to be a ring‐opened polymer(4) by IR, ¹H‐, and ¹³C‐NMR. The yield of the polymer increased with an increase in the initiator concentration. The polymer yield reported in this paper is higher than those of benzocyclobutenes bearing electron‐donating groups, reported previously by the authors. The semi‐empirical molecular orbital calculation supported the contribution of ring‐opening polymerization of spiro‐compounds, rejecting the possibility of 1,4‐polymerization. Lastly, radical copolymerizations of 1 with various comonomers were also performed to obtain the corresponding copolymers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1555–1563, 1999     

Viscoelastic properties of polyacrylonitrile terpolymers during thermo‐oxidative stabilization (cyclization)

A study on the thermo‐oxidative stabilization (cyclization) of polyacrylonitrile (PAN) terpolymers using dynamic mechanical thermal analysis (DMTA) is reported in this article. When the changes in viscoelastic properties were monitored in a rectangular tension mode, besides the tan δ peak characteristic of the glass transition observed below 200°C, the copolymer and the terpolymer displayed a second peak above 200°C due to the cyclization leading to the formation of ladder structures. The initiation temperature of the cyclization process as well as the peak temperature is found to depend on the acid value and the composition of the precursor polymer. The results presented show that monitoring the changes in viscoelastic properties during cyclization provides insight into the material properties as a result of the chemical changes that are taking place. These observations were confirmed by structural characterization using IR spectroscopy, and the observed chemical changes agree with the literature studies, as due to the thermal cyclization process. Copyright © 2008 John Wiley & Sons, Ltd.     

Kinetic study of low‐temperature imidization of poly(amic acid)s and preparation of colorless,transparent polyimide films

Conventional synthesis of polyimides includes high‐temperature (160–350 °C) imidization of poly(amic acid)s. In the present work, imidization has been carried out at much lower temperatures (40–160 °C). 1,2,4,5,‐cyclohexanetetracarboxylic dianhydride (HPMDA) or pyromellitic dianhydride (PMDA) was polymerized with an aromatic diamine, 4,4′‐diaminodiphenylmethane (DDPM), to give poly(amic acid)s, which were then imidized chemically. Imidization was more than 90% complete even at the very low imidization temperature of 40 °C. It was found that the imidization occurs in two steps: an initial rapid cyclization and a subsequent slower cyclization. The activation energy for the rapid process was determined to be 4.3 kJ/mol, and that of the slower process, 4.8 kJ/mol. As the imidization temperature decreases, the transmittance of the resulting polyimides tends to gradually increase, the cutoff wavelength decreases and the color becomes pale. A partially aliphatic polyimide based on HPMDA and DDPM prepared at 40 °C yielded thin films that were highly transparent and colorless, and had good flexibility, solubility and thermal stability. The polyimide films prepared in this study may be good candidates for flexible, transparent plastic substrates in the display industry. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1593–1602     

Polycondensations of Substituted Maleic Anhydrides and 1,6-Hexanediol Catalyzed by Metal Triflates

Polycondensations of methylmaleic anhydride (citraconic anhydride, CiAh), bromomaleic anhydride (BMaAh), and dichloromaleic anhydride (DCMaAh) with 1,6-hexanediol were conducted in bulk. For polycondensations of CiAh, the triflates of aluminum, bismuth, lanthanum, magnesium, samarium, and scandium were used as catalysts and the best results were obtained with Bismuth triflate. MALDI-TOF mass spectra indicated that the chain growth was limited by cyclization and incomplete conversion, but temperatures above 100°C were not advantageous due to side reactions. Bismuth and samarium triflate catalyzed polycondfensations of BMaAh and DCMaAh were less successful than polycondensations of CiAh. For polycondensations of CiAh, BMaAh, and DCMaAh, only a low extent of isomerization to trans isomers (1–3%) was observed at 100°C, but considerably higher extent at 140°C.