Daniel Derouet

Chemical modification of epoxy resins by dialkyl(or aryl) phosphates : Evaluation of fire behavior and thermal stability

The improvement of flame-retardation of thermosetted epoxy–amine resins was attempted by chemically incorporating phosphorus-containing reagents. By reacting 4,4′-diglycidylether of bisphenol A (DGEBA) with dialkyl (or aryl) phosphate, it was possible to chemically modify the epoxy resin and then cure it in the presence of 4,4′-diaminodiphenylsulfone (DDS) to obtain epoxy-amine resin with good flame-retardant and thermal stability behaviors. The quantitative aspect of the addition of dialkyl (or aryl) phosphate onto glycidyle oxiranes was evaluated by elemental analysis of the modified epoxy-amine resins. Flammability and thermal behaviors of modified DGEBA/DDS resins depend on the nature of phosphate groups (the best flame-retardation was observed on resins bearing phenyl phosphate groups) and their concentration in the material. In relation to DGEBA/DDS samples containing additives of the same structure [trialkyl(or aryl) phosphate], cured resins incorporating chemically bonded phosphate groups show a better flame-retardation. On the contrary to the nonomodified DGEBA/DDS [with or without trialkyl (or aryl) phosphate as additive], combustion of modified DGEBA/DDS resins is accompanied by formation of intumescent char. Chemical modification of DGEBA by dialkyl (or aryl) phosphates can be carried out in situ during the curing of epoxy resins without change in the fire behavior.

Application of solid-state NMR (13C and 29Si CP/MAS NMR) spectroscopy to the characterization of alkenyltrialkoxysilane and trialkoxysilyl-terminated polyisoprene grafting onto silica microparticles

The solid-state Nuclear Magnetic Resonance (NMR) was used to characterize surfaces of silica gels chemically modified by alkenyltrialkoxysilanes and trialkoxysilyl terminated 1,4-polyisoprenes. The formation of covalent bonds created between alkoxy functional groups from alkenyltrialkoxysilane or trialkoxysilyl-terminated 1,4-polyisoprene and silanol groups on silica was clearly demonstrated by means of 13C and 29Si CP/MAS NMR spectroscopy. Quantitative data, including calculation of the grafting yields in relation with the initial silanol concentrations, were also obtained by using solid-state 29Si-NMR leading to a final well-defined characterization of the silica surfaces. A relatively good agreement was noticed between the grafting yields calculated from 29Si-NMR spectra and those determined from other analytical techniques such as Wijs titration or elementary analysis. The reactivity of the various silica silanols towards each coupling agent was clearly characterized and estimated, as were the proportions of the various grafted structures formed at the silica surface.

Chemical modifications of polydiene elastomers: A survey and some recent results

By chemical modification of natural rubber, it is possible to modify its basic properties (for instance, improvement of gas permeability, resistance to oils or fire resistance) or to prepare new polymeric materials for specific applications (for instance, photocrosslinkable rubbers and rubbers support of active molecules). The purpose of the present paper is to give an overview of recent works carried out in the field of new rubber derivatives for specific applications, especially dealing with natural rubber degradation and products derived from liquid rubbers of type 1,4-polyisoprene (liquid natural rubbers, synthetic polyisoprenes) and/or their epoxidized forms. A reflexion on the perspectives of researches on natural rubber or its derivatives is given in conclusion.

Chemical modification of 1,4-polydienes by di(alkyl or aryl)phosphates

Abstract Synthesis of 1,4-polydienes bearing di(alkyl or aryl)phosphate groups in side position of the polydiene chains was considered by using a chemical modification procedure. The synthesis was carried out according to a two-step process. Firstly, functionalized intermediate polydienes were prepared by partial epoxidation of 1,4-polyisoprene or 1,4-polybutadiene units. Secondly, the introduction of the di(alkyl or aryl)phosphate groups was realized by using the reactivity of the acidic function P–OH of di(alkyl or aryl)phosphate reagents toward oxirane rings. A preliminary study on model molecules of epoxidized 1,4-polydienes (4,5-epoxy-4-methyloctane for epoxidized 1,4-polyisoprene, 4,5-epoxyoctane for epoxidized 1,4-polybutadiene) allowed the characterization of polymers derived from the various types of polydienes: epoxidized synthetic 1,4-polybutadiene, and epoxidized synthetic 1,4-polyisoprene or epoxidized liquid natural rubber (ELNR), as well as that of the side reactions. In all cases, the oxirane rings are opened and two categories of 1:1 phosphate adducts are obtained: five-membered cycle adducts (2-(alkoxy or aryloxy)-2-oxo-1,3,2-dioxaphospholane structures) and 1:1 β-hydroxyphosphate adducts, in proportions depending on the di(alkyl or aryl)phosphate that is the alkyl (or aryl) group nature (methyl, ethyl, butyl, phenyl), and the model structure. The whole phosphate adduct yields are generally limited to about 80%, due to the side reactions. The addition of di(alkyl or aryl)phosphates on the epoxidized units of epoxidized liquid 1,4-polydienes proceeds in a way nearly similar to that observed on the model molecules. The partial modification of oxirane rings by the phosphate compounds lets unreacted epoxides so available for a latter reaction, for instance for a crosslinking process.

Alcoholysis of epoxidized polyisoprenes by direct opening of oxirane rings with alcohol derivatives 2. Study on epoxidized 1,4-polyisoprene

Alcoholysis of 20% epoxidized 1,4-polyisoprene catalysed by cerium ammonium nitrate was achieved both in bulk and in dichloromethane solution. The oxirane ring opening of epoxidized units simultaneously leads to the formation of alkoxylated units, ketone and allylic alcohols units formed by rearrangement of the epoxide. The trisubstituted allylic alcohol units in the polymer afterwards react with alcohols to give two allylic ether units which increases the total yield of alcohol fixed onto polymer chains. The influence of solvent, alcohol concentration and temperature on the distribution of the epoxide ring opening products and alcohol fixation was studied in the case of 2-phenylethanol addition. The reaction was generalized to various primary alcohols such as benzyl alcohol, 2-propene-1-ol and 2-phenoxyethanol. Comparison with results obtained during a previous study realized with 4,5-epoxy-4-methyloctane selected as a model molecule of epoxidized 1,4-polyisoprene units showed that the reaction on polymer proceeds like on the model. However, an excess of alcohol is necessary for the reaction occurs in case of polymer modification and reaction temperature of 50°C is recommended to obtain a total conversion of epoxidized and allylic alcohol units.

Alcoholysis of epoxidized polyisoprenes by direct opening of oxirane rings with alcohol derivatives. 1. Modelization of the reaction

Abstract Alcoholysis of 4,5-epoxy-4-methyloctane selected as a model molecule of epoxidized 1,4-polyisoprene was achieved with various alcohols using cerium ammonium nitrate as catalyst. Next to the expected alkoxy alcohol, side products resulting from oxirane rearrangement were identified: 5-methyloctan-4-one, 5-methyloct-5-en-4-ol and 2-n-propylhexen-3-ol. One of them, 5-methyloct-3-en-4-ol, was demonstrated to be able to react with alcohols so leading to the formation of allylic ethers. From the diastereoisomer distribution, as well as the effect of temperature and solvent on the reaction, a coordinative SN2 mechanism able to explain the formation of the various products identified, is suggested.

Synthesis of alkoxysilyl-terminated polyisoprenes by means of 'living' anionic polymerization, 2. Synthesis of trialkoxysilyl-terminated 1,4-polyisoprenes by reaction of polyisoprenyllithium with various functional trialkoxysilanes selected as end-capping reagents

In order to prepare trimethoxysilyl (or triethoxysilyl)-terminate 1,4-polyisoprene with good yields, the deactivation of polyisoprenyllithium (M n = 800) was investigated by using various functional trialkoxysilanes (RO) 3 Si-G (G = 3-chloropropyl, methoxy or ethoxy) as silane end-capping agents. The studies were carried out in presence of THF by varying the molar ratio r = [1,4-polyisoprenyllithium chains]/[alkoxysilane]. The resulting products were characterized (qualitatively and quantitatively) by means of 1 H and 29 Si NMR spectroscopies (one or two dimensions), and also by supercritical fluid chromatography. The corresponding rate of substitution depends on r and on the nature of the starting alkoxysilane reagent. It was shown that alkoxy groups at the silicon center are easily substituted by the polyisoprenyl carbanions. Whatever the alkoxysilane used, two families of products are formed: (i) polymer derivatives resulting from nucleophilic substitution reactions of alkoxy groups from alkoxysilane reagent by ω-carbanionic polyisoprene chains, and (ii) substituted side-products issued from substitutions involving n-BuLi. The presence of these side products is explained by the fact that the butyllithium initially introduced to initiate the isoprene anionic polymerization is not totally consumed. The comparison of the results with those previously obtained in hexane with n-BuLi used as model molecule of w-carbanionic polyisoprene shows some differences concerning the order of reactivity of the various reactive substituted species formed during the substitution reaction, as well as the selectivity of the substitution reaction in relation with the alkoxysilane reagent. These differences were attributed to the chelating properties of THF. On the other hand, results are the same concerning the attempts realized with 3-chloropropyltrimethoxysilane which have also shown that nucleophilic attack of polyisoprenyl carbanion occurs exclusively on the silicon atom and not on the carbon bearing the chlorine.

Synthesis of 1,4-polyisoprene support of 2-chloroethylphosphonic acid (ethephon), a stimulating compound for the latex production by the Hevea brasiliensis

Abstract 2-Chloroethylphosphonic acid (ethephon) is a well known stimulating product used to improve the latex production by the rubber tree (Hevea brasiliensis). Its chemical fixation in side position of 1,4-polyisoprene chains by weak chemical bond was considered in order to prepare new derivatives having prolonged stimulating activity. The synthesis was considered by using a chemical modification procedure according to a two-step process. Firstly, an epoxidized 1,4-polyisoprene intermediate was prepared by partial epoxidation of 1,4-polyisoprene. Secondly, the grafting of 2-chloroethylphosphonic acid was achieved by using the reactivity of the P–OH acidic function (or a P–OSiMe3 derived from P–OH) of the reagent toward oxirane rings of epoxidized 1,4-polyisoprene. It was noted that grafting yields are improved when the reaction is carried out in bulk or in a non-polar solvent, and more especially in neutral conditions, that is by replacing ethephon with its trimethylsilylated derivatives [monotrimethylsilyl 2-chloroethylphosphonic acid or, more especially, di(trimethysilyl) 2-chloroethylphosphonate]. With this latter, the addition occurs by the intermediate of the P–OSiMe3 bond, and the formation of 2-oxo-l,3,2-dioxaphospholane structures is highly favored.

Epoxidation of 4-methyloct-4-ene: identification of reaction products and kinetic study

Abstract The performic acid epoxidation of 4-methyloct-4-ene, a model compound for 1,4-polyisoprene, is the subject of this study. The purpose was the optimization of the conditions for the epoxidation reaction by this oxidation agent. The first part of this work is concerned with the identification and characterization of the reaction products through NMR and FTIR spectroscopies. Four main products were characterized. Among them, an epoxide results from the epoxidation of 4-methyloct-4-ene by performic acid and three others, namely a ketone, a diol and a glycol ester, are the result of epoxide rearrangements. The second part is devoted to kinetic studies of the epoxidation reaction through high performance liquid chromatography (HPLC) technique. This investigation allowed an insight into the influence of the reaction parameters (temperature and concentration of hydrogen peroxide) on the conversion rate. The determining step of the epoxidation reaction was the formation of performic acid. The enthalpy of activation was found to be ΔH∗ = 91 kJ/mol and the entropy of activation ΔS ∗ =−34.6 J/mol K.

Synthesis of alkoxysilyl‐terminated polyisoprenes by means of “living” anionic polymerization, 1. Modeling of the termination step by studying the reaction of butyllithium with various alkoxysilane reagents

In order to optimize the preparation of trimethoxysilyl- or triethoxysilyl-terminated 1,4-polyisoprene and to facilitate their characterization, the termination reaction of the anionic polymerization of isoprene was investigated by means of a model reaction. Butyllithium (n-Buli) was used as a model molecule of the “living” ω-carbanionic polymer chain, and its reaction with various alkoxysilyl reagents (3-chloropropyltrimethoxysilane, tetramethoxysilane and tetraethoxysilane) was investigated. Studies were carried out by varying the molar ratio r = [n-BuLi]/[alkoxysilane] (0.5 ≤ r ≤ 3). The corresponding rate of substitution depends on r and on the nature of the coupling reagent. It was shown that alkoxy groups at the silicon center are easily n-Bu-substituted. Whatever, r, higher n-Bu-substituted derivatives are always simultaneously formed with the n-Bu-monosubstituted compound. With tetraalkoxysilane reagents, the formation of butyltrialkoxysilane is always favoured, compared to that of the di- and trisubstituted homologs. This was interpreted in terms of reactivity differences existing between the reacting alkoxysilanes present in the mixture. Attempts realized with 3-chloropropyltrimethoxysilane in order to obtain selective formation of alkyltrimethoxysilane derivative showed that chlorine substitution was impossible because nucleophilic attack of butyl carbanion occurs exclusively on the silicon atom.