Patrick Hémery

Anionic polymerization of octamethylcyclotetrasiloxane in miniemulsion II. Molar mass analyses and mechanism scheme

Abstract Hydroxyl α,ω-functionalized polydimethylsiloxane (PDMS) standards have been synthesized by ring-opening anionic polymerization of octamethylcyclotetrasiloxane (D 4 ) 1 in miniemulsion. Their absolute molar masses as well as Mark–Houwink–Sakurada parameters were then estimated in toluene at 30°C by triple detection size exclusion chromatography (SEC 3 ). MALDI-TOF confirmed the SEC 3 results in addition to providing several clues on chain microstructure and evidence of redistribution reactions. Trimethyl and vinyl α,ω-terminated PDMS oligomers were similarly synthesized in miniemulsion in the presence of functional transfer agents and used together with small dihydroxylated PDMS standards to compare the effect of substituents on the SEC calibration curve. Precise measurement of average molecular weights showed that the existing kinetics model could not account solely for mass control below 70% conversion. A thermodynamic explanation is then proposed where the molar mass control is exerted by the surface activity of the chains which drives their reactivity. Small chains rapidly propagate and backbite at the interface. Once the chains reach a critical DP corresponding to their loss of surface tension activity, they penetrate into the particles where side reactions such as redistribution and condensation hardly occur.

Investigation of ionic surfactant-selective electrodes by EIS

A model of ionic surfactant-selective electrodes with mobile ionic sites is proposed. The steady-state concentrations of the active species are calculated. The electrochemical impedance of the membrane immersed between electrolytic solutions is given. This model is in good agreement with the experimental results obtained on PVC membranes with various amounts of plasticizer sensitive to various ionic surfactants. The conclusion shows that to obtain a good sensitive membrane the dissociation of the species in the membrane is quite important and ionic transfer has to be fast.

Cationic polymerization of 1,3,5,7-tetramethylcyclotetrasiloxane in aqueous emulsion. Part I. Preliminary results

The synthesis of poly(methylhydrogeno)siloxane (PMHS) by cationic polymerization of 1,3,5,7-tetramethylcyclotetrasiloxane (D4H) was studied in aqueous emulsion using dodecylbenzenesulphonic acid (DBSA) as emulsifier/initiator. With pure DBSA, the rates of polymerization are very high and the PMHS formation is not well-controlled (cross-linking reactions, emulsion not very stable). The addition of the sodium salt of DBSA as coemulsifier does not give satisfactory results. In the presence of a neutral coemulsifier (Brij35), the process leads to the formation of linear PMHS of controlled molecular weight in the range 7000–70 000 g mol −1 with a yield of 90%. No cross-linking reaction was observed, showing the good stability of the SiH bond in these experimental conditions. Preliminary investigations lead to the conclusion that the polymer is probably formed by initiation, propagation and termination reactions taking place simultaneously at the surface of 200 nm diameter monomer particles. Nevertheless, the peculiar mechanism of these reactions seems more complicated than those previously found in the case of anionic polymerization of D4 in aqueous emulsion. © 1999 Society of Chemical Industry

An electrogravimetric study of an all-solid-state potassium selective electrode with prussian blue as the electroactive solid internal contact

Due to their fragility, classical ion-selective electrodes (ISEs) can be replaced by a new generation of chemical sensors. In this work, all-solid-state potassium selective electrodes, based on a plasticized poly(vinyl chloride) (PVC) membrane and a Prussian blue film as the solid internal contact, were prepared and characterized. First of all, potentiometric measurements revealed a non-Nernstian response of the electrode. Then, ac electrogravimetric measurements were carried out in KCl solutions under various polarization potentials. It was clearly found that potassium ions enter the PVC membrane regardless of the potential applied. Moreover, small quantities of solvent also enter the membrane at certain studied potentials. Electroacoustic measurements have been also carried out in order to check the validity of Sauerbreys law for the quartz crystal microbalance by estimating the electrical characteristics of the quartz resonator under our experimental conditions.

Poly(vinyl) chloroformate and derivatives

Poly(vinyl chloroformates) of high molecular weight have been successfully modified by amines and by KCN. Poly (vinyl carbonates) have been prepared by polymerization of phenyl vinyl carbonate.

Kinetics of ring opening of propylene sulphide: 1. Alkali metal and cryptated alkali metal carbazyl salts

Abstract The reactivities of free ions and ion pairs have been determined for the ring opening of propylene sulphide with carbazyl salts, in THF at −30°C. A large increase of the ion pair reactivity is observed on increasing the size of the counterion (Na + + + + |222|). Moreover, cryptated carbazyl ion pairs are three times more reactive than free ions in the same manner as thiolate species for the propagation of propylene sulphide in similar conditions.

Study of β-cyclodextrin/dodecyltrimethylammonium bromide complex into water-isopropanol mixtures

Abstract Stability constants of the β-cyclodextrin-dodecyltrimethylammonium complex in water-isopropanol mixtures for compositions between 0 to 34.5 wt% isopropanol were determined by potentiometric measurements. A significant increase in the dissociation constant of the complex was observed when isopropanol was added to its solution in water. Standard Gibbs energies of transfer of the surfactant (DTABr) from water to aqueous mixtures were determined by potentiometric measurements with a junctionless cell. The destabilization of this complex in the presence of alcohol is largely explained by a significant stabilization of the cationic amphiphile.

Cooperative phenomena in semi-dilute solutions of poly(oxyethylene)

Abstract The concentration dependence of the cooperative diffusion coefficient (D) and the cooperative sedimentation coefficient (s) in semi-dilute aqueous solutions of a sharp fraction of poly(oxyethylene) have been studied. It was found that D varies as C 0.60 ± 0.02 and s as C −0.66 ± 0.02 . The results indicate that one should distinguish between a static and an effective dynamical correlation length in order to obtain a consistent overall picture. Furthermore, the data seem to indicate that the POE chains are flexible.

Various types of aggregates in mono- or bifunctional polyisoprenyllithium in hydrocarbons

Abstract The addition of tert -butyllithium on 1,2-bis(isopropenyl-4 phenyl)ethane in pure hexane leads to the quantitative synthesis of a dilithium compound as shown previously by analysis of the reactional mixture. Its characteristics of difunctional initiator for the synthesis of polydienes in pure hydrocarbon solvents was proved by indirect methods. In this study it is shown by mass spectrometry that difunctional oligopolyisoprene is solely obtained at the first stages of the initiation reaction. Viscosimetric measurements show that, for low degree of polymerization and low concentration in Li, cyclic aggregates are preferentially formed by intramolecular association. A 7 Li n.m.r. study in hexane of various mono- and difunctional organolithium compounds confirms their trend to associate in hydrocarbon solvents and, for α, ω-dilithiopolyisoprene, the influence of the chain length on the formation of aggregated species involving more than four lithium atoms. A change in the concentration in Li has little influence on the distribution of the various aggregates in the 4 × 10 −4 − 3 × 10 −3 mol 1 −1 Li concentration range. The addition of LiCl reduces the amount of the most associated species, while the presence of 15 vol% THF leads to their disappearance.

Synthesis of cyclic and multicyclic polyisoprenes

Abstract Cyclic polyisoprenes have been synthesized by reaction of α,ω-dilithiopolyisoprenes with dichlorodimethylsilane or 1,2-bis(isopropenyl-4-phenyl) ethane (BIPE). Using the dihalide compound, the effect of the solvent polarity on the coupling reaction is more pronounced in the case of α,ω-dilithiopolyisoprene than with monofunctional polyisoprenyllithium. The yield in cyclic compound falls from 88% in pure hexane down to 53% in the presence of 15 vol% of tetrahydrofurane (THF). Using the nonconjugated diene (BIPE) as linking agent the addition of THF is required but the formed cycle retains its living character and allows the synthesis of cycles having two arms (after addition of isoprene) and of a bicyclic structure after a second cyclization reaction.