Control of the free‐radical polymerization of styrene was achieved using a series of eight xanthates of the general structure RS(CS)OEt as reversible addition fragmentation chain‐transfer agents. The influence of the nature of the R leaving group was explored. It was found that the transfer ability of the xanthate is markedly improved with increasing stability of R and its steric hindrance. Group R strongly influences the Mn evolution profile during polymerization, but only influences the polydispersities to a small extent. The cyanoisopropyl group was shown to be the best leaving group, leading to an increase in the molecular weight during polymerization that was close to linearity. The living character of the polymerization and the high purity of the chain structures were supported by matrix‐assisted laser desorption ionization time‐of‐flight (MALDI‐TOF) mass spectrometry and 13C NMR spectroscopy. A zero‐order dependence of the kinetics of polymerization on the xanthate concentration was observed.

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We have measured forces between molecularly smooth solid surfaces separated by thin films of molten polydimethylsiloxane. We show that a long-range repulsion reported in earlier work is not an eqUilibrium force, but can be attributed to viscous drag effects. Consistent with previous results, the viscosity of the film can be modeled by assuming that a layer of polymer molecules is immobilized or "pinned" at each surface for a time longer than the time scale of the measurements. We propose that this pinning is a result of entanglement-like effects in the vicinity of a wall.

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Herein we report the living/controlled radical polymerization of ethyl and n-butyl acrylates mediated by β-sulfinyl nitroxides at temperatures below 90 °C. This study examined the effect of the persistent radical stereochemistry on the polymerization kinetics of ethyl acrylate. Experiments were carried out in the presence of alkoxyamines based on 1-phenylethyl transient radicals bearing a 4-hydrogen or a 4-(methyl carboxylate) group, and two enantiomerically pure mixtures of N-tert-butyl-N-(1-tert-butyl-2-ethylsulfinyl)propyl nitroxide (BESN); i.e., a 1:1 S S R β R α/R S S β S α and a 1:1 R S R β R α/S S S β S α racemic mixtures. The dissociation rate constants (k d) of the alkoxyamines and poly(ethyl acrylate) chains capped by both nitroxide racemic mixtures were determined by ESR spectroscopy measurements. Kinetic and ESR studies of ethyl acrylate polymerization experiments allowed the determination of the equilibrium constants (K 90) and the rate constants of combination (k c 90) for each system. The rate constants of combination were found to be slightly dependent on the nitroxide stereochemistry whereas an order of magnitude difference was observed for the rate constants of dissociation. Then, well-defined poly(ethyl acrylate) and poly(n-butyl acrylate) were prepared, at 90 °C, from alkoxyamines based on the R S R β R α/S S S β S α nitroxide diastereomers.

The syntheses of a new nitroxide bearing a sulfoxide group at the β-position (with respect to nitrogen) and the corresponding N-β-sulfinyl alkoxyamine are reported. Styrene was polymerized in bulk in the presence of this new alkoxyamine. The polymerization satisfies the usual criteria of a living/ controlled radical polymerization, with a linear increase of molecular weight vs yield and a constant transient radical concentration with time. However, the polymerization rates were independent of alkoxyamine concentration but much higher than the thermal polymerization ones: R p/Rth ) 2.6 at 90 °C, 3.7 at 100 °C, and 3.5 at 110 °C. Kinetic and ESR studies showed that both transient and persistent radical concentrations do not follow the corresponding theoretical evolutions with time but reach stationary states. The different rate constants (kd and kc) and corresponding activation energies were estimated, showing that the sulfoxide group has a large effect mainly on the combination reaction, the value of which is unusually low for such system (kc ≈ 10 5 L mol -1 s -1 ).

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A diradical initiator containing two thermoreversible bonds was prepared and used for the polymerization of styrene at 90 "C. The monomer consumption and the variation of the molecular weight were monitored with time. The results show that the process can be considered as living and that the polymerization rate is independent of the radical initiator concentration. By elemental analysis of the chain ends it was concluded that the propagation reaction occurs at both ends.