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This Perspective describes the recent developments of polymerization-induced self-assembly of amphiphilic block copolymers based on controlled/living free-radical polymerization (CRP) in water. This method relies on the use of a hydrophilic living polymer precursor prepared via CRP that is extended with a hydrophobic second block in an aqueous environment. The process thus leads to amphiphilic block copolymers that self-assemble in situ into self-stabilized nano-objects in the frame of an emulsion or dispersion polymerization process. Depending on the nature and the structure of the so-formed copolymer, not only spherical particles can be achieved but also all morphologies that can be found in the phase diagram of an amphiphilic block copolymer in a selective solvent. This paper focuses mainly on aqueous emulsion or dispersion polymerization and gives an overview of the CRP techniques used, the general conditions, and the morphologies obtained.

ABSTRACT:Investigations into the kinetics and mechanism of dithiobenzoate-mediated Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerizations, which exhibit nonideal kinetic behavior, such as induction periods and rate retardation, are comprehensively reviewed. The appreciable uncertainty in the rate coefficients associated with the RAFT equilibrium is discussed and methods for obtaining RAFT-specific rate coefficients are detailed. In addition, mechanistic studies are presented, which target the elucidation of the fundamental cause of rate retarding effects.

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Propagation rate coefficients, k p , for free-radical polymerization of butyl acrylate (BA) previously reported by several groups are critically evaluated. All data were determined by the combination of pulsed-laser polymerization (PLP) and subsequent polymer analysis by size exclusion (SEC) chromatography. The PLP-SEC technique has been recommended as the method of choice for the determination of k p by the IUPAC Working Party on Modeling of Polymerization Kinetics and Processes. Application of the technique to acrylates has proven to be very difficult and, along with other experimental evidence, has led to the conclusion that acrylate chain-growth kinetics are complicated by intramolecular transfer (backbiting) events to form a mid-chain radical structure of lower reactivity. These mechanisms have a significant effect on acrylate polymerization rate even at low temperatures, and have limited the PLP-SEC determination of k p of chain-end radicals to low temperatures (<20 8C) using high pulse repetition rates. Nonetheless, the values for BA from six different laboratories, determined at ambient pressure in the temperature range of À65 to 20 8C mostly for bulk monomer with few data in solution, fulfill consistency criteria and show excellent agreement, and are therefore combined together into a benchmark data set. The data are fitted well by an Arrhenius relation resulting in a preexponential factor of 2.21 Â 10 7 L Á mol À1 Á s À1 and an activation energy of 17.9 kJ Á mol À1 . It must be emphasized that these PLP-determined k p values are for monomer addition to a chain-end radical and that, even at low temperatures, it is necessary to consider the presence of two radical structures that have very different reactivity. Studies for other alkyl acrylates do not provide sufficient results to construct benchmark data sets, but indicate that the family behavior previously documented for alkyl methacrylates also holds true within the alkyl acrylate family of monomers.Arrhenius plot of propagation rate coefficients, k p , for BA as measured by PLP-SEC.

Controlled poly(acrylic acid) homopolymers were synthesized for the first time by direct nitroxide-mediated polymerization of acrylic acid. The polymerizations were performed in 1,4-dioxane solution at 120 °C, using an alkoxyamine initiator based on the N-tert-butyl-N-(1-diethyl phosphono-2,2dimethyl propyl) nitroxide, SG1. The kinetics were controlled by the addition of free nitroxide at the beginning of the polymerization and the optimal amount was 9 mol % with respect to the initiator. In this case, whatever the initiator concentration, all polymerizations exhibited the same rate and conversion reached 85-90% within 5 h. Although the rate constant of propagation of acrylic acid is very large, its reactivity is moderated by a low activation-deactivation equilibrium constant between active macroradicals and SG1-capped dormant chains. Various alkoxyamine concentrations were investigated to target different molar masses. At high initiator concentrations, the number-average molar mass, M n, increased linearly with monomer conversion and followed the theoretical values; the polydispersity indexes ranged between 1.3 and 1.5. At low initiator concentration (high target Mn), a deviation from linearity was observed in the Mn vs conversion plot and was clearly assigned to chain transfer to 1,4-dioxane. From these results, the best experimental conditions to obtain well-defined homopolymers with the minimum amount of dead chains were identified.

The average activation-deactivation equilibrium constant, 〈K〉, was determined on a theoretical basis for controlled free-radical copolymerizations operating via a reversible termination mechanism (i.e., nitroxide-mediated polymerization or atom transfer radical polymerization), using the terminal model for the activation-deactivation equilibrium and the terminal model or the implicit penultimate unit effect model for the propagation reaction. From the equation, it was shown that the addition of a small fraction of an appropriate comonomer to a monomer with a very large activationdeactivation equilibrium constant, K, might lead to strong reduction of 〈K〉, providing the added comonomer exhibits a low K. In nitroxide-mediated polymerization, the monomers with a very high K, such as the methacrylic esters, do not lead to controlled polymerization in the presence of nitroxides like SG1, despite the absence of disproportionation reaction between the nitroxide and the growing radical, because of the too fast irreversible self-termination of the propagating radicals present in high concentration. The polymerization stops at low conversion. Consequently, a reduction of K might lead to an enhanced quality of control. The method was indeed successfully applied to the SG1-mediated polymerization of methyl methacrylate at 90 °C. By adding only 4.4 or 8.8 mol % of styrene, the polymerization could be carried out to large conversions, while exhibiting all the features of a controlled system.

A hydrophilic poly(methacrylic acid-co-poly-(ethylene oxide) methyl ether methacrylate) copolymer with a trithiocarbonate reactive group was used in the free-radical, batch emulsion polymerization of styrene. It allowed fast polymerizations and high final conversions to be achieved, and the parameters for a good control over the formation of well-defined amphiphilic diblock copolymers were identified. These diblock copolymers self-assembled in situ into nanoobjects of various morphologies upon chain extension. Achieving a good control over the formed diblock copolymers was shown to be an important step toward a better understanding of the parameters that affect the shape and size of the self-assembled objects, the ultimate goal being the ability to predict and fine-tune them on purpose.