Determination of Addition and Fragmentation Rate Coefficients in RAFT Polymerization via Time‐Resolved ESR Spectroscopy after Laser Pulse Initiation

Summary: A method for simultaneous determination of both the addition and fragmentation rate coefficients of the RAFT equilibrium reactions is presented, which is based on laser single pulse initiation in conjunction with microsecond time‐resolved ESR spectroscopy. The build‐up and subsequent decay in concentration of the intermediate radical are measured and k_ad and k_β values are deduced from fitting the concentration versus time profiles to simple kinetic models.

Normalized ESR signal intensity vs. time after firing an initiating laser pulse in BMPT‐mediated butyl acrylate polymerization.     

Living Free Radical Polymerization of Cyclodextrin Host‐Guest Complexes of Styrene via the Reversible Addition Fragmentation Chain Transfer (RAFT) Process in Aqueous Solution

Summary: Host‐guest complexes of styrene and randomly methylated β‐cyclodextrin (m‐β‐CD) were polymerized in aqueous medium via the reversible addition fragmentation chain transfer (RAFT) process. 3‐Benzylsulfanylthiocarbonylsulfanylpropionic acid (TTC) was used as trithiocarbonate‐type RAFT agent. The results indicate a controlled character of the polymerization of the styrene complexes as the number‐average molecular weight, , increases linearly with monomer to polymer conversion; however, the molecular weights of the obtained polystyrenes deviate to higher values than those theoretically predicted. Nevertheless, the molecular weights can be controlled by variation of the initial RAFT agent concentration. The polystyrenes produced in this system exhibited narrower polydispersities (1.23 < < 2.36) than those produced without RAFT agent (5.24 < < 9.21) under similar conditions. The present contribution represents the first example of RAFT polymerization of a m‐β‐CD‐complexed hydrophobic vinylmonomer (styrene) from homogenous aqueous solution.

Schematic presentation of complexation and RAFT polymerization of m‐β‐CD‐complexed styrene with TTC as RAFT agent and evolution of the full molecular weight distributions in the CD‐mediated styrene free radical RAFT polymerization.     

统计结构含氟丙烯酸酯共聚物的合成与表征

本文对含氟丙烯酸酯(FMA)与甲基丙烯酸丁酯(BMA)的RAFT细乳液共聚合及动力学进行了研究, 计算得到了FMA与BMA的竞聚率并制备出具有统计结构的含氟共聚物乳液.     

Tandem RAFT Polymerization and Click Chemistry: An Efficient Approach to Surface Modification

PS grafted silica nanoparticles have been prepared by a tandem process that simultaneously employs RAFT polymerization and click chemistry. In a single pot procedure, azide‐modified silica, an alkyne functionalized RAFT agent and styrene are combined to produce the desired product. As deduced by thermal gravimetric and elemental analysis, the grafting density of PS on the silica in the tandem process is intermediate between analogous “grafting to” and “grafting from” techniques for preparing PS brushes on silica. Relative rates of RAFT polymerization and click reaction can be altered to control grafting density.

     

Quantum Chemical Mapping of Initialization Processes in RAFT Polymerization

Summary: We present the first ab initio simulation of a reversible addition fragmentation chain transfer (RAFT) polymerization. Using ab initio molecular orbital theory, we calculate the equilibrium constants for the first eight addition–fragmentation steps in the cyanoisopropyl dithiobenzoate‐mediated polymerization of styrene. We then simulate the concentration profiles for the RAFT agent, and its unimeric and dimeric adducts, assuming standard experimental parameters for styrene homopolymerization and the addition of the styryl radical to the RAFT agent. The simulated data show excellent agreement with published experimental data, highlighting the accuracy of quantum chemistry. In contrast, the currently used chain‐length independent models fail to describe even the qualitative trends in the data, regardless of whether the fragmentation reaction is assumed to be fast or slow. The calculated chain‐length dependent equilibrium constants are large, in agreement with the earlier proposed slow fragmentation model.

Ab initio kinetic modelling of concentration profiles during the RAFT initialization period.     

RAFT Miniemulsion Polymerization Kinetics, 1 – Polymerization Rate

The polymerization kinetics of a RAFT‐mediated radical polymerization inside submicron particles (30 < D_p < 300 nm) is considered. When the time fraction of active radical period, ϕ_A, is larger than ca. 1%, the polymerization rate increases with reducing particle size, as for the cases of conventional emulsion polymerization. The rate retardation by the addition of RAFT agent occurs with or without intermediate termination in zero‐one systems. For the particles with D_p < 100 nm, the statistical variation of monomer concentration among particles may not be neglected. It was found that this monomer‐concentration‐variation (MCV) effect may slow down the polymerization rate. An analytical expression describing the MCV effect is proposed, which is valid for both RAFT and conventional miniemulsion polymerizations.

     

Rate Optimization in Controlled Radical Emulsion Polymerization Using RAFT

Summary: Means of improving rates in RAFT‐mediated radical emulsion polymerizations are developed, by setting out strategies to minimize the inhibition and retardation that always are present in these systems. These effects arise from the RAFT‐induced exit of radicals, the desorption of the RAFT‐reinitiating radical from the particles, and the specificity of the reinitiating radical to the RAFT agent. Methods for reducing the inhibition period such as using a more hydrophobic reinitiating radical are predicted to show a significant improvement in the inhibition periods. The time‐dependent behavior of the RAFT adduct to the entering radical and the RAFT‐induced exit (loss) of radicals from particles are studied using a previously described Monte Carlo model of RAFT/emulsion particles. It is shown that an effective way of reducing the rate coefficient for the exit of radicals from the particles is to use a less active RAFT agent. Techniques for improving the rate of polymerization of RAFT/emulsion systems are suggested based upon the coherent understanding contained in these models: the use of an oligomeric adduct to the RAFT agent, a less water‐soluble RAFT re‐initiating group, and a less active RAFT agent.

Populations of the different types of particles (left axis) along with the concentration of the initial RAFT agent, D_R (right axis), as a function of time.     

Kinetic Analysis of Reversible Addition Fragmentation Chain Transfer (RAFT) Polymerizations: Conditions for Inhibition,Retardation, and Optimum Living Polymerization

Careful simulations of conversion vs. time plots and full molecular weight distributions have been performed using the PREDICI® program package in conjunction with the kinetic scheme suggested by the CSIRO group for the reversible addition fragmentation chain transfer (RAFT) process to probe RAFT agent mediated polymerizations. In particular, conditions leading to inhibition and rate retardation have been examined to act as a guide to optimum living polymerization behavior. It is demonstrated that an inhibition period of considerable length is induced by either slow fragmentation of the intermediate RAFT radicals appearing in the pre‐equilibrium or by slow re‐initiation of the leaving group radical of the initial RAFT agent. The absolute values of the rate coefficients governing the core equilibrium of the RAFT process – at a fixed value of the equilibrium constant – are confirmed to be crucial in controlling the polydispersity of the resulting molecular weight distributions: A higher interchange frequency effects narrower distributions. It is further demonstrated that the size of the rate coefficient controlling the addition reaction of propagating radicals to polyRAFT agent, k_β, is mainly responsible for optimizing the control of the polymerization. The fragmentation rate coefficient, k_–β, of the macroRAFT intermediate radical, on the other hand, may be varied over orders of magnitude without affecting the amount of control exerted over the polymerization. On the basis of the basic RAFT mechanism, its value mainly governs the extent of rate retardation in RAFT polymerizations.     

Batch Emulsion Polymerization of Styrene Stabilized by a Hydrophilic Macro‐RAFT Agent

Summary: A well‐defined homopolymer of 2‐(diethylamino)ethyl methacrylate has been synthesized by reversible addition‐fragmentation chain transfer (RAFT) polymerization using (4‐cyanopentanoic acid)‐4‐dithiobenzoate as a chain transfer agent. The corresponding protonated homopolymer with a very reactive dithiobenzoate end group has been used as a water‐soluble macromolecular chain transfer agent in the batch emulsion polymerization of styrene without any surfactant. The reaction leads to a stable latex, as a result of the in‐situ formation of an amphiphilic block copolymer stabilizer, via transfer reaction to the dithioester functions during the nucleation step. The work does not intend to apply controlled free‐radical polymerization in an aqueous dispersed system but takes advantage of the RAFT technique to create a well‐defined polyelectrolyte, with a high chain‐end reactivity.

Schematic of the formation of the stabilized latex by the in situ formation of an amphiphilic block copolymer stabilizer.     

Reversible Addition‐Fragmentation Transfer Polymerization in the Presence of MMT Immobilized Amphoteric RAFT Agent

Summary: A novel reversible addition‐fragmentation transfer (RAFT) agent, 10‐carboxylic acid‐10‐dithiobenzoate‐decyltrimethylammonium bromide (CDDA), was synthesized and intercalated into montmorillonite (MMT). Successively, the CDDA‐intercalated MMT was used as RAFT agent in the in situ RAFT polymerization for preparation of the polystyrene/MMT nanocomposites. After separation of MMT, the polymers obtained have predictable molecular weight and narrow polydispersity. XRD spectra and TEM images of the nanocomposites demonstrated exfoliated structure. Thermal stability of the composites has been noticeably improved.

     

RAFT kinetics revisited: Revival of the RAFT debate

Recently, two electron spin resonance (ESR)‐based methods for the determination of addition and fragmentation rate coefficients in dithiobenzoate‐mediated reversible addition fragmentation transfer (RAFT) polymerization were introduced, one being based on a spin‐trapping method and the other on single‐laser pulse initiation in conjunction with ESR detection at microsecond time resolution. For the RAFT‐intermediate radical fragmentation rate, coefficient data differing by six orders of magnitude were obtained, which cannot be explained by the usual model dependencies, that is the so‐called cross‐termination versus stable intermediate model. Even under consideration of fast cross‐termination in both cases, the large difference persists. Both the experimental designs are thus critically reviewed to identify potential error sources and to explain the vast difference in the individual results. Both techniques appear to be robust and only small interferences could be identified. Finally, recommendations for the refinement of the individual techniques are given to achieve a consistent kinetic picture of the underpinning reaction equilibria. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

A Critical Survey of Dithiocarbamate Reversible Addition‐Fragmentation Chain Transfer (RAFT) Agents in Radical Polymerization

This article provides a critical review of the properties, synthesis, and applications of dithiocarbamates Z′Z″NC(=S)SR as mediators in reversible addition‐fragmentation chain transfer (RAFT) polymerization. These are among the most versatile RAFT agents. Through choice of substituents on nitrogen (Z′, Z″), the polymerization of most monomer types can be controlled to provide living characteristics (i.e., low dispersities, high end‐group fidelity, and access to complex architectures). These include the more activated monomers (MAMs; e.g., styrenes and acrylates) and the less activated monomers (LAMs; e.g., vinyl esters and vinylamides). Dithiocarbamates with balanced activity (e.g., 1H‐pyrazole‐1‐carbodithioates) or switchable RAFT agents [e.g., a N‐methyl‐N‐(4‐pyridinyl)dithiocarbamate] allow control MAMs and LAMs with a single RAFT agent and provide a pathway to low‐dispersity poly(MAM)‐block‐poly(LAM). © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 216–227     

Ab initio Emulsion Polymerization by RAFT (Reversible Addition–Fragmentation Chain Transfer) through the Addition of Cyclodextrins

A novel process to produce homo‐ and copolymers by RAFT polymerization in emulsion is presented. It is known that RAFT‐controlled radical polymerization can be conducted in emulsion polymerization without disturbing the radical segregation characteristic of this process, thus leading to polymerization rates identical to those encountered in the corresponding nonliving systems. However, RAFT agents are often characterized by very low water solubility and, therefore, they diffuse very slowly from the monomer droplets, where they are initially solubilized, to the reaction loci, i.e., the polymer particles. Accordingly, when used in emulsion polymerization, they are practically excluded from the reaction. In this work, we show that cyclodextrins, well‐known for their ability to form water‐soluble complexes with hydrophobic molecules, facilitate the transport across the H₂O phase of the RAFT agent to the polymer particles. Accordingly, chains grow through the entire process in a controlled way. This leads to the production of low‐polydispersity polymers with well‐defined structure and end functionalities as well as to the possibility of synthesizing block copolymers by a radical mechanism.     

RAFT Inverse Miniemulsion Polymerization of Acrylamide

RAFT inverse miniemulsion polymerization is demonstrated for the first time as an alternate way to synthesize hydrophilic polymer latexes. The kinetic behavior of inverse RAFT miniemulsion polymerization of acrylamide is similar to that observed in aqueous RAFT solution polymerization. A water‐soluble initiator provides better control than a lipophilic initiator in inverse RAFT miniemulsion polymerization under the conditions used here.

     

Experimental Requirements for an Efficient Control of Free‐Radical Polymerizations via the Reversible Addition‐Fragmentation Chain Transfer (RAFT) Process

Summary: Reversible addition‐fragmentation chain transfer (RAFT) polymerization is a recent and very versatile controlled radical polymerization technique that has enabled the synthesis of a wide range of macromolecules with well‐defined structures, compositions, and functionalities. The RAFT process is based on a reversible addition‐fragmentation reaction mediated by thiocarbonylthio compounds used as chain transfer agents (CTAs). A great variety of CTAs have been designed and synthesized so far with different kinds of substituents. In this review, all of the CTAs encountered in the literature from 1998 to date are reported and classified according to several criteria : i) the structure of their substituents, ii) the various monomers that they have been polymerized with, and iii) the type of polymerization that has been performed (solution, dispersed media, surface initiated, and copolymerization). Moreover, the influence of various parameters is discussed, especially the CTA structure relative to the monomer and the experimental conditions (temperature, pressure, initiation, CTA/initiator ratio, concentration), in order to optimise the kinetics and the efficiency of the molecular‐weight‐distribution control.

Schematic of the RAFT polymerization.     

Temperature‐Responsive Glycopolymer Brushes Synthesized via RAFT Polymerization Using the Z‐group Approach

Summary: Stimuli‐responsive glycopolymer brushes composed of N‐acryloyl glucosamine (AGA) and N‐isopropylacrylamide (NIPAAm) were prepared using RAFT polymerization. The RAFT agent was immobilized on the surface of a treated silicon waver via covalent attachment using the Z‐group. PAGA and PNIPAAm brushes showed a linear increase in brush thickness with the consumption of monomer in solution. The polymers generated in solution confirm the living behavior with the molecular weight increasing linearly with monomer conversion while the molecular weight distribution remains narrow. Additionally, the ability of PAGA brushes to grow further in the presence of NIPAAm reveals the presence of an active RAFT end group indicative of a living system. PAGA and PNIPAAm homopolymer brushes up to 30 nm were grown using this technique. PAGA brushes were utilized for further chain extension to generate stimuli‐responsive brushes with block structures of PAGA and PNIPAAm. The PAGA‐block‐PNIPAAm brushes were found to grow in size with the consumption of NIPAAm. Contact angle measurements confirm the suggested mechanism showing that the second monomer is incorporated between the first layer and the silicon surface as expected using the Z‐group approach.

Structure of the stimuli‐responsive glycopolymer brushes.     

A Strategy for Synthesis of Ion‐Bonded Supramolecular Star Polymers by Reversible Addition‐Fragmentation Chain Transfer (RAFT) Polymerization

We have developed a novel strategy for the preparation of ion‐bonded supramolecular star polymers by RAFT polymerization. An ion‐bonded star supramolecule with six functional groups was prepared from a triphenylene derivative containing tertiary amino groups and trithiocarbonate carboxylic acid, and used as the RAFT agent in polymerizations of tert‐butyl acrylate (tBA) and styrene (St). Molecular weights and structures of the polymers were characterized by ¹H NMR and GPC. The results show that the polymerization possesses the character of living free‐radical polymerization and the ion‐bonded supramolecular star polymers PSt, PtBA, and PSt‐b‐PtBA, with six well‐defined arms, were successfully synthesized.

     

Kinetic Aspects of RAFT Polymerization

In this short review, selected experimental approaches for probing the mechanism and kinetics of RAFT polymerization are highlighted. Methods for studying RAFT polymerization via varying reaction conditions, such as pressure, temperature, and solution properties, are reviewed. A technique for the measurement of the RAFT specific addition and fragmentation reaction rates via combination of pulsed-laser-initiated RAFT polymerization and µs-time-resolved electron spin resonance (ESR) spectroscopy is detailed. Mechanistic investigations using mass spectrometry are exemplified on dithiobenzoic-acid-mediated methyl methacrylate polymerization.