Reversible additionÀfragmentation chain transfer (RAFT) polymerization, 1À4 mediated by thiocarbonylthio chain transfer agents (or RAFT agents, 1) (Scheme 1), possesses several advantages over other forms of reversible deactivation radical polymerization (RDRP), 5 such as atom transfer radical polymerization (ATRP) 6 and nitroxide mediated polymerization (NMP). 7,8 These include the ability to control the polymerization of a broad range of functional monomers (including vinyl esters and amides) and the absence of potentially toxic transition metal catalysts. RAFT polymerizations are simple to implement, because experimental conditions can mirror those of conventional radical polymerization; differing only by the addition of a RAFT agent. 2À4 To achieve optimal control over a RAFT polymerization, addition of the monomer derived propagating radical (P n • ) to the thiocarbonyl of a RAFT agent 1 and subsequent fragmentation of the RAFT intermediate 2 must occur efficiently. To facilitate this, the selection of a RAFT agent suitable for the monomer system is critical. Because propagating radicals of "more-activated" monomers (MAMs) (e.g., methacrylic, acrylic and styrenic monomers) are somewhat stabilized by conjugation, they are less reactive than propagating radicals derived from the "less-activated" monomers (LAMs) (e.g., vinyl esters and vinylamides). As such, these two classes of monomer require RAFT agents that are tailored to their differing reactivity. For effective control over polymerization of MAMs, dithioesters (Z = alkyl or aryl) or trithiocarbonates (Z = SR) are generally used. When these RAFT agents are used in the polymerization of LAMs, inhibition/retardation is observed, as fragmentation of the more reactive LAMs derived propagating radical is slow with respect to propagation. 9 The presence of O or N as the "Z" group adjacent to the thiocarbonyl, as is the case with xanthates (Z = OR) or dithiocarbamates (Z = NR 2 ), both slows addition of radicals to the RAFT agent and promotes the subsequent fragmentation such that it is not the rate determining step in chain transfer. Hence, these RAFT agents are regularly used for control over LAMs polymerization. Generally, xanthates and dithiocarbamates are relatively unreactive toward MAM derived propagating radicals, 10 making them ineffective control agents for these monomers. However, they may be effective for MAMs when the substituent is part of an aromatic heterocycle 10 or when highly electron withdrawing groups are present on the heteroatom. 11 As the reactivity of commonly used RAFT agents are tailored to either MAMs or LAMs, preparation of low dispersity polyMAM-block-polyLAM is not possible using the conventional RAFT process. While some RAFT agents, such as the N,N-diaryldithiocarbamates, have been reported by Destarac et al. 12 and Malepu et al. 13 for the homopolymerization of both MAMs and LAMs, these RAFT agents give only moderate control with both monomer classes. 12,13 When they were used for the preparation of poly(methyl acrylate)-bloc...

The use of solvents produces the largest amount of auxiliary waste in polymer science. Due to the fact that sustainable chemistry is becoming more and more important in polymer research, alternative reaction media have been investigated in order to reduce or replace the use of organic solvents. The most commonly used green solvents in polymer chemistry are water, supercritical carbon dioxide and ionic liquids. The progress of utilizing these green solvents in polymerization processes is reviewed and discussed in this critical review on the basis of results mainly published during the last five years (216 references).

A series of thermoresponsive diblock copolymers of poly [2-(dimethylamino)ethyl methacrylate-block-di(ethyleneglycol) methyl ether methacrylate], poly(DMAEMA-b-DEGMA), were synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerizations. The series consist of diblock and quasi diblock copolymers. Sequential monomer addition was used for the quasi diblock copolymer synthesis and the macro-chain transfer approach was utilized for the block copolymer synthesis. The focus of this contribution is the controlled variation of the ratios of DMAEMA to DEGMA in the copolymer composition, resulting in a systematic polymer library. One of the investigated block copolymer systems showed double lower critical solution temperature (LCST) behavior in water and was further investigated. The phase transitions of this block copolymer were studied in aqueous solutions by turbidimetry, dynamic light scattering (DLS), variable temperature proton nuclear magnetic resonance ( 1 H NMR) spectroscopy, zeta potential, and cryo transmission electron microscopy (cryo-TEM). The block copolymer undergoes a two-step thermo-induced self-assembly, which results in the formation of multilamellar vesicles after the first LCST temperature and to unilamellar vesicles above the second LCST transition. An interplay of ionic interactions as well as the change of the corresponding volume fraction during the LCST transitions were identified as the driving force for the double responsive behavior.

To reduce the environmental burden of polymer processing, the use of non-toxic solvents is desirable. In this regard, the improved solubility of poly(methyl methacrylate) (PMMA) in ethanol/water solvent mixtures is very appealing. In this contribution, detailed investigations on the solubility of PMMA in alcohol/water solvent mixtures are reported based on turbidimetry measurements. PMMA revealed upper critical solution temperature transitions in pure ethanol and ethanol/water mixtures. However, around 80 wt-% ethanol content a solubility maximum was observed for PMMA as indicated by a decrease in the transition temperature. Moreover, the transition temperatures increased with increasing PMMA molar mass as well as increasing polymer concentration. Careful analysis of both heating and cooling turbidity curves revealed a peculiar hysteresis behaviour with a higher precipitation temperature compared with dissolution with less than 60 wt-% or more than 90 wt-% ethanol in water and a reverse hysteresis behaviour at intermediate ethanol fractions. Finally, the transfer of poly(styrene)-block-poly(methyl methacrylate) block copolymer micelles from the optimal solvent, i.e. aqueous 80 wt-% ethanol, to almost pure water and ethanol is demonstrated.

The moisture uptake of several water-soluble polymers at different humidities was investigated with a thermal gravimetric analyzer equipped with a controlled humidity chamber. The water sorption of poly(acrylic acid) sodium salt, poly(ethylene glycol) and silica, which are known as super absorbers, were examined. In addition, various hydrophilic polymeric materials were selected according to their structural features. These included hydroxyl functions on the side chains (e.g. poly(2-hydroxyethyl methacrylate)), as well as acidic or basic functionalities (e.g. poly (dimethylaminoethyl methacrylate) or poly(vinylimidazole)). In addition, poly(2-methyl-2oxazoline) (P(MeOx)) and poly(2-ethyl-2-oxazoline) (P(EtOx)), which are well-known hydrophilic polymers, were also investigated in this context. More significant weight percent changes were obtained for P(MeOx) (60% at 90% relative humidity (RH)) in comparison to P(EtOx) (35% at 90% RH) as a result of the slight difference in hydrophilicity of the structures. The effect of the chain length on the ability for water uptake was also investigated for both poly(oxazolines). Finally, thermoresponsive polymers with a lower critical solution temperature (LCST) behavior (e.g. poly(N-isopropylacrylamide) and poly(dimethylaminoethyl methacrylate)) were also examined. The measurements for the latter polymers were performed below and above the LCST of each polymer whereby the humidities are varied from 0 to 90% with steps of 10%. Upon increasing humidity, the results revealed relatively high water uptake values (8% and 22% for P(NIPAM) and for P(DMAEMA), respectively) below the LCSTs of the polymers and, contrastingly, a small weight loss above their LCSTs. The present results allow a deeper insight into important structure-property relationships (e.g. the influence of the polymer backbone, functional groups, LCST behavior, etc. on the water-uptake properties), and will in subsequent steps permit the directed design of tailor-made polymers for selected applications.

Controlled radical polymerization using the reversible addition-fragmentation chain transfer approach (RAFT) was successfully conducted under continuous flow processing conditions, provided that steel tubing was used to prevent quenching of the radical process by oxygen. A series of different monomers, including acrylamides, acrylates, and vinyl acetate, were polymerized to high conversions (between 80 and 100%) at temperatures between 70 and 100 °C using various initiators, solvents, and RAFT agents. Low dispersities, typically between 1.15-1.20, and average molecular weights similar to those of batch RAFT polymerizations were obtained. The methodology provides a facile, alternative scale-up route to conventional batch polymerization, which can be challenging because of the oxygen-sensitive nature of the RAFT process.

The structures of the 'Z' and 'R' substituents of a RAFT agent (Z-SCS2-R) determine a RAFT agent's ability to control radical polymerization. In this paper we report new acid/base switchable N-aryl-Npyridyl dithiocarbamates (R= -CH2CN, Z = -N(Py)(Ar)) which vary in substituent at the 4-position of the aryl ring and the use of these to control molecular weight and dispersity. In their protonated form, the new RAFT agents are more effective in controlling polymerization of the more activated monomer, methyl acrylate (MA), whereas in their neutral form they provide more effective control of the polymerization of less activated monomers, N-vinyl carbazole (NVC) and vinyl acetate (VAc). For each polymerization, the apparent chain transfer coefficient (Ctr app ) shows a good correlation with Hammett parameters. Dithiocarbamates with more electron-withdrawing aryl ring substituents have the higher Ctr app . This demonstrates the influence of polar effects on Ctr app and supports the hypothesis that the Scheme 1: Canonical Structures of dithiocarbamates 2 3Recently, we have reported acid/base switchable RAFT agents (4, R'=CH3) which possess a pyridyl moiety capable of moderating the electron density, and hence reactivity, of the thiocarbonyl group (see Scheme 2). [14][15][16] The pyridinium species 5 allows for control over the polymerization of MAMs, whilst the neutral species 4 controls the polymerization of LAMs. Factors that affect the acid/base equilibria and hence the extent of protonation of the RAFT agent in solution have a marked effect on the dispersity of the polymers obtained during MAM polymerization. 16 Importantly, the switchable RAFT protocol also allows for the preparation of block copolymers incorporating both MAM and LAM monomer units (i.e.poly(MAM)-block-poly(LAM)) [14][15][16] which are generally unobtainable using the conventional RAFT methodology.Scheme 2: Acid/base switchable RAFT agents 4 5 Herein, we show that new acid/base "switchable" N-aryl-N-pyridyl dithiocarbamates of structure 4, where R' is aryl or pyridyl, possess enhanced activity in both the acidified and neutral forms when compared to that of the parent class (R' = CH3). These specific RAFT agents (4) were synthesized as the incorporation of aryl substituents as R', due to their electron withdrawing character, should enhance their activity and improve control over molar mass (Mn) and dispersity (Đ). We also demonstrate how manipulation of the RAFT agent structure influences chain transfer kinetics during homopolymerization of methyl acrylate

A series of copolymers containing 50 mol % acrylic acid (AA) and 50 mol % butyl acrylate (BA) but with differing composition profiles ranging from an AA‐BA diblock copolymer to a linear gradient poly(AA‐grad‐BA) copolymer were synthesized and their pH‐responsive self‐assembly behavior was investigated. While assemblies of the AA‐BA diblock copolymer were kinetically frozen, the gradient‐like compositions underwent reversible changes in size and morphology in response to changes in pH. In particular, a diblock copolymer consisting of two random copolymer segments of equal length (16 mol % and 84 mol % AA content, respectively) formed spherical micelles at pH >5, a mix of spherical and wormlike micelles at pH 5 and vesicles at pH 4. These assemblies were characterized by dynamic light scattering, cryo‐transmission electron microscopy and small angle neutron scattering.