Living free‐radical polymerization of sterically hindered monomers: Improving the understanding of 1,1‐disubstituted monomer systems

The sterically hindered, 1,1‐disubstituted monomers di‐n‐butyl itaconate (DBI), dicyclohexyl itaconate (DCHI), and dimethyl itaconate (DMI) were polymerized with reversible addition–fragmentation chain transfer (RAFT) free‐radical polymerization and atom transfer radical polymerization (ATRP). Cumyl dithiobenzoate, cumyl phenyl dithioacetate, 2‐cyanoprop‐2‐yl dithiobenzoate, 4‐cyanopentanoic acid dithiobenzoate, and S‐methoxycarbonylphenylmethyl dithiobenzoate were employed as RAFT agents to mediate a series of polymerizations at 60 °C yielding polymers ranging in their number‐average molecular weight from 4500 to 60,000 g mol^?1. The RAFT polymerizations of these hindered monomers displayed hybrid living behavior (between conventional and living free‐radical polymerization) of various degrees depending on the molecular structure of the initial RAFT agent. In addition, DCHI was polymerized via ATRP with a CuCl/methyl benzoate/N,N,N′,N″,N″‐pentamethyldiethylenetriamine/cyclohexanone system at 60 °C. Both the ATRP and RAFT polymerization of the hindered monomers displayed living characteristics; however, broader than expected molecular weight distributions were observed for the RAFT systems (polydispersity index = 1.15–3.35). To assess the cause of this broadness, chain‐transfer‐to‐monomer constants for DMI, DBI, and DCHI were determined (1.4 × 10^?3, 1.3 × 10^?3, and 1.0 × 10^?3, respectively) at 60 °C. Simulations carried out with the PREDICI program package suggested that chain transfer to monomer contributed to the broadening process. In addition, the experimental results indicated that viscosity had a pronounced effect on the broadness of the molecular weight distributions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3692–3710, 2006     

Reversible addition–fragmentation transfer (RAFT) copolymerization of methyl methacrylate and styrene in miniemulsion

In the reversible addition–fragmentation transfer (RAFT) copolymerization of two monomers, even with the simple terminal model, there are two kinds of macroradical and two kinds of polymeric RAFT agent with different R groups. Because the structure of the R group could exert a significant influence on the RAFT process, RAFT copolymerization may behave differently from RAFT homopolymerization. The RAFT copolymerization of methyl methacrylate (MMA) and styrene (St) in miniemulsion was investigated. The performance of the RAFT copolymerization of MMA/St in miniemulsion was found to be dependent on the feed monomer compositions. When St is dominant in the feed monomer composition, RAFT copolymerization is well controlled in the whole range of monomer conversion. However, when MMA is dominant, RAFT copolymerization may be, in some cases, out of control in the late stage of copolymerization, and characterized by a fast increase in the polydispersity index (PDI). The RAFT process was found to have little influence on composition evolution during copolymerization. The synthesis of the well‐defined gradient copolymers and poly[St‐b‐(St‐co‐MMA)] block copolymer by RAFT miniemulsion copolymerization was also demonstrated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6248–6258, 2004     

Controlled polymerization of N‐isopropylacrylamide with an activated methacrylic ester

A copolymer of N‐isopropylacrylamide with the N‐hydroxysuccinimide ester of methacrylic acid has found use in a variety of applications. Here we report our efforts to gain control over the molecular weight distribution of this copolymer with controlled radical polymerization methods, such as atom transfer radical polymerization, reversible addition–fragmentation transfer (RAFT), and nitroxide‐mediated polymerization. We have found that RAFT is capable of affording these copolymers with a polydispersity index of 1.1–1.2. Our results for all three polymerizations are reported. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6340–6345, 2004     

Reversible addition fragmentation chain transfer polymerization of sterically hindered monomers: Toward well‐defined rod/coil architectures

The sterically hindered monomers dibutyl itaconate (DBI) and dicyclohexyl itaconate (DCHI) were polymerized via reversible addition fragmentation chain transfer (RAFT) free‐radical polymerization. S,S′‐Bis(α,α′‐dimethyl‐α″‐acetic acid) trithiocarbonate, cumyl dithiobenzoate, and cumyl phenyldithioacetate have been used as RAFT agents to mediate a series of polymerizations at 65 °C yielding rod polymers ranging in number average molecular weight from 9000 to 92,000 g mol^?1. The progress of the polymerization was followed via online Fourier transform–near infrared spectroscopy. The polydispersity indices of the obtained rod polymers were relatively high at 1.4–1.7. The RAFT polymerizations of the hindered monomers used in the present study displayed both ideal living and hybrid behavior between conventional and living polymerization, depending on the RAFT agent used. DCHI rod polymers generated via the RAFT process were subsequently reinitiated in the presence of styrene to produce a range of BAAB and A‐B rod‐coil block copolymers of molecular weights up to 164,000 g mol^?1. The chain extension yields molecular weight distributions that progressively shift to higher molecular weights and are unimodal. Thermogravimetric analysis of the pDCHI‐block‐pStyrene copolymers indicates thermal degradation in two separate steps for the pDCHI and pStyrene blocks. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2432–2443, 2004     

Reversible addition–fragmentation chain transfer polymerization of glycidyl methacrylate with 2‐cyanoprop‐2‐yl 1‐dithionaphthalate as a chain‐transfer agent

A reversible addition–fragmentation chain transfer (RAFT) agent, 2‐cyanoprop‐2‐yl 1‐dithionaphthalate (CPDN), was synthesized and applied to the RAFT polymerization of glycidyl methacrylate (GMA). The polymerization was conducted both in bulk and in a solvent with 2,2′‐azobisisobutyronitrile (AIBN) as the initiator at various temperatures. The results for both types of polymerizations showed that GMA could be polymerized in a controlled way by RAFT polymerization with CPDN as a RAFT agent; the polymerization rate was first‐order with respect to the monomer concentration, and the molecular weight increased linearly with the monomer conversion up to 96.7% at 60 °C, up to 98.9% at 80 °C in bulk, and up to 64.3% at 60 °C in a benzene solution. The polymerization rate of GMA in bulk was obviously faster than that in a benzene solution. The molecular weights obtained from gel permeation chromatography were close to the theoretical values, and the polydispersities of the polymer were relatively low up to high conversions in all cases. It was confirmed by a chain‐extension reaction that the AIBN‐initiated polymerizations of GMA with CPDN as a RAFT agent were well controlled and were consistent with the RAFT mechanism. The epoxy group remained intact in the polymers after the RAFT polymerization of GMA, as indicated by the ¹H NMR spectrum. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2558–2565, 2004     

Synthesis of core‐shell poly(divinylbenzene) microspheres via reversible addition fragmentation chain transfer graft polymerization of styrene

Styrene has been grafted from crosslinked poly(divinylbenzene) core microspheres by both reversible addition fragmentation chain transfer (RAFT) polymerization and conventional free radical polymerization. The core microspheres were prepared by precipitation polymerization. Crosslinked poly(DVB) core microspheres containing double bonds on the particle surface can be used directly to graft polymers from the surface by RAFT without prior modification of the core microspheres. The RAFT agent 1‐phenylethyl dithiobenzoate (PEDB) was used: Particle sizes increased from 2 μm up to 3.06 μm, and the particle weight increased by up to 6.5%. PEDB controls the particle weight gain, the particle volume, and the molecular weight of the soluble polymer. PEDB was also used to synthesize core poly(DVB) RAFT microspheres that contain residual RAFT end groups on the surface and within the particle. Styrene was subsequently grafted from the surface of these core poly(DVB) RAFT microspheres. The generated microspheres were characterized by ¹H‐NMR spectroscopy, focused ion beam (FIB) milling, Coulter particle sizing, and size exclusion chromatography. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5067–5076, 2004     

Reversible addition–fragmentation transfer polymerization of p‐nitrophenyl acrylate and synthesis of diblock copolymers poly(p‐nitrophenyl acrylate)‐b‐polystyrene

Poly(p‐nitrophenyl acrylate)s (PNPAs) with different molecular mass and narrow polydispersity were successfully synthesized for the first time by reversible addition–fragmentation transfer (RAFT) polymerization with azobisisobutyronitrile (AIBN) as an initiator and [1‐(ethoxy carbonyl) prop‐1‐yl dithiobenzoate] as the chain‐transfer agent. Although the molecular mass of PNPAs can be controlled by the molar ratio of NPA to RAFT agent and the conversion, a trace of homo‐PNPA was found, especially at the early stage of polymerization. The dithiobenzoyl‐terminated PNPA obtained was used as a macro chain‐transfer agent in the successive RAFT block copolymerization of styrene (St) with AIBN as the initiator. After purification by two washings with cyclohexane and nitromethane to remove homo‐PSt and homo‐PNPA, the pure diblock copolymers, PNPA‐b‐PSt's, with narrow molecular weight distribution were obtained. The structural analysis of polymerization products by ¹H NMR and GPC verified the formation of diblock copolymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4862–4872, 2004     

The future of reversible addition fragmentation chain transfer polymerization

We examine the reversible addition fragmentation chain transfer (RAFT) process with regard to its potential and limits in future industrial applications (including those conducted on a larger scale) as well as materials science. The outlook for the RAFT process is bright: Its unrivaled inherent process simplicity coupled with a wide tolerance to monomer classes and functionalities makes it a prime candidate for the use in large reactors. At the same time, it allows for ready access to complex macromolecular architectures of variable shape and size. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5715–5723, 2008     

Macromolecular design via reversible addition–fragmentation chain transfer (RAFT)/xanthates (MADIX) polymerization

Among the living radical polymerization techniques, reversible addition–fragmentation chain transfer (RAFT) and macromolecular design via the interchange of xanthates (MADIX) polymerizations appear to be the most versatile processes in terms of the reaction conditions, the variety of monomers for which polymerization can be controlled, tolerance to functionalities, and the range of polymeric architectures that can be produced. This review highlights the progress made in RAFT/MADIX polymerization since the first report in 1998. It addresses, in turn, the mechanism and kinetics of the process, examines the various components of the system, including the synthesis paths of the thiocarbonyl‐thio compounds used as chain‐transfer agents, and the conditions of polymerization, and gives an account of the wide range of monomers that have been successfully polymerized to date, as well as the various polymeric architectures that have been produced. In the last section, this review describes the future challenges that the process will face and shows its opening to a wider scientific community as a synthetic tool for the production of functional macromolecules and materials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43:5347–5393, 2005     

Dendrimers as scaffolds for multifunctional reversible addition–fragmentation chain transfer agents: Syntheses and polymerization

The synthesis and characterization of novel first‐ and second‐generation true dendritic reversible addition–fragmentation chain transfer (RAFT) agents carrying 6 or 12 pendant 3‐benzylsulfanylthiocarbonylsulfanylpropionic acid RAFT end groups with Z‐group architecture based on 1,1,1‐hydroxyphenyl ethane and trimethylolpropane cores are described in detail. The multifunctional dendritic RAFT agents have been used to prepare star polymers of poly(butyl acrylate) (PBA) and polystyrene (PS) of narrow polydispersities (1.4 < polydispersity index < 1.1 for PBA and 1.5 < polydispersity index < 1.3 for PS) via bulk free‐radical polymerization at 60 °C. The novel dendrimer‐based multifunctional RAFT agents effect an efficient living polymerization process, as evidenced by the linear evolution of the number‐average molecular weight (M_n) with the monomer–polymer conversion, yielding star polymers with molecular weights of up to M_n = 160,000 g mol^?1 for PBA (based on a linear PBA calibration) and up to M_n = 70,000 g mol^?1 for PS (based on a linear PS calibration). A structural change in the chemical nature of the dendritic core (i.e., 1,1,1‐hydroxyphenyl ethane vs trimethylolpropane) has no influence on the observed molecular weight distributions. The star‐shaped structure of the generated polymers has been confirmed through the cleavage of the pendant arms off the core of the star‐shaped polymeric materials. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5877–5890, 2004     

Synthesis of well‐defined three‐armed polystyrene having thiourethane–isocyanurate as the core structure derived from trifunctional five‐membered cyclic dithiocarbonate

The synthesis of a three‐armed polymer with an isocyanurate–thiourethane core structure is described. Monofunctional reversible addition–fragmentation chain transfer (RAFT) agent 2 and trifunctional RAFT agent 5 were prepared from mercapto‐thiourethane and tris(mercapto‐thiourethane), which were obtained from the aminolysis of mono‐ and trifunctional five‐membered cyclic dithiocarbonates, respectively. The radical polymerization of styrene in the presence of 2,2′‐azobis(isobutyronitrile) and RAFT agent 2 in bulk at 60 °C proceeded in a controlled fashion to afford the corresponding polystyrene with desired molecular weights (number‐average molecular weight = 3000–10,100) and narrow molecular weight distributions (weight‐average molecular weight/number‐average molecular weight < 1.13). On the basis of the successful results with the monofunctional RAFT agents, three‐armed polystyrene with thiourethane–isocyanurate as the core structure could be obtained with trifunctional RAFT agent 5 in a similar manner. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5498–5505, 2005     

Kinetic modeling of controlled living microemulsion polymerizations that use reversible addition–fragmentation chain transfer

Reversible addition–fragmentation chain transfer (RAFT) polymerization is a useful technique for the formation of polymers with controlled architectures and molecular weights. However, when used in the polymerization of microemulsions, RAFT agents are only able to control the polymer molecular weight only at high RAFT concentrations. Here, a kinetic model describing RAFT microemulsion polymerizations is derived that predicts the reaction rates, molecular weight polydispersities, and particle size. The model predicts that at low RAFT concentrations, the RAFT agent will be consumed early in the reaction and that this will result in uncontrolled polymerization in particles nucleated late in the reaction. The higher molecular weight polydispersity that is observed in RAFT microemulsion polymerizations is the result of this uncontrolled polymerization. The model also predicts a shift in the conversion at which the maximum reaction rate occurs and a decrease in the particle size with increasing RAFT concentration. Both of these trends are also consistent with those observed experimentally. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6055–6070, 2006     

Synthesis of statistical and block copolymers containing adamantyl and norbornyl moieties by reversible addition‐fragmentation chain transfer polymerization

The synthesis of statistical and block copolymers, consisting of monomers often used as resist materials in photolithography, using reversible addition‐fragmentation chain transfer (RAFT) polymerization is reported. Methacrylate and acrylate monomers with norbornyl and adamantyl moieties were polymerized using both dithioester and trithiocarbonate RAFT agents. Block copolymers containing such monomers were made with poly(methyl acrylate) and polystyrene macro‐RAFT agents. In addition to have the ability to control molecular weight, polydispersity, and allow block copolymer formation, the polymers made via RAFT polymerization required end‐group removal to avoid complications during the photolithography. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 943–951, 2010     

Living free‐radical polymerization (reversible addition–fragmentation chain transfer) of 6‐[4‐(4′‐methoxyphenyl)phenoxy]hexyl methacrylate: A route to architectural control of side‐chain liquid‐crystalline polymers

Side‐chain liquid‐crystalline polymers of 6‐[4‐(4′‐methoxyphenyl)phenoxy]hexyl methacrylate with controlled molecular weights and narrow polydispersities were prepared via reversible addition–fragmentation chain transfer (RAFT) polymerization with 2‐(2‐cyanopropyl) dithiobenzoate as the RAFT agent. Differential scanning calorimetry studies showed that the polymers produced via the RAFT process had a narrower thermal stability range of the liquid‐crystalline mesophase than the polymers formed via conventional free‐radical polymerization. In addition, a chain length dependence of this stability range was found. The generated RAFT polymers displayed optical textures similar to those of polymers produced via conventional free‐radical polymerization. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2949–2963, 2003     

Kinetic analysis of the cross reaction between dithioester and alkoxyamine by a Monte Carlo simulation

A model reaction of dithioester and alkoxyamine is proposed to probe the reversible addition–fragmentation chain transfer (RAFT) process. The kinetics of the model reaction is analyzed and compared with that of pure alkoxyamine homolysis with a Monte Carlo simulation. Although the pure alkoxyamine obeys the law of persistent radical effect, the model reaction results in higher concentration of the persistent radical during the main period of the reaction. However, for a very fast RAFT process or a very low addition rate constant, the time dependence of the persistent radical concentration is quite close to that of pure alkoxyamine. Furthermore, the cross termination between the intermediate and alkyl radicals causes a retardation effect for the model reaction when the intermediate is relatively long‐lived. The Monte Carlo simulation indicates that it is feasible to measure the individual rate constants of the RAFT process, such as the rate constant of addition, with a large excess of alkoxyamine. In addition, the special feature of the system with different leaving groups in the alkoxyamine and dithioester is also discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 374–387, 2007     

RAFTing down under: Tales of missing radicals,fancy architectures,and mysterious holes

This highlight describes recent developments in reversible addition–fragmentation transfer (RAFT) polymerization. Succinct coverage of the RAFT mechanism is supplemented by details of synthetic methodologies for making a wide range of architectures ranging from stars to combs, microgels, and blocks. In addition, RAFT reactions in different media such as emulsion and ionic liquids receive attention. Finally, a specific example of a novel material design is briefly introduced, whereas polymers prepared via RAFT are adopted for microporous/honeycomb membrane design. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 365–375, 2003     

Use of a trithiocarbonyl RAFT agent without modification as (Co)stabilizer in miniemulsion polymerization

Herein, we demonstrate the effects of using 4‐cyano‐4‐[(dodecylsulfanylthiocarbonyl)sulfanyl] pentanoic acid (CDPA) simultaneously as RAFT chain transfer agent (RAFT‐CTA) and (co)stabilizer in a miniemulsion polymerization process. The novelty of this report includes use this RAFT‐CTA without modification, such as macro‐CTAs, block, or random copolymers. Using an optimized polymerization procedure, it is possible to use distinct formulations and therefore obtain stable latexes comprised of well‐defined spherical nanoparticles. The polymerization kinetics and final polymer properties are directly correlated to the function of the CDPA in the system: RAFT‐CTA and/or (co)stabilizer. Typically, the nanoparticles present average diameters less than 150 nm. The molar mass properties and the kinetic profiles confirm to an expected RAFT process, although some deviations are observable when using the CDPA as only stabilizer. These deviations are a function of the amount of CDPA present within the nano‐droplets, or adsorption on its surface. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1687–1695     

Solution and aqueous miniemulsion polymerization of vinyl chloride mediated by a fluorinated xanthate

Solution and aqueous miniemulsion polymerizations of vinyl chloride (VC) mediated by (3,3,4,4,5,5,6,6,7,7,8,8,8‐tridecafluorooctyl‐2‐((ethoxycarbonothioyl)thio) propanoate) (X1) were studied. The living characters of X1‐mediated solution and miniemulsion polymerizations of VC were confirmed by polymerization kinetics. The miniemulsion polymerization exhibits higher rate than solution polymerization. Final conversions of VC in the reversible addition‐fragmentation chain transfer (RAFT) miniemulsion polymerization reach as high as 87% and are independent of X1 concentration. Initiation process of X1‐mediated RAFT miniemulsion polymerization is controlled by the diffusion–adsorption process of prime radicals. Due to the heterogeneity of polymerization environments and concentration fluctuation of RAFT agent in droplets or latex particles, PVCs prepared in RAFT miniemulsion exhibit relatively broad molecular weight distribution. Furthermore, chain extensions of living PVC (PVC‐X) with VC, vinyl acetate (VAc), and N‐vinylpyrrolidone (NVP) reveal that PVC‐X can be reinitiated and extended, further confirming the living nature of VC RAFT polymerization. PVC‐b‐PVAc diblock copolymer is successfully synthesized by the chain extension of PVC‐X in RAFT miniemulsion polymerization. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2092–2101     

Synthesis of SAN‐containing block copolymers using RAFT polymerization

Copolymerization of styrene and acrylonitrile was carried out via reversible addition‐fragmentation chain transfer process (RAFT) in the presence of cumyl dithiobenzoate with AIBN as initiator. Copolymerization proceeded in a controlled/“living” fashion, and the copolymer composition depended on the feed ratio of monomer pairs. Block copolymers comprising styrene and acrylonitrile (SAN) segments and various functional blocks were synthesized through chain extension using the first blocks as macromolecular chain transfer agents (macroCTAs). Since the polymerization of both blocks proceeded through the RAFT process, the resulting block copolymers exhibited relatively narrow molecular weight distribution, with polydispersity indices in the range of 1.29–1.46. Gel permeation chromatography (GPC), and ¹H NMR and FTIR measurements confirmed the successful synthesis of the functionalized block copolymers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2260–2269, 2006     

Reversible addition–fragmentation chain transfer polymerization of methyl methacrylate in emulsion

Theoretical simulations showed that for controlled/living radical polymerization in an emulsion system, some of the earliest born particles could be superswollen to a size close to 1 μm. We hypothesized that the superswelling of these particles would lead to colloidal instability. Under the guidance of the simulation results, reversible addition–fragmentation chain transfer (RAFT) emulsion polymerization of methyl methacrylate (MMA) was carried out. Experimental results showed that increasing the initiation rate, surfactant level, and targeted molecular weight could improve the colloidal stability of the RAFT polymerization of MMA in an emulsion. The experimental results were in full accord with the theoretical predictions. The poor control of the molecular weight and polydispersity index was found to have a close relationship with the colloidal instability. For the first time, we demonstrated that RAFT polymerization could successfully be implemented with little coagulum, good control of the molecular weight, and a low polydispersity index with the same process used for traditional emulsion polymerization but with higher surfactant levels and initiation rates. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44:2837–2847, 2006