Design strategies for controlling the molecular weight and rate using reversible addition–fragmentation chain transfer mediated living radical polymerization

Living radical polymerization has allowed complex polymer architectures to be synthesized in bulk, solution, and water. The most versatile of these techniques is reversible addition–fragmentation chain transfer (RAFT), which allows a wide range of functional and nonfunctional polymers to be made with predictable molecular weight distributions (MWDs), ranging from very narrow to quite broad. The great complexity of the RAFT mechanism and how the kinetic parameters affect the rate of polymerization and MWD are not obvious. Therefore, the aim of this article is to provide useful insights into the important kinetic parameters that control the rate of polymerization and the evolution of the MWD with conversion. We discuss how a change in the chain‐transfer constant can affect the evolution of the MWD. It is shown how we can, in principle, use only one RAFT agent to obtain a polymer with any MWD. Retardation and inhibition are discussed in terms of (1) the leaving R group reactivity and (2) the intermediate radical termination model versus the slow fragmentation model. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3189–3204, 2005     

Modeling the reversible addition–fragmentation transfer polymerization process

A kinetic model has been developed for reversible addition–fragmentation transfer (RAFT) polymerization with the method of moments. The model predicts the monomer conversion, number‐average molecular weight, and polydispersity of the molecular weight distribution. It also provides detailed information about the development of various types of chain species during polymerization, including propagating radical chains, adduct radical chains, dormant chains, and three types of dead chains. The effects of the RAFT agent concentration and the rate constants of the initiator decomposition, radical addition, fragmentation, disproportionation, and recombination termination of propagating radicals and cross‐termination between propagating and adduct radicals on the kinetics and polymer chain properties are examined with the model. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1553–1566, 2003     

Living radical polymerization of diisopropyl fumarate to obtain block copolymers containing rigid poly(substituted methylene) and flexible polyacrylate segments

Living radical polymerizations of diisopropyl fumarate (DiPF) are carried out to synthesize poly(diisopropyl fumarate) (PDiPF) as a rigid poly(substituted methylene) and its block copolymers combined with a flexible polyacrylate segment. Reversible addition‐fragmentation chain transfer (RAFT) polymerization is suitable to obtain a high‐molecular‐weight PDiPF with well‐controlled molecular weight, molecular weight distribution, and chain‐end structures, while organotellurium‐mediated living radical polymerization (TERP) and reversible chain transfer catalyzed polymerization (RTCP) give PDiPF with controlled chain structures under limited polymerization conditions. In contrast, controlled polymerization for the production of high‐molecular‐weight and well‐defined PDiPF is not achieved by atom transfer radical polymerization (ATRP) and nitroxide‐mediated radical polymerization (NMP). The block copolymers consisting of rigid poly(substituted methylene) and flexible polyacrylate segments are synthesized by the RAFT polymerization. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2136–2147     

Influence of a reversible addition–fragmentation chain transfer agent in the dispersion polymerization of styrene

Dispersion polymerization was applied to the controlled/living free‐radical polymerization of styrene with a reversible addition–fragmentation chain transfer (RAFT) polymerization agent in the presence of poly(N‐vinylpyrrolidone) and 2,2′‐azobisisobutyronitrile in an ethanol medium. The effects of the polymerization temperature and the postaddition of RAFT on the polymerization kinetics, molecular weight, polydispersity index (PDI), particle size, and particle size distribution were investigated. The polymerization was strongly dependent on both the temperature and postaddition of RAFT, and typical living behavior was observed when a low PDI was obtained with a linearly increased molecular weight. The rate of polymerization, molecular weight, and PDI, as well as the final particle size, decreased with an increased amount of the RAFT agent in comparison with those of traditional dispersion polymerization. Thus, the results suggest that the RAFT agent plays an important role in the dispersion polymerization of styrene, not only reducing the PDI from 3.34 to 1.28 but also producing monodisperse polystyrene microspheres. This appears to be the first instance in which a living character has been demonstrated in a RAFT‐mediated dispersion polymerization of styrene while the colloidal stability is maintained in comparison with conventional dispersion polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 348–360, 2007     

α‐Cyanobenzyl dithioester reversible addition–fragmentation chain‐transfer agents for controlled radical polymerizations

A series of new reversible addition–fragmentation chain transfer (RAFT) agents with cyanobenzyl R groups were synthesized. In comparison with other dithioester RAFT agents, these new RAFT agents were odorless or low‐odor, and this made them much easier to handle. The kinetics of methyl methacrylate radical polymerizations mediated by these RAFT agents were investigated. The polymerizations proceeded in a controlled way, the first‐order kinetics evolved in a linear fashion with time, the molecular weights increased linearly with the conversions, and the polydispersities were very narrow (～1.1). A poly[(methyl methacrylate)‐block‐polystyrene] block copolymer was prepared (number‐average molecular weight = 42,600, polydispersity index = 1.21) from a poly(methyl methacrylate) macro‐RAFT agent. These new RAFT agents also showed excellent control over the radical polymerization of styrenics and acrylates. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1535–1543, 2005     

Controlled radical copolymerization of styrene and maleic anhydride and the synthesis of novel polyolefin‐based block copolymers by reversible addition–fragmentation chain‐transfer (RAFT) polymerization

Reversible addition–fragmentation chain transfer (RAFT) was applied to the copolymerization of styrene and maleic anhydride. The product had a low polydispersity and a predetermined molar mass. Novel, well‐defined polyolefin‐based block copolymers were prepared with a macromolecular RAFT agent prepared from a commercially available polyolefin (Kraton L‐1203). The second block consisted of either polystyrene or poly(styrene‐co‐maleic anhydride). Furthermore, the colored, labile dithioester moiety in the product of the RAFT polymerizations could be removed from the polymer chain by UV irradiation. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3596–3603, 2000     

Living/controlled copolymerization of acrylates with nonactivated alkenes

The living/controlled copolymerization of methyl acrylate with 1‐alkenes and norbornene derivatives through several radical polymerization techniques has been achieved. These techniques include atom transfer radical polymerization, reversible addition–fragmentation transfer polymerization, nitroxide‐mediated polymerization, and degenerative transfer polymerization. These systems display many of the characteristics of a living polymerization process: the molecular weight increases linearly with the overall conversion, but the polydispersity remains low. Novel block copolymers have been synthesized through the sequential addition of monomers or chain extension. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6175–6192, 2004     

用ATRP法和RAFT法制备线形嵌段共聚物

嵌段共聚物是将不同性质的聚合物连接在同一分子内，表现出特殊的性质，受到高分子科学家及工业部门的广泛关注。本文简要介绍了嵌段共聚物的结构、性能以及可能的应用。它有多种制备方法，这里着重介绍近年来通过原子转移自由基聚合(ATRP)和可逆加成-裂解链转移(RAFT)法制备嵌段共聚物的研究现状和进展情况。对于加料顺序、大分子引发剂末端基团、单体的反应活性以及大分子引发剂的引发效率、配体种类、大分子链转移剂的链转移常数等对嵌段共聚反应的影响也进行了讨论。     

Synthesis of poly(tert‐butyl acrylate‐block‐vinyl acetate) copolymers by combining ATRP and RAFT polymerizations

The synthesis of poly(tert‐butyl acrylate‐block‐vinyl acetate) copolymers using a combination of two living radical polymerization techniques, atom transfer radical polymerization (ATRP) and reversible addition‐fragmentation chain transfer (RAFT) polymerization, is reported. The use of two methods is due to the disparity in reactivity of the two monomers, viz. vinyl acetate is difficult to polymerize via ATRP, and a suitable RAFT agent that can control the polymerization of vinyl acetate is typically unable to control the polymerization of tert‐butyl acrylate. Thus, ATRP was performed to make poly(tert‐butyl acrylate) containing a bromine end group. This end group was subsequently substituted with a xanthate moiety. Various spectroscopic methods were used to confirm the substitution. The poly(tert‐butyl acrylate) macro‐RAFT agent was then used to produce (tert‐butyl acrylate‐block‐vinyl acetate). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7200–7206, 2008     

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     

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     

Well‐controlled polymerization of 2‐azidoethyl methacrylate at near room temperature and click functionalization

A functional monomer with a pendant azide moiety, 2‐azidoethyl methacrylate (AzMA), was polymerized via reversible addition‐fragmentation chain transfer (RAFT) polymerization with excellent control over the molecular weight distribution (PDI = 1.05–1.15). The subsequent copper‐catalyzed Huisgen 1,3‐dipolar cycloadditions of phenyl acetylene with polyAzMA was achieved at room temperature with high conversion. The resulting functional polymer exhibited identical ¹H NMR and IR spectra with the polymer of the same molecular structure but prepared by a prefunctionalization approach, confirming the retention of the azide side chains during the RAFT polymerization of AzMA. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4300–4308, 2007     

Synthesis of NMP/RAFT inifers and preparation of block copolymers

Two different initiator/transfer agents (inifers) containing an alkoxyamine and a dithiobenzoate were synthetized and used to trigger out either reversible addition‐fragmentation chain transfer (RAFT) polymerization or nitroxide‐mediated polymerization (NMP). α‐Dithiobenzoate‐ω‐alkoxyamine‐difunctional polymers were produced in both cases which were subsequently used as precursors in the formation of block copolymers. This synthetic approach was applied to N‐isopropylacrylamide (NIPAM) or polyethylene oxide methacrylate (EOMA) to form α,ω‐heterodifunctional homopolymers via RAFT at 60°C which were chain extended with styrene by activating the alkoxyamine moiety at 120°C. Under such temperature conditions, it is proposed that a tandem NMP/RAFT polymerization is initiated producing a simultaneous growth of polystyrene blocks at both chain‐ends. Self‐assembled nanostructures of these amphiphilic block copolymers were evidenced by scanning electron microscopy. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Suppressing the long‐chain branching in the synthesis of poly(styrene‐b‐butyl acrylate‐b‐styrene) in RAFT emulsion polymerization by tuning the interfacial properties

Reversible addition‐fragmentation chain transfer (RAFT) emulsion polymerization is becoming an important technique to synthesize the latex of block copolymers. A previous study showed that in the synthesis of polystyrene‐b‐poly(butyl acrylate)‐b‐polystyrene triblock copolymer via RAFT emulsion polymerization using amphiphilic oligo(acrylic acid‐styrene) macroRAFT as surfactant and mediator, the molecular weight distribution could be much broadened to PDI higher than 2. In this study, an in‐depth investigation was performed to decrease PDI. It was found that long‐chain branches could be formed in the synthesis of triblock block copolymer, leading to the appearance of a higher molecular weight shoulder in the GPC curve of the final product. The lower neutralization degree of acrylic acid (AA) units on the macroRAFT and shorter AA chains would help to suppress the formation the long‐chain branches, leading to PDI around 1.5. It is evidenced that the successful suppression is due to the promotion of radical entry as a result of decreased interfacial transport impedance. It is also evidenced that the presence of styrene during the polymerization of butyl acrylate could promote the formation of long chain branches. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1464–1473     

AB and ABA type butyl acrylate and styrene block copolymers via RAFT‐mediated miniemulsion polymerization

Two trithiocarbonate reversible addition fragmentation chain transfer (RAFT) agents are compared in miniemulsion polymerization of styrene and butyl acrylate and the formation of seeded emulsion block copolymers. The order of block synthesis and the number of block segments per polymer are discussed. The use of nonionic surfactants is examined and the type of surfactant in relation to the monomer used is found to have a significant affect on latex formation. Conditions are shown by which AB and ABA type block copolymers can be successfully prepared via a seeded RAFT‐mediated emulsion polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 588–604, 2007     

Stereospecific reversible deactivation radical polymerization of biomass‐based acrylates for precise control of tacticity and molecular weight

Reversible deactivation radical polymerization (RDRP) of biomass‐based acrylates, (S )‐ and (R )‐2‐isopropyl‐5‐methylene‐1,3‐dioxolan‐4‐ones (S‐MiPDO and R‐MiPDO), was successfully performed to produce a well‐defined polymer with simultaneously controlled tacticity and molecular weight, and low dispersity (? < 1.3). In particular, meso triad (mm ) of the polymer was continuously controlled as designed from 28.1% to about 100% by changing the molar ratio of S‐MiPDO/R‐MiPDO in feed. In kinetic studies, the rate of RDRP was strongly influenced by the stereostructures of the propagating radical, and it was much lower in isospecific RDRP than atactic one in reversible chain transfer catalyzed polymerization (RTCP) in contrast to atom transfer radical polymerization (ATRP) where the rate would not change regardless of the tacticity. Increase of molecular weight and low ? of the polymer were also observed in reversible addition‐fragmentation chain transfer (RAFT) polymerization of MiPDO. In addition, block copolymers including stereoblock copolymers were feasibly synthesized by RTCP of styrene and methyl methacrylate using poly(MiPDO) prepolymer. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 445–456     

Synthesis of comb‐like polystyrene with poly(N‐phenyl maleimide‐alt‐p‐chloromethyl styrene) as macroinitiator

The copolymerization of N‐phenyl maleimide and p‐chloromethyl styrene via reversible addition–fragmentation chain transfer (RAFT) process with AIBN as initiator and 2‐(ethoxycarbonyl)prop‐2‐yl dithiobenzoate as RAFT agent produced copolymers with alternating structure, controlled molecular weights, and narrow molecular weight distributions. Using poly(N‐phenyl maleimide‐alt‐p‐chloromethyl styrene) as the macroinitiator for atom transfer radical polymerization of styrene in the presence of CuCl/2,2′‐bipyridine, well‐defined comb‐like polymers with one graft chain for every two monomer units of backbone polymer were obtained. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2069–2075, 2006     

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     

Highly efficient and well‐controlled ambient temperature RAFT polymerization of glycidyl methacrylate under visible light radiation

A range of well‐defined poly(glycidyl methacrylate) (PGMA) polymers and their corresponding block copolymers were synthesized via 2‐cyanoprop‐2‐yl(4‐fluoro) dithiobenzoate or CPFDB‐mediated ambient temperature reversible addition fragmentation chain transfer radical polymerization or RAFT polymerization under environmentally friendly visible light radiation (λ = 405–577 nm), using a (2,4,6‐trimethylbenzoyl) diphenylphosphine oxide photoinitiator. As comparison, CPFDB‐mediated ambient temperature RAFT polymerizations of glycidyl methacrylate (GMA) under both full‐wave radiation (λ = 254–577 nm) and long‐wave radiation (λ = 365–577 nm) were also studied in this article. The results indicated that CPFDB moieties were significantly photolyzed under either full‐wave radiation or long‐wave radiation, thus undermining the controlled behavior of these RAFT processes. Whereas this photolysis was significantly suppressed under visible light radiation, thus CPFDB functionalities exerted well control over RAFT process, leading to a remarkably living behavior up to 90% GMA monomer conversions. This strategy facilitates the facile synthesis of well‐defined PGMA polymers. More importantly, under visible light radiation, a relatively high initial molar ratio of GMA to CPFDB and TPO led to shortening initialization period of RAFT process and accelerating overall polymerization rate. These effects are remarkably in favor of the facile synthesis of well‐defined PGMA polymers and PGMA‐based copolymers with high molecular weights. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5091–5102, 2007     

Reversible Addition Fragmentation Chain Transfer Mediated Dispersion Polymerization of Styrene

Polystyrene microspheres have been synthesized by the reversible addition-fragmentation chain transfer (RAFT) mediated dispersion polymerization in an alcoholic media in the presence of poly(N-vinylpyrrolidone) as stabilizer and 2,2′-azobisisobutyronitrile as a conventional radical initiator. In order to obtain monodisperse polystyrene particles with controlled architecture, the post–addition of RAFT agent was employed to replace the weak point from the pre-addition of RAFT. The feature of preaddition and postaddition of RAFT agent was studied on the polymerization kinetics, particle size and its distribution and on the particle stability. The living polymerization behavior as well as the particle stability was observed only in the postaddition of RAFT. The effects of different concentration on the postaddition of RAFT agent were investigated in terms of molecular weight, molecular weight distribution, particle size and its distribution. The final polydispersity index (PDI) value, particle size and the stability of the dispersion system were found to be greatly influenced by the RAFT agent. This result showed that the postaddition of RAFT agent in the dispersion polymerization not only controls the molecular weight and PDI but also produces stable monodisperse polymer particles.