Reversible Addition-fragmentation Chain Transfer Agent Concentration Research Articles

Thermo-mechanical properties of urethane acrylate networks modulated by RAFT mediated photopolymerization

Reversible addition-fragmentation chain transfer (RAFT) processes in polymer systems have shown great utility in controlling radical polymerizations. Due to the reversible chain transfer mechanism, RAFT agents have been extensively used to synthesize monodisperse linear polymers and block copolymers in solution or emulsion polymerization. On the other hand, little is known about the effects of RAFT agents on the polymerization kinetics and thermo-mechanical properties of cross-linked materials. This work investigates the effect of RAFT agents on photopolymerization behavior and ultimate polymer properties in urethane acrylate systems. Our results indicate that RAFT agent addition has a significantly different impact on network development depending on inherent system characteristics, i.e., systems with glass transition temperature (Tg) below room temperature (LT) or those with Tg above room temperature (HT). When polymerizing with RAFT agents, photopolymerization rate decreases and can be tuned using different concentrations of RAFT agent. The final materials also show increased elongation at break with decreased Young's modulus. However, ultimate thermo-mechanical behavior and mechanical properties are different based on Tg. With RAFT addition decreased Tg is observed from HT polyurethane networks. Tensile toughness is also doubled in comparison to neat HT films. On the other hand, although the Tg of RAFT modified LT polyurethane networks does not change significantly, toughness of the final polymer films varies significantly with RAFT agent concentration. Enhanced toughness is only observed from films with low RAFT agent concentration while toughness decreases with higher loading of RAFT agent. These results demonstrate that the polymer network and thermo-mechanical properties can be modified by introducing RAFT agent which enables dynamic polymer chain rearrangements in both LT- and HT-acrylate network systems.

Carboxyl-Functionalized Polymeric Microspheres Prepared by One-Stage Photoinitiated RAFT Dispersion Polymerization.

Herein, we report a photoinitiated reversible addition-fragmentation chain transfer (RAFT) dispersion copolymerization of methyl methacrylate (MMA) and methyl methacrylic (MAA) for the preparation of highly monodisperse carboxyl-functionalized polymeric microspheres. High rates of polymerization were observed, with more than 90% particle yields being achieved within 3 h of UV irradiation. Effects of reaction parameters (e.g., MAA concentration, RAFT agent concentration, photoinitiator concentration, and solvent composition) were studied in detail, and highly monodisperse polymeric microspheres were obtained in most cases. Finally, silver (Ag) composite microspheres were prepared by in situ reduction of AgNO3 using the carboxyl-functionalized polymeric microspheres as the template. The obtained Ag composite microspheres were able to catalyze the reduction of methylene blue (MB) with NaBH4 as a reductant.

Dependence of cross-termination rate on RAFT agent concentration in RAFT polymerization

Rate retardation is an intrinsic property of reversible addition-fragmentation chain transfer (RAFT) radical polymerization. One of reasons for this phenomenon is cross-termination reaction between intermediate radicals and other active radicals. With the help of Stationary State Model and experimentally controlling on k t,cross , the kinetics of styrene RAFT polymerization were performed at different concentrations of RAFT agent. Results show that there is a difference in two effects of cross-termination rate coefficient and concentration of intermediate radicals on cross-termination at different RAFT agent concentrations: at the low concentration range, the cross-termination reaction is mainly affected by its rate coefficient, and at the high concentration range, the cross-termination reaction is mainly affected by the concentration of the intermediate radicals. It shows that there is an optimal concentrations of RAFT agent for a RAFT polymerization with the least rate retardation by considering the balance between these two effects.

4-Halogeno-3,5-dimethyl-1H-pyrazole-1-carbodithioates: versatile reversible addition fragmentation chain transfer agents with broad applicability

Pyrazole-based dithiocarbamates are versatile reversible addition fragmentation chain transfer (RAFT) agents that provide molar mass and dispersity (Đ) control over the radical polymerization of both more and less activated monomers (MAMs and LAMs). In this paper we report on theoretical and experimental findings demonstrating that their activity as RAFT agents can be significantly enhanced by introducing electron-withdrawing substituents to the pyrazole ring. This enhancement is most noticeable in methyl methacrylate polymerization where product molar masses are more accurately predicted by the RAFT agent concentration, and significantly lower Đ values, with respect to those seen with the parent RAFT agent under similar conditions, are observed. Thus, use of 4-chloro-3,5-dimethyl-1H-pyrazole-1-carbodithioate provides a poly(methyl methacrylate) with the anticipated molar mass and Đ as low as 1.3 at high monomer conversion. Good control is retained for monosubstituted MAMs, styrene, methyl acrylate and N,N-dimethylacrylamide. Low dispersities and less molar mass control are also achieved for homo- and copolymerizations with the LAM vinyl acetate, albeit with some retardation. © 2017 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Functionalization of cellulose with epoxy groups via γ-initiated RAFT-mediated grafting of glycidyl methacrylate

Glycidyl methacrylate (GMA), was grafted from cellulose by the combination of radiation-induced initiation and the reversible addition-fragmentation chain transfer (RAFT) polymerization technique, leading to epoxy functionalized surfaces that enable further modifications. Cumyl dithiobenzoate and 2-cyanoprop-2-yl dithiobenzoate were employed as the RAFT agents. The effects of absorbed dose, monomer and RAFT agent concentrations and solvent choice on grafting yield were investigated. Characterization of the synthesized copolymers by ATR-FTIR spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, thermal analysis and contact angle measurements revealed the grafting of poly(glycidyl methacrylate) (PGMA) from cellulose. Size-exclusion chromatography analysis indicated the difficulty of controlling the polymerization of GMA due to branching and/or crosslinking reactions that might occur in PGMA structure under γ-radiation.

Kinetic investigation of the reversible addition-fragmentation chain transfer polymerization of 1,3-butadiene

Reversible addition-fragmentation chain transfer (RAFT) polymerization of 1,3-butadiene using a trithiocarbonate chain transfer agent, S-1-dodecyl-S′-(r,r′-dimethyl-r″-acetic acid)trithiocarbonate (DDMAT), was investigated. For the first time, comprehensive kinetic study of solution RAFT polymerization of 1,3-butadiene is reported. Effect of some factors such as RAFT agent and initiator concentration and initial molar ratio of monomer to both RAFT agent and initiator on the rate of polymerization and molecular weight distribution (MWD) were examined experimentally and discussed quantitatively. The validity of quasi-steady state approximation (QSSA) was shown and the rate of polymerization’s equation was obtained. Good compatibility with the experimental and theoretical values of molecular weights was obtained. Also, polybutadiene samples with narrow molecular weight distribution were synthesized.

RAFT Copolymerization with Crosslinking of Methyl Methacrylate and Ethylene Glycol Dimethacrylate in Supercritical Carbon Dioxide

An experimental study on the kinetics of the reversible addition-fragmentation chain transfer (RAFT) copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) in supercritical carbon dioxide (scCO2) at 65°C and 172 bar is presented. Azobisisobutyronitrile (AIBN) was used as initiator, S-Thiobenzoyl thioglycolic acid as RAFT agent, and Krytox 257 FSL (Dupont) as stabilizer. A 38 mL, high pressure view cell, equipped with one frontal and two lateral sapphire windows, was used as the reacting vessel. The effect of RAFT agent concentration on polymerization rate, molecular weight development, gel fraction, swelling index and particle morphology was analyzed.

RAFT Copolymerization of Styrene/Divinylbenzene in Supercritical Carbon Dioxide

An experimental study on the kinetics of the reversible addition–fragmentation chain transfer (RAFT) dispersion copolymerization with crosslinking of styrene and divinylbenzene in supercritical carbon dioxide (scCO2) is presented. This is the first time that such a controlled polymer network synthesis is carried out in scCO2. S-Thiobenzoyl thioglycolic acid (TBTGA) and dibenzoyl peroxide were used as RAFT agent and initiator, respectively. The polymerizations were carried out in a high pressure cell with lateral sapphire windows at 80°C. The effect of RAFT agent concentration, including the case without RAFT controller, on polymerization rate, molecular weight development, gel fraction, swelling index, and particle morphology was analysed.

Modeling analysis of chain transfer in reversible addition‐fragmentation chain transfer polymerization

AbstractThe validity of simplifying the reversible addition‐fragmentation chain transfer (RAFT) polymerization as a degenerative chain transfer process was verified in this work. The simplified chain transfer mechanism enabled the direct modeling investigation of chain transfer coefficient in the RAFT polymerization. It also gave the analytical expressions for concentration, chain length, and polydispersity of various chain species. The comparison between the simulations based on chain transfer mechanism and those from general RAFT mechanism showed that this simplified mechanism can accurately predict RAFT polymerization in the absence of side reactions to adduct radicals other than fragmentation. However, significant errors are introduced at high conversion when side reactions to adduct are present. The chain transfer coefficient of RAFT agent is the key factor in RAFT polymerization. The polydispersity is more sensitive to chain transfer coefficient at low conversion. At high conversion, however, the polydispersity is mainly determined by termination, which can be controlled by RAFT agent concentration and the selection of initiator. At last, an analytical equation is derived to directly estimate chain transfer coefficient of RAFT agent from the experimental data. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.

Copolymerization Kinetics of Styrene and Divinylbenzene in the Presence of S‐Thiobenzoyl Thioglycolic Acid as RAFT Agent

AbstractAn experimental study on the kinetics of the reversible addition‐fragmentation chain transfer (RAFT) copolymerization of styrene (STY) and divinylbenzene (DVB) is presented. The experiments were carried out in bulk from a mixture of monomers, S‐thiobenzoyl thioglycolic acid as RAFT agent, and the initiator dibenzoyl peroxide (BPO) at 80 °C. The effect of RAFT agent concentration, including the case without RAFT controller, on polymerization rate, molecular weight development, gel fraction, and swelling index was analyzed.

RAFT polymerization of styrene mediated by naphthalene-containing RAFT agents and optical properties of the polymers

Abstract In this paper, we designed and synthesized five novel reversible addition–fragmentation chain transfer (RAFT) agents bearing naphthyl moieties in the Z or R groups, including 3,4,5-trimethoxy-benzyl dithio-2-naphthalenoate (TOBDN), 4-nitrobenzyl dithio-2-naphthalenoate (NBDN), 1-menaphthyl 4-cyanodithiobenzoate (NCDB), 1-menaphthyl dithiobenzoate (NDB) and 1-menaphthyl dithio-2-naphthalenoate (NDN). The RAFT polymerizations of styrene mediated by these RAFT agents with AIBN as the initiator at 80 °C were conducted and evaluated. Except for NCDB, the RAFT agents showed good control over the polymerization at different RAFT agent concentrations: the M n,GPC increased linearly with the monomer conversion, and the PDIs of the polymers were relatively low (PDI = 1.20–1.50). The structure of RAFT agents bearing three different R groups with naphthyl as the Z group showed less effects on the polymerization rate, while those bearing different Z groups with 1-menaphthyl as the R group presented significant effects on the polymerization rates. The polymerization rate with phenyl as the Z group was higher than that with 2-naphthyl as the Z group, and it decreased significantly when using 4-cycno phenyl as the Z group. Retardation effects were observed with all the RAFT agents. 1 H NMR spectra and chain extension results confirmed that most of the polymer chains were “living”. Ultraviolet (UV) absorption of naphthyl moieties at the R group showed blue shifts compared with those of naphthyl at the Z group. The UV absorption intensity of PS was uniformly lower than that of the corresponding RAFT agent, while the fluorescence intensity of PS was higher than that of the corresponding RAFT agent.

Mechanism of CPDB-Mediated RAFT Polymerization of Methyl Methacrylate: Influence of Pressure and RAFT Agent Concentration

Reversible addition–fragmentation chain transfer (RAFT) polymerizations of methyl methacrylate (MMA) in bulk at 60°C were performed at five pressures up to 200 MPa using 2-(2′-cyanopropyl)dithiobenzoate (CPDB) as RAFT agent at concentrations between 1.5 × 10–3 and 2.0 × 10–2 mol L–1. Applying high pressure during polymerization increases the rate of polymerization, but no effect on polydispersity was observed. Molecular weight distributions and average molecular weights of the final polymer indicated the successful control of MMA polymerization even at low CPDB concentrations. The slight retardation observed is adequately described by the dependence the termination rate coefficient, kt, on the chain-length.

Kinetic Trends in RAFT Homopolymerization from Online Monitoring

Automatic continuous online monitoring of reactions (ACOMP) was used to follow kinetic trends in the reversible addition -fragmentation chain transfer (RAFT) of butyl acrylate (BA) in butyl acetate, using 2-{((dodecylsulfanyl)carbonothioyl)sulfanyl}propanoic acid (DoPAT) as the RAFT agent and 2,2-azobis(isobutyronitrile) as the initiator (AIBN). The goals were to demonstrate the use of ACOMP for RAFT studies and to examine the trend in conversion and evolution of weight-average molar mass Mw and weight-average intrinsic viscosity ( )w for a series of experiments in which the RAFT agent concentration was decreased over a wide range while all the other reaction conditions were held constant; (DOPAT)/(AIBN) ranged from 0 to 2.4. This allowed monitoring the transition from living-like behavior in the controlled radical polymerization (CRP) regime, where sufficient RAFT agent was used, to the noncontrolled radical regime as RAFT agent was progressively reduced to zero. The conversion kinetics were found to be essentially zeroeth order in DoPAT, and deviations from living behavior allowed estimates of radical efficiency. The evolution of Mw vs monomer conversion varied dramatically from near-ideal living behavior at high (DoPAT)/(AIBN) to classical radical behavior at (DoPAT) ) 0. All but one experiment showed a nonzero Mw at f ) 0, and the conversion behavior of Mw points to transfer and branching side reactions, depending on (DoPAT). ACOMP should prove to be a useful tool both for fundamental RAFT research and process development.

Living Radical Polymerization of Isoprene via the RAFT Process

The control over the polymerization of isoprene via reversible addition−fragmentation chain transfer (RAFT) is reported. The effects of RAFT agent, reaction temperature, and monomer concentration over molecular weight control and reaction kinetics were investigated. The optimized conditions were used to produce block copolymers of isoprene with styrene and tert-butyl acrylate.

RAFT synthesis of poly(N‐isopropylacrylamide) and poly(methacrylic acid) homopolymers and block copolymers: Kinetics and characterization

AbstractReversible addition‐fragmentation chain transfer (RAFT) polymerization was used successfully to synthesize temperature‐responsive poly(N‐isopropylacrylamide) (PNIPAAm), poly(methacrylic acid) (PMAA), and their temperature‐responsive block copolymers. Detailed RAFT polymerization kinetics of the homopolymers was studied. PNIPAAm and PMAA homopolymerization showed living characteristics that include a linear relationship between Mn and conversion, controlled molecular weights, and relatively narrow molecular weight distribution (PDI < 1.3). Furthermore, the homopolymers can be reactivated to produce block copolymers. The RAFT agent, carboxymethyl dithiobenzoate (CMDB), proved to control molecular weight and PDI. As the RAFT agent concentration increases, molecular weight and PDI decreased. However, CMDB showed evidence of having a relatively low chain transfer constant as well as degradation during polymerization. Solution of the block copolymers in phosphate buffered saline displayed temperature reversible characteristics at a lower critical solution temperature (LCST) transition of 31°C. A 5 wt % solution of the block copolymers form thermoreversible gels by a self‐assembly mechanism above the LCST. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1191–1201, 2006

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

AbstractSummary: 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, $\overline M _{\rm n}$, 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 < $\overline M _{\rm w} /\overline M _{\rm n}$ < 2.36) than those produced without RAFT agent (5.24 < $\overline M _{\rm w} /\overline M _{\rm n}$ < 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.imageSchematic 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.

RAFT Agent Concentration in Polymer Particles during Emulsion Polymerization

A novel experimental setup to measure the diffusion and the concentration of light-absorbing molecules in particles of polymer dispersions is described. This method allows for instance quantification of the diffusion of hydrophobic reversible addition-fragmentation chain transfer (RAFT) agents from the monomer phase into the polymer particles during ab initio emulsion polymerization. The results clearly show that the diffusion of the RAFT agents strongly depends on their water solubility. This explains their influence on the kinetics of RAFT aqueous heterophase polymerization. Moreover, the retaining or the loss of the color of the RAFT agents after addition of different initiators recorded by this method gives direct experimental evidence that primary initiator radicals enter polymer particles.

Accessing Chain Length Dependent Termination Rate Coefficients of Methyl Methacrylate (MMA) via the Reversible Addition Fragmentation Chain Transfer (RAFT) Process

AbstractSummary: The RAFT‐CLD‐T methodology is demonstrated to be not only applicable to 1‐substituted monomers such as styrene and acrylates, but also to 1,1‐disubstituted monomers such as MMA. The chain length of the terminating macromolecules is controlled by CPDB in MMA bulk free radical polymerization at 80 °C. The evolution of the chain length dependent termination rate coefficient, k, was constructed in a step‐wise fashion, since the MMA/CPDB system displays hybrid behavior (between conventional and living free radical polymerization) resulting in initial high molecular weight polymers formed at low RAFT agent concentrations. The obtained CLD of kt in MMA polymerizations is compatible with the composite model for chain length dependent termination. For the initial chain‐length regime, up to a degree of polymerization of 100, kt decreases with α (in the expression k = k · i−α) being close to 0.65 at 80 °C. At chain lengths exceeding 100, the decrease is less pronounced (affording an α of 0.15 at 80 °C). However, the data are best represented by a continuously decreasing non‐linear functionality implying a chain length dependent α. magnified image

Influence of the chemical structure of dithiocarbamates with different N‐groups on the reversible addition–fragmentation chain transfer polymerization of styrene

AbstractPolymerizations of styrene with azobisisobutyronitrile initiation or thermal initiation have been performed in the presence of dithiocarbamates with different N‐groups, that is, benzyl 4,5‐diphenyl‐1H‐imidazole‐1‐carbodithioate (2a), benzyl 1H‐1,2,4‐triazole‐1‐carbodithioate (2b), benzyl indole‐1‐carbodithioate (2c), benzyl 2‐phenyl‐indole‐1‐carbodithioate (2d), benzyl phenothiazine‐10‐carbodithioate (2e), benzyl 9H‐carbazole‐9‐carbodithioate (2f), and benzyl dibenzo[b,f]azepine‐5‐carbodithioate (2g). The results show that the structure of the N‐group of dithiocarbamates has significant effects on the activity of dithiocarbamates for the polymerization of styrene. 2a, 2b, 2c, 2d, and 2f are effective reversible addition–fragmentation chain transfer (RAFT) agents for the RAFT polymerization of styrene, and the polymerizations have good living characteristics. However, in the cases of 2e and 2g, the obtained polymers have uncontrolled molecular weights and broad molecular weight distributions. The polymerization rate is markedly influenced by the conjugation structure of the N‐group of the dithiocarbamate, and the polymerization rate of 2b is greater than that of 2a. For 2b, the rate of polymerization seems independent of the RAFT agent concentration. However, a significant retardation in the rate of polymerization can be observed in the case of 2c. 2d is more effective than 2c, and the substitution group of phenyl on this dithiocarbamate has obvious effects on the effectiveness of the controlled polymerization of styrene. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4849–4856, 2005

Grafting of n-Butyl Acrylate and N,N'-Dimethyl Acrylamide from Poly(divinylbenzene) Microspheres by RAFT Polymerization

N,N´-Dimethyl acrylamide and n-butyl acrylate have been grafted from crosslinked poly(divinylbenzene) core microspheres by both reversible addition–fragmentation chain transfer (RAFT) polymerization and conventional free radical polymerization. The microspheres were prepared by precipitation polymerization utilizing 55 and 80 wt. % divinylbenzene (DVB), respectively. The RAFT agent cumyl dithiobenzoate (CDB) was utilized and grafting was performed in toluene at 80°C with 2,2´-azoisobutyronitrile (AIBN) as thermal initiator. Particle weights increased up to 6% for poly(n-butyl acrylate) and up to 15% for poly(N,N´-dimethyl acrylamide). Grafting from poly(DVB-55) microspheres results in a higher particle weight increase than grafting from poly(DVB-80) microspheres while using the same RAFT agent concentrations. The particle weight increase is approximately higher by a factor of two. The grafted microspheres were characterized by attenuated total reflection (ATR) IR spectroscopy and scanning electron microscopy.