Enhancing the Mechanical Properties of Matrix-Free Poly(Methyl Acrylate)-Grafted Montmorillonite Nanosheets by Introducing Star Polymers and Hydrogen Bonding Moieties

Matrix-free nanocomposite films of poly(methyl acrylate) (PMA) and montmorillonite (MMT) nanosheets that imitate the microscopic structure of nacre are developed and their mechanical properties are studied in detail via tensile testing. The exfoliated MMT nanosheets are grafted with PMA via a grafting-through radical addition–fragmentation chain transfer polymerization in presence of a surface-anchored ionic monomer. The mechanical properties are precisely tailored a) via the variation of the degree of polymerization of the surface-grafted polymer, illustrating the impact of chain entanglement and the MMT content on the performance of the material, and b) via cross-linking of the surface-grafted polymer, either by partial change of the polymer topology from linear to star-shape or by the introduction of hydrogen-bonding units within the polymer. These experiments demonstrate the strong influence of the chain mobility of the surface-grafted polymer on the mechanical properties of the nanocomposite material.     

RAFT Polymerization of Methyl Acrylate in Carbon Dioxide

Summary: Reversible addition fragmentation chain transfer (RAFT) polymerizations of methyl acrylate (MA) in solution containing either 22 vol.‐% CO₂ or toluene were performed at 80 °C and 300 bar using cumyl dithiobenzoate (CDB) at concentrations between 1.8 × 10^?3 to 2.5 × 10^?2 mol · L^?1 as the RAFT agent. Product molecular weight distributions and average molecular weights indicated the successful control of MA polymerization in CO₂, even at low CDB concentrations. RAFT polymerization rates were strongly retarded by CDB and were lower in CO₂ than in toluene solution. The enhanced fluidity associated with the addition of CO₂ to the polymerizing system provided access to mechanistic details of RAFT polymerization. The data of the present study into MA, together with our recent results on RAFT polymerization of styrene in solution of CO₂ and of toluene, suggest that self‐termination of intermediate RAFT radicals is responsible for retardation in case of high concentrations of this intermediate and in case of enhanced fluidity, which may be achieved by polymerization in solution of CO₂.

     

Influence of reaction parameters on the synthesis of hyperbranched polymers via reversible addition fragmentation chain transfer (RAFT) polymerization

The synthesis of hyperbranched poly(methyl methacrylate) (PMMA) via reversible addition fragmentation chain transfer (RAFT) polymerization was investigated by varying the ratio chain transfer agent (CTA): monomer (methyl methacrylate, MMA): brancher (ethylene glycol dimethyl methacrylate, EGDMA): free radical initiator (AIBN) at various temperatures (50, 55, 60, 65, 70 °C). The rate of polymerization was observed to increase with temperature and concentration in brancher, whilst it was lowered by an increase in chain transfer agent concentration. The molecular weight of the samples increased with the ratios brancher: CTA and monomer: CTA. The polydispersity of the samples increase with conversion, as the level of branching increases. At fixed concentration in brancher, an increase of CTA concentration led to polymers with lower PDI. The variation of enthalpy and entropy relative to the monomer reaction were calculated, and it was observed that an increase in the brancher concentration induced an increase in both and , whilst lower CTA concentrations led to an increase in . The variation in Gibbs energy for the monomer reaction was calculated at 60 °C, and results confirmed the presence of a retardation effect when increasing CTA concentration generally observed in RAFT polymerization.     

Kinetics of RAFT emulsion polymerization of styrene mediated by oligo(acrylic acid‐b‐styrene) trithiocarbonate

The kinetics of ab initio reversible addition‐fragmentation chain transfer (RAFT) emulsion polymerization of styrene using oligo(acrylic acid‐b‐styrene) trithiocarbonate as both polymerization mediator and surfactant were systematically investigated. The initiator concentration was set much lower than that in the conventional emulsion polymerization to significantly suppress the irreversible termination reaction. It was found that decreased rapidly but the nucleation efficiency of micelles increased with the decrease of the initiator concentrations due to the significant radical exit. The particle number ( ) did not follow the classic Smith–Eward equation but was proportional to [I]^?0.4[S]^0.7. It was suggested that RAFT emulsion polymerization could be fast enough for commercial use even at extremely low initiator concentrations and low macro‐RAFT agent concentrations due to the higher particle nucleation efficiency at lower initiator concentration. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2126–2134, 2016     

Reversible addition fragmentation chain transfer polymerization of 3-[tris(trimethylsilyloxy) silyl] propyl methacrylate

Reversible addition fragmentation chain transfer (RAFT) bulk polymerizations of 3-[tris(trimethylsilyloxy)silyl] propyl methacrylate (TRIS) have been carried out at 60 °C, employing cumyl dithiobenzoate (CDB) and 2-cyanoprop-2-yl dithiobenzoate (CPDB) as mediating agents at concentrations ranging from 5.0×10⁻³ to 2.0×10⁻² mol l⁻¹. The monomer conversion vs. time evolution was followed via dilatometry and ¹H NMR spectroscopy. The CDB mediated polymerization displays RAFT agent concentration dependent inhibition and rate retardation phenomena, whereas the CPDB mediated polymerization process is less susceptible to rate retardation and inhibition effects. The different behavior of CDB and CPDB in TRIS polymerization is most likely due to the increased stability of the intermediate macroRAFT radicals in the CDB mediated process. The generated RAFT polymers were analyzed via size exclusion chromatography indicating linear macromolecular growth with respect to monomer conversion and low polydispersities (PDI<1.15) up to high monomer to polymer conversion (>90%).     

Reversible addition‐fragmentation chain transfer (RAFT) polymerization of styrene using novel heterocycle‐containing chain transfer agents

BACKGROUND: Controlled/‘living’ radical polymerization is a new and robust method to synthesize polymers with predetermined molecular weight, narrow polydispersity and tailored architecture. Several methods have been developed but reversible addition‐fragmentation chain transfer (RAFT) has several advantages over the other methods. It has been reported that the effectiveness of RAFT agents depends strongly on the nature of the Z and R groups. RESULTS: Three new dithiocarbamates, namely (2‐ethoxy carbonyl)‐prop‐2‐yl‐pyrrole‐1‐carbodithioate (CTA‐A), (1‐phenyl ethyl)‐pyrazole‐1‐carbodithioate (CTA‐B) and (2‐ethoxy carbonyl)‐prop‐2‐yl‐pyrazole‐1‐carbodithioate (CTA‐C), were synthesized for studying the effect of the Z and R group of a chain transfer agent on the RAFT polymerization of styrene, initiated by 2,2′‐azobisisobutyronitrile. Well‐controlled molecular weight with narrow polydispersity (1.10–1.46) was achieved. The increase in molecular weight with conversion is linear and follows first‐order kinetics. CONCLUSION: The detailed kinetic results show that the structure of the activating (Z) group of dithiocarbamates has significant effects on the reactivity of dithiocarbamates towards the polymerization of styrene. In the homopolymerization of styrene it was found that, from the polydispersity index of polystyrenes obtained and the kinetic results, the pyrazole‐based dithiocarbamates (CTA‐B and CTA‐C) are very effective compared to the pyrrole‐based dithiocarbamate (CTA‐A). All the polymerizations show controlled living characters. Copyright © 2007 Society of Chemical Industry     

Reversible addition–fragmentation chain transfer polymerization of styrene with benzoimidazole dithiocarbamate as a reversible addition–fragmentation chain transfer agent

Two novel dithiocarbamates [2‐Y‐benzoimidazole‐1‐carbodithioic acid benzyl esters: Y = methyl (1b) or phenyl (1c)] were synthesized and successfully used in the reversible addition–fragmentation chain transfer (RAFT) polymerization of styrene in bulk with thermal initiation. The effects of the temperatures and concentration ratios of the styrene and RAFT agents on the polymerization were investigated. The results showed that the polymerization of styrene could be well controlled in the presence of 1b or 1c. The linear relationships between ln([M]₀/[M]) and the polymerization time (where [M]₀ is the initial monomer concentration and [M] is the monomer concentration) indicated that the polymerizations were first‐order reactions with respect to the monomer concentration. The molecular weights increased linearly with the monomer conversion and were close to the theoretical values. The molecular weight distributions [weight‐average molecular weight/number‐average molecular weight (M_w/M_n)] were very narrow from 5.3% conversion up to 94% conversion (M_w/M_n < 1.3). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 560–564, 2006     

A facile strategy for the fabrication of highly stable superhydrophobic cotton fabric using amphiphilic fluorinated triblock azide copolymers

Amphiphilic triblock azide copolymers containing poly(ethylene glycol) (PEG) and poly(2,2,3,4,4,4-hexafluorobutyl acrylate) blocks have been synthesized through room temperature RAFT polymerization using redox initiation and were successfully used to fabricate superhydrophobic cotton fabric by a facile approach. The copolymers were covalently attached to the surface of the cotton fabric by the reaction of azide groups with the cotton fibres based on nitrene chemistry via UV irradiation. Due to introducing the fluorinated polymer chains, the cotton fabric was transformed from hydrophilicity to superhydrophobicity with a water contact angle of 155°. Since the fluorinated polymer chains were covalently attached on the surface of the cotton fabric, the superhydrophobic cotton fabric possessed high stability and chemical durability.     

Effect of monomer composition on apparent chain transfer coefficient in reversible addition fragmentation transfer (RAFT) copolymerization

The effects of monomer composition on the apparent chain transfer coefficient (〈C_tr〉) in reversible addition fragmentation transfer (RAFT) copolymerization were investigated. The studied RAFT systems included methyl methacrylate (MMA)/butyl acrylate (BA) mediated by 1-phenylethyl phenyldithioacetate (PEPDTA) (i.e. MMA/BA-PEPDTA), MMA/BA by 2-cyanoprop-2-yl dithiobenzoate (i.e. MMA/BA-CPDTB), and styrene (St)/BA by benzyl dithioisobutyrate (i.e. St/BA-BDTiB). The R groups of the RAFT agents were first converted to the corresponding copolymer oligomers having the same composition to facilitate the measurement of the main RAFT equilibrium transfer coefficients. It was found that there exist minimum values in the 〈C_tr〉 ∼ f₁ curves in MMA/BA-CPDTB and St/BA-BDTiB at f₁ = 0.75 and 0.25, respectively. The apparent transfer coefficients of the copolymerization systems within some composition range were lower than their homopolymerization values. The lower 〈C_tr〉 values resulted in broader copolymer molecular weight distributions. The composition dependence of 〈C_tr〉 was determined by the comonomer reactivity ratios and the Z group functionality of the RAFT agent. The experimental data could be well described by a simple equation derived from the terminal model:

     

Tuning the Mechanical Properties of Multiblock Copolymers Generated by Polyfunctional RAFT Agents

When employing polyfunctional reversible addition–fragmentation chain transfer (RAFT) agents (e.g., polytrithiocarbonates) in a reversible‐deactivation radical polymerization, a redistribution of the RAFT groups being connected to polymer segments occurs, which leads to a characteristic distribution of blocks in the polymer. The authors show that by adding bifunctional RAFT agents to such a system, the average number of blocks and their distribution may be tailored, proving that in principle any RAFT agent may be combined with a polyfunctional RAFT agent to tailor its topology. The authors thus add star‐shaped RAFT agents and develop multiblock copolymers of styrene and n‐butyl acrylate having incorporated star‐shaped topological features and investigate the materials via tensile testing. Using this novel mixing approach, the material toughness is substantially increased compared to multiblock copolymers obtained from pure polyfunctional RAFT agent, and stress whitening is prevented. Importantly, the approach yields copolymers with a significantly higher toughness compared to conventional blends of star and multiblock copolymers.

     

An in-depth analytical approach to the mechanism of the RAFT process in acrylate free radical polymerizations via coupled size exclusion chromatography-electrospray ionization mass spectrometry (SEC-ESI-MS)

Coupled size exclusion chromatography (SEC)-electrospray ionization mass spectrometry (ESI-MS) was applied to carefully map the product spectrum of a series of acrylate free radical polymerizations mediated via the reversible addition fragmentation chain transfer (RAFT) process. The product stream of a significantly rate retarded RAFT system (i.e. n-butyl acrylate (BA)/cumyl dithiobenzoate (CDB)) was compared with the less rate retarded RAFT polymerizations of BA mediated by cumyl phenyl dithioacetate (CPDA) and methyl acrylate (MA)/CPDA. In each case excellent agreement between the theoretical and experimental masses, as well as the simulated isotopic peak distributions, of polymeric species in the product stream was observed. Although conventional disproportionation and combination bimolecular termination products were clearly identified within the product spectra, the presence of irreversibly terminated RAFT intermediates, i.e. 3-armed star polymers, was not observed. The mass spectroscopic results are compared to modeling estimations (carried out via the PREDICI^® program package) of the concentration ratios of 3-armed stars vs. conventional termination products. It is demonstrated that the occurrence of conventional termination products should be accompanied by a significant product stream associated with 3-armed star polymer material if cross termination was operational—at least under the current reaction conditions. The absence of three armed star polymer products in the polymers stream suggests that irreversible cross termination reactions may be of minor importance in the present systems.     

Modeling RAFT polymerization kinetics via Monte Carlo methods: cumyl dithiobenzoate mediated methyl acrylate polymerization

Cumyl dithiobenzoate (CDB) mediated methyl acrylate (MA) bulk polymerizations at 80 °C, using CDB concentrations between 1.5×10⁻² and 5.0×10⁻² mol L⁻¹, were modeled via a novel Monte Carlo simulation procedure with respect to experimental time-dependent conversions, X, number average molecular weights, M_n, and weight average molecular weights, M_w. The simulations were based upon individual treatment of 5×10⁸ discrete molecules in accordance to their actual reaction pathways. The kinetic scheme employed includes termination reactions of intermediate RAFT radicals with propagating radicals and reaction steps of the RAFT pre-equilibrium, which are different from those of the RAFT main equilibrium. The equilibrium constant of the main equilibrium of the CDB/MA system at 80 °C was found to be K=1.2×10⁴ L mol⁻¹, indicating a relatively stable intermediate radical. The concentration of the intermediate RAFT radical, although not employed as experimental input data for the modeling, was calculated by using the obtained set of kinetic parameters as being in excellent agreement with experimental electron spin resonance spectroscopic data.     

Synthesis of polydimethylsiloxane‐containing block copolymers via reversible addition fragmentation chain transfer (RAFT) polymerization

Low polydispersity polydimethylsiloxane (PDMS) was end functionalized with a reversible addition fragmentation chain transfer (RAFT) agent by the esterification of hydroxyl terminated PDMS with a carboxylic acid functional RAFT agent. These PDMS‐RAFT agents were able to control the free radical polymerization of styrene and substituted styrene monomers to produce PDMS‐containing block copolymers with low polydispersities and targeted molecular weights. A thin film of polydimethylsiloxane‐block‐polystyrene was prepared by spin coating and exhibited a microphase separated morphology from scanning force microscopy measurements. Controlled swelling of these films in solvent vapor produced morphologies with significant long‐range order. This synthetic route will allow the straightforward production of PDMS‐containing block copolymer libraries that will be useful for investigating their thin film morphological behavior, which has applications in the templating of nanostructured materials.© 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Surface grafting of poly(pentafluorostyrene) on the iron and iron oxide particles via reversible addition fragmentation chain transfer (RAFT) polymerization

A surface grafting technique is reported for synthesis of poly(pentafluorostyrene) via reversible addition fragmentation chain transfer onto iron (iron oxide) particles. 4‐Methoxydithiobenzoate is used for the RAFT chain transfer agent. The molecular weight, surface morphology, thickness, thermal properties, and monomer conversion of the grafted polymer are reported. The grafted poly(pentafluorostyrene)–iron particles show a higher thermal transition temperature compared to the nongrafted polymer because it is speculated that the covalent bond between the polymer backbone and the surface of the iron particles restricts the molecular mobility. The monomer conversion increases in proportion to the amount of chain transfer agent (CTA) concentration at early polymerization time. The grafted poly(pentafluorostyrene) shows a “hairy” like polymer architecture with fibril thickness in the range of 80 to 100 nm. A thin coating is expected to maintain the magnetic saturation properties of iron particles. To the best of our knowledge, this is the first time that poly(pentafluorostyrene) has been grafted onto the iron particles utilizing RAFT and 4‐methoxydithiobenzoate as a CTA. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44898.     

Synthesis of thermoresponsive block and graft copolymers via the combination of living cationic polymerization and RAFT polymerization using a vinyl ether-type RAFT agent

A novel vinyl ether-type RAFT agent, benzyl 2-(vinyloxy)ethyl carbonotrithioate (BVCT) was synthesized for various block copolymers via the combination of living cationic polymerization of vinyl ethers and reversible addition−fragmentation chain transfer (RAFT) polymerization. The novel BVCT–trifluoroacetic acid adduct play an important role to produce well-defined block copolymers, which is both as a cationogen under EtAlCl₂ initiation system in the presence of ethyl acetate for living cationic polymerization and a RAFT agent for blocks by RAFT polymerization. The resulting polymer, poly(vinyl ether)s, by living cationic polymerization had a high number average α-end functionality (≥0.9) as determined by both ¹H NMR and MALDI-TOF-MS spectrometry. In addition, this poly(vinyl ether)s worked well as a macromolecular chain transfer agent for RAFT polymerization. The RAFT polymerization of radically polymerizable monomers was conducted in toluene using 2,2′-azobis(isobutyronitrile) at 70 °C. For example, a double thermoresponsive block copolymer (MOVE₆₁-b-NIPAM₁₅₀) consisting of 2-methoxyethyl vinyl ether (MOVE) and N-isopropylacrylamide (NIPAM) was prepared via the combination of living cationic polymerization and RAFT polymerization. The block copolymer reversibly formed and deformed micellar assemblies above the phase separation temperature (T_ps) of poly(NIPAM) block in water. This BVCT is not only functioned as an initiator, but also acted as a monomer. When BVCT was copolymerized with MOVE by living cationic polymerization, followed by graft copolymerization with NIPAM via RAFT polymerization, well-defined graft copolymers (MOVE_n-co-BVCT_m)-g-NIPAM_x (n = 62–73, m = 1–9, x = 19–214) were successfully obtained. However, no micelle formed in water above T_ps of poly(NIPAM) graft chain unlike the case of block copolymers.     

Living Radical Polymerization and Molecular Imprinting: Improving Polymer Morphology in Imprinted Polymers

Living radical polymerization (LRP) techniques and their ability to improve the morphology of crosslinked polymer networks by controlling polymer chain growth are reviewed. Recent successes in the creation of improved molecularly imprinted polymer networks are also discussed. LRP offers the ability to control molecular weight, polydispersity, and tacticity while reducing microgel formation in polymers created via free‐radical polymerization (FRP). The improved network architecture of polymers created via LRP has great potential, especially when considering imprinted networks which have traditionally been plagued by heterogeneity in network morphology and binding affinities. Using LRP can considerably improve template recognition and further delay template transport in imprinted polymers.

     

Modeling and theoretical development in controlled radical polymerization

Controlled radical polymerization (CRP) systems have gained increasing interests for the past two decades. Numerous publications may be found in the literature reporting experimental and modeling work on various CRP processes, including their use in surface modification through grafting. Knowledge of underlying mechanism behind polymerization systems is valuable for product design and process optimization. This information may be obtained through the combination of modeling and experimental studies. In this review, published studies on kinetic and stochastic based modeling for CRP systems are summarized. Their relevance in model discrimination of proposed mechanisms is discussed. This review also includes various parameter estimation studies, that is crucial to obtain accurate simulation predictions. Existing issues on the fundamental mechanism in CRP processes are also addressed.     

Living free radical polymerization (RAFT) of dodecyl acrylate: Chain length dependent termination, mid-chain radicals and monomer reaction order

The reversible addition fragmentation chain transfer-chain length dependent-termination (RAFT-CLD-T) methodology was employed to map chain length dependent termination rate coefficient, , in dodecyl acrylate (DA) free radical polymerization at 60 and 80 °C. The chain length of the propagating DA radicals was controlled by the RAFT agents methoxycarbonylethyl phenyldithioacetate (MCEPDA) and dimethoxycarbonylethyl trithiocarbonate (DMCETC). In addition, the reaction order of the polymerization process with respect to the monomer concentration was determined at both temperatures and found to be close to 1.55 (60 °C) and 1.75 (80 °C), commensurate with the increased presence of mid-chain radicals. A modeling study demonstrates that the obtained data for the reaction order can be transferred to RAFT polymerization systems. The RAFT-CLD-T procedure was modified to account for the determined reaction orders. The obtained chain length dependence of k_t in dodecyl acrylate polymerizations is in good agreement with the composite model for chain length dependent termination, showing two distinct regions: For the initial chain-length regime up to a degree of polymerization of 20, k_t decreases rapidly with α (in the expression ) being close to 1.15 at 80 °C. At chain lengths exceeding 20, the decrease is significantly less pronounced (α close to 0.22 at 80 °C). At 60 °C, the chain length dependence in both regions is somewhat more pronounced. The RAFT agent DMCETC may not be as suited to map out CLD k_t values in the DA system, since it induces some limited rate retardation effects.     

Poly(N-isopropylacrylamide) hydrogels with improved shrinking kinetics by RAFT polymerization

Functional poly(N-isopropylacrylamide) (PNIPAM) hydrogels were prepared by reversible addition fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) in the presence of N,N-methylenebisacylamide (BIS) as a cross-linker and 4-cyanopentanoic acid dithiobenzoate as chain transfer reagent (CTA). The swelling behaviors were investigated and the hydrogels by RAFT polymerization (RAFT gels) showed accelerated shrinking kinetics and higher swelling ratio comparing with conventional hydrogel (CG). It could be attributed to the presence of dangling chains mainly caused by CTA, which could retard the crosslinking reaction rate greatly. Another CTA, 3-(trithiocarbonyl) propanoic acid, was adopted to further investigate the effect of CTA. It showed the similar effect except the different accelerated degree to the shrinking kinetics. Furthermore, the living character of the RAFT process was used to polymerize a new batch of monomer (NIPAM) from functional RAFT gels to introduce grafted structure. The PNIPAM-g-PNIPAM hydrogels indicted further accelerated shrinking kinetics than functional backbone hydrogels.