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     

基于RAFT聚合策略合成功能化聚烯烃嵌段聚合物的研究进展

首先介绍了可逆加成-断裂链转移聚合（RAFT）的聚合机理及其常用的RAFT试剂,并与其它两种活性可控自由基聚合[氮氧化合物媒介的自由基聚合（NMP）和原子转移自由基聚合（ATRP）]进行了简单的优缺点对比。其次,介绍了近些年在基于RAFT聚合制备功能化聚烯烃嵌段聚合物研究中取得的进展,重点综述了制备功能化聚烯烃嵌段聚合物时所采用的6种方法,包括①烯烃配位聚合与RAFT聚合相结合;②阴离子聚合与RAFT聚合相结合;③阳离子聚合与RAFT聚合相结合;④Click反应与RAFT聚合相结合;⑤开环聚合与RAFT聚合相结合;⑥叶立德活性聚合与RAFT聚合相结合。最后,对基于RAFT聚合策略设计合成功能化聚烯烃嵌段聚合物的研究前景与实际应用进行了展望。     

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     

Functionalization of poly(ester‐urethane) surface by radiation‐induced grafting of N‐isopropylacrylamide using conventional and reversible addition–fragmentation chain transfer‐mediated methods

The functionalization of poly(ester‐urethane) (PUR) surface was conducted using radiation‐induced grafting. A thermosensitive layer constructed from N‐isopropylacrylamide (NIPAAm) was introduced onto a polyurethane film and characterized using attenuated total reflection Fourier transform infrared and X‐ray photoelectron spectroscopies and contact angle measurements. Size exclusion chromatography was used to analyse the PUR‐graft‐PNIPAAm copolymers and homopolymers formed in solution. Additionally, reversible addition–fragmentation chain transfer (RAFT) polymerization was performed in order to obtain PNIPAAm‐grafted surfaces with well‐defined properties. Atomic force microscopy was used to evaluate the surfaces synthesized via conventional and RAFT‐mediated grafting methods. The results of various techniques confirmed the successful grafting of NIPAAm from PUR film. © 2015 Society of Chemical Industry     

Swelling and hydrolytic degradation behaviour of pH‐responsive hydrogels of poly[(N‐isopropylacrylamide)‐co‐(methacrylic acid)] crosslinked by biodegradable polycaprolactone chains

BACKGROUND: Stimuli‐responsive hydrogels are typically obtained from non‐biodegradable monomers. The use of biodegradable crosslinkers can overcome this limitation. In this context, the main aim of this work was to use modified polycaprolactone as a crosslinker in the preparation of pH‐responsive hydrogels based on N‐isopropylacrylamide and methacrylic acid to give poly[(N‐isopropylacrylamide)‐co‐(methacrylic acid)] (P(N‐iPAAm‐co‐MAA)). RESULTS: Poly(caprolactone) dimethacrylate macromonomer was synthesized and successfully employed as crosslinker with various ratios in the synthesis of well‐known pH‐responsive hydrogels of P(N‐iPAAm‐co‐MAA). The swelling properties of these degradable hydrogels were investigated. They practically do not swell at pH = 2, but exhibit a very high swelling capacity in distilled water and in solutions of pH = 7. In addition, degradation studies at pH = 12 showed that the hydrolysis of the ester groups in the polycaprolactone chains produces, after a relatively short time, the total solubilization of the polymer chains. CONCLUSION: The hydrogels under study have certain characteristics that could make them good candidates for use as matrices in controlled drug delivery. On the one hand, they do not swell in acid pH solution (stomach conditions) but they swell extensively at neutral pH. On the other hand, they became rapidly water soluble following degradation. Copyright © 2009 Society of Chemical Industry     

Controlled/living RAFT polymerization of N‐phenyl maleimide and synthesis of its block copolymers

The controlled/living radical polymerization of N‐phenyl maleimide (NPMI) was achieved using 2,2′‐azobisisobutyronitrile as the initiator and 2‐cyanopropyl‐2‐yl dithiobenzoate as the reversible addition‐fragmentation chain transfer agent at 75°C in dichloroethane/ethylene carbonate (60/40, w/w) mixed solvent. The block copolymers of polystyrene‐b‐polyNPMI and poly(n‐butyl methacrylate)‐b‐polyNPMI were successfully prepared by chain extension from dithiobenzoate‐terminated polystyrene and poly (n‐butyl methacrylate) to NPMI, respectively. The obtained NPMI‐based (co)polymers were characterized by gel permeation chromatography and ¹H‐NMR spectroscopy. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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     

RAFT synthesis of cellulose‐g‐polymethylmethacrylate copolymer in an ionic liquid

We demonstrate for the first time the feasibility in conducting the graft copolymerization of methylmethacrylate (MMA) with cellulose by the means of the reversible addition‐fragmentation chain transfer (RAFT) polymerization in an ionic liquid [1‐N‐butyl‐3‐methylimidazolium chloride] (BMIMCl). Cellulose was first converted to a macromolecular chain transfer agent to which MMA was grafted by RAFT in BMIMCl. The success of the occurrence of different reactions was validated by elemental analyses, Fourier transform infrared and nuclear magnetic resonance spectroscopies. The results demonstrate that the MMA polymer chains were grafted onto the cellulose while the use of the ionic liquid as a reaction medium enhanced the polymerization rate to a moderate extent. Gel permeation chromatography analysis of poly(MMA) chains cleaved from the cellulose by acidic hydrolysis indicated low polydispersity indices (ca. 1.3) that were consistent with the “living” nature of the RAFT. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Thermoresponsive sodium alginate‐g‐poly(N‐isopropylacrylamide) copolymers III. Solution properties

Stimuli‐responsive biocompatible and biodegradable materials can be obtained by combining polysaccharides with polymers exhibiting lower critical solution temperature (LCST) phase behavior, such as poly(N‐isopropylacrylamide) (PNIPAAm). The behavior of aqueous solutions of sodium alginate (NaAl) grafted with PNIPAAm (NaAl‐g‐PNIPAAm) copolymers as a function of composition and temperature is presented. The products obtained exhibit a remarkable thermothickening behavior in aqueous solutions if the degree of grafting, the concentration, and the temperature are higher than some critical values. The sol–gel‐phase transition temperatures have been determined. It was found that at temperatures below LCST the systems behave like a solution, whereas at temperatures above LCST, the solutions behave like a stiff gel, because of PNIPAAm segregation. This behavior is reversible and could find applications in tissue engineering and drug delivery systems. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

One-step synthesis of poly(2-oxazoline)-based amphiphilic block copolymers using a dual initiator for RAFT polymerization and CROP

A range of poly(2-oxazoline) (POx)-based amphiphilic block copolymers were synthesized using 4-cyano-4-(dodecylthiocarbonothioylthio)pentyl-4-methylbenzenesulfonate (CDPS) as a dual initiator for reversible addition-fragmentation chain transfer (RAFT) polymerization and cationic ring-opening polymerization (CROP) in a one-step procedure. Methyl (meth)acrylate, butyl (meth)acrylate, tert-butyl (meth)acrylate, and N-isopropylacrylamide were polymerized for the hydrophobic block, and 2-methyl-2-oxazoline and 2-ethyl-2-oxazoline were used as the hydrophilic block. RAFT polymerization and CROP proceeded independently in a controlled manner and resulted in amphiphilic block copolymers with a narrow molecular weight distribution. CDPS was found to be a useful dual initiator for the one-step synthesis of POx-based amphiphilic block copolymers via a combination of RAFT polymerization and CROP.     

Enhanced emission from Alq3 in amphiphilic block copolymers PEG‐b‐PVK‐co‐Alq3 synthesized via RAFT polymerization

A series of amphiphilic poly(N‐vinyl carbazole) (PVK) block copolymers containing tris(8‐hydroxyquinoline) aluminum (Alq₃) have been synthesized via reversible addition‐fragmentation chain transfer (RAFT) radical polymerization with trithiocarbonate terminated polyethylene glycol (PEG) as a RAFT agent. The chemical structures of the block copolymers have been identified by ¹H nuclear magnetic resonance and Fourier transform infrared spectrometer. The analysis of gel permeation chromatography indicates that the polydispersity indices of the block copolymers are lower than 1.30. The results of thermogravimetric analysis show that the decomposition temperatures of the copolymers are higher than 330 °C. The optical properties of the copolymers have been investigated and the obviously enhanced emission from Alq₃ has been found due to resonance energy transfer from PVK to Alq₃. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44573.     

Wetting behaviour and direct observation of thermally responsive polystyrene‐block‐poly(N‐isopropylacrylamide)‐block‐polystyrene electrospun fibres in aqueous environment

Electrospun fibres of thermally responsive triblock copolymer polystyrene‐block‐poly(N‐isopropylacrylamide)‐block‐polystyrene were prepared. Fibre morphology and swelling were studied below and above the lower critical solution temperature of poly(N‐isopropylacrylamide) (PNIPAM) using cryo‐electron microscopy. Cryo‐transmission electron microscopy showed that the fibre diameter increased up to 150% after immersion in water at 20 °C. In contrast, at 45 °C the fibre diameter increased considerably less. The sessile drop technique was used to characterize temperature‐dependent wetting of fibre mats. Contact angle (θ_CA) measurements revealed that a block copolymer fibre mat changed from hydrophobic (θ_CA > 90°) to hydrophilic (θ_CA < 90°) state within seconds after applying a water droplet on it at 20 °C. At 40 °C the initial contact angle was measured to be higher (135°) and it decreased much less than at 20 °C during the first minute of measurement. We observed using scanning electron microscopy that the electrospun fibres of the block copolymer having 77 wt% of PNIPAM lost their cylindrical shape and changed from fibres to thin sheets at both 20 and 40 °C within seconds after applying water on the fibres. Fibres having 55 wt% of PNIPAM were observed to be stable in water at both 20 and 40 °C, which resulted, surprisingly, in fibre mats with the strongest effects on thermally sensitive wetting. We discuss the surprising results and the implications that the evolution of fibre surface roughness has on the long‐term wetting behaviour, demonstrating a self‐adaptable hydrophilicity/hydrophobicity nature of the fibre mats. © 2013 Society of Chemical Industry     

Thermoresponsive behaviour in aqueous solution of poly(maleic acid‐alt‐vinyl acetate) grafted with poly(N‐isopropylacrylamide)

The synthesis of a thermoresponsive graft copolymer consisting of a maleic acid/vinyl acetate alternating copolymer backbone (MAc‐alt‐VA) and poly(N‐isopropylacrylamide) (PNIPAM) side chains is reported. Turbidimetric measurements in dilute aqueous solutions showed that no macroscopic phase separation takes place when the temperature is raised above the lower critical solution temperature (LCST) of PNIPAM, even at pH = 2. Moreover, in semi‐dilute aqueous solutions, a pronounced thermally induced viscosity increase (thermothickening) was observed. This thermoresponsive behaviour has been attributed to the interconnection of the hydrophilic MAc‐alt‐VA graft copolymer backbones by means of the hydrophobic PNIPAM side chain aggregates formed as the temperature increases above the LCST of this polymer. Copyright © 2004 Society of Chemical Industry     

Lower critical solution temperatures of thermo‐responsive poly(N‐isopropylacrylamide) copolymers with racemate or single enantiomer groups

BACKGROUND: Thermo‐responsive copolymers with racemate or single enantiomer groups are attracting increasing attention due to their fascinating functional properties and potential applications. However, there is a lack of systematic information about the lower critical solution temperature (LCST) of poly(N‐isopropylacrylamide)‐based thermo‐responsive chiral recognition systems. In this study, a series of thermo‐responsive chiral recognition copolymers, poly[(N‐isopropylacrylamide)‐co‐(N‐(S)‐sec‐butylacrylamide)] (PN‐S‐B) and poly[(N‐isopropylacrylamide)‐co‐(N‐(R,S)‐sec‐butylacrylamide)] (PN‐R,S‐B), with different molar compositions, were prepared. The effects of heating and cooling processes, optical activity and amount of chiral recognition groups in the copolymers on the LCSTs of the prepared copolymers were systematically studied. RESULTS: LCST hysteresis phenomena are found in the phase transition processes of PN‐S‐B and PN‐R,S‐B copolymers in a heating and cooling cycle. The LCSTs of PN‐S‐B and PN‐R,S‐B during the heating process are higher than those during the cooling process. With similar molar ratios of N‐isopropylacrylamide groups in the copolymers, the LCST of the copolymer containing a single enantiomer (PN‐S‐B) is lower than that of the copolymer containing racemate (PN‐R,S‐B) due to the steric structural difference. The LCSTs of PN‐R,S‐B copolymers are in inverse proportion to the molar contents of the hydrophobic R,S‐B moieties in these copolymers. CONCLUSION: The results provide valuable guidance for designing and fabricating thermo‐responsive chiral recognition systems with desired LCSTs. Copyright © 2008 Society of Chemical Industry     

Temperature‐sensitive switch from composite poly(N‐isopropylacrylamide) sponge gels

Thermally sensitive polymers change their properties with a change in environmental temperature in a predictable and pronounced way. These changes can be expected in drug delivery systems, solute separation, enzyme immobilization, energy‐transducer processes, and photosensitive materials. We have demonstrated a thermal‐sensitive switch module, which is capable of converting thermal into mechanical energy. We employed this module in the control of liquid transfer. The thermally sensitive switch was prepared by crosslinking poly(N‐isopropylacrylamide) (PNIPAAm) gel inside the pores of a sponge to generate the composite PNIPAAm/sponge gel. This gel, contained in a polypropylene tube, was inserted into a thermoelectric module equipped with a fine temperature controller. As the water flux through the composite gel changes from 0 to 6.6 × 10² L m⁻² h, with a temperature change from 23 to 40°C, we can reversibly turn on and off the thermally sensitive switch. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75:1735–1739, 2000     

Synthesis of well‐defined polystyrene‐graft‐poly(methyl methacrylate)

A well‐defined graft copolymer, polystyrene‐graft‐poly(methyl methacrylate), was synthesized in two steps. In the first step, styrene and p‐vinyl benzene sulfonyl chloride were copolymerized via reversible addition–fragmentation chain transfer polymerization (RAFT) in benzene at 60 °C with 2‐(ethoxycarbonyl)prop‐2‐yl dithiobenzoate as a chain transfer agent and 2,2′‐azobis(isobutyronitrile) as an initiator. In the second step, poly[styrene‐co‐p‐(vinyl benzene sulfonyl chloride)] was used as a macroinitiator for the atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) in toluene at 80 °C with CuCl as a catalyst and 2,2′‐bipyridine as a ligand. With sulfonyl chloride groups as the initiating sites for the ATRP of MMA, high initiation efficiencies were obtained. Copyright © 2006 Society of Chemical Industry     

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.     

A novel strategy to branched polyacrylonitrile via one‐pot RAFT copolymerization of acrylonitrile and an asymmetrical divinyl monomer

Branched polyacrylonitriles were prepared via the one‐pot radical copolymerization of acrylonitirle and an asymmetric divinyl monomer (allyl methacrylate) that possesses both a higher reactive methacrylate and a lower reactive allyl. RAFT technique was used to keep a low‐propagation chain concentration via a fast reversible chain transfer euilibration and thus the cross‐linking was prevented until a high level of monomer conversions. This novel strategy was demonstrated to engenerate a branched architecture with abundant pendant functional vinyl and nitrile groups, and controlled molecular weight as a behavior of controlled/living radical polymerization characteristics. The effect of the various experimental parameters, including temperature, brancher to monomer molar ratio, and chain transfer agent to initiator molar ratio, on the control of moleculer dimension (molecular weight and polydispersity indices) and the degree of branching were investigated in detail. Moreover, ¹H NMR and gel permeation chromatography confirm the branched architecture of the resultant polymer. The intrinsic viscosity of the copolymer is also lower than the linear counterpart.© 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Synthesis and characterization of pH‐ and temperature‐sensitive silk sericin/poly(N‐isopropylacrylamide) interpenetrating polymer networks

Interpenetrating polymer networks (IPNs) composed of silk sericin (SS) and poly(N‐isopropylacrylamide) (PNIPAAm) were prepared simultaneously. The properties of the resultant IPN hydrogels were characterized by differential scanning calorimetry and SEM as well as their swelling behavior at various temperatures and pH values. The single glass transition temperature (T_g) presented in the IPN thermograms indicated that SS and PNIPAAm form a miscible pair. The swollen morphology of the IPNs observed by SEM demonstrated that water channels (pores present in SEM micrographs) were distributed homogeneously through out the network membranes. The swelling ratio of the IPNs depended significantly on the composition, temperature and pH of the buffer solutions. The dynamic transport of water into the IPN membrane was analyzed based on the Fickian equation. Copyright © 2006 Society of Chemical Industry     

One‐step synthesis of triarm block copolymers via simultaneous reversible‐addition fragmentation chain transfer and ring‐opening polymerization

One‐step synthesis of star copolymers by reversible addition–fragmentation chain transfer (RAFT) and ring‐opening polymerization (ROP) by using a novel dual initiator is reported. Triarm block copolymers comprising one polystyrene (or polyacrylamide) arm and two poly(β‐butyrolactone) arms were synthesized in one‐step by simultaneous RAFT polymerization of styrene (St) (or acrylamide, designated as AAm) and ROP of β‐butyrolactone (BL) in the presence of a novel trifunctional initiator, 1,2‐propanediol ethyl xanthogenate (RAFT‐ROP agent). This dual initiator was obtained through the reaction of 3‐chloro‐1,2‐propanediol with the potassium salt of ethyl xanthogenate. The principal parameters such as monomer concentration, initiator concentration, and polymerization time that affect the one‐step polymerization reaction were evaluated. The characterization of the products was achieved using Fourier‐transform infrared spectroscopy (FTIR), ¹H‐nuclear magnetic resonance (¹H‐NMR), ¹³C‐nuclear magnetic resonance (¹³C‐NMR), Gas chromatography–mass spectrometry (GC–MS), gel‐permeation chromatography (GPC), thermogravimetric analysis (TGA), and fractional precipitation (γ) techniques. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010