Reversible addition fragmentation transfer (RAFT) polymerization of styrene in a miniemulsion: A mechanistic investigation

The RAFT polymerization of styrene in miniemulsion using 1-phenylethyl phenyl-dithioacetate (PEPDTA) as a RAFT agent was investigated, in attempt to reveal the mechanism for the often observed inferior performance such as low polymerization rate, broad molecular weight distribution and particle size distribution in the RAFT miniemulsion polymerization with regular levels of surfactant and co-stabilizer (1 wt% sodium dodecyl sulfate and 2 wt% hexadecane). It is strongly evident that a few of large oligomer particles consisting of oligomer, RAFT agent (RAFT agent refers to the original RAFT agent), and monomer would be formed in the early stage of the polymerization due to the superswelling of the first nucleated droplets. With the regular levels of surfactant and co-stabilizer, the observed low polymerization rate, broadened molecular weight distribution, slow conversion of the RAFT agent, lower N_p, and broadened particle size distribution could be well explained by the formation of these large oligomer particles and their prolonged existence. When the formation of the oligomer particles was suppressed by increasing surfactant and co-stabilizer levels and wise selection of types of RAFT agent, the molecular weight distribution could be narrowed to around 1.3 and particle size distribution could be close to that of the conventional non-living miniemulsion polymerization.     

Comparison of RAFT polymerization of methyl methacrylate in conventional emulsion and miniemulsion systems

In this study, we have conducted the reversible addition-fragmentation chain transfer (RAFT) polymerization of methyl methacrylate (MMA) in two heterogeneous systems, i.e. conventional emulsion and miniemulsion, with identical reaction conditions. The main objective is to compare the living character in both systems according to the nucleation mechanism, the latex stability, the particle sizes and particle size distributions of latexes, the molecular weights and molecular weight distributions (or polydispersity index, PDI) of PMMA, and the kinetics of the RAFT polymerization. The RAFT agent used in both systems was 2-cyanoprop-2-yl dithiobenzoate (CPDB). The effects of an oil-soluble initiator 2,2′-azobisisobutyronitrile (AIBN) and a water-soluble initiator kalium persulfate (KPS) on the RAFT/emulsion and RAFT/miniemulsion polymerizations were investigated. Methyl-β-cyclodextrin (Me-β-CD) was used as a solubilizer. The average molecular weights and molecular weight distributions (PDIs) of dried PMMA samples were characterized by gel permeation chromatography (GPC). The experimental results showed that the RAFT/miniemulsion polymerization of MMA exhibited better living character than that of RAFT/emulsion polymerization under the conditions of our experiment. The PDI of PMMA in RAFT/miniemulsion polymerization was decreased with the addition of Me-β-CD. However, Me-β-CD did not have influence on the PDI of PMMA prepared in RAFT/emulsion polymerization.     

Miniemulsion copolymerization of styrene and butyl acrylate initiated by redox system at lower temperature: Reaction kinetics and evolution of particle‐size distribution

The kinetics of the miniemulsion copolymerization of styrene (St) and butyl acrylate (BA) initiated by redox initiators, (NH₄)₂S₂O₈/NaHSO₃, at lower temperature (45°C) was studied. The polymerization rate in miniemulsion copolymerization is lower than that of the corresponding conventional emulsion copolymerization. In regard to the rate of polymerization, the initiator concentration plays a more important role in miniemulsion copolymerization than in conventional emulsion polymerization, while the surfactant concentration has a more important role in conventional emulsion polymerization than in miniemulsion polymerization. These are attributed to their different nucleation mechanisms, which are the same as those found in the miniemulsion polymerization carried out at higher temperatures. While by eliminating nucleation via micelle and ensuring against homogeneous nucleation, miniemulsion polymerization can be carried out by the sole nucleation mechanism—monomer droplet nucleation—at lower temperature. Because of this, the particles become narrower during the polymerization and, finally, monodisperse polymer particles are obtained. The result of the particle numbers indicated that a continuous nucleation will cease at about 60% conversion. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 315–322, 1999     

Comparison of styrene reversible addition–fragmentation chain‐transfer polymerization in a miniemulsion system stabilized by ammonlysis poly(styrene‐alt‐maleic anhydride) and sodium dodecyl sulfate

In this study, we conducted the reversible addition–fragmentation chain‐transfer (RAFT) polymerization of styrene (St) in a miniemulsion system stabilized by two different stabilizers, ammonlysis poly(styrene‐alt‐maleic anhydride) (SMA) and sodium dodecyl sulfate (SDS), with identical reaction conditions. The main objective was to compare the polymerization kinetics, living character, latex stability, and particle morphology. The macro‐RAFT agent used in both systems was SMA, which was obtained by RAFT solution polymerization mediated by 1‐phenylethyl phenyldithioacetate. The experimental results show that the St RAFT miniemulsion polymerization stabilized by SDS exhibited a better living character than that stabilized by ammonlysis SMA. The final latices were very stable in two systems, but different stabilizers had an obvious effect on the polymerization kinetics, living character, and particle morphology. All of the particles obtained by RAFT miniemulsion polymerization stabilized by SDS were solid, but an obvious core–shell structure was observed in the miniemulsion system stabilized by ammonlysis SMA. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Kinetics of reversible addition–fragmentation transfer (RAFT) miniemulsion polymerization of styrene using dibenzyl trithiocarbonate as RAFT reagent and costabilizer

The roles of dibenzyl trithiocarbonate (DBTTC) as both costabilizer and reversible addition–fragmentation transfer (RAFT) reagent in RAFT miniemulsion polymerizations of styrene were investigated. The effectiveness of DBTTC costabilizer in retarding Ostwald ripening involved in the storage stability of miniemulsion is comparable to that of conventional low‐molecular‐weight costabilizers such as cetyl alcohol, but inferior to that of hexadecane. The major variables chosen for studying kinetics of RAFT miniemulsion polymerizations include the type of initiators and levels of DBTTC and surfactant. At a constant level of DBTTC, the rate of polymerization for benzoyl peroxide (BPO)‐initiated polymerization is slower than that for sodium persulfate (SPS)‐initiated polymerization. Furthermore, the polymerization rate decreases with increasing level of DBTTC for polymerizations initiated by BPO (or SPS). It is the monomer droplet nucleation that governs BPO‐initiated polymerizations. In contrast, for SPS‐initiated polymerizations, the probability for homogeneous nucleation to take place is greatly increased, especially for polymerizations with lower levels of DBTTC and higher levels of surfactant. © 2015 Society of Chemical Industry     

Kinetics of UV‐initiated RAFT crosslinking polymerization of dimethacrylates

The UV‐initiated RAFT polymerizations of a series of poly(ethylene glycol) dimethacrylates (PEGDMA) were investigated using differential scanning photocalorimetry (DPC) at room temperature. The rate of the RAFT system was much lower than that of a conventional free radical polymerization. A mild autoacceleration occurred as the addition reaction became diffusion controlled. The influence of the spacer length (CH₂CH₂O)_x between the vinyl moieties of the dimethacrylates on the polymerization kinetics was examined. The polymerization rate of PEGDMA decreased with an increased x value from 4 to 9, but it increased with a further increased x value from 9 to 14. Mechanical properties of the resulting polymers were also examined by dynamic mechanical analysis (DMA). It was concluded that the presence of the RAFT agent during polymerization of multifunctional monomers did not have an effect on the heterogeneity of the polymer network. In comparison with three different PEGDMAs, the PEGDMA with the longest spacer formed the most homogeneous networks with a lower crosslinking density. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis and characterization of PMMA/SiO2 organic–inorganic hybrid materials via RAFT‐mediated miniemulsion polymerization

In this article, we first carried out the surface modification of SiO₂ using silane coupling agent KH570, and then prepared PMMA/SiO₂ organic–inorganic hybrid materials by conventional free radical polymerization and RAFT polymerization in miniemulsion, respectively. The kinetics comparisons of these two polymerizations were studied. PMMA/SiO₂ hybrid materials were characterized by gel permeation chromatography, differential scanning calorimetry and thermogravimetric analysis. Experimental results indicated that the polymerization behavior of MMA in miniemulsion showed controlled/living radical polymerization characteristics under the control of RAFT agent. Incorporation of RAFT agent and SiO₂ nanoparticles improved the thermal properties of polymers, the thermal stability of polymers increased with increasing content of SiO₂ nanoparticles. The structures and morphologies of SiO₂, modified SiO₂, and PMMA/SiO₂ hybrid materials were characterized by FT‐IR and TEM. TEM results showed that the addition of modified SiO₂ nanoparticles to miniemulsion polymerization system obtained different morphology latex particles. Most of modified SiO₂ nanoparticles were wrapped by polymer matrix after polymerization. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers     

RAFT miniemulsion polymerization of methyl methacrylate

It has been well documented that RAFT miniemulsion polymerization has broader molecular weight distribution, compared with its bulk polymerization counterpart. Interestingly, it was found that the PDI value of RAFT miniemulsion polymerization of methyl methacrylate (MMA) mediated by 2-cyranoprop-2-yl dithiobenzoate (CPDB) was still as low as its corresponding bulk polymerization did. PDI could be as low as 1.13 even with typical sodium dodecyl sulfate (SDS, 1 wt%, surfactant) and n-hexadecane (HD, 2 wt%, costablizer) concentrations. When the polymerization was carried out at 60 °C, a dramatic increase in PDI (>1.4) was observed after 80% monomer conversion since RAFT addition reaction became diffusion-controlled. Increasing the polymerization temperature to 80 °C could reduce the PDI to 1.2 even at 100% monomer conversion. The compartmentalization effect of radicals was surprisingly absence before 30% monomer conversion but became pronounced afterwards in the miniemulsion polymerization. Thus, it still took less time to finish the miniemulsion polymerization with the increase of the surfactant levels.     

Miniemulsion vs. conventional emulsion polymerization for pressure-sensitive adhesives production

A systematic study of the production of poly(2-ethyl hexyl acrylate/methyl methacrylate/acrylic acid) pressure-sensitive adhesives (PSAs) via conventional emulsion and miniemulsion polymerization was carried out in order to discern and compare the influence of copolymer composition, chain transfer agent (CTA) and surfactant concentrations on the kinetics and microstructure of the resulting adhesive films. The results showed that miniemulsion polymerization proceeded solely under droplet nucleation for a certain set of initial reaction conditions. The miniemulsion-based latexes presented a polymer microstructure that was different to that found in latexes produced by conventional emulsion polymerization, as observed in the frequency master curves and PSA performance analysis. Batch miniemulsion polymerization was able to produce films with entanglement densities (M_w/M_e) from 2 up to 11, which were strongly correlated with loop tack, peel strength and shear strength. Conversely, under the same reaction conditions, batch conventional emulsion polymerization was only able to produce M_w/M_e ratios below 2.     

细乳液聚合研究进展

在简单介绍细乳液制备方法的基础上,重点综述了细乳液中各组分(如单体、乳化剂、助稳定剂、引发剂、聚合物等)对细乳液的稳定性及对细乳液聚合行为的影响。     

Preparation of star polymers based on polystyrene or poly(styrene-b-N-isopropyl acrylamide) and divinylbenzene via reversible addition-fragmentation chain transfer polymerization

Star polymers based on styrene/divinyl benzene (St/DVB) and PSt-b-poly(N-isopropyl acrylamide) (NIPAAM)/DVB have been successively prepared by ‘arm-first’ method via reversible addition-fragmentation chain transfer (RAFT) polymerization. The linear macro RAFT agent PSt-SC(S)Ph was prepared by RAFT polymerization of St using benzyl dithiobenzoate and AIBN as RAFT agent and initiator. Successive RAFT polymerization of NIPAAM with PSt-SC(S)Ph as macro RAFT agent to afford diblock copolymer, PSt-b-PNIPAAM-SC(S)Ph. The coupling reactions of PSt-SC(S)Ph or PSt-b-PNIPAAM-SC(S)Ph in the presence of DVB produced the star copolymers, C(PSt)_n or C(PSt-b-PNIPAAM)_n. The molar ratio of DVB/PSt-SC(S)Ph and polymerization time influenced the yields, molecular weight and distribution of the star-shaped polymers, which was characterized by ¹H NMR and IR spectra, GPC measurements as well as DLS.     

Synthesis of styrene based liquid-filled polymeric nanocapsules by the use of RAFT-mediated polymerization in miniemulsion

Nanocapsules (below 100 nm) with liquid cores and molar mass controlled polystyrene shells were synthesized by an in situ miniemulsion polymerization reaction in the presence of a RAFT (reversible addition fragmentation chain transfer) agent. The formation of structured particles with the targeted core/shell morphology, i.e. liquid cores and polymeric shells, is extremely dependent on the type of RAFT agent used in conjunction with the type of initiator used. Different RAFT agents lead to different polymerization rates, thus resulting in different chain lengths as a function of time. This will influence viscosity and consequently chain mobility and can therefore cause a deviation from the desired morphology. The type of initiator used influences the surface activity of entering oligomers and is therefore also an important factor in obtaining the correct structure. Results showed that a RAFT agent that causes no rate retardation (phenyl 2-propyl phenyl dithioacetate, PPPDTA), used in conjunction with a surface active initiating species (potassium persulfate, KPS), is able to lock the locus of polymerization at the droplet/water interface. This results in entering oligomers being anchored at the droplet/water interface with consequent core/shell (nanocapsule) formation only if the RAFT agent used leads to a sufficiently rapid increase in chain length with time and thus a restriction of chain mobility of the mediated species.     

Miniemulsion copolymerizations of styrene and methyl methacrylate in the presence of reactive costabilizer

Miniemulsion stability of three‐component disperse phase systems comprising styrene [ST (1)], methyl methacrylate [MMA (2)], and stearyl methacrylate [SMA (3)] was investigated. The Ostwald ripening rate (ω) increases with increasing MMA content in the monomer mixture. The empirical equation 1 /ω = k(φ₁/ω₁ + φ₂/ω₂) + φ₃/ω₃ was proposed to adequately predict the miniemulsion stability data. The empirical parameter k was determined to be 555.77, and the Ostwald ripening rate (ω₃) and water solubility of SMA were estimated to be 8.77 × 10^?21 cm³/s and 1.90 × 10^?9 mL/mL, respectively. A water‐insoluble dye was used as a molecular probe to study particle nucleation mechanisms in the miniemulsion copolymerizations. In addition to the primary monomer droplet nucleation, homogeneous nucleation also plays an important role in the formation of particle nuclei, and this mechanism becomes more important for the polymerization systems with higher MMA contents as a result of the enhanced aqueous phase polymer reactions. The polymer composition data suggest that, during the early stage of polymerization, MMA is consumed more rapidly by free radical polymerization compared with ST. The final latex particle surface potential data also support this conclusion. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

RAFT‐mediated emulsion polymerization of chloroprene and impact of chain‐end structure on chloroprene rubbers

The reversible addition‐fragmentation chain transfer (RAFT) polymerization of chloroprene (CP) in an emulsion system using a dithiocarbamate‐type RAFT agent was studied. The controlled RAFT‐mediated emulsion polymerization was achieved by the appropriate combination of a RAFT agent and nonionic surfactant (polyoxyethylene phenyl ether) using a water‐soluble initiator (VA‐044) at 35 °C. An almost linear first‐order kinetic plot was observed until relatively high conversion (>80%) with molecular weights between 22,300 and 33,100 and relatively narrow molecular weight distributions (M_w/M_n ≦ 1.5) were achieved. The amount of the emulsifier used and the pH of the system were found to affect the controlled character, polymerization rate, and induction period, which are related to the size of the emulsion particles. Large‐scale RAFT‐mediated emulsion polymerization was also employed to afford industrially applicable poly(CP) (M_w > 25 × 10⁴, resulting product > 2300 g). The vulcanized CP rubber obtained from the RAFT‐synthesized poly(CP) exhibited better physical properties, particularly tensile modulus and compression set, which may be due to the presence of the reactive end groups and the absence of low‐molecular‐weight products. We also evaluated the impact of the chain‐end structure on the mechanical and physical properties of these industrially important CP rubbers with carbon black. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46008.     

Poly(dimethylsiloxane‐b‐styrene) diblock copolymers prepared by reversible addition‐fragmentation chain transfer polymerization: Kinetic model

Well‐defined polydimethylsiloxane‐block‐polystyrene (PDMS‐b‐PS) diblock copolymers were prepared by reversible addition‐fragmentation chain transfer (RAFT) polymerization using a functional PDMS‐macro RAFT agent. The RAFT polymerization kinetics was simulated by a mathematical model for the RAFT polymerization in a batch reactor based on the method of moments. The model described molecular weight, monomer conversion, and polydispersity index as a function of polymerization time. Good agreements in the polymerization kinetics were achieved for fitting the kinetic profiles with the developed model. In addition, the model was used to predict the effects of initiator concentration, chain transfer agent concentration, and monomer concentration on the RAFT polymerization kinetics. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Preparation of poly(tert‐butyl acrylate)‐poly(acrylic acid) amphiphilic copolymers via radiation‐induced reversible addition–fragmentation chain transfer mediated polymerization of tert‐butyl acrylate

Poly(tert‐butyl acrylate) (PtBA) is a versatile hydrophobic macromolecule usually preferred in the development of new materials for a host of applications. PtBA homopolymers with well‐defined structure and controlled molecular weight in a wide range were successfully synthesized via radiation‐induced reversible addition–fragmentation chain transfer (RAFT) polymerization in the presence of a trithiocarbonate type RAFT agent. The polymerization of tBA was performed under ⁶⁰Co γ‐irradiation in the presence of 2‐(dodecylthiocarbonothioylthio)‐2‐methylpropionic acid (DDMAT) as the RAFT agent in toluene at room temperature with three [tBA]/[DDMAT] ratios (400, 600 and 1000) and different irradiation times. Radiation‐induced polymerization of tBA displayed controlled free radical polymerization characteristics: a narrow molecular weight distribution (M_w/M_n ~ 1.1), pseudo first order kinetics and controlled molecular weights. The system followed the RAFT polymerization mechanism even at very low amounts of RAFT agent ([tBA]/[DDMAT] = 1000), and molecular weights up to 113 900 with narrow dispersity (Ð =1.06) were obtained. PtBA was further hydrolysed into different amphiphilic PtBA‐co‐poly(acrylic acid) (PAA) copolymers by low (27.5%) and high (77.3%) degrees of hydrolysis. The pH sensitivity of the two copolymers was investigated by dynamic light scattering at pH 2 and pH 9 (above and below the pK_a value of PAA) and their hydrodynamic diameters and zeta potential values were determined. © 2020 Society of Chemical Industry     

黄原酸酯调控的氯乙烯的可逆-加成断裂链转移聚合

以三种不同结构的黄原酸酯为调控剂,进行氯乙烯（VC）溶液和细乳液可逆加成-断裂链转移（RAFT）聚合,发现O-乙基黄原酸丙酸乙酯对VC聚合的调控效果良好,氯乙烯RAFT细乳液聚合速率明显大于溶液聚合,VC聚合12 h转化率大于90%,但聚氯乙烯（PVC）的分子量分布宽于溶液聚合产物。核磁共振和紫外可见吸收光谱分析证明合成的PVC具有黄原酸酯基端基结构,结构缺陷少。含黄原酸酯基PVC可进一步调控VC及醋酸乙烯酯聚合,进行扩链或得到嵌段共聚物。结合聚合动力学,说明黄原酸酯调控的氯乙烯聚合具有活性特征。     

Living radical dispersion photopolymerization of styrene by a reversible addition-fragmentation chain transfer (RAFT) agent

In this study, an addition-fragmentation chain transfer agent bearing dithioester group is synthesized and applied to conventional dispersion photopolymerization of styrene in ethanol medium in the presence of poly(N-vinylpyrrolidone) stabilizer with varying amounts of the RAFT agent and optionally with conventional initiator, azobisisobutyronitril (AIBN) at various temperatures. Monomer conversion, molecular weight evolution, polydispersity index (PDI), and final particle sizes are measured. The PDI of the formed polymer is between 1.5 and 2.5 in the presence of RAFT agent. Higher concentration of RAFT agent or elevated temperature leads to the acceleration of the polymerization rate resulting in fast conversion, and reducing molecular weight and PDI. Stable polystyrene beads above 1 μm in diameter are successfully prepared by means of RAFT method applied in dispersion polymerization. The weight average particle sizes are between 1.08 and 2.04 μm, and the uniformity (D_w/D_n) is ranged from 1.26 to 2.51.     

Particle formation mechanism in radical polymerization in miniemulsion based on in situ surfactant formation without high energy homogenization

Conventional radical polymerization of styrene at 70 °C in aqueous miniemulsion generated using the in situ surfactant technique, without use of high energy mixing, has been investigated in detail. The surfactant potassium oleate was formed in situ by reaction between oleic acid and potassium hydroxide at the styrene/water interface. The particle formation mechanism was investigated by use of pyrene as a probe, revealing that under suitable conditions with an oil-phase initiator, particle formation occurs primarily via monomer droplet nucleation. The droplet/particle stability is however inferior to that in a typical miniemulsion generated employing ultrasonication, as manifested by a marked increase in droplet/particle size with conversion and a bimodal droplet/particle size distribution by weight. The droplet/particle stability increases with increasing amount of oleic acid, hexadecane, water, and the ratio potassium hydroxide:oleic acid, respectively.     

RAFT polymerization of acrylamide manipulated with trithiocarbonates in poly(ethylene glycol) solution

The reversible addition fragmentation chain transfer (RAFT) polymerization of acrylamide (AM) in aqueous two‐phase system was successfully carried out in polyethylene glycol (PEG) aqueous solution. Because of phase transition involved in the polymerization process, the ln([M]₀/[M])‐time plots were indicated in two‐stages significantly. Both the initial homogeneous polymerization and the subsequent heterogeneous polymerization were under good control. The effects of various synthesis parameters such as polymerization temperature, concentration of CTA, and initiator on RAFT polymerization behaviors have been investigated. Furthermore, the evolution process of the droplet morphologies after separation was examined by transmission electron microscope. The results showed that the nuclei were formed throughout the whole heterogeneous polymerization and stable sphere particles with an average size of about 1 μm were produced finally. More importantly, it was also found that the viscosity played a significant role in the stabilization of the dispersion of polymer particles. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43000.