Miniemulsion and conventional emulsion copolymerization of styrene and butadiene: A comparative kinetic study

The kinetics of conventional and miniemulsion copolymerizations of styrene and butadiene were compared using the Mettler RC1 calorimeter. A two‐step homogenization procedure was applied to obtain miniemulsions of these monomers with hexadecane as the costabilizer. The results indicated that the miniemulsion polymerizations proceeded mainly by nucleation in the monomer droplets, while in the conventional emulsion polymerizations, particle formation occurred by a combination of micellar and homogeneous nucleation. The overall rate of miniemulsion polymerization was faster than the corresponding conventional emulsion system if the surfactant concentration was below the critical micelle concentration (cmc) and slower if the surfactant concentration was above the cmc. The homogenization process is important for making stable miniemulsion systems, but had no effect on the conventional emulsion system (without hexadecane), most likely because of the second stage addition of the butadiene monomer. The dependencies of the rate of polymerization (heat of reaction) and number of particles on the surfactant concentration differed for the two types of polymerization systems. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2304–2312, 2006     

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.     

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.     

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.     

大分子链转移剂控制下的苯乙烯RAFT细乳液聚合

以十二烷基硫酸钠为乳化剂,十八烷基甲基丙烯酸酯为助乳化剂,偶氮二异丁腈为引发剂,本体聚合得到的大分子链转移剂控制苯乙烯RAFT细乳液聚合,考察聚合过程中动力学、粒径及温度对其的影响,发现聚合过程符合活性/可控自由基聚合一般规律且乳液较稳定,随反应温度升高反应速度加快。     

Monte Carlo Simulation of Droplet Nucleation in RAFT Free Radical Miniemulsion Polymerization

Abstract

The early stages of the reversible addition/fragmentation transfer (RAFT) miniemulsion polymerization were simulated, focusing on the effect of the RAFT agent on droplet nucleation. For highly reactive RAFT agents, a large number of free radicals (N_c ) needed to be captured by a droplet in order to initiate polymerization in the droplet, which was totally different from the behavior of regular miniemulsion polymerization. More interestingly, it was found that droplet size had a significant influence on N_c value. It was shown that the RAFT agent has a significant influence on miniemulsion polymerization, leading to long induction periods and retardation of polymerization. In addition, miniemulsion droplets with different sizes are nucleated at different times, which could lead to very low nucleation efficiency. The results would be very helpful in understanding and designing a RAFT miniemulsion polymerization system.     

Styrene miniemulsion polymerization stabilized by carboxylated polyurethane. II. Kinetics

A styrene miniemulsion was prepared using carboxylated polyurethane as the sole costabilizer and sodium dodecyl sulfate as the surfactant. The effects of the amount of carboxylated polyurethane, the amount of the initiator and surfactant, the presence of a water‐phase inhibitor (sodium nitrite), and the reaction temperature on the kinetics of the miniemulsion polymerization were investigated. The evolution of the particle size during the polymerization was measured. The results show that the polymerization rate was proportional to the 0.21 power of the surfactant concentration and the 0.30 power of azobisisobutyronitrile. The droplet nucleation and homogeneous nucleation were found to be coexistent in the polymerization. The hydrophility of the particle surface plays a key role in the nucleation of the particle and, therefore, has an important effect on the kinetics of the polymerization. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1941–1947, 2003     

Facile synthesis of one‐to‐one copy of monomer droplet to latex particle via equilibrium stabilized miniemulsion polymerization

Miniemulsion polymerization has been traditionally used to synthesize latex particles with a high homogenization energy to prepare an oil/water miniemulsion followed by further polymerizations. However, the exact copy of monomer droplets to latex particles depends critically on emulsion formulation, homogenization condition as well as the stability of the miniemulsified droplets after homogenization. In this study, we demonstrated experimentally for the first time that one‐to‐one copy of monomer droplets to latex particles can be synthesized via polymerization of a miniemulsion prepared from a less stringent preparation process including formulation without costabilizer and low homogenization energy. The criterion to obtain narrow size distribution of monomer droplets was established by equilibration of a low energy homogenized emulsion for different keeping time and the bulk homogenized emulsion subsequently phase separated into two layers. Top layer is the polydispersed monomer‐rich phase. The bottom layer is the equilibrium‐stabilized monomer droplet emulsion, in which the size distribution of droplets is narrow. The equilibrium‐stabilized emulsion is stable for days and subsequent polymerization exhibits nearly 100% droplet nucleation. Furthermore, the effect of surfactant/costabilizer and initiator on the one‐to‐one feature for the synthesis of latex particles was investigated in details. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Particle nucleation and growth mechanisms in miniemulsion polymerization of styrene

Styrene miniemulsion polymerizations stabilized by sodium lauryl sulfate in combination with a reactive costabilizer, lauryl methacrylate (LMA) or stearyl methacrylate (SMA), were studied. A small amount of extremely hydrophobic dye was incorporated into monomer droplets (10² nm in diameter) to investigate particle nucleation and growth mechanisms. In addition to monomer droplet nucleation, particle nuclei generated in the aqueous phase (homogeneous nucleation) also play an important role in both LMA‐ and SMA‐containing polymerization systems. The way that these two nucleation mechanisms compete with each other is closely related to the water solubility of the costabilizer (LMA > SMA). The fraction of latex particles originating from homogeneous nucleation increases with decreasing hydrophobicity of the costabilizer. Zeta potential data of latex particles and the molecular weight and molecular weight distribution of emulsion polymers provide supporting evidence for the proposed competitive particle nucleation and growth mechanisms. © 2002 Society of Chemical Industry     

Mass transfer and radical flux effects in dispersed‐phase polymerization of isooctyl acrylate

The kinetics of dispersed phase polymerization of a highly water‐insoluble monomer (isooctyl acrylate) were explored in emulsion, miniemulsion, and microsuspension polymerization. The effects of monomer water solubility and choice of initiator (oil‐ vs. water‐soluble) strongly impact the final product (particle size and molecular weight distribution). For emulsion polymerization, as the surfactant concentration was increased, there was a transition from homogenous to micellar nucleation near the CMC, then a drop in nucleation rate at high surfactant concentration due to insufficient radical flux to support more nucleation. For miniemulsion polymerization, a slow rate of growth of (droplet) nucleation with surfactant concentration was found, followed (at the CMC) by an increase in the rate of nucleation with added surfactant as the mode of nucleation switched to micellar. The conversion‐time kinetics of microsuspensions could be modeled with a bulk polymerization model. IOA is sufficiently insoluble in the aqueous phase that emulsion polymerization may or may not be reaction limited. The presence of a stabilizer such a PAA, the use of an oil‐soluble initiator such as BPO, and the insolubility of IOA in the aqueous phase all push the polymerization locus toward droplet (microsuspension) nucleation and bulk kinetics.© 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5649–5666, 2006     

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。     

Interfacially confined RAFT miniemulsion copolymerization of styrene and butadiene

In this study, ammonolyzed poly(styrene‐alt‐maleic anhydride) terminated with dithioester group can be self‐assembled into an amphiphilic macro‐reversible addition‐fragmentation chain transfer (RAFT) agent, and RAFT group will be located in the interface of oil and water. RAFT polymerization of styrene (S) and butadiene (B) will be confined in the interface. The main work is to study the effect of degree of aminolysis, reaction temperature, and ratio of S/B on the polymerization kinetics and living characters. The experimental results revealed that aminolysis of dithioester group would lead to retardation and loss of living characters under higher degree of aminolysis. Interfacially confined RAFT miniemulsion polymerizations were of relatively good controlled/living characters under lower degree of aminolysis before gelation. Increase of reaction temperature and ratio of S/B will accelerate the formation of gelation. Finally, styrene/butadiene copolymer nanoparticles with uniform particle size were formed, and because of microphase segregation “core–shell” morphology with polybutadiene core and polystyrene shell was seen obviously. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Multiblock copolymers synthesized in aqueous dispersions using multifunctional RAFT agents

Triblock copolymers were synthesized in aqueous dispersions in two polymerization steps using a low molar mass difunctional dithiocarbamate-based RAFT agent, and in merely one polymerization step using a macromolecular difunctional dithiocarbamate-based RAFT agent. Segmented block copolymers containing several alternating blocks of different polarities were synthesized in miniemulsion in merely two polymerization steps by applying multifunctional, dithiocarbamate-based RAFT agents. All polymerizations showed a linear increase of with monomer conversion. Evidence that this novel synthetic concept resulted in the formation of (multi)block copolymers was obtained by combining normal gel permeation chromatography with gradient polymer elution chromatography.     

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。     

Microemulsion polymerization kinetics and mechanisms

The mechanisms of microemulsion polymerizations stabilized by sodium dodecyl sulfate in combination with pentanol were investigated with a water‐insoluble dye as the probe. The major parameters chosen for study were the types of initiators [water‐soluble sodium persulfate (SPS) vs oil‐soluble 2,2′‐azobisisobutyronitrile (AIBN)] and the polarity of the monomers [relatively hydrophobic styrene (ST) vs relatively hydrophilic methyl methacrylate (MMA)]. Both continuous particle nucleation and limited particle flocculation had a significant influence on the polymerization kinetics. For the polymerizations investigated in this work, the relatively low initiation efficiency of AIBN resulted in a reaction system showing a quite different particle nucleation mechanism than that of the ST polymerization with SPS. The formation of particle nuclei in water was suppressed to some extent, and microemulsion droplet nucleation predominated in the ST polymerization initiated by AIBN. Homogeneous nucleation played an important role, and a mixed mode of particle nucleation (microemulsion droplet nucleation and homogeneous nucleation) was operative in the MMA polymerization. The MMA polymerization experienced stronger particle flocculation than its ST counterpart. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2005–2013, 2005     

Nucleation mechanism and morphology of polystyrene/Fe3O4 latex particles via miniemulsion polymerization using AIBN as initiator

In this study, oil‐based magnetic Fe₃O₄ nanoparticles were first synthesized by a coprecipitation method followed by a surface modification using lauric acid. Polystyrene/Fe₃O₄ composite particles were then prepared via miniemulsion polymerization method using styrene as monomer, 2,2′‐azobisisobutyronitrile (AIBN) as initiator, sodium dodecyl sulfate (SDS) as surfactant, hexadecane (HD) or sorbitan monolaurate (Span20®) as costabilizer in the presence of Fe₃O₄ nanoparticles. The effects of Fe₃O₄ content, costabilizer, homogenization energy during ultrasonication, and surfactant concentration on the polymerization kinetics (e.g., conversion), nucleation mechanism, and morphology (e.g., size distributions of droplets and latex) of composite particles were investigated. The results showed that at high homogenization energy, an optimum amount of SDS and hydrophobic costabilizer was needed to obtain composite particles nucleated predominately by droplet nucleation mechanism. The morphology of the composite particles can be well controlled by the homogenization energy and the hydrophobicity of the costabilizer. The magnetic composite particles can be made by locating Fe₃O₄ inside the latex particles or forming a shell layer on their PS core surface depending on the aforementioned polymerization conditions. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Synthesis of triblock copolymer having alternating structures and kinetics study on RAFT‐mediated bulk,miniemulsion and seed miniemulsion polymerizations

We have conducted reversible addition‐fragmentation chain transfer (RAFT) polymerizations of styrene (St) and maleic anhydride (MAh) and n‐butyl acrylate (BA) to produce a well‐defined triblock copolymer having alternating structure, P(St‐alt‐MAh)‐b‐PSt‐b‐PBA, via bulk, miniemulsion and seed miniemulsion polymerizations. The polymerization kinetics and living characters were investigated. The results followed by gel permeation chromatography (GPC) showed that bulk and miniemulsion polymerizations exhibited controlled nature such as narrow polydispersity index (PDI), controlled molecular weight, and first‐order polymerization kinetics, whereas triblock copolymer owned a rather wider PDI. Comparison of GPC RI and UV traces revealed that alternating copolymer and diblock copolymer have a very high percentage of living chains. For seed miniemulsion polymerization, when the molecular weight of triblock copolymer is more than 36,000 g/mol, the formation of homopolymer of BA resulted in broadening of PDI. ¹H NMR method was used to identify the compositions of block copolymers. Differential scanning calorimetry analysis showed that the copolymers exhibited distinct glass temperatures. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

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     

Preparation of narrowly distributed nanoparticles of poly(n‐butyl methacrylate‐co‐vinyl pyrrolidone) through microemulsion polymerization

Narrowly distributed nanoparticles of poly (n‐butyl methacrylate‐co‐vinyl pyrrolidone) were prepared through microemulsion polymerization with a nonionic surfactant of Tween‐80 as emulsifier (6 wt % of the latex) and n‐butanol as coemulsifier. The polymerizations were initiated with benzoylperoxide (BPO), potassium persulfate (KPS), KPS/ferric sulfate (FeSO₄), and BPO/FeSO₄, respectively, where the initiation in the case of BPO/FeSO₄ took place mainly at the interphase between the oil phase and the reaction media. Namely, this interfacial‐initiated microemulsion polymerization resulted in larger particles with relatively narrower particle size distribution as well as higher limiting monomer conversion but lower polymerization rate compared with the polymerization initiated with KPS/FeSO₄. In this article, the influences of initiation method, monomer ratio, and addition of water‐soluble components on microemulsion polymerization and latex particle size were studied to discuss the mechanism of interfacial‐initiated microemulsion polymerization. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2334–2340, 2004     

Nanoencapsulation via interfacially confined reversible addition fragmentation transfer (RAFT) miniemulsion polymerization

Oligomer of styrene and maleic anhydride synthesized by bulk RAFT polymerization (SMA-RAFT) was used to construct a novel strategy for robust nanoencapsulation via interfacially confined controlled/living radical miniemulsion polymerization. After ammonolysis, SMA-RAFT becomes amphiphilic and can be used as a surfactant to prepare miniemulsion. The ammonolyzed SMA-RAFT molecules would self-assemble on the interface of water/droplets. This self-assembly property combining with the RAFT living polymerization chemistry demands the polymer chains to grow inwards gradually in particles, leading to the formation of a polymer shell. The hydrophilicity of ammonolyzed SMA-RAFT agent tuned by the ammonolyzed degree or structures of SMA-RAFT agent was found to play a key role in the final morphology. The well-defined nanocapsules with little solid particles can be obtained by using partially ammonolyzed SMA-RAFT with 0.5 wt% SDS as a co-surfactant.