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.     

甲基丙烯酸甲酯的RAFT微乳液聚合

该文合成了一种双官能团的RAFT试剂——S,S'-二(α,α'-二甲基-α″-乙酸)三硫代碳酸酯(BDAT)。以其为链转移剂,在微乳液体系中进行了甲基丙烯酸甲酯的RAFT聚合。分别讨论了聚合反应温度和链转移剂浓度对聚合反应的影响,并对相关的聚合反应动力学常数进行了计算。研究结果表明,在微乳液中进行的RAFT聚合具有显著的活性聚合的特征。聚合产物的相对分子质量(简称分子量,下同)随着转化率的提高而线性增加,同时聚合产物具有较窄的分子量分布,聚合过程随着链转移剂浓度的增加而逐渐可控。另外,利用透射电子显微镜对链转移剂浓度对微乳液粒子尺寸的影响也进行了考察,扫描电镜照片表明,微乳液聚合所得乳液粒子呈现单分散性状态,并且粒子尺寸随着链转移剂浓度的增加而逐渐增加。     

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.     

Synthesis of poly(methyl methacrylate)-silica nanocomposites using methacrylate-functionalized silica nanoparticles and RAFT polymerization

Well-defined poly(methyl methacrylate)-silica nanocomposites were produced by “grafting through” using reversible addition-fragmentation chain transfer (RAFT) polymerization. The surface of silica nanoparticle was modified covalently by attaching methacryl group to the surface using 3-methacryloxypropyldimethylchlorosilane. Polymerization of methyl methacrylate (MMA) using the 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanyl pentanoic acid RAFT agent, produced the PMMA-SiO₂ nanocomposites. Characterization of these well-defined nanocomposites included FT-IR, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), transmission electron microscopy (TEM) and dynamic mechanical analysis. These results show that the T_g values are higher and the mechanical strength of the PMMA-SiO₂ nanocomposites is slightly improved when compared to bulk PMMA. Further, the molecular weight of the PMMA (up to M_n = 100,000) is controlled and the SiO₂ are well dispersed in the PMMA matrix.     

Synthesis of structured nanoparticles of styrene/butadiene block copolymers via RAFT seeded emulsion polymerization

The structured nanoparticles of styrene (St) and butadiene (Bd) block copolymers were prepared by RAFT seeded emulsion polymerization of butadiene. It was confirmed that the block copolymers of PSt-b-P(St-co-Bd) was formed with controlled molecular weight and rather low PDI at low composition of the P(St-co-Bd) segment. With more incorporation of butadiene, the branching reaction of polybutadiene became obvious, leading to higher PDI and positive deviation of M_n from the theoretical predication. At the gel point, the composition of the P(St-co-Bd) segment was estimated to be 0.72. After this, the gel fraction increased quickly. The morphology of structured nanoparticles could be largely tuned simply by the copolymer composition. With the composition of the P(St-co-Bd) segment increased from 0.37 to 0.92, the morphology within the structured particles changed from the polybutadiene domains-in-polystyrene matrix, perforated concentric-spherical layer, concentric-spherical multi-layers, bi-continuous, to broken layers of polystyrene in polybutadiene matrix. It was found that the morphology of the block copolymer within nanoparticles was dependent on d/L values, which was in excellent agreement with the theoretical prediction.     

Atom transfer radical suspension polymerization of methyl methacrylate catalyzed by CuCl/bpy

Atom transfer radical suspension polymerization (suspension ATRP) of methyl methacrylate (MMA) was carried out using 1-chloro-1-phenylethane (1-PECl) as initiator, copper chloride/bipyridine (CuCl/bpy) as catalyst. The polymerization was accomplished with a mechanical agitator under the protection of nitrogen atmosphere. Apart from the dispersing agent (1% PVA), NaCl was also used in the water phase to decrease the diffusion of CuCl/bpy to water and the influence of the concentration of NaCl was investigated. Subsequently, the kinetic behavior of the suspension ATRP of MMA at different temperatures was studied. At 90 and 95 °C, the polymerization showed first order with respect to monomer concentration until high conversion. The molecular weight (M_n) of the polymer increased with monomer conversion. However, at lower temperatures, different levels of autoacceleration was observed. The polymerization deviated from first order with respect to monomer concentration when the conversion was up to some degree. The lower the temperature was, the more the deviation displayed. On comparison with bulk ATRP of MMA, the rate of suspension ATRP was much faster.     

Synthesis of novel random and block copolymers of tert-butyldimethylsilyl methacrylate and methyl methacrylate by RAFT polymerization

Hydrophobic-hydrolysable copolymers consisting of methyl methacrylate (MMA) and tert-butyldimethylsilyl methacrylate (TBDMSMA) have been synthesized for the first time by Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization technique using cumyl dithiobenzoate (CDB) and cyanoisopropyl dithiobenzoate (CPDB) as chain transfer agents (CTAs). The monomer reactivity ratios for TBDMSMA (r₁ = 1.40 ± 0.03) and MMA (r₂ = 1.08 ± 0.03) have been determined using a non-linear least-squares fitting method. Well-defined random copolymers PMMA-co-PTBDMSMA have been prepared. Then, the versatility of the RAFT process to synthesize silylated block copolymers with controlled molecular weights and low polydispersities has been demonstrated using two strategies: the synthesis of PMMA-SC(S)Ph or PTBDMSMA-SC(S)Ph as macro-chain transfer agent (macro-CTA) for use in a two step method or an one-pot method which consists in the successive addition of the two monomers. Diblock copolymers with narrow molecular weight distributions (PDI < 1.2) were obtained from the one-pot method with number-average molecular weight values within the range 10,000-22,000 g mol⁻¹.     

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.     

甲基丙烯酸甲酯-甲基丙烯酸三氟乙酯共聚物细乳液的合成及其表征

以甲基丙烯酸甲酯(MMA)和甲基丙烯酸三氟乙酯(TFEMA)为反应单体进行细乳液聚合,制得了MMA-co-TFEMA共聚物乳液。利用FTIR和GPC表征了共聚物的结构和分子量,采用激光光散射法及透射电镜对乳胶粒子的粒径大小及形貌进行了表征,并通过接触角法对共聚物膜的表面性能进行了研究。结果表明,与常规乳液相比,以细乳液方法制得的MMA-co-TFEMA共聚物乳液稳定性好,粒径分布窄,聚合物的分子量分布窄,并且共聚物膜随着氟含量的增加疏水疏油性增强,表面能降低。     

High solids Pickering miniemulsion polymerization

Surfactant free 50 wt% solids content latexes were synthesized by means of miniemulsion copolymerization of vinyl acetate and Veova10 using modest amount (≤10 wt% based on monomer) of silica as a Pickering stabilizer. The silica was surface modified with PEO chains to improve its adsorption on the miniemulsion droplets. Coagulum free latexes were produced using oil-soluble initiators. The latexes were stabilized by modified silica particles aggregated at the surface of the polymer particles and yielded glossy films with a minimum film forming temperature of 26 °C.     

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.     

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.     

Half-sandwich samarium (III) diketiminate bromide as a catalyst for methyl methacrylate polymerization

Half-sandwich samarium(III) diketiminate bromide was successfully synthesized and was shown to be active in methyl methacrylate (MMA) polymerization. The effects of temperature, polymerization time and catalyst concentration were studied. Activities of ca. 18 kg of polymethacrylate (PMMA) per mol of samarium per hour were obtained under optimum conditions (0 °C and a MMA/catalyst molar ratio of 100/1), giving a polymer with a molar mass M_n>24,000 g mol⁻¹ and a molar mass distribution (M_w/M_n)<1.4. After 1 h of polymerization, conversions of MMA as high as 96% were observed.     

ATR‐UV monitoring of methyl methacrylate miniemulsion polymerization for determination of monomer conversion

Attenuated Total Reflection (ATR) UV spectroscopy has been used to monitor monomer conversion in methyl methacrylate miniemulsion polymerization. It was found that the vinylic groups of methyl methacrylate strongly absorb the UV light with a maximum absorption at 225 nm. This absorption peak decreases as monomer is converted to polymer. The polymer has a strong absorption at a lower UV region. The results from this feasibility study indicate that ATR‐UV sensor technique has a great potential to be used for on‐line or in‐line process monitoring in emulsion and miniemulsion polymerization. With a partial least square (PLS) calibration model, very good prediction the monomer conversion was obtained. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1471–1475, 2006     

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.     

Polymerization of methyl methacrylate by iron(II) pyridinebisimine complexes

Fe(II) pyridinebisimine complexes activated with trialkylaluminium or modified methylaluminoxane (MMAO) as catalysts were employed for the polymerization of methyl methacrylate. Polymer yields, activities and polymer molecular weights as well as molecular weight distributions can be controlled over a wide range by the variation of the structures of the Fe(II) pyridinebisimine complexes and the reaction parameters such as Al/Fe molar ratio, monomer/catalyst molar ratio, monomer concentration, reaction temperature and time applied to the polymerization of methyl methacrylate. Under optimum condition, the catalytic activity of Fe(II) complex is of up to 74.5 kg_polym/mol_Fe h.     

Poly(ethylene glycol) as a ‘green solvent’ for the RAFT polymerization of methyl methacrylate

We demonstrate the use of a range of poly(ethylene glycol)s (PEGs) to control the polymerization of methyl methacrylate (MMA) using reversible addition-fragmentation chain transfer (RAFT) polymerization. The use of PEG as the solvent (M_n = 4600 g mol⁻¹) resulted in an increase in the rate of the reaction over that of other solvents by a factor of 5 at 60 °C, allowing MMA to be polymerized to high conversions with a DP of 100 much more rapidly than in standard solvents, while maintaining control over the molecular weight with polydispersities as low as 1.05. Interestingly, whilst the same rate increase is seen when polymerizing to a DP of 500, PEG appears to limit the achievable molecular weight to differing degrees depending on its chain length. Advantages of using PEG include its very low toxicity and other environmentally friendly aspects of its nature that allow it to be classed as a ‘green’ solvent.     

Thermal polymerization of methyl (meth)acrylate via reversible addition-fragmentation chain transfer (RAFT) process

Thermal polymerization of methyl (meth)acrylate (MMA) was carried out using 2-cyanoprop-2-yl-1-dithionaphthalate (CPDN) and cumyl dithionaphthalenoate (CDN) as chain transfer agents. The kinetic study showed the existence of induction period and rate retardation, especially in the CDN mediated systems. The molecular weights of the polymers increased linearly with the monomer conversion, and the molecular weight distributions (M_w/M_ns) of the polymers were relatively narrow up to high conversions. The maximum number-average molecular weights (M_ns) reached to 351?900 g/mol (M_w/M_n = 1.47) and 442?400 g/mol (M_w/M_n = 1.29) in the systems mediated by CPDN and CDN, respectively. Chain-extension reactions were also successfully carried out to obtain higher molecular weight PMMA and PMMA-block-polystyrene (PMMA-b-PSt) copolymer with controlled structure and narrow M_w/M_n. Thermal polymerization of methyl acrylate (MA) in the presence of CPDN, or benzyl (2-phenyl)-1-imidazolecarbodithioate (BPIC) also demonstrated “living”/controlled features with the experimented maximum molecular weight 312?500 g/mol (M_w/M_n = 1.57). The possible initiation mechanism of the thermal polymerization was discussed.     

Synthesis of carboxylic acid functionalized nanoparticles by reversible addition-fragmentation chain transfer (RAFT) miniemulsion polymerization of styrene

In this study, an addition-fragmentation chain transfer agent bearing carboxylic acid, 4-toluic acid dithiobenzoate (TADB), was used to synthesize carboxylic acid functionalized PS nanospheres via the miniemulsion polymerization. In addition, non-functionalized RAFT agent, benzyl dithiobenzoate (BDB), was also used to compare the surface properties of the PS nanoparticles. For the TADB system, the rate of polymerization was approximately two-fold faster than the BDB system, while the molecular weights and PDI of PS remain intact.With increasing the molar ratio of [TADB]/[AIBN] from 0 to 3.0, the average particle diameter is substantially increased from 90 to 126 nm. The absolute value of zeta potential and conductivity also correspondingly increase from 49.1 mV and 3.47 mS/cm to 53.9 mV and 4.21 mS/cm, respectively. The results indicate that the surface of PS nanospheres could be functionalized by means of a carboxylic acid group on the RAFT agent and the stability of the PS miniemulsion latex could be significantly improved.     

通过细乳液聚合方法制备自交联型有机硅乳液

苯乙烯与甲基丙烯酸-3-三甲氧基硅丙酯(M PS)经细乳液聚合,制得了稳定的自交联型的水分散涂料,用透射电子显微镜(TEM)、动态光散射仪(DLS)和红外光谱(IR)表征了乳胶粒的形态和结构。发现与传统乳液聚合相比,细乳液聚合法可显著提高乳液的稳定性,且M PS加入量的增加会导致乳液稳定性降低。