Soap-free emulsion polymerization of styrene using poly(methacrylic acid) macro-RAFT agent
Graphical abstract
Introduction
Nano-sized polymeric particles with monodisperse size distribution have been utilized in various applications such as ion adsorbents [1], fillers in polymers, pigments, instrument calibration standards [2], diagnostics and drug carriers [3], reaction catalysis [4], environmental protection [5], etc. Water-borne polymerizations such as emulsion and suspension polymerizations are of great importance in industrial application as they provide environmental friendly processes, remove the reaction heat easily during polymerization, and assure the feasible handling of the final product having a low viscosity [6], [7], [8].
Research in controlled/living free-radical polymerization (LFRP) has been increased significantly during the past two decades. Based on the radical capping mechanism and agents, controlled/living radical polymerizations are generally classified as the nitroxide-mediated polymerization (NMP) [9], [10], metal catalyzed atom transfer radical polymerization (ATRP) [11], [12], and the reversible addition–fragmentation chain transfer (RAFT) [13], [14] methods. Among the LFRPs, the RAFT polymerization allows the controlled polymerization of a wide variety of monomers at convenient temperatures. In the previous studies, we have reported the LFRP techniques using NMP [15], [16] and RAFT [17], [18], [19], [20], [21], [22] emulsion/dispersion polymerization to synthesize polymers with well-defined end groups and narrow molecular weight distributions. All these methods rely on a reversible activation–deactivation of the growing polymer chains. Although LFRP in bulk or solution polymerization was well studied previously, the implementation of LFRP in the dispersed aqueous media and especially in the emulsion polymerization has reported one or more of the following problems; poor colloidal stability, high polydispersity, poor molecular weight control. The first attempts to conduct LFRP in aqueous dispersions logically used emulsion polymerization. However these attempts generally failed, leading to the use of miniemulsions, which have proven to be quite robust for all forms of LFRP. In recent years, there have been several exciting and innovative developments in LFRP aqueous dispersions. Understanding of the interaction of the LFRP chemistry within a colloidally dispersed environment has progressed to the stage where processes are approaching commercial viability [13], [23], [24], [25].
With the advance of LFRP methods, a variety of amphiphilic block copolymers have been studied as potential stabilizers in the emulsion polymerization. Common surfactants such as sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide possess critical micelle concentrations (CMCs) on the order of 10−3–1 mol L−1 [26]. In the heterogeneous polymerization, lower concentration of amphiphilic block copolymers can be used than that of the conventional surfactants or stabilizers [27]. Amphiphilic block copolymers having both hydrophobic and hydrophilic blocks can form a micellar structure. For the efficient anchoring in amphiphilic block copolymer, the hydrophobic block has to be long enough, however, this might lead to a difficulty in solubilizing the block copolymer in water solution prior to polymerization. An alternative route is then to employ a reactive hydrophilic polymer capable to react during the polymerization course, hence affording an in situ amphiphilic block copolymer stabilizer.
Charleux et al. [28], [29] reported a new approach to carry out miniemulsion polymerization. The amphiphilic diblock copolymer is used to emulsify the monomer phase in water, stabilize the particles, and initiate the polymerization of a third monomer for the preparation of ABC triblock copolymers using AGET (activator generated by electron transfer) ATRP. In addition, the RAFT-mediated non-aqueous dispersion polymerization of methyl acrylate in isododecane, which is a nonsolvent for poly(methyl acrylate), was carried out by using soluble poly(2-ethylhexyl acrylate) macromolecular RAFT agents, containing either a dithiobenzoate reactive function or a trithiocarbonate one. The soap-free emulsion polymerization of styrene was acted in the presence of comonomer (sodium acrylate) and RAFT agent (dibenzyltrithiocarbonate). The new process based on a spontaneous phase inversion mechanism overcomes the slow diffusion of the RAFT agent [30]. The water-soluble polymers carrying a thio end group such as poly(dimethylaminoethyl methacrylate), poly(ethylene oxide), and poly(ethylene oxide)-b-poly(dimethylaminoethyl methacrylate) have been estimated as precursors of stabilizers in soap-free emulsion polymerization of styrene under acidic conditions to form ionically stabilized polystyrene latex particles [31], [32].
In our previous study, the crosslinked poly(divinylbenzene) [PDVB] particles were synthesized in the presence of poly(styrene-block-4-vinylpyridine) copolymer in aqueous media. The block copolymer consisting of the RAFT agent is used as an emulsifier and reactive stabilizer as well, but the process is carried out using two-step polymerizations [33].
In this study, we were able to obtain submicron-sized monodisperse spherical particles using a small amount of block copolymer of the poly(methacrylic acid) (PMAA) macro-RAFT agent as a precursor of emulsifier in the in situ one-step polymerization. The mechanism is proposed on the basis of the formation of particle, particle size, and the content of oxygen on the particle surface and follows the previously proposed micelle formation in our previous study [31]. This work does not only intend to apply the controlled free-radical polymerization in an aqueous dispersed system but also takes advantage of the RAFT technique to create a well-defined emulsifier with a high chain-end reactivity. Such reactive hydrophilic polymers can be considered as a new class of macromolecular emulsifier, which will be able to afford new developments in soap-free emulsion polymerization.
Section snippets
Materials
Styrene (Junsei Chemicals, Tokyo, Japan) and methacrylic acid (99% purity, Aldrich Chemical Co., WI, USA) were purified using an inhibitor removal column (Aldrich Chemical Co., WI, USA) and stored at −5 °C prior to use. 2,2-Azobis(isobutyronitrile) (AIBN, Junsei Chemicals, Tokyo, Japan) was used without further purification. Double-distilled deionized (DDI) water was used as the polymerization medium. Tetrahydrofuran (THF, Acros, Gheel, Belgium) employed in the synthesis of a RAFT agent was
Effect of the presence of PMAA macro-RAFT agent on the emulsion polymerization
The weight-average molecular weight (Mw) and polydispersity index (PDI) of PMAA macro-RAFT agent were measured using GPC; the Mw of the macro-RAFT agent is 316,160 g/mol and the PDI is 1.21 (Fig. 1). The conversion was 38% in 12 h. The low molecular weight distribution of PMAA confirms that the RAFT method successfully worked for the preparation of macro-RAFT agent.
1H NMR spectroscopy is used to verify the chemical structure of the synthesized PMAA macro-RAFT agent, polystyrene (PS), and PS
Conclusions
In this study, poly(methacrylic acid) (PMAA) macro-RAFT agent has been synthesized by the bulk polymerization using 4-toluic acid dithiobenzoate as a RAFT agent and used in the soap-free emulsion polymerization of styrene. In order to prevent this phenomenon that water-soluble initiator such as potassium persulfate (KPS) forms oligomeric radical and reacts with hydrophobic monomer as an emulsifier in emulsion polymerization, water-insoluble initiator, AIBN, is used for the polymerization. The
Acknowledgements
This work is financially supported by the Ministry of Education Science and Technology (MEST), the Ministry of Knowledge Economy (MKE), and the Ministry of Labor (MOLAB) through the fostering project of the Lab of Excellency during 2005–2008.
References (38)
- et al.
Int J Pharm
(2004) - et al.
Prog Polym Sci
(2002) - et al.
Polymer
(2004) - et al.
Polymer
(2003) - et al.
Polymer
(2005) - et al.
Polymer
(2006) Prog Polym Sci
(2008)- et al.
Eur Polym J
(2004) - et al.
Macromol Rapid Commun
(2001) - et al.
J Nanopart Res
(2000)
Biotechnol Bioeng
Environ Sci Technol
Emulsion polymerization: a mechanistic approach
Emulsion polymerization
Macromolecules
J Ind Eng Chem
Macromolecules
Polym Int
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