2-Hydroxyethyl Methacrylate Radical Inverse Miniemulsion Polymerization

2-Hydroxyethyl methacrylate radical Inverse Miniemulsion Polymerization Wang Guoxian, Wu Hu

（College of Chemistry and chemical engineering, Hunan Institute of Science and Technology, Yue yang

414000, Hunan province, P.R. China） Abstract: In miniemulsion process, Surfactants are used to stabilize the latexes and control the particle size. They play a very important role during the polymerization process and also during the subsequent storage. However, conventional surfactants have some drawbacks: the first is that the water sensitivity of the final coating increases and affects gloss and adhesive, the second is that the surfactants are physically adsorbed on the polymer particles and easily desorbed from the particle surface. To overcome these drawbacks, the polymeric surfactants have drawn increasing interest. The use of polymeric surfactants for stabilisation of cosmetic and personal care formulations is described in terms of their adsorption and conformation at the solid/liquid and liquid/liquid interface. The mechanism by which these polymeric surfactants stabilise suspensions and emulsions is briefly discussed in terms of their interaction on approach of particles or droplets. This provides very strong repulsion, that is referred to as steric stabilisation. This polymeric surfactant was used for stabilisation or oil-in-water (O/W) emulsions both in aqueous media and in the presence of high electrolyte concentrations. The amount of polymeric surfactant required for maintenance of stability (for more than one year at ambient temperature) was relatively low (of the order of 1 w/w% based on the oil phase). In addition, the polymeric surfactant shows no skin irritation, no stickiness or greasiness and it gave an excellent skin feel. Alkali-soluble random copolymers are a kind of polymeric surfactant (ASR). Alkali-soluble resins are random copolymers that contain both hydrophobic groups and a large number of carboxylic acid groups, which are widely used as the sole stabilizers in the production of waterborne dispersed polymers because

1 they remarkably improve the end-use properties of the latexes. However, they reduce the radical entry rate. Emulsion polymerization stabilized with ASRs has not been deeply investigated. The behaviour and the role of polymeric surfactant aggregates in emulsion polymerization may be expected to be somewhat different from those of conventional emulsifier micelles. Hydroxyethyl methacrylate (HEMA) is perhaps the most widely studied and used neutral hydrophilic monomer. The monomer is soluble, its homopolymer is water-insoluble but plasticized and swollen in water. This monomer is the basis for many hydrogel products such as soft contact lenses, as well as polymer binders for controlled drug release, absorbants for body fluids and lubricious coatings. As a comonomer with other ester monomers, HEMA can be used to control hydrophobicity or introduce reactive sites. Hydrogels based on poly (2-hydroxyethyl methacrylate) (PHEMA) are widely studied for biomedical applications due to their swellability, oxygen permeability, and biocompatibility. Poly (2-hydroxyethyl methacrylate) is a kind of biocompatible macromolecule with good transparence and flexibility, which would be a promising material used as artificial cornea． No effect has been devoted to investigate systematically the polymerization kinetics and mechanism of sterically stabilized miniemulsion systems of HEMA. In this work, the mechanisms for radical entry in ASR-stabilized systems have been investigated. It has been found that the mechanism responsible for the reduction of the rate of radical entry depends on the type of ASR used, the type of radical produced from the initiator, the phase where the radicals are produced. To some extent the results are helpful to the understanding of the nucleation mechanism for the miniemulsion polymerization of soluble monomers. The miniemulsion polymerization of 2-hydroxyethyl methacrylate (HEMA) was conducted at 70℃, using either an alkali-soluble random copolymer (ASR) or

2 sodium dodecyl sulfate (SDS) as the emulsifier. The kinetics of the miniemulsion polymerization was investigated by the effects of initiator (including 2,2’-azobisisbutyronitrile (AIBN), Potassium persulfate (KPS)), pH of the aqueous phase, and surfactant type. ASR could form the aggregates like micelles in the polymerization. The polymerization rate using ASR was lower than that using SDS. This is due to a hairy ASR layer around the particles surface to decrease the entry rate of radical through the region. This factor results in reducing the efficiency of these radicals to initiate the polymerization. There is significant aqueous phase polymerization, and frequent particle nucleation and coagulation during the polymerization. Increasing pH, the polymerization rate does decrease proportionally to the particle number.

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