The Synthesis of Semipermeable Membrane Microcapsules using in situ Cyanoacrylate Ester Polymerization

j. microencapsulation, 2002, vol. 19, no. 6, 699±724 The synthesis of semipermeable membrane microcapsules using in situ cyanoacrylate ester polymerization R. J. HOLL and R. P. CHAMBERS* Chemical Engineering Department, Auburn University, AL 36849, USA (Received 9 August 1999; accepted 23 November 2000 ) A new material for the microencapsulation of biological systems was discovered and characterized with regards to the e ects of reaction conditions on product yield. By using poly(cyanoacrylate ester), membrane microcapsules were produced with su cient strength and porosity to be e ective in a process environment for the immobilization and protection of encapsulated material. After synthesizing numerous monomeric cyanoacrylate esters, the n-butyl derivative was discovered to give the best results with regards to microcapsule formation. Microcapsules were prepared by a droplet technique in which an aqueous solution is sprayed into an organic solvent containing the cyanoacrylate ester monomer. By pre-treating the cyanoacrylate ester monomer with an anion exchange resin (Amberlyst A-21, Rohm and Haas, Philadelphia, USA), a signi®cant reduction in the amount of acidic impurities which can adversely a ect results was achieved. The use of polyvinylpyrrolidone as a polymerization initiator gave the best results of a variety of polymeric and non-polymeric initiators investigated. Successful encapsulations were achieved using a solvent mixture of 60% (v/v) iso-octane/40% trichloroethylene, 0.1% (w/v) polyvinylpyrrolidone initiator, pH 6.5 aqueous encapsulation solution, and 5% (v/v) methylcyanoacrylate/A-21 treated n-butylcyanoacrylate (added separately to solvent) made to a 4% (v/v) solution in solvent. Ester monomers were synthesized and used to prepare polymer membranes. Keywords: Microencapsulation, semipermeable membrane, biological immobilization, polymerization. Materials and methods All chemicals used in the synthesis of cyanoacrylate monomers were of reagent grade or better. Cyanoacetic acid and paraformaldehyde were obtained from Fisher Scienti®c (Pittsburgh, USA); piperidine, tricresyl phosphate, and hydroquinone were obtained from Fisher Scienti®c (Pittsburgh, USA); and phosphorus pentoxide was obtained from Aldrich Chemical Company (Milwaukee, USA). In the formation of microcapsules, solvents used were of reagent grade or better and obtained from Fisher Scienti®c (Pittsburgh, USA), except decahydronapthalene which was obtained from Aldrich Chemical Company (Milwaukee, USA). Isobutyl cyanoacrylate, Tween 20 (polyoxyethylene sorbitan monolaurate), and polyvinylpyrrolidone (MW: 40 000) were obtained from Fisher Scienti®c (Pittsburgh, USA). Methoxypropyl cyanoacrylate was provided by Permabond Corporation (Bridgewater, NJ, USA). Amberlyst A-21 ion-exchange resin and * To whom correspondence should be addressed: Journal of Microencapsulation ISSN 0265±2048 print/ISSN 1464±5246 online # 2002 Taylor & Francis Ltd http://www.tandf.co.uk/journals DOI: 10.1080/02652040110055225 700 R. J. Holl and R. P. Chambers poly (4-vinylpyridine) were purchased from Aldrich Chemical Company (Milwaukee, USA). Additional ion-exchange resins, A-1P and Dowex 1-X8, were obtained from J. T. Baker Chemical Company (Phillipsburg, NJ, USA). Cyanoacrylate ester synthesis Cyanoacrylate esters were prepared by the method outlined by Jeremias (1954). Cyanoacetate ester required by this method was synthesized by an acid catalyzed condensation reaction between cyanoacetic acid and the alcohol providing the ester side group. The reaction can be summarized as: To this end, 125 g cyanoacetic acid was placed in a 1 l reaction ¯ask possessing a thermometer well. Approximately 250 ml of alcohol (enough to dissolve all of the cyanoacetic acid) was added. When all of the cyanoacetic acid was dissolved, 5 ml concentrated sulphuric acid was added to the reaction mixture, and the ¯ask connected to a total re¯ux condenser. The mixture was heated and a vacuum applied until constant boiling was obtained. The vacuum was adjusted to give a boiling temperature of ¹ 90 °C. The mixture was allowed to re¯ux for more than 4 h, at which time it was cooled and neutralized with saturated potassium carbonate. After ®ltering to remove precipitated sulphates, the reaction solution was allowed to stand overnight in a 500 ml separatory funnel. The formed organic phase was collected and vacuum distilled repeatedly to achieve a high purity product; the aqueous phase was discarded. Boiling points, densities, and yields of the synthesized cyanoacetate esters are given in table 1. Cyanoacrylate esters were prepared by a condensation reaction between formaldehyde and the respective cyanoacetate ester. The cyanoacrylate ester monomers prepared in this manner polymerized spontaneously; therefore, a monomer was obtained by changing the reaction conditions to achieve depolymerization. The reaction can be summarized as: Synthesis of semipermeable membrane microcapsules Table 1. 701 Boiling points, densities, and yields of various cyanoacetate esters. Ester side group n-propyl Iso-propyl Allyl n-butyl Iso-amyl n-hexyl Cyclohexyl n-octyl 2-ethyl hexyl n-decyl Benzyl Tb (pb vacuum) 100 °C 118 °C 118 °C 108 °C 114 °C 139 °C 144 °C 161 °C 165 °C 196 °C 185 °C (740 mmHg) (740 mmHg) (740 mmHg) (750 mmHg) (745 mmHg) (740 mmHg) (750 mmHg) (740 mmHg) (730 mmHg) (740 mmHg) (740 mmHg) Density (g/ml) Yield (g) 1.01 1.01 1.06 0.99 0.97 0.97 1.05 0.94 0.94 0.92 1.10 70.6 97.1 64.4 89.5 116.8 78.9 84.0 146.6 72.5 138.5 115.3 The amount of paraformaldehyde necessary to give ¹1.5 £ the moles of ester available was weighed and added to a three-neck 1 l reaction ¯ask to which a DeanStark trap and total re¯ux condenser, a thermometer, and a 500 ml separatory funnel were attached. Methanol (150 ml) was added through the Dean-Stark trap, and 0.5 ml piperidine to the reaction ¯ask. Heat was applied until re¯ux was achieved. The cyanoacetate ester prepared by the aforementioned procedure was placed in the separatory funnel and added dropwise to the reaction mixture after re¯ux was achieved. The addition rate of the ester and the heating rate to the apparatus were adjusted to maintain a constant boiling condition in the reaction ¯ask. After the addition was completed, 15 ml tricresyl phosphate was added to the reaction ¯ask, and the pH of the resulting mixture was tested to insure the solution was basic. If the solution was not basic (¹pH 8), then piperidine was added sparingly until a proper pH was achieved. Since the polymerization of cyanoacrylate esters is anionic, a basic reaction solution insures a favourable depolymerization condition by stabilizing the anionic species formed when the terminal group is removed. Heating rate was increased and methanol slowly siphoned o the DeanStark trap. When the reaction temperature reached 80 °C and ¹100 ml of methanol collected, 165 mL benzene was slowly added to the reaction mixture. Azeotropic distillation was continued until most of the water formed by the reaction and methanol were removed from the system by siphoning the Dean-Stark trap. Volumes of distillate collected were monitored to insure complete removal of a methanol and water. Upon complete removal of the methanol and water, the apparatus was converted to a long-path vacuum distillation unit by removal of the Dean-Stark trap/total re¯ux condenser and the thermometer and connecting a gas addition tube and Clasien distillation head with a 300 mm West condenser. After slightly cooling the reaction mixture, 5.6 g phosphorus pentoxide and 4.2 g hydroquinone were added and sulphur dioxide bled through the system. Benzene used in the azeotropic distillation was distilled until most of the volume added was collected. The apparatus was then converted to a short-path distillation unit with the connection of the vacuum adapter directly to the Claisen head. A long neck, 100 ml receiver, half-®lled with glass beads and into which small amounts of phosphorus pentoxide and hydroquinone were added, was connected to the vacuum adapter and chilled in an iced water bath. High vacuum (