High Yield Preparation Of Uniform Polyurea Microspheres Through Precipitation Polymerization

Uniform polymer microspheres have been widely used in various fields,such as drug delivery and release,liquid crystal display,immobilization of enzyme,and packing materials for chromatography.Different methods have been developed for the preparation of uniform microspheres including emulsion polymerization,suspension polymerization,Shirasu porous glass membrane emulsification,and precipitation polymerization.Among these methods,precipitation polymerization has gained particular attention because no any surfactants and stabilizerswere needed in the process.The surface of the polymer microspheres thus prepared is very clean,and the preparation and post-treatment processes are simple.However,owing to the low monomer concentration allowed to maintain the stability of the particles because of the absence of surfactants and stabilizers,the efficiencies of the microsphereproduction and the solvent utilization are usually very low in this process,which seriously restricts its large-scale production and application.Based on step-growth polymerization of diisocyanate,an alternative for the fabrication of highly uniform polyurea?PU?microspheres has been reported.Uniform PU microspheres are prepared through precipitation polymerization of isophorone diisocyanate?IPDI?reaction with water in the solvent mixture.In this work,highly uniform PU microspheres are prepared by precipitation polymerization of IPDIin a binary mixture of H₂O-acetonitrile?AN?.By optimizing the preparation process,including composition of the solvent mixture and reaction temperature in particular,the maximal IPDI concentration allowed to achieve uniform PU microspheres is increased to 28.0 wt%,with a high yield of 94.8%.The productivity of microspheres thus obtained is almost 27 times that by free radical precipitation polymerization,and triple of that previously reported on the same polymerization in the mixed solvent of H₂O-acetone?AT?.In addition,the optimization of the experimental conditions show that IPDI solubility in the solvent is the key factor for the production of the uniform microspheres.That is,only when IPDI concentration is lower than or equal to the IPDI solubility in the solvent mixture used under the same experimental conditions,uniform PU microspheres can be prepared.By increasing AN amount in the mixed solvent or thepolymerization temperature,IPDI solubility is improved,so is the maximal IPDI concentration allowed to have uniform PU microspheres.However,there exist upper limits for AN amount in the solvent mixture and the polymerization temperature,preparation of uniform PU microspheres is failed whenever one of the limits is exceeded.When N,N'-dimethylformamide?DMF?is added to the solvent mixture,IPDI reaction with water is catalyzed because DMFis of high electron donating ability.Using binary mixture of H₂O/DMF as the solvent,uniform PU microspheres are quickly preparedwith IPDI as the monomer.The catalysis of DMF is confirmed by measuring the solubility of IPDI andits polymer in different solvents,and the catalytic mechanism of DMF in thepolymerization is proposed.With AT in H₂O/AT partially substituted by DMF,a significant acceleration effect is observed in the preparation of PU microspheres.Uniform PU microspheres are then prepared in ternary solvent mixture of H₂O/DMF/AT,and the preparation conditionsare optimized with regard to composition of the ternary solvent,polymerization temperature and IPDI concentration.The results show that,with H₂O/DMF/AT mass ratio at 25/10/65and polymerization done at 60°C,the highest IPDI concentration allowed to have highly uniform PU microspheres is 13.0 wt%,which represents an increase of 3 wt%compared tothat observed in the binary solvent of H₂O/AT.This is also accomplished in an obviously shortened time.Uniform PU microspheres arealso obtained quickly when DMF is added to the binary solvent H₂O/AN,i.e.when the preparationis conducted in a ternary solvent of H₂O/DMF/AN.That is to say that the polymerization of IPDI reaction with water is accelerated by addition of DMF to the binary solvent H₂O/AN.However,owing to a higher solubility of polyurea in DMF,the highest IPDI concentration to obtain uniform PU microspheres is 24.0 wt%in ternary solvent mixture of H₂O/DMF/AN,which is lower than thatachievedin the binary solvent H₂O/AN?28.0 wt%?under otherwise exactly the same experimental conditions.