Study On Preparation Of Monodisperse Micron-size Poly (Vinyl Acetate) Microspheres By Dispersion Polymerization

Dispersion polymerization has proven to be a useful polymerization technique forpreparing monodisperse micron-size polymer microspheres in a single step.In this paper,dispersion polymerization of vinyl acetate(VAc) by potassium persulphate(KSP) –sodiumbisulfite redox initiation in aqueous alcohol media have been studied.Firstly,stabledispersion polymerization system was chosen,and then ,the effects of polymerizationparameters on polymer microspheres size , microspheres size distribution and relativemolecular weight of poly(vinyl acetate) (PVAc) were systematically investigated tofind outthe reasonable polymerization way of PVAc monodisperse microspheres .Based onexperimental results,the polymerization mechanism and kinetics features were discussed.A series of experiments were conducted to get the effects of polymerizationparameters on polymer microspheres size , microspheres size distribution and relativemolecular weight of PVAc. In the isopropanal/water system,PVAc microspheres size andmicrospheres size distribution coefficient increased with the increase of ratio ofalcohol/water,initiatior concentration,monomer concentration,polymerization temperatureand they also increased with the decrease of dispersant(PVP) concentration . polymermicrospheres size decreased and microspheres size distribution increased with the increaseof agitation velocity.Relative molecular weight of PVAc decreased with the increase of ratioof alcohol/water, initiatior concentration and polymerization temperature and it alsodecreased with the decrease of dispersant (PVP) concentration .Its relative molecularweight reached a maximum with the increase monomer concentration.Only the existence of physics absorption-stabilization of dispersant was found in thepolymer microspheres by the FTIR and X-ray photoelectron spectroscopy(XPS) .Theabsorption mechanism of precipitated growing short chain radical were preferred in thissystem.In the beginning,monomer,dispersant and initiator all dissolved in the dispersedmedia,the reaction system was homogeneous prior to dispersion polymerization.As soon asthe polymerization started,primary radicals,generated by decmposition of the initiator,grewin the continuous phase by the addition of monomer until they reached their critical chainlength where they precipitated to form nuclei.These nuclei were unstable and quiklyaggregated with each other;concurrently,they adsorbed the dispersant and finally becamestable particles.The existing particles continued to grow by capturing the nuclei ang shortchain radicals which will either continue to grow inside the particles ,no more particlesformed and the particles formation stage will stop owing to the exhaustion of the monomer.The standard recipes for preparing 2.0μm ,4.0 μm and 5.0μm PVAc monodispersemicrospheres have been gotten,the PVAc with viscosity average molecularweight( M η>100×104) and PVA with average degree of polymerization of 4800 also havebeen prepared.