2Emulsion Polymerizations Economically importantWestern countries 108 tons/year30% of all polymers made by free radical methodsemulsion polymers accounts for 40-50% of thisFirst employed during WWII for production of synthetic rubberToday: MMA, VC, vinylidene chloride, styrene, fluoropolymers, vinyl acetate, EVA, SA, SBR, chloroprene, etc
3Emulsion Polymerization Recipe Water (continuous phase)Water-insoluble monomerWater-soluble initiatorSurfactant (detergent)
17Qualitative Details I II III Conversion Micelles Monomer Droplets ParticleNumberSizeCommentsI0 – 15%presentincreasesNucleation period, Increasing RpII15 – 80%absentconstantConstant # of particles,Cp = constantIII80 – 100%roughly constantCp = decreasing
18Emulsion Polymerization Kinetics Once inside a particle, radical propagates as rp = kp[M]Overall rate: Rp = kp[M][P.][P.] = N’ñ (where N’ = the sum of micelle and particle concentrations and ñ = average # of radicals per particle)Therefore,Increase N’ to increase rate!
19Emulsion Kinetics, cont. Smith-Ewart Kinetics:Case 2: ñ = 0.5 (MOST CASES!)1 radical per particleHalf of the particles active, half not activeCase 1: ñ<0.5Radical can diffuse out of the particleMonomer with higher water solubilityCase 3: ñ>0.5Termination constant is lowHigh viscosity, initiator; large particles
20Emulsion Polymerization Kinetics How to increase Rp?Increase N’ to increase rateIncrease surfactant concentration to increase N’
21Molecular Weight in Emulsion Polymerizations Molecular weight determined by rate of growth of a chain divided by rate of radical entry (ri)How to increase molecular weight?rpRi= ——Nri= ——DP= kp[M]rpriN kp [M]Ri= ———DP
22Free Radical Solution Polymerizations RecallTo increase molecular weight…Increase monomer concentrationDecrease initiator concentrationTo increase Rate of PolymerizationIncrease initiator concentration٧ =kp [M]2 (kt kd f [I])1/2= —————Rp = kp [M] (kd f [I] / kt)1/2Can’t do both!