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Polymer Synthesis CHEM 421 Odian Book: Chapter 4 Emulsion Polymerizations Polymer Synthesis CHEM 421 • Economically important • Western countries 108 tons/year • 30% of all polymers made by free radical methods –emulsion polymers accounts for 40-50% of this • First employed during WWII for production of synthetic rubber • Today: MMA, VC, vinylidene chloride, styrene, fluoropolymers, vinyl acetate, EVA, SA, SBR, chloroprene, etc Emulsion Polymerization Recipe • Water (continuous phase) • Water-insoluble monomer • Water-soluble initiator • Surfactant (detergent) Polymer Synthesis CHEM 421 Surfactants Polymer Synthesis CHEM 421 H2O Hydrophobic / Lipophilic core Critical Micelle Concentration (CMC) Unimers Micelles Surfactant Concentration Surfactants Types - Anionic - Cationic - Amphoterics - Non-ionics Polymer Synthesis CHEM 421 Emulsion Polymerization Recipe Polymer Synthesis CHEM 421 Emulsion Polymerizations Polymer Synthesis CHEM 421 Polym’z Rate Critical Micelle Concentration Surfactant Concentration Kinetics of Emulsion Polymerization Polymer Synthesis CHEM 421 III Percent Conversion II I Time Kinetics of Emulsion Polymerization Polymer Synthesis CHEM 421 Rate I II % Conversion III Before Initiation M Monomer Droplet ca. 1 micron diameter conc = 1011/mL stabilized by soap M Micelle Containing M Monomer I ca. 75 Å diameter M conc = 1018/mL M I M Initiation of micelles statistically favored M M I I M I M M M I M M M I Relative surface area 1 : 560 M Polymer Synthesis CHEM 421 M I M M I Interval One: 0 – 15 % Conversion M Polymer Synthesis CHEM 421 M M I I• I Inactive latex particles M Active Active latex latex particles particle I M M P• I MM M I M P• M I Micelles I • Containing Monomer M M M M I Qualitative Details Conversion I II III 0 – 15% Micelles present Polymer Synthesis CHEM 421 Monomer Particle Particle Droplets Number Size present increases increases Comments Nucleation period, Increasing Rp Interval Two: 15 – 80% Conversion Active M latex particles Polymer Synthesis CHEM 421 M M P• I• I I P• M Number of particles I constant, M therefore Rp = constant P• M I M P• M I• No I micelles I I Inactive latex particles I M M M I• M Kinetics of Emulsion Polymerization 1018 I II Polymer Synthesis CHEM 421 III 1015 Number of Polymer Particles Number of Micelles 0 0 Time Qualitative Details Conversion I II III 0 – 15% 15 – 80% Micelles present absent Polymer Synthesis CHEM 421 Monomer Particle Particle Droplets Number Size present present increases constant increases increases Comments Nucleation period, Increasing Rp Constant # of particles, Cp = constant Interval Three: 80 – 100% Conversion M No monomer droplets Polymer Synthesis CHEM 421 M M P• I• I P• I• M P• M M P• M M P• I P• I• I I M No I micelles M M I M Qualitative Details Conversion I II III 0 – 15% 15 – 80% 80 – 100% Micelles present absent absent Polymer Synthesis CHEM 421 Monomer Particle Particle Droplets Number Size present present absent increases constant constant increases increases roughly constant Comments Nucleation period, Increasing Rp Constant # of particles, Cp = constant Constant # of particles, Cp = decreasing Emulsion Polymerization Kinetics Polymer Synthesis CHEM 421 • 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, Rp N ' nkp[ M ] –Increase N’ to increase rate! Emulsion Kinetics, cont. • Smith-Ewart Kinetics: –Case 2: ñ = 0.5 (MOST CASES!) » 1 radical per particle » Half of the particles active, half not active –Case 1: ñ<0.5 » Radical can diffuse out of the particle » Monomer with higher water solubility –Case 3: ñ>0.5 » Termination constant is low » High viscosity, initiator; large particles Polymer Synthesis CHEM 421 Emulsion Polymerization Kinetics Polymer Synthesis CHEM 421 • How to increase Rp? Rp N ' nkp[ M ] –Increase N’ to increase rate »Increase surfactant concentration to increase N’ Molecular Weight in Emulsion Polymerizations Polymer Synthesis CHEM 421 • Molecular weight determined by rate of growth of a chain divided by rate of radical entry (ri) DP = rp —— r i DP = ri Ri = —— N N kp [M] ——— Ri –How to increase molecular weight? rp = kp[M] Free Radical Solution Polymerizations Polymer Synthesis CHEM 421 • Recall Rp = kp [M] (kd f [I] / kt)1/2 kp [M] ٧ == ————— 2 (kt kd f [I])1/2 – To increase molecular weight… » Increase monomer concentration » Decrease initiator concentration – To increase Rate of Polymerization » Increase monomer concentration » Increase initiator concentration Can’t do both!