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Polymer Synthesis CHEM 421 Odian Book: Chapter 4.

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Presentation on theme: "Polymer Synthesis CHEM 421 Odian Book: Chapter 4."— Presentation transcript:

1 Polymer Synthesis CHEM 421 Odian Book: Chapter 4

2 Polymer Synthesis CHEM 421 Emulsion Polymerizations Economically important Western countries 10 8 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

3 Polymer Synthesis CHEM 421 Emulsion Polymerization Recipe Water (continuous phase) Water-insoluble monomer Water-soluble initiator Surfactant (detergent)

4 Polymer Synthesis CHEM 421 Surfactants H2OH2O Hydrophobic / Lipophilic core Surfactant Concentration Unimers Micelles Critical Micelle Concentration (CMC)

5 Polymer Synthesis CHEM 421 Surfactants Types - Anionic - Cationic - Amphoterics - Non-ionics

6 Polymer Synthesis CHEM 421 Emulsion Polymerization Recipe

7 Polymer Synthesis CHEM 421 Emulsion Polymerizations Polymz Rate Surfactant Concentration Critical Micelle Concentration

8 Polymer Synthesis CHEM 421 Kinetics of Emulsion Polymerization Percent Conversion Time I II III

9 Polymer Synthesis CHEM 421 Kinetics of Emulsion Polymerization Rate % Conversion IIIIII

10 Polymer Synthesis CHEM 421 Before Initiation I I I I I I I I I M M M M M M M M M M M M M M M M M M M Monomer Droplet ca. 1 micron diameter conc = /mL stabilized by soap Micelle Containing Monomer ca. 75 Å diameter conc = /mL Relative surface area 1 : 560 Initiation of micelles statistically favored

11 Polymer Synthesis CHEM 421 Interval One: 0 – 15 % Conversion I I I I I I I I I M M M M M M M M M M MM M P P M M Micelles Containing Monomer Active latex particle Micelles Containing Monomer Micelles Containing Monomer Active latex particles Inactive latex particles Inactive latex particles

12 Polymer Synthesis CHEM 421 Qualitative Details ConversionMicellesMonomer Droplets Particle Number Particle Size Comments I 0 – 15%present increases Nucleation period, Increasing Rp II III

13 Polymer Synthesis CHEM 421 Interval Two: 15 – 80% Conversion I I I I I I I M M M M M M MM M P P I I M M P Inactive latex particles Inactive latex particles Inactive latex particles Active latex particles Active latex particles I I M P Active latex particles No micelles Number of particles constant, therefore R p = constant

14 Polymer Synthesis CHEM 421 Kinetics of Emulsion Polymerization Number of Micelles Time IIIIII Number of Polymer Particles

15 Polymer Synthesis CHEM 421 Qualitative Details ConversionMicellesMonomer Droplets Particle Number Particle Size Comments I 0 – 15%present increases Nucleation period, Increasing Rp II 15 – 80%absentpresentconstantincreases Constant # of particles, C p = constant III

16 Polymer Synthesis CHEM 421 Interval Three: 80 – 100% Conversion I I I I I I M M M M M M M P P I M M P I M P M P M P I No monomer droplets No micelles

17 Polymer Synthesis CHEM 421 Qualitative Details ConversionMicellesMonomer Droplets Particle Number Particle Size Comments I 0 – 15%present increases Nucleation period, Increasing Rp II 15 – 80%absentpresentconstantincreases Constant # of particles, C p = constant III 80 – 100%absent constantroughly constant Constant # of particles, C p = decreasing

18 Polymer Synthesis CHEM 421 Emulsion Polymerization Kinetics Once inside a particle, radical propagates as r p = k p [M] Overall rate: R p = k p [M][P. ] [P. ] = Nñ (where N = the sum of micelle and particle concentrations and ñ = average # of radicals per particle) Therefore, –Increase N to increase rate!

19 Polymer Synthesis CHEM 421 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

20 Polymer Synthesis CHEM 421 Emulsion Polymerization Kinetics How to increase Rp? –Increase N to increase rate »Increase surfactant concentration to increase N

21 Polymer Synthesis CHEM 421 Molecular Weight in Emulsion Polymerizations Molecular weight determined by rate of growth of a chain divided by rate of radical entry (r i ) –How to increase molecular weight? DP rprp = riri RiRi = N riri = k p [M] rprp N k p [M] RiRi = DP

22 Polymer Synthesis CHEM 421 Free Radical Solution Polymerizations Recall –To increase molecular weight… »Increase monomer concentration »Decrease initiator concentration –To increase Rate of Polymerization »Increase monomer concentration »Increase initiator concentration ٧ = k p [M] 2 ( k t k d f [I] ) 1/2 = Cant do both! R p = k p [M] ( k d f [ I ] / k t ) 1/2


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