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Polymer SynthesisCHEM 421
Odian Book: Chapter 4
Polymer SynthesisCHEM 421
Emulsion Polymerizations
• 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
Polymer SynthesisCHEM 421
Emulsion Polymerization Recipe
• Water (continuous phase)
• Water-insoluble monomer
• Water-soluble initiator
• Surfactant (detergent)
Polymer SynthesisCHEM 421
Surfactants
H2O
Hydrophobic /Lipophilic core
SurfactantConcentration
Unimers Micelles
Critical MicelleConcentration
(CMC)
Polymer SynthesisCHEM 421
Surfactants
Types- Anionic
- Cationic
- Amphoterics
- Non-ionics
Polymer SynthesisCHEM 421
Emulsion Polymerization Recipe
Polymer SynthesisCHEM 421
Emulsion Polymerizations
Polym’zRate
Surfactant Concentration
Critical Micelle Concentration
Polymer SynthesisCHEM 421
Kinetics of Emulsion Polymerization
PercentConversion
Time
I
II
III
Polymer SynthesisCHEM 421
Kinetics of Emulsion Polymerization
Rate
% Conversion
I II III
Polymer SynthesisCHEM 421
Before Initiation
II
I
II
I
I I
I
M
M
MM
MM
M
MM M
MM
M
M
M
M
M
M
M
Monomer Dropletca. 1 micron diameter
conc = 1011/mLstabilized by soap
Micelle Containing Monomer
ca. 75 Å diameterconc = 1018/mL
Relative surface area1 : 560
Initiation of micelles statistically favored
Polymer SynthesisCHEM 421
Interval One: 0 – 15 % Conversion
I •I
I
II
I
I • I
I
M
M
MM
MM M
M
M
M
MM
M
PP••
PP••
M M
MicellesContaining Monomer
Activelatex particle
MicellesContaining Monomer
MicellesContaining Monomer
Activelatex particles
Inactivelatex particles
Inactivelatex particles
Polymer SynthesisCHEM 421
Qualitative Details
Conversion Micelles Monomer
Droplets
Particle
Number
Particle
Size
Comments
I0 – 15% present present increases increases
Nucleation period,
Increasing Rp
II
III
Polymer SynthesisCHEM 421
Interval Two: 15 – 80% Conversion
I
II
I
I • I
I
M
M
MM
M
M
MM
M
PP••
PP•• I •I
M
M
PP••
Inactivelatex particles
Inactivelatex particles
Inactivelatex particles
Activelatex particles
Activelatex particles
I •I
M
PP••Active
latex particles
No micelles
Number of particles constant, therefore
Rp = constant
Polymer SynthesisCHEM 421
Kinetics of Emulsion Polymerization
Number ofMicelles
Time
I II III
Number ofPolymerParticles
1018
0
1015
0
Polymer SynthesisCHEM 421
Qualitative Details
Conversion Micelles Monomer
Droplets
Particle
Number
Particle
Size
Comments
I0 – 15% present present increases increases
Nucleation period,
Increasing Rp
II15 – 80% absent present constant increases
Constant # of particles,
Cp = constant
III
Polymer SynthesisCHEM 421
Interval Three: 80 – 100% Conversion
I
II
I
I • I
M
M
MM
M
M
M
PP••
PP••I
M
M
PP••
I •M
PP••M
PP••
M
PP••
I •
No monomer droplets
No micelles
Polymer SynthesisCHEM 421
Qualitative Details
Conversion Micelles Monomer
Droplets
Particle
Number
Particle
Size
Comments
I0 – 15% present present increases increases
Nucleation period,
Increasing Rp
II15 – 80% absent present constant increases
Constant # of particles,
Cp = constant
III80 – 100% absent absent constant roughly
constant
Constant # of particles,
Cp = decreasing
Polymer SynthesisCHEM 421
Emulsion 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!
][' MknNR pp
Polymer SynthesisCHEM 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
Polymer SynthesisCHEM 421
Emulsion Polymerization Kinetics
• How to increase Rp?
–Increase N’ to increase rate
»Increase surfactant concentration to increase N’
][' MknNR pp
Polymer SynthesisCHEM 421
Molecular 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?
DPrp = ——ri
Ri = ——N
ri = kp[M]rp
N kp [M]
Ri
= ———DP
Polymer SynthesisCHEM 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 =kp [M]
2 (kt kd f [I])1/2 = —————
Can’t doboth!
Rp = kp [M] (kd f [I] / kt)1/2