Polymer Synthesis CHEM 421 Chapter 3.9 (Odian)

Polymer Synthesis CHEM 421 Free Radically Polymerized Monomers

Polymer Synthesis CHEM 421 Thermodynamics of Chain Polymerizations Relief of “strain” is the driving force –Exothermic process

Polymer Synthesis CHEM 421 Thermodynamics ΔG, ΔH, and ΔS ΔG = ΔH - T ΔS Chain polymerizations: »Enthalpy »Exothermic (- ΔH) »Entropy »Negative ΔS Polymer favored from enthalpic considerations but un-favored from entropic considerations

Polymer Synthesis CHEM 421 Thermodynamics ΔH polymerization depends upon differences in: –Resonance stabilization of polymer versus the monomer –Steric strain in monomer versus the polymer –Hydrogen bonding or dipolar interaction in monomer versus the polymer

Polymer Synthesis CHEM 421 Thermodynamic Trends Ethylene Styrene α-Methyl styrene or Ethylene methyl acrylate methyl methacrylate PTFE (!) –Most exothermic polymerization known (kJ/mole) (J/°K-mole)

Polymer Synthesis CHEM 421 Equilibrium Considerations For most polymerizations, there is a temperature where the reaction becomes reversible The position for the monomer / polymer equilibrium will be dependent on the temperature ΔG = ΔH - T ΔS –Polymerization:ΔS = –De-polymerization:ΔS = With increasing temperature the equilibrium will shift? k p k dp

Polymer Synthesis CHEM 421 Equilibrium Considerations When R p = R dp –Ceiling temperature

Polymer Synthesis CHEM 421 Thermodynamics The reaction isotherm: ΔG = ΔG° + RTlnK is applicable. Where ΔG° is the ΔG of polymerization for the monomer and the polymer in the appropriate standard states –Monomer Std. State: pure liquid –Polymer Std. State: crystalline state if possible, otherwise amorphous state

Polymer Synthesis CHEM 421 Thermodynamics At equilibrium, ΔG = 0 by definition: ΔG° = - RTlnK Equilibrium constant is defined by K eq = k p / k dp K eq = ————— = ——— [M n+1] [M n][M] 1

Polymer Synthesis CHEM 421 Thermodynamics Combine: [M] c is the equilibrium monomer concentration as a function of reaction temperature –The monomer concentration in equilibrium with the polymer increases as the temperature increases T c = ————— ΔH° ΔS° + Rln[M] c

Polymer Synthesis CHEM 421 Ceiling Temperature Poly(α-methyl styrene) –T g = 170 °C –T ceil = 61 °C Processing temperature?

Polymer Synthesis CHEM 421 Ceiling Temperature Poly(methyl methacrylate) –T g = 125 °C –T ceil = 164 °C Recycle… PMMA 99% MMA monomer 300 °C

Polymer Synthesis CHEM 421 Heats of Polymerization and Ceiling Temperatures MonomerHeat of Polymerization (kcal/mol) Ceiling Temperature (°C) Styrene α-Methyl styrene-761 Methyl acrylate Methyl methacrylate Ethylene Propylene Isobutene-1750

Polymer Synthesis CHEM 421 Polymerization Processes Solution Polymerization Bulk (Mass) Polymerization Heterogeneous Polymerizations

Polymer Synthesis CHEM 421 Solution Polymerizations Ingredients –Monomer –Solvent –Initiator R p = k p [M] ( k d f [ I ] / k t ) 1/2 k p [M] 2 (k t k d f [I] ) 1/2 = ————— k p [M] [M] 2 k t [M] 2 = ——— R p R i R t ٧ = — = —

Polymer Synthesis CHEM 421 Solution Polymerization Solvent, monomer & initiator Polymer remains soluble in the solvent Easy temperature, viscosity, MW control Free radical kinetics apply Solvent M MM M M M M M M M I I I I I hνhν or Δ Solvent M M M M I I I P P P P P P P P

Polymer Synthesis CHEM 421 Solution Polymerization Considerations: –Chain transfer to solvent –Purity of polymer (difficulty in removing solvent) Used for: vinyl acetate, acrylonitrile, and esters of acrylic acid

Polymer Synthesis CHEM 421 Bulk (Mass) Polymerizations Ingredients: Monomer and Initiator only Kinetics follows solution polymerization kinetics…R p and ٧

Polymer Synthesis CHEM 421 Bulk (Mass) Polymerization Considerations: –Hard to control: high activation energies, gel effect –Equipment: elaborate, strong stirring due to viscosity increase –Temperature Control: local hot spots »Can lead to degradation, discoloration, and broad MW distribution »“Runaway” reactions –Used for styrene and methyl methacrylate (Chain Growth) »Low conversion and separation/recycling of un-reacted monomer