Polymerization Techniques: Homogenous Systems and Heterogeneous Systems

1. Chain length distribution Crucial in the derivation of the CLD is the chance on formation of an i-mer chance on propagation: thus: chance on chain stop: 1-p During the growth of a single chain p is constant disproportionation mol fraction i-mer: mass fraction i-mer: which yields:

2. combination which yields:

3. Copolymerization k11 k12 k21 k22 —M1• + M1 —M1• —M1• + M2 —M2• —M2• + M1 —M1• —M2• + M2 —M2• } }

4. Copolymerization fi: fraction of monomer i in reaction mixture f1 = [M1] / ([M1] + [M2]) Fi: fraction of monomer i built into polymer F1 = d[M1] / (d[M1] + d[M2]) Average copolymerisation rate: Long chain assumption (ki, kd ignored; kp, kt not ~ chain length) Reactivity ratios independent of environmental factors

5. Ideal copolymerisation Composition drift If f1≠ F1 → f1 changes → F1 changes What does composition drift mean for the polymer that is formed?

6. Polymerization techniques Sometimes for one monomer several techniques of polymerizing are available. Choice of a specific technique depends on a number of factors: • Kinetic / mechanistic factors related to chain length, chain composition • Technological factors e.g. heat removal, reaction rate, viscosity of the reaction mixture, morphology of the product • Economic factors; production costs, enviromental aspects, purification steps etc.

7. Polymerization techniques Homogeneous systems • Bulk polymerization • Solution polymerization Heterogeneous systems • Suspension polymerization • Emulsion polymerization • Precipitation polymerization • Polymerization in solid state • Polymerization in the gas phase

8. Bulk polymerization Polymerization of the undiluted monomer. Viscosity increases dramatically during conversion. Heat removal and hot spots Advantages Disadvantages * Pure products * heat control * Simple equipment * dangerous * No organic solvents * molecular weights very disperse Applications Polymers through step reactions (nylon 6) PMMA-plates

9. Solution polymerization Monomer dissolved in solvent, formed polymer stays dissolved. Depending on concentration of monomer the solution does not increase in viscosity. Advantages Disadvantages * Product sometimes * Contamination directly usable with solvent * Controlled heat * Chain transfer to release solvent * Recycling solvent Applications Acrylic coating, fibrespinning, film casting

10. Suspension polymerization • Water insoluble monomers are dispersed in water. • Initiator dissolved in monomer. • Stabilization of droplets/polymer particles with non-micelle forming emulsifiers like polyvinylalcohol or Na-carboxymethylcellulose. • Equivalent to bulk polymerization, small droplets dispersed in water. • Product can easily be separated, particles 0.01-1mm. • Pore sizes can be controlled by adding a combination of solvent (swelling agent) and non-solvent. • Viscosity does not change much. Qualitative description of emulsion polymerization kinetics

11. Emulsion Polymerization • A micelle forming emulsifier is used. • Initiator is water soluble. • The formed latex particles are much smaller than suspension particles (0.05-2 µm). • Kinetics differ considerable from other techniques. • Polymer is formed within the micelles and not in the monomer droplets.