Polymer Chemistry Free Radical Polymerization

1. Polymer ChemistryFree Radical Polymerization Jihperng (Jim) Leu

2. Chain Polymerization Free-radical polymerization What is free-radical • Each addition reproduces the reactive group. • most widely practised method of polymerization Three-Stage • Initiation • Propagation • Termination

4. Initiation • The formation of free radicals • The addition of one of these free radicals to a molecule of monomer • Paths to generate free radicals • Homolysis of a single-bond • Application of heat • Peroxide (-O-O-) or azo (-N=N-) linkage • 50-100 C • Examples • Application of radiation (UV) - photoinitiators • Single electron transfer to or from an ion or molecule (redox reaction) • Widely used when a low-temperature polymerization is desired • An active center is created when a free radical generated from an initiator attacks the pi-bond of a molecule • Examples

5. Initiation • Thermal initiator • Redox initiators • Photochemical • Ionizing radiation • Self-initiation:

6. Thermal Initiators • Most common • Unimolecular decomposition. • First order kinetics. • Most common examples: peroxides or azo compounds. Peroxides azo compound

7. Others Photochemical • Thin Films • Examples • Ionizing Radiation (hv) • X-ray, gamma-ray. • Random destruction leads to radical formation. • Used only in very special cases.

8. Propagation • The Steady State • Initiation is relatively slow but continuous. • Termination speeds up as active radical concentration builds. • Termination removes (kills) active radicals. • Results: a steady-state concentration of radicals is established early in the reaction. • The concentration of radicals is very small (ca. 10-8 M) and nearly constant throughout. • Propagation is Fast! • Time needed to reach 10E6 in MW: • Styrene7.6 s • Methyl methacrylate 1.5 s • Vinyl chloride0.13 s

9. The following graph illustrates the effect for PMMA polymerization in benzene solution at various conecntrations. Note that at low concentreations (40% or more dilute in this example), the polymerization proceeds smoothly with no unusual effects. However, at higher concentrations, a pronounced rate acceleration after partial conversion of monomer to polymer. The more concentrated the solution, the earlier the acceleration occurs.

10. To explain the effect, recall that initiation, propagation, and termination are completely different chemical reactions with different responses to conditions. Termination involves the reaction between two chain ends. However, in concentrated solutions, the viscosity of the reaction mixture becomes high as polymer chains form. This high viscosity hinders the diffusion of chains because of entanglements, so the rate of termination slows considerably. However, the diffusion of small molecular monomers is hardly affected by viscosity, so propagation proceeds as before. In addition, initiator continues to add more free radicals to the system. The rates of initiation and propagation come out of balance. What was once a low, steady state concentration of radicals gives way to increasing concentration. Chains grow without termination, so the conversion is rapid and the MW is high. In dilute solutions, the viscosity never builds up to the point where the diffusion of chains is slowed, so autoacceleration does not occur. For neat monomer (i.e., 100% in the graph), often in cases where the polymer formed is a high Tg material, there can come a point at which even the diffusion of monomer is slow. The mixture has become a hard glass, and unreacted radicals become trapped inside. The reaction shuts down at less than 100% conversion, as depicted in the curve.

11. Chain Transfer • The Essence of Chain Transfer • Chain termination occurs when two radical species (each odd-electron) react to form one or two new molecules without radical (even-electron). Chain transfer occurs when a radical species reacts with a nonradical species. The result must be at least one radical species. In the most common occurence, the chain end radical attacks a weak bond. An atom gets transferred to the chain end. • After this happens, the current chain is terminated. A new chain may start or not, depending on reactivity of new radical.

12. Chain Transfer agent • In many cases, a chain transfer agent is added deliberately to the reaction mixture. Many compounds work well for this purpose, but mercaptans (also known as thiols) are the most general. Example for styrene + butyl mercaptan: • The sulfur-centered radical reinitiates very efficiently. The result is a dimunition of the molecular weight without changing the overall rate of conversion of monomer to polymer. (Using more initiator is another way to decrease MW, but the reaction rate would increase proportionally, a possibly dangerous situation.)

13. Chain Transfer • Naturally, there are many even-electron species present in the reaction mixture (i.e., monomer, initiator, solvents, polymer chains, etc.), and all of these may participate in transfer reactions, depending on the relative reactivities of the structures involved. Here is an example of transfer to initiator featuring acrylonitrile and benzoyl peroxide (BPO): • One chain is terminated, but another one initiates. This particular reaction reduces MW and wastes initiator (i.e., an initiator molecule is consumed, but no new chains are begun). Sometimes this process is called induced decomposition of the initiator. It is a common side reaction for the peroxy initiators, but happens less often with the azo initiators.

14. Inhibition and Retardation • Inhibitor • Retarder • Most commercial monomers are packaged with traces of inhibitor to prevent premature polymerization. The inhibitor can be removed prior to polymerization by distillation, chromatography, or extraction. In many cases, it is simply left alone, and additional initiator is used to overwhelm the inhibitor. • Inhibitors are added in minute quantities to many other chemicals (e.g., ether, THF) to interrupt radical chain reactions that lead to decomposition. They are also used in foods to slow oxidation that leads to spoilage.

15. The role of Oxygen • Molecular oxygen presents peculiar behavior toward free radical polymerization. • O2 is a very potent inhibitor for most common vinyl monomers. Polymerization media are usually thoroughly purged with inert gas to remove O2, otherwise the reaction may not work. • Traces of O2 can initiate free radical vinyl polymerization (indirectly). O2 can react with some monomers or with trace impurities to form peroxy compounds that are thermal initiators.

16. Termination • Combination • Two radicals at the chain termini simply join to form a single bond, as shown here in an example with styrene: • Disproportionation • The radical at the end of one chain attacks a hydrogen atom at the second-to-last carbon atom in the second chain, as shown here in an example with methyl methacrylate (MMA): Replaced with correct example

17. Rate of Polymerization • Degree of Polymerization

18. Polymerization Process • Bulk – with monomer only • High rate of polymerization and DP • High MW and purity • High viscosity and heat removal issue • Solution – in a solvent • Lower the viscosity, better heat transfer, avoid autoacceleration • DP (reduced conc. And chain transfer) • Used in solution

19. Polymerization Process (cont.) • Suspension – with monomer dispersed in an aqueous phase • Better heat transfer • Reaction mixtures suspended as droplets in an inert medium (agitation, dispersion stabilizers) • High surface area in droplets (0.1-2 mm diameter as mini-reactors) • PMMA, PVC and PS • Water insoluble monomer. • Water insoluble initiator. • Suspending agent (optional). • Emulsion • Initiator must not soluble in monomers, but only in the aqueous dispersion medium (Water insoluble monomer; Water soluble initiator) • 0.05-1 um polymer particle • Complicated mechanism.

20. Comparison

22. Important Vinyl Polymersprepared by free radical polymerization • LDPE • PVC • PS • Polychloroprene (neoprene rubber) • PMMA • PVA • Poly(vinylidene chloride) • Polyacrylamide • Polytetrafluoroethylene • SBR • ABS • SAN • SMA • EVA (ethylene-vinyl acetate copolymer) • Acrylonitrile-vinyl chlorodie copolymer